Electronic spectral shifts, reorganization energies, and local density augmentation of Coumarin 153 in supercritical solvents

10 September 1999

Chemical Physics Letters 310 Ž1999. 485–494 www.elsevier.nlrlocatercplett

Electronic spectral shifts, reorganization energies, and local density augmentation of Coumarin 153 in supercritical solvents R. Biswas, J.E. Lewis, M. Maroncelli

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Department of Chemistry, 152 DaÕey Laboratory, The PennsylÕania State UniÕersity, UniÕersity Park, PA 16802, USA Received 25 May 1999; in final form 29 June 1999

Abstract Solvent reorganization energies and local densities of Coumarin 153 ŽC153. in supercritical C 2 H 6 , CO 2 , and CHF3 ŽTc s 5 K. are measured using fluorescence spectroscopy. Reorganization energies are 320 " 70 and 770 " 70 cmy1 in CO 2 and CHF3 , respectively – nearly independent of density Ž0.3 F rrrc F 2.1.. The spectral shifts imply similar effective local densities in all three solvents. These effective densities can exceed 3–5 times the bulk density. In CO 2 and CHF3 , the maximum density augmentation calculated from emission shifts is ; 20% greater than that calculated from excitation shifts. No such difference is found in C 2 H 6 . q 1999 Elsevier Science B.V. All rights reserved.

1. Introduction Supercritical solvents, solvents at temperatures slightly above their critical points ŽTc ., have become a topic of intense current interest 1. The unique feature of fluids in this region of the phase diagram is that their high compressibility allows for large and continuous variation of density with only modest changes in applied pressure. This tunability enables selection or variation of solubilities, solvation energetics, and transport rates so as to optimize conditions for a number of practical applications. This same attribute also renders supercritical solvation

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Corresponding author. Fax: q1-814-863-5319; e-mail: [email protected] 1 A thematic issue of Chemical ReÕiews on supercritical fluids edited by R. Noyori w1x provides an excellent overview of much of the recent work in this area.

distinct from solvation in typical liquid solvents. In particular, as a result of attractive solute–solvent interactions, solvent densities in the vicinity of a solute often appear to be much higher than that of the bulk fluid. Since local solvent density presumably dictates the behavior of many of the variables one would like to control, this phenomenon of ‘local density augmentation’ has received considerable attention from both experimental and theoretical perspectives w2,3x. However, a fully quantitative understanding of how solute and solvent properties determine local density augmentation in supercritical fluids has yet to be achieved. The present Letter describes experimental measurements we hope will be of use in this context. We report fluorescence measurements of excitation and emission shifts of the polarity probe Coumarin 153 Žhereafter designated C153. in the supercritical solvents C 2 H 6 , CO 2 , and CHF3 at a reduced temperature ŽTr s TrTc . of 1.02. From these measurements we deduce the den-

0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 8 3 8 - 6

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sity dependence of the Stokes shift Žor reorganization energy. of the S 0 ™ S 1 transition and estimate the extent of local density augmentation in both the S 0 and S 1 states. These data will be compared to the results of molecular dynamics computer simulations, which will be published separately w4x. Preliminary measurements of the electronic shifts of C153 in supercritical solvents have been previously reported w5,6x. An earlier study from our group w5x was aimed at demonstrating that the reorganization energies of C153 are dominated by interactions between the large dipolar change in the S 0 ™ S 1 transition of C153 and the permanent charge distributions of solvent molecules. In that study we included measurements in both CO 2 and CHF3 at liquid-like densities. The present measurements extend this initial work down to low solvent densities, where the effects of local density augmentation become evident. During the course of our new measurements, Takahashi et al. w6x reported on the density dependence of the Stokes shifts Žthe equivalent of the reorganization energy. of C153 in CO 2 , CHF3 , and C 2 H 6 at Tr s 1.03. Their estimates of Stokes shifts at high densities differed considerably from our previous estimates. Takahashi et al. also reported decreasing Stokes shifts with increasing solvent density in CO 2 as well as a non-monatonic variation with density in supercritical CHF3 . Both observations are contrary to what would be expected – a continuous increase in the solvent reorganization energy with increasing solvent density. Given this unexpected behavior, and in order to provide values of local density augmentation for comparison to simulation, further study of the C153 system seemed warranted. As will be described below, our results for reorganization energies differ quantitatively from those reported by Takahashi et al. w6x However, the differences appear to be mainly attributable to differences in analysis methods, rather than to differences in primary data. Our analysis indicates that the Stokes shifts in both CO 2 and CHF3 are nearly invariant to solvent density over a wide range, 0.3 F rrrc F 2.1 – still a surprising result. In this Letter we also compare the excitation and emission shifts in these supercritical fluids to results previously measured in liquid solvents w5,7x in order to estimate effective Žlocal. densities, something not done in previous studies.

