Can attachment of tert-butyl substituents to methoxycarbazole moiety induce efficient TADF in diphenylsulfone-based blue OLED emitters?

A new series of TADF emitters with donor-acceptor or donor-acceptor-donor structure containing 2,7-dimethoxycarbazole donor moiety and diphenylsulfone acceptor unit were synthesized and employed in non-doped sky-blue organic light emitting diodes with the external quantum efficiency reaching 9.0%. Thermal, optical and photophysical properties of for D-A and D-A-D compounds containing one or two tert-butyl groups attached to C-3 and C-6 positions of 2,7-dimethoxycarbazole moiety were studied and compared with those of the corresponding compounds containing no tert-butyl groups. Studies of photophysical properties of the compounds confirmed TADF nature of their emission. Computational studies revealed that small modification of the donor moiety could have a significant impact on the TADF mechanism turning it from vibronically induced TADF mediated by the T₂ state to the common TADF which occurs through the direct reverse intersystem crossing between the S₁ and T₁ states. New principle of purposeful of molecular design of TADF emitters with controlled efficiency of reverse intersystem crossing by only small functionalization of donor fragments is proposed.