WO2021074275A1 - Effective photomodulation of lanthanide (iii) ion luminescence in the visible or near-infrared range - Google Patents

Abstract

The invention relates to photochromic complexes of lanthanide (III) ions of chemical formula (I) and their use in different fields of application, in particular in security systems for identification, authentication and/or protection against imitation, falsification or counterfeiting of a security document, document of value, commercial asset document or other item. The invention also relates to methods and kits for implementing said security systems.

Description

Efficient Photo-Modulation of the Luminescence of Lanthanide (III) Ions in the Visible or the Near Infrared Parent Patent Application The present patent application claims the priority of the French patent application number FR 1911472 filed on October 15, 2019. The content of the French patent application is incorporated by reference in its entirety. Field of the Invention The present invention relates to the field of rare earth complexes and more particularly lanthanide complexes. The invention relates more specifically to photochromic complexes of lanthanide (III) ions which find application in numerous fields, in particular in the fight against counterfeiting. Background of the Invention The counterfeiting of valuable documents, currency products and branded products is a thorny problem that has serious economic, safety and health implications for governments, businesses and consumers around the world. whole. It is estimated that counterfeiting represents a multi-billion dollar underground economy and that counterfeit products are produced on a larger scale each year. “Trends in Trade in Counterfeit and Pirated Goods”, the 2019 report of the OECD (Organization for Economic Co-operation and Development) and the Intellectual Property Office of the European Union estimates at $ 509 billion per value of imported counterfeits in the world, based on customs seizure data in 2016. These figures do not include counterfeit and pirated products distributed via the Internet. The trade in counterfeit products, which infringes trademarks and copyrights, generates profits for organized criminal organizations at the expense of businesses and governments. In addition, counterfeit items, such as medical supplies, auto parts, toys, food and cosmetic brands, industrial parts in the air transport, aerospace and defense sectors, as well as electrical appliances include various health and / or safety risks. Counterfeiting is an increasingly technologically advanced crime and therefore calls for high-tech solutions to prevent and deter acts of counterfeiting. Over the past decades, a wide variety of security technologies have been developed, including watermarks, holograms, radio frequency identification (RFID), optical systems, and more. (Yoon et al., J. Mater. Chem. C., 2013, 1: 2388-2403; Kumar et al., Nanoscale, 2016, 8: 14297-14340). However, these methods are relatively well known to counterfeiters. Much attention has been paid to the development of colorimetric or fluorometric materials for advanced anti-counterfeiting and encryption. Luminescent systems, which can be easily observed by irradiation, appear to be attractive tools for security labels, identification markers and bar code systems (d'Aleo et al., Coord. Chem. Rev., 2012, 256 (15-16): 1604-1620; Moore et al., Acc. Chem. Res., 2009, 42 (4): 542-552; Elisseva et al., Chem. Soc. Rev., 2010, 39 (1): 189-227; Armelao et al., Coord. Chem. Rev., 2010, 254 (5-6): 487-505). The idea behind these apps is that luminescent labels are invisible (or barely visible) to the naked eye and are only revealed by illumination. For example, euro banknotes carry hidden pigments containing europium (III), which emit a characteristic red-orange color upon UV excitation. However, conventional anti-counterfeiting and encryption systems based on colorimetric or fluorometric materials are limited by spectral overlap and background interference and can also be easily identified by counterfeiters and decoders. In this context, functional molecules have significant advantages, such as the density and complexity of information allowing more advanced encryption. This can be achieved by turning on / off color / fluorescence in a single-mode stimulus response. A practical approach is based on a reversible fluorescence switching with derivatives of fluorophores most often associated with photochromic units (Raymo et al., PCCP, 2013, 15: 14840-14850; Irie et al., Chem. Rev., 2014 , 114: 12174-12277; Klajn et al., Chem. Soc. Rev., 2014, 43: 148-184; Zhang et al., Advanced Optical Materials, 2018, 6: 1701278) via resonance energy transfer intramolecular (FRET) or an electron transfer mechanism (Fukaminato et al., J. Am. Chem. Soc., 2011, 133: 4987-4990; Cheng et al., J. Am. Chem. Soc., 2013 , 135: 10190-10193). Such systems have already been integrated into various anti-counterfeit security media. The present inventors have previously described an organometallic system with redox activity comprising a ytterbium (III) ion, the system being able to activate / deactivate the electrochemically luminescence of Yb (III) (Norel et al., Organometallics, 2014, 33 (18): 4824-4835). More recently, they developed a system containing an original photochromic diethenylethene (DTE) unit, the system being able to be efficiently transformed from a colorless, unconjugated form to a colored conjugate form by UV irradiation and then reverted to the form. initial by irradiation in the visible (He et al., Inorg. Chem., 2016, 55: 12635-12643). Although this system is extremely interesting from a fundamental point of view due to its multifunctional combination and the first observation of pure NIR luminescence optical switching, its potential application in real life remains limited due to the instability. long-term inorganic assembly and low luminescence produced, which is only detectable with powerful devices. Although efforts to develop advanced technologies to combat counterfeiting and forgery have yielded positive results, the need for improved security materials continues to increase. Summary of the Invention The present inventors have developed complexes combining three units of a photochromic entity of diethenylethene (DTE) and a lanthanide (III) ion through a β-diketonate entity. These complexes are useful as effective and efficient anti-counterfeiting systems because they display a dual optical encryption method: (1) a colored code which can be induced and erased under UV irradiation and visible irradiation, respectively, the colored code being due. to the staining / discoloration of the photochromic entities and (2) at the same time, the gradual disappearance and gradual restoration of the luminescence of the lanthandide ion. This system is applicable both with lanthanide (III) ions emitting in the visible range (such as the europium ion, Eu ³⁺ ) and with lanthanide (III) ions emitting in the near infrared range (such as by example ytterbium ion, Yb ³⁺ ) - i.e. both in the case of emissions visible to the naked eye and in the case of emissions invisible to the naked eye. To the knowledge of the present inventors, this system is the very first to allow efficient reproducible modulation of luminesence in the pure near infrared. The system has also been shown to be just as effective when integrated into a polymer (PDMS). Consequently, the present invention relates to a photochromic lanthanide complex of the following chemical formula (I):

(I) where: M is a lanthanide ion (III), Ph.L. is a photochromic ligand; is a bidentate chelating agent; and R is a chemical group (see below). In some embodiments, the lanthanide ion (III) is selected from the group consisting of: Ce ³⁺ , Pr ³⁺ , Nd ³⁺ , Pm ₃ Sm ³⁺ , Er ³⁺ , Gd ³⁺ , Tb ³⁺ , Dy ³⁺ , Ho ³⁺ , Er ³⁺ Tm ³⁺ , and Yb ³⁺ . In certain embodiments, the chemical group R is chosen from the group consisting of: a hydrogen atom (H), a group C _n H _{2n + 1} , where n is an integer between 1 and 14, limits included; a group C _n F _{2n + 1} , where n is an integer between 1 and 14 limits inclusive; a C ₆ H ₅ group, a C ₆ F _{5 group} , a naphthyl, an anthracenyl, a 2-anthracenyl, a 9-anthracenyl, a 1-phenanthrenyl, a 2-phenanthrenyl, a 3-phenanthrenyl, a 4-phenanthrenyl, 9-phenanthrenyl, thiophenyl, and furanyl. In some embodiments, the bidentate chelator is selected from the group consisting of 1,10-phenanthroline, 2,2'-bipyridine, 2,2'-biquinoline, 2- (2-pyridyl) benzimidazole , 2- (1H-imidazol-2-yl) pyridine and their derivatives. In certain embodiments, the photochromic entity is a P-type photochromic entity or a T-type photochromic entity. For example, the photochromic entity is selected from the group consisting of: diarylethenes, in particular dithienylethenes, spiropyranes, spiro-oxazines, fulgides, fulgimides, stilbenes, aromatic azo compounds in particular azobenzenes, aminoazobenzenes, and pseudo-stilbenes, polycyclic quinones, benzopyranes, naphthopyranes, spirodihydroindolizines, viologinesadienpirocyclones triarylmethanes, and anils. In particular, the present invention relates to a photochromic lanthanide complex chosen from the 4ooo complex and the 5ooo complex having the following chemical formula:

. The present invention also relates to a combination comprising a photochromic complex according to the invention and at least one other luminescent compound. In certain embodiments, the at least one other luminescent compound is at least one photochromic lanthanide complex according to the invention, the photochromic lanthanide complexes of chemical formula (I) which make up the combination being all different from each other. In other embodiments, the at least one other luminescent compound is selected from the group consisting of: luminescent organic compounds and luminescent inorganic compounds, and combinations thereof. The present invention also relates to the use of a photochromic complex according to the invention or of a combination according to the invention, as a security compound for a security, identification, authentication, and / or application. protection against imitation, falsification or counterfeiting of security paper, valuable document, commercial property or other object. The present invention also relates to a security composition comprising a photochromic complex according to the invention or a combination according to the invention, and a vehicle. In some embodiments, the security composition is a security ink or a security coating composition. In other embodiments, the vehicle is a substrate selected from the group consisting of polymers, plastics, composite materials, metals, alloys, metallized materials, textile fibers, leather, cardboard and the like. packaging materials, glass, ceramics, wood, stone, paper and their combinations. The present invention also relates to a security element comprising a photochromic complex according to the invention or a combination according to the invention or a security composition according to the invention. In some embodiments, the security element is selected from the group consisting of: security threads, security films, security fibers, security tapes, bands or tapes, security sheets, papers security, security boards, security patches, security labels, security transfers, security laminates and security inserts. The present invention finally relates to a kit comprising a combination according to the invention, characterized in that the combination is provided in a single container or aliquoted in several containers or characterized in that the components of the combination are provided in separate containers. A more detailed description of some preferred embodiments of the invention is provided below. Brief Description of the Drawings Figure 1. Chemical formulas of two complexes according to the invention before irradiation at 350 nm (4ooo and 500o open form of DTE units) and after irradiation at 350 nm (4ccc and 5ccc - closed form of DTE units). The irradiation in the visible (^ = 650 nm) of the closed forms gives again the open forms. Figure 2. Diagram of the syntheses of the 4ooo and 5ooo complexes. Figure 3. Variations of absorption spectra (A) of the 4ooo complex and (B) of the 5ooo complex (each in dichloromethane (CH ₂ Cl ₂ ) at a concentration of about 10 ^-5 M) under UV irradiation (λ = 350 nm). The initial spectra were recovered after irradiation at λ = 650 nm, which causes discoloration of the solutions. Figure 4. (A) Colorless initial form of 500 complex (open state of DTE units) and blue form of formed state (photostationary equilibrium) in dichloromethane. (B) Variations in absorptions of the 5ooo complex at 350 nm (upper part of the graph) and at 625 nm (lower part of the graph) under cyclic UV (λ = 350 nm) / visible (λ = 650 nm) irradiation. Figure 5. (A) Variations in the emission of the 4ooo complex during UV irradiation (λ = 350 nm) in dichloromethane at a concentration of about 10 ^-5 M. The initial spectrum was recovered after irradiation in the visible (λ = 650 nm) and a concomitant discoloration of the solution. (B) Decrease in the emission of the 4ooo complex at 610 nm under the same conditions as for (A). Three successive discolorations are reported. Figure 6. Emission spectrum of the 5ooo complex in dichloromethane captured using a CCD camera (λ _ex = 350 nm). Figure 7. (A) Decrease in emission of the 5ooo complex at 1000 nm during UV irradiation (λ = 350 nm) in dichloromethane (black line). The second, third, and fourth cycles are shown as gray circles, light gray triangles, and black squares, respectively. The initial spectrum was recovered after irradiation in the visible (λ = 650 nm). (B) Emission spectrum obtained with a CCD camera with an integration time of 5 seconds (λ _ex = 350 nm). The first (black line), second to sixth (gray lines) and 38th spectra (corresponding to the photostationary state with approximately 92% of the DTE ligands in closed form) are shown. Figure 8. (A) Photographs showing the transparency of the initial films and the staining upon closure at steady state (λ = 350 nm). (B) Evolution of the absorption spectrum of 4ooo (top left) and 5ooo (top right) in a PDMS polymer (0.2% by weight) during UV irradiation in a stationary state (λ = 350 nm). Fatiguability tests were carried out over 50 (bottom left) and 20 (bottom right) opening and closing cycles by checking absorption in the UV and visible ranges). Figure 9. (A) Photographs of the 4ooo complex in PDMS films (0.2% by weight) in initial open form (left) and in closed form (right) obtained using a camera under UV excitation . (B) Decrease in emissions obtained using λ _ex = 350 and (C) specific decrease at 1000 nm with 10 cycles. The reopening was carried out with a halogen lamp with a high pass filter at 455 nm for 3 minutes. Figure 10. (A) Decrease in emission of the 5ooo complex in PDMS films (0.2% by weight) recorded using a CCD camera with an integration time of 5 seconds (λ _ex = 350 nm), and (B) measured at 1000 nm using a camera with 0.2 second of integration time during an excitation at λ _ex = 378 nm. The reopening was carried out with a halogen lamp and a high pass filter at 455 nm for 3 minutes. Detailed Description of the Invention As mentioned above, the present invention provides a new family of photochromic complexes of lanthanide (III) ions which can find application in a variety of fields, in particular in the field of anti-counterfeiting. I - Lanthanide Complexes A lanthanide complex according to the present invention has the chemical formula (I) and consists of a lanthanide ion (III) complexed with a bidentate chelating agent and with three β-diketonate units, each of the β units -diketonate being covalently linked to a photochromic entity.

