INFLUENCE OF ISOLATED CHROMOPHORES ON THE TEMPERATURE-DEPENDENCE OF THE EXCIMER EMISSION IN STEADY-STATE AND TIME-RESOLVED FLUORESCENCE OF POLYSILOXANES

Steady-state fluorescence spectra and time-correlated emissions of a copolymer of dimethyl-siloxane and methylphenylsiloxane were measured, as a function of temperature, in the range -70 to 30-degrees-C. The stationary results indicate that the system is always, apparently, in the low-temperature limit, with a very low activation energy for excimer formation. Monomer decays are triexponential, and one of the components shows, at any temperature, a lifetime very similar to the monomeric analog. Si-29 NMR spectra demonstrate that the chromophore sequence distribution is very close to random and that about 79 % of the phenyl units are isolated. The fluorescence results can be explained in light of a kinetic scheme which takes into account the existence of isolated chromophores (64%, as calculated from the monomeric decays) and chromophores which form excimers with the same photophysical characteristics as in a homopolymer diad. Such a kinetic scheme has been used to develop the equations for steady-state and time-resolved experiments, and with use of them, apparent and corrected values of some kinetic parameters, such as the rate constant for excimer formation and the corresponding activation energy, have been determined.