Mohammed A. H.
Alamiry
,
Andrew C.
Benniston
,
Graeme
Copley
and
Anthony
Harriman
*
Molecular Photonics Laboratory, School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, UK NE1 7RU. E-mail: anthony.harriman@ncl.ac.uk; Fax: +44 191222 8660; Tel: +44 191222 8660
First published on 6th January 2012
The viscosity of 1,2-dichloroethane increases steadily with increasing pressure, as does the density, refractive index and polarizability of this solvent. The pressure dependence for each of these properties can be monitored by a combination of absorption and fluorescence spectroscopy carried out in the presence of a fluorescent molecular rotor that responds to changes in the local environment. At 20 °C, dichloroethane freezes under an applied pressure of ca. 370 MPa, causing sudden extinction of the fluorescence of the molecular rotor due to the opaque nature of the frozen solvent. However, this same emission is enhanced dramatically if a small amount of inert polymer is present in the solution. The behaviour is interpreted in terms of the polymeric solute promoting establishment of a glassy matrix with reasonably good optical transparency for emission spectroscopy.
There have been several earlier studies of pressure effects on the trans–gauche equilibrium for DCE in the neat liquid8 and in alkane solvents.9 Thus, using vibrational spectroscopy, it has been established9 that DCE undergoes a significant decrease in molar volume as the pressure is raised. This has been interpreted in terms of internal rotation from the trans form to the gauche conformer and accompanying overlap of the chlorine atoms as their mutual separation distance decreases. This equilibrium process is affected8,9 by changes in solvent polarity, with the gauche form being somewhat stabilized by polar solvents. Since the gauche conformer is expected to exhibit a more significant dipole moment than the corresponding trans isomer, increased pressure should lead to a modest increase in the dielectric constant. The combined changes in refractive index and dielectric constant will influence the photophysical properties of any dissolved dye molecule.10,11 It has also been shown8,9 that the freezing point of DCE is elevated at higher pressures, at least in 2-methylpentane, causing precipitation at ambient temperature for applied pressures of around 1 GPa. This is an unusually large elevation of the freezing point, augmented by the strong decrease in molar volume, that pushes the freezing point from −35 °C at atmospheric pressure to >30 °C at 5 GPa. Most of these prior studies were aimed at evaluating the equilibrium step, rather than establishing bulk properties of DCE, and were carried out in the absence of a chemical probe.
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Fig. 1 Molecular formula of the fluorescent rotor used in this work. |
In the first instance, dilute solutions of ROBOD in DCE (typically in the order of 2–10 μM) were subjected to increasingly higher pressures and monitored by absorption spectroscopy at 20 °C. At moderate pressures (0 < P < 350 MPa), there is a progressive decrease in the baseline level, a small (i.e., 4 nm) red shift for the absorption maximum of the dye, and a significant (i.e., 25%) increase in absorbance at the peak maxima (Fig. 2). The change in the transmission level is due to a pressure-induced increase in the solvent refractive index (n) while the variation in the properties of the dye are due to the anticipated changes in polarisability (σ) and density (ρ) of DCE that accompany raising the pressure. Indeed, ρ and σ are related by way of the Lorentz–Lorenz expression.15 The original spectral profile is recovered on release of the applied pressure. The increased absorbance, this being due entirely to compression of the solvent, followed the same pattern for both the first-and second-allowed transitions found at 497 and 415 nm, respectively. These various effects are nonlinear with respect to applied pressure and tend towards saturation at high pressure (Fig. 3). Similar behaviour has been reported for certain other solvents16 and also for DCE,17 although data for this particular solvent are highly limited.