(I) where: M is a lanthanide ion (III), Ph.L. is a photochromic ligand;

is a bidentate chelating agent; and R is a chemical group (see below). The terms “complex” and “coordination complex” are used interchangeably herein. They refer to a chemical association between a central atom or ion (here: the lanthanide (III) ion) - called the coordination center - and at least one organic part called a ligand or complexing agent. In a complex, the coordination center and the ligand are connected by weak electrostatic bonds rather than by covalent bonds. A. Ions lanthanide (III) The term "rare earth" is used here to designate any of the metals of the ^6th period having atomic numbers ranging from 57 to 71, from lanthanum to lutetium. These elements, along with chemically similar elements such as scandium and yttrium, are collectively referred to as rare earth elements. Lanthanides, often represented by the symbol Ln, more specifically include: lanthanum (La), cerium (Ce), praseodymium (Pr),), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). The terms “lanthanide (III) ion”, “Ln (III) ion”, “LnIII ion” and “Ln3 +” are used interchangeably herein. They denote the trivalent form of a lanthanide, which is the most stable oxidation state of the lanthanides. Ln ³⁺ ions have the following electronic configuration: 4f ⁿ 5s ₂ 5p ⁶ , where n ranges from 0 to 14. The partially filled inner layer 4f is responsible for the characteristic optical properties of lanthanides, such as long-lived luminescence and very narrow absorption and emission lines. Luminescence is one of the remarkable characteristics of trivalent lanthanide ions. Indeed, with the exception of La ³⁺ and Lu ³⁺ , all the other trivalent lanthanide ions exhibit ubiquitous luminescence. Therefore, the lanthanide ion (III) present in a photochromic complex of chemical formula (I) can be selected from the group consisting of: Ce ³⁺ , Pr ³⁺ , Nd ³⁺ , Pm ³ Sm ³⁺ , Er ^{3 +} , Gd ³⁺ , Tb ³⁺ , Dy ³⁺ , Ho ³⁺ , Er ³⁺ Tm ³⁺ , and Yb ³⁺ . In designing a lanthanide complex according to the present invention, the choice of the lanthanide (III) ion will generally depend on the desired luminescence properties, and more specifically on the desired emission spectral range. Thus, a lanthanide ion (III) can be chosen for its luminescence in the ultraviolet (UV), in the visible or in the infrared (IR), in particular in the near infrared (NIR). In the context of the present invention, UV radiation is to be understood as ranging from 200 nm to 400 nm, visible radiation from 400 nm to 700 nm and IR radiation as having a wavelength greater than 700 nm and generally between 700 nm and 1 mm. NIR radiation should be understood to range from 700 nm to 2500 nm (2.5 μm). The emission spectra of trivalent lanthanides are known in the art (Bünzli, Chem. Rev., 2010, 110: 2729). Due to the effective protection of 4f electrons, the 4f energy levels of Ln ³⁺ ions are only slightly affected by the chemical environment. Consequently, the absorption and emission spectra of Ln ³⁺ ions more or less retain their atomic character, regardless of the organic ligand. More than half of the trivalent lanthanide ions exhibit NIR luminescence with emission wavelengths greater than 700 nm: Nd ³⁺ , Yb ³⁺ and Er ³⁺ , which are the most studied lanthanide (III) ions and Pr ³⁺ , Sm ³⁺ , Dy ³⁺ , Ho ³⁺ and Tm ³⁺ , which have been much less explored. Several of the lanthanide (III) ions are characterized by both visible and NIR emissions, including Pr ³⁺ , Sm ³⁺ , Dy ³⁺ , Ho ³⁺ and Er ³⁺ . Thus, when one wishes a luminescence in the near infrared, the lanthanide ion (III) present in a complex according to the invention can be chosen from the group consisting of Nd ³⁺ , Yb ³⁺ , Er ³⁺ , Pr ³⁺ , Sm ³⁺ , Dy ³⁺ , Ho ³⁺ and Tm ³⁺ . In some embodiments, the lanthanide ion (III) present in a complex of chemical formula (I) exhibits only luminescence with a wavelength greater than 700 nm and is selected from the group consisting of: Nd ³⁺ and Yb ³⁺ . The NIR emission of Nd ³⁺ takes place in three distinct spectral regions: 0.87-0.92 ^ μm, 1.06-1.09 ^ μm and 1.32-1.39 ^ μm; and Yb ³⁺ has a single emission band at 0.96-1.03 μm. In other embodiments, the lanthanide ion (III) present in a complex according to the invention emits both in the NIR and in the visible and is selected from the group consisting of Pr ³⁺ , Sm ³⁺ , Dy ³⁺ , Ho ³⁺ , Tm ³⁺ and Er ³⁺ . Pr ³⁺ has two main emission bands in the NIR at 1.01-1.04 μm and 1.44 μm. Sm ³⁺ has three NIR bands at 0.88 μm, 1.02-1.04 μm and 1.16-1.17 μm. Dy ³⁺ has bands in the NIR at 0.93 μm, 1.0 μm, 1.15 μm, 1.33 μm, 1.50 μm, and 1.70 μm. For Tm ³⁺ , three main emission bands appear at 1.40-1.51 µm, 1.75-1.90 µm and 2.33 µm. Ho ³⁺ has four main NIR emission bands at 0.98-0.99 μm, 1.16-1.19 μm, 1.48-1.51 μm and 1.63-1.68 μm, distinct from two other low energy NIR bands at 1.98-2.10 μm and 2.39-2.45 μm. Er ³⁺ has two NIR emission bands at 1.54-1.60 μm, approximately 1.7 μm and approximately 2.7 μm. When luminescence in the visible range is desired, the lanthanide ion (III) present in a complex according to the present invention can be chosen from the group consisting of Pr ³⁺ , Sm ³⁺ , Eu ³⁺ , Tb ^{3 +} , Ho ³⁺ , Er ³⁺ , Dy ³⁺ and Tm ³⁺ . Eu ³⁺ excited electonically produces a red emission (λ _em = 615 nm), Tb ³⁺ a green emission (λ _em = 540 nm), Sm ³⁺ an orange emission (λ _em = 590 nm), and Dy ³⁺ a emission at 580 nm. Electonically excited Pr ³⁺ produces a red emission (λ _em = 600-650 nm), Tm ³⁺ a blue emission (λ _em = 450 nm), and Ho ³⁺ a red emission (λ _em = 650 nm). B. Chelation of Ln ³⁺ - β-Diketonate Motif and N, N-Bidentate Chelating Agent Since ff transitions are prohibited, ^{unbound Ln 3+} ions have remarkably low molar absorption coefficients, which makes their direct photoexciation difficult. . This can be overcome by using the transfer of energy from organic or organometallic chromophores (antenna effect) to lanthanide ions. In this context, β-diketonates are particularly important because they can efficiently absorb ultraviolet (UV) light and transfer the absorbed energy to the ^{central Ln 3+} ions in an efficient and appropriate manner. In addition, the replacement of the solvent molecules coordinated around the ^{central Ln 3+} by an appropriate auxiliary ligand avoids the effects of non-radiative de-excitation due to the presence of high frequency oscillators. Thus, in a complex according to the present invention, the lanthanide ion (III) is complexed, by chelation, to three β-diketonate units and to a bidentate chelating agent acting as an auxiliary ligand. “Chelation” occurs when a metal ion is combined with a molecule containing two or more electron donating groups, so that at least one ring structure is formed. Molecules containing the electron donor groups are called chelating agents or ligands. The metal ion is generally bonded to strong Lewis bases such as nitrogen (N), oxygen (O) or sulfur (S) atoms. Ligands having two donor sites are called bidentate, those having three donor sites, tridentate, etc. 1. β-diketonate motif. The terms “β-diketone unit” and “β-diketonate unit” are used interchangeably herein. They refer to a 1,3-diketonate anionic entity containing two ketone groups (deprotonated 1,3-propanedione and derivatives), which is commonly used for the synthesis of heterocyclic compounds and which is known to form complexes with many ions of transition metals via a reaction where the two oxygen atoms bond to the metal to form a six-membered chelate ring. More specifically, in the context of the present invention, a β-diketonate unit is capable of chelating a lanthanide (III) ion to give a ternary lanthanide β-diketonate complex. In a β-diketonate unit present in a photochromic complex of chemical formula (I) of the present invention, the first carbon atom is bonded so covalent to the photochromic ligand (see below) and the third carbon atom is covalently bonded to a chemical group R. In formula (I), the chemical group R can be any chemical group not affecting the complexing properties of the ^ -diketonate unit with respect to a lanthanide ion (III) and not disturbing the transfer of energy to the central ^{Ln 3+.} The introduction of broad or fluorinated substituents in β-diketonates has been reported to significantly improve lanthanide luminescence, especially in the case of NIR emission, by suppressing non-radiative processes due to OH, NH and vibrations. CH near Ln ³⁺ centers. Thus, in certain embodiments, R can be any one of: a hydrogen atom (H), C _n H _{2n + 1} , where n is an integer between 1 and 14 (limits included), in particular n = 1 (CH ₃ ), n = 2 (-CH ₂ -CH ₃ ), or n = 3 (-CH ₂ -CH ₂ -CH ₃ ); C _n F _{2n + 1} , where n is an integer between 1 and 14 (limits included), in particular n = 1 (CF ₃ ), n = 2 (-CF ₂ -CF ₃ ), or n = 3 (- CF ₂ -CF ₂ -CF ₃ ); C ₆ H ₅ (phenyl), C ₆ F ₅ (fully fluorinated phenyl), naphthyl, anthracenyl, 2-anthracenyl, 9-anthracenyl, 1-phenanthrenyl, 2-phenanthrenyl, 3- phenanthrenyl, 4-phenanthrenyl, 9-phenanthrenyl, thiophenyl, and furanyl. In certain preferred embodiments, R is a fluorinated substituent selected from the group consisting of C _n F _{2n + 1} , where n is an integer from 1 to 14 (limits included), in particular n = 1 (CF ₃ ), n = 2 (-CF ₂ -CF ₃ ), n = 3 (-CF ₂ -CF ₂ -CF ₃ ), and C ₆ F ₅ (fully fluorinated phenyl). In a preferred embodiment, R is CF ₃ . 2. Bidentate Chelating Agent. As used herein, the term "N, N-bidentate chelating entity" refers to a chelating agent containing two donor nitrogen atoms. Examples of bidentate chelating agents suitable for practicing the present invention include, but are not limited to, 1,10-phenanthroline, 2,2'-bipyridine, 2,2'-biquinoline, 2- (2-pyridyl) benzimidazole, 2- (1H-imidazol-2-yl) pyridine and their derivatives. Examples of suitable derivatives of 1,10-phenanthroline include, without limitation, the following compounds: 5-nitro-1,10-phenanthroline, 5-amino-1,10-phenanthroline, 4,7-dimethyl-phenanthroline, 4 , 7-diphenyl-1,10-phenanthroline (or bathophenanthroline), 5,6-dihydro-1,10-phenanthroline, 1,10-phenanthrolinedisulfonate, 2-phenyl-imidazo [4,5-f] -1,10- phenanthroline, 2- (4'-dialkylaminophenyl) imidazo [4,5-f] - 1,10-phenanthroline, and 3-alkyl-2- (4'-dialkylamino-phenyl) imidazo [4,5-f] -1,10-phenanthroline. Examples of suitable 2,2'-bipyridine derivatives include, without limitation, 3,3'-dimethyl-2,2'-bipyridine and 4,4'-dimethyl-2,2'-bipyidine. Examples of suitable 2,2'-biquinoline derivatives include, without limitation, 4,4'-dimethyl-2,2'-biquinoline, 6,6'-dimethyl-2,2'-biquinoline, 8 , 8'-dimethyl-2,2'-biquinoline, 4,4'-diphenyl-2,2'-biquinoline, 6,6'-diphenyl-2,2'-biquinoline, and 8,8'- diphenyl-2,2'-biquinoline. In some preferred embodiments, the N, N-bidentate chelating agent is 1,10-phenanthroline. C. Photochromic Entity In a complex of chemical formula (I) according to the present invention, each β-diketonate unit is covalently linked to a photochromic entity. As used herein, the terms “photochromic entity” and “photochromic motif” refer to a molecule capable of reversibly transforming between two states or isomers, A and B, having different absorption spectra, the transformation being induced in a direction or in both directions by absorption of electromagnetic radiation. The spectral change produced is usually, but not necessarily, a visible change in colors. The terms 'switch-on' and 'switch off' are sometimes used to denote the two directions of the photochromic reaction, and the photochromic molecules are then called “photoswitches” or “molecular switches”. Thus, for example, in the context of the present invention, the photochromic entity may be colorless in the first state and colored in the second state (e.g. blue, red, green, etc.). Alternatively, the photochromic entity may have a first color in the first state and a second color in the second state, the first and second colors being different colors (e.g. blue and red) or distinguishable colors, preferably. to the naked eye (eg red and dark red). Photochromic systems are classified as either 'P-type' or 'T-type'. In a “P-type photochromic system”, the two isomers are thermodynamically stable and the transformation is carried out in both directions by light irradiation. So, for example, a P-type system can change color when irradiated with a specific wavelength range, then can remain in that state after removal of the stimulus. It is only when subjected to light of the appropriate set of wavelengths that it reverts to its original color. In a “T-type photochromic system,” light drives change in one direction. A T-type photochromic system generally returns to its original state, through a thermal feedback reaction when it is no longer exposed to the light source. In the context of the present invention, the photochromic entity may be a P-type photochromic agent or a T-type photochromic agent. Preferably, the photochromic entity exhibits high resistance to fatigue. Indeed, it is desirable that a photochromic material can undergo the transformation between the two states (or cycle of “coloring / discoloration”) a large number of times, so that the photochromic capacities of a document or an article containing the lanthanide complex are not quickly lost. However, since for most photochromic compounds the “coloring / discoloration” cycle involves a chemical conversion or reaction, degradation, for example by the action of unwanted side reactions, can occur over time. The greater the number of “stain / discolour” cycles of a photochromic compound, the greater the fatigue resistance of the compound. The term “high fatigue strength photochromic compound” denotes a photochromic compound capable of undergoing from about 10 to about 10,000,000 “staining / discoloration” cycles, for example from about 20 to about 1,000,000 “staining / discoloration or bleaching” cycles. example about 30 to about 500,000 "stain / bleach" cycles. A “low fatigue strength photochromic compound” means a photochromic compound which shows no photochromic activity after less than 10 radiation exposures, for example after 1 radiation exposure, or after approximately 3 radiation exposures or after approximately 5 radiation exposures. radiation. The term “about”, as used herein in connection with a quantity, includes the indicated value and has the meaning dictated by the context (for example, it includes at least the degree of error associated with the measurement of the particular quantity). When used with a specific value, it should be considered as revealing that value as well. For example, the phrase “about 2” also includes the value 2, and the range “about 2 to about 4” also describes the range “2 to 4”. In certain situations, the term "envirion", as used herein with reference to a number, generally includes numbers that are within a range of 10% in either direction of the number (greater than or less than the number ), except when this number exceeds 100% of a possible value. Any photochromic compound which exhibits sufficient fatigue resistance for the intended use of the lanthanide complex is suitable for use in the context of the present invention. Several varieties of photochromic molecules have been designed and synthesized. The reversible transformation between the two states A and B of these photochromic molecules can be carried out by a process of cis / trans isomerization, valence isomerization, cycloaddition, tautomerization, or the like. Depending on the type of chemical process involved, photochromic molecules can generally be divided into several classes (Bouas-laurent et al., Pure Appl. Chem., 2011, 73: 639- 665): (1) Photochromic molecules that undergo pericyclic reactions, such as spiropyrans, spiro-oxazines, fulgides, fulgimides, and diarylethenes, or a cycladdition such as certain aromatic compounds; (2) Photochromic molecules which undergo cis / trans isomerization, such as aromatic azo compounds and stilbenes; (3) Molecules which undergo intramolecular transfer of hydrogen or another chemical group, such as anils and polycyclic quinones; (4) Photochromic molecules which undergo photoinduced bond cleavage, such as triarylmethanes and perchlorotoluene; and (5) Photochromic molecules which undergo electron transfer (redox) such as viologens. The photochromic entity present in a lanthanide complex of chemical formula (I) can be chosen from any one of these different classes. Thus, for example, the photochromic entity can be selected from the group consisting of diarylethenes (especially dithienylethenes), spiropyranes, spiro-oxazines, stilbenes, azo aromatic compounds (especially azobenzenes, aminoazobenzenes and pseudo -stilbenes), benzopyranes, naphthopyranes, spirodihydroindolizines, polycyclic quinones, permidinespirocyclohexadienones, viologens, fulgides, fulgimides, triarylmethanes, and anils. 1. Diarylethenes. In some preferred embodiments, the photochromic entity present in a lanthanide complex of chemical formula (I) is a diarylethene. Diarylethenes were first introduced by Irie (Irie, Chem. Rev., 2000, 100: 1685–1716; Irie et al., Chem. Rev., 2014, 114: 12174-12277) and have since elicited a considerable interest, due to their high thermodynamic stability. They operate via a 6- π electrocyclic reaction, the thermal analogue of which is impossible due to steric hindrance. Diarylethenes are known for their resistance to fatigue. Thus, in certain preferred embodiments, the photochromic entity present in a lanthanide complex according to the invention has the following chemical formula (1) and undergoes the following reversible cyclization:

