Transient Absorption Dynamics of Sterically Congested Cu(I) MLCT Excited States

Subpicosecond through supra-nanosecond transient absorption dynamics of the homoleptic Cu­(I) metal-to-ligand charge transfer (MLCT) photosensitizers including the benchmark [Cu­(dmp)2]+ (dmp =2,9-dimethyl-1,10-phenanthroline) chromophore, as well as [Cu­(dsbp)2]+ (dsbp =2,9-di­(sec-butyl)-1,10-phenanthroline and [Cu­(dsbtmp)2]+ (dsbtmp =2,9-di­(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline) were investigated in dichloromethane and tetrahydrofuran solutions. Visible and near-IR spectroelectrochemical measurements of the singly reduced [Cu­(dsbp)2]+ and [Cu­(dsbtmp)2]+ species were determined in tetrahydrofuran, allowing for the identification of redox-specific phenanthroline-based radical anion spectroscopic signatures prevalent in the respective transient absorption experiments. This study utilized four different excitation wavelengths (418, 470, 500, and 530 nm) to elucidate dynamics on ultrafast times scales spanning probe wavelengths ranging from the UV to the near-IR (350 to 1450 nm). With the current time resolution of ∼150 fs, initial excited state decay in all three compounds was found to be independent of excitation wavelength. Not surprisingly, there was little to no observed influence of solvent in the initial stages of excited state decay in any of these molecules including [Cu­(dmp)2]+, consistent with results from previous investigators. The combined experimental data revealed two ranges of time constants observed on short time scales in all three MLCT chromophores and both components lengthen as a function of structure in the following manner: [Cu­(dsbtmp)2]+ < [Cu­(dsbp)2]+ < [Cu­(dmp)2]+. The molecule with the most inhibited potential for distortion, [Cu­(dsbtmp)2]+, possessed the fastest ultrafast dynamics as well as the longest excited state lifetimes in both solvents. These results are consistent with a small degree of excited state distortion, rapid intersystem crossing, and weak vibronic coupling to the ground state. The concomitant systematic variation in both initial time constants, assigned to pseudo-Jahn–Teller distortion and intersystem crossing, suggest that both processes are intimately coupled in all molecules in the series. The variability in these time scales illustrate that strongly impeded structural distortion in Cu­(I) MLCT excited state enables more rapid surface crossings in the initial deactivation dynamics.