The Best of Both Worlds: ΔDFT Describes Multiresonance TADF Emitters with Wave-Function Accuracy at Density-Functional Cost

With their narrow-band
emission, high quantum yield, and good chemical
stability, multiresonance thermally activated delayed fluorescence
(MR-TADF) emitters are promising materials for OLED technology. However,
accurately modeling key properties, such as the singlet–triplet
(ST) energy gap and fluorescence energy, remains challenging. While
time-dependent density functional theory (TD-DFT), the workhorse of
computational materials science, suffers from fundamental issues,
wave function-based coupled-cluster (CC) approaches, like approximate
CC of second-order (CC2), are accurate but suffer from high computational
cost and unfavorable scaling with system size. This work demonstrates
that a state-specific ΔDFT approach based on unrestricted Kohn–Sham
(ΔUKS) combines the best of both worlds: on a diverse benchmark
set of 35 MR-TADF emitters, ΔUKS performs as good as or better
than CC2, recovering experimental ST gaps with a mean absolute deviation
(MAD) of 0.03 eV at a small fraction of the computational cost of
CC2. When combined with a tuned range-separated LC-ωPBE functional,
the excellent performance extends to fluorescence energies and ST
gaps of MR- and donor–acceptor TADF emitters and even molecules
with an inverted ST gap (INVEST), rendering this approach a jack of
all trades for organic electronics.