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Definitions

• the present invention is directed to the provision of Thermally Activated Delayed Fluorescence (TADF) compounds for use in OLEDs or other light emitting devices.
• TADF Thermally Activated Delayed Fluorescence
• OLEDs Organic Light Emitting Diodes
• OLEDs have come to the fore as the state-of-the-art technology for visual displays and lighting.
• OLEDs are desirable as they are light- weight, flexible, provide better contrast and possess large viewing angle.
• OLEDs are also more power efficient than traditional lighting sources and thus their wide adoption can alleviate significantly the strain on current energy demand because lighting alone constitutes about 20% of energy consumption worldwide.
• the "first generation" OLEDs were based on organic fluorescent emitters whose efficiency was intrinsically capped at 25% due to only being able to recruit singlet excitons.
• the "second generation” OLEDs employed organometallic phosphorescent emitters, which harvest both singlet and triplet excitons for emission due to the enhanced intersystem crossing (ISC) mediated by the large spin-orbit coupling of heavy metals such as iridium (III) and platinum(ll). Despite their highly desirable performance characteristics, the rarity of these metals, their high cost and their toxicity are important detracting features that inhibit large-scale, worldwide adoption of OLED technology.
• the “third generation” OLEDs were recently first reported by Adachi and co-workers.
• TADF thermally activated delayed fluorescence
• the principle of TADF relies on a small energy gap between the lowest singlet and triplet excited states ( ⁇ 3 ⁇ ). Under these conditions, the electrons in the triplet state can return to the singlet state by reverse intersystem crossing (RISC) using thermal energy, followed by radiative fluorescence (Reference 1 a).
• RISC reverse intersystem crossing
• the small AE S T IS realized by spatial separation between HOMO and LUMO to minimize the electronic repulsion between these orbitals.
• a large number of organic TADF emitters have been reported to date. They can make use of donor and acceptor moieties of various types within the molecule to achieve the desired small energy gap between the lowest singlet and triplet excited states ( ⁇ 3 ⁇ ). The majority of these molecules are based on a twisted intramolecular charge transfer (TICT) design in which the donor and acceptor moieties are designed to be nearly orthogonal to each other (References 1 a, 1 c and 3).
• OLEDs organic light emitting diodes
• LEECs light emitting electrochemical cells
• the present invention provides a compound according to formula I :
• each Het is an aromatic heterocyclic moiety and each D is a donor moiety.
• the compound according to formula I exhibits TADF behaviour.
• the aromatic heterocyclic groups Het act as acceptor groups and the donor moieties D act as donor groups.
• the spatial separation of Het and D by the ring II provides segregation of the HOMO (centred on donors D) and LUMO (centred on acceptors Het) that provides efficient TADF behaviour.
• the ring II may be a benzene ring and thus the compound of the invention may be according to formula III:
• R * may be , independently for each occurrence, selected from the group consisting of -H, alkyl, aryl or heteroaryl (for example substituted or unsubstituted C1 -C20 or even C1 -C10).
• aromatic heterocyclic moieties Het may be, independently for each occurrence
• -L represents the bonding position to ring II or III
• X represents O, S or NR 2 and groups R 2 are, independently for each occurrence, selected from the group consisting of -H, substituted or unsubstituted primary , secondary or tertiary alkyl, that may be cyclic and may be unsaturated (for example C1 -C10 or even C1 -C4); substituted or unsubstituted aryl or heteroaryl;
• groups R 1 are, independently for each occurrence, selected from the group consisting of -H, substituted or unsubstituted primary , secondary or tertiary alkyl, that may be cyclic and may be unsaturated (for example C1 -C10 or even C1 -C4); substituted or unsubstituted aryl or heteroaryl, -CF 3 , -OMe, -SF 5 , -N0 2 , halo (e.g.
• group R 1 is amino it may be -NH 2 , -NHR or -NR 2 , where the substituents R on the nitrogen may be alkyl, aryl or heteroaryl (for example substituted or unsubstituted C1 -C20 or even C1 -C10).
