Local Molecular Motion of Polystyrene Model Compounds Measured by Picosecond Pulse Radiolysis. 2. Multicomponent Fluorescence Decay Curves of Diastereomeric Styrene Trimers

The dynamics of intramolecular excimer formation in isotactic (mm), syndiotactic (rr), and heterotactic (mr) 4,6,8-triphenylundecanes (PS3), which are diastereomeric trimer model compounds of polystyrene, were investigated by picosecond pulse radiolysis. The monomeric fluorescence of rr PS3 decays single exponentially with a time constant of 7.4 ns, while those of mm PS3 and mr PS3 decay dual exponentially with time constants of ~0.5 and 4 ns. The multicomponent fluorescence decays are influenced by local molecular motion (conformational change). The results suggest that the phenyl groups do not rotate independently around the carbon-carbon bonds of the backbone but that their rotation involves a cooperative motion of backbone chain bonds. The existence of singlet energy migration is established by the fact that the monomeric fluorescence decay curve of mr PS3 does not have as long a decay constant as the curves of the racemic isomers r PS2 and rr PS3 have.