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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 1998-07, Vol.102 (30), p.6101-6106
• Main Authors: Spence, Thomas G, Trotter, Brett T, Burns, Thomas D, Posey, Lynmarie A
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp982335x

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.