Insights into the excited state dynamics of Fe( II) polypyridyl complexes from variable- temperature ultrafast spectroscopy

In an effort to better define the nature of the nuclear coordinate associated with excited state dynamics in first-row transition metal-based chromophores, variable-temperature ultrafast time-resolved absorption spectroscopy has been used to determine activation parameters associated with ground state recovery dynamics in a series of low-spin Fe(ii) polypyridyl complexes. Our results establish that high-spin (T-5(2)) to low-spin ((1)A(1)) conversion in complexes of the form [Fe(4,4-di-R-2,2-bpy)(3)](2+) (R = H, CH3, or tert-butyl) is characterized by a small but nevertheless non-zero barrier in the range of 300-350 cm(-1) in fluid CH3CN solution, a value that more than doubles to approximate to 750 cm(-1) for [Fe(terpy)(2)](2+) (terpy = 2,2:6,2-terpyridine). The data were analyzed in the context of semi-classical Marcus theory. Changes in the ratio of the electronic coupling to reorganization energy (specifically, H-ab(4)/) reveal an approximately two-fold difference between the [Fe(bpy)(3)](2+) complexes (approximate to 1/30) and [Fe(terpy)(2)](2+) (approximate to 1/14), suggesting a change in the nature of the nuclear coordinate associated with ground state recovery between these two types of complexes. These experimentally-determined ratios, along with estimates for the T-5(2)/(1)A(1) energy gap, yield electronic coupling values between these two states for the [Fe(bpy)(3)](2+) series and [Fe(terpy)(2)](2+) of 4.3 +/- 0.3 cm(-1) and 6 +/- 1 cm(-1), respectively, values that are qualitatively consistent with the second-order nature of high-spin/low-spin coupling in a d(6) ion. In addition to providing useful quantitative information on these prototypical Fe(ii) complexes, these results underscore the utility of variable-temperature spectroscopic measurements for characterizing ultrafast excited state dynamics in this class of compounds.