Intramolecular excimer formation of diastereoisomeric model compounds of polystyrene in fluid solution. Their local molecular motion and photophysical properties

The dynamic process of intramolecular excimer formation in diastereoisomeric oligomers [dimers: meso (m)- and racemic (r)-4,6-diphenylnonane (PS2), trimers: mm-, mr- and rr-4, 6,8-triphenylundecane (PS3), and tetramers: mmm-, mmr-, mrm-, rrm-, and rrr-4, 6,8, 10-tetraphenyltridecane (PS4)], model compounds of polystyrene, was investigated by using a picosecond pulse radiolysis technique. Monomer fluorescence of all-racemic isomers (r PS2, rr PS3, rrr PS4) and rrm PS4 decays single-exponentially, while that of other isomers decays dual-exponentially. Multicomponent fluorescence decay curves are supposed to be mainly induced by conformational changes. The results suggest that the excimer in oligostyrenes (or polystyrene) is formed mainly in meso diad. It is proved that there exists singlet energy migration in styrene trimer and tetramer systems. The conformational change in PS3 and PS4 is concluded to occur by cooperative motions in backbone chains bond such as a crankshaft transition, but not by way of independent rotation around each carbon-carbon bond of the backbone chain.