Local Molecular Motion of Polystyrene Model Compounds Measured by Using Picosecond Pulse Radiolysis. 1. Diastereoisomeric Styrene Dimers: Multicomponent Fluorescence Decay Curves, Concentration Dependence, and Alkyl End-Group Effect on Excimer Formation

The dynamic process of intramolecular excimer formation in meso and racemic 4,6-di-phenylnonanes (PS2(propyl», diastereoisomeric dimer models of polystyrene, was investigated by using a picosecond pulse radiolysis technique. In dilute cyclohexane solution, the monomeric fluorescence of racemic PS2(propyl) decays single exponentially with a time constant of 11 ns, while that of meso PS2(propyl) was found to decay dual exponentially with time constants of 6 and 0.5 ns. These multicomponent fluorescence decays were found to be explicable by the local motion of conformational change. In conclusion, each time required for the conformational change (g⁺t/tg^-) → tt, (g⁺t/tg^-) ₒ g⁺g^-, and g⁺g^- → (g⁺t/tg^-) in meso PS2(propyl) is estimated to be 0.7, 2.5 and 6 ns, respectively, at room temperature. The proposition that fluorescence decay of diastereoisomeric dimers is influenced by molecular motion (conformational change) is also valid for explaining the profile of the transient fluorescence spectra of racemic PS2(propyl). The present result is an indication that multicomponent fluorescence decay observed in some polymer systems may be mainly due to the conformational change motion in meso dyads. The concentration dependence of the time constants of meso and racemic PS2(propyl) was also measured. In addition, an alkyl end-group effect on intramolecular excimer formation in styrene dimers is found to exist by comparing time constants of styrene dimers with methyl ends, propyl ends, and pentyl ends.