Can TDDFT render the electronic excited states ordering of Azine derivative? A closer investigation with DLPNO-STEOM-CCSD

Abstract TADF materials depend on the small energy gap between the lowest excited singlet and triplet state for an efficient reverse intersystem crossing process. Herein, the first excited singlet (S1) and triplet (T1) state and their gap (ΔEST) are computed based on TDDFT and recently developed coupled-cluster DLPNO-STEOM-CCSD method for Azine derivative. Considerations of higher-order excitations in DLPNO-STEOM-CCSD assign the correct excited state energy level ordering of Azine derivative while DFT functionals fall short in this context.