Effects of Added CO2 on the Conformation of Pyrene End-Labeled

Wendy E. Gardinier, Gary A. Baker, Sheila N. Baker, and Frank V. Bright. Macromolecules 2005 38 (20) ... Chase A. Munson, Phillip M. Page, and Frank V...
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J. Phys. Chem. B 2000, 104, 8585-8591

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Effects of Added CO2 on the Conformation of Pyrene End-Labeled Poly(dimethylsiloxane) Dissolved in Liquid Toluene Maureen A. Kane, Gary A. Baker, Siddharth Pandey, E. Peter Maziarz III, David C. Hoth, and Frank V. Bright* Department of Chemistry, Natural Sciences Complex, UniVersity at Buffalo, The State UniVersity of New York, Buffalo, New York 14260-3000 ReceiVed: April 4, 2000; In Final Form: June 27, 2000

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 CO2 (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 CO2 pressure from 0 to 70 bar. E/M begins to decrease gradually as the CO2 pressure is increased above 70 bar. Timeresolved fluorescence spectroscopy reveals three important points. First, the rate of Py-PDMS-Py tail-tail unfolding (kunfolding) is essentially independent of added CO2. Second, the rate that describes the intramolecular Py-PDMS-Py tail-tail cyclization (kcyclization) increases 5-6-fold between 0 and ∼90 bar CO2. Above ∼90 bar CO2, kcyclization decreases with increasing CO2 pressure. Finally, the apparent excited-state equilibrium constant (K* ) kcyclization/kunfolding) increases with added CO2 up to ∼90 bar and then decreases above 90 bar. The independence of kunfolding on adding CO2 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 CO2 affects the PDMS chain cyclization probability which is a manifestation of changes in chain conformation brought on by the addition of CO2. Together these results show that the addition of CO2 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 CO2, the mean free distance between the pyrene-labeled termini begins to increase. This arises from an increase in the solvent quality with increasing CO2 density.

Introduction In polymer systems, there are a variety of dynamical events that occur on the nanosecond or faster time scale.1-8 In dilute solution, polymer chain conformation and chain/residue dynamics also depend on the physicochemical properties of the solvent.9 For example, experimental work has shown that polymer tail-tail cyclization and unfolding kinetics are governed by the polymer type and chain length,10-15 solvent,10,13,16,17 temperature (e.g., above or below the θ temperature),15-17 and hydrostatic pressure.18,19 Polymer tail cyclization dynamics in dilute liquids have also been investigated theoretically.20-24 In this paper we aim to determine how the addition of CO2 influences the tail-tail dynamics of poly(dimethylsiloxane) (PDMS) dissolved at low concentration in liquid toluene. CO2 was investigated as an additive here because it is an environmentally responsible solvent25-31 and it has been used as an anti-solvent for the controlled precipitation of polymers from solution.32-34 Toward these ends, we have selectively labeled the PDMS chain termini with the fluorescent probe pyrene (PyPDMS-Py, Figure 1). We use steady-state and time-resolved * Corresponding author. Phone: (716) 645-6800, ext. 2162 (voice). Fax: (716) 645-6963. E-mail: [email protected].

Figure 1. Chemical structure of pyrene end-labeled PDMS.

fluorescence spectroscopy to investigate the pyrene monomer and excimer emission, and we determine the Py-PDMS-Py tail-tail cyclization and unfolding kinetics as a function of added CO2.35,36 Theory Section Inter- and intramolecular excimer emission are often modeled within a Birks framework (Figure 2).35,36 For a simple intramolecular excimer, all the rate coefficients shown in Figure 2 are unimolecular. Following electronic excitation by a short pulse of electromagnetic radiation, Figure 2 predicts that the monomer time-resolved fluorescence intensity (IM(t)) will decay as the sum of two exponentials, the excimer time-resolved emission intensity (IE(t)) will decay as the difference between

10.1021/jp001301y CCC: $19.00 © 2000 American Chemical Society Published on Web 08/09/2000

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Figure 2. Birks scheme for the formation of an intramolecular excimer. Symbols represent: kcyclization, unimolecular rate for tail-tail cyclization; kunfolding, unimolecular rate for tail-tail unfolding; kM, unimolecular decay rate for the pyrene monomer residue; and kE, unimolecular decay rate for the intramolecular pyrene excimer.

two exponentials, and the decay constants (λi) are identical for the IM(t) and IE(t) decay profiles.

