Solvent-Dependent Dynamics of a Series of Rhenium Photoactivated Catalysts Measured with Ultrafast 2DIR

The spectral dynamics of a series of rhenium photocatalysts, fac-Re(4,4'-R-2-bpy)(CO)(3)Cl, where R = H, methyl, t-butyl, and carboxylic acid, as well as Re(1,10-phenanthroline)(CO)(3)Cl were observed in multiple aprotic solvents using two-dimensional infrared spectroscopy (2DIR). The carbonyl vibrational stretching frequencies showed slight variations due to the electron-donating or -withdrawing nature of the substituents on the bipyridine. The different substituents had minimal to no influence on the spectral diffusion time scales of the compounds within a particular solvent, but among the three different solvents investigated (DMSO, THF, and CH3CN), we find the spectral diffusion times to correlate with the solvents donor number (DN). Because the donicity is a measure the Lewis basicity of the solvent, these findings may help establish a more complete dynamical picture of the photocatalysis, where the first chemical step following optical excitation is electron transfer from a sacrificial donor to the rhenium complex.