Taking into account third-generation (TADF) OLED emitters, various structures possessing electron-donating and electron-accepting fragments are published. By forming new emitters or hosts such donors as carbazole, phenoxazine, phenothiazine and acridine are widely used. In literature, those fragments, except carbazole, rarely possess any type of additional groups, which, for example, could increase (in case of methoxy) or decrease (in case of halogens) electron density in aromatic system. Methoxycarbazole-based derivatives are most widely investigated and employed in OLED structure. It is proven that by attaching methoxy groups to emitter structure the density of hydrogen bonding in the layer is increasing. This type of weak bonding is resulting higher morphological stability of amorphous layer and positively affecting charge transportation between molecules. Therefore, considering published data, it would be scientifically interesting to investigate the impact of new type electron-donating moieties, such as methoxyacridine, methoxyphenoxazine and methoxyphenotiazine, on emitters thermal, photophysical, photoelectrical, charge transporting properties.
Project aim is to modify acridine, phenoxazine or phenothiazine moieties by substituting aromatic hydrogens with one or two methoxy groups following the formation of new emitters with benzonitrile or diphenylsulphone electron accepting fragments. In addition, thermal and photophysical properties will be compared.
Project funding:
Project is funded by EU Structural Funds according to the 2014–2020 Operational Programme for the European Union Funds’ Investments priority “Development of scientific competence of researchers, other researchers, students through practical scientific activities” under Measure No. 09.3.3-LMT-K-712.
Project results:
To investigate the effect of methoxy groups onto the morphological and photophysical properties of organic emitters, four new compounds using electron-accepting benzonitrile fragment and electron-donating moieties, such as phenothiazine, acridine, 2-methoxyphenothiazine and 2,7-dimethoxyacridine were synthesized. The potential of the target compounds was evaluated using quantum chemistry methods. All derivatives possess modest overlap between HOMO/LUMO orbitals and small singlet-triplet splitting energy (0,1-0,2 eV) which is beneficial for the T1-to-S1 transitions in TADF molecules. Compounds were obtained by a nucleophilic substitution reaction using 4-fluoro-3- (trifluoromethyl)benzinotrile and corresponding phenothiazine or acridine chromophores. The structures of the final compounds were identified by mass spectrometry, X-ray analysis (XRD) and NMR technique. The XRD data showed higher density of intermolecular (hydrogen) bonding between methoxy-substituted compounds in the crystal lattices. The study of thermal properties proved that only methoxy-substituted compounds (57 °C – 58 °C) are able to form stable molecular glasses. The photophysical investigation revealed, that the edge of the absorption spectra of the toluene solutions of the final compounds were found in the range from 440 nm to 470 nm. The peak values of the fluorescence curves recorded from the layers, toluene and THF solutions fall in the following range: 481 – 572 nm, 511 – 628 nm and 551 – 649 nm, respectively. In addition, the phenomenon of solvatochromism was detected, i.e. the fluorescence peaks of the compounds in polar solvent (THF) were red-shifted by 40-60 nm compared with spectra recorded from toluene solutions of the corresponding derivatives. Photoluminescence (FL) quantum yield values recorded from amorphous layers of target compounds are reaching 20%. The FL lifetime values determined from FL decay curves recorded from toluene solutions of materials were found in nanosecond range. Additional photophysical studies will be performed to investigate the phenomenon of thermally activated delayed fluorescence phenomenon.
Period of project implementation: 2020-11-03 - 2021-04-30
Project coordinator: Kaunas University of Technology
