Direct observation of mass transfer at solid-liquid interface by laser flash photolysis of the interface probe molecules

Abstract

Transient absorption signal delayed by a magnitude of 100 μs with respect to the exciting laser shot is observed after UV laser flash photolysis of degassed benzene, acetone, and acetonitrile solutions of an interface probe complex, i.e., gold(I) complex [{Au[P(C 6H 4OMe-p) 3]} 2-(μ-C≡C)] (μ-ethynylene-bis{tris(4-methoxyphenyl)-phosphine}gold]). Chemical reactions leading to the transient absorbance change are confirmed to be occurring at the solid-liquid interface, and the delay time is believed to arise from the photogenerated intermediate species in the bulk solution crossing the diffusion layer, which ultimately undergo interfacial reactions resulting in the observed transient absorbance change. The delay time is suggested as a direct measure for the thickness of the diffusion layer. The thickness of the diffusion layer around 0.2 μm, estimated by this method, is comparable to that from the sonovoltammetric study. Oscillations in transient absorbance kinetics are also observed, which can be attributed to the coupling between the interfacial chemical reactions and a photoacoustic effect; oscillation due to a single physical process arising from the schlieren effect under certain condition is also discussed. Similar transient phenomena are observed in another luminescent complex, i.e., a hexanuclear Cu(I) cluster Cu 6(t-NS) 6 (t-NS = 4-tert-butylpyridine-2-thiolate). The characterization of the photochemical reaction processes of the Cu(I) complex by means of transient absorbance difference spectra reveals that a consecutive biphotonic ionization process occurs after the laser flash. The intermediate species with a lifetime of 65 μs responsible for the interfacial reaction is tentatively assigned as a charge separation pair. A reaction scheme involving surface-assisted ionization of the charge separation pair is proposed to account for the coupling between the interfacial chemical reaction and the photoacoustic effect. © 2000 American Chemical Society.