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(dimet hylsiloxane) that is end-labeled with pyrene (Py-PDMS-Py) when it is dissol ved at low concentration in liquid toluene (a good solvent) as a function o f added CO2 (0-204 bar). The pyrene excimer emission provides information o n the PDMS tail-tail cyclization and unfolding kinetics and the conformatio n of the polymer chains. Under the aforementioned conditions, the Py-PDMS-P y excimer emission is entirely intramolecular in nature; there is no eviden ce for any inter- or intramolecular ground-state preassociation of the pyre ne residues. However, the pyrene excimer-to-monomer intensity ratio (E/M) i ncreases by similar to 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 abov e 70 bar. Time resolved fluorescence spectroscopy reveals three important p oints. First, the rate of Py-PDMS-Py tail-tail unfolding (k(unfolding)) is essentially independent of added CO2. Second, the rate that describes the i ntramolecular Py-PDMS-Py tail-tail cyclization (k(cyclization)) increases 5 -6-fold between 0 and similar to 90 bar CO2. Above similar to 90 bar CO2, k cyclization decreases with increasing CO? pressure. Finally, the apparent e xcited-state equilibrium constant (K* = k(cyclization)/k(unfolding)) increa ses with added CO2 up to similar to 90 bar and then decreases above 90 bar. The independence of k(unfolding) on adding CO2 suggests that this rate coe fficient reports on a local process that is not influenced to any significa nt extent by chain conformation or the viscosity of the medium. The large c hange in K* argues that the addition of CO2 affects the PDMS chain cyclizat ion probability which is a manifestation of changes in chain conformation b rought on by the addition of CO2. Together these results show that the addi tion of CO2 to liquid toluene (up to similar to 90 bar) results in a system atic decrease in the mean free distance between the pyrene-labeled PDMS ter mini. 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 similar to 90 bar CO2 th e mean free distance between the pyrene-labeled termini begins to increase. This arises from an increase in the solvent quality with increasing CO2 de nsity.