Distinguishing the Quantum Yield and Lifetime of Carbazole‐Based Room‐Temperature Phosphorescence Materials: QM/MM Study.

Commercial carbazole derivatives are widely used as typical electron donors in room‐temperature phosphorescence materials. The high phosphorescence quantum yield (Φp) and short lifetime (τp) of the carbazole‐based (9H‐carbazol‐9‐yl)isonicotinonitrile (P‐34N) crystal are distinguished by quantum mechanics and molecular mechanics method. Two inducements are accounted for the high Φp of P‐34N: i) large spin–orbit coupling (0.63 cm−1) induces fast intersystem crossing (ISC) of the lowest singlet excited state (S1) to the lowest triplet excited state (T1), ii) the intermolecular π–π interactions stabilize triplet excitons. Meanwhile, the 3(n, π*) →1(π, π*) of P‐34N favors the nonradiative decay of T1to the ground state (S0), and thus, causes short τp. In contrast to P‐34N, the low Φp of 6‐(9H‐carbazol‐9‐yl)nicotinonitrile (P‐24N) crystal results from the effective reverse ISC process induced by small ΔEST (0.3 eV) between S1 state and the higher triplet state (T2), and the long τp is due to the unfavorable orbital reversals of T1 (π, π*)→S0 (π, π*) transition and the internal conversion process of T2 → T1. This work presents a reasonable interpretation of the Φp and τp of two carbazole‐based crystals. [ABSTRACT FROM AUTHOR]