Characterization of Excited States in a Multiple-Resonance-Type Thermally Activated Delayed Fluorescence Molecule Using Time-Resolved Infrared Spectroscopy

We investigated the correlation between the photophysical properties and detailed excited-state characteristics of a multiple-resonance-type thermally activated delayed fluorescence (TADF) molecule, DABNA-1, using time-resolved infrared vibrational spectroscopy. By comparing the distinctive vibrational spectra in the fingerprint region (1000–1700 cm−1) to the simulated spectra, we found the optimal calculation conditions for density functional theory calculations to retrieve the vibrational spectra. Based on the calculations, the excited-state geometries and molecular orbitals in the lowest excited singlet (S1) and triplet (T1) states, as well as the ground state (S0), were determined. Consequently, we revealed that the similarity between the potential surfaces of T1 and S0 suppressed non-radiative decay and improved the high fluorescence quantum yield via TADF. Furthermore, we calculated the spin-orbit coupling matrix elements (SOCMEs) considering the experimentally confirmed geometries, and revealed that twisting of the main skeleton increases the SOCMEs.