Mechanism evolution from normal fluorescence to thermally activated delayed fluorescence and color tuning over visible light range: Effect of intramolecular charge transfer strength.

A group of D-π-A type emitters were designed and synthesized by using triazine (Trz) as electron acceptor (A), carbazole (Cz), 10-dimethylacridine (DMAC) or 10H-phenoxazine (PXZ) as electron donors (D), and phenylene as π-bridge between D and A. The electron withdrawing ability of A unit and thus the intramolecular charge transfer (ICT) strength play the most essential role to determine the luminescence mechanism and the emission color. By gradually enhancing the ICT extent through selecting strong electron donating D and/or replacing methyl with electron withdrawing cyano group on the π-bridge, the luminescence is tuned from normal fluorescence to thermally activated delayed fluorescence (TADF) and the emission color is successfully tuned from pure-blue (434 nm) to orange-red (616 nm). Organic light-emitting diodes (OLEDs) based on these emitters exhibit bright electroluminescence covering the major visible-light range. An external quantum efficiency (η ext) of 4.8% with excellent color coordinate of (0.16, 0.08) is obtained for pure blue fluorescent OLED of TrzCz-Me. TrzCz-CN, TrzDMAC-Me, and TrzPXZ-Me exhibited higher η ext values of 14.40% (19.73 cd A−1), 19.01% (41.12 cd A−1), 16.50% (51.17 cd A−1) in their blue, sky-blue, and green TADF OLEDs. This study provides a practical strategy related to ICT strength to facilitate generation of TADF emitters and tune the emission colors over the visible light range. A group of D-π-A emitters were developed by combining triazine acceptor with different donors and incorporating either cyano or methyl on phenylene bridge. The triazine/carbaozle based blue emitter was tuned from normal fluorescence to thermally activated delayed fluorescence (TADF) by incorporating cyano group, and the emission color was tuned from deep blue to orange-red. It was proved the intramolecular charge-transfer strength played the most important role to facilitate both the emission mechanism evolution and color tuning. The TADF emitters exhibited high quantum efficiencies of 14–19% in organic light-emitting diodes. Image 1 • A series of D-p-A type emitters were developed with various donors. • Cyano or methyl was grafted on phenylene bridge to tune the acceptor strength and molecular steric conformation. • The triazine/carbazole compound was tuned from normal fluorescence to TADF by incorporating cyano group. • The emission color was continuously tuned from deep blue to orange-red by tuning the intramolecular charge-transfer strength. [ABSTRACT FROM AUTHOR]