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

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 pi-pi 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%.