• groups R * , R 1 or R 2 are described as substituted they may be independently substituted for each occurrence. For example once, twice, or three times, e.g. once, i.e. formally replacing one or more hydrogen atoms with substituents such as halo (e.g.
• substituent is amino it may be NH 2 , NHR or NR 2 , where the substituents R on the nitrogen may be alkyl, aryl or heteroaryl (for example substituted or unsubstituted C1 -C20 or even C1 - C10).
• aryl is meant herein a radical formed formally by abstraction of a hydrogen atom from an aromatic compound.
• heteroaryl moieties are a subset of aryl moieties that comprise one or more heteroatoms, typically O, N or S, in place of one or more carbon atoms and any hydrogen atoms attached thereto.
• exemplary aryl substituents for example, include phenyl or naphthyl that may be substituted.
• Exemplary heteroaryl substituents for example, include pyridinyl, furanyl, pyrrolyl and pyrimidinyl.
• heteroaromatic rings include pyridazinyl (in which 2 nitrogen atoms are adjacent in an aromatic 6-membered ring); pyrazinyl (in which 2 nitrogens are 1 ,4-disposed in a 6-membered aromatic ring); pyrimidinyl (in which 2 nitrogen atoms are 1 ,3-disposed in a 6-membered aromatic ring); or 1 ,3,5-triazinyl (in which 3 nitrogen atoms are 1 ,3,5-disposed in a 6-membered aromatic ring).
• group R * , R 1 or R 2 includes one or more rings they may be cycloalkyi they may be for example cyclohexyl or cyclopentyl rings.
• the cyclohexyl or cyclopentyl groups if present may be saturated or unsaturated and may be substituted as described above.
• oieties Het include 1 ,3,4 oxadiazoles:
• -Alkyl represents a substituted or unsubstituted primary , secondary or tertiary alkyl, that may be cyclic and may be unsaturated (for example C1 -C10 or even C1 -C4); and wherein groups R 3 , R 4 and R 5 are, independently for each occurrence selected from the group consisting of :
• substituted or unsubstituted primary, secondary or tertiary alkyl that may be cyclic and may be unsaturated (for example C1 -C10 or even C1 -C4); substituted or unsubstituted aryl or heteroaryl, -CF 3 , -OMe, -SF 5 , -N0 2 , halo (e.g.
• D are donor moieties that may take several different forms as known in the field of TADF compounds. They may be the same or different for each occurrence
• donor moieties D may be:
• each group R 6 , R 7 , R 8 and R 9 is, independently for each occurrence, selected from the group consisting of -H, substituted or unsubstituted primary , secondary or tertiary alkyl, that may be cyclic and may be unsaturated (for example C1 -C10 or even C1 -C4); substituted or unsubstituted aryl or heteroaryl, -CF 3 , -OMe, -SF 5 , -N0 2 , halo (e.g.
• R 6 , R 7 , R 8 and R 9 is amino it may be -NH 2 , -NHR or - NR 2 , where the substituents R on the nitrogen may be substituted or unsubstituted alkyl, aryl or heteroaryl (for example substituted or unsubstituted C1 -C20 or even C1 - C10).
• R 6 , R 7 , R 8 and R 9 is phosphine oxide or phosphine sulphide it may be selected from the group consisting of: and where the substituents R on the phosphorus may be substituted or unsubstituted alkyl, aryl or heteroaryl (for example substituted or unsubstituted C1 -C20 or even C1 -C10).
• the phosphine oxide or phosphine sulphide substituent may be para to the nitrogen of the carbazole structure i.e. one or both of R 8 may be a phosphine oxide or phosphine sulphide substituent. Conveniently where both R 8 are a phosphine oxide or phosphine sulphide substituent they may be the same.
• the phosphine oxide or phosphine sulphide substituent may have phenyl or substituted phenyl groups R on the phosphorus.
• substituents are contemplated for donor moieties D.
• Phosphine oxide or phosphine sulphide may be used as acceptor moieties, or part of acceptor moieties (substituents on acceptor moieties) in the structure of a TADF molecule, such as the TADF compounds described herein.
