Ten-nanosecond step-scan FT-IR absorption difference time-resolved spectroscopy: Applications to excited states of transition metal complexes
Ten-nanosecond time resolution has been achieved with step-scan FT-IR absorbance difference spectroscopy (S2FT-IR ΔA TRS) and demonstrated by measuring ΔA spectra of fac-[Re(bpy)(CO)3Cl] and cis-[Os(bpy)2(CO)(4,4′-bpy)]2+ (bpy = 2,2′-bipyridine; 4,4′-bpy =4,4′-bipyridine) in CH3CN solution, following 355-nm laser excitation. In both complexes, the large shifts in v̄(CO) to higher energy are consistent with the assignment that the lowest-energy excited states are metal-to-ligand charge transfer in nature. For [Os(bpy)2(CO)(4,4′-bpy)]2+, it is also possible to measure the excited-state decay kinetics, again with 10-ns resolution. In addition, ΔA bands are observed that are related to excited-state vibrations of the bipyridine ligands. ΔA spectra of good signal-to-noise ratio can be obtained for complexes with lifetimes as short as 10 ns.
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