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

Multiresonance thermally activated delayed fluorescence (MR-TADF) emitters have gained popularity given their potential of attaining negligible singlet-triplet energy gaps, i.e., Delta E-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 Delta E-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 LOCa, 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-0 -> S-1, S-0 -> T-1, and S-1 -> T-1. For the studied cases, these descriptors are independent of the choice of the electronic structure method and optimal geometry. We show that the adiabatic Delta E-ST decreases with the increase of S-1 -> T-1 Q(a)(t), while Delta E-ST increases with an increase of the S-0 -> T-1 Q(a)(t). These trends showcase how the Q(a)(t) values can act as guiding descriptors to design new MR-TADF emitters with small Delta E-ST values.