(1) where Z is independently O, S or NR; and R ₁ , R ₂ , R ₃ , R ₄ , R ₅ , R ₆ , R ₇ and R ₈ are independently a hydrogen atom, an alkyl, aryl, arylalkyl, heteroaryl, nitro, sulfo, cyano, halogen group, amine, hydroxyl, alkoxy, aryloxy, alkylthio, alkylsulfo, arylthio, arylsulfo, aldehyde, ketone, ester, amide, carboxylic acid, sulfonic acid, substituted alkyl, substituted aryl, substituted arylalkyl, substituted heretoroaryl, substituted alkoxy, substituted aryloxy, , substituted alkylsulfo, substituted arylthio or substituted arylsulfo, and in which two adjacent R groups can be joined to form a ring. For example, R ₂ and R ₃ can form a phenyl ring, and / or R ₅ and R ₆ can form a phenyl ring. In some preferred embodiments, R ₇ and R ₈ form a ring. For example R ₇ and R ₈ can form one of the following cycles:

In some particular embodiments, R ₁ and R ₄ are independently an alkyl, alkoxy or aryl group. In some preferred embodiments, the photochromic entity is a dithienylethene or DTE, i.e. a thiophene derivative of diaryethene. Some specific examples of dithienylethene include, without limitation, 1,2-bis- (2,4-dimethyl-thiophen-3-yl) per-fluorocyclopentene; 1,2-bis- (3,5-dimethylthiophen-3-yl) perfluoro-cyclopentene; 1,2-bis- (2,4-diphenylthiophen-3-yl) perfluorocyclopentene, etc. Other dithienylethene compounds suitable for use in the context of the present invention include those described below:

In some embodiments, the photochromic entity is a dithienylethene having the following chemical formula (2) or a derivative thereof, which undergoes reversible cyclization from a colorless state to a colored state:

