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)/(2D(op) + 1), where D-op is the optical dielectric constant of the bulk solvent. A linear relationship between E-op and (1 - D-op)/(2D(op) + 1) is observed not only in the bulk solvents, as anticipated, but also in clusters containing as few as four solvent molecules.