2. Experimental C153 was used as obtained from Exciton. Supercritical grade C 2 H 6 Ž99.0%., CO 2 Ž99.99%., and CHF3 Ž99.995%. were purchased from Scott Specialty Gases and further purified by passage through an oxygen trap prior to use. Spectroscopic measurements were performed using a stainless-steel high-pressure cell with quartz windows and an inner volume of ; 10 ml. Measurements in CHF3 , CO 2 , and C 2 H 6 were performed at fixed temperatures of 304.3, 309.3, and 311.9 K, respectively. Temperature was maintained to within "0.1 K by the flow of thermostated water through the optical cell. Gases were pumped into the cell and maintained at a desired pressure using a syringe pump ŽISCO. with an accuracy of "1 psi. Samples were prepared by pipetting an aliquot of stock solution of C153 in methanol into the cell and then driving off the methanol solvent with a stream of dry nitrogen gas. The concentration of the stock solution was chosen to produce a concentration of 2 = 10y6 M when the C153 was completely dissolved. A series of spectra were typically recorded as the pressure was varied stepwise from low to high pressure. Between closely spaced pressure points the solution inside the cell was stirred for at least 5 min using a magnetic stir bar. Two series of spectra were recorded in C 2 H 6 and three each in CO 2 and CHF3 and the best two runs were analyzed to provide the data presented here. Fluorescence excitation and emission spectra were recorded using a SPEX Fluorolog F212 fluorimeter with instrumental parameters chosen to provide resolution of 2 nm. The spectra were corrected for instrument converted to a frequency representation prior to analysis. Several different measures of spectral frequency were examined in this work, but unless specified otherwise the first moment of the frequency spectrum is used to measure peak shifts and to compute the local density augmentation. The equations of state reported by Younglove and Ely w8x for C 2 H 6 , Ely et al. w9x for CO 2 , and Rubio et al. w10x for CHF3 were used to convert pressure and temperature readings into densities. These parameterizations give critical constants of: Tc s 305.34 K, rc s 6.875 mol dmy3 , Pc s 4.871 MPa for C 2 H 6 ; Tc s 304.1 K, rc s 10.63 mol dmy3 , Pc s 7.375 MPa

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for CO 2 ; and Tc s 299.01 K, rc s 7.556 mol dmy3 , Pc s 4.816 MPa for CHF3 . Values of the refractive indices of C 2 H 6 and CO 2 were determined from the densities using the empirical equations given by Besserer and Robinson w11x. The dielectric constants of C 2 H 6 and CO 2 were assumed to be equal to the square of their refractive indices. This is an excellent approximation in the case of C 2 H 6 w9x and sufficient for our purposes in the case of CO 2 Žwhere a dielectric analysis is inapplicable in any case.. Dielectric parameters of CHF3 were obtained from fits to collected literature data of the form w12x: R Ž n2 . '

n2 y 1 n2 q 2

f 0.05347rr y 3.136 = 10y4rr2 y 1.534 = 10y4rr3

Ž 1.

and RŽ ´ . '

´y1 ´q2

s 0.1723 rr q 100.6

rr T

Fig. 1. Representative fluorescence emission Žleft. and excitation spectra Žright. of C153 in supercritical CHF3 , CO 2 , and C 2 H 6 at three solvent densities.

exp Ž y0.1530 rr2 . ,

Ž 2. where rr is the reduced density rrrc .

