Photophysical Properties of Thermally Activated Delayed Fluorescent Materials upon Distortion of Central Axis of Donor Moiety

Abstract

In this study, we showed the distortion of the central axis of the donor moiety can switch critically the rate of the reverse intersystem crossing (k(RISC)) process, which is the trigger point to modulate the lifetime of delayed fluorescence. To achieve k(RISC), what we desired (10(5) to 10(6) s(-1)) in a series of donor-acceptor-donor (D-A-D) type thermally activated delayed fluorescence (TADF) materials, the donor groups (phenoxazine and phenothiazine) were selectively introduced. Maintaining the near orthogonality between donor and acceptor (benzonitrile) moiety, the occurrence of the distortion of the central axis of the donor moiety could make the effect of locally excited triplet state ((LE)-L-3). In other words, the interaction between (LE)-L-3 and the charge transfer counterparts (i.e., (CT)-C-1 and (CT)-C-3) contributes an opposite propensity of k(RISC) for each of the target TADF materials when those are dissolved in solution and condensed in solid state. Herein, we have theoretically and experimentally shown the photophysical behavior of common D-A-D type TADF upon the different systems.