Electron transfer reactions in ternary systems on silica gel surfaces: evidence for radical cation diffusion

Abstract

Electron transfer reactions in ternary systems on silica gel surfaces: evidence for radical cation diffusion. Photochemical and Photobiological Sciences, 9 (7), pp. 937 -941.Additional Information:• This article was published in the journal, Photochemical and Photobio- Electron transfer reactions have been studied between 9-anthracenecarboxylic acid co-adsorbed with perylene on silica gel surfaces employing azulene as a molecular shuttle in order to facilitate hole transfer. In this paper we present for the first time a ternary system that unambiguously demonstrates an appreciable mobility of radical cations on the silica gel surface. Rates of hole transfer from the 9-anthracenecarboxylic acid radical cation to perylene via azulene have been studied using diffuse 10 reflectance laser flash photolysis spectroscopy. Azulene has been shown to enhance the rate of electron transfer in the ternary system, demonstrating significant mobility of the azulene and its radical cation species on silica gel surfaces. The data shows that the azulene radical cation can diffuse at an appreciable rate on the silica gel surface. Introduction 15The photochemistry and photophysics of molecules adsorbed to oxide surfaces and contained within zeolites has been the subject of a number of studies . The rapid mobility of molecules on oxide surfaces has been previously reported 2,6-11 and studied through both energy and electron transfer 20 reactions. The latter have also been studied on other porous materials such as clays 2,43-46 . Silica gel, which was employed in this study, is widely used in industry as a catalytic support and a greater understanding of reaction kinetics on these materials is significant in optimising these applications. 25Multi-photon ionisation of arenes and the subsequent electron transfer reactions of their radical cations on silica gel have been previously reported 3,4,13,14 and the influence of the nature and loading of the electron donor and electron acceptor compounds on the reaction rate investigated. The kinetics of 30 decay of radical ions on silica gel is not straightforward since the surface is heterogeneous in nature and rates rarely conform to simple exponential kinetics. We have used the model described by Albery et al 4,[12][13][14][15]50 to characterise our data sets, ensuring a comprehensive exploration of the 35 parameter space in order to obtain a global optimum value for the rate constants 13,14 .We have previously shown that energy and electron transfer reactions in these systems are dependent upon the rate of diffusion 13,14 , and in the case of azulene as energy acceptor / 40 electron donor, rates of reaction are largely determined by the rate of diffusion of this relatively more mobile species. We have demonstrated also that energetics play a role in determining electron transfer rates, and have shown a Marcustype dependence of rate on the free energy for electron 45 transfer 4,14 . In this study we also found evidence for some steric influence on reaction rates.In previous work we studied t...

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

“…In a previous publication 8 , we described a termolecular system which for the first time unambiguously demonstrated radical cation diffusion on silica gel, and indeed showed that this diffusion rate in the case of the azulene radical cation was 50 approximately a factor of 10 slower than that of the neutral molecule. Here we extend this study to encompass activation energies, since the activation energy for the diffusion of an uncharged species can be compared with that of its radical cation through the study of either the decay of the primary 55 electron acceptor or the rise of the tertiary donor.…”

“…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 .…”

Probing the interplay between factors determining reaction rates on silica gel using termolecular systems

“…Previously we have chosen anthracene, azulene and perylene to investigate electron transfer in a ter-molecular system 3,4 . In that study, evidence for the mobility of the azulene radical cation was 65 found based upon the relative rates of the rise of the perylene radical cation absorption in the presence and absence of azulene.…”

“…The excitation and multi-photon ionisation of arenes and their subsequent energy and electron transfer reactions on silica gel 25 have been previously reported [1][2][3][4]7,8,[12][13][14][15] . The decay of excited state and radical ion species on silica gel is complicated due to the heterogeneity of the surface and rates rarely conform to simple exponential kinetics.…”