EP4111510A1

EP4111510A1 - Method of synthesising metal organic framework and metal organic framework

Info

Publication number: EP4111510A1
Application number: EP21709781.5A
Authority: EP
Inventors: Mario Gutiérrez Tovar; Jin-Chong Tan
Original assignee: Oxford University Innovation Ltd
Current assignee: Oxford University Innovation Ltd
Priority date: 2020-02-28
Filing date: 2021-02-26
Publication date: 2023-01-04
Also published as: GB202002886D0; WO2021171033A1

Abstract

The invention relates to a process for producing a metal organic framework, wherein the metal organic framework comprises one or more metal ions and one or more organic linkers, wherein the process comprises contacting the one or more metal ions with the one or more organic linkers in the presence of water to form a precipitate of the metal organic framework, wherein the process is performed at a temperature that does not exceed 50°C.

Description

METHOD OF SYNTHESISING METAL ORGANIC FRAMEWORK AND METAL

ORGANIC FRAMEWORK

The work leading to this invention has received funding from the European Research Council under the European Union's HORIZON 2020 research and innovation programme (grant agreement no. 771575).

FIELD OF INVENTION

The invention provides a process for producing a metal organic framework, a metal organic framework and devices comprising a metal organic framework. The invention also provides for producing a mixed-metal organic framework, a mixed metal organic framework and devices comprising the mixed-metal organic framework. The invention also provides a method for detecting the presence of a volatile organic compound using the mixed-metal organic framework and a method for analysing a sample of breath from a subject.

INTRODUCTION

Luminescent multifunctional smart-nanomaterials have aroused great interest because of their strong impact on the development of efficient and cost-effective devices in different technologies such as luminescent sensors or solid-state lighting. A good luminescent nanomaterial needs to fulfil a number of requirements for being integrated into real-world devices, such as being facile and cost-effective to fabricate, have high quantum yield and be robust and reliable, among others.

During the last few decades, the development of luminescent smart nanomaterials has been instigated by their potential application in technologies such as optoelectronics, anti-counterfeiting or bioimaging (Kumaret al., Future prospects of luminescent nanomaterial based security inks: from synthesis to anti-counterfeiting applications. Nanoscale 2016, 8 (30), 14297-14340; Jiang et al., Triple-Mode Emission of Carbon Dots: Applications for Advanced Anti-Counterfeiting. 2016,

55 (25), 7231-7235; Dingke Xue et al., High-bandwidth white-light system combining a micro-LED with perovskite quantum dots for visible light communication. ACS Appl. Mater. Interfaces 2018, 10, 5641; Yang et al., Recent advances in ultra-small fluorescent Au nanoclusters toward oncological research. Nanoscale 2019). Luminescent phototunable nanomaterials that experience changes in response to external chemical or mechanical stimuli have gained much interest, as they are ideal candidates for the fabrication of efficient non-invasive sensor devices (e.g. sensors of pollutants, volatile organic compounds, pH, pressure or thermometers) (Walekar et al., Functionalized fluorescent nanomaterials for sensing pollutants in the environment: A critical review. TrAC Trends in Analytical Chemistry 2017, 97, 458-467; Zhang et al., Luminescent sensors based on metal-organic frameworks. Coordination Chemistry Reviews 2018, 354, 28-45; Brites et al., Lanthanide-Based Thermometers: At the Cutting-Edge of Luminescence Thermometry. 2019, 7 (5), 1801239). Similarly, there has been a rapidly growing interest concerning the fabrication of new electroluminescent nanomaterials which can be used as luminescent layers in energy-efficient light-emitting diodes (LEDs) (Das et al., White light emitting diode based on purely organic fluorescent to modern thermally activated delayed fluorescence (TADF) and perovskite materials. 2019, 6 (1), 31.; Xu et al., A comprehensive review of doping in perovskite nanocrystals/quantum dots: evolution of structure , electronics , optics , and light-emitting diodes. Materials Today Nano 2019, 6, 100036). Among all the possibilities, metal-organic frameworks (MOFs), a class of hybrid ordered porous materials, have emerged as one of the most prominent over the last two decades. The combination of organic and inorganic moieties along with their high porosity and large surface area have made MOFs promising candidates for a wide range of applications (Burtch et al., Mechanical Properties in Metal-Organic Frameworks: Emerging Opportunities and Challenges for Device Functionality and Technological Applications. 2018, 30 (37), 1704124. Yan et al., Metal/covalent-organic frameworks-based electrocatalysts for water splitting. Journal of Materials Chemistry A 2018, 6 (33), 15905-15926.). Particularly, the field of luminescent MOFs (FMOFs) has been boosted over the last years through the development of an almost infinite number of new FMOF materials with potential for real-world applications like chemical sensors (i.e. VOC, nitroaromatics or antibiotic sensing), optoelectronics, biomedicine or thermometers (Hu et al., Luminescent metal-organic frameworks for chemical sensing and explosive detection. Chemical Society Reviews 2014, 43 (16), 5815-5840; Cui et al., Metal- organic frameworks for luminescence thermometry. Chemical communications 2015, 51 (35), 7420-7431; Fustig et al., Metal-organic frameworks: functional luminescent and photonic materials for sensing applications. Chemical Society Reviews 2017, 46 (11), 3242-3285; Zhang, Z et al., Metal-organic frameworks for multimodal bioimaging and synergistic cancer chemotherapy. Coordination Chemistry Reviews 2019, 399, 213022). For example, a wide number of luminescent thermochromic MOFs have been investigated because of their abilities to be exploited in the fabrication of a new generation of luminescent thermometers, which can overcome the intrinsic limitations of conventional thermometers (Cui, Y et al., Metal-organic frameworks for luminescence thermometry, Chemical communications 2015, 51 (35), 7420-7431; Rocha et al., Lanthanide Organic Framework Luminescent Thermometer, 2016, 22 (42), 14782-14795; Yue et al., Ratiometric near infrared luminescent thermometer based on lanthanide metal-organic frameworks. Journal of Solid State Chemistry 2016, 241, 99-104). Commercially available thermometers based on temperature-induced electrical resistance changes have inherent problems such as slow response and high sensitivity towards electric and magnetic fields (Xu et al., Highly sensitive optical thermometry through thermally enhanced near infrared emissions from Nd3+/Yb3+ codoped oxyfluoride glass ceramic, Sensors and Actuators B: Chemical 2013, 178, 520-524; Manzani et al., A portable luminescent thermometer based on green up-conversion emission of Er3+/Yb3+ co-doped tellurite glass, Scientific Reports 2017, 7, 41596). On the other hand, liquid-based thermometers are fragile, work by contact, and are not very accurate and cannot be used in environments involving extremely high or low temperatures. All these disadvantages make conventional thermometers useless in many applications where an accurate control of the temperature is essential, such as electrical transformers in power stations, in oil refineries, coal mines or building fires (Xu et al., Highly sensitive optical thermometry through thermally enhanced near infrared emissions from Nd3+/Yb3+ co-doped oxyfluoride glass ceramic, Sensors and Actuators B: Chemical 2013, 178, 520-524; Manzani et al., A portable luminescent thermometer based on green up-conversion emission of Er3+/Yb3+ co-doped tellurite glass, Scientific Reports 2017, 7, 41596). On the contrary, luminescent thermometers offer fast response, non-invasive operation and are inert to electric and magnetic fields. Moreover, the development of nano-sized luminescent thermometers has also attracted great attention due to their promising potential for understanding heat-transfer mechanisms occurring in living cells or integrated electronic circuits (Vetrone et al., Temperature Sensing Using Fluorescent Nanothermometers. ACS Nano 2010, 4 (6), 3254-3258; Marciniak et al., A new generation of highly sensitive luminescent thermometers operating in the optical window of biological tissues, J Mater Chem C 2016, 4 (24), 5559-5563; Brites et al., Thermometry at the nanoscale, Nanoscale 2012, 4 (16), 4799-4829). Although thermochromic LMOFs and nanomaterials have been extensively reported, most of them are based on expensive and non-environmentally friendly rare earth metals and/or complicated synthetic methodologies (Cui et al., Metal-organic frameworks for luminescence thermometry, Chemical communications 2015, 51 (35), 7420-7431); Dingke Xue et al., High- bandwidth white-light system combining a micro-LED with perovskite quantum dots for visible light communication, ACS Appl. Mater. Interfaces 2018, 10, 5641; Rocha et al., Lanthanide Organic Framework Luminescent Thermometers, 2016, 22 (42), 14782-14795; Yue et al., Ratiometric near infrared luminescent thermometer based on lanthanide metal-organic frameworks, Journal of Solid State Chemistry 2016, 241, 99-104).

Another complex challenge that scientists are facing is the reduction of global energy consumption. The fabrication of new energy-efficient LED devices free of expensive and toxic rare-earth metals is one possible solution to that issue. Hitherto, even though a large number of LMOFs have been developed, there are only a few examples in the literature of electroluminescent MOFs that can be employed as emissive layers in the fabrication of new LEDs, and these mostly employ rare-earth metals (Haider et al., Electrically Driven White Light Emission from Intrinsic Metal-Organic Framework, ACS Nano 2016, 10 (9), 8366-8375. 24. Chen et al., Electrical conductivity and electroluminescence of a new anthracene-based metal-organic framework with π-conjugated zigzag chains, Chemical communications 2016, 52 (10), 2019-2022. 25; Gutierrez et al., New OLEDs Based on Zirconium Metal-Organic Framework, 2018, 6 (6), 1701060.). Thus, the development of versatile new cost-effective smart LMOF materials having potential for multiuse purposes in technologies such as sensing or LEDs is urgently needed.

There is also a need to develop an energy-efficient synthetic route to producing MOFs, without the need for toxic and/or expensive solvents. At present, the synthesis of MOF materials is usually time-consuming and involves harmful solvents (DMF, DMA, etc) and harsh reaction conditions (high temperature and/or pressure). These reaction conditions are typically hard to scale up and may preclude the preparation of MOFs in bulk (multi-gram) scale.

Sun et al., Syntheses and characterizations of a series of silver-carboxylate polymers , Inorganica Chimica Acta 2004, 357 (4), 991-10011) describes an Ag-polymer produced via a synthetic protocol which involves the slow growth of large crystals via a layered water/methanol system over one week in the dark.

Y. Wu et al. Facile fabrication of Ag2(bdc)@Ag nano-composites with strong green emission and their response to sulfide anion in aqueous medium, Sensors and Actuators B 255 (2018) 3163-3169 describes an Ag network that containing Ag NPs synthesised via a route using DMF and heating to 85 °C. The emission of this material arises from the Ag nanoparticles.

Liu et al., Four silver-containing coordination polymers based on bis (imidazole) ligands , Journal of Coordination Chemistry 2008, 61 (22), 3583-3593 describes the hydrothermal synthesis of Ag- containing materials at 100 °C for 36 hours. Hydrothermal conditions consume a lot of energy due to the high temperatures/long time frames needed and also are difficult to scale-up.

Cui, Y et al., Metal-organic frameworks for luminescence thermometry , Chemical communications 2015, 51 (35), 7420-7431IV) Y. Cui et al. Chem. Commun., 2015, 51, 7420 — 7431 concerns with the use of different luminescent MOFs as thermometers. The MOFs of interest are based on expensive rare-earth and/or complex synthetic methodologies.

WO2013/186542 relates to anti-bacterial applications of MOFs. WO2013/186542 does not investigate or describe the luminescent properties of the materials synthesised. There therefore exists a need to develop MOFs with useful properties such as photoluminescence, electroluminescence, thermochromism and mechanochromism, which may be used in sensing devices. Ideally such MOFs will not use expensive and toxic elements such as rare earth metals and will be easy to synthesise via an environmentally friendly low-energy route that can readily be scaled up.

SUMMARY OF THE INVENTION

The present application provides an easy, cost-effective, eco-friendly, fast, and scalable route to highly photoluminescent, multistimuli-responsive (thermochromic, mechanochromic) and electroluminescent MOFs. These MOFs can be synthesized following a fast and simple step reaction using an eco-friendly solvent (typically water) and mild conditions (room temperature). The synthesis is easily scaled-up, allowing the fast production of large quantities of material. With this new methodology, large amounts (10 g) of useful MOF materials, for instance highly luminescent silver-containing MOFs, can be fabricated quickly (30-60 min) using mild conditions (room temperature) and an eco-friendly solvent (water). This methodology has also been extrapolated to the development of other MOF materials, providing unequivocal proof that this simple and green synthetic protocol can be straightforwardly extended to numerous other Ag-MOF materials.

Moreover, the MOFs produced via the synthetic route of the invention are very robust and exhibit exceptional photophysical properties. For example, they may be highly luminescent and exhibit high quantum yield in the solid-state (~60%). The MOFs produced via the synthetic route of the invention also exhibit fast and reliable responses to changes in temperature and pressure, and therefore could be deployed in the fabrication of luminescent non-invasive sensors. As mentioned above, commercially available thermometers have a series of inconveniences like slow response, contact/invasive operation and high sensitivity to electromagnetic fields, which make them ineffective in demanding technological applications where an accurate temperature control is crucial at power stations, refineries, pipelines and coal mines. Luminescent thermometers can overcome all these problems as they will provide a fast response, working in a non-invasive mode and are unaffected by electric fields. Although some luminescent thermometers have been reported, most of them are based on expensive and non-environmentally friendly rare -earths and/or other materials obtained through a very complex, expensive and/or non-environmentally friendly synthetic protocols. Therefore, this cost-effective method allows the fabrication of a cheap materials that exhibit excellent thermochromic, mechanochromic and pH responses. Solvatochromic responses can also be achieved for sensing of chemicals such as volatile organic compounds (VOCs).

Additionally, the MOFs described herein behave as a good electroluminescent materials, which has permitted fabrication of a new MOF-LED (light emitting diode) using the MOF as the electroluminescent layer. This is one of very few examples of electroluminescent MOFs reported hitherto. These results illustrate the huge potential for the MOFs synthesised to be deployed in real- word technologies such as thermometers, electroluminescent devices such as LEDs, chemical sensing (for instance in medical applications), force/pressure sensing, motion sensing, vibration damage detection and also as data storage devices and optical memory devices.

The invention therefore provides a process for producing a metal organic framework (MOF), wherein the metal organic framework comprises one or more metal ions and one or more organic linkers, wherein the process comprises contacting the one or more metal ions with the one or more organic linkers in the presence of water to form a precipitate of the metal organic framework, wherein the process is performed at a temperature that does not exceed 50°C.

The invention also provides a metal organic framework comprising one or more metal ions and one or more organic linkers, wherein the metal organic framework is obtainable by a process which comprises contacting the one or more metal ions with the one or more organic linkers in the presence of water to form a precipitate of the metal organic framework, wherein the process is performed at a temperature that does not exceed 50°C.

The invention also provides a metal organic framework comprising a silver (Ag) ion and one or more organic linkers, wherein the metal organic framework has a photoluminescence quantum yield of greater than 50%.

