The modelling of multi-resonant thermally activated delayed fluorescence emitters – properly accounting for electron correlation is key!

With the surge of interest in multi-resonant thermally activated delayed fluorescent (MR-TADF) materials it is important that there exist computational methods to accurately model their excited states. Here, building on our previous work, we demonstrate how the Spin-Component Scaling second-order approximate Coupled-Cluster (SCS-CC2), a wavefunction-based method, is robust at predicting the ΔEST (i.e., the energy difference between the lowest singlet and triplet excited states) of a large number of MR-TADF materials, with a mean average deviation (MAD) of 0.04 eV compared to experimental data. Time-Dependent Density Functional Theory calculations with the most common DFT functionals as well as the consideration of the Tamm-Dancoff approximation (TDA) consistently predict a much larger ΔEST owing to the absence of an explicit account of double (or higher order) excitations. This contribution is key in order to describe more precisely the Coulomb correlation that results in a stabilization of the S1 state. We also employed SCS-CC2 to evaluate donor-acceptor systems that contain a MR-TADF moiety acting as the acceptor and show that the broad emission observed for some of these compounds arises from the solvent-promoted stabilization of a higher-lying charge transfer (CT) singlet state (S2). This work highlights the importance of using wavefunction methods in relation MR-TADF emitter design and associated photophysics.