Ion–electron recombination on silica gel surfaces: experiment and modelling

Abstract

Kinetics on silica gel and other solid, porous surfaces are often complex. In this paper we have studied the decay kinetics of radical cations produced following multiphoton ionisation on silica gel, and have characterised these using an empirical model. Trends in kinetics have been observed both as a function of concentration and of temperature. Concentration dependent studies suggest heterogeneity of surface adsorption, both in terms of the nature of adsorption sites and aggregation effects. Temperature dependent studies show that the activation energies for surface diffusion correlate with the size of the radical cation, suggesting that its movement rather than that of the electron dominates the observed kinetics. Monte Carlo simulations have been shown to give useful qualitative insights into the interpretation of the extracted parameters, in particular into how apparent distributions of rate constants can arise as a result of low surface dimensionality.

Activation energies of photoinduced unimolecular, bimolecular and termolecular processes on silica gel surfaces

“…5 Rate constants for the ion-electron recombination of 2,3-DMO and 2,3-DDO radical cations were measured as a function of temperature. Both diffuse reflectance and fluorescence recovery techniques were used, the results from the 10 diffuse reflectance having been previously reported 1 . The fluorescence spectra of 2,3-didecyloxyanthracene (adsorbed on silica gel) at various time delays following laser excitation are shown in figure 1.…”

“…The compound(s) of interest were dissolved in n-hexane or acetonitrile and a known weight added to the dried silica gel. The mixture was allowed to equilibrate for a period of one hour, with Sample concentrations for all systems studied were kept low corresponding to a homogeneous 2D surface concentration of less than 1% of a monolayer as discussed in previous publications [1][2][3][4][5] . 30 The following chemicals were used as supplied: silica gel (Davisil grade 635, 60 -100 mesh, 6 nm pore size, surface area 480 m 2 g -1 ; Aldrich Chemical Co.), acetonitrile (spectrophotometric grade; Aldrich Chemical Co.), n-hexane (spectrophotometric grade; Aldrich Chemical Co.), anthracene 35 (scintillation grade, minimum 99%, Sigma), 2-methylanthracene (HPLC grade; ≥ 99%; Fluka), 9-anthracenecarboxylic acid (minimum 99%, Aldrich Chemical Co.), (azulene (99%; Aldrich Chemical Co.), perylene (99%; Aldrich Chemical Co.), 2,3-didecyloxyanthracene (2,3-DDO) and 2,3-dimethoxyanthracene 40 (2,3-DMO) were kindly supplied by Professors Bouas-Laurent and Desvergne of the University of Bordeaux and prepared as described in the literature 42 .…”

“…In previous work the activation energy for back electron transfer from the surface to the radical cations of anthracene 50 derivatives was reported 1,4 . There was a correlation between the activation energy and the size of the radical cation which suggested that movement of the radical cation and not of the electron dominated the observed kinetics.…”

“…The excitation and multi-photon ionisation of arenes and their subsequent energy and electron transfer reactions on silica gel have been previously reported [1][2][3][4][6][7][8] including investigation of the influence of the nature and loading of the 25 donor and acceptor compounds on the reaction. The radical cation decays on short timescales (of the order of a few milliseconds) via geminate recombination 6 and at longer timescales by diffusion of the radical cation and electron followed by ion-electron recombination.…”

“…However, the study of the kinetics of decay or reactions of "dark" states such as triplets and radical ions has been through the use of diffuse reflectance spectroscopy [1][2][3][5][6][7][8]12,[16][17][18]21,23,26 . This latter 60 technique has been used by ourselves to study the long time decay (greater than 30 seconds) of anthracene radical cations 1 . This technique works well as most anthracene derivatives have a distinctive radical cation absorption peak in the 700-800 nm region 4 .…”