Efficiency of hot carrier trapping by outer-sphere redox probes at quantumdot interfaces

The action spectra of the colloidal TiO2-photosensitized oxidations of bifu nctional aromatics in 1 mM phosphate colloidal media provide firm evidence that electron transfer from outer-sphere donors can compete with excited ho le relaxation at nanoparticle interfaces. The possibility and extent of the competition are largely determined by the dependence of Marcus nuclear fac tors on the donors' reversible redox potentials E-D/D(+.) relative to the v alence band edge. Good electron donors are degraded by OH radicals produced in the oxidation of water by thermal holes, whereas direct electron transf er into excited holes (the pathway favored by less oxidizable substrates) l eads to enhanced quantum efficiencies at short wavelengths. The ultimate de cline of the quantum efficiency for the oxidation of phthalate (the most en doergic donor of the set) by lambda less than or equal to 320 nm photons in dicates that the relaxation of highly excited carriers takes place in discr ete steps commensurate with electron transfer reorganization energies. The latter observation is consistent with the opening of low order multiphonon channels for the disposal of kinetic energy quanta larger than the depth of surface (v) over bar(O-H) similar to 3700 cm(-1) vibrational sinks.