Regulating the photophysical properties of highly twisted TADF emitters by concurrent through-space/-bond charge transfer

Abstract

A series of thermally activated delayed fluorescence (TADF) emitters with folded-layout, namely SF12oTz, SF23oTz and SF34oTz are strategically designed and synthesized by substituting 4,6-diphenyl-1,3,5-triazine (Tz) with indoline-fused spirobifluorene donors at the ortho position. The substitution position of the spirobifluorene-based donors changes the steric hindrance and conjugation degree of molecules, which dominate the excited state levels (singlet and triplet) and intramolecular charge transfer (ICT) state properties, resulting in simultaneous through bond charge transfer (TBCT) and through space charge transfer (TSCT) transitions in one single molecule. Among them, SF23oTz and SF12oTz both show dual emissions with LE (local excited) and CT (charge transfer) features in different solvents, whereas SF34oTz possesses single emission from TSCT (96.8%) contributed S1. Furthermore, SF12oTz exhibits more TBCT (32.0%) contributed emission compared with SF23oTz (20.6%), which results in much higher ratio of delayed fluorescence (79.1% vs. 39.2%) because of the more effective channel of TBCT over TSCT to realize charge transfer from donors to acceptor. In devices, SF12oTz and SF23oTz with more contribution of TBCT achieve high EQEmax of 22.4% and 19.6% with reduced efficiency roll-off. The mechanism studies reveal that the control of TBCT and TSCT in these molecules play a significant role in regulating photophysical properties and improving the device performance of TADF-OLEDs.