The invention also provides a process for producing a mixed-metal organic framework (MMOF), wherein the mixed-metal organic framework comprises a first metal ion, a second metal ion and one or more organic linkers, wherein the process comprises contacting a metal organic framework comprising a first metal ion and the one or more organic linkers with a compound comprising the second metal ion in the presence of a polar protic solvent to form the mixed-metal organic framework, preferably where the polar protic solvent is water. This process provides a further low-cost, low energy, green route to mixed-metal organic frameworks which have useful properties, such as luminescence and sensitivity to chemical stimuli (solvatochromic response).

The invention also provides a mixed-metal organic framework comprising a first metal ion, a second metal ion and one or more organic linkers, wherein the first metal ion is a zinc ion; and the second metal ion is a silver ion.

The invention also provides a method for detecting the presence of a volatile organic compound, the method comprising contacting a mixed-metal organic framework as described herein with the volatile organic compound and measuring a property of the mixed-metal organic framework.

The invention also provides a method for analysing a sample of breath from a subject, wherein the subject has, or is suspected of having, diabetes, said method comprising contacting the sample of breath with a mixed-metal organic framework as described herein, and measuring a property of the mixed-metal organic framework to determine the level of acetone in the sample.

The invention also provides a device comprising a metal organic framework or a mixed-metal organic framework as described herein, wherein the device is selected from a light emitting device, a photoluminescent device, a electroluminescent device, a luminescent thermometer, a mechanical force sensor, a chemical sensor, a motion sensing system, a data storage device, security paper, optical memory devices and vibration damage detectors.

The invention also provides a luminescent thermometer comprising a thermochromic metal organic framework or a mixed-metal organic framework, wherein the metal organic framework or a mixed- metal organic framework is as described herein.

The invention also provides a mechanical force sensor comprising a mechanochromic metal organic framework or a mixed-metal organic framework, wherein the metal organic framework or a mixed-metal organic framework is as described herein.

The invention also provides a chemical sensor comprising a metal organic framework or a mixed- metal organic framework as described herein.

The invention also provides an antibacterial material comprising a metal organic framework or a mixed-metal organic framework as described herein. The invention also provides a device comprising a metal organic framework or a mixed-metal organic framework as described herein, wherein the device is an optoelectronic device, preferably wherein the device is an electroluminescent device.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 shows powder X-ray diffraction (PXRD) patterns of the different OX-2 MOFs from Example 1 synthesized in methanol (OX-2_m), methanol but using different ratio of organic linker and metal salt, 1:2 BDC:AgN03 (OX-2_m:1/2), DMF (OX-2_DMF) and water (OX-2_w). The inset are photos of each powder showing the white colour of all materials with the exception of the brownish OX-2_m:1/2.

Figure 2 shows field emission scanning electron microscopy (FESEM) micrographs of (A-B) OX- 2_W and (C-D) OX-2_m:1/2. Figures 2 (E-J) show energy-dispersive X-ray (EDX) FESEM (FESEM- EDX) images of the different crystals found in OX-2_m:1/2 MOF, showing the distribution of Ag (Figures 2F and 21) and C (Figures 2G and 2J) in the selected crystals.

Figure 3A shows the excitation-emission map representation of OX-2_m, Figure 3B shows the excitation-emission map representation of OX-2_m:1/2, Figure 3C 3B shows the excitation-emission map representation of OX-2_DMF and Figure 3D 3B shows the excitation-emission map representation of OX-2_w recorded in the solid-state. The quantum yield recorded using an excitation wavelength of 330-nm, along with a photo of each sample under UV-irradiation (365- nm) are depicted as inset.

Figure 4A and 4B show PXRD patterns and emission spectra of OX-2 pellets compressed at different pressures (indicated as inset). Figures 4C and 4D show PXRD patterns and emission spectra of OX-2 powders obtaining by grinding the former pellets.

Figure 5A shows the emission spectra of OX-2 collected by exposing it to increasing temperatures. The inset is a representation of the I₀-I vs T showing a linear response offered by OX-2 to changes in the temperature. Figure 5B shows the emission spectra of OX-2 recorded at 25 and 100°C by heating-up and cooling down successively the material. The inset is a representation of the excellent repeatability and reproducibility exhibited by OX-2 to changes in the temperature. Figures 6A and 6B show photographs of the OX-2 pellet (A) and film (B) upon cyclically exposing to high (200-220°C) and room (25°C) temperature conditions.

Figure 7 shows the Fourier transform infrared (FTIR) spectra of OX-2m, OX-2m:1/2, OX-2DMF and OX-2w.

Figure 8A shows the FESEM micrograph of OX-2_m MOF. Figure 8B shows the FESEM micrograph of OX-2_DMF MOF.

Figures 9A and 9B show AFM topography images of the elongated nanoplates of OX-2_w. Figures 9C and 9D show thickness profiles of the OX-2_w elongated nanoplates obtained from the AFM image in 9A and 9B respectively, showing a thickness of tens of nm.

Figures 10 A to C show FESEM-EDX micrographs of OX-2_m:1/2. Figures 10 D to F show FESEM- EDX micrographs of OX-2_w, showing the “flower” shaped crystals and the homogeneously distributed elongated nanoplates, respectively.

Figure 11 shows nanosecond-picosecond emission decays of 11A) BDC linker, 12B) OX-2_DMF,

11C) OX-2_m, 11D) OX-2_m:1/2 and 11E) OX-2_w in solid-state. The observation wavelengths are indicated as inset and the samples were excited at 365 nm. The solid lines are from the best-fit using a multiexponential function.

Figure 12 shows a comparison of the PXRD pattern of the [Ag-(BDC)_1/2]_n reported and those of OX-2_w MOFs obtained through a small scale synthesis (hundreds of mg) and a scaled-up synthesis (10 mg).

Figure 13 shows the excitation-emission map of the OX-2_w obtained through the scaled-up method.

Figure 14 shows PXRD patterns of OX-2_w after being soaked in water for several periods (up to 21 days).

Figure 15 shows PXRD patterns of OX-2_w MOF as synthetized and after being exposed to ambient conditions in the lab (day light, ~40% humidity, solvent vapours, etc) for 70 days. Figure 16 shows emission of spectra of OX-2w MOF before and after being exposed to ambient conditions in the laboratory (day light, ~40% humidity, solvent vapours, etc) for different periods (up to 70 days). The inset shows a minimal decrease in the quantum yield values from 60% to 57%.

Figures 17A and 17B show PXRD patterns and emission spectra of OX-2 pellets compressed at different pressures for second time. Figures 18C and 18D show PXRD patterns and emission spectra of OX-2 powders obtaining by grinding the former pellets.

DETAILED DESCRIPTION

Definitions

The term “metal organic framework” or “MOF” is known in the art, and takes its normal meaning herein. Thus, the term refers to a compound comprising metal ions coordinated to organic ligands to form an extended one-, two-, or three-dimensional structure. Often, the structure is an extended two- or three-dimensional structure. It may for instance be an extended three-dimensional structure.

A “mixed-metal organic framework” refers to a MOFs comprising at least two types of metal ion.

The term “crystalline” as used herein indicates a crystalline compound, which is a compound having along-range ordered structure. A crystalline compound is typically in the form of crystals or, in the case of a polycrystalline compound, crystallites (i.e. a plurality of crystals having particle sizes of less than or equal to 1 μm). The crystals together often form a layer. The crystals of a crystalline material may be of any size.

The term “room temperature”, as used herein, refers to the conventional definition of room temperature of between 15 and 25°C.

The term “organic linker” as used herein indicates an organic molecule comprising two or more coordination sites suitable for coordinating to metal ions. Thus, an organic linker typically comprises two or more functional groups capable of coordinating to metal ions.

The term “nanoparticle”, as used herein, means a microscopic particle whose size is typically measured in nanometres (nm). A nanoparticle typically has a particle size of from 0.1 nm to 1000 from 0.1 nm to 1000 nm, for instance from 1 nm to 750 nm, from 10 nm to 500 nm. Typically, a nanoparticle is a particle having a size of from 50 to 500 nm, from 100 nm to 500 nm, or from 250 nm to 500 nm.

The term “halide” as used herein indicates the singly charged anion of an element in group VIIA of the periodic table. “Halide” includes fluoride, chloride, bromide and iodide.

The term “alkyl”, as used herein, refers to a linear or branched chain saturated hydrocarbon radical. An alkyl group may be a C_1-20 alkyl group, a C_1-14 alkyl group, a C_{1- 10} alkyl group, a C_1-6 alkyl group or a C_1-4 alkyl group. Examples of a C_{1- 10} alkyl group are methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl or decyl. Examples of C_1-6 alkyl groups are methyl, ethyl, propyl, butyl, pentyl or hexyl. Examples of C_1-4 alkyl groups are methyl, ethyl, i-propyl, n-propyl, t-butyl, s-butyl or n-butyl. If the term “alkyl” is used without a prefix specifying the number of carbons, it typically has from 1 to 6 carbons (and this also applies to any other organic group referred to herein).

The term “aryl”, as used herein, refers to a monocyclic, bicyclic or polycyclic aromatic ring which contains from 6 to 14 carbon atoms, typically from 6 to 10 carbon atoms, in the ring portion. Examples include phenyl, naphthyl, indenyl, indanyl, anthrecenyl and pyrenyl groups. The term “aryl group”, as used herein, includes heteroaryl groups. The term “heteroaryl”, as used herein, refers to monocyclic or bicyclic heteroaromatic rings which typically contain from six to ten atoms in the ring portion including one or more heteroatoms. A heteroaryl group is generally a 5- or 6- membered ring, containing at least one heteroatom selected from O, S, N, P, Se and Si. It may contain, for example, one, two or three heteroatoms. Examples of heteroaryl groups include pyridyl, pyrazinyl, pyrimidinyl, pyridazinyl, furanyl, thienyl, pyrazolidinyl, pyrrolyl, oxazolyl, oxadiazolyl, isoxazolyl, thiadiazolyl, thiazolyl, isothiazolyl, imidazolyl, pyrazolyl, quinolyl and isoquinolyl.

The term “alkylene group” as used herein, refers to a substituted or unsubstituted bidentate moiety obtained by removing two hydrogen atoms, either both from the same carbon atom, or one from each of two different carbon atoms, of a hydrocarbon compound having from 1 to 20 carbon atoms (unless otherwise specified), which may be aliphatic or alicyclic, and which may be saturated, partially unsaturated, or fully unsaturated. Thus, the term "alkylene" includes the sub-classes alkenylene, alkynylene, cycloalkylene, etc., discussed below. Typically it is C_{1- 10} alkylene, for instance C_1-6 alkylene. Typically it is C_1-4 alkylene, for example methylene, ethylene, i-propylene, n-propylene, t-butylene, s-butylene or n-butylene. It may also be pentylene, hexylene, heptylene, octylene and the various branched chain isomers thereof. An alkylene group may be substituted or unsubstituted, for instance, as specified above for alkyl. Typically a substituted alkylene group carries 1, 2 or 3 substituents, for instance 1 or 2.

In this context, the prefixes (e.g., C_1-4, C_1-7, C_1-20, C_2-7, C_3-7, etc.) denote the number of carbon atoms, or range of number of carbon atoms. For example, the term " C_1-4alkylene," as used herein, pertains to an alkylene group having from 1 to 4 carbon atoms. Examples of groups of alkylene groups include C_1-4 alkylene ("lower alkylene"), C_1-7 alkylene, C_1-10 alkylene and C_1-20 alkylene.

Examples of linear saturated C_1-7 alkylene groups include, but are not limited to, -(CH₂)_n- where n is an integer from 1 to 7, for example, -CH₂- (methylene), -CH₂CH₂- (ethylene), -CH₂CH₂CH₂- (propylene), and -CH₂CH₂CH₂CH₂- (butylene).

Examples of branched saturated C_1-7 alkylene groups include, but are not limited to, -CH(CH₃)-, -CH(CH₃)CH₂-, -CH(CH₃)CH₂CH₂-, -CH(CH₃)CH₂CH₂CH₂-, -CH₂CH(CH₃)CH₂-, -

CH₂CH(CH₃)CH₂CH₂-, -CH(CH₂CH₃)-, -CH(CH₂CH₃)CH₂-, and -CH₂CH(CH₂CH₃)CH₂-.

Examples of linear partially unsaturated C_1-7 alkylene groups include, but are not limited to, - CH=CH- (vinylene), -CH=CH-CH₂-, -CH₂-

CH=CH₂-, -CH=CH-CH₂-CH₂-, -CH=CH-CH₂-CH₂-CH₂-, -CH=CH-CH=CH-, -CH=CH-CH=CH- CH₂-, -CH=CH-CH=CH-CH₂-CH₂-, -CH=CH-CH₂-CH=CH-, and -CH=CH-CH₂-CH₂-CH=CH-.

Examples of branched partially unsaturated C_1-7 alkylene groups include, but are not limited to, -C(CH₃)=CH-, -C(CH₃)=CH-CH₂-, and -CH=CH-CH(CH₃)-.

Partially unsaturated alkylene groups comprising one or more double bonds may be referred to as alkenylene groups. Partially unsaturated alkylene groups comprising one or more triple bonds may be referred to as alkynylene groups (for instance -C≡C-, CH₂-C≡C-, and -CH₂-C≡C-CH₂-).

Examples of alicyclic saturated C_1-7 alkylene groups include, but are not limited to, cyclopentylene (e.g., cyclopent- 1 ,3 -ylene), and cyclohexylene (e.g., cyclohex-1, 4-ylene).

Examples of alicyclic partially unsaturated C_1-7 alkylene groups include, but are not limited to, cyclopentenylene (e.g., 4-cyclopenten-1,3-ylene), cyclohexenylene (e.g., 2-cyclohexen-1, 4-ylene; 3-cyclohexen-1,2-ylene; 2, 5-cyclohexadien-1, 4-ylene). Such groups may also be referred to as “cycloalkylene groups”.

The term “arylene group”, as used herein, refers to a substituted or unsubstituted bidentate moiety obtained by removing two hydrogen atoms, either both from the same carbon atom, or one from each of two different carbon atoms, of an aryl group, as defined herein. Thus, the term "arylene" includes phenylene, naphthylene, indenylene, indanylene, anthrecenylene and pyrenylene groups, and also heteroarylene groups such as pyridylene, pyrazinylene, pyrimidinylene, pyridazinylene, furanylene, thienylene, pyrazolidinylene, pyrrolylene, oxazolylene, oxadiazolylene, isoxazolylene, thiadiazolylene, thiazolylene, isothiazolylene, imidazolylene, pyrazolylene, quinolylene and isoquinolylene.

The term “substituted”, as used herein in the context of substituted organic groups, refers to an organic group which bears one or more substituents selected from C_{1- 10} alkyl, aryl (as defined herein), cyano, amino, nitro, C_{1- 10} alkylamino, di(C_1-10)alkylamino, arylamino, diarylamino, aryl( C_1- ₁₀)alkylamino, amido, acylamido, hydroxy, oxo, halo, carboxy, ester, acyl, acyloxy, C_{1- 10} alkoxy, aryloxy, halo( C_{1- 10}) alkyl, sulfonic acid, thiol, C_{1- 10} alkylthio, arylthio, sulfonyl, phosphoric acid, phosphate ester, phosphonic acid and phosphonate ester. Examples of substituted alkyl groups include haloalkyl, perhaloalkyl, hydroxyalkyl, aminoalkyl, alkoxyalkyl and alkaryl groups. When a group is substituted, it may bear 1, 2 or 3 substituents. For instance, a substituted group may have 1 or 2 substituents.

