Dynamics of cyclic methylphenyltrisiloxane in the picosecond to nanosecond time range

The dynamics of the cyclic trimer of methylphenyl-substituted siloxane (1,3,5-triphenyl-1,3,5-trimethyl-cyclotrisiloxane; CMPS3) in dilute methylcyclohexane solution was probed with picosecond time-resolved and steady-state fluorescence in a wide range of temperatures (20 to -100 degrees C) from the high-temperature limit to the low-temperature limit. The crossover between these two regimes is found around -30 degrees 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 x 10(9) s(-1) at 20 degrees 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 (similar to 0.23) in total agreement with the fraction of trans phenyl rings (0.23 determined from H-1 NMR) and also in giving a rate constant for excited monomer energy transfer independent of temperature.