3. Results and discussion Representative excitation and emission spectra of C153 in C 2 H 6 , CO 2 , and CHF3 are shown in Fig. 1. In all cases one observes the expected red-shift of the spectra with increasing solvent density. The shapes of the spectra in these supercritical fluids do not differ appreciably from those observed in typical liquid solvents of comparable polarities. To a first approximation the only change with solvent density is a spectral shift. However, there are also subtle changes to the vibronic structure near the peaks of the bands, especially in excitation. Such changes were also observed in the spectra of Takahashi et al. w6x. Characteristics of the spectra are plotted as functions of solvent density in Fig. 2. The top panel of this figure displays the average Ž1st moment. emission frequencies. Two independent sets of data are

shown in order to illustrate the reproducibility of the measurements, which is roughly "70 cmy1 . The large point at zero density is the gas-phase frequency extrapolated from liquid solvent data Žsee below.. At the lowest densities accessible Ž rrrc f 0.2. the frequency shifts from the gas phase are substantial in the polar solvents CO 2 and CHF3 : 900 and 2600 cmy1 , respectively. Thus, even at the lowest densities allowed by the limited solubility of C153 in supercritical solvents, the solvation environment is far from gas-like in these. The greater shift in CHF3 reflects the greater polarity of this dipolar solvent Ž m s 1.65 D. compared to the quadrupolar solvent CO 2 and the non-polar solvent C 2 H 6 . The emission frequencies in all three solvents exhibit the sigmoidal density dependence Žsee Fig. 3. characteristic of the ‘three-density region’ behavior often noted for spectral data in supercritical fluids w13x. Although not shown for the sake of brevity, the excitation frequencies exhibit all of the same qualitative features as the emission frequencies. Summaries of the density dependence of both the excitation and emission fre-

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Fig. 2. Density dependence of various spectral characteristics of C153 in supercritical C 2 H 6 , CO 2 , and CHF3 . Upper panel: Emission frequencies Žfirst moments of the emission spectra.. The filled and open symbols represent two independent sets of data. For C 2 H 6 , only one set of data are shown. Middle panel: Spectral bandwidths Žfull width at half maximum intensity. of the fluorescence excitation Ž‘Exc.’. and emission Ž‘Em.’. spectra. The squares denote CHF3 data, the triangles data in C 2 H 6 , and the circles data in CO 2 . Lower panel: Relative Stokes shifts calculated from Eq. Ž4. using three different frequency measures. The squares denote the shifts calculated from the average of the high- and low-frequency half-height points of the spectra, the circles from the first frequency moments, and the triangles from the ‘00’ edge frequencies Žsee text.. The solid lines are fits to the average of these three estimates.

quencies are provided in Table 1, which lists parameters of the fits of multiple sets of data to the polynomial: Õ s A q B rr q C rr2 q Drr3 q E rr4 ,

Ž 3.

with rr s rrrc The widths of the spectra Ž G ; full widths at half maximum intensity. are plotted in the middle panel of Fig. 2. As is the case in liquid solvents, the excitation Žor absorption. spectra are broader than the emission spectra by 15–20%. As discussed in Ref. w7x, the absorption widths of C153 in liquid

solvents show the expected increase with increasing solvent polarity, whereas the emission widths actually narrow. The latter behavior has been attributed to small changes in the vibronic structure of the emission with solvent. The supercritical data in Fig. 2 are roughly consistent with this interpretation of liquid solvent spectra. Thus, the excitation widths in CHF3 are greater than those in C 2 H 6 and CO 2 , which would be anticipated based on the greater spectral shift Žand reorganization energy. in CHF3 . The emission widths on the other hand are much more similar in the three solvents. Also in keeping with the polarity trends observed in liquid solvents is

R. Biswas et al.r Chemical Physics Letters 310 (1999) 485–494

Fig. 3. Frequencies of C153 observed and predicted on the basis of Eq. Ž5.. Data in nonpolar and dipolar liquid solvents Žopen circles. and in supercritical C 2 H 6 Žopen diamonds., CO 2 Žopen triangles., and CHF3 Žfilled triangles. are plotted. The large filled symbols at high n denote the extrapolated gas-phase value Ž n 0 ..

