Modeling MR-TADF Emitters: Excited-State Decay Rate Constants and Wave Function Descriptors

Abstract

Multiresonance thermally activated delayed fluorescence (MR-TADF) emitters have gained popularity given their potential of attaining negligible singlet−triplet energy gaps, i.e., ΔE ST , without hindering emission, thus increasing the reverse and direct intersystem crossing rates without affecting fluorescence. This is achieved due to the singlet and triplet states' short-range charge transfer character (SRCT). Thus, obtaining quantitative information about SRCT would help develop new MR-TADF emitters. This work studies three different families of MR-TADF emitters: DOBOA, DiKTa, and OQAO. First, we compute their adiabatic ΔE ST with four different methods (TDA-CAM-B3LYP, STEOM-DLPNO-CCSD, ADC(2), and SCS-CC2). Then, we compute fluorescence (k r ), direct (k ISC ), and reverse intersystem crossing rate constants. For k r , we assessed the effect of different levels of approximations on the rate calculations. We show that k r does not depend significantly on the different harmonic models (adiabatic Hessian or vertical Hessian), coordinate systems, and broadening widths. Moreover, Herzberg−Teller effects are negligible for k r but are the main contribution for k ISC and k RISC . The computed rate constants agree well with the experimental results. Moreover, we propose the use of two wave function descriptors, Q a t and LOC a , based on the 1-particle transition density matrix, which assigns the amount of charge centered on the atoms. We compute these descriptors for three transitions: S