Insight into through-space conjugation in rotation-restricted thermally activated delayed fluorescence compounds

Abstract

Two pairs of compounds, BPAPTC/mBPAPTC and BPAMTC/mBPAMTC, are designed and synthesized in order to deeply understand thermally activated delayed fluorescence (TADF) with through-space charge transfer. BPAPTC and BPAMTC contain a sandwich structural unit composed of two di(4-tert-butylphenyl)amine donors and one inserted 1,3,5-triazine acceptor hitched at the 1,8,9-positions of a single carbazole, whereas mBPAPTC and mBPAMTC involve an open sandwich structural unit consisting of one donor and one acceptor fixed at the 1,9-positions of the carbazole. The strong π–π conjugations between the donor and acceptor groups bring about efficient TADF in the compounds. Each pair of compounds show almost identical photophysical properties and electrochemical behaviors, but a clear contrast in the electroluminescent (EL) performance. BPAPTC and BPAMTC achieve the maximum external quantum efficiencies (EQEs) of 23.3% and 14.7% with the low efficiency roll-off rates, while the maximum EQEs of mBPAPTC and mBPAMTC just reach 17.8% and 9.5%, respectively, in their solution-processed devices. In the meantime, the effect of a substituent group on EL behaviour was obviously observed. The devices of the emitters with the bulky acceptor attain a higher luminance, while the compound MPAPTC with methoxyl replacing tert-butyl on the donors of BPAPTC produces a yellow EL emission with a red-shifted wavelength over 40 nm relative to BPAPTC and BPAMTC, and meanwhile achieves a maximum EQE of 9.1%.