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

摘要

A group of D-pi-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 pi-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 pi-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 (eta(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 eta(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.