Effects of Added CO₂ on the Conformation of Pyrene End-Labeled Poly(dimethylsiloxane) Dissolved in Liquid Toluene

Abstract

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. Timeresolved 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 cyclization /k unfolding ) increases with added CO 2 up to ∼90 bar and then decreases above 90 bar. The independence of k unfolding 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.

Synthesis, spectroscopy, electrochemistry, and coordination chemistry of substituted phosphine sulfides and selenides

“…The fact that well-defined vibrational peaks (i.e., peaks I, II, IV, and V) can still be observed for the pyrene substituted at the 1-position is unusual. 40,41 Indeed, the analogous pyrenyl phosphine oxide studied previously 18 showed an emission profile more typical of 1-substituted pyrenyl compounds (i.e., two well-resolved monomeric bands near 375 nm and 400 nm). We note that the relative fluorescence intensity of the sulfide analog (4) was twice that of the selenide analog (8), for an identical absorbance at the excitation wavelength (325 nm).…”

Temperature-Dependent Tail−Tail Dynamics of Pyrene-Labeled Poly(dimethylsiloxane) Oligomers Dissolved in Ethyl Acetate

“…The normalized steady-state emission wavelength-dependent excitation spectra provide a convenient diagnostic to determine the origin of the observed excimer. [30][31][32]41 If the ground state is homogeneous and the excimer is formed dynamically in the excited state, then all of the normalized emission wavelength-dependent excitation spectra will be superimposable. However, if the ground state is heterogeneous, as would be the case for Py-residue preassociation, then the spectra will not be superimposable.…”

“…g Binding energy for excimer formation. [30][31][32] that PDMS backbone rotation is not the limiting barrier to cyclization in this system. We estimate an internal activation barrier for Py-PDMS-Py in ethyl acetate that is in line with the internal barrier rotation for PDMS backbone rotation.…”

“…In this paper we use steady-state and time-resolved fluorescence spectroscopy to investigate the tail-tail dynamics and activation energies of linear poly(dimethylsiloxane) (PDMS) polymers that have been end-labeled with pyrene (Py-PDMS-Py (M n ) 3100; M w /M n ) 1.07), Figure 1) [30][31][32] in ethyl acetate (a theta solvent for PDMS) between 255 and 323 K. The upper critical solution theta temperature (θ u ) for PDMS in ethyl acetate is 278 K. 33 This particular system was selected for investigation because the internal barrier to backbone rotation is about 1 order-of-magnitude smaller for PDMS in comparison to PS (<2.5 kJ/mol 34 vs >21 kJ/mol 35 ). We have also studied the Py-PDMS-Py system in pure supercritical CO 2 and CO 2 -dilated toluene (at good PDMS solvent), ethyl acetate (a theta solvent for PDMS), and methanol (a poor solvent for PDMS).…”

“…We have also studied the Py-PDMS-Py system in pure supercritical CO 2 and CO 2 -dilated toluene (at good PDMS solvent), ethyl acetate (a theta solvent for PDMS), and methanol (a poor solvent for PDMS). [30][31][32] Theory Section Figure 1 presents a simplified energy-level diagram describing the behavior of a flexible molecule like Py-PDMS-Py undergoing tail-tail cyclization (i.e., intramolecular excimer formation) within the idealized Birks framework. 2,10,28 Here, isolated Py-PDMS-Py molecules (1) absorb a photon (hν ex ) to produce excited-state (*) monomers (2).…”