INFLUENCE OF SEQUENTIAL SOLVATION ON METAL-TO-LIGAND CHARGE-TRANSFER IN BIS(2,2',2''-TERPYRIDYL)IRON(II) CLUSTERED WITH DIMETHYL-SULFOXIDE

The first direct measurements of the contributions of individual solve nt molecules to solvent reorganization associated with photoinitiated charge transfer in a coordination complex are reported for gas-phase c lusters of bis(2,2',2 ''-terpyridyl)iron(II), [Fe(terpy)(2)](2+), with dimethyl sulfoxide (DMSO) prepared by electrospray ionization. Excita tion of [Fe(terpy)(2).(DMSO)(n)](2+) clusters at wavelengths correspon ding to the metal-to-ligand charge-transfer (MLCT) absorption characte ristic of Fe(II)-polypyridine complexes in solution triggers evaporati on of solvent molecules, permitting the application of laser photofrag mentation mass spectrometry to monitor MLCT absorption. The energy of the optical transition corresponding to MLCT E-op is reported for [Fe( terpy)(2).(DMSO)(n)](2+) clusters (n = 1-11), and the cluster-size dep endence of E-op is examined following Jortner's cluster-size-equation approach (Jortner, J. Z. Phys. D 1992, 24, 247). The first 11 DMSO mol ecules contribute 54-63% of the bulk solvent reorganization energy, ba sed on estimates that E-op for the gas-phase [Fe(terpy)(2)](2+) comple x falls between 1.860 x 10(4) and 1.876 x 10(4) cm(-1).