Metal-to-Ligand Charge Transfer in the Gas-Phase Cluster Limit

The solvent dependence of metal-to-ligand charge transfer (MLCT) in the bis(2,2‘,2‘‘-terpyridyl)iron(II) complex, [Fe(terpy) 2 ] 2+, isolated in small gas-phase clusters with one or four molecules of the polar, organic solvents acetone, acetonitrile, dimethyl sulfoxide, N, N -dimethylformamide, methanol, and pyridazine is reported. The shift in the maximum of the MLCT band, E op, for [Fe(terpy) 2 ·(solvent) 1 ] 2+ clusters, measured using laser photofragmentation mass spectrometry, relative to the corresponding values in bulk solution ranges from +601 cm -1 for acetone to +764 cm -1 for pyridazine. The solvent dependence of the outer-sphere reorganization energy predicted by a dielectric continuum model provides a context for comparing E op values determined for MLCT in [Fe(terpy) 2 ·(solvent) n ] 2+ clusters ( n = 1, 4) with those measured in solution. A model derived from Kirkwood's equation for the mutual electrostatic interaction energy of an ion and a polar medium predicts that the solvent reorganization energy associated with MLCT in [Fe(terpy) 2 ] 2+ is a linear function of (1 − D op )/(2 D op + 1), where D op is the optical dielectric constant of the bulk solvent. A linear relationship between E op and (1 − D op )/(2 D op + 1) is observed not only in the bulk solvents, as anticipated, but also in clusters containing as few as four solvent molecules.