Bimolecular processes on silica gel surfaces: energetic factors in determining electron-transfer rates

Abstract

Triplet state and radical cation formation is observed following laser excitation of anthracene, phenanthrene and naphthalene (and their derivatives) adsorbed on silica gel. Energy- and electron-transfer reactions of these compounds with co-adsorbed azulene have been studied using a time-resolved diffuse reflectance laser flash photolysis technique. Triplet energy transfer from the arene derivative to azulene and electron transfer from azulene to the arene radical cation have been investigated in order to distinguish between diffusional and energetic control in these systems. Energy and electron transfer can be studied independently due to differing absorption properties and energy dependencies of production of the triplet states and radical cations. Transient decay kinetics for both electron and energy transfer have been modelled using two different rate constant distributions: a log Gaussian and a symmetrical Levy stable distribution. The latter model has also been demonstrated to be applicable to the decay of radical cations in the absence of an electron donor, which cannot be adequately described by the Gaussian model. Energy-transfer rates between the arene derivatives and azulene have been found to be close to the diffusion-controlled limit; however, in most cases, the rate of electron transfer is considerably lower. A correlation between the bimolecular rate constant and free energy of electron transfer has been found, indicating a Marcus inverted region. Compounds with bulky substituents show a further reduction in the rate of electron transfer, suggesting that an additional steric factor is involved in this process.

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

“…Energetics were also demonstrated to have a significant role in determining electron transfer rates 2,6 , and we have shown a Marcus-type dependence of rate on the 45 free energy for electron transfer 2,6 . In these studies we have also found some evidence for an additional influence from steric factors 2 .…”

“…Previous studies have shown evidence for energy and electron transfer reactions in these systems as being largely dependent upon the rates of diffusion 2,3 , and in the case of 40 anthracene / azulene systems, rates of reaction are governed by the rate of diffusion of azulene which is more mobile than the anthracene moeities. Energetics were also demonstrated to have a significant role in determining electron transfer rates 2,6 , and we have shown a Marcus-type dependence of rate on the 45 free energy for electron transfer 2,6 .…”

“…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.…”

“…The decay of excited 30 state and radical ion species on silica gel is not uncomplicated due to the heterogeneity of the surface and rates rarely conform to simple exponential kinetics. The model described by Albery et al 41 has been used to characterise the data sets, ensuring a comprehensive exploration of the parameter space 35 in order to obtain a global optimum value for the rate parameters 2,3 .…”

“…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 .…”