(2) This compound switches between a colorless state, upon exposure to visible light such as sunlight or ambient light (incandescent light), and a dark blue state, upon exposure to UV radiation. . The closed cycle, that is, the colored form of this compound, is, like other diaryethenes, thermally stable in the dark, which means that a printed document can retain the colored state for a very long time. period away from exposure to visible light. 2. Spiropyrans and Spirooxazines. The photochromic entity of a lanthanide complex of chemical formula (I) can be a spiropyran or a spirooxazine, which are T-type photochromic compounds. Photochromes of this class are thermodynamically unstable in one form and revert to the stable form. in the dark unless cooled to a low temperature. Their lifespan can also be affected by exposure to UV rays. Literally, the term spiro refers to a bicyclic molecule whose two rings share a quaternary carbon atom. The photochromic reaction is a heterolytic cleavage of the covalent bond between the spiro carbon and the oxygen atom (spiropyran) or the nitrogen atom (spiro-oxazine), inducing the opening of the ring: it results in the formation of the zwitterionic isomer, which has a characteristic absorption in the visible while the spiro form is generally colorless. The reverse reaction can be thermo- or photo-induced (T-type photochromism). Historically, spiropyrans have been gradually replaced by spirooxazines, which show better stability. Specific examples of spiropyrans include, without limitation, spiro [2H-1-benzopyran-2,2'-indolines], spiroindolinonaphthopyran, spiro [2H-1-benzopyran-2,2 'benzothiazolines], spiropyranopyranes , aza-spiroindolinopyranes, spiro [2H-1-benzopyran-2,2 '(benzoxazolines], spiro (quinolinopyran), and spiro (pyridinopyran). Specific examples of spirooxazines include spiro [indoline-2,3' - [3H] - naphtho [2,1-b] -1,4-oxazines] and spiro [2H-1,4-benzoxazine-2,2'-indolines]. Specific examples include, without limitation, 1, -dihydro-1, 3,3-trimethylspiro [2H- indole-2,3 '- [3H] naphth [2,1-b) - [1,4] oxazine], 1,3-dihydro-1,3,3-trimethyl- 5-nitrospiro- [2H-indole-2,3 '- [3H] naphth [2,1-b) - [1,4] oxazine], 1,3-dihydro-1,3,3-trimethyl-5 , 6'- dinitrospiro [2H-indole-2,3 '- [3H] naphth [2,1-b) - [1,4] oxazine], 1,3-dihydro-1,3,3-trimethyl- 5-methoxy, 5'-methoxy-spiro [2H-indole-2,3 '- [3H] naphth [2,1-b) - [1,4] oxazine], and 1,3-dihydro-1- ethyl-3,3-dimethyl-5'-nitrospiro [2H-indole-2,3 '- [3H] naphth [2,1-b) - [1,4] oxazine]. 3. The Fulgides and Fulgimides. As with spiropyrans and related compounds, photochromism in fulgides and fulgimides is based on a cyclization reaction. More specifically, a conversion between the ^ and ^ bonds results in electrocyclization, and the main difference with spiropyrans is that the open form is colorless and the cyclized form is the colored species. The first fulgides developed were T-type photochromic compounds. But during the 1980s, the introduction of heterocyls such as aryl groups (e.g., furylfulgides and indolinofulgides) allowed thermal stabilization of the closed form and P-type photochromic compounds were obtained. Thus, in certain embodiments, the photochromic entity present in a lanthanide complex of formula (I) is a fulgide or a fulgimide. Specific examples of fulgides or fulgimides include, without limitation, the following compounds: 1- (p-methoxyphenyl) -ethylidene (isopropylidene) succinic anhydride; 2- [1-2,5-dimethyl-3-furyl) -2-methylpropylidene] -3-isopropylidene succinic anhydride; and (1,2-dimethyl-4-isopropul-5-phenyl) -3-pyrrylethylidene (isopropylidene) succinic anhydride. 4. The Stilbenes. Stilbenes undergo isomerization of the C = C double bond under UV irradiation. The color change between the two forms is generally not very pronounced due to a small difference in conjugation between the two states. Usually the cis form is colorless or weakly colored and the trans form is colored. Thus, in certain embodiments, the photochromic entity present in a lanthanide complex of chemical formula (I) is chosen from the class of stilbenes. Specific examples of stilbenes include, without limitation, the following compounds: stilbene (unconjugated), 3-methylstilbene, 4-methoxystilbene, 3-methoxy-stilbene, 4-aminostilbene, 4-fluorostilbene, 3-fluorostilbene, 4-chlorostilbene , 3-chlorostilbene, 4-bromostilbene, 3-bromostilbene, 4-iodostilbene, 3-iodostilbene, 4-cyano-stilbene, 3-cyanostilbene, 4-acetylstilbene, 4-benzylstilbene, 4-phenacylstilbene, 4-nitro , 3-nitrostilbene, 3-nitro-3'-methoxystilbene, 3-nitro-4-dimethylaminostilbene, 4,4'-dinitrostilbene, 4-nitro-4'-methoxystilbene, 4-nitro-3'-methoxystilbene, 4-nitro -4'- aminostilbene, 4-nitro-4'-dimethylaminostilbene, 2,2 ', 4,4', 6,6'-hexamethylstilbene, ^ - methylstilbene, α, α'-dimethylstilbene, α, α'-difluorostilbene, and α, α'-dichlorostilbene, 2,4,6-trimethylstilbene. 5. Azo Aromatic Compounds. In some embodiments, the photochromic entity is selected from the class of aromatic azo (or azo) compounds, which overwhelm isomerization of the -N = N- double bond. In certain preferred embodiments, the photochromic entity is chosen from the class of azobenzenes. The wavelengths at which isomerization of azobenzenes occurs depend on the particular structure of each azo molecule. These are generally grouped into three classes: molecules of the azobenzene type, aminoazobenzenes, and pseudo-stilbenes. These azo compounds are yellow, orange and red, respectively due to subtle differences in their absorption spectra. The unsubstituted azobenzene compounds exhibit a low intensity n- π * type absorption in the visible range and a much higher intensity π- π * type absorption in the ultraviolet. Azo compounds which are ortho or para substituted by electron donor groups (such as amino groups) are classified as aminoazobenzenes and tend to have closely spaced n- π * and π- π * bands in the visible. The class of pseudo-stilbenes is characterized by the substitution of the 4 and 4 'positions of the two azo rings by electron donor and election attractor groups (i.e. the two opposite ends of the aromatic systems are functionalized). The presence of a 'push-pull' configuration leads to a strongly asymmetric electronic distribution, which modifies many optical properties. In particular, it shifts the spectra absorption of trans and cis isomers, so that they overlap effectively. Thus, for these compounds, a single wavelength of light in the visible region induces both forward and reverse isomerization. Under illumination, these molecules cycle between two isomeric states. Photoisomerization of azobenzene is extremely rapid and occurs at picosecond time scales. The rate of thermal relaxation varies considerably depending on the compound: generally, in hours for molecules of the azobenene type, in minutes for aminoazobensenes and in seconds for pseudo-stilbenes. Specific examples of azo aromatic compounds include the following compounds: azobenzene, 2-methoxyazobenzene, 2-hydroxyazobenzene, 4-bromoazobenzene, 4-carbomethoxy azobenzene, 4-acetylazobenzene, 4-carboxyazobenzene, 4-cyanoazobenzene, 4-ethoxy-azobenzene, 4-ethoxy-azobenzene, 4-carbomethoxy azobenzene, 4-acetylazobenzene, 4-carboxyazobenzene, 4-cyanoazobenzene, 4-ethoxy-azobenzene , 4-dimethylaminoazobenzene, 4-amino-azobenzene, 4-trimethylammonium azobenzene (with any suitable anion accompanying the ammonium cation; 4-dimethyl-amino-4'-phenylazobenzene, 4-dimethylamino-4'-hydroxylazobenzene, 4,4'- bis- (dimethylamino) azobenzene, 4-dimethylamino-4 '- [13- (p- aminophenyl) ethyl] -azo-benzene, 2,6-dimethyl-4-hydroxylazobenzene, 2,2', 4 ', 6,6 '- pentamethyl-4-hydroxylazo-benzene, 2,2', 4 ', 6'-tetrachloro-4-hydroxylazobenzene, 3-sulfonate-4-hydroxyazo-benzene, 4,4-dichloroazobenzene, 4-methoxy-3'- sulfonate- azobenzene, 2,2'-azopyridine, 2-phenylazopyridine, 6,6'-azoquinoline, 1-phenyl-azonaphthalene, 1,1-azonaphthalene, 1-phenylazo-4-naphthole, 1-phenylazo-4-methoxynaphthalene, 3-methylazobenzene, 3-nit roazobenzene, 3-methoxyazobenzene, 3-hydroxyazobenzene, 4-iodoazobenzene, 4-methylazobenzene, 4-acetylazobenzene, 4-methoxyazobenzene, 4-nitroazobenzene, 4-acetamidoazobenzene, 4-dimethylamin-4'-dimethylaminophen-4'-dimethylaminophen-4'-dimethylamino-4'-dimethylamino-4'-dimethylamino-4'-dimethylaminophen '-acetamidophenylazobenzene, 4-dimethyl-amino-4'-p-aminobenzyl-azobenzene, 4-hydroxyazobenzene, 2-methyl-4-hydroxyl-azobenzene, 4-hydroxyl-4'-methylazobenzene, 2,6-dimethyl-2' , 4 ', 6'-trichloro-4- hydroxylazobenzene, 4-hydroxyl-4'chloro-azobenzene, 2,2'-dimethoxy-azobenzene, 3,3'-dinitroazobenzene, 3,3'-dimethylazobenzene, 4,4' -dimethylazo-benzene, and 4,4'-dimethoxyazobenzene. 6. The Photochromic Quinones. Photochromic quinones are a class of photochromic organic compounds which, compared to other compounds, are only recently known for their photochromic properties. These properties have were discovered during the synthesis and study of anthraquinone derivatives. It has been found that anthraquinone derivatives having an aryloxy group peri-position relative to the carbonyl group change color reversibly in the presence of UV irradiation. Since then, many photochromic quinones have been developed, such as naphthoquinone derivatives, substituted anthraquinones, naphthacenequinones, pentacenequinones, and phthaloylpyrenes. Specific examples of photochromic quinones include, without limitation, 1-phenoxy-2,4-dioxy-anthraquinone; 6-phenoxy-5,12-naphthacenequinone; 6-phenoxy-5,12-pentacene-quinone; and 1,3-dichloro-6-phenoxy-7,12-phthaloylpyrene. 7. The Chromènes. In some embodiments, the photochromic entity present in a lanthanide complex of chemical formula (I) is selected from the class of chromenes (benzopyran and naphthopyran). Colored forms of chromenes absorb in the range 400-500 nm. Specific examples of chromenes include, without limitation, the following compounds: 3,3-diphenyl-3H-naphtho [2,1-b] pyran; 2-methyl-7,7-diphenyl-7H-pyrano- [2,3-g] -benzo-thyazole; and 2,2'-spiroadamantylidene-2H-naphtho- [1,2-b] pyran. 8. Other Photochromic Entities. In certain embodiments, the photochromic entity is a bisimidazole, for example chosen from the following specific examples: 2,2 ', 4,4', 5,5'-hexaphenyl bisimidazole, 2,2 ', 4,4' , 5,5'-hexa-p-tolyl bisimidazole, 2,2 ', 4,4', 5,5'-hexa-p-chlorophenyl bisimidazole, 2,2 ', di-p-chlorophenyl- 4,4' , 5,5'-tetraphenyl bisimidazole, and 2,2 ', di-p-anisyl-4,4', 5,5'-tetraphenyl bisimidazole. In other embodiments, the photochromic entity present in a lanthanide complex of chemical formula (I) is selected from the class of spirodihydroindolizines and related systems such as tetrahydro- and hexahydroindolizine. Specific examples of suitable spirodihydroindolizines include, without limitation, 4,5-dicarbomethoxy-3H-pyrazole- (3-spiro-9) -fluorene; 1'H- 2 ', 3'-6-tricarbomethoxy-spiro- (fluorine-9-1'-pyrrolo- [1,2-b] -pyridazine]; and 1'H-2', 3'-dicyano- 7-Methoxycarbonyl-spiro- [fluorine-9,1'-pyrrolo- [1,2-b] -pyridine In yet other embodiments, the photochromic entity present in a lanthanide complex of formula (I) is selected from the class of permidin-spirocyclohexadienones. Specific examples of permidinespirocyclohexadienones include, without limitation, the following compounds: 2,3-dihydro-2-spiro-4 '- (2', 6'-di-ter- butylcyclohexadien-2 ', 5'-one) perimidin; 1-methyl-2,3-dihydro-2-spiro-4 '- (2', 6'-di-tert-butylcyclohexadien-2 ', 5'-one) -perimidin; 2,3-dihydro-2-spiro-4 '- [(4H) -2'-tert-butyl-naphthalen-1'-one] -perimidin; and 5,7,9-trimethyl-2,3-dihydro-2-spiro-4 '- (2', 6'-di-tert-butylcylohexadien-2 ', 5'-one) -pyrido- [4.3 , 2, d, e] quinazoline. In still other embodiments, the photochromic entity present in a lanthanide complex of formula (I) is selected from the class of viologens (derivatives of 4,4'-bipyridine) which vary greatly in color (from white to very dark blue) reversibly. Specific examples of viologens include, without limitation, the following compounds: N, N'-dimethyl-4,4'-bipyridinium dichloride; N, N'-diethyl-4,4'-bipyridinium dibromide; and N-phenyl, N'-phenyl-4,4-bipyridinium dichloride. In yet other embodiments, the photochromic entity present in a lanthanide complex of chemical formula (I) is selected from the class of triarylmethanes or from the class of anils (T-type photochromes). In the above chemical formulas, the term “alkyl” denotes a radical composed entirely of carbon atoms and hydrogen atoms. The alkyl radicals can be saturated or unsaturated. The alkyl radical can be linear, branched or cyclic. The alkyl radical can be univalent or divalent, that is, it can bind to one or two different non-hydrogen atoms. Examples of alkyl radicals include methyl, ethyl, vinyl, allyl and propynyl radicals. The term "alkanyl" is used herein to denote saturated alkyl radicals, i.e. C _n H _{2n + 1} . Alkanyl radicals can have from 1 to about 50 carbon atoms, for example from 1 to about 30 carbon atoms, from about 1 to about 25 carbon atoms, from about 1 to about 15 carbon atoms, or from about 1 to about 15 carbon atoms. The term “aryl” denotes an aromatic radical composed entirely of carbon atoms and hydrogen atoms. When an aryl group is described in relation to a numerical range of carbon atoms, it should not be interpreted as including substituted aromatic radicals. For example, the term 'aryl containing 6 to 10 carbon atoms' should not be interpreted as including substituted aromatic radicals. For example, the expression 'aryl containing 6 to 10 carbon atoms' should be interpreted as meaning a phenyl group (6 carbon atoms) or a group Naphthyl (10 carbon atoms) only, and should not be interpreted as including a methylphenyl group (7 carbon atoms). The aryl radical can be univalent or divalent. Aryl radicals can contain from about 5 to about 30 carbon atoms, for example from about 5 to about 20 carbon atoms. The term "arylalkyl" denotes an aryl radical which is located at one end of an alkyl radical. An example of an arylalkyl radical is benzyl (-CH ₂ -C ₆ H ₅ ). Arylalkyl radicals can contain a total of from about 6 to about 40 carbon atoms, for example from about 6 to about 30 carbon atoms. The term "heteroaryl" denotes an aromatic radical composed of carbon atoms, hydrogen atoms, and one or more heteroatoms. Carbon atoms and heteroatoms are present in a ring ring or in the backbone of the radical. The heteroatoms are selected from O, S and N. Examples of heteroaryl radicals include thienyl and pyridyl radicals. It should be noted here that heteroaryl groups are not a subset of aryl groups. Heteroaryl radicals can contain from about 5 to about 30 carbon atoms, for example from about 5 to about 20 carbon atoms. Examples of heteroaryl groups include, without limitation, pyridyl, indolyl, indazolyl, quinoxalinyl, quinolinyl, isoquinolinyl, benzothienyl, benzofuranyl, furanyl, pyrrolyl, thiazolyl, benzothiazolyl, oxazolyl, isoxazolyl, triazolyl, tetrazolyl, pyrazolyl, imidazolyl, thienyl, etc. . The term “nitro” refers to the –NO ₂ radical. The term “sulfo” denotes the –SO ₄ - radical. The term “cyano” refers to the –CN radical. The term "halogen" refers to fluorine, chlorine, bromine, and iodine. The term “amine” denotes the radical –NR ₂ , where each R is independently a hydrogen atom or an alkyl atom. The term "alkoxy" refers to an alkyl radical which is bonded to an oxygen atom (eg, -OR). The alkoxy radicals can contain from 1 to about 50 carbon atoms, for example from 1 to about 30 carbon atoms. Alkoxy groups include, for example, methoxy, ethoxy, propoxy, isopropoxy, butoxy, 2-butoxy, isobutoxy, sec-butoxy, tert-butoxy, pentoxy, hexoxy, etc. The term "aryloxy" denotes an aryl radical which is bonded to an oxygen atom. An example of an aryloxy radical is a phenoxy (-OC ₆ H ₅ ). The term "alkylthio" denotes an alkyl radical which is bonded to a sulfur atom (eg -SR). Alkylthio radicals can contain from 1 to about 50 carbon atoms, for example from 1 to about 30 carbon atoms. The term "alkylsulfo" denotes an alkyl radical which is attached to a sulfate group (i.e. RO-SO ₂ -O-). Alkylsulfo radicals can have a total of from about 1 to about 50 carbon atoms, for example from about 1 to about 30 carbon atoms. The term "arylthio" denotes an aryl radical which is attached to a sulfur atom (i.e. -SC ₆ H ₅ ). Arylthio radicals can contain a total of from about 5 to about 30 carbon atoms, for example from about 5 to about 20 carbon atoms. The term “arylsulfo” denotes an aryl radical which is attached to a sulfate group (i.e. Ar-O-SO ₂ -O-). Arylsulfo radicals can contain from about 5 to about 30 carbon atoms, for example from about 5 to about 20 carbon atoms. The term “aldehyde” denotes a radical which ends in an aldehyde group, for example –R-CHO or –CHO. The term “ketone” denotes a carbonyl group which is located between two radicals and binds to the molecule through one of the radicals, for example R ₁ -CO-R ₂ . The term “ester” denotes a radical of formula –R1-COO-R ₂ or –R1-OCO-R2. The term “amide” denotes a radical of formula –R ₁ -CO-NR ₂ R ₃ or R ₂ -NR ₃ -CO- R ₁ . The term “carboxylic acid” denotes a radical which ends in a carboxyl group, for example –COOH or –R-COOM, where M is a suitable cation, such as hydrogen, sodium or potassium. The term “sulfonic acid” denotes a radical which ends with a sulfonic acid group, for example -SO ₃ H or –R-SO ₃ H. The term “substituted” denotes at least one hydrogen atom of the named radical, which is substituted (or replaced) by another functional group. An alkyl radical can be substituted by nitro, sulfo, cyano, halogen, amine, hydroxyl, aldehyde, ketone, ester, amide, carboxylic acid or sulfonic acid groups. Aryl radicals, arylalkyl, heteroaryl, alkoxy, aryloxy, alkylthio, alkylsulfo, arylthio, and arylsulfo can be substituted by alkyl, nitro, sulfo, cyano, halogen, amine, hydroxyl, alkoxy, aryloxy, alkylthio, alkylthio, alkylsulfo, arylthio, arylsulfo radicals ketone, ester, amide, carboxylic acid or sulfonic acid. An example of a substituted alkyl group is a perhaloalkyl group, in which one or more hydrogen atoms in an alkyl group are replaced by halogen atoms. Examples of substituted aryl groups include methylphenyl and methoxyphenyl groups. Examples of substituted heteroaryl groups include 3-methylthienyl. D. Specific Lanthanide Complexes A lanthanide complex of chemical formula (I) can be designed using any combination of a lanthanide ion (III), a β-diketonate unit, an N, N-bidentate chelating agent and a photochromic entity, as defined above. The present invention relates in particular to two lanthanide (III) complexes designed and synthesized by the inventors. These lanthanide complexes, called 4ooo, which is a europium (IIII) complex and 5ooo, which is an ytterbium (III) complex, have the formulas shown in Figure 1. They undergo an open state transformation. colorless and unconjugated in a closed state conjugate deep blue upon UV irradiation (e.g. at λ = 350 nm) and revert to their original form upon irradiation in the visible (e.g. at λ = 650 nm) . E. Preparation of a Lanthanide Complex (I) A lanthanide photochromic complex of chemical formula (I) can be prepared using any suitable chemical synthesis method. Methods for synthesizing metal complexes and more specifically lanthanide (III) complexes are known in the art (Bünzli, Chem. Rev., 2010, 110 (5): 2729-2755; dos Santos et al., Coord. Chem. Rev., 2008, 252 (23-24): 2512-2527; Moore et al., Acc. Chem. Res., 2009, 42 (4): 542-552). Those skilled in the art know how to modify, adapt and / or optimize such processes to prepare a complex of chemical formula (I). An example of such a synthetic process employed by the present inventors to synthesize the Eu ³⁺ 4ooo complex and the Yb ³⁺ 5ooo complex is described below in the experimental part. This process comprises three steps: a first step in wherein the β-diketonate is covalently linked to photochromic dithienylethene (DTE) to obtain a β-diketonate-DTE unit; a second step in which the lanthanide ion (III) is complexed with three equivalents of the β-diketonate-DTE unit to obtain a bis-aqua complex; and a third step where the final complex is obtained by coordinating 1,10-phenanthroline with the bis-aqua complex. II — Combinations of Lanthanide (III) Ion Complexes and Kits A lanthanide (III) complex according to the present invention can be used alone or alternatively, it can be used in combination with one or more other luminescent compounds. Indeed, a monochrome broadcast hardly meets the criteria of multi-level and high fidelity anti-counterfeiting applications. One of the simplest strategies to improve the level of security is to widen the range of emission colors. For example, by mixing, in appropriate ratios, compounds emitting different colors, an arbitrary emission color can be obtained depending on the percentages of the compounds. Accordingly, the present invention relates to a combination comprising a photochromic lanthanide complex of formula (I) and at least one other luminescent compound. As used herein, the term "luminescent compound" refers to a molecule or nanoparticle which is capable of spontaneously emitting radiation when electronically or vibrationally excited. Upon irradiation (eg, UV irradiation), the lanthanide complex of formula (I) and the other luminescent compound can produce luminescence at distinct emission wavelengths. Alternatively, the emission of the lanthanide (III) complex according to the invention and the emission of other luminescent compound require irradiations of different wavelengths. A combination according to the present invention can comprise a lanthanide complex of formula (I) and another luminescent compound. Alternatively, a combination according to the present invention may comprise a lanthanide complex of formula (I) and two or more (eg three, four or more) other luminescent compounds. The lanthanide complex of chemical formula (I) and the luminescent compound (s) can be present in any ratio or ratio in the combination. For example, when a combination consists of a complex of lanthanide of formula (I) and another luminescent compound, the ratio or ratio can vary from about 1:99 to about 99: 1, for example about 5:95, about 10:90, about 15:85, about 20: 80, about 25:75, about 30:70, about 35:65, about 40:60, about 45:55, about 50:50, about 55:45, about 60:40, about 65:35, about 70: 30, about 75:25, about 80:20, about 85:15, about 90:10 and about 95: 5 (by weight: wt or mol: mol. Likewise, when a combination according to the present invention consists of a lanthanide complex of formula (I), a first luminescent compound and a second luminescent compound, the ratio or ratio may vary from about 1: 1: 98, to about 1: 98: 1 to about 98: 1: 1 (in weight: weight or mol: mol). Those skilled in the art will know how to select a ratio making it possible to optimize the luminescent signature of the combination. In certain embodiments, the luminescent compound of a combination according to the present invention is another complex. photochromic of lanthanide of formula (I). Thus, in a particularly advantageous embodiment, at least two lanthanide complexes of formula (I) which differ from each other only by the lanthanide ion (III) will be used. For example, the first complex of formula (I) may contain the Yb ³⁺ ion, which exhibits luminescence in the NIR and the second complex of formula (I), identical to the first complex but containing, instead of Yb ³⁺ , the Eu ³⁺ ion, which produces a red emission. Or, the first complex of formula (I) can contain Eu ³⁺ , which produces a red emission and the second complex of formula (I), identical to the first complex, can contain Tb ³⁺ , which produces a green emission. Therefore, it is possible to obtain very sophisticated color codes, i.e. emission spectra, with a mixture of chemical molecules having practically identical chemical structures. One of the advantages of this approach is that it does not require the synthesis of several chromophoric molecules with different chemical structures that cannot even be excited at the same wavelength. In addition, the overall emission will evolve over time under UV irradiation with the evolution of the emission of the photochromic compound, and can then be regenerated under visible irradiation. Thus, the present invention provides an easy way to create many optical codes due to the number of possible combinations between lanthanide ions, photochromic ligands, and bidentate chelating agents, the optical codes also being variable depending on the amounts of each of the. complex, length excitation wave and emission intensities. The emission intensities can also be easily adjusted by a person skilled in the art, taking into account the optical luminescence efficiency of each complex and the intensity of the signals which they emit. The use of a mixture of at least two photochromic lanthanide complexes differing only in the nature of the lanthanide (III) ion is also advantageous in that the complexes have different lifetimes in the excited state. The emission spectrum of such combinations varies with time and can be followed by time resolved spectrometry. This provides a further means of varying the production of complex optical codes which are very difficult to reproduce, forge or forge and to increase the diversity of possible optical codes. In embodiments where the luminescent compound present in a combination according to the invention is not a lanthanide complex of formula (I), the luminescent compound may be a luminescent organic compound or a luminescent inorganic compound. Examples of organic luminescent compounds are known in the art and include, without limitation, polycyclic aromatic compounds, such as those comprising a perylene diimide nucleus (known as peri-dinaphthalene bis-diimide, perilene or dibenz [ de, kl] anthacene); linear conjugate systems such as polyparaphenylene / arylene; planar systems based on benzene such as rubrene, couraminic dyes; and metalloorganic compounds containing iridium (Ir), boron (B), aluminum (Al) or platinum (Pt) (for example BODIY, i.e. dipyrromethene-boron; Alq ³ i.e. tris (8-hydroxylquinoline) aluminum), and other metals such as Ru and Cu. An inorganic luminescent compound can consist of metal oxide nanoparticles doped with rare earths (for example, zirconium oxide, yttrium oxide, zinc oxide, copper oxide, lanthanum oxide, gadolinium oxide, praseodymium oxide, etc. and their combinations, which are doped with rare earths like lanthanum, cerium, praseodynium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, the thulian, and their combinations); metallic nanoparticles (e.g. gold, silver, platinum, palladium, iridium, rhenium, mercury, ruthenium, osmium, copper, nickel and their combinations); semiconductor nanoparticles, for example, semiconductors of groups II-VI (such as ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgTe, MgTe, etc.), of groups III-V (such as as GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, AlAs, A1P, AlSb, AÏS, etc.), group IV (such as Ge, Si and the like), groups I-VIII (such as CuCl, Agi and the like), their alloys and mixtures (eg ternary or quaternary mixtures) and combinations thereof.

