Effects of added CO₂ on the conformation of pyrene end-labeled poly(dimethylsiloxane) dissolved in liquid toluene

We report on the tail-tail cyclization and unfolding kinetics of poly(dimethylsiloxane) that is end-labeled with pyrene (Py-PDMS-Py) when it is dissolved at low concentration in liquid toluene (a good solvent) as a function of added CO₂ (0-204 bar). The pyrene excimer emission provides information on the PDMS tail-tail cyclization and unfolding kinetics and the conformation of.the polymer chains. Under the aforementioned conditions, the Py-PDMS-Py excimer emission is entirely intramolecular in nature; there is no evidence for any inter- or intramolecular ground-state preassociation of the pyrene residues. However, the pyrene excimer-to-monomer intensity ratio (E/M) increases by ~5-fold as we increase the CO₂ pressure from 0 to 70 bar. E/M begins to decrease gradually as the CO₂ pressure is increased above 70 bar. Timeresolved fluorescence spectroscopy reveals three important points. First, the rate of Py-PDMS-Py tail-tail unfolding (Awoiding) is essentially independent of added CO₂. Second, the rate that describes the intramolecular Py-PDMS-Py tail-tail cyclization (k_cyclization) increases 5-6-fold between 0 and ∼90 bar CO₂. Above ∼90 bar CO₂, A_cyciization decreases with increasing CO₂ pressure. Finally, the apparent excited-state equilibrium constant (K* = k_cyclization/k_unfolding) increases with added CO₂ up to ∼90 bar and then decreases above 90 bar. The independence of k_unfolding on adding CO₂ suggests that this rate coefficient reports on a local process that is not influenced to any significant extent by chain conformation or the viscosity of the medium. The large change in K* argues that the addition of CO₂ affects the PDMS chain cyclization probability which is a manifestation of changes in chain conformation brought on by the addition of CO₂. Together these results show that the addition of CO₂ to liquid toluene (up to ∼90 bar) results in a systematic decrease in the mean free distance between the pyrene-labeled PDMS termini. This change in the mean free tail-tail distance is consistent with an excluded volume argument which is in line with Monte Carlo simulations and small-angle neutron scattering experiments. Above ∼90 bar CO₂. the mean free distance between the pyrene-labeled termini begins to increase. This arises from an increase in the solvent quality with increasing CO₂ density.