Osnabrück 2002 – scientific programme

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MO: Molekülphysik

MO 2: Femtosekundenspektroskopie I: Ladungstransfer

MO 2.1: Talk

Monday, March 4, 2002, 14:00–14:15, HS 15/E07

Pump-Probe Spectroscopy of Trimetallic Mixed-Valence Complexes: Solvent Dependence of Back Electron Transfer Lifetime — •Elmar Schreiber¹, David F. Watson² und Andrew B. Bocarsly² — ¹Center for Ultrafast Laser Applications — ²Department of Chemistry, Princeton University, Princeton, NJ 08544 (USA)

The photo-induced ultrafast dynamics of trinuclear mixed-valence complexes [(NC)₅M^II-CN-Pt^IV(NH₃)₄-NC- M^II(CN)₅]⁴⁻, where M = Ru or Fe, are investigated. The electronic absorption spectra of these complexes exhibit broad M^II→Pt^IV metal-to-metal charge transfer (MMCT) bands. Irradiation into these bands leads to the dissociation of the complexes into 2 equiv of M(CN)₆³⁻ and 1 equiv of Pt(NH₃)₄²⁺, via a net two-electron- transfer process. Pump-probe experiments (400 nm pump, 400 nm or 532 nm probe) were performed on these systems. The timescale of back electron transfer (BET) following M^II→Pt^IV MMCT was found to vary with solvent and the identity of the M^II centers. The BET timescale of the Ru system ranged from 476±23 fs in water to 1110±288  fs in a water/DMSO mixture (C_DMSO = 0.1). The BET timescale of Fe system in water was approximately 3300 fs. The timescale of ground state vibrational relaxation following BET was approximately 2 ps. Variation of the BET timescale is explained in terms of the dependence of the ground state free energy, and therefore the activation barrier to BET, on solvent and the identity of the M^II centers. Solvent-induced shifts of the ground electronic state in the Ru system enable systematic tunability of electron transfer dynamics.