Determination of the photoinduced electron transfer rate constant in Langmuir-Blodgett films by time-resolved fluorescence

The feasibility of a relevant analysis of the fluorescence decays of Langmuir-Blodgett-Kuhn films where electron and energy transfer occur simultaneously is demonstrated. Starting from a stretched exponential, describing Forster-type energy transfer, a model describing simultaneous photoinduced electron transfer and energy transfer is developed. In this model a binomial distribution of electron donors (pyrene) is assumed in the layer adjacent to the excited probe (Rhodamine G). When the fluorescence decays of two or three samples with different concentrations of electron donors are analyzed simultaneously with the decay of a sample containing no electron donor, reliable values of the global parameters (the fluorescence decay time of the unquenched probe, the rate constant for electron transfer, and the rate constant for energy transfer to a trap) could be obtained. The local parameters (the average number of electron donors in a site opposite the probe and the fraction of the monolayer containing an electron donor) are, however, highly correlated. For the concentration range investigated, the decay parameters do not depend on the emission wavelength.