Dynamics of siloxane chains bearing phenyl chromophores

Molecular dynamics simulations are used to model the conformational motions associated with transitions between rotational states which may be respons ible for the coupling between chromophores (excimer forming states) in the chains of poly(methylphenylsiloxane). Analysis of the trajectories show tha t the rotations around backbone bonds occur much faster than the coupling-u ncoupling between the lateral chromophores, Although fast, the rotations ar ound backbone bonds are strongly coordinated, each pair of Si-O-Si bonds ro tating in a concerted fashion, such that the lateral phenyl groups (attache d to the Si atoms) remain coupled over long times (many backbone rotations) . The quantitative assessment of these results is established by calculatin g the correlation times for the backbone rotational angles (Si-O), for the distance separating chromophores (Ph ... Ph), and for the lateral Si-Ph bon ds. The relevance for the kinetics of excimer emission is discussed. (C) 19 99 Society of Chemical Industry.