Accurate Singlet–Triplet\nExcited States Energy\nGap Can Be Mastered by Time-Dependent Density Functional Theory Calculations\nBased on a Dielectric-Screened Range-Separated Hybrid Functional

The energy gap between the lowest singlet and lowest\ntriplet excited\nstates of molecular emitters is a key property affecting their thermally\nactivated delayed fluorescence (TADF) functionality. In cases in which\nthese states are marked by internal charge transfer, the molecular\nenvironment of the emitter significantly stabilizes these states.\nTo address this challenging relationship, a recently developed density\nfunctional theory (DFT) framework that alleviates the fundamental\norbital gap in the gas phase can be employed, offering a highly effective\nand predictive description of these states. These modern functionals,\nrange-separated hybrid functionals, are based on a generalized Kohn–Sham\nexchange functional expression where electronic interactions are separated\ninto long and short ranges. For representing the significant condensed\nphase effects, especially in the case of charge transfer states, dielectric\nscreening can be affected by functional expression. Dielectric-screened\nrange-separated functionals that are invoked within a polarizable\ncontinuum model were shown to properly address the orbital gap challenge\nin the condensed phase and consequently benchmark well in calculating\ntriplet and charge transfer excited states. We address the excited\nsinglet–triplet state energy gap using such a screened range-separated\nfunctional and find that the calculated gaps tend to fall within 0.1\neV of the measured energies for a widely addressed benchmark set.\nHere, we emphasize the success of the dielectric-screened DFT framework\nin calculating these important energy gaps, reproducing well the benchmark\nvalues and therefore alleviating concerns about the applicability\nof existing functionals. We posit that dielectric-screened calculations\nwill bear increasing impact on the design of organic materials aiming\nto enhance optoelectronic applications and in particular of realized\nTADF efficiencies.