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 CO 2 (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 CO 2 pressure from 0 to 70 bar. E/M begins to decrease gradually as the CO 2 pressure is increased above 70 bar. Time-resolved fluorescence spectroscopy reveals three important points. First, the rate of Py−PDMS−Py tail−tail unfolding ( k unfolding ) is essentially independent of added CO 2 . Second, the rate that describes the intramolecular Py−PDMS−Py tail−tail cyclization ( k cyclization ) increases 5−6-fold between 0 and ∼90 bar CO 2 . Above ∼90 bar CO 2, k cyclization decreases with increasing CO 2 pressure. Finally, the apparent excited-state equilibrium constant ( K * = k c yclization / k unfolding ) increases with added CO 2 up to ∼90 bar and then decreases above 90 bar. The independence of k unfoldin g on adding CO 2 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 CO 2 affects the PDMS chain cyclization probability which is a manifestation of changes in chain conformation brought on by the addition of CO 2 . Together these results show that the addition of CO 2 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 CO 2, the mean free distance between the pyrene-labeled termini begins to increase. This arises from an increase in the solvent quality with increasing CO 2 density.