The term “optoelectronic device”, as used herein, refers to devices which source, control, detect or emit light. Light is understood to include any electromagnetic radiation. Examples of optoelectronic devices include photovoltaic devices, photodiodes (including solar cells), phototransistors, photomultipliers, photoresistors, light emitting devices, electroluminescent devices, light emitting diodes and charge injection lasers. Often, an “optoelectronic device” that is referred to herein is an electroluminescent device.

The term “electroluminescent device”, as used herein, refers to a light-emitting device comprising a material that accepts charge, both electrons and holes, which subsequently recombine and emit light.

The term “n-type layer” refers to a layer of an electron-transporting (i.e. an n-type) material. An electron-transporting (i.e. an n-type) material could, for instance, be a single electron-transporting compound or elemental material. An electron-transporting compound or elemental material may be undoped or doped with one or more dopant elements.

The term “p-type layer” refers to a layer of a hole-transporting (i.e. a p-type) material. A hole- transporting (i.e. a p-type) material could be a single hole-transporting compound or elemental material, or a mixture of two or more hole-transporting compounds or elemental materials. A hole- transporting compound or elemental material may be undoped or doped with one or more dopant elements.

The term “electroluminescent material”, as used herein, refers to a material that accepts charge, both electrons and holes, which subsequently recombine and emit light.

The term “photoluminescent material”, as used herein, refers to a material that is able to absorb photons and undergo photoexcitation, then emit photons.

The phrase “imidazole-based linker” refers to any organic linker comprising a substituted or unsubstituted imidazole group, or an ionic version thereof, for instance a substituted or unsubstituted imidazolate anion.

The phrase “aromatic carboxylate ion linker” refers to any organic linker comprising a carboxylate group and an aromatic group.

Process

The present invention provides a process for producing a metal organic framework (MOF), wherein the metal organic framework comprises one or more metal ions and one or more organic linkers, wherein the process comprises contacting the one or more metal ions with the one or more organic linkers in the presence of water to form a precipitate of the metal organic framework, wherein the process is performed at a temperature that does not exceed 50°C.

Typically the MOF is crystalline. Thus, typically the process comprises contacting the one or more metal ions with the one or more organic linkers in the presence of water to form a crystalline precipitate.

Typically, the precipitate of the metal organic framework forms within 10 minutes of contacting the one or metal ions with the one or more linkers, for instance within 5 minutes, within 2 minutes, within 1 minute or within 30 seconds of contacting the one or metal ions with the one or more linkers. Thus, the formation of the metal organic framework by the process described herein is distinguished from processes in which large crystals (hundreds of microns in size) are slowly grown from solution over a period of hours or even days. Typically, the metal organic framework is precipitated as a bulk solid. A bulk solid is an assembly of solid particles that is large enough for the statistical mean of any property to be independent of the number of particles.

Thus, the metal organic framework typically comprises a large number of small crystals. Typically the crystals of the metal organic framework in the precipitate have an average maximum dimension of less than 500 μm, for instance of less than 100 μm, of less than 50 μm, preferably less than 10 μm. Preferably, the crystals of the metal organic framework in the precipitate have an average maximum dimension of less than 5 μm, more preferably less than 2 μm. Typically, the nanoparticles are plate-shaped.

Typically, the metal organic framework precipitates as nanoparticles. Thus, the metal organic framework typically consists of particles having an average maximum dimension of from 0.1 nm to 1000 nm, for instance from 1 nm to 750 nm, from 10 nm to 500 nm, or for example from 50 to 500 nm, from 100 nm to 500 nm, or from 250 nm to 500 nm. Preferably, the metal organic framework consists of particles having an average maximum dimension of from 250 to 450 nm, for instance from 300 to 400 nm. Typically the nanoparticles are in the form of nanoplates.

The process is performed at a temperature that does not exceed 50°C. Typically, the process is performed at a temperature that does not exceed 40°C, for instance at a temperature that does not exceed 30°C. Preferably the process is performed at a temperature of from 0 to 30°C, typically from 10 to 30°C, for instance from 20 to 30°C. The process is typically performed at room temperature, for instance at a temperature of from 15 to 25°C, typically about 20°C. In the context of the temperature, the process refers to the process for producing the metal organic framework, rather than any subsequent processing that may be performed on the metal organic framework e.g. drying.

Metal ions

By “one or more metal ions” is meant one or more kinds of metal ions, i.e. the MOF may comprise only one kind of metal ion or may comprise two or more kinds of metal ions. The kinds of metal ions may differ by charge and/or element. Typically, the MOF comprises one kind of metal ion or two kinds of metal ions.

Typically, the one or more metal ions comprise one or more metal ions of group 9, 10, 11, 12 or 14 of the periodic table. Thus, the one or more metal ions may consist of ions of metals selected from the group consisting of cobalt (Co), rhodium (Rh), iridium (Ir), nickel (Ni), palladium (Pd), platinum (Pt), copper (Cu), silver (Ag), gold (Au), zinc (Zn), cadmium (Cd), mercury (Hg) and lead (Pb). The one or more metal ions may be transition metal ions. Typically, the one or more metal ions comprise one or more ions selected from Ag, Au, Cu, Cd, Ir, Pt, Pd and Pb ions. Preferably, the one or more metal ions comprise Ag ions, typically Ag⁺ ions. The one or more metal ions may consist of Ag ions, typically an Ag⁺ ions.

Typically, the process comprises a step of dissolving a salt of the one or more metal ions in an aqueous solution. The salt comprises one or more metal ions, as described herein, and a counterion. Typically, the counterion is selected from the group consisting of halide ions, nitrate ions, sulfate ions, hydroxide ions, or organic anions such as carboxylate anions. For instance, the counterion may be selected from the group consisting of fluoride, chloride, bromide, iodide, nitrate or ethanoate (acetate). Thus, the salt may be selected from a salt comprising a metal selected from groups 9, 10, 11,12 or 14 of the periodic table, and a counterion selected from the group consisting of halide ions, nitrate ions, sulfate ions, hydroxide ions, or organic anions such as carboxylate anions, preferably from the group consisting of fluoride, chloride, bromide, iodide, nitrate or ethanoate. For instance, the salt may be a salt comprising an Ag, Au, Cu, Cd, Ir, Pt, Pd or Pb ion, preferably an Ag ion, and a counterion selected from the group consisting of fluoride, chloride, bromide, iodide, nitrate or ethanoate. For instance, the salt may be silver nitrate (AgNO₃), silver chloride (AgCl) or silver acetate (CH₃CO₂Ag).

The aqueous solution is typically water. Thus, preferably the process comprises a step of dissolving a salt of the one or more metal ions, as defined herein, in water.

Organic linkers

By “one or more organic linkers” is meant one or more kinds of organic linkers i.e. the metal organic framework may comprise only one kind of organic linker or may comprise two or more kinds or organic linker. Typically, the metal organic framework comprises one kind of organic linker or two kinds of organic linker.

Typically, the one or more organic linkers are selected from carboxylate ion linkers and imidazole- based linkers. Preferably the one or more organic linkers are selected from aromatic carboxylate ion linkers and imidazole-based linkers. The metal organic framework may comprise one kind of organic linker or two kinds of organic linker. For instance, the metal organic framework may comprise one kind of organic linker that is a carboxylate ion linker. The metal organic framework may comprise one kind of organic linker that is an imidazole-based linker. The metal organic framework may comprise both carboxylate ion linkers and an imidazole-based linkers.

When the one or more organic linkers comprise aromatic carboxylate ion linkers, the process typically comprises a step of deprotonating an aromatic carboxylic acid with a base to produce the aromatic carboxylate ion linker. The base may be any base known to the skilled person suitable for deprotonating an aromatic carboxylic acid to produce the carboxylate anion. For instance, the base may be an amine or a metal hydroxide. Typically the base is trimethylamine (NEt₃) or sodium hydroxide (NaOH).

When the one or more organic linkers comprise aromatic carboxylate ion linkers, the process may comprise a step of deprotonating an aromatic carboxylic acid by dissolving the aromatic carboxylic acid in a solvent, preferably in water, to form the aromatic carboxylate ion linker. The process may comprise dissolving a salt of an aromatic carboxylic acid in a solvent, preferably water, to form the aromatic carboxylate ion linker. In this case, the aromatic carboxylic acid or the salt of an aromatic carboxylic acid dissociates to form the aromatic carboxylate ion linker in solution in the solvent.

When the one or more organic linkers comprise an imidazole-based linker, the process may comprise a step of deprotonating an imidazole derivative with a base to produce the imidazole- based linker. For instance, when the one or more organic linkers comprise an imidazole-based linker, the process may comprise a step of deprotonating an organic linker comprising a substituted or unsubstituted imidazole group to produce an organic linker comprising a substituted or unsubstituted imidazolate group. Typically, the base is an amine or a metal hydroxide. Typically the base is trimethylamine (NEt₃) or sodium hydroxide (NaOH).

Typically, the aromatic carboxylate ion linker comprises two or more carboxylate groups. For instance, the aromatic carboxylate ion linker may comprise three or more carboxylate groups. Typically, the aromatic carboxylate ion linker comprises two or three carboxylate groups.

The aromatic carboxylate ion linker typically comprises a substituted or unsubstituted aryl group, typically a substituted or unsubstituted aryl group selected from the group consisting of substituted or unsubstituted phenyl, substituted or unsubstituted napthyl, substituted or unsubstituted biphenyl, substituted or unsubstituted indenyl, substituted or unsubstituted indanyl, substituted or unsubstituted anthracenyl, substituted or unsubstituted pyrenyl and substituted or unsubstituted 1 , 3, 5-triphenyl benzene. Typically the substituted or unsubstituted aryl group is selected from the group consisting of substituted or unsubstituted phenyl, substituted or unsubstituted napthyl, substituted or unsubstituted biphenyl and substituted or unsubstituted 1, 3, 5-triphenyl benzene. Typically, the aromatic carboxylate ion linker comprises a phenyl group, a napthyl group, a biphenyl group or 1, 3, 5-triphenyl benzene substituted with two or more carboxylate groups. For instance, the aromatic carboxylate ion linker may comprise a phenyl group, a napthyl group, a biphenyl group or 1, 3, 5-triphenyl benzene substituted with three or more carboxylate groups. Typically, the aromatic carboxylate ion linker comprises a phenyl group, a napthyl group, a biphenyl group or 1, 3, 5-triphenyl benzene substituted with two or three carboxylate groups.

Thus, the aromatic carboxylate ion linker is typically selected from an ion of a substituted or unsubstituted benzene dicarboxylic acid, an ion of a substituted or unsubstituted benzene tricarboxylic acid, an ion of a substituted or unsubstituted naphthalene dicarboxylic acid, an ion of a substituted or unsubstituted biphenyl dicarboxylic acid, or an ion of a substituted or unsubstituted 1,3,5-triphenylbenzene tricarboxylic acid.

Preferably, the aromatic carboxylate ion linker is selected from an ion of a compound from the group consisting of

wherein R and R’ are selected from H, OH, NH₂, CH₃, CN, NO₂, F, Cl, Br, I, -OC₃H₅, OC₇H₇. Typically, the aromatic carboxylate ion linker is benzene-1, 4-dicarboxylate (terephthalate), benzene-1, 3-dicarboxylate (isophthalate), benzene-1, 2-dicarboxylate (phthalate), naphthalene-2, 6- dicarboxylate or naphthalene-1, 4-dicarboxylate. Preferably, the aromatic carboxylate ion linker is benzene-1 , 4-dicarboxylate (terephthalate).

Typically, the imidazole-based linker is an imidazolate linker of formula (I) or an imidazole linker of formula (II):

Wherein R₁, R₂, R₃, R₄, R₅, R₆, and R₇ are each independently selected from hydrogen, C_{1- 10} alkyl, C_{2- 10} alkenyl, C_{2- 10} alkynyl, cyano, and halogen, provided that R₂ and R₃ or R₆ and R₇ may be joined so as to form a substituted or unsubstituted ring, preferably wherein R₂ and R₃ or R₆ and R₇ are joined to form a 5 or 6-membered ring, optionally wherein the ring comprises 1, 2 or 3 heteroatoms selected from O, N and S, and provided that any one of R₁ to R₇ may be a hydrocarbon linker bonded to a further substituted or unsubstituted imidazole or imidazolate ring.

When one of R₁ to R₇ is a hydrocarbon linker bonded to a further substituted or unsubstituted imidazole, the hydrocarbon linker may be selected from the group consisting of alkylene, arylene, alkylene- arylene, alkylene-arylene-alkylene-, for instance from the group consisting of C_1-10 alkylene, C_6-10 arylene, C_1-10 alkylene -C_6-10 arylene, C_1-10 alkylene -C_6-10 arylene - C_1-10 alkylene-. Thus, the imidazole-based linker may comprise two substituted or unsubstituted imidazole or imidazolate rings joined by a hydrocarbon linker.

Typically, the imidazole-based linker is an imidazolate linker of formula (I) or an imidazole linker of formula (II), wherein R₁, R₂, R₃, R₄, R₅, R₆, and R₇ are each independently selected from hydrogen, C_1-4 alkyl, C_2-4 alkenyl, C_2-4 alkynyl, cyano, and halogen, provided that R₂ and R₃ or R₆ and R₇ may be joined so as to form a substituted or unsubstituted 5 or 6-membered ring, optionally wherein the ring comprises 1, 2 or 3 heteroatoms selected from O, N and S.

Preferably, the imidazole-based linker is selected from the group consisting of wherein R₆, and R₇ are selected from H, methyl, CN and Cl. For instance, the imidazole-based linker may be selected from the group consisting of imidazole, 2-methylimidazole and 4,5- dichloroimidazole.

Typically, the one or more metal ions comprise an Ag ion, and the one or more organic linkers comprise a terephthalate linker. Preferably the one or more metal ions are Ag ions and the one or more organic linkers are terephthalate linkers. Typically, the metal organic framework further comprises water of crystallization. For instance, the metal organic framework may comprise water molecules coordinated to the one or more metal ions and/or the one or more organic linkers. For example, the metal organic framework may comprise Ag ions and the water molecules may be coordinated to the Ag ions. Typically, the only solvent molecules present in the metal organic framework are water molecules.

Typically, the process comprises contacting an aqueous solution of the one or more metal ions with an aqueous solution of the one or more organic linkers. For instance, the process may comprise mixing an aqueous solution of the one or more metal ions with an aqueous solution of the one or more organic linkers.

Typically the aqueous solution of the one or more metal ions is a solution of the one or more metal ions in a solvent comprising at least 50% water, at least 75% water, at least 90% water, for instance at least 95% water or at least 99% water. Typically, the aqueous solution of the one or more metal ions is a solution of the one or more metal ions in water.

Typically the aqueous solution of the one or more organic linkers is a solution of the one or more organic linkers in a solvent comprising at least 50% water, at least 75% water, at least 90% water, for instance at least 95% water or at least 99% water. Typically, the aqueous solution of the one or more organic linkers is a solution of the one or more organic linkers in water.