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Fig. 2 Effect of applied pressure (0 < P < 370 MPa) on the absorption spectral profile recorded for ROBOD in DCE at 20 °C. The upper panel shows the absorbance at 500 nm over the full pressure range. |
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Fig. 3 Effect of applied pressure on some relevant properties of the system at 20 °C: (a) density of DCE, (b) wavenumber of the absorption peak recorded for ROBOD and (c) refractive index of DCE. In each case, the solid line drawn through the data points corresponds to a fit to the appropriate equation as discussed in the text. |
The pressure-induced change in absorbance, analysed globally across the spectral window, can be used to compute the corresponding variation in ρ (Fig. 3). Thus, it was demonstrated that ROBOD follows the Beer–Lambert law in DCE at 20 °C, at least over the concentration range of interest, such that the increased absorbance can be assigned to the compressibility, k, of the solvent. This latter term is related to the molar volume, VM, of DCE according to the Tait expression18 (eqn (1)). Here, the coefficients B and C are independent of pressure, P. Indeed, B is sensitive to the nature of the solvent and temperature while C is independent of temperature and remains closely comparable across series of similar solvents.19 Fitting the absorbance data to eqn (1) indicates that DCE undergoes a total decrease in VM of ca. 14 cm3; at atmospheric pressure VM0 = 73.3 cm3. Some of this reduced volume can be attributed8a to the switch from trans to gauche conformers since quantum chemical calculations give an accompanying change in molar volume of 5 cm3. The main factor responsible for the fall in VM, however, is closer packing of neighbouring DCE molecules in the fluid. Non-linear, least-squares analysis leads to estimates for B and C of 42.23 and 0.21, respectively (see ESI†). These values appear to be in line with estimates made for related solvents,20 and allow calculation of the density at any applied pressure over the relevant range.
The molar density, ρM, can be related to the refractive index, n, according to the Lorentz–Lorenz expression15 (eqn (2)), where A is the molar refractivity. This latter term, in turn, is related to the molecular polarisability, α. On the basis that α is independent of pressure, the changes in absorbance can be used to calculate the pressure-induced effect on n. From this analysis, we find that n increases from 1.4421 at atmospheric pressure to a value of 1.5212 at an applied pressure of 330 MPa (see ESI†). Changes in refractive index also cause the red shift found for the absorption maximum of ROBOD (Fig. 3). Here, the shift given in terms of wavenumber, Δν, can be related to refractive index by way of the so-called Bayliss expression21 (eqn (3)), where R (= 7 ± 2 Å) is the radius of the cavity housing the chromophore, νABS (= 20120 cm−1) is the absorption maximum in cm−1, and μTD (= 5.8 D) is the transition dipole moment calculated22 from the absorption spectrum recorded for ROBOD in DCE at atmospheric pressure.
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Curve-fitting analyses carried out for ROBOD in DCE indicate that the absorption spectral profile does not change with increasing pressure, other than the red shift. In particular, pressure has no effect on either the half-width (FWHM = 640 cm−1) of the underlying vibronic bands or the Huang–Rhys23 (S = 0.31) factor. This insensitivity towards pressure can be used to argue that the cavity holding the dye molecule does not change significantly during the pressure cycle. As such, the absorption spectral changes are likely to reflect the pressure effect on the refractive index. As above, n increases progressively but nonlinearly with pressure (see ESI†) and shows a net enhancement of ca. 10% over the available pressure range. Using this information together with eqn (2) it becomes possible to estimate a value of 0.127 for the molecular refractivity. An independent estimate of the pressure dependence for n was made using a Michelson-type interferometer with 632.8 nm illumination after calibration with pure toluene.24 The two sets of data agree reasonably well and are compiled in the ESI.† We emphasize the pressure effect on n because this term is extremely important in many optical processes, including fluorescence and Förster-type electronic energy transfer.25
The molecular rotor used for this study is a member of the unhindered boron dipyrromethene class of dyes14 and fluoresces strongly in viscous media. The emission quantum yield (ΦF = 0.052) and excited-singlet state lifetime (τS = 0.39 ns) are set by the ease of rotation of the meso-phenylene ring, which itself is determined by frictional forces with the surrounding media. It has been shown14 that the rate constant (kNR = (1 − ΦF)/τS) for nonradiative decay of the first-excited singlet state of ROBOD follows eqn (4) where η is the bulk viscosity of DCE, χ is a limiting pressure (in the region of ca. 2 GPa) and EA (being typically in the region of 2.5 kJ mol−1) is the activation energy for internal rotation of the phenylene ring in that solvent. The dimensionless coefficient δ allows for the fact that ROBOD resides in a cavity within the solvent structure such that the full effect of viscosity is not observed. Over a wide range of solvents, linear log–log plots of kNRvs. η have been observed14 with δ = 0.44. On this basis, ROBOD can be used to monitor changes in viscosity at applied pressures, provided there are no specific solvation effects.
