On the performance of DFT/MRCI for singlet–triplet gaps and emission energies of thermally activated delayed fluorescence molecules

This work investigates the performance of the density functional theory multireference configuration interaction (DFT/MRCI) method for the donor-acceptor and multi-resonance thermally activated delayed fluorescence (TADF) emitters of the recent STGABS27 benchmark set [L. Kunze, A. Hansen, S. Grimme and J.-M. Mewes, <i>J. Phys. Chem. Lett.</i>, 2021, <b>12</b>, 8470-8480]. Comparing the accurate experimental singlet-triplet energy gaps (Δ<i>E</i>_ST) and fluorescence energies (<i>E</i>_EM) to values computed with DFT/MRCI reveals a robust performance without large or systematic errors. Specifically in the vertical approximation without a solvation model, DFT/MRCI achieves mean absolute deviations (MADs) for singlet-triplet gaps and emission energies of 0.06 eV and 0.21 eV, respectively. Surprisingly, these values do not improve systematically when geometric relaxation and state-specific solvation effects are included. Apparently, part of these effects are absorbed in the parameterization of DFT/MRCI and attempting to include them explicitly <i>via</i> a ROKS+PCM reaction field leads to an imbalanced treatment. As a result, the simplest approach of running calculations in the vertical approximation in gas phase turns out to be the most accurate. Albeit less accurate and more computationally demanding than state-specific orbital-optimized DFT, DFT/MRCI has the advantage that all low-lying excited states are obtained in a single calculation, including transition properties between them. At the same time, the aforementioned performance for the Δ<i>E</i>_ST and <i>E</i>_Em values is achieved without molecule-specific or state-specific adjustments like optimal tuning that is often necessary for time-dependent DFT. Hence, we conclude that DFT/MRCI is particularly useful during the initial stage of computational investigations of TADF emitters to screen for the Δ<i>E</i>_ST and identify the relevant states, whose energies can then be refined with accurate state-specific DFT methods like ROKS or (Δ)UKS with MADs for Δ<i>E</i>_ST below 0.03 eV.