• phosphine oxide or phosphine sulphide acts to moderate the character of the donor and can therefore alter the photo physical behaviour of a TADF compound, for example resulting in a change in colour and or intensity of emission.
• the use of phosphine oxide or phosphine sulphide as a substituent on a donor moiety D of a TADF compound, such as the compounds described herein constitutes another aspect of the invention.
• moieties D may also be selected from:
• -L represents the bonding position to ring II or I II , that is para to the nitrogen in structures C, D, G and H;
• X 1 is selected from the group consisting of O, S, NR, SiR 2 , PR and CR 2 , wherein each R is independently selected from the group consisting of -H, alkyl, aryl or heteroaryl (for example substituted or unsubstituted C1 -C20 or even C1 -C10);
• each Ar is independently for each occurrence selected from the group consisting of substituted or unsubstituted aryl or heteroaryl;
• n ( ) indicates the optional presence of saturated -CH 2 - groups in the rings annelated to the benzene ring, wherein n is independently for each occurrence, 0, 1 , or 2.
• Substituents on -Ar and , where present can include phosphine oxide or phosphine sulphide, as discussed above, to moderate the donor properties.
• moieties D may be selected from:
• n 1 :
• Exemplary compounds include structures IV, V and VI , below, where the moieties Het are 1 ,3,4 oxadiazoles and the donor moieties D are carbazole:
• the groups Het can be formed on an existing TADF structure by reaction at a nitrile.
• Other means of building or attaching groups Het to an aromatic (spacer) ring are well known.
• methods for attaching donor moieties D to an aromatic (spacer) ring are available to the skilled person. (For example in: Name reactions in heterocyclic chemistry 2005 - Jie jack Li, editor; Wiley; and Strategic Applications of Organic Named Reactions in Organic Synthesis 2005 - by Laslo Kurti and Barbara Czako; Academic Press. The content of these documents are incorporated by reference herein).
• the addition of donor moieties D may be by nucleophilic aromatic substitution reactions such as are employed to provide carbazole substituents on prior art TADF molecules like 2CzPN.
• the use of different donor moieties D on the compounds of the invention affords the opportunity to adjust the photo physical properties of TADF structures. More generally the donor moieties described herein can provide useful alternatives to the carbazole or modified carbazole moieties typically employed in some prior art TADF structures such as 2CzPN.
• the present invention provides a compound according to formula Ic:
• each Acc is an acceptor moiety with each acceptor moiety Acc bonded to adjacent carbon atoms
• each D is a donor moiety and each donor moiety D bonded to adjacent carbon atoms
• one of the donor moieties D is according to formula A and the other is according to formula A or according to formula B :
• X 1 is selected from the group consisting of O, S, NR, SiR 2 , PR and CR 2 , wherein each R is independently selected from the group consisting of -H, alkyl, aryl or heteroaryl (for example substituted or unsubstituted C1 -C20 or even C1 -C10); and f Ar
• ⁇ — represents, independently for each occurrence a substituted or unsubstituted aryl or heteroaryl ring fused to the central ring of structures A or B, for example a five or a six membered substituted or unsubstituted aryl or heteroaryl ring.
• Substituents on — ' can include phosphine oxide or phosphine sulphide, as discussed above, to moderate the donor properties.
• the ring I I may be a benzene ring and thus the compound of formula Ic may be according to formula Illc:
• R* may be , independently for each occurrence, selected from the group consisting of -H, alkyl, aryl or heteroaryl (for example substituted or unsubstituted C1 -C20 or even C1 -C10
• the compounds according to formula Ic or Illc exhibit TADF behaviour.
• the acceptor moieties Acc act as acceptor groups and the donor moieties D act as donor groups.
• the spatial separation of Acc and D by the ring II provides segregation of the HOMO (centred on donors D) and LUMO (centred on acceptors Acc) that provides efficient TADF behaviour.
• the donor moieties according to formula A may moderate the photo physical behaviour of the TADF structure and can provide improved robustness to the molecules.
• Prior art typical TADF structures make use of only carbazole derived donor moieties D when having a similar "cruciform" structure - two acceptor groups and two donor groups as depicted in formula lc.