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Increasing pressure causes a small (i.e., 5 nm) red shift for the emission maximum and a steady increase in the emission quantum yield (Fig. 4). The red shift is comparable to that observed by absorption spectroscopy, and is a consequence of the change in polarisability of DCE under high pressure. The increase in emission yield, which amounts to a factor of ca. two-fold, can be assigned14 to a fall in kNR and, after correction for changes in refractive index and absorbance, it can be concluded that the viscosity of DCE increases only slightly with increasing pressure over the range 0 < P < 370 MPa, despite the significant increase in density found over the same range (see ESI†). The magnitude of this pressure-induced increase in viscosity can be expressed in terms of a modified Tait equation26 with B = 4320 and C = 0.63 (see ESI†). These derived parameters permit estimation of the viscosity of DCE at any reasonable pressure, although analysis is restricted to 20 °C.
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Fig. 4 Effect of applied pressure on the fluorescence spectral profile recorded for ROBOD in DCE at 20 °C. The upper panel shows the integrated emission yield as a function of applied pressure. |
At an applied pressure of 370 MPa, fluorescence from ROBOD suddenly decreases in intensity and falls to a value well below that recorded at atmospheric pressure (Fig. 4). Further increases in pressure have no effect on the fluorescence intensity but release of the pressure quickly restores the system to its original state. The pressure cycle can be repeated numerous times without change. Clearly, the switching of the fluorescence yield is associated with freezing of the solvent. Again, the effect occurs over a narrow pressure range.
In the low pressure regime, at any given concentration of PMMA, there is an almost linear relationship between kNR and the applied pressure (Fig. 5). The effective gradients of such plots, however, exhibit a clear dependence on the concentration of PMMA. This is an important finding, not least because it raises doubts about using fluorescent rotors to monitor the concentration of dissolved polymers. The implication here is that applied pressure alters the average conformation of PMMA dissolved in DCE. This is not the case, however, because the compressibility of the solution increases systematically with increasing concentration of PMMA. When the latter effect is taken into account, it is found that pressure has little effect on the intrinsic viscosity of the solution relative to that of DCE. In particular, there is no indication that the radius of gyration of the dissolved polymer is affected by pressure.
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Fig. 5 Effect of applied pressure on the rate constant for nonradiative decay of the first-excited singlet state for ROBOD in DCE (○) and in the presence of 6% w/w PMMA (●). |
In the presence of PMMA, there is a marked perturbation of the fluorescence yield at pressures where the solvent is expected to freeze. Thus, the presence of PMMA causes fluorescence from ROBOD to increase dramatically upon freezing (Fig. 6), in marked contrast to the virtual extinction of emission found in the absence of polymer. This effect on the fluorescence yield is fully reversible on releasing the pressure but causes no discernible shift of the peak maximum. The full impact of fluorescence enhancement is observed with quite low concentrations (i.e., 10% w/w) of PMMA (Fig. 7) and corresponds to an emission quantum yield of ca. 0.6. This is probably a lower limit because the optical transparency of the matrix is far from perfect but means that ROBOD must reside within a very high viscosity medium.14 The most likely explanation for this highly unusual effect is that the PMMA facilitates formation of glassy regions within the mixture that combine high viscosity with reasonable optical transparency. At all concentrations of PMMA, further increases in applied pressure cause a small but distinct loss of fluorescence (Fig. 3). We attribute this particular effect to restructuring of the glassy matrix with resultant degradation of the optical transparency. Although this effect loses fluorescence, the residual emission greatly exceeds that found in the absence of PMMA.
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Fig. 6 Effect of increasing pressure on the fluorescence from ROBOD in DCE containing 12% w/w PMMA. |
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Fig. 7 Effect of PMMA concentration on the fluorescence quantum yield for ROBOD in DCE as measured immediately after the sudden increase in fluorescence at the equilibrium freezing pressure. |
Footnote |
† Electronic supplementary information (ESI) available: Energy-minimised conformation of ROBOD, pressure effect on refractive index, viscosity and molar volume. See DOI: 10.1039/c2ra00848c |
This journal is © The Royal Society of Chemistry 2012 |