IM(t) ) a1 exp(-λ1t) + a2 exp(-λ2t)

(1)

IE(t) ) a3 [exp(-λ1t) - exp(-λ2t)]

(2)

Although numerous methods exist for recovering the desired kinetic terms (Figure 2) from IM(t) and IE(t), we have opted to use a global analysis strategy.37 In this approach, the monomer and excimer decay profiles are simultaneously fit to a single model (e.g., Figure 2) such that a self-consistent set of kinetic terms are fit for and recovered. We use the global analysis strategy here because it provides a much more rigorous set of fitting criteria, it yields all the kinetic parameters directly, and the accuracy of the recovered kinetic terms is improved significantly because orthogonal data sets are being modeled simultaneously.37 Experimental Section Chemicals and Reagents. Aminopropyldimethyl-terminated poly(dimethylsiloxane) (NH2-PDMS-NH2) of 2500 g/mol average molecular weight (Mn) was purchased from United Chemical Technologies. 1-Pyrenebutanoic acid succinimidyl ester and 1-ethylpyrene were purchased from Molecular Probes, Inc. Anhydrous toluene (HPLC grade, 99.8+%) was a product of Sigma-Aldrich. CO2 (SFC grade) and Ar (99.9%) were from Scott Specialty Gases. All chemicals and reagents were used as received. Instrumentation. A home-built gel permeation chromatography (GPC) system with variable wavelength UV-Vis (λ ) 336 nm) and refractive index (RI) detectors placed in series was used for isolating the desired product (Py-PDMS-Py) from side products and starting materials. Three 7.8 mm ID × 30 cm long GPC columns (American Polymer Standard; Cat. Nos. AM Gel 104/10, AM Gel 500/10, and AM Gel 100/10) were used in series to effect the separation. The crude reaction mixture was passed through a 0.2 µm Teflon membrane filter prior to injection into the GPC. The GPC mobile phase was toluene, and the flow rate was 1.0 mL/min. The UV-Vis detector served to follow the pyrene chromophore, and the RI detector was used to detect the polymer. When both detectors registered a signal, this marked the presence of labeled PDMS (i.e., Py-PDMS-Py or Py-PDMS-NH2). Fractions (cuts)

were collected at the appropriate time to isolate Py-PDMSPy from the side products and starting materials. UV-Vis absorbance data were acquired using a Milton-Roy model 1201 and 1-cm2 quartz cuvettes. Infrared spectra were recorded on a Perkin-Elmer model 1760 FT-IR by coating the GPC-purified product onto a NaCl window. All matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) data were acquired with a PerSeptive Biosystems model DE-STR linear time-of-flight mass spectrometer, operating in the reflector mode. Ions were formed by laser desorption using a nitrogen laser (337 nm, 3 ns pulse width, 106 W/cm2, 100 mm beam diameter). The acceleration voltage was kept at 25 kV for all experiments. During the ionization process, a delay time of 150 ns was applied before acceleration and the grid and guide wire voltages were set to 70.0% and 0.010% of the applied acceleration voltage, respectively, to focus the ion beam. Typically, 256 laser shots were averaged for each spectrum. We used dithranol or 2,5-dihydroxybenzoic acid as the ionization matrixes. Ionization matrix solutions were prepared at 20 mg/mL in THF with 1% (v/v) LiCl. The matrix solution and each sample were mixed 1:1, and 1 µL of this mixture was manually spotted onto the sample plate. Data were collected with the laser power adjusted to just above the matrix ionization threshold to minimize fragmentation and maximize resolution. Steady-state excitation and emission experiments were performed with an SLM-AMINCO model 48000 MHF fluorometer (Spectronic Instruments) using a 450 W xenon arc lamp as the excitation source and single grating monochromators as wavelength selection devices. All emission and excitation spectra were background corrected using appropriate blanks. Timeresolved fluorescence measurements were carried out by using an IBH model 5000W SAFE time-correlated single photon counting fluorescence lifetime instrument. The excitation source was a N2-filled flashlamp, operating at a 40 kHz repetition rate. The excitation wavelength was set to 337 nm and the excitedstate fluorescence intensity decay data were acquired at the desired emission wavelengths. Single grating monochromators were used for wavelength selection. All decay profiles were recorded under magic angle polarization conditions. The typical time resolution for an experiment was 0.47 ns/channel, and we used 1024 total channels within the multichannel analyzer (MCA). To avoid pulse pile-up, the count rate at the reference and emission detectors was always less than 2% of the flash lamp repetition rate. Data were acquired until there were at least 104 counts in the peak MCA channel. The instrument response and the monomer and excimer decay data were acquired simultaneously. All data analysis was performed using a commercial software package from Globals Unlimited. All reported results represent the average of at least three discrete experiments performed on different days. The high-pressure system consists of an Isco model 260-D microprocessor-controlled syringe pump that is purged and charged with CO2, stainless steel tubing and plumbing, a highpressure optical cell with fused silica optical windows and a stainless steel cell body,38 a home-built temperature controller ((0.1 °C), and a pressure monitoring system ((0.2 bar). The high-pressure cell internal volume is 3.5 mL. For these experiments the cell is charged with 3.000 mL of toluene. Synthesis of Py-PDMS-Py. NH2-PDMS-NH2 was reacted in anhydrous toluene with a 10-fold molar excess of

