Ir- and Ru-based photosensitizers dominate photocatalysis research, as they have for half a century. Given the cost of Ir and Ru, replacing them with non-precious metals has been a long standing objective. Replacing Ru with isoelectronic Fe would meet this objective, but simple substitution of Fe for Ru reduces the CT lifetime by up to seven orders of magnitude. The resultant 100 femtosecond lifetimes render Fe-based complexes poor photosensitizers. Attempts to identify new synthetic schemes to extend CT lifetimes have been pursued, but progress has been slow. Heteroleptic approaches to influencing CT relaxation dynamics will be presented. Motivated by the work of Winkler and Sutin, we have investigated the non-radiative relaxation mechanism for MLCT excited states in [Fe(CN)4(L)]2- , where L are a variety of polypyridyl ligands, with ultrafast x-ray spectroscopy and x-ray scattering. For L = 2,2’-bipyridine (bpy) complexes in aprotic solvents, the MLCT exited state lifetime increasing to 19 ps, from the 0.1 ps lifetime of the CT excited state in [Fe(bpy)3]2+. For [Fe(CN)4(bpy)]2- dissolved in water, the relaxation dynamics differ significantly with the MLCT state lifetime being similar to that of [Fe(bpy)3]2+. Unlike [Fe(bpy)3]2+, the ultrafast relaxation of the MLCT state of [Fe(CN)4(bpy)]2- dissolved in water forms a triplet metal-centered excited state with a 12 ps lifetime. We attribute the strong solvent dependence of the relaxation dynamics to destablization of the MLCT excited state resulting from water hydrogen bonding to the cyanide ligands. Interestingly, further redshifting of the MLCT excitation by replacing bpy with larger polypyridyl ligands does not lead to systematic variations in the MLCT lifetimes.

Ultrafast X-ray laser characterization of electronic excited states in first row transition metal complexes

256th National Meeting (2018)

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