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 skyblue 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 C3 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.