Controlling factors in electron and energy transfer reactions on silica gel surfaces

Energy and electron transfer reactions between co-adsorbed molecules on silica gel have been studied using nanosecond time-resolved diffuse reflectance laser flash photolysis. The systems under investigation are anthracene and 9-carboxylic acid anthracene co-adsorbed with azulene, which undergo both triplet-triplet energy transfer and electron transfer from azulene to the anthracene radical cation following laser excitation. The decay traces have been analysed using a model which assumes a log gaussian distribution of rate constants and the methodology behind the optimisation of the fitting parameters is described. Bimolecular rate constants for energy and electron transfer between anthracene (and its derivative) and azulene have been obtained. Ground state association between anthracene and azulene has been observed, and an equilibrium constant for the process determined. The kinetic data is corrected for these ground state association effects which reduce the free azulene concentration. For both systems and for both the energy and electron transfer processes, analysis of the quenching data yields the same quenching constant. This indicates that the rate of reaction of anthracene (and the 9-carboxylic acid anthracene) on silica gel is predominantly governed by the rate of diffusion of the quencher.