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 ((5)T(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, CH(3), or tert-butyl) is characterized by a small but nevertheless non-zero barrier in the range of 300–350 cm(–1) in fluid CH(3)CN solution, a value that more than doubles 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)/λ) reveal an approximately two-fold difference between the [Fe(bpy′)(3)](2+) complexes (∼1/30) and [Fe(terpy)(2)](2+) (∼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 (5)T(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.