CO2 Effects on Conformation of Py-PDMS-Py

Figure 3. Typical MALDI-TOF spectra of the Py-PDMS-Py reaction product following a GPC cleanup step. The lower section shows the entire oligomer distribution associated with the GPC fraction. The upper section shows an expanded view and illustrates where unlabeled (b) and singly labeled (O) PDMS would be expected to appear. No unlabeled or single labeled PDMS is detectable.

1-pyrenebutanoic acid succinimidyl ester for 24 h with stirring in the dark under a N2 atmosphere. Product Confirmation. To determine the efficiency of pyrene end-labeling prior to executing our fluorescence experiments, we performed a series of MALDI-TOF MS experiments. Before discussing these data, it is instructive to bear in mind that free amines (such as those found in NH2-PDMS-NH2) are effective nucleophiles toward the central carbonyl (position 9) of dithranol. In our hands, NH2-PDMS-NH2 yielded a strongly colored (brown) imine adduct with the dithranol matrix. Although only a qualitative indicator, the lack of a visually detectable colored product for our GPC-purified product when it was mixed with the dithranol matrix suggests little free amine in our GPC-purified samples. Figure 3 presents a typical MALDI-TOF MS spectrum for GPC-purified Py-PDMS-Py using a dithranol matrix. A similar spectrum was recorded when we used 2,5-dihydroxybenzoic acid as the matrix (data not shown). The quasimolecular lithium adducts for the PDMS oligomers with a 74 Da repeat unit are evident. In the expanded spectrum (top) we mark the expected positions for Py-PDMS-NH2 (b) and NH2-PDMSNH2 (O) species. If either of these undesirable species is present in the GPC-purified sample, their concentration falls within the baseline noise. A representative MALDI-TOF MS spectrum displaying fully resolved isotopic peaks for the Py-PDMSPy 32-mer is illustrated in Figure 4. The resolving power (m/ ∆m50%) achieved in this spectrum is approximately 6880. The theoretical isotopic distribution (O) correlates remarkably well with the experimental isotopic profile. In fact, the experimental di-isotopic mass (3015.5282 Da) agrees within 144 ppm of the theoretical di-isotopic mass (3015.9628 Da). Additional work using PDMS and 1-ethylpyrene standard solutions in concert with UV-Vis and RI detection confirmed that the molar ratio of Py to PDMS in the GPC isolate was 1.95 ( 0.05. FT-IR spectra (not shown) of the GPC-purified samples did not exhibit any peak for the amine group; only the peptide bound was detectable. All these results argue that we have mostly Py-PDMS-Py in our samples. Sample Preparation for Fluorescence Studies. All PyPDMS-Py solutions were freeze-pump-thaw degassed four times to remove dissolved O2 which would quench the pyrene emission. All sample transfer operations were performed within an Ar-purged drybox (Vacuum Atmospheres Company, model no. HE-43-2) to ensure that O2 did not contaminate the sample

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Figure 4. Typical high-resolution MALDI-TOF spectra for one particular Py-PDMS-Py oligomer showing the actual isotope distribution profile along with the predicted profile (O). The agreement between the actual spectrum and the prediction for a Li+/Py-PDMS-Py oligomer is excellent.