Other examples of inorganic luminescent compounds include, without limitation, carbon quantum dots (small carbon nanoparticles less than 10 nm in size with some form of surface passivation), and phosphors (c 'that is to say compounds of transition metals or rare earths, such as barium magnesium aluminate doped with europium (BaMgAl ₁₀ O ₁₇ : EU ²⁺ OR BAM) or other elements, for example BAM: Ce ²⁺ ; YAG: Ce ³⁺ ; (Sr, Ba) SiO ₄ : Eu ²⁺ ; LuAG: Ce ³⁺ ).

A combination according to the present invention can be in the form of a mixture (i.e. an admixture of a lanthanide complex of formula (I) and at least one other luminescent compound), for example provided in a single container (or aliquoted in several containers). Alternatively, the components of a combination according to the invention can be supplied in separate containers - the different components being intended to be mixed in the desired ratio to form a suitable luminescent mixture before use.

Accordingly, the present invention also relates to a kit comprising at least one combination described above supplied in a single container or aliquoted in several containers. The present invention also relates to a kit comprising the components of at least one combination described above, each component being provided in a separate container. Thus, the kit may include components (i.e. at least one lanthanide complex of formula (I) and luminescent compounds) which can be used to prepare different combinations. A kit according to the present invention may also contain reagents and / or protocols for preparing and / or using the luminescent combinations.