Thus, when the one or more metal ions contact the one or more organic linkers, the only solvent present may be water. Preferably, the only solvent used in the process is water.

The molar ratio of metal ion to linker used in the process is typically between 1 : 1 and 1:10. For instance, the molar ratio of metal ion to linker may be between 1 : 1 and 1 : 5, for instance between 1 : 1 and 1 :3. The molar ratio of metal ion to linker may be about 1:5, 1 :4, 1 :3 or 1 :2. Typically, the molar ratio of metal ion to linker is 1 :2.

The process may further comprise recovering the metal organic framework. The skilled person would be well of methods for recovering the precipitate of the metal organic framework. For instance, the process may comprise isolating the precipitate of the metal organic framework by filtration, by centrifugation or by removing the surrounding solvent. The step of recovering the metal organic framework may comprise one or more optional washing steps, where the metal organic framework is washed with a solvent, preferably where the metal organic framework is washed with water. The step of recovering the metal organic framework may comprise one or more drying steps where the solvent is removed.

Metal organic framework

The invention also provides a metal organic framework comprising one or more metal ions, as defined herein, and one or more organic linkers, as defined herein, wherein the metal organic framework is obtainable by a process which comprises contacting the one or more metal ions with the one or more organic linkers in the presence of water to form a precipitate of the metal organic framework, wherein the process is performed at a temperature that does not exceed 50°C. The process may contain any of the process features described herein.

The invention also provides a metal organic framework comprising a silver ion and one or more organic linkers, wherein the metal organic framework has a photoluminescence quantum yield of greater than 50%.

The metal organic framework may have a photoluminescence quantum yield of at least 55%, preferably at least 60%. Typically, the photoluminescence quantum yield is measured at an excitation wavelength of 330nm.

Typically, the metal organic framework is crystalline. Typically, the metal organic framework is in the form of a bulk solid. A bulk solid is an assembly of solid particles that is large enough for the statistical mean of any property to be independent of the number of particles. Thus, the metal organic framework typically comprises a large number of small crystals. Typically the crystals of the metal organic framework in the precipitate have an average maximum dimension of less than 500 μm, for instance of less than 100 μm, of less than 50 μm, preferably less than 10 μm. Preferably, the crystals of the metal organic framework have an average maximum dimension of less than 5 μm, more preferably less than 2 μm.

Typically, the metal organic framework is in the form of nanoparticles. Thus, the metal organic framework typically consists of particles having an average maximum dimension of from 0.1 nm to 1000 nm, for instance from 1 nm to 750 nm, from 10 nm to 500 nm, or for example from 50 to 500 nm, from 100 nm to 500 nm, or from 250 nm to 500 nm. Preferably, the metal organic framework consists of particles having an average maximum dimension of from 250 to 450 nm, for instance from 300 to 400 nmTypically the nanoparticles are plate -shaped. The one or more organic linkers are as defined herein. Typically the one or more organic linkers comprise an ion of a substituted or unsubstituted benzene dicarboxylic acid, an ion of a substituted or unsubstituted benzene tricarboxylic acid, an ion of a substituted or unsubstituted naphthalene dicarboxylic acid, an ion of a substituted or unsubstituted biphenyl dicarboxylic acid, or an ion of a substituted or unsubstituted 1,3,5-triphenylbenzene tricarboxylic acid. Preferably the one or more organic linkers comprise a terephthalate linker. Thus, the metal organic framework may comprise a silver ion and a terephthalate linker.

Typically, the metal organic framework comprises water of crystallisation. For instance, the metal organic framework may comprise water molecules coordinated to the one or more metal ions and/or the one or more organic linkers. For example, when the metal organic framework comprises Ag ions, the metal organic framework may also comprise water molecules coordinated to the Ag ions.

The invention also provides a metal organic framework comprising one or more metal ions, as defined herein, and one or more organic linkers, as defined herein and water of crystallisation. Typically, the only solvent molecules present in the metal organic framework are water molecules.

Process for mixed-metal organic framework

The invention also provides a process for producing a mixed-metal organic framework (MMOF), wherein the mixed-metal organic framework comprises a first metal ion, a second metal ion and one or more organic linkers, wherein the process comprises contacting a metal organic framework comprising a first metal ion and the one or more organic linkers with a compound comprising the second metal ion in the presence of a polar protic solvent to form the mixed-metal organic framework.

Typically, the polar protic solvent is a solvent selected from the group consisting of a C1 -10 alcohol, water or mixtures thereof. For instance, the polar protic solvent may be a solvent selected from the group consisting of methanol, ethanol, propanol, butanol, water and mixtures thereof. Preferably the polar protic solvent is water.

The process usually comprises contacting the metal organic framework with a compound comprising the second metal ion and the polar protic solvent. Preferably the process comprises contacting the metal organic framework with a compound comprising the second metal ion and water. Typically, the process comprises mixing the metal organic framework with a compound comprising the second metal ion to create a mixture, then contacting the mixture with the polar protic solvent to form the mixed-metal organic framework. For instance, the process may comprise mixing the metal organic framework with a compound comprising the second metal ion to create a mixture, then contacting the mixture with water to form the mixed-metal organic framework.

Typically, the metal organic framework and the compound comprising the second metal ion are both solids. Thus, the process may comprise grinding the metal organic framework and the compound comprising the second metal ion together to form a mixture, then contacting the mixture with a polar protic solvent, preferably water, to form the mixed-metal organic framework. The metal organic framework and the compound comprising the second metal ion may both be powders. Thus, the process may comprise mixing a powder of the metal organic framework with a powder of a compound comprising the second metal ion to create a mixture, then contacting the mixture with a polar protic solvent, preferably water, to form the mixed-metal organic framework.

The polar protic solvent may be present in liquid form, or may be present in the surrounding atmosphere as a vapour. Thus, when the polar protic solvent is water, the humidity of the air may be sufficient to facilitate conversion of the metal organic framework to a mixed-metal organic framework. For instance, the process may comprise contacting the metal organic framework with a compound comprising the second metal ion and a vapour of the polar protic solvent, preferably water vapour. On the other hand, the polar protic solvent may be added to the metal organic framework and the compound comprising the second metal ion as a liquid. For instance, the process may comprise contacting the metal organic framework with a compound comprising the second metal ion and the polar protic solvent in liquid form, preferably liquid water.

The metal organic framework comprising a first metal ion and the one or more organic linkers may be any metal organic framework as defined herein. Typically, the first and second metal ions are metal ions of group 9, 10, 11, 12 or 14 of the periodic table. Thus, the first and second metal ions may be selected from the group consisting of cobalt (Co), rhodium (Rh), iridium (Ir), nickel (Ni), palladium (Pd), platinum (Pt), copper (Cu), silver (Ag), gold (Au), zinc (Zn), cadmium (Cd), mercury (Hg) and lead (Pb). The first and second metal ions may be transition metal ions. Typically the first and second metal ions are selected from the group consisting of Zn, Ag, Au, Cu, Cd, Ir, Pt, Pd and Pb ions. Preferably the first metal ion is a Zn ion, for instance a Zn²⁺ ion, and the second metal ion is an Ag ion, for instance a Ag⁺ ion.

Typically, the compound comprising the second metal ion is a salt of the second metal ion. The salt comprises the second metal ion, as described herein, and a counterion. Typically, the counterion is selected from the group consisting of halide ions, nitrate ions, sulfate ions, hydroxide ions, or organic anions such as carboxylate anions. For instance, the counterion may be selected from the group consisting of fluoride, chloride, bromide, iodide, nitrate or ethanoate (acetate). Thus, the salt may be selected from a salt of a metal selected from groups 9, 10, 11, 12 or 14 of the periodic table, with a counterion selected from the group consisting of halide ions, nitrate ions, sulfate ions, hydroxide ions, or organic anions such as carboxylate anions, preferably from the group consisting of fluoride, chloride, bromide, iodide, nitrate or ethanoate. For instance, the salt may be a salt of an Zn, Ag, Au, Cu, Cd, Ir, Pt, Pd or Pb ion, preferably Ag, with a counterion selected from the group consisting of fluoride, chloride, bromide, iodide, nitrate or ethanoate. For instance, the salt may be silver nitrate (AgNO₃), silver chloride (AgCl) or silver actetate (CH₃CO₂Ag).

Typically, the salt of the second metal ion is a salt of formula MX_n, wherein M represents the second metal ion; Xis an anion selected from halide, nitrate or CH₃COO; and where depends on the charge of the second metal ion, M, and the charge of the anion, X. Thus, if M has a charge of +1, n=1 and the salt of the second metal ion is a salt of formula MX wherein M represents the second metal ion; Xis an anion selected from halide, nitrate or CH₃COO-.

The one or more organic linkers may be any organic linkers as defined herein. Typically, the one or more organic linkers are selected from aromatic carboxylate ion linkers and imidazole-based linkers.

Typically, the aromatic carboxylate ion linker is benzene-1, 4-dicarboxylate (terephthalate) or benzene 1,3-dicarboxylate (isophthalate). Preferably, the aromatic carboxylate ion linker is benzene-1 , 4-dicarboxylate (terephthalate).

The invention also provides a mixed-metal organic framework obtainable by the process for producing a mixed-metal organic framework as defined herein.

Mixed-metal organic framework

The invention also provides a mixed-metal organic framework comprising a first metal ion, a second metal ion and one or more organic linkers, wherein the first metal ion is a zinc ion; and the second metal ion is a silver ion. The one or more organic linkers may be any organic linkers as defined herein. Typically, the one or more organic linkers are selected from aromatic carboxylate ion linkers and imidazole-based linkers. Typically, the aromatic carboxylate ion linker is benzene-1, 4-dicarboxylate (terephthalate) or benzene 1,3-dicarboxylate (isophthalate). Preferably, the aromatic carboxylate ion linker is benzene-1 , 4-dicarboxylate (terephthalate).

Chemical sensing using a mixed-metal organic framework

The invention provides a device comprising a mixed-metal organic framework as defined herein, wherein the device is a chemical sensor. A chemical sensor is a device for detecting the presence of a particular chemical or group of chemicals. Typically, the mixed-metal organic framework in the chemical sensor undergoes a change when exposed to a chemical of interest. For instance, the mixed-metal organic framework may be photoluminescent, and the photoluminescence properties of the mixed-metal organic framework change in the presence of the chemical of interest. For instance, the emission intensity may increase or decrease, and/or the colour of the emitted light may change upon exposure of the mixed-metal organic framework to the chemical of interest.

Typically, the device is a chemical sensor for detecting a volatile organic compound (VOC). Preferably, the device is a chemical sensor for detecting acetone.

Preferably, the device comprises a mixed-metal organic framework comprising a first metal ion, a second metal ion and one or more organic linkers, wherein the first metal ion is a zinc ion; and the second metal ion is a silver ion. Typically, the one or more organic linkers are selected from aromatic carboxylate ion linkers, preferably from an ion of a substituted or unsubstituted benzene dicarboxylic acid, an ion of a substituted or unsubstituted benzene tricarboxylic acid, an ion of a substituted or unsubstituted naphthalene dicarboxylic acid, an ion of a substituted or unsubstituted biphenyl dicarboxylic acid, or an ion of a substituted or unsubstituted 1,3,5-triphenylbenzene tricarboxylic acid. Typically, the aromatic carboxylate ion linker is benzene-1, 4-dicarboxylate (terephthalate) or benzene 1,3-dicarboxylate (phthalate). Preferably, the aromatic carboxylate ion linker is benzene-1, 4-dicarboxylate (terephthalate).

Thus the chemical sensor typically comprises a mixed-metal organic framework comprising zinc ions, a silver ions and terephthalate ions. Method for detecting a volatile organic compound

The invention therefore also provides a method for detecting the presence of a volatile organic compound, the method comprising contacting a mixed-metal organic framework as defined herein with the volatile organic compound and measuring a property of the mixed-metal organic framework.

Contacting the mixed-metal organic framework with the volatile organic compound may comprise contacting the mixed-metal organic framework with a liquid comprising the volatile organic compound, or contacting the mixed-metal organic framework with a vapour of the volatile organic compound.

Typically, the mixed-metal organic framework is photoluminescent, and the property of the mixed- metal organic framework is photoluminescence, for instance fluorescence. Typically, the photoluminescence properties of the mixed-metal organic framework may change in the presence of the chemical of interest, for example, the emission intensity may increase or decrease, and/or the colour of the emitted light may change upon exposure of the mixed-metal organic framework to the volatile organic compound of interest. Typically, the emission is decreased (quenched) in the presence of the volatile organic compound of interest.

Thus, the property may be fluorescence and measuring a property of the mixed-metal organic framework comprises measuring the emission spectrum of the metal organic framework.

Preferably, the volatile organic compound is acetone.

The mixed-metal organic framework may be any mixed-metal organic framework as described herein. Typically, the mixed-metal organic framework comprises a first metal ion, a second metal ion and one or more organic linkers, wherein the first metal ion is a zinc ion; and the second metal ion is a silver ion. Typically, the one or more organic linkers are selected from aromatic carboxylate ion linkers, preferably from ions of a substituted or unsubstituted benzene dicarboxylic acid, ions of a substituted or unsubstituted benzene tricarboxylic acid, ions of a substituted or unsubstituted naphthalene dicarboxylic acid, ions of a substituted or unsubstituted biphenyl dicarboxylic acid, or ions of a substituted or unsubstituted 1,3,5-triphenylbenzene tricarboxylic acid. Typically, the aromatic carboxylate ion linker is benzene-1, 4-dicarboxylate (terephthalate) or benzene 1,3-dicarboxylate (phthalate). Preferably, the aromatic carboxylate ion linker is benzene - 1 , 4-dicarboxylate (terephthalate). Thus the method for detecting the presence of a volatile organic compound may comprising contacting a mixed-metal organic framework comprising zinc ions, silver ions and terephthalate ions with a volatile organic compound and measuring a property of the mixed-metal organic framework. Typically, the method is a method for detecting acetone may comprising contacting a mixed-metal organic framework comprising zinc ions, silver ions and terephthalate ions with acetone and measuring a property of the mixed-metal organic framework. Preferably, the property is fluorescence and measuring a property of the mixed-metal organic framework comprises measuring the emission spectrum of the metal organic framework. Generally the emission of the mixed-metal organic framework is decreased (i.e. is quenched) in the presence of acetone.

Method for analysing a sample of breath

In diabetes, acetone in the breath is an indicator of elevated levels of ketones in the blood (diabetic ketoacidosis), which indicate a lack of insulin. A lack of insulin in the bloodstream allows unregulated fatty acid release from adipose tissue which increases fatty acid oxidation to acetyl CoA, some of which is diverted to ketogenesis. This raises ketone levels significantly above what is seen in normal physiology. Serious side effects can result if insulin is not administered. It is therefore useful to have a quick and reliable method for detecting acetone on the breath to determine whether a subject who has, or is suspected of having, diabetes, might require medical attention.

