Internal Dynamics of Poly(Methylphenylsiloxane) Chains as Revealed by Picosecond Time Resolved Fluorescence

Abstract

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 a (20 °C) ) 1.4 × 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 k u (20 °C) ) 1.2 × 10 9 s -1 and E u ) 5.6 kcal mol -1 . The fractions of "frozen" monomers, β ) 0.04, of ground-state dimers, R ) 0.05, and the rate of energy transfer between "frozen" neighbor phenyl rings, k t ) 5.6 × 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.

Ion Pairing and Anion-Driven Aggregation of an Ionic Liquid in Aqueous Salt Solutions

“…Emission and Absorption Spectra. Fluorescence spectra of pTSA and [emim][TOS] dissolved in deionized water and in NaCl aqueous solutions show monomer and excimer bands 19,20 that overlap partially (Figure 1); the excimer band becomes evident upon subtraction of the emission of very dilute solutions, which corresponds to only monomer emission. The apparent red shift of the monomer emission (285−295 nm with increasing concentration) is due to the increasing relative intensity of the excimer band, which is centered at 319 nm in any case.…”

“…In contrast to pTSA, the excimer-to-monomer fluorescence ratio of [emim][TOS] solutions does not grow throughout the whole range of chromophore concentrations, but draws a sigmoid and levels off at about 0.5 M. The different dependence on concentration of the two compounds reveals some differences in the mechanism of excimer formation, linked to the cation. Excimers may be formed by diffusion of two p -toluenesulfonate groups through a short distance, or by direct absorption of light by pre-existing dimers or aggregates , and, therefore, the different I E / I M values at the larger concentrations could arise from differences in the diffusion and/or aggregation of the two compounds. Let us consider these two possibilities.…”

Self-Assembly of a Hydrophobically Modified Naphthalene-Labeled Poly(acrylic acid) Polyelectrolyte in Water:Organic Solvent Mixtures Followed by Steady-State and Time-Resolved Fluorescence

“…15,34,35 These were attributed to long-and short-range phenyl interactions. To establish the two types of interactions, one has to be able to differentiate them.…”

“…In their case, [34][35][36] it involves a sequential process where two monomerlike species are directly connected, and the last gives rise to the excimer. We believe that our system behaves differently, although we cannot discard one mechanism relative to the other since phenomenologically the two are indistinguishable.…”

Viscosity Dependence of Intramolecular Excimer Formation with 1,5-Bis(1-pyrenylcarboxy)pentane in Alkane Solvents as a Function of Temperature

“…Examples are 1Py(CH 2 OCH 2 )1Py and 2Py(CH 2 OCH 2 ) n 2Py ( n = 1−5) with their meso/racemic derivatives 1PyCH(CH 3 )OCH(CH 3 )1Py and 2PyCH(CH 3 )OCH(CH 3 )2Py, poly(dimethylsiloxanes), , ologosilanes 1Py[Si(Me) 2 ] n 1Py ( n = 2, 3, 4, 6), 1-pyrenylalanines, , 1Py(CH 2 NHCOCH 2 )1Py, and 1Py(CH 2 ) 10 COO(CH 2 ) 6 1Py . Intramolecular excimer formation has likewise been employed to investigate the end-to-end cyclization dynamics of poly(ethylene oxide) polymers , and polystyrenes substituted at both ends by a 1-pyrenylbutyric acid ester group, as well as of 1-pyrenyl disubstituted poly(methylphenylsiloxane) chains.…”