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 dimethylphenyldisiloxane (DS) and of poly-(methylphenylsiloxane) (PMPS), in methylcyclohexane 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-temperature 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 evidence for excimer dissociation, compatible with an isoemissive point observed in the range 5-27-degrees-C. An excimer rise time could be observed for DS at the lowest temperatures, but the sum of the preexponential factors is larger than zero, proving that a certain fraction of the excimers come from ground-state dimers. In order to account for the presence of such preformed dimers, the kinetic scheme has been modified and the proper equations for this modified scheme have been derived. The steady-state results for PMPS indicate that about 50% of the chromophores form part of the ground-state preformed dimer conformations. Fluorescence decays of PMPS are too complex to yield kinetic parameters with physical meaning, but the steady-state results allow the extraction of the rate constant for excimer formation and its activation energy. A complete set of the Birks scheme rate constants was obtained as a function of temperature for DS, both from steady-state and transient experiments, with a good coincidence of the results obtained by the two methods. Apparent and corrected values of the rate constant for excimer formation, taking into account the contribution of ground-state preformed dimers, were also obtained.