Dynamics of siloxane chains bearing phenyl chromophores

Horta, Arturo; Maçanita, António L.; Freire, Juan J.; Piérola, Inés F.

doi:10.1002/(sici)1097-0126(199908)48:8<665::aid-pi208>3.0.co;2-#

Cited by 3 publications

(17 citation statements)

References 4 publications

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“…Previous results on the distribution of rotational angles for the backbone bonds in PMPS have shown that all values of the rotational angle have a significant probability . Although the distribution function has maxima close to the traditional states t , g + , and g - , the valleys in between are at a considerable height . This poses a difficulty for the definition of transitions between states, as we shall see when analyzing the results of the MD trajectories.…”

Section: Introductionmentioning

confidence: 77%

“…The rotational angle distribution is a rather smooth function, which nowhere vanishes, so that a nonzero baseline remains between the peaks . As an illustration of this baseline, we give, in Table , the distribution at regular intervals of the rotational angle.…”

Section: Resultsmentioning

confidence: 99%

“…This calculation is not so straightforward here as it is in C−C chains, because the conformational states of the siloxanes are not sharply defined and, therefore, no clear jumps between them serve to define transitions. Previous results on the distribution of rotational angles for the backbone bonds in PMPS have shown that all values of the rotational angle have a significant probability . Although the distribution function has maxima close to the traditional states t , g + , and g - , the valleys in between are at a considerable height .…”

Section: Introductionmentioning

confidence: 85%

“…The kinetics of excimer formation between two consecutive chromophores (diad) is usually analyzed considering a one bond rotation of the backbone from a nonexcimer conformation to an excimer-forming one. However, in PMPS the two backbone bonds of the diad rotate in a coordinated fashion, such that the lateral groups are not displaced from each other at the same rate as the individual bonds rotate . So, the time needed for a bond rotation is roughly 1 order of magnitude shorter than the time needed for the approach of two phenyls to an excimer-forming conformation .…”

Section: Introductionmentioning

confidence: 99%

“…However, in PMPS the two backbone bonds of the diad rotate in a coordinated fashion, such that the lateral groups are not displaced from each other at the same rate as the individual bonds rotate . So, the time needed for a bond rotation is roughly 1 order of magnitude shorter than the time needed for the approach of two phenyls to an excimer-forming conformation . This poses another difficulty for the analysis of the transitions in the simulation.…”

Section: Introductionmentioning

confidence: 99%

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Molecular Dynamics of Methylphenylsiloxane Chains

2000

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Molecular dynamics is used to analyze the motions of backbone and lateral groups that lead to the formation and dissociation of excimer-forming conformations in methylphenylsiloxane. Several structures are studied: two fragments of linear poly(methylphenylsiloxane) with 14 (P14) and six (P6) monomer units, the cyclic trimer (C3) in its two configurations, cis and trans, and the disiloxane (DS) as a model for a single diad. It was found that the distribution of distances between C 1 atoms in a diad of any structure and the corresponding rotational angle distribution do not show the clear distinctions between different states that are typical of RIS models. Nevertheless, excimer-forming sites can be defined as diad conformations with distances between neighbor C1 atoms below 4.2 Å, which correspond to ample ranges of the rotational angles (0-110°in DS), to angles between phenyl rings of about 15°with a certain staggering of the aromatic groups, and to angles between Si-C1 bonds in neighbor rings below 45°. This conformation of the excimer-forming site is very different from the one observed in hydrocarbon analogous polymers. Large differences can also be observed in the set of movements necessary to form excimer sites in both types of polymers. In DS, P14, and P6, the two bonds of a diad rotate in a coordinated way, keeping the sum of dihedral angles about constant. Changes of φ 1 + φ2 from any value to 30°are accompanied by changes in the angle between phenyl rings from around 90°to 15°and by the approach of the two chromophores to a distance smaller than 4.2 Å. During this time, φ1 and φ2 may suffer more than 20 random changes. These complex chain motions are quite different from the simple rotation through a backbone bond where either φ1 or φ2 changes by 120°, which is the type of motion usually associated with the excimer formation in polystyrene-like chains. Transitions to form excimer sites are also discussed in term of time-dependent distributions of the conformational variables. The frequency of transitions between excimer sites and the other conformational states is larger for C3 and for DS than for P14 and P6, and calculated relative values of the rate constant for excimer formation are in agreement with photophysical experimental results. The distribution of transition times can also help to interpret the existence of a fraction of isolated chromophores in the polymer.

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Section: Introductionmentioning

confidence: 77%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 85%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Molecular Dynamics of Methylphenylsiloxane Chains

2000

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

Lima

Horta

2001

J. Phys. Chem. A

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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.

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Dynamics of Cyclic Methylphenyltrisiloxane in the Picosecond to Nanosecond Time Range

et al. 1999

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The dynamics of the cyclic trimer of methylphenyl-substituted siloxane (1,3,5-triphenyl-1,3,5-trimethylcyclotrisiloxane; CMPS3) in dilute methylcyclohexane solution was probed with picosecond time-resolved and steady-state fluorescence in a wide range of temperatures (20 to -100 °C) from the high-temperature limit to the low-temperature limit. The crossover between these two regimes is found around -30 °C. Monomer and excimer decays are triexponential, with one of the three components coming from the monomer that is unable to form excimer with its neighboring chromophores (the lone phenyl ring in the trans isomer of CMPS3). A kinetic mechanism is developed that takes into account preformed dimers, lone monomers, and also energy transfer from these lone monomers to excimer-forming ones. With such a mechanism, the rate constants for excimer formation (k a ) and excimer dissociation (k d ), as well as the corresponding activation energies (E a , E d ), are obtained from the decays. The rate constants are high (k a ) 13.7 × 10 9 s -1 at 20 °C) and the activation energies are low (E a ) 2.2 kcal mol -1 ) compared with C-C molecules; however, their values for this small cycle are very similar to those for long linear chains of poly(methylphenylsiloxane). Thus, although the cycle is somewhat strained and has a greater fraction of isolated monomers and a smaller fraction of preformed dimers than the linear polymer, the main factor that determines excimer kinetics in both types of structures is their common conformational flexibility of the siloxane backbone. The kinetic mechanism developed succeeds in giving a fraction of photophysically hindered monomers (∼0.23) in total agreement with the fraction of trans phenyl rings (0.23 determined from 1 H NMR) and also in giving a rate constant for excited monomer energy transfer independent of temperature.

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Dynamics of siloxane chains bearing phenyl chromophores

Cited by 3 publications

References 4 publications

Molecular Dynamics of Methylphenylsiloxane Chains

Molecular Dynamics of Methylphenylsiloxane Chains

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

Dynamics of Cyclic Methylphenyltrisiloxane in the Picosecond to Nanosecond Time Range

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