Typically, the mixed-metal organic framework is photoluminescent, and the property of the mixed- metal organic framework measured may be photoluminescence, for instance fluorescence. Typically, the photoluminescence properties of the mixed-metal organic framework may change in the presence of acetone, for example, the emission intensity may increase or decrease, and/or the colour of the emitted light may change upon exposure of the mixed-metal organic framework to the volatile organic compound of interest. Typically, the emission is decreased (quenched) in the presence of the acetone. Typically, the property is fluorescence and measuring a property of the mixed-metal organic framework comprises measuring the emission spectrum of the mixed-metal organic framework. The mixed-metal organic framework may be any mixed-metal organic framework as described herein. Typically, the mixed-metal organic framework comprises a first metal ion, a second metal ion and one or more organic linkers, wherein the first metal ion is a zinc ion; and the second metal ion is a silver ion. Typically, the one or more organic linkers are selected from aromatic carboxylate ion linkers, preferably from ions of a substituted or unsubstituted benzene dicarboxylic acid, ions of a substituted or unsubstituted benzene tricarboxylic acid, ions of a substituted or unsubstituted naphthalene dicarboxylic acid, ions of a substituted or unsubstituted biphenyl dicarboxylic acid, or ions of a substituted or unsubstituted 1,3,5-triphenylbenzene tricarboxylic acid. Typically, the aromatic carboxylate ion linker is benzene-1, 4-dicarboxylate (terephthalate) or benzene 1,3-dicarboxylate (isophthalate). Preferably, the aromatic carboxylate ion linker is benzene-1 , 4-dicarboxylate (terephthalate).

Thus, the method may comprise contacting the sample of breath with a mixed-metal organic framework comprising zinc ions, silver ions and terephthalate ions, and measuring a property of the mixed-metal organic framework to determine the level of acetone in the sample. Preferably, the property is fluorescence and measuring a property of the mixed-metal organic framework comprises measuring the emission spectrum of the metal organic framework. Generally the emission of the mixed-metal organic framework is decreased (i.e. is quenched) in the presence of acetone.

Devices

The metal organic framework or mixed-metal organic framework as described herein may be thermochromic and/or mechanochromic.

Thermochromic materials change colour in response to a change in temperature. Mechanochromic materials change colour in response to stress in the solid state, for instance stress caused by mechanical grinding, crushing and milling; by friction and rubbing, or by high pressure or sonification.

The metal organic framework or mixed-metal organic framework may be a photoluminescent and/or electroluminescent material. The term “electroluminescent material”, as used herein, refers to a material that accepts charge, both electrons and holes, which subsequently recombine and emit light. The term “photoluminescent material”, as used herein, refers to a material that is able to absorb photons and undergo photoexcitation, then emit photons. Typically, the metal organic framework or mixed-metal organic framework as described herein exhibits photoluminescence. Thus, the photoluminescence of the metal organic framework or mixed-metal organic framework may change in response to a change in temperature, or in response to mechanical stress. Thus, the metal organic framework or mixed-metal organic framework may be a luminescent thermochromic metal organic framework or mixed-metal organic framework.

The metal organic framework or mixed-metal organic framework may be a luminescent mechanochromic metal organic framework or mixed-metal organic framework. These properties lead to various applications of the metal organic framework or mixed-metal organic framework as described herein in devices that are able to respond to a stimulus and produce a detectable signal.

Whether or not a particular material is thermochromic or mechanochromic may easily be established by method known to the skilled person. For instance, the material could be heated up or cooled down under a ultraviolet lamp and observed for changes in photoluminescence.

Similarly, for mechanochromic materials the photoluminescent properties of the material can be established in the absence of mechanical stress, then mechanical stress may be induced by, for example, compressing the material into a pellet and observing whether or not the photoluminescent properties change.

The invention provides a device comprising a metal organic framework or mixed-metal organic framework as described herein, wherein the device is selected from a light emitting device, a photoluminescent device, a electroluminescent device, a luminescent thermometer, a mechanical force sensor, a chemical sensor, a motion sensing system, a data storage device, security paper, optical memory devices and vibration damage detectors. The skilled person would be aware of now to construct such devices and would readily be able to incorporate the metal organic framework or mixed-metal organic framework, for instance as a pellet or as a thin film, into such devices.

Luminescent thermometer

When the metal organic framework or mixed-metal organic framework is a thermochromic material, the metal organic framework or mixed-metal organic framework may have useful applications a thermometer, preferably in a luminescent thermometer. The invention therefore also provides a luminescent thermometer comprising a thermochromic metal organic framework or mixed-metal organic framework, wherein the metal organic framework or mixed-metal organic framework is as described herein. The skilled person would be well aware of how to construct a luminescent thermometer which comprises the metal organic framework or mixed-metal organic frameworks described herein. For instance, a luminescent thermometer device may readily be constructed by disposing the metal organic framework or mixed-metal organic framework on a substrate, then irradiating the metal organic framework or mixed-metal organic framework on the substrate with an LED. The emission of the metal organic framework or mixed-metal organic framework may be filtered using a long- pass filter and then measured using a photodiode. The emission will be translated into a numerical signal displayed in a small screen to display the temperature. The luminescent thermometer may be self-calibrated.

The metal organic framework or mixed-metal organic framework in the luminescent thermometer may be any metal organic framework or mixed-metal organic framework as described herein. Typically, the metal organic framework or mixed-metal organic framework in the luminescent thermometer comprises a silver ion and/or a lead ion and one or more organic linkers. Typically, the one or more organic linkers are selected from aromatic carboxylate ion linkers, preferably from ions of a substituted or unsubstituted benzene dicarboxylic acid, ions of a substituted or unsubstituted benzene tricarboxylic acid, ions of a substituted or unsubstituted naphthalene dicarboxylic acid, ions of a substituted or unsubstituted biphenyl dicarboxylic acid, or ions of a substituted or unsubstituted 1,3,5-triphenylbenzene tricarboxylic acid. Typically, the aromatic carboxylate ion linker is benzene-1, 4-dicarboxylate (terephthalate) or benzene 1,3-dicarboxylate (phthalate). Preferably, the aromatic carboxylate ion linker is benzene-1, 4-dicarboxylate (terephthalate). Thus, metal organic framework or mixed-metal organic framework typically comprises silver ions and/or lead ions and terephthalate ions. Preferably, the luminescent thermometer comprises a thermochromic metal organic framework comprising one or more metal ions that are silver ions or lead ions and one or more organic linkers which are terephthalate ions.

Mechanical force sensor

When the metal organic framework or mixed-metal organic framework is a mechanochromic material, the metal organic framework or mixed-metal organic framework may have useful applications a force sensor. The invention therefore also provides a mechanical force sensor comprising a mechanochromic metal organic framework or mixed-metal organic framework, wherein the metal organic framework or mixed-metal organic framework is as described herein.

The skilled person would be well aware of how to construct a mechanical force sensor which comprises the metal organic framework or mixed-metal organic frameworks described herein. For instance, the mechanical force sensor may comprise a film of the metal organic framework or mixed-metal organic framework disposed on a solid substrate.

Typically, one side of the film of the metal organic framework or mixed-metal organic framework is in contact with the substrate, either directly or via an intermediate layer. The solid substrate may act as a surface against which the film of the metal organic framework or mixed-metal organic framework can be compressed, for instance in response to a compressive force or pressure. The response of the metal organic framework or mixed-metal organic framework may be determined by irradiating the metal organic framework or mixed-metal organic framework on the substrate with an LED. The emission of the metal organic framework or mixed-metal organic framework may be filtered using a long-pass filter and then measured using a photodiode. Changes in emission intensity may then be used to locate areas of the substrate where a force has been applied and/or to quantify the magnitude of the force applied.

The metal organic framework or mixed-metal organic framework in the mechanical force sensor may be any metal organic framework or mixed-metal organic framework as described herein. Typically, the metal organic framework or mixed-metal organic framework in the mechanical force sensor comprises a silver ion and/or a lead ion and one or more organic linkers. Typically, the one or more organic linkers are selected from aromatic carboxylate ion linkers, preferably from ions of a substituted or unsubstituted benzene dicarboxylic acid, ions of a substituted or unsubstituted benzene tricarboxylic acid, ions of a substituted or unsubstituted naphthalene dicarboxylic acid, ions of a substituted or unsubstituted biphenyl dicarboxylic acid, or ions of a substituted or unsubstituted 1,3,5-triphenylbenzene tricarboxylic acid. Typically, the aromatic carboxylate ion linker is benzene-1, 4-dicarboxylate (terephthalate) or benzene 1,3-dicarboxylate (phthalate). Preferably, the aromatic carboxylate ion linker is benzene-1, 4-dicarboxylate (terephthalate). Thus, metal organic framework or mixed-metal organic framework typically comprises silver ions and/or lead ions and terephthalate ions. Preferably, the mechanical force sensor comprises a mechanochromic metal organic framework comprising one or more metal ions that are silver ions or lead ions and one or more organic linkers which are terephthalate ions.

Chemical sensor

The invention also provides a chemical sensor comprising a metal organic framework or mixed- metal organic framework as described herein. Typically, the metal organic framework or mixed- metal organic framework exhibits a change in photoluminescence when exposed to a particular chemical. For instance, the photoluminescence properties of the mixed-metal organic framework may change in the presence of the chemical of interest. For instance, the emission intensity may increase or decrease, and/or the colour of the emitted light may change upon exposure of the metal organic framework or mixed-metal organic framework to the chemical of interest.

Typically, the metal organic frameworks or mixed-metal organic frameworks of the present invention are stable in water. Thus, the chemical sensor may be suitable for sensing the presence of a chemical in an aqueous sample. For instance, the chemical sensor comprising the metal organic framework or mixed-metal organic framework may be suitable for sensing a chemical selected from the group consisting of chemical pollutants and drugs in an aqueous sample.

Antibacterial material

The invention also provides antibacterial material comprising a metal organic framework or a mixed-metal organic framework as described herein.

Typically, the antibacterial material comprises a metal organic framework or mixed-metal organic framework comprising silver ions. Typically, the antibacterial material comprises a metal organic framework or mixed-metal organic framework comprising an imidazole-based linker, preferably an imidazolate linker of formula (I) or an imidazole linker of formula (II), as described herein. For instance, the antibacterial material may comprise a metal organic framework or mixed-metal organic framework comprising silver ions and an imidazole-based linker, preferably an imidazolate linker of formula (I) or an imidazole linker of formula (II), as described herein.

Optoelectronic device

The invention also provides a device comprising a metal organic framework or a mixed-metal organic framework, as defined herein, wherein the device is an optoelectronic device.

Preferably the device is an electroluminescent device. Typically, the device is an electroluminescent device comprising an electroluminescent layer, wherein the electroluminescent layer comprises the metal organic framework or mixed-metal organic framework. Typically, the device comprises one or more additional components selected from the group consisting of a p-type layer, an n-type layer and one or more electrodes.

Preferably the device comprises a first electrode and a second electrode. The first and second electrode may comprise any suitable electrically conductive material. The first electrode typically comprises one or more metals. The first electrode typically comprises a metal selected from silver, gold, copper, aluminium, platinum, palladium, or tungsten, preferably aluminium.

The second electrode typically comprises a transparent conducting oxide. Typically, the second electrode typically comprises a transparent conducting oxide and the second electrode typically comprises one or more metals. The transparent conducting oxide typically comprises fluorine- doped tin oxide (FTO), indium tin oxide (ITO) or aluminium-doped zinc oxide (AZO), and typically ITO.

Preferably the device comprises an n-type layer and a p-type layer. Typically, the n-type layer comprises an organic electron-transporting material. Preferably, the n-type layer comprises 2-(4- tert-Butylphenyl) -5 -(4-biphenylyl) -1,3,4 -oxadiazole (p-PBD) .

Typically, the p-type layer comprises a organic hole-transporting material. Preferably, the p-type layer comprises poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS).

Preferably the electroluminescent layer comprises the metal organic framework or mixed-metal organic framework dispersed in a conducting polymer matrix, preferably wherein the conducting polymer matrix comprises poly(9-vinylcarbazole) (PVK) polymer.

Typically, the metal organic framework or mixed-metal organic framework in the electroluminescent device comprises silver ions and/or lead ions and one or more organic linkers. Typically, the one or more organic linkers are selected from aromatic carboxylate ion linkers, preferably from ions of a substituted or unsubstituted benzene dicarboxylic acid, ions of a substituted or unsubstituted benzene tricarboxylic acid, ions of a substituted or unsubstituted naphthalene dicarboxylic acid, ions of a substituted or unsubstituted biphenyl dicarboxylic acid, or ions of a substituted or unsubstituted 1,3,5-triphenylbenzene tricarboxylic acid. Typically, the aromatic carboxylate ion linker is benzene-1, 4-dicarboxylate (terephthalate) or benzene 1,3- dicarboxylate (phthalate). Preferably, the aromatic carboxylate ion linker is benzene-1, 4- dicarboxylate (terephthalate). Thus, the metal organic framework or mixed-metal organic framework typically comprises silver ions and/or lead ions and terephthalate ions. Preferably, the electroluminescent comprises a metal organic framework comprising one or more metal ions that are silver ions or lead ions and one or more organic linkers which are terephthalate ions. For instance, the device may comprise a first electrode, an n-type layer, an electroluminescent layer comprising the metal organic framework or mixed-metal organic framework, a p-type layer and a second electrode. The first electrode is typically in contact with the n-type layer. The second electrode is typically in contact with the p-type layer. Therefore, the electroluminescent device may comprise the following layers in the following order:

I. a first electrode as defined herein;

II. an n-type layer as defined herein;

III. electroluminescent layer comprising the metal organic framework or mixed-metal organic framework as defined herein;

IV. a p-type layer as defined herein; and

V. a second electrode as defined herein.

The electroluminescent device may comprise the following layers in the following order:

I. a first electrode comprising aluminium;

II. an n-type layer comprising p-PBD; III. electroluminescent layer comprising the metal organic framework or mixed-metal organic framework as defined herein dispersed in conducting polymer matrix, preferably wherein the conducting polymer matrix comprises poly(9- vinylcarbazole) (PVK) polymer;

IV. a p-type layer comprising PEDOT:PSS; and V. a second electrode comprising ITO.

EXAMPLES

The advantages of the invention will hereafter be described with reference to some specific examples.

Example 1

The work in this example is also discussed in the journal article by Gutierrez et al, “Scalable eco- friendly synthesis paving the way for photonics sensors and electroluminescent devices” Applied Materials Today 21 (2020) 100817, the contents of which are incorporated by reference.

Materials

Terephthalic acid (+99%), AgNO₃, triethylamine (99%), N,N-Dimethylformamide (99%) and methanol were purchased from fisher scientific and used without further purification. Synthesis of OX-2

Synthesis 1 : methanol (OX-2_m)

6.0 mmol of terephthalic acid (BDC) were deprotonated in a solution of 12.0 mmol of triethylamine (NEt₃) in 20 mL of methanol. Another solution was prepared by sonicating 3.0 mmol of AgNO₃ in 20 mL of methanol. The latter solution was added to the former one and a white suspension was promptly formed. The sample was sonicated for 5 minutes and then washed two times with methanol. The white solid sample was collected by centrifugation (8000 rpm) and dried at 80 °C for 2 hours. Following this procedure 430 mg of OX-2 was obtained as a white powder.

