During the last few years, the phenomenon of thermally activated delayed fluorescence (TADF) finds more and more applications in numerous fields of technology, especially in organic light emitting diodes (OLED) giving rise to variously shaped and flexible optoelectronic devices and displays. Further development of products using OLED technology requires emitters exhibiting deep-blue TADF with relatively short emission lifetimes and high chemical stability. The state-of-the-art pure organic TADF emitters comprise donor and acceptor fragments separated spatially. This enables very small gap between the lowest singlet and triplet excited states of charge-transfer (CT) character. However, as these states differ only by multiplicity, the transition between them is forbidden by the selection rules. According to the current understanding of TADF mechanism, the efficient conversion of nonradiative triplet excitons to the emissive singlet ones occurs when a locally excited triplet state (3LE) is energetically close to the 1CT and 3CT states. The photophysical principles described above work very well for design of efficient yellow, green, and sky-blue TADF materials. To achieve deep-blue TADF, the emitter should comprise highly energetic CT and 3LE states, which is a challenging requirement. To face this problem we investigated two approaches of increasing excited states’ energies: (i) reduction of conjugation of the acceptor fragment by its deplanarization and (ii) introduction of electron-releasing substituents in the acceptor fragment. 2-Phenyl-s-triazine and 9,10- dihydro-9,9-dimethylacridine were selected as acceptor and donors fragments, respectively. Our results show that both approaches efficiently increase energies of the CT and acceptor 3LE states, maintaining the rate constant of reverse intersystem crossing in the 105–106 s–1 range. This presentation will focus on details of the above mentioned investigations including characteristics of the OLED devices.
Synthesis and properties of novel excited-state intramolecular proton transfer (ESIPT) materials recently developed in our group are described. Highly efficient ESIPT in polymeric system has been investigated theoretically and experimentally with a semi-rigid polyquinoline possessing an intramolecular tautomerizable hydrogen bond. Poly(aryl ether) dendrimers of three different generations that are cored with photo-tautomerizable quinoline (QGn, n=1,2,3) were also synthesized and characterized to investigate the effect of dendritic architecture on the ESIPT activity. Stimulated emission and amplified spontaneous emission in these organic materials system are discussed in terms of ESIPT activity.
Strongly fluorescent organic nonlinear optical materials containing triphenyl amine as a core and 1-(2-ethyl-hexane-1-sulfonyl)-4-vinylbenzene or 4-[4-(2-ethyl-hexane-1-sulfonyl)-1-ethyl] styrene group as the arms which were mono-,di-, and tri-substituted at the para position of nitrogen were synthesized. Photorefractivity of these molecular glasses were measured and correlated with their structure and symmetry. Large two-photon absorption cross-section and strong up-conversion fluorescence emission were also observed in these materials.