Figure 5. Effects of added CO2 on the Py-PDMS-Py emission in toluene at 35 °C. (Panel A) Normalized emission spectra of 1-PyPDMS-1-Py dissolved in toluene at 0, 34.0, 68.0, and 204.1 bar CO2. The spectrum for 1-ethylpyrene dissolved in neat toluene is also shown. (Panel B) Py-PDMS-Py excimer-to-monomer intensity ratio as a function of added CO2 (b) or Ar (O). λex ) 320 nm. ∆λem ) 2 nm.

in the high-pressure optical cell. The lack of any significant O2 contamination was confirmed by comparing the Py-PDMSPy emission spectra in the high-pressure optical cell to those recorded in a standard quartz freeze-pump-thaw cuvette. Spectra were identical when we performed all transfers inside the drybox. The Py-PDMS-Py fluorescence spectra were independent of the Py-PDMS-Py concentration between 0.1 and 25 µM. All experiments were performed at 10 µM chromophore. Experiments were performed from 0 up to 204 bar CO2. CO2 was pumped into the toluene solution with stirring, and equilibrium was established within 20-30 min following a pressure change. Equilibrium was evidenced by the pump flow going to zero ((3 µL/min), indicating that the toluene solution was no longer taking up any CO2 and by Py-PDMS-Py emission spectra that were time invariant. There was no evidence for hysteresis on slow decompression and reequilibration. Results and Discussion Steady-State Fluorescence Studies. Figure 5A presents typical emission spectra for Py-PDMS-Py dissolved in toluene (T ) 35 °C) at several CO2 pressures along with a spectrum for 1-ethylpyrene dissolved in neat liquid toluene. The monomer emission is clearly observed between 370 and 430 nm in all samples. The excimer emission is only seen for the Py-PDMSPy samples in the 440-550 nm region and the excimer-tomonomer intensity ratio (E/M) is a strong function of the amount of added CO2. No excimer emission is evident for 1-ethylpyrene under these experimental conditions.

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Figure 6. Normalized emission wavelength-dependent excitation spectra for Py-PDMS-Py dissolved in toluene at 35 °C at 0 (Panel A), 34.0 (Panel B), 68.0 (Panel C), and 204.1 bar CO2 (Panel D). The numbered spectra correspond to λem ) 378 (1), 400 (2), 425 (3), 450 (4), and 480 nm (5). The excitation and emission spectral band-passes were set at 2 and 8 nm, respectively.

Figure 5B summarizes the effects of added CO2 on the PyPDMS-Py E/M (b). These data show that the addition of CO2 to a toluene solution leads to a 5-fold increase in the PyPDMS-Py E/M up to ∼70 bar. However, as we increase the CO2 pressure above 70 bar, we see that the Py-PDMS-Py E/M gradually decreases. Control experiments with Ar(g) (O) in place of CO2 demonstrate that the observed change in E/M arises from a CO2-induced effect and not an effect of gas absorption alone. The marked increase in E/M on adding CO2 to the toluene solution suggests that addition of CO2 somehow facilitates the Py-PDMS-Py tail segment cyclization. One must be very cautious when studying “excimer” emission and carefully distinguish between a classic excimer that forms in and exists only in the excited state and related species that look like an excimer but are formed from preassociated interor intramolecular pyrene species prior to optical excitation.39 A convenient way to discriminate between these so-called static dimers and true dynamic excimers is to acquire the emission wavelength-dependent excitation scans at several emission wavelengths.40 If the system is described by a classic excimer model (i.e., Figure 2), the normalized emission wavelengthdependent excitation scans will be independent of the emission wavelength.39,40 Figure 6 presents a series of normalized emission wavelength-dependent excitation scans for Py-PDMSPy dissolved in toluene as a function of added CO2. Within our measurement precision, these spectra demonstrate39,40 that the observed excimer is of the classic type and there is no evidence for inter- or intramolecular pyrene residue preassociation in this system. Thus, the model presented in Figure 2 should suffice to describe the Py-PDMS-Py kinetics in toluene + CO2. Farinha et al.41 have studied polystyrene (PS, Mn ) 10 600) dissolved in cyclohexane that was end-labeled with 4-(1pyrenyl)butanoamide to form peptide bounds identical the ones in our Py-PDMS-Py samples. Interestingly, Farinha et al. found that their spectra were inconsistent with a homogeneous

Figure 7. Typical normalized fluorescence intensity decay profiles for 1-ethylpyrene dissolved in toluene at 35 °C in the presence of 13.6 (b) and 170.0 bar CO2 (O). (Upper Panel) Data plus fits (solid curves) to a single-exponential decay model. (Lower Panels) Residuals between the data and the model. χ2 at 13.6 bar ) 1.084; χ2 at 170 bar ) 1.106. λex ) 337 nm; ∆λex ) 4 nm; λem ) 378 nm; ∆λem ) 8 nm. IRF ) instrument response function.

