Measurements of energy dispersion at liquid–solid interfaces: Fluorescence quenching of pyrene bound to fumed silica

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Abstract
The observation of a stretched exponential response in chemical kinetics at solid–liquid interfaces is an indicator of solid surface disorder. In this contribution, we review earlier relaxation kinetics studies of solid surface disorder and the statistical criteria analyzing kinetic data that do not fit single exponential. We draw on these concepts to interpret the fluorescence decay kinetics of pyrene covalently attached to the surface of fumed silica particles that are suspended in methanol. This surface probe exhibits stretched exponential behavior of the form f(t) = exp[−(kt)β] in the decay of its excited-state populations. The addition of iodine quencher increases the average decay rate, but does not alter the nonlinear exponent β. These results, along with the diffusional length of the iodine quencher, the photophysics of the probe, and the chemistry of the interface, indicate that the kinetic inhomogeneity is dominated by dispersion of surface energies, rather than by diffusional excursions of the quencher on a fractally aggregated surface. Numerical Laplace inversion of the experimental decay curves is used to determine the distribution of rates which reveal that the quenching process is inhomogeneous and correlated with the unquenched decay rates of the probe.

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