• Moderation of the photo physical properties can include shifting of the emission spectra towards the red end of the spectrum. This effect can be related to the increased donor strength provided by donor groups D.
• this aspect of the invention allows the production of light emitting devices based on the general structure of formula lc with red shifted, including red, emission spectra.
• the acceptor moieties Acc may be selected from the group consisting of acceptor moieties Het as described herein, -CN and other acceptors such as sulfoxide, imine, amide, sulfone, acridine, acridinium, carboxylate ester, phosphine oxide, phosphine sulfide, ketone and aldehyde. Conveniently the acceptor moieties may both be the same. Alternatively they may be different.
• the moieties D of formula A in formula lc may be selected from:
• Examples of compounds according to formula lc include compounds of formulas VIII, IX and X:
• acceptor groups Acc are -CN and donor moieties D employed are phenoxazine (VIII), phenothiazine (IX) and both phenothiazine and carbazole (X).
• donor moieties D employed are phenoxazine (VIII), phenothiazine (IX) and both phenothiazine and carbazole (X).
• the nitrile acceptor moieties -CN may be modified as described herein and in accordance with the first aspect of the invention to be acceptor moieties -Het.
• the present invention provides a charged organic species according to formula la:
• each Het represents an aromatic spacer ring with each Het bonded to adjacent carbon atoms and each moiety D bonded to adjacent carbon atoms; wherein each Het is an aromatic heterocyclic moiety and each D is a donor moiety.
• the charged organic species according to formula la exhibits TADF behaviour.
• the aromatic heterocyclic groups Het act as acceptor groups and the donor moieties D act as donor groups.
• the spatial separation of Het and D by the ring II provides segregation of the HOMO (centred on donors D) and LUMO (centred on acceptors Het) that provides efficient TADF behaviour.
• C is a counter ion with opposite charge to that provided on the ring structure including the groups Het and D, and n is at least one
• TADF counter ion with opposite charge to that provided on the ring structure including the groups Het and D
• n is at least one
• the TADF species la can provide the benefits in terms of high efficiencies found with OLED devices but in use in a LEEC, by virtue of the charge and corresponding counter ions. (i.e. salts lb).
• the charged organic thermally activated delayed fluorescence (TADF) species la and its accompanying counter ions can present the advantage of good solubility allowing solution processing, for example inkjet type printing when fabricating display devices, especially when fabricating large displays.
• Other benefits of LEECs can be realised, such as the ability to fabricate the device in air and the ability to use air stable electrodes.
• the charged species of the invention may be according to formula Ilia: wherein D and Het have the same meaning as for formula la.
• R * may be, independently for each occurrence, selected from the group consisting of -H, alkyl, aryl or heteroaryl (for example substituted or unsubstituted C1 -C20 or even C1 - C10; the aromatic heterocyclic moieties Het may be, independently for each
• NR 2 and groups R 2 are, independently for each occurrence, selected from the group consisting of -H, substituted or unsubstituted primary , secondary or tertiary alkyl, that may be cyclic and may be unsaturated (for example C1 -C10 or even C1 -C4); substituted or unsubstituted aryl or heteroaryl;
• groups R 1 are, independently for each occurrence, selected from the group consisting of -H, substituted or unsubstituted primary , secondary or tertiary alkyl, that may be cyclic and may be unsaturated (for example C1 -C10 or even C1 -C4); substituted or unsubstituted aryl or heteroaryl, -CF 3 , -OMe, -SF 5 , -N0 2 , halo (e.g.
• the donor moieties D includes a charged group Z directly attached or attached via a linking group L.
• Moieties D and Het in the charged species may take the form described above with respect to the first aspect of the invention.
• the linking group L is optional for each occurrence of groups Z. Conveniently where linking groups L are employed, one is used for each group Z.
• the linking group L may, independently for each occurrence, comprise or consist of a hydrocarbylene chain, for example C1 to C30 or even C1 to C10, that may be substituted or unsubstituted, hydrocarbylene or unsaturated hydrocarbylene.
• the hydrocarbylene chain can include substituted or unsubstituted saturated, unsaturated or aromatic rings.
