Dynamics of conformational transitions to isomeric states favoring intramolecular excimer formation in polymeric chains. Application to dimer models of polystyrene

The dynamic rotational isomeric states model, which has been recently proposed to investigate the dynamics of local conformational transitions in polymers, is elaborated to formulate the increase in the number of excimer-forming sites through rotational sampling. The transition rate matrix governing the rate of passage between the subsets {ø}_a and {ø}_b, consisting of excimer-forming and non-excimer-forming conformations, respectively, is modified such that microstates in {ø}_b, are nonrecurrent, in contrast to the stationary process of conformational transitions in chains in equilibrium. The theory allows for the calculation of the fraction P¹ (a) of sites that undergo transition to excimer-forming states at least once prior to time t, as well as mean first passage time from {ø}_b, to {ø}_a, as a function of real chain conformational characteristics and structural properties. Application of the model to the meso and racemic dyads in polystyrene confirms the fact that conformational mobility of the chain plays a major role in intramolecular excimer formation. Comparison with experiments demonstrates that the decay of the monomer fluorescence in styrene dimers is predominantly governed by the process of conformational transitions.