Intra- and Inter-Molecular Charge Transfer Dynamics of Carbene-Metal-Amide Photosensitizers

A series of steady state and time-resolved spectroscopies were performed on a set of carbene metal amide (cMa) complexes, where M = Cu and Au, that could be used as photosensitizers for photosensitized electrocatalytic reactions. Using ps-to-ns and ns-to-ms transient absorption spectroscopies (psTA and nsTA, respectively), the excited state kinetics from light absorption, intersystem crossing, and eventually intermolecular charge transfer were thoroughly characterized. Ultrafast intersystem crossing (ISC) rates for these compounds were obtained from time correlated single photon counting (TCSPC) experiments utilizing a thermally activated delayed fluorescence (TADF) model, leading to ~3-20 x 10^9 s^(-1) rate constants for ISC (S1/T1). These rates were corroborated with psTA, while also confirming previously instrument limited ISC rates for gold complexes (80-130 x 10^9 s^(-1)). The psTA additionally abstracted an early time (0.2-0.8 x 10^12 s^(-1)) relaxation rate attributed to solvent relaxation and vibrational cooling. The nsTA experiments for a gold-based cMa complex demonstrated efficient intermolecular charge transfer from the excited cMa to either an electron acceptor or donor. Spectroelectrochemical experiments allow us to identify products observed in the nsTA as the formation of the oxidized and reduced forms of the cMa sensitizer, respectively.