• the hydrocarbylene chain may include or consist of substituted or unsubstituted cyclopentane-1 ,3-diyl, cyclohexane-1 ,4-diyl, 1 ,4- phenylene or 4,4'-biphenylene moieties.
• Aromatic rings where present may be aryl or heteroaryl.
• linking group L may be independently substituted for each occurrence. For example once, twice, or three times, e.g. once, i.e. formally replacing one or more hydrogen atoms of the hydrocarbylene chain.
• substituents are halo (e.g. fluoro, chloro, bromo and iodo),-SF 5 , -CF 3 , -OMe,
• substituent is amino it may be NH 2 , NHR or NR 2 , where the substituents R on the nitrogen may be alkyl, aryl or heteroaryl (for example substituted or unsubstituted C1 - C20 or even C1 -C10).
• substitution options for linking groups L may be employed for other groups or substituents that may be substituted or unsubstituted as described herein.
• groups that may be substituted may be, for example, substituted once, twice, or three times, e.g. once, i.e. formally replacing one or more hydrogen atoms of the group.
• substituents are halo (e.g.
• substituent is amino it may be NH 2 , NHR or NR 2 , where the substituents R on the nitrogen may be alkyl, aryl or heteroaryl (for example substituted or unsubstituted C1 -C20 or even C1 - C10).
• linking group includes one or more rings they may be cycloalkyl they may be for example cyclohexyl or cyclopentyl rings.
• the cyclohexyl or cyclopentyl groups if present may be saturated or unsaturated and may be substituted as described above.
• a linking group L may also include heteroatoms in a hydrocarbylene chain, for example by substituting one or more carbon atoms in the chain e.g. one, two, or three carbon atoms with any one of O, N, or S for example.
• Examples of unsubstituted hydrocarbylene chains for group L include:
• n is from 0 to 10 or even 0 to 5 and optionally containing one or more unsaturations; cyclopentane-1 ,3-diyl; cyclohexane-1 ,4-diyl; 1 ,4-phenylene; 4,4'-biphenylene.
• Non-metal charged groups Z may be, independently for each occurrence, positively or negatively charged. Counter ions C will have the opposite charge. Where groups Z are positively charged they may be, independently for each occurrence, selected from the group consisting of quaternary nitrogen cations, and quaternary phosphorus cations. Conveniently all groups Z will be the same.
• groups Z are negatively charged they may be provided with anionic substituents such as carboxylate, sulfonate, sulfinate, phosphonate, cyanide and thiocyanate.
• quaternary nitrogen groups Z include:
• R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 are, independently for each occurrence, selected from the group consisting of -H, substituted or unsubstituted primary , secondary or tertiary alkyl, that may be cyclic and may be unsaturated (for example C1 -C10 or even C1 -C4); substituted or unsubstituted aryl or heteroaryl, -CF 3 , -OMe, -SF 5 , -N0 2 , halo (e.g.
• the quaternary nitrogen is pyridynyl as in structure 3
• the attachment to linking group L or directly to a TADF moiety may be to a carbon rather than to the nitrogen
• the quaternary nitrogen group Z may take the form of structure 5:
• R 8 , R 9 , R 10 and R 11 bonds to a linking group L or directly to a TADF moiety and the others of R 8 , R 9 , R 10 and R 11 , independently for each occurrence selected from the group consisting of -H, substituted or unsubstituted primary , secondary or tertiary alkyl, that may be cyclic and may be unsaturated (for example C1 -C10 or even C1 -C4); substituted or unsubstituted aryl or heteroaryl, -CF 3 , -OMe, - SF 5 , -N0 2 , halo (e.g.
• R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 are aryl, heteroaryl or cycloalkyl and are substituted, they may be substituted with substituted or unsubstituted primary , secondary or tertiary alkyl, that may be cyclic and may be unsaturated (for example C1 -C10 or even C1 -C4); substituted or unsubstituted aryl or heteroaryl, -CF 3 , -OMe, -SF 5 , -N0 2 , halo (e.g.
