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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Internal Dynamics of Poly(Methylphenylsiloxane) Chains as Revealed by Picosecond Time Resolved Fluorescence
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Internal Dynamics of Poly(Methylphenylsiloxane) Chains as Revealed by Picosecond Time Resolved Fluorescence

机译：皮秒时间分辨荧光揭示的聚甲基苯基硅氧烷链的内部动力学

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摘要

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 K0. 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 f 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_a(20 deg C) = 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 configuratin), dur to retardation imposed by the polymer, giving the slower rate k)u(20 deg C) = 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.
机译：用皮秒时间分辨荧光探测了在稀甲基环己烷溶液中线性聚甲基苯基硅氧烷链的动力学。在宽范围（193-293 K0）的温度下，对一个平均骨架键数为25的单分散样品进行了实验。在单体和准分子发射波长处观察到三倍指数衰减，解释了三个弛豫时间并在动力学方案的基础上进行了全面分析，其中涉及三种处于激发态的动力学耦合物种：受激准分子（E）和两种不同类型的单体（M_（nh）和M_h）。在不同的速率下，m_（nh）通过一个快速跃迁（k_a），M_h通过一个较慢的跃迁（k_u）两种生色团接近准分子构型的分子动力学模拟表明，可以归结为两种时间模式仅由一个二倍体水平的局部键旋转控制的快速到不受限制的运动，而d的局部键旋转的较慢的从慢速运动到延迟的运动。仅在由于二重体与附连链的偶联引起的延迟时间之后才发生。相应的理论倒数弛豫时间与k_a和k_u的实验相对值在质量上一致。这些结果表明动力学动力学受到其余骨架的影响，这可能是造成聚合物中复杂的准分子形成动力学的原因。测量了链的这两个过渡模式的速率和活化能：导致两个相邻苯环接近近距离受激准分子构型的许多Si-O-Si双（同步）旋转很快发生，例如k_a（20℃）= 1.4 X 10〜（10）s〜（-1）和E_a = 2.2 kcal mol〜（-1）的单二元体，但有一些滞后（如冻结在非准分子配置素中）在聚合物施加的阻滞作用下，给出较慢的速率k）u（20℃）= 1.2 X 10〜9 s〜（-1），E_u = 5.6 kcal mol〜（-1）。基态二聚体的“冻结”单体的分数＃beta＃= 0.04，＃alpha＃= 0.05，以及“冻结”相邻苯环之间的能量转移速率k_t = 5.6 X 10〜8 s〜（ -1），也进行了测量。通过使用拟议的动力学方案和从时间分辨结果中评估的参数，可以准确地再现稳态荧光结果。

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