PHOTOPHYSICS OF SILOXANES - INFLUENCE OF PREFORMED DIMERS AND TRANSITION FROM LOW-TEMPERATURE TO HIGH-TEMPERATURE BEHAVIOR OF DIMERIC AND POLYMERIC METHYLPHENYLSILOXANE

Fluorescence spectra and fluorescence decays of dimethylphenyldisiloxa ne (DS) and of poly-(methylphenylsiloxane) (PMPS), in methylcyclohexan e dilute solution, were measured in a range of temperatures (-132 to 50-degrees-C). This range is shown to cover both the low- and high-tem perature limits and the crossover region. The steady-state results for DS show a typical Arrhenius behavior, compatible with a simple Birks kinetics. DS monomer fluorescence decays are biexponential, showing ev idence for excimer dissociation, compatible with an isoemissive point observed in the range 5-27-degrees-C. An excimer rise time could be ob served for DS at the lowest temperatures, but the sum of the preexpone ntial factors is larger than zero, proving that a certain fraction of the excimers come from ground-state dimers. In order to account for th e presence of such preformed dimers, the kinetic scheme has been modif ied and the proper equations for this modified scheme have been derive d. The steady-state results for PMPS indicate that about 50% of the ch romophores form part of the ground-state preformed dimer conformations . Fluorescence decays of PMPS are too complex to yield kinetic paramet ers with physical meaning, but the steady-state results allow the extr action of the rate constant for excimer formation and its activation e nergy. A complete set of the Birks scheme rate constants was obtained as a function of temperature for DS, both from steady-state and transi ent experiments, with a good coincidence of the results obtained by th e two methods. Apparent and corrected values of the rate constant for excimer formation, taking into account the contribution of ground-stat e preformed dimers, were also obtained.