Synthesis 2: Different reactants proportion in methanol (OX-2_m:1/2)

Similar procedure to the above was followed, but in this case, the amount (in mmol) of BDC was reduced to a half compared to AgNO₃. Briefly, 1.5 mmol of BDC and 3 mmol of NEt₃ were dissolved in 20 mL of methanol and this solution was added to a 3.0 mmol AgNO₃ solution in 20 mL of methanol. The mixture was sonicated for 5 minutes, washed with methanol and dried at 80°C for 2 hours. 480 mg of a brownish OX-2 powder were obtained.

Synthesis 3: N,N-dimethylformamide (OX-2_DMF)

Similar methodology to “synthesis 1” was followed by using N,N-dimethylformamide (DMF) instead of methanol as solvent. Typically, 6.0 mmol of BDC and 12.0 mmol of NEt₃ were dissolved in 20 mL of DMF and then added to another solution prepared by sonicating 3.0 mmol of AgNO₃ in 20 mL of DMF. The mixture was sonicated for 5 minutes and then washed with DMF and methanol, and dried at 80°C for 2 hours. 420 mg of a white OX-2 powder were obtained.

Synthesis 4: water (OX-2_w)

Similarly, 6.0 mmol of BDC and 12.0 mmol of NEt₃ were dissolved in 20 mL of deionized water and this solution was added to another prepared by dissolving 3.0 mmol of AgNO₃ in 20 mL of deionized water. The instantaneously formed white suspension of OX-2 was sonicated for 5 minutes, washed with deionized water, collected by centrifugation and dried at 100°C for 3 hours. 450 mg of white OX-2 powder were obtained.

Scalable synthesis of OX-2 in water

The above described procedure was adapted to scale-up the synthesis of OX-2. Briefly, 120 mmol of BDC and 240 mmol of NEt₃ were dissolved in 300 mL of deionized water. Another solution was prepared by dissolving 60 mmol of AgNO₃ in 300 mL of deionized water. After that, the latter solution was added to the former one and a white suspension of a highly green luminescent OX-2 was instantaneously formed. The mixture was sonicated for 5 minutes, then washed with copious amount of deionized water, collected by centrifugation and dried at 100°C for 3 hours. Following this procedure 10 g of OX-2w MOF was obtained.

Materials Characterization

The crystalline structure, morphology and luminescent properties of OX-2 MOFs were characterized by a combination of X-ray, spectroscopy and microscopy techniques. Powder X-ray diffraction (PXRD) experiments were carried out in a Rigaku Smart Lab diffractometer with a Cu Kα source (1.541 Å). The diffraction data were collected using 0.01° step size, 1° min-1 and at 2θ angle ranging from 2° to 32°. Field emission scanning electron microscopy (FESEM) and energy- dispersive X-ray (EDX) images and spectra were obtained using the Carl Zeiss Merlin equipped with a high-resolution field emission gun. Micrographs were attained under high vacuum with an accelerating voltage of 10 kV and in secondary electron imaging mode. FTIR spectra were recorded on a Nicolet™ iS™ 10 FTIR Spectrometer. The FTIR spectrum of each sample was collected 3 times and then averaged. Steady-state fluorescence spectra, excitation-emission maps, luminescence quantum yields and time-resolved emission decays were recorded using a FS5 spectrofluorometer (Edinburgh Instruments) equipped with different modules for each specific experiment (i.e. integrating sphere for quantum yield, heated sample module to measure the emission of powders at different temperatures and a standard solid holder for powder experiments). For time -resolved measurements, the samples were pumped with a 365-nm centred pulsed diode laser. The instrumental response function (IRF, ~800 ps) was used to deconvolute the emission decays. The decays were fitted to a multiexponential function and the quality of the fit was estimated by the χ2, which was always below 1.2.

Results and Discussion

Materials characterization

In this study, 4 different approaches for obtaining new luminescent OX-2 MOF materials were developed. The synthetic conditions mainly differ in the solvent used for the reaction and in the ratio of reactants. To unravel how those changes could affect the physicochemical characteristics of these materials, the crystalline structure, morphology and spectroscopic properties have been explored by means of PXRD, FESEM-EDX, FTIR and steady-state and time-resolved fluorescence spectroscopy. Firstly, the crystalline structure of the different OX-2 materials was explored. As shown in Figure 1, the PXRD patterns of all synthesized OX-2 MOFs (in methanol (OX-2_m), in methanol but using different ratio of organic linker and metal salt, 1 :2 BDC:AgNO₃ (OX-2_m:1/2), in DMF (OX-2_DMF) and in water (OX-2_w)) are very similar, showing no significant changes neither in the peak intensity nor in the peak position, and resembling to a previously reported Ag-carboxylate coordination polymer.²⁶ The molecular formula of this Ag network corresponds to [Ag(BDC)_1/2]_n and the crystalline structure reveals a 3D framework with short Ag-Ag distances (2.901 Å). Basically, binuclear Ag₂(BDC)₂ may be considered as the building units, which are connected head-to-tail giving place to 1D chains, which in turn are linked by Ag-O bonds with other BDC linkers to form the 2D wave -like layers. Finally, those layers are interweaved one each other to result in a 3D framework (Sun, D et al., Syntheses and characterizations of a series of silver- carboxylate polymers, Inorganica Chimica Acta 2004, 357 (4), 991-1001). The FTIR spectra of the different OX-2 MOFs depicted in Figure 7 show no remarkable differences apart from a broad band at ~1700 cm^-1 for OX-2_DMF which can be attributed to u< o of remaining DMF molecules in OX-2 MOF (Shastri, A et al., Spectroscopy ofN,N-dimethylformamide in the VUV andIR regions: Experimental and computational studies, J. Chem. Phys. 2017, 147 (22), 224305). The FTIR spectra of all OX-2 exhibit bands at -1520, 1360, 1300, 1150, 1090, 1010, 890, 820 and 740 cm^-1. Following previous assignments, the bands at 1520 and 1360 cm^-1 can be ascribed to asymmetric and symmetric stretching vibrations of the carboxylic groups of the BDC linker coordinated to the Ag metal centre (Biemmi, E et al., Synthesis and characterization of a new metal organic framework structure with a 2D porous system: (H2NEt2)2[Zn3(BDC)4] •3DEF. Solid State Sciences 2006, 8 (3), 363-370). Moreover, the lack of bands in the region between 1750-1680 cm^-1 indicates a complete deprotonation of terephthalic acid as expected. Finally, the bands observed in the region between 1150-740 cm^-1 are attributable to C=C stretching, β(CCH) and γ(CCC) bending of the benzene ring (Tellez et al., Fourier transform infrared and Raman spectra , vibrational assignment and ab initio calculations of terephthalic acid and related compounds, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2001, 57 (5), 993-1007). Although no significant structural variations and FTIR response have been detected for the different OX-2 materials, simple visual changes have been noticed both under daylight and UV exposures. OX-2_m, OX-2_DMF and OX-2_w MOFs are very white powders (see photos in Figure 1), however, under UV irradiation there is a significant difference in the emission intensity of these powders, that decreases from the OX-2 synthesized in water > methanol > DMF. On the other side, OX-2_m:1/2 presents a brownish colour under daylight (see photo in Figure 1). Different explanations can be used to justify this effect, as the aggregation of the undissolved silver to form nanoparticles as it has been previously shown a brownish colour (Wu et al., Facile fabrication of Ag2(bdc)@Ag nano- composites with strong green emission and their response to sulfide anion in aqueous medium , Sensors and Actuators B: Chemical 2018, 255, 3163-3169). However, it can be also possible that the different ratio of silver nitrate and BDC linker could induce dissimilar MOF morphology or structures, as the OX-2. In order to unveil which assumption is most plausible, FESEM experiments were performed. Figures 2 A-D and Figure 8 display the micrograph of OX-2_w. OX- 2m: 1/2, OX2_m and OX-2_DMF, where some appreciable changes were perceived. While the FESEM images of OX-2_w (Figures 2A-B), OX2_m and OX-2_DMF (Figure 8) show very homogenous distribution of elongated nanoplates (height 60-100 nm, Figure 9), OX-2_m:1/2 presents three different morphologies: i) elongated nanoplates, ii) aggregation of a second type of nanoplates creating a kind of “flower” shaped crystals, and iii) a kind of micro-sized columns (Figures 2 C-D). To get more insights about the molecular composition of these three different morphologies, FESEM-EDX measurements on the OX-2_m:1/2 and OX-2_w MOFs were performed (Figures 2 E-G, Figures 10A-C and Figures 10D-F, respectively) as they exhibit the most notorious dissimilar characteristics (white and highly luminescent and brownish and almost non-emissive powders, respectively). Interestingly, whereas the elemental composition of the needles and flowers indicates the presence of homogeneously distributed Ag, C and O atoms, the crystals with a column morphology are mainly based on C and O atoms and no traces of Ag apart from the background were detected. Moreover, the needles and flower crystals also presents important differences in the Ag/C atomic ratio, being 1/16 and 1/30, respectively (Table 1).

Table 1. Representation of the %weight and %atomic of C, Ag and O elements found in the FESEM-EDX micrographs indicated in the table. Therefore these results indicate the differences is more likely related to the formation of additional substructures to OX-2 MOF when the linker/metal ratio in the synthesis is 1/2 instead of the silver nanoparticle formation. This assumption is further confirmed by the emission spectrum of our material which completely differs to the silver nanoparticles reported (Wu et al., Facile fabrication of Ag2(bdc)@Ag nano-composites with strong green emission and their response to sulfide anion in aqueous medium, Sensors and Actuators B: Chemical 2018, 255, 3163-3169). Additionally, it is possible to anticipate that the presence of those new assemblies will have a strongly negative impact on the photoluminescence properties.

Photoluminescent characteristics of OX-2 MOFs

When OX-2 MOFs are exposed to UV irradiation (λ= 365nm) they display a strongly green-yellow emission, with the exception of OX-2_m:1/2 which is barely emissive (see photos of Figure 3). To decipher the luminescent properties of those MOFs, steady-state excitation-emission map, fluorescence quantum yield and time -resolved emission experiments were carried out. The excitation-emission maps of OX-2_m, OX-2_m:1/2, OX-2_DMF and OX-2_w reveal no very significant differences, with all of them showing a vibrationally resolved emission, having maxima located at 485, 520 and 560 nm that correspond to an excitation maximum of ~330 nm (Figure 3). Despite the emission spectra of all OX-2 MOFs are similar, there are evident vicissitudes in the photo luminescence quantum yield (φ_{Exc( 330nm)}) value, which changes from 63% > 46% > 15% > 1% for OX-2_w, OX-2_m, OX-2_DMF and OX-2_m:1/2 bulk powders, respectively. The huge difference between the φ value of OX-2_w and OX-2_m:1/2 are easily explainable by the different crystalline structures found in the latter ( vide supra). As described above, whereas OX-2_w bulk powder consist on homogenously dispersed elongated nanoplates, OX-2_m:1/2 is composed by elongated nanoplates and “flowers” and “column” shaped crystals. The elongated nanoplates of OX-2_m:1/2 are similar to those observed for OX-2_w and it is assumed that those are the emissive ones. However, the flowers presents different proportion of Ag/C and the column shaped crystals are composed only by C and O atoms. Therefore, those species must be causing the decrease in the emission φ value of OX- 2_m:1/2 mainly by two mechanisms: through the absorption of part of the photons from the irradiation source (brownish colour) and then deactivating by means of non-radiative channels, and by absorbing part of the light emitted by the elongated nanoplates. More intriguing are the differences observed in the φ values of OX-2_w, OX-2_m and OX-2_DMF, as all of them are white solid powders with no morphological or structural differences. Therefore, if the synthesis of those MOFs was analogous and in all the cases the same structure and morphology without any evidence of silver nanoparticles were obtained, the only interchangeable parameter, the solvent, is a keystone in the emission of OX-2 MOFs. It has been recently reported that the luminescence properties of Ag nanoclusters confined into zeolites strongly depends on the coordinated water molecules (Grandjean et al., Origin of the bright photoluminescence of few-atom silver clusters confined in LTA zeolites, Science 2018, 361 (6403), 686-690). In fact, their optical properties are originated from a confined two-electron superatom quantum system by a hybridization of Ag and water O orbitals, giving place to a promotion of one electron from the s-type highest occupied molecular orbital to the p-type lowest unoccupied molecular one upon excitation (Grandjean et al., Origin of the bright photoluminescence of few-atom silver clusters confined in LTA zeolites, Science 2018, 361 (6403), 686-690). This could explain why the quantum yield of OX-2_w is the highest (63%) as water molecules could be coordinated to Ag atoms. Similarly, OX-2_m also exhibits a high φ (46%) as the O atoms of methanol molecules may be hybridized with the Ag atoms of the framework.

The emission of all OX-2 MOFs resembles to that previously reported for the [ Ag( L ¹ )]₂(p- bdc)•8H₂O coordination polymer with maxima at ~485, ~520 and ~560 nm (Liu., Four silver- containing coordination polymers based on bis (imidazole) ligands, Journal of Coordination Chemistry 2008, 61 (22), 3583-3593). In that study, the band at ~520-nm was assigned to a metal- to-ligand charge -transfer transition (MLCT), whereas the ~485-nm one was attributed to intra- ligand emission as it was more or less similar to a weak emission at 466 nm previously reported for terephthalic acid (Liu., Four silver-containing coordination polymers based on bis (imidazole) ligands, Journal of Coordination Chemistry 2008, 61 (22), 3583-3593). However, this assumption is not fully supported by the following facts: i) the intra-ligand emission of BDC is 20-nm blue shifted (466-nm) compared to the observed in OX-2 (485-nm). ii) The intra-ligand BDC emission band is much broader (FWHM ~ 3250cm^-1) (Fun et al., A three-dimensional network coordination polymer , (terephthalato)(pyridine)cadmium, with blue fluorescent emission, Journal of the Chemical Society, Dalton Transactions 1999, (12), 1915-1916) that the band at 485-nm observed for OX-2 MOFs (FWHM ~ 1050cm^-1, deconvoluted from OX-2 emission), whose narrowness matches better with a vibrationally-resolved structure, and iii) it is possible to anticipate that the three vibrationally-resolved bands response in a comparable way to external stimuli. Based on that, it is probable to discard that the band at 485-nm is originated by the intra-ligand BDC emission and it can be proposed that the vibrationally-resolved emission is due to a transition from a unique excited state origin, which is generated by a ligand-to-metal charge transfer (LMCT) and/or a ligand-metal-metal charge transfer (LMMCT) transitions. It has been extensively demonstrated that LMCT is one of the most plausible mechanism for the luminescence of d¹⁰ metal complexes (Wing-Wah Yam et al., Luminescent polynuclear d10 metal complexes, Chemical Society Reviews 1999, 28 (5), 323-334). Additionally, short Metal-Metal (M-M) distances may induce the emission from LMMCT transitions (Chen et al., Photoemission Mechanism of Water-Soluble Silver Nanoclusters: Ligand-to-Metal-Metal Charge Transfer vs Strong Coupling between Surface Plasmon and Emitters, J Am Chem Soc 2014, 136 (5), 1686-1689; Jia et al., Supramolecular self- assembly of morphology-dependent luminescent Ag nanoclusters, Chem. Commun. 2014, 50 (67), 9565-9568). For instance, the emission of Ag-carboxylate nanoclusters have been previously attributed to LMMCT from Ag(I)-carboxylate complexes to Ag atoms (Chen et al., Photoemission Mechanism of Water-Soluble Silver Nanoclusters : Ligand-to-Metal-Metal Charge Transfer vs Strong Coupling between Surface Plasmon and Emitters, J Am Chem Soc 2014, 136 (5), 1686- 1689). Therefore, taking into account that the Ag-Ag distances in OX-2 MOFs are just 2.901 A, the possibility of luminescent transitions originated from LMMCT and/or LMCT phenomena is reinforced. To shed more light on this attribution, time-correlated single photon counting (TCSPC) experiments on the 4 different OX-2 samples and the BDC linker were performed (Figures 11A-D and Tables 2-3).

