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

Multiresonance\nthermally activated delayed fluorescence (MR-TADF)\nemitters have gained popularity given their potential of attaining\nnegligible singlet–triplet energy gaps, i.e., Δ<i>E</i><sub>ST</sub>, without hindering emission, thus increasing\nthe reverse and direct intersystem crossing rates without affecting\nfluorescence. This is achieved due to the singlet and triplet states’\nshort-range charge transfer character (SRCT). Thus, obtaining quantitative\ninformation about SRCT would help develop new MR-TADF emitters. This\nwork studies three different families of MR-TADF emitters: DOBOA,\nDiKTa, and OQAO. First, we compute their adiabatic Δ<i>E</i><sub>ST</sub> with four different methods (TDA-CAM-B3LYP,\nSTEOM-DLPNO-CCSD, ADC(2), and SCS-CC2). Then, we compute fluorescence\n(<i>k</i><sub>r</sub>), direct (<i>k</i><sub>ISC</sub>), and reverse intersystem crossing rate constants. For <i>k</i><sub>r</sub>, we assessed the effect of different levels of approximations\non the rate calculations. We show that <i>k</i><sub>r</sub> does not depend significantly on the different harmonic models (adiabatic\nHessian or vertical Hessian), coordinate systems, and broadening widths.\nMoreover, Herzberg–Teller effects are negligible for <i>k</i><sub>r</sub> but are the main contribution for <i>k</i><sub>ISC</sub> and <i>k</i><sub>RISC</sub>. The\ncomputed rate constants agree well with the experimental results.\nMoreover, we propose the use of two wave function descriptors, <i>Q</i><sub>a</sub><sup>t</sup> and LOC<sub>a</sub>, based on the 1-particle transition density\nmatrix, which assigns the amount of charge centered on the atoms.\nWe compute these descriptors for three transitions: S<sub>0</sub> →\nS<sub>1</sub>, S<sub>0</sub> → T<sub>1</sub>, and S<sub>1</sub> → T<sub>1</sub>. For the studied cases, these descriptors\nare independent of the choice of the electronic structure method and\noptimal geometry. We show that the adiabatic Δ<i>E</i><sub>ST</sub> decreases with the increase of S<sub>1</sub> →\nT<sub>1</sub> <i>Q</i><sub>a</sub><sup>t</sup>, while Δ<i>E</i><sub>ST</sub> increases with an increase of the S<sub>0</sub> → T<sub>1</sub> <i>Q</i><sub>a</sub><sup>t</sup>. These trends showcase how the <i>Q</i><sub>a</sub><sup>t</sup> values can act\nas guiding descriptors to design new MR-TADF emitters with small Δ<i>E</i><sub>ST</sub> values.