An identifier (eg barcode, radio frequency, identification labels, etc.) may be present in or on the kit. The identifier can be used, for example, to uniquely identify the kit for quality control, inventory control, movement tracking between workstations, etc. III - Uses of Lanthanide Complexes and Their Combinations Photochromic lanthanide complexes of chemical formula (I), alone or in combination with at least one other luminescent compound as described above, have a wide range of applications, such as for example in anti-counterfeiting technologies, biomedical technologies, display technologies, etc. A. Anti-Counterfeiting Systems As used here, the term “counterfeiting” refers to both forgeries of banknotes and valuable documents (such as identity documents (driver's license, passports, identity cards). , visas and others), credit cards, smart cards, diplomas, etc.), as well as imitations of items such as clothing, handbags, shoes, pharmaceuticals, pesticides, aviation or automotive spare parts, watches, electronic products (spare parts or finished products), software, works of art (paintings, sculptures, photographs, and other collectibles), toys , foods and beverages (e.g. powdered baby milk, cheese, coffee, olive oil, pasta, spirits and others), tobacco products, etc. With their unique optical signatures, the lanthanide complexes of formula (I) and their combinations (as described above) can be used to distinguish genuine items from counterfeit items. A lanthanide complex according to the present invention, or a combination, can be used as such or in an anti-counterfeiting system known in the art. Preferably, the lanthanide complexes used in the identification methods have high thermal stability. They generally do not degrade faster than the document or object to which they are applied or of which they are a part, and can be used for authentication purposes long after their initial application. In such applications, the lantanide complex is included in or applied to a substrate used in the manufacture of the document or object to be secured, or is included in or applied to in the document or object during its manufacture. or after its manufacture. Then, for identification purposes, the document or object is irradiated at the correct wavelength. During irradiation, the color change of the complex photochromic of the lanthanide of chemical formula (I), or of the combination, and the change in luminescence make it possible to identify, without ambiguity, the document or the article. 1. Security Compounds. Accordingly, the present invention relates to the use of a photochromic lanthanide complex of chemical formula (I), or a combination as described above, as a safety compound for safety application, identification, authentication, and / or protection against imitation, forgery or counterfeiting of security paper, valuable document, commercial property, or other object. As used herein, the term “security paper” denotes a paper already equipped with at least one security element but which is not yet circulating and which is an intermediate product in the manufacture of a document. valuable. The term “valuable document” refers to a paper document (which may not contain only paper) which is suitable for circulation. Valuable documents include, without limitation, identity documents (such as identity cards, visas, passports, driver's licenses, etc.), fiduciary items (such as banknotes, bank checks , traveller's checks, bank cards, credit cards and other payment cards, stocks, bonds, etc.), official documents (such as diplomas, certificates of ownership, registration documents, etc. .), identifiers (such as customer or loyalty cards, health cards, chip cards, company identifiers, membership identifiers, service cards, subscription cards, etc. ), transport tickets, access cards, lottery tickets, tickets or tickets for a cultural or sporting event, fiscal stamps, brand authentication labels, tamper-evident labels, seals, and the like. The term “commercial good or other object” generally designates any article having a commercial or other value which one wishes to protect against imitation, falsification or counterfeiting. Examples of such commercial goods or other objects include, without limitation, clothing, handbags, shoes, sporting goods, pharmaceuticals, pesticides, spare parts for land, air or nautical transport equipment. , watches, electronics (spare parts or finished products), software, works of art (paintings, sculptures, photographs, and other collectibles), toys, food and drink (e.g. milk baby powder, cheese, coffee, olive oil, pasta, alcohols, etc.), cosmetics, perfumes, tobacco products, etc. 2. Safety Compositions. The present invention also provides a safety composition comprising a lanthanide complex of chemical formula (I), or a combination as described above, and a vehicle. A safety composition can be liquid or semi-liquid. Alternatively, a security composition according to the invention can be solid. In the context of the present invention, by “vehicle” is meant any medium or additive which does not interfere with the effectiveness of the lanthanide complex or of the combination and, in particular, which does not interfere with these photochromic properties and luminescence. at. Liquid and Semi-Liquid Safety Compositions. In some embodiments, the safety composition is in liquid form or in fluid or semi-liquid form (i.e., which is neither solid nor thick, which flows easily). In particular, a liquid or semi-liquid security composition may be a security ink or a security coating composition or it may be used to manufacture a security ink or a security coating composition. The term “security composition” as used herein means a liquid (“security ink”) or semi-liquid (“security coating composition”) solution that imparts unique identifiable characteristics to the substrate on which it is coated. applied or incorporated into it for authentication or counterfeiting prevention. The application of a security ink can be carried out using any suitable method known in the art, such as, for example, printing, stamping, scoring, spraying, jet marking. ink, coating, soaking, soaking, and impregnation. The application of a safety coating composition can be accomplished by any suitable method known in the art, such as, for example, by means of spin, spray, curtain, flow, spray application. immersion, by injection, by casting, or by means of application by roller or wire or by overmolding. In a liquid or semi-liquid safety composition, the vehicle is generally chosen for its properties of dissolving or dispersing the lanthanide complex of chemical formula (I) or of the combination. In certain embodiments, in particular in the case of security inks, the vehicle is chosen from the group consisting of water, organic solvents and their mixtures. The organic solvents can be protic or aprotic and can be chosen from alcohols, polyols, amines, amides, esters, acids, acetones, ethers, saturated or unsaturated hydrocarbons and their mixtures. Specific examples of organic solvents include for example linear or branched C1-C4 alcohols (in particular methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, ter-butanol ), acetone, 2-butanone, N, N-dimethylacetamide, N, N-dimethylformamide, dimethylsulfoxide, 1,4-dioxane, tetrahydrofuran, ethyl acetate, dichloromethane, chloroform, 1,2-dichloroethane, benzene, toluene, diethyl ether, ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, dipropylene glycol, acetonitrile, N-methylpyrrolidinone and their mixtures. In other embodiments, in particular in the case of safety coating compositions, the vehicle is selected from the group consisting of binder polymers such as polyurethanes, polyacrylics, polyvinyl alcohols, polyvinyl acetates, resins epoxy, aminoplate resins, poly (meth) acrylate resins, polyanhydride resins, and other resins known to be used to produce paints and pigmented pastes. The binder can be transparent. The binder can have adhesion properties, being for example thermoadhesive. A security ink or a security coating composition according to the invention may further comprise one or more additives, such as, for example adhesion promoters, rheology modifiers, surfactants, wetting angle modifiers , humectants, crystallization inhibitors, binders, pH adjusters, etc. A security ink can also include a protective coating material, such as a varnish, a polymer or a lacquer. The composition (constitution) of a security ink or security coating composition and the concentration of the lanthanide complex (I), or a combination, depends on several factors such as the intensity of the desired color. , the nature of the vehicle, the nature of the additives, the intended purpose of the security ink, the nature of the substrate on which the ink is intended to be applied, the application method used, etc. Those skilled in the art know how to select the appropriate components and optimize their respective quantities to design and manufacture a safety composition taking into account these different factors. The total concentration of the lanthanide complex of formula (I), or a combination, in a safety composition can be any suitable proportion, for example at least about 0.01%, for example at least 0.05%, at least about 0.1% by weight, at least about 0.5% by weight, at least about 1% by weight, at least about 2% by weight, for example at least about 5% by weight, at least about 10% by weight, at least about 15% by weight, at least about 20% by weight, or at least about 40% by weight, based on the total weight of the security composition. In some embodiments, it may be preferable that the total concentration of the lanthanide complex of formula (I), or a combination, in a safety composition is less than about 75% by weight, for example less than about 60. wt%, less than about 50 wt%, less than about 45 wt% based on the total weight of the security composition. When applied to the surface of a substrate, document or object, a security composition can be applied in the form of a pattern. The term "pattern" as used herein refers to any type of distribution of visual elements of any type, preferably in a two-dimensional arrangement which forms an image, an element or part of an image, a character, in particular. an alphanumeric character, a symbol, an emblem, a formula, a logo, a bar code, or the like. b. Strong Safety Compositions. In some embodiments, the security composition is in the form of a solid (including in the form of a flexible solid), and the vehicle is a substrate which can be of different types. The term “substrate” is understood here to mean any substance which can serve as a support. Examples of suitable substrates include, without limitation, polymers; plastics; composite materials (such as, for example, paper / plastic or paper / polymer composites); metals; alloys (such as alloys used to make golf clubs or coins) or metallized materials (for example in the form of a box or other type of container intended to hold various items, such as beverages or food products, or in the form of an automobile part for the purpose of authenticating body parts); textile fibers (for example in order to identify a product from the textile industry); leather (eg for authenticating luxury goods such as leather bags or clothing); cardboard or other materials packaging; glass (for example in the form of a container such as a bottle intended to contain various articles, such as drinks or food products, or as a bottle intended to contain a perfume or a cosmetic product), fiberglass ( for example used in the manufacture of printed circuit boards); ceramics; wood; stone (like marble); cellulose or cellulose fibers, in particular cotton; and paper (eg for making security paper). As indicated above, the safety compounds according to the invention (that is to say the lanthanide complexes of formula (I) and the combinations described here) can be applied to at least a part of the surface of a substrate using a security ink or a security coating composition. Alternatively, the security compounds according to the invention can be incorporated into a substrate. As used herein, the term "incorporated into a substrate" means physically and / or chemically combined with a substrate. Thus, a safety compound described herein can be physically associated with part of a substrate, for example, and without limitation, by mixing or combining the safety compound and the substrate (eg, by dissolving or dispersing the safety compound. in a substrate for example before extrusion or lamination), by impregnating the substrate with the security compound, by casting in place or by inserting the security compound in the substrate or even by soaking the substrate with the security compound. A safety compound can also be chemically associated with part of a substrate, for example and without limitation, by copolymerization or otherwise, covalently or non-covalently linking the safety compound to the substrate. In some embodiments, the safety compound described herein can be applied to the host material as part of a coating composition or a sheet comprising the safety compound. As used herein, the term "coating" refers to a supported film derived from a flowable composition, the film may or may not have a uniform thickness. As used herein, the term "sheet" refers to a preformed film having a generally uniform thickness and capable of self-support. Examples of methods of applying sheets comprising a safety compound according to the invention include, without limitation, laminating, melting, mold casting, adhesive bonding of the polymeric sheet and at least a portion of the polymeric sheet. substrate. In-mold casting can be performed by overmolding (a process in which the sheet is placed in a mold and the substrate is formed (e.g. by casting) on at least part of the surface of the sheet); or by injection molding (a process in which the substrate is formed around the sheet). In some embodiments, the substrate is a polymer. Examples of suitable polymers include, without limitation, cellulosic polymers, such as cellulose acetate (CA) and nitrocellulose; polyolefin polymers, such as polyethylenes (PE) and polypropylenes (PPE or PP); vinyl polymers, such as poly (vinyl chlorides) (PVC), poly (vinyl acetates) (PVAC), and polyvinyl alcohols (PVAL); styrenic polymers; saturated polyesters, such as poly (ethylene terephthalates (PET), poly (butylene terephthalates (PBT), poly (ethylene naphthalates) (PEB); unsaturated polyesters (UP); polyamides (PA) ; saturated or crosslinked polyurethanes (PUR); polycarbonates (PC), acrylic polymers, such as poly (methyl methacrylates) (PMMA), and polyacrylonitriles (PAN); aminoplasts; phenoplasts; polyacetates, such as polyoxymethylenes (POM); silicones (SI); polyimides (PI), halogenated polymers; polydimethylsiloxane (PDMS); epoxides (EP); thermostable polymers, such as polyetheretherketone (PEEK) and liquid crystal polymers ( LCP); elastomers; thermoplastic elastomers (TPE), such as styrene-butadiene-styrene (SBS) block copolymers and thermoplastic polyurethanes (TPU); electroactive polymers (EAP); and mixtures and combinations thereof. realization, the substrate is a film, in particular a flexible plastic film, such as for example a film of PI, PP, monoaxially oriented polypropylene (MOPP), PE, polyphenylene sulfide (PPS), PEEK, polyetherketone (PEK), polyetherimide (PEI), polysulfone (PSU), LCP, poly (ethylene naphthalate) (PEN), PBT, PET, PA, PC, cyclic olefin copolymers (COC), POM, acrylonitrile butadiene styrene (ABS), or PVC. The support films can have a thickness of 5 to 700 μm, preferably 5 to 200 μm, or even 5 to 50 μm. Typical examples of composite materials include, without limitation, multilayer structures or laminates of paper and at least one plastic or polymeric material. Thus, for example paper or composites containing paper, for example paper / plastic composites with a basis weight of 20 to 20 to 500 g / m ² , for example 40 to 200 g / m ² , can be used as substrates. supports. The concentration of the safety compound in a substrate may vary depending on the nature of the substrate, its form, its future use, etc. Thus, the total concentration of the safety compound in a substrate can be any suitable proportion. Thus, in some embodiments, the safety compound may be present, in the substrate, at a concentration of from about 10 parts per billion (ppb - parts per billion) to about 5000 parts per million (ppm) in the substrate. substrate, for example from about 15 ppb to about 3000 ppm, such as from about 20 ppb to about 1000 ppm, or from about 25 ppb to about 750 ppm, or from about 30 ppb to about 5000 ppm, or about 30 ppb to about 250 ppm, or about 30 ppb to about 100 ppm, or about 30 ppb to about 50 ppm, or about 30 ppb to about 10 ppm. In other embodiments, the safety compound may be present in a substrate from about 0.01 wt% to about 50 wt%, or from about 0.1 wt% to about 45 wt%, or about 1 wt% to about 40 wt%, or about 2 wt% to about 35 wt%, or about 5 wt% to about 30 wt%, or about 10 wt% by weight to about 20% by weight or about 12% by weight to about 17% by weight, based on the total weight of the substrate. 3. Security Elements. A liquid or semi-liquid security composition according to the invention, such as a security ink or a security coating composition, can be used to “mark” a solid substrate which then itself becomes a security element or which can be used in the manufacture of a security element. Likewise, a solid security composition according to the invention can itself be a security element or can be used in the manufacture of a security element. Consequently, the present invention also relates to a security element comprising a photochromic lanthanide complex of chemical formula (I), or a combination as described above, or else a security composition according to the invention. As used herein, the term “security feature” refers to any feature that can be applied to an article, or be incorporated into an article or be used to manufacture an article, so as to make possible identification or removal. authentication of this article and / or to protect the article against imitation, forgery or forgery. In certain embodiments, the security element forms an integral part of the document or of the object to be secured and is introduced during the manufacture of the document or of the object. This is particularly important because if the item is an integral part of the document or object, it will be very difficult for counterfeiters to reproduce it. Examples of security elements include, without limitation, security threads, security films, security fibers, security tapes, bands or tapes, security boards, security sheets, security papers , security patches, security labels, security transfers, security laminates and security inserts. Security threads, bands, bands and tapes are generally thin bands (plastic, metal or other material) that are incorporated into the substrate during the manufacturing process. They are typically incorporated into papers and polymers used to make valuable documents, so as to provide resistance to counterfeiting or illegal reproduction. Alternatively, the security threads, bands, bands and tapes may be incorporated into the security substrate as a window security element or may be disposed on the surface of the substrate. When the substrate is security paper, the security threads, bands, bands and tapes can be at least partially incorporated into the security paper during manufacture by techniques commonly used in the paper industry. For example, they can be pressed into wet paper fibers while the paper fibers are neither consolidated nor pliable, resulting in the security threads, bands, bands and tapes being fully integrated into the resulting security paper. . The security threads, bands, bands and tapes can also be fed into a paper machine for the molding of cylinders, which results in partial encrustation of the security threads, bands, bands and tapes in the body of the finished paper (i.e. i.e. windowed paper). Alternatively, the security threads, bands, bands and tapes can be arranged on the surface of the substrate acting as a transfer element. In this case, the wires, bands, bands and security tapes can be mounted on the surface of the substrate by any known technique, including the application of a pressure-sensitive adhesive on a surface of the wire, the band, the band or security tape, the application of a heat activated adhesive to a surface of the security wire, tape, band or tape, or using thermal transfer techniques. Security fibers are generally made of PET, PA or viscose. They are designed for use in various authentication applications, for example for protect valuable documents from reproduction, imitation or forgery - banknotes and checks often include security fibers. Safety boards are small disks that can be integrated (or dispersed) in a paper medium during its manufacture. The boards are integrated in the same way as the security fibers. The boards can be metallic or transparent. Security films and security laminates (or laminates) have traditionally been used to protect documents or packages to ensure that the underlying elements are not tampered with. They are particularly useful for identity papers such as driver's licenses and passports, as well as other important documents such as certificates of ownership, identity cards, access cards, etc. Security films and security laminates (or laminates) are also useful as protective labels, for example on medicaments. They can be affixed or applied to a substrate or article by pressure or by a thermal process. The present invention also relates to a method of authenticating an article comprising a security element supposed to contain a security compound according to the invention, the method comprising the steps of: (1) irradiating the article (or part of it). the article including the security element); (2) capture the photochromic and emission properties of the security element; (3) compare the captured properties with the known photochromic and emission properties of the safety compound; and (4) identify the article as genuine if there is complete or sufficient correspondence between the captured properties and the known properties. In such a method, the article can be security paper, a document of value, a commercial good or other object, as defined above. In the context of this method, the irradiation of the article is carried out at a specific wavelength (or with a specific wavelength range), in most cases chosen preferably in the UV. UV irradiation can be done using a simple manual UV lamp. By “capturing the photochromic and emission properties” is meant the observation, detection or measurement of photochromic properties (eg color change due to the photochromic entity) and emission properties of the lanthanide ion ( III) complexed during irradiation. The capture can be carried out with the naked eye, for example in the case of a change of color in the visible and / or in the case of an Ln ³⁺ ion emitting in the visible. Alternatively or additionally, the capture can be carried out using an instrument, such as for example an NIR detector. Those skilled in the art know how to select the most appropriate capture method as a function of the known photochromic and emission properties of the lanthanide complex of chemical formula (I). B. Biological and Medical Applications The photochromic lanthanide complexes of chemical formula (I) possess advantageous properties which can be taken advantage of in biological and biomedical applications. Indeed, the optical analysis and the imaging of biological systems are confronted with two major problems: the important autofluorescence of the tissues when they are irradiated with UV or blue light, and a low photostability of the generally organic luminescent probes. used. Replacing the latter with lanthanide-based probes considerably alleviates these drawbacks, in the sense that lanthanide probes are significantly more photostable and their slow luminescence (long lifespan) can be detected in temporal resolution mode, which Significantly improves the signal-to-noise ratio. The lanthanide complexes of chemical formula (I), and the combinations described here, can therefore be used in different techniques, such as flow cytometry, active fluorescence sorting, immunofluorescence, immunohistochemistry, fluorescence multiplexing , single molecule imaging, single particle tracking, protein folding, protein rotation dynamics, DNA and gene analysis, protein analysis, protein analysis, metabolites, liquid analysis, FRET sensors, high throughput screening, cell detection, bacteria detection, virus detection, biomarker detection, cell imaging, in vivo imaging, labeling bioorthogonal, Click reactions, fluorescence-based bioassays such as immunoassays and enzyme assays, as well as various other fluorescence techniques for assays and biological measurements. Other aspects and advantages of the present invention are described in the following figures and examples, which should be considered as illustrative and not limiting the scope of the invention. Examples The following examples describe certain embodiments of the present invention. However, it is understood that the examples and the figures are presented for illustrative purposes only and in no way limit the scope of the invention. Materials and Methods The chemical reactions were carried out under an inert atmosphere using the techniques of Schlenk. The solvents were dried and distilled under argon by standard procedures. The ¹ H, ¹³ C, ¹⁹ F and ³¹ P-NMR spectra were recorded using a Bruker Avance III 400 MHz spectrometer or a Bruker Avance III HD MHz spectrometer. HR-MS spectra were recorded using a Bruker MicrO-Tof-Q 2 spectrometer. YbCl ₃ , 6H ₂ O (99.998%) and EuCl ₃ , 6H ₂ O (99.99%) were purchased from Aldrich. The precursor A of DTE was obtained as indicated above (Hiroki Kamiya et al., Organic Letters, 2011, 13 (24): 6394-6397). Routine UV-vis irradiations were performed in UV cells or NMR tubes with an LS series Light Source light source from ABET Technologies, Inc. (150 W xenon lamp), with full length light filters. single wave “350FS 10-25” and “650FS 10”. Alternatively, 365nm photoirradiations were performed with fluorescent tubes used for chromatographic plate revealing and white light from an Ikea 1.5W LED lamp was used for opening. Luminescence spectra were measured using a spectrofluorimeter Horiba-Jobin-Yvon Fluorolog-3 ^®, equipped with a monochromator dual-homed excitation with dispersions of 2.1 nm / mm (1200 grooves / mm). The steady-state luminescence was obtained by excitation with unpolarized light emitted from a 50 W continuous wave xenon lamp and detected at an angle of 90 ° for measurements in dilute solutions by a sensitive Hamamatsu R928 photomultiplier tube. to red. The spectra were corrected by reference both for the variation in the light intensity of the excitation source (lamp and grating) and for the emission spectral response (detector and grating). Near infrared spectra were recorded at an angle of 90 ° using a solid indium / gallium / arsenic detector cooled with liquid nitrogen (850-1600 nm) through an RG830-2 mm filter. The selective excitation at 350 nm required the use of a CG400 filter in order to perfectly suppress the excitation of the second harmonic (700 nm) located in the absorption band of the closed form of the DTE. Synthetic Route of Compound 1o Synthesis of Compound B: 1- (4- (3,3,4,4,5,5-hexafluoro-2- (2-methyl-5-phenylthiophen-3-yl) cyclopent-1-en -1-yl) -5-methylthiophen-2-yl) ethanone. In a well pre-dried Schenk tube, the deprotected DTE compound A (0.5 g, 1.2 mmol) was dissolved in 10 mL of dry dichloromethane. The solution was degassed for 5 minutes, then acetic anhydride (0.45 mL, 4.8 mmol) was added to the solution. Then tin chloride (SnCl ₄ ) (1 M, 3.71 mmol) was added slowly. After two hours at room temperature, the reaction mixture was quenched with 20 mL of cold 0.1 M HCl). The product was extracted with 3x15 mL of ethyl acetate. The organic phase was collected, dried with MgSO ₄ , and dried in vacuo. Column chromatography (silica / 0.2: 0.8 diethyl ether / petroleum ether, Rd = 0.2) was carried out and gave the expected product in the form of a yellowish solid (0.57 g, 97%). 1H NMR (400 MHz, CDCl ₃ ): δ (ppm) = 7.67 (s, 1H, CH ₂ ), 7.54 (d, J = 6.69 Hz, 2H, CH _o ), 7.42 ( t, J = 7.76Hz, 2H, CH _m ), 7.34 (t, J = 7.77Hz, 1H, CH _p ), 7.23 (s, 1H, CH ₁ ), 2.56 ( s, 3H, -COCH ₃ ), 2.04 (s, 3H, H ₃ ), 1.97 (s, 3H, CH ₃ ). 1 ³ C NMR (300 MHz, CDCl ₃ ): δ (ppm) = 189.89, 150.5, 142.8, 142.5, 141.2, 133.1, 132.1, 129.0, 128 , 1, 126.2, 125.6, 125.4, 122.1, 30.9, 26.5, 15.23, 14.5. 1 ⁹ F NMR (300 MHz, CDCl ₃ ): δ (ppm) = -131.9 (s, 2F), -110.3 (s, 2F), -110.0 (s, 2F). HR-MS (ESI - CH Cl): m + 2 ₂ / z 509.0439 {M + Na}, Calculated (C ₂₃ H ₁₆ F ₆ OS ₂ + Na) 509.0444. Elementary Analysis. Calculated for C ₂₃ H ₁₆ F ₆ OS ₂ : H, 3.32; C, 56.78; S, 13.18; Measured: H; 3,410; C, 56.69; S, 12.96.

