Electron Transfer on Insulator Surfaces:  Exciplex Emission and the Role of Electron Diffusion in Determining Radical Deactivation Rates

Abstract

The kinetics of electron transfer on silica gel between the anthracene radical cation and the electron donors triphenylamine (TPA) and N,N,N‘,N‘-tetramethyl-1,4-phenylenediamine (TMPD) have been investigated using the technique of diffuse reflectance laser flash photolysis. The mean rate of decay of the anthracene radical cation, determined as the maximum of the rate constant distribution, correlates with the surface concentration of the electron donor in both cases up to a loading of 2 μmol g-1. In the case of triphenylamine as donor, using 355 nm excitation, plots of mean electron-transfer rate constant versus electron-donor concentration deviate positively from linearity at higher donor concentrations. In the case of TMPD, linearity is preserved at higher loadings with 355 nm excitation, where TMPD does not absorb appreciably and is not therefore directly photoionized, but again a positive deviation ensues with 266 nm excitation that causes direct photoionization of TMPD. In both cases the increased rate of decay of the anthracene radical cation correlates with appreciable direct photoionization of the electron donor. The enhanced rate of decay is explained as being due to donor photoionization increasing the local concentration of electrons on the surface and electron diffusion competing efficiently with donor diffusion leading to increased anthracene radical cation decay. In addition, we have demonstrated the formation between anthracene and TPA coadsorbed on silica gel of an emissive exciplex and have found no evidence that relaxation of this exciplex results in radical ion pair formation.

Spectroscopy and Ion−Electron Recombination Kinetics of Radical Ions of Anthracenes and Substituted Anilines on Silica Gel

“…Time-resolved transient emission measurements were performed using a gated intensified photodiode array system (EG&G Princeton Applied Research) described in detail elsewhere. 3 Anthracene (Anth) was scintillation grade (Sigma) and used as received. 9-Methylanthracene (9-Me) was supplied by Eastman Kodak and used as received.…”

Energy and electron transfer reactions on silica gel and titania–silica mixed oxide surfaces

“…The singlet excited state of anthracene may undergo fluorescence and intersystem crossing. The triplet excited state of anthracene interacts with azulene present in the system by energy transfer or by molecular aggregation [64,65], with exciplex formation occurring in the latter case [2,13].…”

“…However, on a reactive surface, such as titania or titania-silica mixed oxides the surface can directly interact with the excited states and modify the observed chemistry [10,[25][26][27][28][29][30]. The mobility of molecules adsorbed on silica gel and also on titania-silica surfaces have been previously reported [11][12][13][27][28][29][30] and studied through both energy and electron transfer reactions.…”

“…The kinetics of decay of radical ions on silica gel and titania-silica surface is not simple 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 [31] to characterise our data sets [2][3][4][5][6][13][14][15], employing a comprehensive exploration of the parameter space in order to obtain a global optimum value for the rate constants 1 3 Energy and electron transfer reactions on silica gel and titania-… [2,3]. Additionally, we have used Monte-Carlo simulations to model the molecular behaviour and have found good correlations with experiment [15].…”

Methylene blue based protein solder for vascular anastomoses: An in vitro burst pressure study

“…The spectral profile of the laser diode was measured by using the gated photodiode array system. The use of this apparatus has previously been described [44].…”