Dynamics of Cyclic Methylphenyltrisiloxane in the Picosecond to Nanosecond Time Range

Abstract

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.

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.…”

“…This pattern was also found for different kinds of polymers 14 and can also be due to the existence of high-energy barriers separating the two types of monomers. 15 …”

Equilibrium modeling of removal of cadmium ions by olive stones

“…Finally, for the sake of comparison, we mention the copolymers based on CH 3 PhSiO and (CH 3 ) 2 SiO units, the photophysics of which have been extensively studied 6–15. In light of the available data, the photophysics of CH 3 PhSiO‐based copolymers seem to be very similar to those of Ph 2 SiO‐based copolymers.…”

Intramolecular Excimer Formation of the Copolymers Based on Methylphenylsiloxane and Dimethylsiloxane Units in Solution

“…[1][2][3][4][5] It is known that oligomers and polymers containing methylphenylsiloxane (MePhSiO) units exhibit both the monomer and excimer fluorescence through an interaction between the phenyl chromophores and that the intensity ratio of these emission depends on the molar fraction of MePhSiO unit in the copolymers. [6][7][8][9][10][11][12][13] Horta et al have measured fluorescence spectra and decay of a homopolymer, poly(methylphenylsiloxane), and copolymers, poly(methylphenylsiloxane-co-dimethylsiloxane) with three different compositions at different temperatures in dilute organic solvents. 8,11 The intensity ratio of the excimer to monomer fluorescence of the copolymer shows linear Arrhenius plots (i.e., the logarithmic plots of the intensity ratio against the reciprocal of the absolute temperature, 1/T) with a positive or a negative slope depending on the MePhSiO content of the copolymer.…”