• quaternary nitrogen groups Z include:
• quaternary phosphorus groups Z include:
• R 1 , R 2 and R 3 have the same meaning as for the corresponding quaternary nitrogen group 1 discussed above and wherein -L indicates the position of bonding to a linking group L or directly to a TADF moiety.
• quaternary phosphorus groups Z include:
• the counter ion or counter ions C may be selected from the group consisting of: halide, PF 6 “ , BF 4 " , BR 4 " ; wherein R is an aryl group, for example phenyl; OTf “ , OTs “ , SbX 6 " wherein X is halide, NTf 2 " N0 3 " , C0 3 2” ; cations of first and second group elements in the periodic table and quaternary ammonium cations.
• Exemplary structures according to the second aspect of the invention may take the same form as those discussed above with respect to the first aspect of the invention, with the addition of at least one charged group Z, each optionally attached by a linking group L.
• the charged species may take the general form VII:
• Synthesis of the charged organic thermally activated delayed fluorescence (TADF) species and sufficient counter ions to balance the charge on the charged organic thermally activated delayed fluorescence (TADF) species can be carried out by a skilled person.
• salts may be made by modification of the TADF species to provide charged species and the desired counter ion or counter ions may be present in the synthetic route to the charged species or introduced by suitable ion exchange procedures.
• carbazole or similar donor moieties D of the TADF molecules may be modified before and/or after synthesis of the TADF molecule to provide a charged group or groups.
• This general approach can be used to provide other TADF species with charged groups, optionally connected by linking groups such as hydrocarbylene linkers.
• the other donor moieties D discussed herein may be modified by synthetic routes akin to that of scheme A to add charged groups, optionally connected by linking groups.
• Quaternisation at N or P provides a convenient route to charged (cationic) species.
• anionic charged species such as carboxylate, sulfonate, sulfinate, phosphonate, cyanide and thiocyanate can be readily done by the skilled person.
• Modification of the -CN groups to produce -Het groups can be carried out before or after addition of the donor groups.
• the present invention also provides an OLED or other light emitting device, such as a LEEC comprising a compound or charged organic species of the invention.
• the present invention provides a TADF compound wherein at least one donor moiety (D) is substituted by at least one substituent selected from the group consisting of phosphine oxide and phosphine sulphide.
• at least one donor moiety (D) is substituted by at least one substituent selected from the group consisting of phosphine oxide and phosphine sulphide.
• substituents selected from the group consisting of phosphine oxide and phosphine sulphide.
• more than one phosphine oxide or phosphine sulphide substituent may be the same. Alternatively they may be different.
• all the donor moieties D may be provided with one or more phosphine oxide or phosphine sulphide substituents.
• the phosphine oxide or phosphine sulphide substituent may be selected from the group consisting of: P P
• R R and R R where the substituents R on the phosphorus may be alkyl, aryl or heteroaryl (for example substituted or unsubstituted C1 -C20 or even C1 -C10).
• the phosphine oxide or phosphine sulphide substituent may have phenyl or substituted phenyl groups R on the phosphorus.
• TADF compound according to the fourth aspect may be according to formula Id:
• each Acc is an acceptor moiety with each acceptor moiety Acc bonded to adjacent carbon atoms
• each D is a donor moiety and each donor moiety D bonded to adjacent carbon atoms
• the donor moieties D are independently selected from the group consisting of formula A and formula B :
• X 1 is selected from the group consisting of O, S, NR, SiR 2 , PR and CR 2 , wherein each R is independently selected from the group consisting of -H, alkyl, aryl or ryl (for example substituted or unsubstituted C1 -C20 or even C1 -C10); and represents, independently for each occurrence a substituted or unsubstituted aryl or heteroaryl ring fused to the central ring of structures A or B, for example a five or a six membered substituted or unsubstit aryl or heteroaryl ring; and wherein at least one substituent on at least one present in the compound is phosphine oxide or phosphine sulphide.
• Figure 1 shows cyclic voltammetry studies of compounds of the invention.
• Figure 2 shows the structure of OLEDs in schematic cross section.