the fact that the excitation widths appear to decrease slightly with decreasing solvent density Žpolarity. whereas the emission widths in C 2 H 6 and CHF3 increase slightly. It should be admitted that all of these width variations are comparable in magnitude to the confidence limits Ž"100 cmy1 . we would place on these data. Nevertheless, they are relevant for interpreting the unusual behavior observed for the Stokes shifts of C153 in supercritical solvents. The bottom panel of Fig. 2 shows the relative Stokes shifts determined using three different frequency measures. These shifts are defined by: DDÕ Ž r . ' Õexc Ž r . y Õem Ž r . y Õexc Ž ref . y Õem Ž ref . ,

solvent, this subtraction removes the vibrational contribution and provides a measure of the solvent component to the nuclear reorganization energy of the S 0 l S1 transition: lsolv s 12 DD n . In studies of liquid solvents w7x, we found that the DD n determined in this manner 2 predict the time-dependent reorganization observed in polar solvents to an accuracy of better than ; 200 cmy1 w7,14x. ŽDynamical shifts are not observed in nonpolar solvents w7x like pentane and are probably smaller than 50 cmy1 .. Returning to the Stokes shift results in Fig. 2 we note two surprising features. First, the data indicate a non-negligible Stokes shift in C 2 H 6 relative to pentane. This is true for all three of the frequency measures examined, the first moment frequency, the average of the high- Ž nq . and low-frequency Ž ny . half-height points, n s Ž nqq ny .r2, and the ‘00’ edge frequencies, nexc s ny and nem s nq. Even if the solvent contribution to the reorganization energy of C153 in nonpolar solvents were non-negligible, the effect should be larger for the more dense solvent pentane than for C 2 H 6 . Thus, the apparent reorganization energy of 50–350 cmy1 in C 2 H 6 relative to pentane implies a breakdown of the assumption of solvent-independent vibronic structure. In other words, the intramolecular reorganization energy of C153 is not independent of solvent conditions — a result also suggested by the opposite solventrdensity dependence of the emission and excitation widths. This coupling between intra- and inter-molecular effects ultimately limits the accuracy with which we can estimate solvent reorganization energies in these supercritical systems. Fig. 2 also shows that the Stokes shifts derived from different measures of spectral frequency differ from one another much more than the shifts vary with solvent density. As a best guess for the ‘correct’ values we use the average of these three indicators, which is shown using linear fits to Eq. Ž3. in Fig. 2. ŽThe fit parameters are provided in Table 1.. These

2

Ž 4.

where ‘ref’ denotes a nonpolar reference solvent, n-pentane in the present case. To the extent that one can neglect differences in vibronic structure with

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Actually, prediction of ‘time-zero’ spectra in Refs. w7,14x involves a slightly more sophisticated analysis, which properly accounts for the effects of inhomogeneous broadening and different frequency-weighting of absorption and emission spectra. However, this more detailed analysis would not significantly change the results presented here.

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Table 1 Parameters summarizing the density dependence of various observablesa Solvent

Observable

B

C

D

E

Std. Err.

C2 H6

nexc Ž10 3 cmy1 . nem Ž10 3 cmy1 . DD n Žcmy1 . Ž reffrr .exc Ž reffrr .em

A 28.233 23.522 244 4.52 3.81

y4.025 y3.549 y49 2.06 1.88

4.503 3.922 – – –

y2.137 y1.858 – – –

0.3500 0.3078 – – –

0.010 0.009 11 0.09 0.11

CO 2

nexc Ž10 3 cmy1 . nem Ž10 3 cmy1 . DD n Žcmy1 . Ž reffrr .exc Ž reffrr .em

27.730 22.656 764 3.95 5.76

y3.111 y3.426 y8 2.05 2.46

3.191 4.119 – – –

y1.611 y2.262 – – –

0.2989 0.4363 – – –

0.006 0.023 5 0.07 0.09

CHF3

nexc Ž10 3 cmy1 . nem Ž10 3 cmy1 . DD n Žcmy1 . Ž reffrr .exc Ž reffrr .em

26.961 21.337 1664 4.51 5.91

y3.761 y4.864 y8 2.20 2.32

3.727 5.168 – – –

y1.689 y2.427 – – –

0.2720 0.4042 – – –

0.009 0.012 3 0.06 0.06

a

The excitation Ž nexc . and emission frequencies Ž nem . and the relative Stokes shifts Ž DD n . are fit to the polynomial in reduced density shown in Eq. Ž3.. Relative effective densities Ž reffrnr . are fit to the exponential form in Eq. Ž6.. All fits are meant to accurately represent the experimental data only over the experimental range, 0.3 F rr F 2.1.

