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 CO 2

We report on the steady-state and time-resolved fluorescence from poly(dimethylsiloxane) oligomers that have been end-labeled with pyrene (Py-PDMS-Py) when it is dissolved in supercritical CO 2 between a reduced density (ρ r = ρ experiment /ρ critical ) of 0.7 and 2.3. In a good solvent like liquid toluene, the Py-PDMS-Py excimer formation follows a classic Birks scheme with a homogeneous ground state. In supercritical CO 2 there are at least two forms of Py-PDMS-Py (constrained and unconstrained) in the ground state prior to photoexcitation. The constrained species arises from intramolecular hydrogen bonding between the opposing peptide segments within the Py-PDMS-Py terminal region that were introduced during labeling. This ground-state, hydrogen-bonded species precludes normal excimer formation. The unconstrained species leads to classic excimer formation. The constrained/intramolecularly hydrogen-bonded species dominates at lower CO 2 densities (86 ± 2% contribution at ρ r = 1.40); however, its dominance wanes as the CO 2 density increases (14 ± 2% contribution at ρ r = 1.95). There is also evidence that the known changes in the PDMS chain correlation length with CO 2 density influence the Py-PDMS-Py tail−tail dynamics. The mean-free distance between the Py-PDMS-Py termini also depends on the CO 2 density. At low CO 2 densities, the Py residues on the unconstrained Py-PDMS-Py molecules are closer together on average than they are at higher CO 2 densities. However, in supercritical CO 2, 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 continuous space Monte Carlo simulations. Our results are also consistent with the presence of upper and lower critical solution pressures/densities for dilute PDMS dissolved in supercritical CO 2 .