B Synthesis of Compound 1o: 4,4,4-trifluoro-1- (4- (3,3,4,4,5,5-hexafluoro-2- (2-methyl-5-phenylthiophen-3-yl) cyclopent- 1-en-1-yl) -5-methylthiophen-2-yl) butan-1,3-dione. In a well pre-dried Schenk tube, Compound B (0.57 g, 1.18 mmol) was dissolved in 12 mL of freshly dried THF. The system was cooled to -78 ° C, after which LDA (2 M in THF, 1.3 mmol) was slowly added. The reaction medium took on a dark green color during the addition. The mixture was kept under stirring for 2 hours at the same temperature. Then, ethyl 2,2,2-trifluoroacetate CF ₃ COOEt (0.15 mL, 1.3 mmol) was added slowly. The solution was kept overnight with stirring to reach room temperature. The reaction was neutralized by adding 15 mL of HCl (1M), then the solvent was extracted with 2x15 mL of diethyl ether. The organic phase was collected, dried with MgSO4, and then dried in vacuo. Column chromatography was carried out with first of all a 0.5: 1.5: 0.8 mixture of ₃ N Et / EtOAc / pentane, then a 0.5: 0.5: 0.1 mixture: 0.8 of Et ₃ N / MeOH / EtOAc / pentane as eluent. The final product 10 was obtained as a bluish white solid (0.44 g, 65%). 1H NMR (300 MHz, CDCl ₃ ): δ (ppm) = 7.71 (s, 1H, CH ₂ ), 7.55 (d, J = 6.69 Hz, 2H, CH _o ), 7.38 ( t, J = 7.76Hz, 2H, CHm), 7.32 (t, J = 7.77Hz, 1H, CH _p ), 7.29 (s, 1H, CH ₁ ), 6.13 (s, 1H, CH), 2.03 (s, 3H, CH ₃ ), 1.95 (s, 3H, CH ₃ ) - the enol form was observed. 1 ⁹ F NMR (300 MHz, CDCl ₃ ): δ (ppm) = -133.5 (s, CF ₂ ), -111.7 (s, CF ₂ ), -111.6 (s, CF ₂ ), -77.4 (s, CF ₃ ). 1 ³ C NMR (400 MHz, CDCl ₃ ): δ (ppm) = 182.6, 170.6, 152.8, 143.4, 142.2, 138.0, 133.5, 132.9, 129 , 4, 128.5, 127.4, 126.0, 125.5, 122.5, 94.1, 15.6, 14.8. HR-MS (ESI - CH Cl): m / z 581.0301 {M + 2 ₂ + H}, Calculated (C ₂₅ H ₁₄ O ₂ F ₉ S ₂ ) 582.370.

1o Synthetic Route of Compound 2ooo Compound 1o (56 mg, 0.0961 mmol) was dissolved in the minimum amount of methanol (10 mL). Then, water (9 mL) was added and the solution became a little cloudy. Several drops of methanol were then added to obtain a solution limpid. The pH of the solution was between 6 and 6.5 (otherwise the pH was kept within this range by adding NH ₄ Cl). EuCl ₃ , 6H ₂ O (0.032 mmol) was dissolved in the minimum of water (1 mL). The metal salt solution was then added to compound 10, and a bluish white solid appeared immediately. The solution was kept under stirring for 30 minutes, the pH did not change any more. Water (40 mL) was added to the suspension which was further stirred for 10 minutes. The solid was filtered off and washed with 2 x 10 mL of water and 0.5 mL of pentane. Compound 200 was obtained in the form of a bluish-white powder (60 mg, 97% yield). 1H NMR (400 MHz, CDCl ₃ ): δ (ppm) = 7.84 (bs, 3H, H ₂ ), 7.60 (bm, 6H, H _ph ), 7.45 (bm, 3H, H _ph ) , 7.37 (bm, 3H, H _ph ), 3.06 (bs, 3H, H ₁ ), 2.62 (bs, 9H, CH _ph ), 1.77 (bs, 9H, CH _ph ). 1 ⁹ F NMR (400 MHz, CD ₂ Cl ₂ ): δ (ppm) = -81.45 (s, 9F, CF ₃ ), -110.60 (s, 6F, CF ₂ ), -110.67 ( s, 6F, CF ₂ ), -132.33 (s, 6F, CF ₂ ).

2ooo Synthetic Route of the Compound 3ooo The compound DTE 1o (50 mg, 0.086 mmol) was dissolved in the minimum amount of methanol (10 mL). Then water (9 mL) was added and the solution became a little cloudy. Several drops of methanol were then added to obtain a clear solution. The pH of the solution was between 6 and 6.5 (otherwise the pH was kept within this range by adding NH ₄ Cl). YbCl ₃ , 6H ₂ O (0.028 mmol) was dissolved in the minimum amount of water (0.5 mL). The metal salt solution was then added to the solution containing compound 10, and a bluish white solid appeared immediately. The solution was kept under stirring for 30 minutes, the pH did not change any more. Water (40 mL) was added to the suspension which was further stirred for 10 minutes. The solid was filtered off and washed with 2 x 10 mL of water and 0.5 mL of pentane. The compound 3ooo was obtained in the form of a powder of bluish white color (53 mg, yield 96%). 1H NMR (400 MHz, CD ₂ Cl ₂ ): δ (ppm) = 7.86 (bd, 6H, H _ph ), 7.85 (bd, 6H, H _ph ), 7.69 (bt, 3H, H _ph ), 7.68 (bs, 3H, H _2/3 ) [integration of the whole zone shows 22 protons, which correspond to the aromatic region], 1.81 (bs, 9H, CH _m ), 1, 26 (bs, 9H, CH _m ), -17.45 (bs, 3H, H ₁ ). 19F NMR (400 MHz, CD ₂ Cl ₂ ): δ (ppm) = -91.93 (s, 9F, CF ₃ ), -110.70 (s, 6F, CF ₂ ), - 110.89 (s, 6F, CF ₂ ), -132.46 (s, 6F, CF ₂ ). HR-MS ESI - CH + 2Cl ₂ ): m / z 1917.02578 {M - 2H ₂ O}, Calculated (C ₇₅ H ₄₂ O ₆ F ₂₇ S ₆ Yb) 1917.0242.

3ooo Synthetic Route of the Compound 4ooo A mixture of the compound 2ooo (113 mg, 0.058 mmol) and 1.10-phenanthroline (10.56 mg, 0.058 mmol) was dissolved in 10 mL of dichloromethane and kept under stirring for 24 hours before stirring. evaporate the solvent under reduced pressure. Compound 4ooo was obtained without further purification in quantitative yield (115 mg, 95% yield). 1H NMR (400 MHz, CD ₂ Cl ₂ , 297 K): δ (ppm) = 11.16 (bs, 2H, H _phen ), 10.05 (bs, 2H, H _phen ), 9.28 (bs, 2H, H _phen ), 8.46 (bs, 2H, H _phen ), 7.54 (d, 6H, H _Ph ), 7.43 (d, 6H, H _Ph ), 7.35 (s, 3H, H _Ph ), 7.19 (s, 3H, H ₃ ), 5.81 (s, 3H, H ₂ ), 3.05 (s, 3H, CH ₁ ), 2.45 (s, 9H, CH ₃ ), 1.64 (s, 3H, CH ₃ ). 1 ⁹ F NMR (376 MHz, CD ₂ Cl ₂ , 297 K): δ (ppm) = -80.07 (s, 9F, CF ₃ ), -110.64 (s, 12F, CF ₂ ), -132 , 31 (m, 6F, CF ₂ ).

4ooo Synthetic Route of Compound 5ooo A mixture of compound 3ooo (37 mg, 0.019 mmol) and 1.10-phenanthroline (3.4 mg, 0.019 mmol) was dissolved in 5 mL of dichloromethane in a round bottom flask and stirred for 24 hours. Compound 500 was obtained without further purification in quantitative yield (40 mg, 98% yield) after evaporation of the solvent. 1H NMR (400 MHz, CD ₂ Cl ₂ , 297 K): δ (ppm) = 18.27 (bs, 2H, H _phen ), 7.46 (bs, 2H, H _phen ), 12.96 (bs, 2H, H _phen ), 10.41 (bs, 2H, H _phen ), 7.54 (d, 15H, aromatics), 7.43 (m, 3H, H ₁ ), 6.85 (s, 3H, H ₂ ), 1.36 (s, 9H, CH ₃ ), 1.34 (s, 9H, CH ₃ ), -10.14 (s, 9H, CH ₁ ); 19F NMR (376 MHz, CD ₂ Cl ₂ , 297 K): δ (ppm) = -88.36 (s, 9F, CF ₃ ), -111.03 (s, 6F, CF ₂ ), -111.69 (s, 6F, CF ₂ ), -132.83 (m, 6F, CF ₂ ).