average Stokes shifts vary between 230 and 145 cmy1 in C 2 H 6 , but are essentially constant in CO 2 Ž750 " 10 cmy1 . and CHF3 Ž1660 " 10 cmy1 . over the density range 0.3–2.1 rc . The estimates of DD n in CO 2 and CHF3 are close to the values derived by us previously on the basis of a slightly more sophisticated analysis, 650 " 100 and 1550 " 200 cmy1 , respectively w5x. They differ by more than anticipated uncertainties from the values of Takahashi et al. w6x, who reported relative Stokes shifts that varied between 250 and 400 cmy1 in CO 2 , and between 1100 and 1400 cmy1 in CHF3 . ŽWe believe that the differences may be due to some differences in emission correction procedures, and more importantly to differences in the choice of frequency measures 3.. Perhaps the most surprising feature of the Stokes shift data is their density independence in CO 2 and

3

Takahashi et al. employed peak frequencies determined by fitting the top one-third of the spectra w15x. The small changes in vibronic structure near the peak of the spectra noted in Fig. 1 are probably responsible for the variations in Stokes shift with density they report. The difference in the overall magnitude of the relative Stokes shifts reported Ž ; 300 cmy1 . is likely due to the effects of the substantial structure present in the nonpolar solvents used to extract the solvent component of the Stokes shift via Eq. Ž4..

CHF3 . Given the changes in solvent environment signaled by the large frequency shifts of the spectra, one would expect a substantial increase in the solvent reorganization energies with increasing density in these polar fluids. In the nonpolar solvent C 2 H 6 on the other hand, one would expect a negligible density dependence. The fact that the relative Stokes shift in C 2 H 6 actually decreases significantly with increasing solvent density is another manifestation of the intramolecular changes with solvent noted above. This trend is contrary to what is expected for a solvent effect and it may be partially responsible for the density invariance observed in the polar solvents CO 2 and CHF3 . Thus, if instead of measuring Stokes shifts in these latter solvents relative to pentane, one refers them to the density-dependent values observed in C 2 H 6 , the estimates for the solvent contribution to the Stokes shifts in CO 2 and CHF3 would be found to increase with solvent density: from 530 to 600 cmy1 in CO 2 , and from 1430 to 1500 cmy1 in CHF3 over the experimental range. This density variation is now of the expected sign, but it is still only between 5 and 10% of the total, i.e. it is still surprisingly small. It is not clear whether this use of the C 2 H 6 data is any more correct than use of a single-reference value, and for this reason estimates

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of the solvent contribution to the Stokes shift or the solvent reorganization energy remain uncertain. As a final assessment we conclude only that the solvent reorganization energies Ž lsolv s 12 DD n . of C153 in the polar solvents CO 2 and CHF3 should lie within the ranges 320 " 70 and 770 " 70 cmy1 , respectively, and to be nearly independent of density over the range 0.3 F rrrc F 2.1. We now turn to the estimates of local densities derived from the emission and excitation shifts. To calibrate the behavior expected in the absence of density inhomogeneities we employ the frequencies of C153 previously measured in a wide range of liquid solvents w5x. These calibrations are illustrated in Fig. 3 in the form of fits of the excitation and emission frequencies to the dielectric continuum expression w7x: Õcalc s Õ 0 q a R Ž n2 . q b R Ž ´ . ,

Ž 5.

with RŽ x . '

xy1 xq2

where n is the refractive index and ´ the dielectric constant of the solvent, and a , b , and n 0 are fitting parameters. Fits to the liquid data yield: Õexc Ž 10 3 cmy1 . s 27.91 y 2.53 R Ž n 2 . y 5.65R Ž ´ . , Õem Ž 10 3 cmy1 . s 23.12 y 5.06 R Ž n 2 . y 1.50 R Ž ´ . . As illustrated in Fig. 3 this dielectric representation correlates the excitation Žor absorption. and emission frequencies observed in 30 dipolar and nonpolar liquid solvents Žopen circles. to an accuracy of ; " 270 and ; " 180 cmy1 , respectively. In addition, as discussed in Ref. w7x, the fitted frequencies n 0 are equal to the gas-phase frequencies estimated on the basis of high-temperature data w16x, to within these same uncertainty limits. Thus, Eq. Ž5. should provide good estimates of the frequencies expected in homogeneous polar fluids. In the case of C 2 H 6 and CHF3 Žfilled triangles and open diamonds, respectively, in Fig. 3. the

