Intramolecular excimer formation of oligostyrenes from dimer to tridecamer: The measurements of rate constants for excimer formation, singlet energy migration, and relaxation of internal rotation

The emission properties of styrene oligomers from dimer to tridecamer have been investigated by picosecond pulse radiolysis at room temperature. The oligomers are precisely fractionated and are considered to be models of atactic polystyrene. The rate constant for excimer formation k_DM of the order of 10⁸ s^-1, increases with increasing number of monomer units n for 2 ≤ n ≤ 8, then levels off at the value for polystyrene. The rate constant for singlet energy migration between adjacent chromophores in polystyrene was determined to be 3×10¹⁰ s^-1 (∼30 ps). An average time of 7.2 ns was found to be necessary for two adjacent chromophores to assume the excimer conformation (g^-t or tg ^- in a racemo dyad; tt in a meso dyad) from their initial equilibrium distribution of conformations. The present result is an indication that one may investigate the dynamics of internal rotation by photophysical techniques. Moreover, from a comparison of the results of photostationary measurements with those of transient methods, it is found that there is a discrepancy between them. This discrepancy is assumed to be caused by a molecular weight dependence either of the rate constant for the radiative deactivation from the excited monomer singlet k_FM or of intramolecular self-quenching.