Table 2. Values of time constants ( τ_i) , pre -exponential factors (a_i) and normalized (to 100) fractional contributions (c_i = τ_ia_i) obtained from the fit of the emission decays of BDC linker in solid-state upon excitation at 365 nm and observation as indicated.

Table 3. Values of time constants ( τ_i), pre -exponential factors (a_i) and normalized (to 100) fractional contributions (c_i = τ_ia_i) obtained from the fit of the emission decays of OX-2_m, OX-2_m:1/2, OX-2_DMF and OX-2_w in solid-state upon excitation at 365 nm and observation as indicated.

Few important findings can be extracted from the analysis of the emission decays. Firstly, while the BDC signal recorded at 485, 520 and 560 nm decays multiexponentially with time constants of τ₁ = 1.7 ns, τ₂ = 4.2 ns and τ₃ = 16.5 ns, those of all OX-2 exhibit a biexponential behaviour with time constants of τ₁ = 350-540 ps and τ₂ = 2.0-3.5 ns (Table 3). This observation unequivocally proofs that the band at 485-nm cannot be attributable to a BDC intra-ligand emission, and it also supports the previous assignment that all the vibrationally-resolved bands aroused from a common origin. Secondly, the decrease in the lifetimes observed for OX-2_m:1/2 compared with OX-2_w (from τ₁ = 540 ps and τ₂ ~ 3.5 ns to τ₁ = 350 ps and τ₂ = 2.0 ns, respectively) agrees with our assumption that the “flowers” and “column” shaped crystals are quenching the emission of the elongated nanoplates. Thirdly and more remarkably, upon the excitation of BDC linkers with UV light, the green- yellowish emission of OX-2 MOFs decays similarly to a previously reported Ag nanocrystals, supporting the idea that the provenance of the OX-2 luminescence is the LMCT and/or LMMCT transitions (Shamsipur, M et al., Photoluminescence Mechanisms of Dual-Emission Fluorescent Silver Nanoclusters Fabricated by Human Hemoglobin Template: From Oxidation- and Aggregation-Induced Emission Enhancement to Targeted Drug Delivery and Cell Imaging, ACS Sustainable Chemistry & Engineering 2018, 6 (8), 11123-11137).

Scalable synthesis and robustness of OX-2

Stimulated by the easy synthesis and excellent photophysical properties of OX-2 MOF and in order to get large quantities of this material for a real-world applications, a scaled-up of the previous synthetic methodology was performed. More than lOg of highly luminescent OX-2 material were easily achieved by increasing the amount of initial reactants using water as solvent (see experimental section for more information). Scaling-up the synthetic protocol for MOF fabrication usually harvests amorphous materials whose physicochemical properties are completely different from the crystalline material obtained in a small scale reaction. Herein, this scalable approach produces identical OX-2_w to that synthesized in a small scale, as shown by the comparable PXRD (Figure 12) and luminescence properties with the expected vibrationally-resolved bands at 485, 520 and 560nm (Figure 13) and a φ_{Exc( 330nm)} value of ~60±3%. The potential applicability of any material has to satisfy certain described conditions, hence, the chemical stability of this OX-2 in water and at ambient conditions has been also tested. OX-2_w was soaked in water for 1, 4 , 8 and 21 days and then dried at 80°C under vacuum for 2 hours. The stability of OX-2 was then confirmed by means of PXRD (Figure 14), which exhibits no variations in the peak position nor in the peak intensity, indicating that its crystalline structure remains unaltered. Moreover, a fraction of OX-2_w was exposed to ambient conditions in the lab (day light, ~40% humidity, solvent vapours, etc) and checked its stability by carrying out PXRD and photoluminescent measurements (Figures 15 and 16, respectively). The PXRD patterns of OX-2_w MOF show no changes upon its exposure to ambient conditions (Figure 15). Similarly, the photoluminescence properties of OX-2_w were progressively followed by measuring the quantum yield, whose minimal decrease from an initial value of 60% to 57% in 70 days (meaning just a decrease of 5% over the total signal) indicates the high stability and robustness of OX-2 MOF in normal conditions (Figure 16).

Luminescent OX-2 for Multistimuli Detection

Encouraged by the scalable, fast, cost-effective and eco-friendly synthesis of OX-2 alongside its extremely high stability in water and in ambient conditions and its high quantum yield in solid state, the luminescent response of OX-2 to different external stimuli like temperature or mechanical compression have been explored for its possible integration in the fabrication of non-invasive sensors.

Response of OX-2 under compressive stress

To investigate the effect of mechanical stress to the properties of OX-2, 4 batches of 200 mg each were compressed into pellets under 0.074, 0.148, 0.222 and 0.297 GPa, respectively, and further characterized by PXRD and fluorescence spectroscopy. As shown in Figure 4A, upon mechanical compression, the pellets undergo significant changes in the intensity of the PXRD peaks. Specifically, there is a decrease in the intensity of the peak at ~13° corresponding to the (100) plane. By making a correlation between the two most intense peaks located at ~13° and ~31° (corresponding to (100) and (121) planes) there is an unequivocally diminishing in the ratio with the stress (value presented in Figure 4A), however, even at high pressures, OX-2 pellets conserved their original crystalline structure as no significant shifting in the peak positions were detected. Mechanical compression also induces important fluctuations in the emission intensity of OX-2 pellets which is obviously translated into a variations in the quantum yield value (Figure 4B). At lower stress (0.074 GPa), the emission intensity of the pellet is comparable to the powder of OX-2, with a quantum yield value of 59% in both cases. However, upon increasing the pressure, there is a continuous decrease of the emission intensity, and thus, in the quantum yield values from 57% > 47% > 45% for pressures corresponding to 0.148, 0.222 and 0.297 GPa, respectively. Interestingly, the changes observed above can be reversed by grinding the pellets into a fine powder in a mortar using a pestle. The PXRD of the OX-2 powders from the pellets in Figure 4C illustrates the recovery in the peak intensity at 13° (plane (100)), comparable to that observed for the as- synthetized OX-2 powder. This may indicate a certain plasticity of OX-2 under compression, as if there was a severe structural modification, it would be rather complicated to obtain the original crystalline structure by simply grinding the pellet. What is even more astonishing is the recovery of the emission intensity, especially for the powders gotten from pellets compressed at 0.222 and 0.297 GPa, whose quantum yield changes from 47 and 45% in the pellet form to 57 and 55% in powders, respectively (Figure 4D). This exceptional behaviour along with the compressive stress confirm that the LMMCT and LMCT processes are the responsible of the photoluminescence characteristics, because when the MOF structure distortion happens, the distances between the clusters and the ligands are altered, and hence those CT processes are less efficient and the emission intensity decreases, similar observations were found for other LMOFs (Lustig et al., Metal-organic frameworks: functional luminescent and photonic materials for sensing applications, Chemical Society Reviews 2017, 46 (11), 3242-3285; Chen et al., All Roads Lead to Rome: Tuning the Luminescence of a Breathing Catenated Zr-MOF by Programmable Multiplexing Pathways, Chemistry of Materials 2019, 31 (15), 5550-5557). Extraordinarily, when the compress test and the grinding were repeated for a second time on the same material, comparable results were obtained. Firstly, the compression caused again a decrease in the intensity of the 13° peak of the PXRD spectra along with a diminishing in the emission intensity and fluorescence quantum yield (Figures 16A-B). After that, when the pellets were ground into fine powders both the intensity of the 13° peak of the PXRD spectra, the emission intensity and quantum yield were recovered (Figures 16 C-D). All these results highlight the sensitivity of OX-2 MOF towards exogenous mechanical compression and the repeatability of this procedure, opening an exciting via for the integration of OX-2 into deformation-based sensors.

Luminescent Response of OX-2 to Temperature

The ability of OX-2 to detect changes in the temperature has also been tested. Upon increasing the temperature, the emission spectrum of OX-2 recorded at 25, 50, 75 and 100°C reveals a gradual quenching of its intensity, giving place to a very linear dependence of the I₀-I vs T (R²= 0.993, Figure 5A). A precise linear response of the luminescence to variations in the temperature is one of the most paramount qualities of a luminescent material to be deployed in the fabrication of luminescent thermometers. However, others parameters like the reproducibility or repeatability have a similar or even higher importance. To further comprehend how reproducible and repeatable is the luminescent response of OX-2 towards changes in the temperature, we have cyclically heated and cooled down the OX-2 powder while recording its emission. As shown in Figure 5B, when the sample was heated to 100°C the emission is quenched and when the sample was cooled down to 25 °C the emission is fully recovered as expected. However, what is most remarkable is that the emission of OX-2 does not exhibit significant losses with each cycle (10 in total, inset of Figure 5B), meaning that the response of this OX-2 MOF is very reproducible and repeatable. All those facts indicate that the reason behind this temperature effect should be connected with the MOF structure modification. When the temperature is risen the vibrational modes of the MOFs are enlarged causing an increment of the non-radiative recombination (Yan et al., Metal/covalent- organic frameworks-based electrocatalysts for water splitting, Journal of Materials Chemistry A 2018, 6 (33), 15905-15926) and at the same time an alteration of the distances and the coupling between the ligands and the clusters as it has been previously observed for other LMOFs (Dong et al., A Flexible Fluorescent SCC-MOF for Switchable Molecule Identification and Temperature Display, Chemistry of Materials 2018, 30 (6), 2160-2167).

Luminescent Thermometer: Test stability over pellets/films

The outstanding abilities to reversibly detect changes in the temperature jointly to the easy, cheap, scalable and eco-friendly synthesis converts OX-2 into a promising candidate for being integrated in a luminescent thermometer. Encouraged by these exceptional properties, we have fabricated and examined pellets and films of OX-2 as they will be easily handled for a future fabrication of luminescent thermometers. On one hand, and as explained in the above section, we have fabricated a pellet by applying mechanical compression to 200 mg of OX-2 powder under 0.074 GPa, as at this pressure the quantum yield is similar to the powder form of OX-2. On the other hand, homogenous films of OX-2 were prepared by dispersing 20 mg of the MOF into 20 mL of water under sonication, and then drop casting this solution into a glass plate. Both the film and the pellet were exposed to high temperatures (200 and 220°C, respectively) in a hot plate, showing a total disappearance of the emission at higher temperatures and the subsequent recovery when the samples were cooled down (Figure 6). Moreover, the promptly response of OX-2 powder, pellets or films to changes in the temperature was demonstrated by filming the process. In the attached videos, the powder and pellet of OX-2 was placed and maintained onto the hot plate, so the recovery of the emission is a long process as the cooling of the hot plate takes around 30 min. However, in the case of the film, the sample was cyclically deposited and removed from the hot plate, and thus, a simple visual inspection of this video could shed an idea about how fast and reproducible is the response of OX-2 material to changes in the temperature. Conclusions

In this study, 4 synthetic approaches for the fabrication OX-2 MOFs have been developed and then the obtained materials were characterized by a combination of XRD, microscopies and spectroscopies techniques. The OX-2 synthesized in water showed the highest quantum yield (~60%), possibly because of a hybridization between the Ag atoms of the MOF and the oxygen orbitals of water molecules. This is an enormous advantage as the cost-effective synthesis of OX-2 has been scaled-up using an eco-friendly solvent (water) and mild conditions (room temperature), allowing the easily fabrication of at least 10g in a reduced period of time. Moreover, OX-2 MOF has proved to be a very robust material with no remarkable changes neither in its crystalline structure nor in its photo luminescence properties upon soaked in water for a period of 21 days and after being exposed to ambient conditions in the lab (day light, ~40% humidity, solvent vapours, etc) for a total of 70 days.

OX-2 MOF also exhibits an excellent luminescent response to mechanical compression (pelletizing the powder) and to changes in the temperature. The thermochromic behaviour showed a linear response of the emission intensity with the temperature, and those changes are very reproducible and repeatable. In addition to all above, OX-2 MOF also behaves as a good electroluminescent material as their excited state mechanism is based on LMCT and LMMCT processes. Therefore, this work shed light on the easy synthesis of this new luminescent nanomaterial, remarking the outstanding potential of OX-2 to be integrated in multiple real-word technologies.

Details of other MOFs prepared using the aqueous synthesis method:

Another 5 MOFs have been synthesized following exactly the same protocol described for OX-2_w, but using different organic linkers. The BDC (terephthalic acid) linker was then substituted by: 1) imidazole; 2) 2-Methylimidazole; 3) 4,5-dichloroimidazole; 4) phthalic acid; 5) isophthalic acid.

Example 2

The synthesis of the material consists of two steps: i) synthesis of OX-1 MOF (already reported); and ii) substitution of Zn atoms by Ag in OX-1 MOF leading to a new mixed-metal MOF (MM- MOF) material.

Part 1: Synthesis of OX-1 ( already reported: Chaudhari , A.K., et al., Optochemicallv Responsive 2D Nanosheets of a 3D Metal-Orsanic Framework Material. Adv Mater. 2017. 29(27): 1701463.) Two solutions containing (A) 6 mmol of Zn(NO₃)•6H₂O and (B) 12 mmol of terephthalic acid and 24 mmol of triethylamine (NEt₃) in 20 mL of methanol each were prepared. Once dissolved, the solution (A) was mixed with the solution (B) and a white gel was promptly formed. This gel was then thoroughly washed with methanol, and the solid was collected by centrifugation (8000 rpm) and dried at 90°C for 3 hours.

Part 2: Substitution of Zn by Ag (unreported invention)

250 mg of OX-1 MOF and 1 mmol (169 mg) of AgNO₃ powders were mixed and ground using a mortar and a pestle for 2 minutes. Then, a few micro liters of water (from 50 to 500 μL) were added to the above mixture and blended using a spatula for 30 seconds. The initial OX-1 MOF which is non-emissive is converted to a highly photoluminescent mixed-metal organic framework. After that, the sample was dried at 90°C overnight.

Acetone Sensing Experiments

The luminescent response of the mixed-metal organic framework to different VOCs was carried out by adding 2 mg of the mixed-metal organic framework material to 4 mL of each solvent. The mixed-metal organic framework suspensions were sonicated for 10 minutes and then their emission was collected by using a FS-5 spectrofluorometer. The emission spectra of the mixed-metal organic framework in the presence of different VOCs remain almost unaltered, however in presence of acetone, the emission is completely supressed.