• Optically dilute solutions of concentrations on the order of 10 "5 or 10 "6 M were prepared in HPLC grade solvent for absorption and emission analysis. Absorption spectra were recorded at room temperature on a Shimadzu UV-1800 double beam spectrophotometer with a 1cm quartz cuvette. Molar absorptivity values were determined from at least four solutions followed by linear regression analysis.
• Cyclic Voltammetry (CV) analysis was performed on an Electrochemical Analyzer potentiostat model 600D from CH Instruments. Samples were prepared as MeCN solutions, which were degassed by sparging with MeCN-saturated argon gas for 15 minutes prior to measurements. All measurements were performed using 0.1 M MeCN solution of tetra-n-butylammonium hexafluorophosphate. An Ag/Ag + electrode was used as the reference electrode while a platinum electrode and a platinum wire were used as the working electrode and counter electrode, respectively. The redox potentials are reported relative to a saturated calomel electrode (SCE) with a ferrocenium/ferrocene (Fc7Fc) redox couple as the internal standard (0.38 V vs SCE).
• SCE saturated calomel electrode
• Fc7Fc ferrocenium/ferrocene
• a pre-patterned ITO glass substrate ( Figure 2) was treated by ultrasonic cleaning in acetone and propan-2-ol consecutively and then being treated by oxygen plasma.
• a PEDOT:PSS layer was spin-coated on the ITO substrate and subsequently baked at 120 °C for 10 minutes inside the glove-box to remove the residual moisture.
• a layer of electron-transporting material, B3PYMPM was thermally deposited.
• Ca/AI as the composite cathode was thermally deposited through a shadow mask in the vacuum chamber under a pressure of -2.0 ⁇ 10 ⁇ 6 mbar. All the devices were encapsulated with UV epoxy resin inside the glove-box. The luminance- current-voltage characteristics were measured in ambient environment using Keithley 2400 source meter and a 2000 multi-meter connected to a calibrated Si photodiode. The external quantum efficiency was calculated with the assumption of a Lambertian distribution. The electroluminence spectrum was captured by an Andor DV420-BV CCD spectrometer.
• V LUMO -2.80
• the electrochemistry of IV, V and VI was studied by CV in degassed MeCN solutions under argon, Figure 1.
• the HOMO levels of the dyes are practically identical (-5.83 to - 5.84 eV) because they have the same carbazole donor. Yet, their LUMO levels are strongly influenced by the acceptor strength induced by the groups attached to the oxadiazole.
• the LUMO level of V is lower by 0.1 eV compared to IV as a result of increased conjugation length afforded by the phenyl group.
• Emitter VI has the most stabilized LUMO (-2.86 eV) due to the strong electron-withdrawing effect of the trifluoromethyl group.
• Table 2 shows a summary of photo physics measurements.
• the photoluminescence quantum yields ( ⁇ range from 12-39% and do not significantly change with solvent choice.
• the quantum yields in degassed solutions are always higher than those in aerated solutions, which suggests that there is a contribution from the triplet excited state towards emission, a typical observation of TADF materials.
• the emission lifetimes (T e ) of IV, V, VI consist of both prompt (13.2- 25.0 ns) and delayed (556-1310 ns) components, which are characteristic of TADF emission .
• the weaker acceptor strength of oxadiazole compared to the cyano group causes a desired blue-shifted emission in emitters IV, V, VI compared with 2CzPN.
• the LUMO levels of IV, V, VI range from -2.70 eV to -2.86 eV while the LUMO of 2CzPN was found to be -2.99 eV, and the emission is blue-shifted as a consequence.
• IV has an emission maximum at 448 nm in toluene, blue-shifted by 30 nm compared with 2CzPN (478 nm) in the same solvent.
• the emission lifetimes of delayed components for 1-3 range from 0.5 to 1 .3 ⁇ , which are significantly shorter than that observed for 2CzPN (14.9 ⁇ ).
• the short delayed component of the emission lifetimes reveal a smaller singlet-triplet energy gap, which is a consequence of greater electronic separation between donor (HOMO) and acceptor (LUMO) units in IV, V, and VI compared with 2CzPN.