ground state and they argued that the Py-PS-Py was undergoing intrachain hydrogen bonding wherein the amide hydrogen at one chain termini hydrogen bonded to the carbonyl residue at the other termini of the same PS chain. This resulted in two forms of Py-PS-Py (i.e., non-hydrogen bonded and intramolecularly hydrogen bonded) existing simultaneously in the ground state prior to photoexcitation. Our experimental data are inconsistent with the existence of anything other than the nonhydrogen bonded form of Py-PDMS-Py. The difference between our results and those of Farinha et al. may arise from differences in the chain flexibility of PS vs PDMS, the differences in chain length, and/or the relative differences in solvent quality (toluene is a good solvent for PDMS whereas cyclohexane is a θ solvent for PS below 35 °C). Time-Resolved Fluorescence Studies. Figure 7 presents typical time-resolved fluorescence intensity decay profiles for 1-ethylpyrene dissolved in toluene at 13.6 and 170.0 bar CO2. These decay traces and all other decay traces for 1-ethylpyrene dissolved in toluene as a function of added CO2 are well described (χ2 e 1.113) by a single excited-state fluorescence lifetime (τM ) 1/kM) and τM decreases as we add CO2 (Table 1). Figure 8 presents typical time-resolved fluorescence intensity decay profiles at the Py-PDMS-Py monomer (M) and excimer (E) emission wavelengths in toluene at 13.6 and 170.0 bar CO2.

CO2 Effects on Conformation of Py-PDMS-Py

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TABLE 1: Effects of Added CO2 on the Py-PDMS-Py Monomer (kM) and Excimer (kE) Decay Rates, the Tail Cyclization (kcyclization) and Unfolding (kunfolding) Rates, and Excited-State Equilibrium Constant (K*) for Folding in Liquid Toluene at 35 °Ca CO2 (bar)

kM (107 s-1)b

kE (107 s-1)

kcyclization (108 s-1)

kunfolding (108 s-1)

K*c

0 6.8 13.6 20.4 27.2 34.0 40.8 47.6 68.0 102.0 136.1 170.1

1.10 ( 0.01 1.28 ( 0.02 1.46 ( 0.02 1.70 ( 0.03 2.10 ( 0.04 2.74 ( 0.08 3.70 ( 0.14 4.78 ( 0.23 7.52 ( 0.57 8.13 ( 0.66 9.52 ( 0.91 9.80 ( 0.96

1.51 ( 0.10 2.12 ( 0.21 2.47 ( 0.40 7.08 ( 1.82 5.13 ( 0.85 6.05 ( 0.73 8.32 ( 0.90 7.95 ( 1.03 9.29 ( 1.24 9.14 ( 1.64 8.98 ( 2.11 9.29 ( 2.42

0.67 ( 0.10 0.72 ( 0.12 1.13 ( 0.13 0.48 ( 0.11 1.57 ( 0.13 1.62 ( 0.14 1.09 ( 0.12 2.76 ( 0.12 2.65 ( 0.11 5.09 ( 0.15 6.03 ( 0.15 3.81 ( 0.15

1.28 ( 0.14 1.32 ( 0.14 1.69 ( 0.13 0.82 ( 0.15 1.63 ( 0.13 1.77 ( 0.14 1.94 ( 0.14 2.18 ( 0.16 1.52 ( 0.13 1.75 ( 0.14 2.15 ( 0.10 1.88 ( 0.10

0.52 ( 0.10 0.55 ( 0.11 0.67 ( 0.09 0.58 ( 0.17 0.96 ( 0.11 0.92 ( 0.11 0.56 ( 0.07 1.27 ( 0.11 1.74 ( 0.17 2.91 ( 0.25 2.80 ( 0.15 2.03 ( 0.13

a The reported uncertainties represent (1 standard deviation. b From τ for 1-ethylpyrene (kM ) 1/τ1-ethylpyrene). c Excited-state equilibrium constant for folding (K* ) kcyclization/kunfolding).

Figure 9. Effects of added CO2 on the Py-PDMS-Py unimolecular monomer (kM) and excimer decay rates (kE) in toluene at 35 °C.

