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 (∆E_ST). 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 ∆E_ST values between the S₁ and T₁ states were calculated by the DFT and TD-DFT methodes, and the k_UISC rates were estimated by using the semiclassical Marcus theory. The present studies indicate that as the π-conjugation in the D unit enhances, the ∆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 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.