Strategy for tuning the up-conversion intersystem crossing rates in a series of organic light-emitting diodes emitters relevant for thermally activated delayed fluorescence

Accurate prediction on the up-conversion intersystem crossing rate (k(UISC)) is a critical issue for the molecular design of an efficient thermally activated delayed fluorescence (TADF) emitter, and the k(UISC) rate is considered to be mainly determined by the spin-orbit coupling matrix element (SOCME) and the singlet-triplet energy difference (Delta E-ST). In the present contribution, we strategically designed a series of organic molecules, bearing an isoindoledione core as the electron acceptor (A) unit and dinitrocarbazolyl, carbazolyl, diphenylcarbazolyl, dicarbazolyl and tercarbazolyl groups as the electron donor (D) units, respectively. Their SOCME and Delta E-ST values between the S-1 and T-1 states were calculated by the DFT and TD-DFT methodes, and the k(UISC) rates were estimated by using the semidassical Marcus theory. The present studies indicate that as the n-conjugation in the D unit enhances, the Delta E-ST value gradually decreases, and the k(UISC) rate gradually increases. The molecule using tercarbazolyl as the D moiety is found to exhibit the largest k(UISC) in the present computations, as high as 122 x 10(6)s(-1), which is of the same order of magnitude as an experimentally observed highly-efficient TADF emitter using a 4-benzoylpyridine as the A unit and the same tercarbazolyl group as the D moiety. The present results sufficiently prove the necessity of introducing strong electron-rich substituent groups when designing highly efficient TADF emitters. (C) 2019 Elsevier B.V. All rights reserved.