Effects of density on the intramolecular hydrogen bonding, tail-tail cyclization, and mean-free tail-to-tail distances of pyrene end-labeled poly(dimethylsiloxane) oligomers dissolved in supercritical CO2

We report on the steady-state and time-resolved fluorescence from poly(dime thylsiloxane) oligomers that have been end-labeled with pyrene (Py-PDMS-Py) when it is dissolved in supercritical CO2 between a reduced density (rho ( r) = rho (experiment)/rho (critical)) of 0.7 and 2.3. In a good solvent lik e liquid toluene, the Py-PDMS-Py excimer formation follows a classic Birks scheme with a homogeneous ground state. In supercritical CO2 there are at l east two forms of Py-PDMS-Py (constrained and unconstrained) in the ground state prior to photoexcitation. The constrained species arises from intramo lecular hydrogen bonding between the opposing peptide segments within the P y-PDMS-Py terminal region that were introduced during labeling. This ground -state, hydrogen-bonded species precludes normal excimer formation. The unc onstrained species leads to classic excimer formation. The constrained/intr amolecularly hydrogen-bonded species dominates at lower CO2 densities (86 /-2% contribution at rho (r) = 1.40); however, its dominance wanes as the C O2 density increases (14 +/-2% contribution at rho (r) = 1.95). There is al so evidence that the known changes in the PDMS chain correlation length wit h CO2 density influence the Py-PDMSPy tail-tail dynamics. The mean-free dis tance between the Py-PDMS-Py termini also depends on the CO2 density. At lo w CO2 densities, the Py residues on the unconstrained Py-PDMS-Py molecules are closer together on average than they are at higher CO2 densities. Howev er, in supercritical CO2, the unconstrained Py-PDMS-Py termini are never as far apart, on average, as they are when Py-PDMS-Py is dissolved in a good solvent like liquid toluene. This result is consistent with previous contin uous space Monte Carlo simulations. Our results are also consistent with th e presence of upper and lower critical solution pressures/densities for dil ute PDMS dissolved in supercritical CO2.