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high-density data Žlow-frequency points. are in good agreement with the liquid-phase correlation. However, at most densities the data deviate markedly. We interpret these deviation as reflecting solvent inhomogeneities, i.e. as indicating that the local density in the neighborhood of C153 deviates from that of the bulk fluid and is thus not described by Eq. Ž5. when one uses values of dielectric parameters appropriate to the bulk solvent density. If we assume that Eq. Ž5. holds locally but with values of n and ´ appropriate to some other Žlocal. density, we can use the relationships RŽ n 2 . s f Ž r . and RŽ ´ . s f X Ž r . in Eqs. Ž1. and Ž2., respectively, to determine this other, effective density via reff ' r Ž nobs ., where r Ž n . is the inverse of n w f Ž r ., f X Ž r .x. This inversion procedure 4 has been carried out to yield the effective densities in C 2 H 6 and CHF3 shown in Fig. 4. The same inversion cannot be effected for the CO 2 data since in this case the observed frequencies deviate from the dielectric continuum predictions even at high densities. This deviation is to be expected for the reason that the continuum reaction field factor RŽ ´ . does not describe the polarity of quadrupolar liquids like CO 2 w5x. However, effective densities can still be derived in this case by making some reasonable assumptions about how the correct ‘RŽ ´ .’ function would behave. First, we note that RŽ n 2 . is very nearly proportional to solvent density in CO 2 over the range of experimental interest. The same is true of both RŽ n 2 . and RŽ ´ . in CHF3 . It is therefore reasonable to assume that the correct ‘RŽ ´ .’ would also be proportional to density. In this case one would expect n s n 0 q K r in the absence of density inhomogeneities. We further assume that at the highest densities studied Ž rr ; 2. the system is indeed homogeneous, so that the bulk density and the observed frequency determine the value of K. Effective or local densities can therefore be determined from the observed frequencies using the relation reff ' Ž nobs y n 0 .rK. Results obtained in this manner were used to calculated the CO 2 results shown in Fig. 4. Support for the validity of this simplified procedure comes from the fact that its use with the CHF3 data yields identical results to those

4

Inversions of this sort have been performed by many authors. One of the earliest discussions is given in Ref. w17x.

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Fig. 4. Effective densities determined from excitation Žopen symbols. and emission Žsolid symbols. shifts as described in the text. Values of the density augmentation Ž reff y r . and relative densities Ž reffrr . are shown in the left- and right-hand panels, respectively. Squares and triangles denote two independent sets of data Žexcept for ethane.. The curves through the Ž reff y r . data are densities determined from the frequency fits to Eq. Ž3. Žthree data sets.. Curves through the Ž reffrr . data are fits to Eq. Ž6..

obtained with the more complete inversion process described above. Fig. 4 provides two representations of the effective densities implied by the spectral shifts. The left-hand panels display values of the density augmentation Ž reff y r . and the right-hand panels the relative densities reffrr . Filled and open symbols denote values derived from emission and excitation measurements, respectively. A summary of these

data are provided in Table 1 using fits to the function:

reffrr s 1 q A exp Ž yB rr . .

Ž 6.

Fig. 4 shows that the apparent density augmentation goes through a maximum at a density near to rcr2 in all cases. At this maximum the magnitude of the density increase Ž reff y r . amounts to 0.7–0.9 rc , or to effective densities that are 3- to 4-fold higher

R. Biswas et al.r Chemical Physics Letters 310 (1999) 485–494

than the bulk density. At lower densities reffrr is even larger, but the absolute magnitude of the augmentation decreases as a result of the lower bulk solvent density. The primary conclusion to be drawn from Fig. 4 is that the effective densities measured by C153 are remarkably similar in these three solvents. This observation is surprising given that C153 is a highly polar solute Žespecially in S 1 . and one would anticipate the strength of solute–solvent interactions to be appreciably different in these solvents since their electrical characteristics are widely different. Looking more closely at the data, one does find differences in the apparent values of the local density augmentation in Fig. 4. In CO 2 and CHF3 the emission frequencies report consistently higher effective densities than the excitation frequencies. Such a distinction is not found in C 2 H 6 . In addition, the maximal augmentation occurs at a slightly higher bulk density in C 2 H 6 compared to the other two solvents. Both observations may indicate the presence of subtle differences in local density induced by C153 as a function of solvent and electronic state. However, two things should be kept in mind when considering the results presented in Fig. 4. First, the differences among the different curves reflect variations of only 100–200 cmy1 in the frequency shifts. The behavior of the widths and the Stokes shifts discussed above suggests that some or all of these subtle differences could result from changes in vibronic structure with density. Second, the ‘densities’ reported in Fig. 4 are only effective densities, not true densities. They reflect only the particular view of the solute’s surroundings afforded by electronic spectroscopy. Simulations show that electronic shifts may sense slightly different portions of the solvent environment andror vary differently with density depending on solvent polarity and electronic state w4x.

