Dye-polymer interactions controlling the kinetics of fluorescein photobleaching reactions in poly(vinyl alcohol)

This work presents results for the temperature dependence of the photobleaching reactions of Fluorescein dissolved in poly(vinyl alcohol) (PVA) irradiated by a continuous 150 W Hg high pressure are lamp. We observed that the photobleaching process efficiency is dependent on: lamp power, dye concentration and temperature. Although the photobleaching processes can only be described by a single-exponential function for lower concentration samples, a biphasic model must be employed for intermediate concentrations, resulting in one slower and one faster processes, while, for higher concentration samples, even multiple-exponential functions are unable to fit the experimental data. This complex behavior is probably due to the co-existence of many different types of energy transfer and photochemical processes with different rate constants. Therefore, a model based on a sequential distribution of site occupation by the dye molecules in the matrix, controlled by dye-polymer interaction strengths, is proposed to explain the complex-exponential kinetic behavior. Moreover, photobleaching process efficiency is somewhat increased by temperature and, above the PVA glass transition temperature, T-g, (350 K), it follows a first order kinetics, fitted by a mono-exponential function. (C) 1998 Elsevier Science S.A. All rights reserved.