ARTICLES Dynamics of Cyclic Methylphenyltrisiloxane in the Picosecond to Nanosecond Time Range Fernando B. Dias, ² Joa ˜ o C. Lima, ² Antonio L. Mac ¸ anita, ²,‡ Arturo Horta,* ,§ and Ine ´ s F. Pie ´ rola § Instituto de Tecnologia Quı ´mica e Biolo ´ gica (ITQB), Oeiras, Portugal, Instituto Superior Te ´ cnico (IST), UTL, Lisboa, Portugal, and Departamento de Fisicoquı ´mica (CTFQ), UniVersidad a Distancia (UNED), Madrid, Spain ReceiVed: August 25, 1999; In Final Form: October 21, 1999 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 °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. Introduction Siloxanes are well-known materials with application in many different fields. Their conformational and dynamic properties have been recently reviewed. 1 Cyclization 2 is an important characteristic of polysiloxanes that has been theoretically and experimentally measured by the cyclization equilibrium constant. Experimental results 2 agree with calculated values (with a modified Monte Carlo method) 3,4 for intermediate and large cycles, but theoretical predictions fail for the smallest cycles as cyclic trisiloxanes. These shortest cycles require particular sequences of mutually interdependent rotational angles that have negligible probability in the rotational isomeric states (RIS) model employed in MC calculations. In fact, the RIS model has been questioned for polysiloxanes. 5-7 With these confor- mational peculiarities it may be of special interest to determine dynamic magnitudes of cyclic trisiloxanes as (1,3,5-triphenyl- 1,3,5-trimethyl-cyclotrisiloxane; CMPS3). This cyclic trimer is very much used as monomer to obtain the polymer by ring- opening polymerization. It has two isomers: cis and trans (Chart 1). 2 Time-resolved fluorescence has been demonstrated to be useful in determining through direct measurements the rate constant of polymer segmental motions occurring in the nanosecond to picosecond time range. 8 Such is the time range ² Instituto de Tecnologia Quı ´mica e Biolo ´gica. ‡ Instituto Superior Te ´cnico. § Universidad a Distancia. CHART 1: Structures of the Isomers (Cis and Trans) of the Cyclic Trimer CMPS3, the Model Dimer DS, and the Model Monomer MS 17 J. Phys. Chem. A 2000, 104, 17-24 10.1021/jp993041u CCC: $19.00 © 2000 American Chemical Society Published on Web 12/09/1999