DECAY OF POLY(PHENYLSILOXANE) FLUORESCENCE EMISSION - KINETIC-PARAMETERS AND ROTATIONAL MOTION

Time-correlated single-photon-counting measurements were performed on the monomer and excimer emissions of very dilute solutions of methylphenylsiloxane polymers. The homopolymer (PMPS) and three copolymers of methylphenylsiloxane with dimethylsiloxane of different compositions have been studied. Results in two solvents of widely different viscosity (cyclohexane and heavy oil) and at two temperatures (21 and 40-degrees-C) were obtained. The decays are in all cases multiexponential. No rise time of excimer emission was observed in the low-viscosity solvent. In heavy oil, a rise time of a few hundred picoseconds is detected but, still, there remains an important contribution of excimer species with no rise time. The analysis of the decays is made in the light of a kinetic scheme that is consistent with a previous ground-state conformational analysis of PMPS. In such a kinetic scheme, a significant part of the excimers come from preformed dimers in the diads of the polymer. The possible distinction of two excimeric states, one due to meso diads and the other due to racemic diads, is discussed. The rate constant for the formation of excimer by rotational motion (k(a) = 10(9)-10(10) s-1) is larger than in hydrocarbons polymers.