Further embodiments of the invention are defined in the following numbered clauses:

1. A process for producing a metal organic framework (MOF), wherein the metal organic framework comprises one or more metal ions and one or more organic linkers, wherein the process comprises contacting the one or more metal ions with the one or more organic linkers in the presence of water to form a precipitate of the metal organic framework, wherein the process is performed at a temperature that does not exceed 50°C .

2. The process according to clause 1, wherein the one or more metal ions comprise one or more metal ions of group 9, 10, 11, 12 or 14 of the periodic table, preferably wherein the one or more metal ions comprise one or more ions selected from Ag, Au, Cu, Cd, Ir, Pt, ,Pd and Pb ions .

3. The process according to clause 1 or clause 2 wherein the one or more metal ions comprise an Ag ion.

4. The process according to any preceding clause wherein the process comprises a step of dissolving a salt of the one or more metal ions in an aqueous solution , preferably wherein the process comprises a step of dissolving a salt of the one or more metal ions in water.

5. The process according to any preceding clause wherein the one or more organic linkers are selected from carboxylate ion linkers and imidazole-based linkers, and preferably from aromatic carboxylate ion linkers and imidazole-based linkers.

6. The process according to clause 5 wherein the one or more organic linkers are aromatic carboxylate ion linkers and the process comprises a step of deprotonating an aromatic carboxylic acid with a base to produce the aromatic carboxylate ion linker.

7. The process according to clause 5 or clause 6 wherein the aromatic carboxylate ion linker comprises two or more carboxylate groups, or wherein the aromatic carboxylate ion linker comprises three or more carboxylate groups.

8. The process according to any one of clauses 5 to 7 wherein the aromatic carboxylate ion linker is selected from an ion of a substituted or unsubstituted benzene dicarboxylic acid, an ion of a substituted or unsubstituted benzene tricarboxylic acid, an ion of a substituted or unsubstituted naphthalene dicarboxylic acid, an ion of a substituted or unsubstituted biphenyl dicarboxylic acid, or an ion of a substituted or unsubstituted 1,3,5-triphenylbenzene tricarboxylic acid.

9. The process according to any one of clauses 5 to 8 wherein the aromatic carboxylate ion linker is selected from an ion of a compound from the group consisting of

wherein R and R’ are selected fromH, OH, NH2, CH3, CN, NO2, F, Cl, Br, I, -OC3H5, OC7H7. 10. The process according to any one of clauses 5 to 9 wherein the aromatic carboxylate ion linker is benzene-1, 4-dicarboxylate (terephthalate).

11. The process according to clause 5 wherein imidazole-based linker is an imidazolate linker of formula (I) or an imidazole linker of formula (II): Wherein R1, R2, R3, R4, R5, R6, and R7 are each independently selected from hydrogen, C1-10 alkyl, C2 10 alkenyl, C2-10 alkynyl, cyano, and halogen, provided that R2 and R3 or R6 and R7 may be joined so as to form a substituted or unsubstituted ring, preferably wherein R2 and R3 or R6 and R7 are joined to form a 5 or 6-membered ring, optionally wherein the ring comprises 1, 2 or 3 heteroatoms selected from O, N and S, and provided that any one of R1 to R7 may be a hydrocarbon linker bonded to a further substituted or unsubstituted imidazole or imidazolate ring. 12. The process according to clause 5 or clause 11 wherein the imidazole-based linker is selected from the group consisting of wherein R6 and R7 are selected from ,H methyl, CN and Cl. 13. The process according to any preceding clause wherein the metal organic framework comprises one type of linker.

14. The process according to any preceding clause wherein the one or more metal ions are an Ag ion and the one or more organic linkers are a terephthalate linker.

15. The process according to any preceding clause, wherein the metal organic framework further comprises water of crystallization.

16. The process according to any preceding clause wherein the process comprises contacting an aqueous solution of the one or more metal ions with an aqueous solution of the one or more organic linkers.

17. The process according to any preceding clause wherein the only solvent used in the process is water.

18. The process according to any preceding clause wherein the process is performed at a temperature which does not exceed 40°C, preferably at a temperature that does not exceed 30°C, more preferably at room temperature.

19. The process according to any preceding clause wherein the molar ratio of metal ion to linker is between 1 : 1 and 1:10, optionally between 1 : 1 and 1 :5, preferably between 1 : 1 and 1 :3 , more preferably wherein the molar ratio of metal ion to linker is 1 :2.

20. The process according to any preceding clause wherein said process further comprises recovering the metal organic framework.

21. A metal organic framework comprising one or more metal ions and one or more organic linkers, wherein the metal organic framework is obtainable by a process which comprises contacting the one or more metal ions with the one or more organic linkers in the presence of water to form a precipitate of the metal organic framework, wherein the process is performed at a temperature that does not exceed 50°C, optionally wherein the process is as further defined in any one of claims 2 to 20.

22. A metal organic framework comprising a silver ion and one or more organic linkers, wherein the metal organic framework has a photo luminescence quantum yield of greater than 50%.

23. The metal organic framework according to clause 21 or clause 22 wherein the one or more organic linkers are as defined in any one of clauses 5 to 12.

24. A metal organic framework according to any one of clauses 21 to 23, wherein the metal organic framework comprises a silver ion and a terephthalate linker.

25. A metal organic framework according to any one of clauses 21 to 24 wherein the metal organic framework comprises water of crystallisation.

26. A metal organic framework according to any one of clauses 21 to 25 wherein the metal organic framework is in the form of nanoparticles.

27. A metal organic framework according to any one of clauses 21 to 26 wherein the metal organic framework is thermochromic and/or mechanochromic.

28. A device comprising a metal organic framework as defined in any one of clauses 21 to 27, wherein the device is selected from a light emitting device, a photoluminescent device, a electroluminescent device, a luminescent thermometer, a mechanical force sensor, a chemical sensor, a motion sensing system, a data storage device, security paper, optical memory devices and vibration damage detectors.

29. A luminescent thermometer comprising a thermochromic metal organic framework, wherein the metal organic framework is as defined in any one of clauses 21 to 27.

30. A mechanical force sensor comprising a mechanochromic metal organic framework, wherein the metal organic framework is as defined in any one of clauses 21 to 27.

31. A chemical sensor comprising a metal organic framework as defined in any one of clauses 21 to 27.

32. An antibacterial material comprising a metal organic framework as claimed in any one of clauses 21 to 27.

33. A device comprising a metal organic framework as defined in any one of clauses 21 to 27, wherein the device is an optoelectronic device, preferably wherein the device is an electroluminescent device.

34. A device according to clause 33 wherein the device is an electroluminescent device comprising an electroluminescent layer, wherein the electroluminescent layer comprises the metal organic framework, preferably wherein the electroluminescent layer comprises the metal organic framework dispersed in a conducting polymer matrix, preferably wherein the conducting polymer matrix comprises poly(9-vinylcarbazole) (PVK) polymer.

35. A process for producing a mixed-metal organic framework (MMOF), wherein the mixed- metal organic framework comprises a first metal ion, a second metal ion and one or more organic linkers, wherein the process comprises contacting a metal organic framework comprising a first metal ion and the one or more organic linkers with a compound comprising the second metal ion in the presence of a polar protic solvent to form the mixed-metal organic framework, preferably where the polar protic solvent is water.

36. The process according to clause 35 wherein the first and second metal ions are selected from group 9, 10, 11, 12 or 14 metal ions, preferably wherein the first and second metal ions are selected from Zn, Ag, Au, Cu, Cd, Ir, Pt, Pd or Pb ions, more preferably wherein the first metal ion is a Zn ion and the second metal ion is an Ag ion.

37. The process according to clause 35 or clause 36 wherein the one or more organic linkers are as defined in any one of clauses 5 to 12.

38. The process according to any one of clauses 35 to 37, wherein the process comprises mixing the metal organic framework with a compound comprising the second metal ion to create a mixture, then contacting the mixture with the polar protic solvent to form the mixed-metal organic framework.

39. The process according to clause 37, wherein the compound comprising the second metal ion is a salt of the second metal ion, preferably wherein the salt of the second metal ion is a salt of formula MXn, wherein M represents the second metal ion; X is an anion selected from halide, nitrate or CH3COO-; and wherein n depends on the charge of the second metal ion, M, and the charge of the anion, X.

40. A mixed-metal organic framework obtainable by the process of any one of clauses 35 to 39.

41. A mixed-metal organic framework comprising a first metal ion, a second metal ion and one or more organic linkers, wherein the first metal ion is a zinc ion; and the second metal ion is a silver ion.

42. The mixed-metal organic framework according to clause 41 wherein the one or more organic linkers are as defined in any one of clauses 5 to 12.

43. A device comprising a mixed-metal organic framework as defined in any one of clauses 40 to 42, wherein the device is a chemical sensor.

44. A method for detecting the presence of a volatile organic compound, the method comprising contacting a mixed-metal organic framework as defined in any one of clauses 40 to 42 with the volatile organic compound and measuring a property of the mixed-metal organic framework. 45. The method according to clause 44 wherein the property is fluorescence and measuring a property of the mixed-metal organic framework comprises measuring the emission spectrum of the metal organic framework.

46. The method according to clause 44 or 45 wherein the volatile organic compound is acetone.

47. A method for analysing a sample of breath from a subject, wherein the subject has, or is suspected of having, diabetes, said method comprising contacting the sample of breath with a mixed-metal organic framework as defined in any one of clauses 40 to 42, and measuring a property of the mixed-metal organic framework to determine the level of acetone in the sample.

48. The method according to clause 47 wherein the property is fluorescence and measuring a property of the mixed-metal organic framework comprises measuring the emission spectrum of the mixed-metal organic framework.

49. A device comprising a mixed-metal organic framework as defined in any one of clauses 40 to 42, wherein the device is selected from a light emitting device, a photoluminescent device, a luminescent thermometer, a mechanical force sensor, a chemical sensor, a motion sensing system, a data storage device, security paper, optical memory devices and vibration damage detectors.

50. A luminescent thermometer comprising a thermochromic mixed-metal organic framework, wherein the mixed-metal organic framework is as defined in any one of clauses 40 to 42.

51. A mechanical force sensor comprising a mechanochromic mixed-metal organic framework, wherein the mixed-metal organic framework is as defined in any one of clauses 40 to 42.

52. An antibacterial material comprising a mixed-metal organic framework as claimed in any one of clauses 40 to 42.

53. A device comprising a mixed-metal organic framework as claimed in any one of clauses 40 to 42, wherein the device is an optoelectronic device, preferably wherein the device is an electroluminescent device.

Claims

1. A process for producing a metal organic framework, wherein the metal organic framework comprises one or more metal ions and one or more organic linkers, wherein the process comprises contacting the one or more metal ions with the one or more organic linkers in the presence of water to form a precipitate of the metal organic framework, wherein the process is performed at a temperature that does not exceed 50°C.

2. The process according to claim 1 wherein the process comprises a step of dissolving a salt of the one or more metal ions in an aqueous solution, preferably wherein the process comprises a step of dissolving a salt of the one or more metal ions in water.

3. The process according to claim 1 or claim 2 wherein the process comprises contacting an aqueous solution of the one or more metal ions with an aqueous solution of the one or more organic linkers.

4. The process according to any preceding claim wherein the only solvent used in the process is water.

5. The process according to any preceding claim wherein the process is performed at a temperature which does not exceed 40°C, preferably at a temperature that does not exceed 30°C, more preferably at room temperature.

6. A metal organic framework comprising one or more metal ions and one or more organic linkers, wherein the metal organic framework is obtainable by a process which comprises contacting the one or more metal ions with the one or more organic linkers in the presence of water to form a precipitate of the metal organic framework, wherein the process is performed at a temperature that does not exceed 50°C, optionally wherein the process is as further defined in any one of claims 2 to 5.

7. A metal organic framework comprising a silver ion and one or more organic linkers, wherein the metal organic framework has a photoluminescence quantum yield of greater than 50%.

8. A device comprising a metal organic framework as defined in claim 6 or claim 7, wherein the device is selected from a light emitting device, a photoluminescent device, a electroluminescent device, a luminescent thermometer, a mechanical force sensor, a chemical sensor, a motion sensing system, a data storage device, security paper, optical memory devices and vibration damage detectors.

9. A luminescent thermometer comprising a thermochromic metal organic framework, wherein the metal organic framework is as defined in claim 6 or claim 7.

10. A mechanical force sensor comprising a mechanochromic metal organic framework, wherein the metal organic framework is as defined in claim 6 or claim 7.

11. A chemical sensor comprising a metal organic framework as defined in claim 6 or claim 7.

12. An antibacterial material comprising a metal organic framework as defined in claim 6 or claim 7.

13. A device comprising a metal organic framework as defined in claim 6 or claim 7, wherein the device is an optoelectronic device, preferably wherein the device is an electroluminescent device.

14. A device according to claim 13 wherein the device is an electroluminescent device comprising an electroluminescent layer, wherein the electroluminescent layer comprises the metal organic framework, preferably wherein the electroluminescent layer comprises the metal organic framework dispersed in a conducting polymer matrix, preferably wherein the conducting polymer matrix comprises poly(9-vinylcarbazole) (PVK) polymer.

15. A process for producing a mixed-metal organic framework (MMOF), wherein the mixed- metal organic framework comprises a first metal ion, a second metal ion and one or more organic linkers, wherein the process comprises contacting a metal organic framework comprising a first metal ion and the one or more organic linkers with a compound comprising the second metal ion in the presence of a polar protic solvent to form the mixed-metal organic framework, preferably where the polar protic solvent is water.

16. A mixed-metal organic framework obtainable by the process of claim 15.

17. A mixed-metal organic framework comprising a first metal ion, a second metal ion and one or more organic linkers, wherein the first metal ion is a zinc ion; and the second metal ion is a silver ion.

18. A device comprising a mixed-metal organic framework as defined in claims 16 or claim 17, wherein the device is a chemical sensor.

19. A method for detecting the presence of a volatile organic compound, the method comprising contacting a mixed-metal organic framework as defined in claim 16 or claim 17 with the volatile organic compound and measuring a property of the mixed-metal organic framework.

20. A method for analysing a sample of breath from a subject, wherein the subject has, or is suspected of having, diabetes, said method comprising contacting the sample of breath with a mixed-metal organic framework as defined in claim 16 or claim 17, and measuring a property of the mixed-metal organic framework to determine the level of acetone in the sample.

21. A device comprising a mixed-metal organic framework as defined in claims 16 or claim 17, wherein the device is selected from a light emitting device, a photo luminescent device, a luminescent thermometer, a mechanical force sensor, a chemical sensor, a motion sensing sy stem, a data storage device, security paper, optical memory devices and vibration damage detectors.

22. A luminescent thermometer comprising a thermochromic mixed-metal organic framework, wherein the mixed-metal organic framework is as defined in claim 16 or claim 17.

23. A mechanical force sensor comprising a mechanochromic mixed-metal organic framework, wherein the mixed-metal organic framework is as defined in claim 16 or claim 17.

24. An antibacterial material comprising a mixed-metal organic framework as claimed in claim 16 or claim 17.

25. A device comprising a mixed-metal organic framework as claimed in claim 16 or claim 17, wherein the device is an optoelectronic device, preferably wherein the device is an electroluminescent device.