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

Abstract

Accurate prediction on the up-conversion intersystem crossing rate (kUISC) is a critical issue for the molecular design of an efficient thermally activated delayed fluorescence (TADF) emitter, and the kUISC rate is considered to be mainly determined by the spin-orbit coupling matrix element (SOCME) and the singlet-triplet energy difference (∆EST). In the present contribution, we strategically designed a series of organic molecules, bearing an isoindole-dione 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 ∆EST values between the S1 and T1 states were calculated by the DFT and TD-DFT methodes, and the kUISC rates were estimated by using the semiclassical Marcus theory. The present studies indicate that as the π-conjugation in the D unit enhances, the ∆EST value gradually decreases, and the kUISC rate gradually increases. The molecule using tercarbazolyl as the D moiety is found to exhibit the largest kUISC in the present computations, as high as 1.22 × 106 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.