Oral: Revealing the Spin-Orbit Coupling in Thermally Assisted Delayed Fluorescence of Carbene-Metal-Amides

Photosensitizers based on earth-abundant elements that can efficiently harvest solar energy are vital in achieving our mission of sustainable solar fuel production. When a photon is absorbed by the sensitizer, the long-lived charge-separated states can transfer the charge to an electrocatalyst, driving reactions such as water splitting to produce hydrogen. The excited state lifetime of μs, tunable redox potential, and excellent visible photon absorption coefficient make carbene-metal-amide (cMa) complexes a strong candidate as sensitizers for capturing solar energy. The spin-orbit coupling (SOC) enhanced intersystem crossing (ISC) in cMas enables access to the triplet population and emits through thermally assisted delayed fluorescence (TADF) from an inter-ligand charge transfer state. TADF is a process in which triplet population ISC to singlet state using thermal energy and emit photon. Prior studies revealed that singlet to triplet ISC rates vary significantly (~ 3 to 120 x 10⁹ s^-1) based on the changes in carbene, carbazole, and metal in cMas. However, the contribution of spin-orbit coupling in the ISC process and the complex interplay between SOC, ISC, and TADF in cMa’s has not yet been revealed, limiting our detailed understanding of the photophysical effects of changes in the molecular architecture. From a series of high-throughput transient absorption experiments, time-correlated single photon counting measurements, and theoretical calculations, we demonstrate the influence of different molecular architectural motifs of cMas on the spin-orbit coupling constants (SOCCs). We experimentally determined SOCCs using Arrhenius and Marcus-Hush theories. Further, the experimentally determined SOCCs are compared with the theoretically determined SOCCs values. Thus, our work opens a dialog between the experimental and theoretical approach in determining SOCCs and contributes to understanding the photophysics of cMa’s and a wider class of molecules that emit through TADF.