Figure 8. Typical normalized excited-state fluorescence intensity decay profiles for Py-PDMS-Py dissolved in toluene at 35 °C in the presence of 13.6 (Left Panel) and 170.0 bar CO2 (Right Panel) at the 378 nm (monomer, b) and 480 nm (excimer, O). The solid curve passing through the data represent the regression lines for the Birks scheme (Figure 2). (Lower Panels) Corresponding residuals between the data and the model. χ2global at 13.6 bar ) 1.025; χ2global at 170 bar ) 1.087. λex ) 337 nm; ∆λex ) 4 nm; ∆λem ) 2 nm at 378 and 16 nm at 480 nm. IRF ) instrument response function.

These intensity decay traces are complex and they cannot be described well (χ2 > 227) by a single-exponential decay model. Simultaneous global analysis37 of these E and M data (at a given CO2 pressure) in concert with the independent kM data from the 1-ethylpyrene experiments (vide supra) was used to model the E and M data. The Birks scheme (Figure 2) adequately (χ2global e 1.093) described the E and M data at all CO2 pressures. To illustrate this point, the solid traces that pass through the data points in Figure 8 represent the best fits between the experimental E and M data and the Birks model. Table 1 reports all the recovered Py-PDMS-Py kinetic terms as a function of added CO2 while Figures 9-11 summarize the key results graphically. Figure 9 illustrates the effects of added CO2 on the kM and kE. These results show that the Py-PDMS-Py monomer and excimer de-excitation rates parallel each other; increasing by about 8-9-fold as the CO2 pressure is raised from 0 to 204 bar. Figure 10 summarizes the effects of added CO2 on the Py-

Figure 10. Effects of added CO2 on the Py-PDMS-Py unimolecular cyclization (kcyclization) and unfolding rates (kunfolding) in toluene at 35 °C.

PDMS-Py kcyclization and kunfolding. These results illustrate that kunfolding is essentially independent of added CO2 ( ) 1.7 × 107 s-1), but kcyclization increases by ∼5-fold on addition of CO2 to toluene. Figure 11 illustrates the effects of added CO2 on the apparent excited-state equilibrium constant (K* ) kcyclization/kunfolding) for Py-PDMS-Py dissolved in toluene. As the CO2 pressure increases, K* increases up to ∼90 bar whereupon K* decreases above ∼90 bar. If the added CO2 influenced the Py-PDMS-Py chain dynamics, kcyclization and kunfolding would change to the same degree, and K* would be independent of added CO2.42 The fact that kunfolding changes little with added CO2 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* (Figure 11) shows that

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Figure 11. Effects of added CO2 on the apparent Py-PDMS-Py excited-state equilibrium constant (K*) in toluene at 35 °C.

the addition of CO2 affects the chain cyclization probability which is a manifestation of a change in chain conformation brought on by the addition of CO2. Thus, the addition of CO2 to liquid toluene (up to ∼90 bar) results in a systematic change in the PDMS chain conformation such that the mean free distance between the pyrene-labeled PDMS termini decreases. This change in the mean free tail-tail distance is consistent with the known “anti-solvent” nature of CO2.32-34 Above ∼90 bar CO2, the mean free distance between the pyrene-labeled termini begins to increase. This increase could arise from an increase in the solvent quality with increasing CO2 density. The proposed scenario is based on an excluded volume argument.13,14,43 Additional support for this scenario comes also from recent small-angle neutron scattering experiments44 on PDMS dissolved in neat supercritical CO2 where the authors report that the polymer radius of gyration decreased with fluid density and the intermolecular correlation length increased dramatically near the upper critical solution pressure and Monte Carlo simulations45 which showed polymer/oligomer chain collapse in a supercritical fluid when one decreases the fluid density at constant temperature. Conclusions We have determined the effects of added CO2 on the tailtail cyclization and unfolding dynamics of Py-PDMS-Py dissolved in liquid toluene. The Py-DMS-Py tail-tail dynamics are well-described by a Birks model wherein the tail-tail cyclization rate increases 5-6-fold on addition of CO2 and the unfolding rate is essentially independent of CO2. These results show that the addition of CO2 (an anti-solvent) modulates the PDMS conformation in liquid toluene. Acknowledgment. This work was generously supported by the Department of Energy (DEFG0290ER14143). We thank Gary Sagerman and Gary Nottingham of the Machine Shop at University at Buffalo, State University of New York, for their help in constructing portions of our high-pressure equipment. We also thank the two Reviewers who helped to open our eyes. References and Notes (1) Bailey, R. T.; North, A. M.; Pethrick, R. A. Molecular Motion in High Polymers; Clarendon Press: Oxford, 1981.

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