• the delayed component emission lifetimes of IV, V and VI are among the shortest of the organic TADF materials known.
• high quality thin films were prepared by spin-coating DCM solution of 10 wt% dye in PMMA. All the emission maxima are blue-shifted by -20 nm and the profiles slightly sharper compared with those measured in toluene solution.
• the thin film quantum yields (40-75%) are significantly higher than in solution as a result of the more rigid environment.
• the quantum yields of the thin films are higher under a nitrogen atmosphere than when exposed to air, suggesting the presence of TADF in solid state.
• the emission lifetimes (T e ) of IV, V, VI in thin film consist of both prompt (1 1 -12 ns) and delayed (1582-1989 ns) components, which are characteristic of TADF emission.
• a series of OLEDs based on the three emitters were fabricated with the multi-layer architecture shown in Figure 2, where PVK, mCP and OXD-7 were used as the hole transporting layer (HTL), electron blocking layer and the exciton blocking layer (EBL), respectively while B3PYMPM was used as the electron transporting layer (ETL) layer.
• Ca was used to reduce the injection barrier.
• This multi-layer structure provides a compact emissive layer (EML) where the excitons were finely confined to enhance radiative recombination rate.
• A1 mCP:OXD-7:IV (70:20:10), A2: mCP: IV (90:10), A3: IV(without additives); B1 : mCP:OXD-7:V (70:20:10), B2: mCP: V (90:10), B3: V;
• PEDOT:PSS poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)
• PVK poly(A/-vinylcarbazole)
• mCP 3,5'-A/,W-dicarbazole-benzene
• OXD-7 1 ,3-bis[(4-tert- butylphenyl)-1 ,3,4-oxadiazolyl] phenylene
• B3PYMPM bis-4,6-(3,5-di-3- pyridylphenyl)-2-methylpyrimidine
• Scheme 2 shows the synthetic route to TADF structures employing different nitrogen containing donor moieties D.
• the route is similar to that of Scheme 1 for compounds 2CzPN, VIII and IX, with minor differences.
• Table 4 lists in Summary the absorption and electrochemistry data for compounds VIII, IX, X, XI, XII and 2CzPN.
• Table 5 shows a summary of photo physics measurements.
• phenothiazine was found to act as the strongest donor with an emission maximum in IX of 674 nm and phenoxazine promoted a smaller red-shift with an emission maximum found at 650 nm in VIII.
• the emission profile for X is blue-shifted by 18 nm compared to VIII, pointing to a HOMO in X localized on the PTZ ring but whose energy is modulated by the carbazole, most likely via ⁇ -stacking interactions.
• the decay profiles of all the compounds show prompt and delayed components in degassed toluene and doped thin films, indicating TADF materials.
• the lifetimes of 2CzPN and XI are similar because of their similar structures.
• the delayed components are much shorter (279-1000 ns) compared with 2CzPN.
• OLED devices for compounds VIII, IX, X, XI, XII and 2CzPN were fabricated with the same device architecture as shown in figure 2 with the emissive layer comprising mCP:OXD-7:Emitter material (70:20:10, 20 nm thickness).
• the emission color significantly shifts from bluish-green to deep-red, e.g., peak wavelength of 629 nm and CIE coordinates of (0.60, 0.39) for the emitter VIII.
• Tests at different current densities show that the compounds with phenoxazine and phenothiazine donors, as well as brominated carbazole donors, show extremely stable EL spectra, which are independent of the current density applied. This is in contrast with carbazole containing compounds where peak wavelength was found to shift depending on current density applied.
• n-tetrabutylammonium fluoride (1.34 g, 5.1 mmol, 3.0 equiv.) was added and the mixture was allowed to stir for further 30 min.
• the mixture was added to water (10 mL) and extracted by DCM (3x 10 mL).
• the combined organic layer was dried with anhydrous sodium sulphate.
• the concentrated organic layer was purified by column chromatography using EA/Hexanes (v/v 2:1) as the eluent. White solid. Yield: 71%.
• Compound 8 was prepared from 7 in the same way as 4 and 5 were prepared from 2 and 3.

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