Internal dynamics of poly(methylphenylsiloxane) chains as revealed by picosecond time resolved fluorescence

The dynamics of linear polymethylphenylsiloxane chains in dilute methylcyclohexane solution was probed with picosecond time-resolved fluorescence. Experiments were performed, for one monodisperse sample with an average number of skeletal bonds, equal,to 25, at temperatures covering a wide range (193-293 K). Triple exponential decays were observed at the monomer and excimer emission wavelengths. The three relaxation times were interpreted and full analyzed, on the basis of a kinetic scheme, which involves three kinetically coupled species in the excited state: the excimer (E) and two different types of monomers (M-nh and M-h). The transition of these monomers to excimer occurs at different rates, M-nh by a fast transition (k(a)), and M-h by a slower transition (k(u)). Molecular dynamics simulations for the approach of two chromophores to the excimer configuration suggest that there are two time regimes that can be ascribed to these transitions. The fast one to unrestricted motions controlled just by local bond rotations at the level of a single dyad, and the slower one to retarded motions in which the local bond rotations of the dyad occur only after a delay time caused by the coupling of the dyad to the attached chain. The corresponding to theoretical reciprocal, relaxation times are in qualitative agreement with the experimental relative values of k(a) and k(u). These results reveal that the dynamics of dyads is influenced by the rest of the backbone, something that can be responsible for the generally complex excimer formation kinetics in polymers. The rates and activation energies of these two transition modes of the chain were measured: Many of the Si-O-Si double (synchronized) rotations leading to the approach of two neighbor phenyl rings to the close distance excimer configuration occur fast, as in a single diad, with k(8)(20 degreesC) = 1.4 x 10(10) s(-1) and E-a = 2.2 kcal mol(-1), but a few suffer a lag (like frozen in the nonexcimer configuration), due to retardation imposed by the polymer, giving the slower rate ku(20 degreesC) = 1.2 x 10(9)s(-1) and E-u = 5.6 kcal mol(-1). The fractions of "frozen" monomers, beta = 0.04, of ground-state dimers, alpha = 0.05, and the rate of energy transfer between "frozen" neighbor phenyl rings, k(t) = 5.6 x 10(8) s(-1), were also measured. Steady state fluorescence results are accurately reproduced by using the proposed kinetic scheme and the parameters evaluated from time-resolved results.