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

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 (105 to 106 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 (3LE). In other words, the interaction between 3LE and the charge transfer counterparts (i.e., 1CT and 3CT) 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.