Local Molecular Motion of Polystyrene Model Compounds Measured by Picosecond Pulse Radiolysis. 3. Diastereomeric Styrene Dimer, Trimer, and Tetramer in Poly(methyl methacrylate) Solid Solution

The photophysical properties of diastereomeric oligostyrenes, meso (m) and racemic (r) 4,6- diphenylnonanes (PS2); mm-, mr-, and rr-4,6,8-triphenylundecanes (PS3); and mmm-, mmr-, mrm-, rrm-, and rrr-4,6,8,10-tetraphenyltridecanes (PS4), in poly(methyl methacrylate) (PMMA) were studied by photostationary and transient measurements. The fluorescence spectra of PS2, PS3, and PS4 with meso diads exhibited excimer fluorescence even in PMMA solid solution where conformational changes are restricted, suggesting that the population of tt, an excimer conformer, in the meso diad is noticeable even in low molecular weight compounds such as PS2, PS3, or PS4. Monomer fluorescence of oligostyrenes in PMMA decayed single exponentially with the same time constant as that of cumene. The observed monomer fluorescence is assumed to come from a molecule that has no excimer conformer while a molecule having excimer conformers does not exhibit monomer fluorescence. Fluorescence polarization measurements at 77 K and at room temperature demonstrated that the reorientational motion of phenyl groups was not perfectly restricted in PMMA solid solution. The efficiency of the energy migration among the phenyl rings in PS2, PS3, and PS4 was not so outstanding that it could induce the fluorescence depolarization.