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 c opolymer of dimethyl-siloxane and methylphenylsiloxane were measured, as a function of temperature, in the range -70 to 30-degrees-C. The st ationary results indicate that the system is always, apparently, in th e low-temperature limit, with a very low activation energy for excimer formation. Monomer decays are triexponential, and one of the componen ts shows, at any temperature, a lifetime very similar to the monomeric analog. Si-29 NMR spectra demonstrate that the chromophore sequence d istribution 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 isolate d chromophores (64%, as calculated from the monomeric decays) and chro mophores which form excimers with the same photophysical characteristi cs as in a homopolymer diad. Such a kinetic scheme has been used to de velop the equations for steady-state and time-resolved experiments, an d with use of them, apparent and corrected values of some kinetic para meters, such as the rate constant for excimer formation and the corres ponding activation energy, have been determined.