4. Summary and conclusions The observations made here concerning solvation of C153 in supercritical C 2 H 6 , CO 2 , and CHF3 are comparable to results from previous studies using other solvatochromic probes w2x. Under near-critical conditions ŽTr f 1.02; r F rc . the electronic spectral

493

shifts of C153 report effective densities greatly in excess Ž reffrr ) 4. of those existent in the bulk fluid. The effective densities deduced for C153 in all three solvents are surprisingly similar. One might have anticipated that the stronger interactions between the highly polar solute C153 and the dipolar solvent CHF3 would lead to greater augmentation than in CO 2 or C 2 H 6 . Yet, the maximal augmentation values derived from the spectra are nearly identical. A similar invariance to the details of the solute–solvent interactions has been previously noted by Sun and coworkers w13x and in our own work on substituted anthracenes in various solvents w12x. There does appear to be a small Ž; 20%. and probably significant difference in the apparent augmentation Ž reff y r . of the S 0 and S 1 electronic states of C153 in CO 2 and CHF3 but not in C 2 H 6 , as revealed by differences in the emission and excitation shifts. We are aware of only one other study that has directly compared the density augmentation measured in two different electronic states of the same solute. In that work, Rice et al. w18x reported a much greater difference between the augmentation deduced for the S 0 and S 1 states of pyrene in supercritical CO 2 Ž reffrr s 1.6 vs. 2.4 in S 0 vs. S1.. Given the apparent invariance to many details of solute–solvent interactions noted above, these effects of electronic excitation are interesting and warrant further study. Finally, we have attempted to use the differences between the excitation and emission shifts to measure the solvent contribution to the reorganization 1 energies Ž lsolv f DD n . of the S 0 l S1 transition 2 of C153. These values should be indicative of the reorganization energies expected for electron transfer processes in supercritical solvents, about which little is known at the present time w6x. We have found relatively large solvent reorganization energies in the polar supercritical solvents: 320 " 70 and 770 " 70 cmy1 in CO 2 and CHF3 , respectively. These values are comparable to the reorganization energies previously found in the liquid solvents diethyl ether and ethyl acetate, respectively. What is curious about the observed reorganization energies is that they exhibit very little variation with fluid density over the range 0.3 F rrrc F 2.1. The fact that the excitation and emission spectra shift appreciably Žby ) 1000 cmy1 in the case of CHF3 . indicates that the solvent

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environment undergoes substantial changes over this density range. There is of course, an important distinction between the individual frequencies and their difference, which determines the reorganization energy. The absorption and emission frequencies Žlike the solvation energy. are sensitive to both the electronic and the nuclear polarizabilities of the solvent, whereas only the nuclear polarizability, i.e. that part of the solvent polarization requiring nuclear motion, affects the reorganization energies. It could be that changes in local densityrsolvation structure lead to a variation in the electronic polarizability of the solute’s surroundings while the nuclear polarizability remains unchanged. However, it is difficult to envision how such a difference would occur. Our companion simulation studies of C153 in supercritical fluids w4x predict both components to change with solvent density in a parallel manner. It is also interesting to note that at least two previous studies have reported unusual behavior when comparing absorption and emission shifts in supercritical solvents w19,20x. However, unlike the C153 case, these studies involved solutes known to have overlapping absorption bands or other complications, which renders interpretation in terms of solvent reorganization energies difficult. Further study of reorganization energies in supercritical fluids, especially with other simple solutes, is needed.

Acknowledgements The authors would like to thank Kenji Takahashi and Charles Jonah for sharing the results of their studies prior to publication, as well as helpful discussions on the analysis methods they employed, and also to Abby Robinson for her contributions to the

initial stages of this work. Partial support of this research by the US Office of Naval Research is also gratefully acknowledged.

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