Diazine-based thermally activated delayed fluorescence chromophores

International audienceDiazines are electron deficient six-membered aromatic rings heterocycles which have successfully been employed in the design of a wide range of push-pull chromophores for various applications in the field of organic electronics and optoelectronics. For instance, some have been specifically designed to be inserted in the electroluminescent emission layer of organic light emitting diodes (OLED). OLED technology has continuously evolved and is currently used in industry for display and lighting purposes, offering many advantages including low turn-on voltage and power consumption. In the prospect to further reduce energy waste, a third generation of OLED based on thermally activated delayed fluorescence (TADF), which allows harvesting both triplet and singlet excitons, attracts increasing attention from the scientific community. In this review, we provide a comprehensive overview of the present status of diazine-based dyes developed for TADF. The pyrimidine core is by far the most used and has demonstrated deep blue, blue or sky-blue and green emission when combined with carbazole, acridan and phenoxazine fragments, respectively. Further design of TADF emitters is desirable to afford red OLED and both the pyrazine and pyridazine cores have been scarcely explored

Are Styrylpyrimidines dyes promising candidates for TADF ?

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Controlling Cation-Cation Interactions in Uranyl Coordination Dimers by Varying the Length of the Dicarboxylate Linker

International audienceThe chemistry of linear uranyl(V/VI) dioxo cations, [Oyl-U-Oyl] +/2+ , is dominated by coordination of uranium in the equatorial plane. Effects of this constraint were evaluated by experiment and theory for gas-phase mixed-valence U V/VI coordination dimers in which uranyl moieties are linked by alkyl dicarboxylates, [(UO2 +)(UO2 2+)(OOC-(CH2)n-2-COO 2-)2]-(n = 3-12). Faster O2-addition to dimers with short linkers n = 3 and 4, versus n ≥ 5, suggests a structural difference. Computed structures with the shortest linkers have bridging dicarboxylates and nearly parallel, noninteracting uranyls. Longer linkers, n = 5-7, accommodate uranyl orientations with distinct U V-U VI end-on cation-cation interactions (CCIs), whereby Lewis base Oyl from U V coordinates to the acid U VI , denoted as U V Oyl … U VI. The dimer structure for n = 8 has a U V-U VI side-on diamondshape CCI, with U V Oyl … U VI and U VI Oyl … U V interactions. Addition of O2 to the n = 4 and 5 dimers yields [(UO2 2+)2(OOC-(CH2)n-2-COO 2-)2(O2-)]-, with U V oxidized to U VI and O2 reduced to O2-. Whereas O2 can associate to and oxidize the exposed U V center for dimers with n = 3 and 4, the more crowded U V site in the CCI structures inhibits O2 addition. The results demonstrate rational structural control of uranyl-uranyl bonding and reactivity in small coordination complexes

Theoretical and experimental study of the influence of (de)protonation of phenol-substituted diazine for White OLEDs

International audienceIn recent years, lighting has become more and more energy consuming, now reaching 20% of the global electricity consumption. Thus, there is need to properly design new environment friendly lighting devices and organic light emitting devices (OLEDs), in particular White OLEDs (WOLEDs), are more relevant than ever.[1] The major issue is that obtaining white light is not an easy task, and is in most cases achieved by using multilayer devices or a combination of different emissive molecules.[1,2] In this contribution, we present an innovative way of obtaining white light emission by modulating the protonation[3,4] and deprotonation of organic fluorophores. To this end, a series of 8 push-pull phenol substituted hydroxystyrildiazines has been designed by varying the nature of the acceptor diazinic heterocycle (pyrimidine/pyrazine) to be protonatedand of the linker moieties (thiophene, vinylene, thienylenevinylene, vinylphenylene). The photo-physical investigation of this series of compounds in solution showed a systematic bathochromic shift of the emission compared to the neutral form, not only upon the protonation, but also after deprotonation. Extensive Density Functional Theory (DFT) and TD-DFT calculations were performed to rationalize this behavior and understand the impact of (de)protonation on the different optical transitions characteristic of this family of fluorophores. Such computations showed that the (de)protonation impacts not only the energies of the vertical transitions, but also the nature of these transitions, with a significant increase in the Intramolecular Charge Transfer (ICT) character of the (de)excitations. Furthermore, playing with the nature of the diazinic moiety and the linker in the push-pull skeleton allows to fine tune the emission wavelength of the two forms and reach a complementary emission in solution.To the best of our knowledge, modulation of the emission by deprotonation has not yet been reported as an alternative strategy for obtaining WOLEDs. Our experimental and theoretical results on the phenol-substituted diazinic chromophores demonstrate that this approach proves successful, at least in solution. Thus, it opens a new route for the design of efficient WOLEDs and shall prompt further investigations up to device fabrication

Influence on photophysical properties of (de)protonation of phenol-substituted diazine chromophores: experimental and theoretical studies

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Styrylpyrimidine chromophores with bulky electron-donating substituents: experimental and theoretical investigation

International audienceStyrylpyrimidine with bulky 9,9-dimethylacridan, phenoxazine and phenothiazine electron-donating fragment were designed. Thermally activated delayed fluorescence (TADF) properties were expected for these structures. These chromophores exhibit peculiar emission properties. For 9,9-dimethylacridan and phenoxazine derivatives, a single emission highly sensitive to the polarity is observed in solution whereas for phenothiazine derivative a dual emission is observed in solution and is attributed to the coexistence of quasi-axial (Qax) and quasi-equatorial (Qeq) conformers. This study intends to understand with theoretical and experimental works, why the studied chromophores do not exhibit TADF properties, contrary to what was expected. The absence of phosphorescence both at room temperature and 77K tends to indicate the impossibility to harvest triplet states in these systems. Wave-function based calculations show that for both conformers of the three chromophores the S1-T1 splitting is significantly larger than 0.2 eV. The second triplet state T2 of Qeq conformers is found very close in energy to the singlet S1 state, but S1 and T2 states possess similar charge transfer characters. This prevents efficient spin-orbit coupling between the states, which is consistent with the absence of TADF

Influence of (de)protonation on the photophysical properties of phenol-substituted diazine chromophores: experimental and theoretical studies

International audienceIn this contribution, a series of seven new push–pull systems has been designed by combining a protonable diazine heterocycle (pyrimidine/pyrazine) with a deprotonable phenol unit through various π-conjugated linkers (phenylene, thienylene, thienylenevinylene, and phenylenevinylene). The (de)protonation in solution resulted in a systematic bathochromic shift both in the absorption and emission maxima compared to the neutral forms. Extensive Density Functional Theory (DFT) and its Time Dependent counterpart (TD-DFT) calculations were performed to rationalize this behavior and understand the impact of (de)protonation on the different optical transitions. These computations showed that (de)protonation affects both the energy and the nature of the vertical transitions, with a significant increase in the Intramolecular Charge Transfer (ICT) character of the (de)excitations. Some of the compounds remained moderately luminescent after (de)protonation, giving a mixture of complementary emitting species that were used to obtain white light emission

A molecular loaded dice: When the π conjugation breaks the statistical addressability of an octastate multimodal molecular switch

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