Effects of Nonradiative Energy Transfer on Photodimerization of a Stilbazolium Cation on Ag and GaAs Substrates: Infrared Reflection Absorption and Emission Spectroscopic Studies

Article abstract of DOI:10.1021/j100011a037

Infrared reflection absorption (IRA) and fluoroscence spectroscopies were applied to elucidate photodimerization processes induced by irradiation at 340 nm of stilbazolium cations (C18S) incorporated in Langmuir-Blodgett (LB) films of fully deuterated arachidic acid (DA) on the silver and GaAs substrates.A spacer layer consisting of monomolecular LB films of DA was inserted between the substrates and the mixed LB monolayers (C18S:DA = 1:4).Both the rate and the extent of photodimerization increased with the thickness of the spacer layer (d).The quantum yield of excimer fluorescence also increased with d.Analyses of these results confirmed that photodimerization proceeded through an excimer of C18S and that the reduction in the dimerization rate observed for the samples with smaller numbers of spacer layers was mainly due to an increase in the rate of nonradiative energy transfer to the substrates.The energy transfer on the GaAs substrate depended on d^-3, which conformed to a well-known classical theory based on the standard Foerster-type energy transfer.On the other hand, the energy transfer rate on the silver substrate was proportional to d^-1, which did not agree with the classical theory.Change in the intensity of an excimer fluorescence band near 490 nm was observed from 15 monolayers of a mixed LB film (C18S:DA = 1:4) on a silver substrate as a function of t in the temperature range 77-320 K.Above 300 K, the intensity decreased precipitously with t, indicating the advance of photodimerization; the activation energy of the process was determined to be 28.4 kJ/mol.On the contrary, the intensity showed appreciable increase below 200 K and dimerization hardly proceeded.This phenomenon was interpreted as due to a photoinduced orientation change of C18S, resulting in a state which was more favorable for the excimer formation and hence stronger fluorescence.However, only very few excimers had thermal energy larger than the activation energy so that dimers were scarcely produced below 200 K.In the temperature range 200-300 K, the intensity at first increased and then decreased gradually; the result could be reproduced by assuming that both the orientation change and the photodimerization occurred at the same time.