ELECTRON-TRANSFER ON INSULATOR SURFACES - EXCIPLEX EMISSION AND THE ROLE OF ELECTRON-DIFFUSION IN DETERMINING RADICAL DEACTIVATION RATES

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 us ing the technique of diffuse reflectance laser flash photolysis. The m ean 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 mu mol g(-1). In the case of triphenylamine as donor, using 355 nm e xcitation, plots of mean electron-transfer rate constant versus electr on-donor concentration deviate positively from Linearity at higher don or concentrations. In the case of TMPD, linearity is preserved at high er loadings with 355 nm excitation, where TMPD does not absorb appreci ably and is not therefore directly photoionized, but again a positive deviation ensues with 266 mm excitation that causes direct photoioniza tion of TMPD. In both cases the increased rate of decay of the anthrac ene radical cation correlates with appreciable direct photoionization of the electron donor. The enhanced rate of decay is explained as bein g due to donor photoionization increasing the local concentration of e lectrons on the surface and electron diffusion competing efficiently w ith donor diffusion leading to increased anthracene radical cation dec ay. In addition, we have demonstrated the formation between anthracene and TPA coadsorbed on silica gel of an emissive exciplex and have fou nd no evidence that relaxation of this exciplex results in radical ion pair formation.