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

The dynamics of linear polymethylphenylsiloxane chains in dilute methylcycl ohexane solution was probed with picosecond time-resolved fluorescence. Exp eriments 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 kinetical ly 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 excim er 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 tw o time regimes that can be ascribed to these transitions. The fast one to u nrestricted 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 b ond rotations of the dyad occur only after a delay time caused by the coupl ing of the dyad to the attached chain. The corresponding to theoretical rec iprocal, relaxation times are in qualitative agreement with the experimenta l 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 r esponsible for the generally complex excimer formation kinetics in polymers . The rates and activation energies of these two transition modes of the ch ain were measured: Many of the Si-O-Si double (synchronized) rotations lead ing to the approach of two neighbor phenyl rings to the close distance exci mer 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 (lik e frozen in the nonexcimer configuration), due to retardation imposed by th e 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 g round-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 res ults.