Investigation on the effect of connected bridge on thermally activated delayed fluorescence property for DCBPy emitter

One thermally activated delayed fluorescence (TADF) molecule DCBPy (compound 1) has been reported. Based on it, four compounds 2-5 have been designed by modifying the connected bridge between electron-donor (D) and electron-acceptor (A) units. The calculated normal mode reorganization energy (A) for the non-radiative decay process shows that the A of compounds 2-5 in the high-frequency region is noticeably reduced compared with compound 1, suggesting that the high-frequency C=0 stretching vibration is hindered through modifying the connected bridge between D and A units. Moreover, the radiative decay rate constant (k(r)) values of compound 2-5 are nearly one order of magnitude higher than that of pristine compound 1. Besides, for TADF material, the reverse intersystem crossing (RISC) is dependent on a small singlet-triplet energy gap (Delta E-ST) and large spin-orbital coupling matrix elements <S-n|H-SOC|T-n>. For our designed compounds, modifying the connected bridges noticeably increase their <S-n|H-SOC|T-n> values, although the Delta E-ST values of compound 2-5 are greater than that of compound 1. Especially, compound 2 has a comparable Delta E-ST with compound 1 and larger k(r) and <S-n|H-SOC|T-n> than compound 1, which exhibits the best TADF efficiency among these compounds. As a consequence, for this kind of DCBPy emitter, modifying the connected bridge between D and A units is a valid approach to improve their TADF performances. (C) 2017 Published by Elsevier Ltd.