5ooo Results and Discussion Syntheses of the Complexes. The synthesis of the new lanthanide complexes was carried out in two steps. First, the bis-aqua precursors [M (1o) ₃ .2H ₂ O] 2ooo (M = Eu ³⁺ ) and 3ooo (M = Yb ³⁺ ) were prepared by complexing the Eu ³⁺ ion or the ion Yb ³⁺ with three equivalents of DTE- β diketone 1o. The final 4ooo (M = Eu ³⁺ ) and 5ooo (M = Yb ³⁺ ) complexes were then obtained by coordinating phenanthroline with the bis-aqua complex 2ooo and with the bis-aqua complex 3ooo, respectively in dichloromethane. These novel phenanthroline adducts were characterized using NMR and IR spectroscopy, elemental analysis and high resolution mass spectrometry (HR-MS). First, informative 1H NMR spectra showing paramagnetic shifts (pseudo-contact shifts) were obtained for the two complexes (Lapadula et al., Chem. Mater., 2014, 26: 1062-1073; Pintacuda et al., Chem. Mater., 2014, 26: 1062-1073; Pintacuda et al. al., Acc. Chem. Res., 2007, 40: 206-212; Ahmed et al., Inorg. Chim. Acta, 2010, 363: 2606-2615; Bourdolle et al., Chemphyschem, 2013, 14: 3361- 3367). They show in particular that the protons of phenanthroline and β-diketonate, located closest to the paramagnetic center, are those which undergo the greatest chemical shifts. ¹⁹ F NMR spectra with signals corresponding to the CF ₃ groups (- 88.4 ppm) and to the central (-132.8 ppm) and external _{CF 2 groups (-111.0 and -111.7 ppm) were} observed for the 5ooo complex. Likewise for the 4ooo complex, signals corresponding to CF ₃ (-80.0 ppm) and to the central (-132.3 ppm) and external ( _{-110.6 ppm) CF 2 groups were observed.} It should be noted that in these spectra, a set of unique and well resolved signals appears for the 3 diketonate ligands, which indicates a rapid exchange between them. Absorption Properties and Photochromic Properties of 4ooo and 5ooo Complexes. The absorption spectra of the colorless 4ooo and 5ooo complexes were found to be identical and the characteristic photochromism of the DTE unit was observed (see Figure 3). The strong absorption bands at λ _max = 274 nm and λ _max = 349 nm are attributed to the π- π * transitions of the DTE ligand, which probably overlap with the transitions based on the phenanthroline ligand. When irradiating 4ooo and 5ooo solution at λ = 350 nm, a decrease in the absorption bands was observed and a strong absorption assigned to an intra-ligand (IL) transition centered on the closed form of the DTE fragment d 'an intense blue appeared at λ _max = 632 nm, with a shoulder at about 390 nm. The two compounds thus irradiated are of a shade of blue. The solutions were brought back to the open and colorless form under irradiation with visible light (λ = 650 nm), as evidenced by the quantitative recovery of the initial spectrum. It should be noted that absorption studies were performed using concentrations in the range of 10 ^-5 M without observation of uncoordinated species in the spectra of the open and closed forms of the complexes, indicating the integrity of each complex within this concentration range. Several cycles of irradiation have were carried out and did not cause any degradation of the complexes (see Figure 4 in the case of the 5ooo complex). Isomerization processes were further monitored by ¹ H NMR in deuterated dichloromethane (CD ₂ Cl ₂ ) at a concentration of approximately 2 × 10 ^-3 M. Integration of CH ₃ proton signals from DTE units shows unambiguously that the photochromic events, during UV irradiation, are almost over since about 92% of the DTE units are closed in a steady state. Likewise, the presence of only two different signals for the α-H protons on the phenanthroline unit and their integration rates suggest the presence of 76% of the completely closed complex and 24% of the complex comprising only two closed photochromic units. (the third DTE unit being open). It should be noted that in these cases, the stability of the systems allows direct irradiation to the steady state at the NMR concentration, without noticeable decomposition. Specifically, in both the open and closed states, (i) NMR signals from uncoordinated phenanthroline are not detected, and (ii) well resolved signals are observed. Crystals of quality suitable for determining the X-ray structure of the 5cccc complex were obtained. The complex crystallizes in the P-1 space group. The bond lengths and angles are in agreement with those of previously published complexes, such as [Yb (phen) (pfnp) ₃ ] (where Hpfnp = 4,4,5,5,5-pentafluoro-1- (2- naphthyl) -1,3-butane-dione) (Feng et al., Dalton Trans., 2009, 13: 2406-2414) with the Yb ³⁺ ion in a slightly distorted square antiprism coordination polyhedron. It should be noted that, in the solid state, for the three DTE units, the DC bond resulting from the electrocyclization is between 1.500 Å and 1.602 Å. Emission Properties of Complexes 4ooo and 5ooo. The emission spectra of the 4ooo and 5ooo complexes were studied in dichloromethane solutions. With a conventional fluorimeter equipped with a solid indium / gallium / arsenic detector (850-1600 nm) or a red sensitive photomultiplier tube, it is difficult to measure their full emission spectrum due to the concomitant closure of the units. photochromic with UV excitations at 350 nm. However, in the case of the 4ooo compound (M = Eu ³⁺ ), under excitation with a small slit opening, allowing the excitation power to be reduced, and with a fast scanning speed, the characteristics Typical Eu (III) ion emission were detected, although the resolution was low (see Figure 5 (A)). Therefore, a sharp transition located at 611 nm ( ⁵ D ₀ → ⁷ F ₂ transition) and wider bands at 590 nm (J = 0), 596 nm (J = 1), 651 nm (J = 3) (transition ⁵ D ₀ → ⁷ F _J ) were observed. For the reasons discussed above, the intensity of the emission was probably lower than expected because the spectrum was recorded upon deactivation due to the closed isomer of DTE. The efficiency of the photomodulation process is clearly demonstrated by a further loss of intensity during the successive measurement of several scans. In order to demonstrate this luminescence quenching during the isomerization of DTE, the decrease in luminescence at a specific wavelength (610 nm) was also measured as a function of time during continuous irradiation at 350 nm. (see Figure 5 (B)).

In the case of the compound 5ooo (M = Yb ³⁺ ), although the kinetic parameters of isomerization were essentially the same, a higher excitation power and longer integration times were required to obtain a signal-to-noise ratio. acceptable, because the emission of the Yb (III) ion is generally less intense than the emission of the Eu (III) ion. However, under these conditions, the closing of the DTE units is too fast and no luminescence could be detected, which clearly shows that 5ccc is non-emissive. To obtain informative spectra of the complex in open form, the present inventors have used a CCD camera, which allows simultaneous and rapid detection over the entire spectral range and avoids spectrum distortion when measuring an evolving system. As 5ccc is non-emissive, the spectrum obtained (Figure 6) can be attributed to the 5ooo complex with an emission characteristic of a Yb (III) complex with the transition maximum ² F _5/2 → ² F _7/2 detected at 970 nm (λ _ex = 350 nm). It is also interesting to note that whatever the type of detection used and unlike a previously published complex associating a DTE and a Yb (III) center (He et al., Inorg. Chem., 2016, 66: 12635-12643 ), no emission was detected when 5ccc was excited with light at 600nm.

Then, the extinction of luminescence with the isomerization of DTE was followed either by following the decrease in luminescence at a specific wavelength (1000 nm), during continuous irradiation at λ _ex = 350 nm ( Figure 7 (A)); or a successive acquisition of several spectra measured with CCD detection (Figure 7 (B)). The two measurements demonstrated without ambiguity that the photoisomerization of DTE leads to a very efficient attenuation (quenching) of the emission of Yb (III). In addition, to evaluate the stability of the photoinduced emission, several decreases in luminescence were successively measured, with regeneration of the completely open and colorless 5ooo complex by irradiation with white light between each measurement. As shown in Figure 7, the system response is highly reproducible for 4 cycles.

The quenching mechanism of fluorescence with DTE units most often involves a non-radiative process ranging from the excited state of the fluorophore (donor) to the photochromic unit (acceptor), generally in its conjugated state, which requires that the characteristic absorption band of the acceptor overlaps the emission of the donor (FRET, Förster Resonance Energy Transfer). Thus, the extinction of the luminescence of Eu (III) by the closed form of the DTE antenna can easily be explained by this overlap, knowing that the low excited state of the DTE (550-580 nm, 17,000-18 000 cm ^-1 ) is less _than ^{the most accepting 5} D 1 state and has almost the same energy as the europium ion ( ⁵ D ₁ at 19,000 cm ^-1 and ⁵ D ₀ at 17,400 cm ^-1 ). This mechanism is systematically proposed for this extinction of luminescence of Eu (III). As regards the Yb (III) ion, this overlap does not exist, the respective bands being 300 nm apart. The position of the excited state ² F _5/2 of Yb (III) (10200 cm ^-1 ) being at a much lower energy than that of Eu (III), the extinction mechanism can come directly from the triplet state of the closed DTE ligand.

Manufacture and Study of the Polymer Film. The excellent properties of Eu (III) and Yb (III) complexes make them ideal candidates for the construction of high quality luminescent polymers. Therefore, the two lanthanide complexes (in their open forms) were incorporated into polymethylsiloxane polymers (PDMS) with different fillers (0.1, 0.2, 0.4% by weight for 4ooo and 0.1 and 0.2% by weight for 5ooo).

The photographs in Figure 8 (A) show the transparency of the initial PDMS films loaded with 0.2% of the 4ooo complex and the staining upon stationary state closure (λ = 350 nm). The same coloring was obtained with the 5ooo complex. The absorption spectra revealed that the properties of the complexes were preserved in the films in open form and in closed form compared to the respective solutions. Several open / close cycles were performed and demonstrated the stability of the complexes in the polymer film during manufacture. Further studies of the luminescence properties were carried out on the Eu (III) (Figure 9) and Yb (III) (Figure 10) complexes incorporated into the polymers. Again, the emission spectra were found to be similar to those obtained in solution and the photomodulation of luminescence was fully operational with an extinction rate of 92% (at λ = 617 nm) and 96% (at λ = 975 nm) for films of Eu (III) and Yb (III), respectively, under UV irradiation. The reversibility of the system has been demonstrated by performing several close / open cycles. Under UV irradiation, the reversible extinction of luminescence was perfectly visible to the naked eye in the case of europium. It is then possible to adjust the charge of the new lanthanide complexes in the polymer in order to allow the naked eye detection of the two concomitant processes (blue coloration and loss of red phosphorescence) with a very fast response time (a few seconds). Conclusion The innovative system, which is described here, combines three DTE units and a lanthanide (III) ion via a β-diketonate unit. This system provides efficient quenching of lanthanide ion luminescence, which can be highly reproducibly reversed by irradiation with visible light. This can be achieved with lanthanide ions emitting in the visible, such as the europium ion, but also with a lanthanide ion emitting in the near infrared, such as the ytterbium ion - therefore observable or not with the naked eye. In fact, to the knowledge of the present inventors, this system is the very first to allow efficient reproducible modulation of pure NIR luminescence. The system has also been found to be effective when integrated into a PDMS polymer for practical application. Consequently, a complex according to the invention can be used as an effective anti-counterfeiting system displaying a double optical encryption method: (i) a colored code which can be traced in blue and then erased under UV and visible irradiation, respectively - the code colored being visible to the naked eye like many systems for coloring / decolouring photochromic entities, and (ii) simultaneously, the gradual disappearance and gradual restoration of the luminescence of the associated lanthanide ions can be revealed under UV light. The mixing of several lanthanide (III) complexes in a specific ratio will allow the production of systems with specific optical signatures and temporal response. Modulation of the DTE unit may also allow color adjustment.

Claims

Claims 1. Photochromic lanthanide complex of the following chemical formula (I):

(I) where: M is a lanthanide ion (III), Ph.L. is a photochromic ligand;

is a bidentate chelating agent; and R is a chemical group.

2. Photochromic lanthanide complex according to claim 1, characterized in that the lanthanide ion (III) is chosen from the group consisting of: Ce ³⁺ , Pr ³⁺ , Nd ³⁺ , Pm ³ Sm ³⁺ , Er ^{3 +} , Gd ³⁺ , Tb ³⁺ , Dy ³⁺ , Ho ³⁺ , Er ³⁺ Tm ³⁺ , and Yb ³⁺ .

3. A photochromic lanthanide complex according to claim 1 or claim 2, characterized in that the chemical group R is chosen from the group consisting of: a hydrogen atom (H), a group C _n H _{2n + 1} , where n is an integer between 1 and 14, limits included; a group CnF2n + 1, where n is an integer between 1 and 14 limits inclusive; a C ₆ H ₅ group, a C ₆ F _{5 group} , a naphthyl, an anthracenyl, a 2-anthracenyl, a 9-anthracenyl, a 1-phenanthrenyl, a 2-phenanthrenyl, a 3-phenanthrenyl, a 4-phenanthrenyl, 9-phenanthrenyl, thiophenyl, and furanyl.

4. Photochromic lanthanide complex according to any one of claims 1 to 3, characterized in that the bidentate chelating agent is selected from the group consisting of 1,10-phenanthroline, 2,2'-bipyridine, 2 , 2'-biquinoline, 2- (2-pyridyl) benzimidazole, 2- (1H-imidazol-2-yl) pyridine and their derivatives.

5. Lanthanide photochromic complex according to any one of claims 1 to 4, characterized in that the photochromic ligand is a P-type photochromic entity or a T-type photochromic entity.

6. Lanthanide photochromic complex according to claim 5, characterized in that the photochromic entity is selected from the group consisting of: diarylethenes, in particular dithienylethenes, spiropyrans, spirooxazines, fulgides, fulgimides, stilbenes, azo aromatic compounds in particular azobenzenes, aminoazobenzenes, and pseudo-stilbenes, polycyclranic quinones, benzopyrans, naphthenes spirodihydroindolizines, permidinespirocyclohexadienones, viologens, triarylmethanes, and anils.

7. Lanthanide photochromic complex according to claim 1 selected from the 4ooo complex and the 5ooo complex having the following chemical formula:

.

8. A combination comprising a photochromic lanthanide complex according to any one of claims 1 to 7 and at least one other luminescent compound.

9. Combination according to claim 8, characterized in that the at least one other luminescent compound is at least one photochromic lanthanide complex according to any one of claims 1 to 7, the photochromic lanthanide complexes of chemical formula (I) which make up the combination being all different from each other.

10. Combination according to claim 8, characterized in that at least one other luminescent compound is selected from the group consisting of: luminescent organic compounds and luminescent inorganic compounds, and combinations thereof.

11. Use of a photochromic lanthanide complex according to any one of claims 1 to 7 or of a combination according to any one of claims 8 to 10, as a safety compound for a safety application, of. identification, authentication, and / or protection against imitation, forgery or counterfeiting of security paper, valuable document, commercial property or other object.

12. A security composition comprising a photochromic lanthanide complex according to any one of claims 1 to 7 or a combination according to any one of claims 8 to 10, and a vehicle.

13. Security composition according to claim 12, characterized in that the security composition is a security ink or a security coating composition.

14. Security composition according to claim 12, characterized in that the vehicle is a substrate and the substrate is selected from the group consisting of polymers, plastics, composite materials, metals, alloys, metallized materials, fibers. textiles, leather, cardboard and other packaging materials, glass, ceramics, wood, stone, paper and their combinations.

15. A security element comprising a photochromic lanthanide complex according to any one of claims 1 to 7 or a combination according to any one of claims 8 to 10 or a composition according to any one of claims 12 to 14.

16. Security element according to claim 15, characterized in that the security element is selected from the group consisting of: security threads, security films, security fibers, bands, bands or security tapes , Security Sheets, Security Papers, Security Plates, Security Patches, Security Labels, Security Transfers, Security Laminates and Security Inserts.

17. Kit comprising a combination according to any one of claims 8 to 10, characterized in that the combination is provided in a single container or aliquoted in several containers or characterized in that the components of the combination are provided in separate containers. .