Preferential Solvation in Carbonyl-Twisted PRODAN Derivatives

Article pubs.acs.org/JPCA

Preferential Solvation in Carbonyl-Twisted PRODAN Derivatives Yuliia Y. Nikitina, Emil S. Iqbal, Hye Joo Yoon, and Christopher J. Abelt* Department of Chemistry, College of William and Mary, Williamsburg, Virginia, 23185, United States S Supporting Information *

ABSTRACT: The Rosés and Bosch model for preferential solvation is used to analyze the fluorescence behavior of two PRODAN derivatives in binary solvents with one or two protic components. The preferential solvation results suggest that the excited PRODAN derivatives form two H-bonds. The model allows for determining the characteristics of the singly Hbonded excited states. They show red-shifted fluorescence but relatively little quenching. In contrast, the doubly H-bonded excited states are significantly quenched when the protic solvent is a strong H-bond donor (large SA value). With two protic solvents there is little preferential interaction even though the solvents have very different H-bond-forming ability.

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INTRODUCTION Small fluorescent molecules that suffer a large increase in their dipole moment upon formation of an emissive excited state are powerful polarity probes. The increased dipole moment interacts with surrounding solvent molecules and gives rise to large, polarity-dependent Stokes shifts. PRODAN (6-propionyl2-dimethyaminonaphthalene)1 is one example of this class of fluorophores. It forms a charge-transfer excited state with a dipole moment that is double that of the ground state. Specific H-bonding interactions with the carbonyl group also affect its Stokes shift.2−5 Results from the Kamlet−Taft solvatochromic comparison method show that the Stokes shift depends equally on the polarity/polarizability (π*) parameter and the hydrogen bond donor ability (α) parameter of the solvent.6 Because of this complication, care must be used in correlating the Stokes shifts with micropolarity.7,8 PRODAN derivatives bearing carbonyl groups that are twisted out-of-plane (Figure 1) show strong quenching in

Scarlata and Zurawsky examined the preferential solvation of PRODAN before the general phenomenological model used in this paper and described below was developed.10 Their analysis showed that no preferential solvation occurs with mixtures of aprotic solvents, but that a protic solvent (methanol) interacts strongly with the excited state. They proposed the following equilibrium to explain the emission behavior: PRODAN + n MeOH ⇆ PRODAN−(MeOH)n. Determining the ratio of the H-bonded and non-H-bonded fluorophores ratios as a function of mole fraction methanol allowed them to calculate a value of 1.1 for n and 12 for the equilibrium constant in acetonitrile/ methanol. In the preferential solvation model of Rosés and Bosch11−18 and the similar model of Connors,19 the solvation of the indicator I is described by two solvent-exchange equilibria (eqs 1 and 2). The equations invoke three solvated species: two surrounded by either one (S1) or the other (S2) solvents and a third surrounded by an equal number of both solvent molecules (S12). The equilibrium constants, f 2/1 and f12/1, are for the complete exchange and half-exchange, respectively. f12/1

I(S1)m + m /2S2 HoooI I(S12)m + m /2S1

(1)

f2/1

I(S1)m + mS2 HooI I(S2)m + mS1 Figure 1. Structures of carbonyl-twisted PRODAN derivatives 1 and 2.

While the expressions involve any number of surrounding solvent molecules, most systems are modeled adequately with m = 2. In this case the two equilibria expressions describe double and single solvent exchange. A solvatochromic indicator, most commonly Reichardt’s betaine dye,20 is used to sense the

hydroxylic solvents.9 Indeed, the degree of quenching can be related to Catalán’s solvent acidity parameter for that solvent. Clearly the quenching involves H-bonding, but little more is revealed from the empirical correlation. This paper uses competitive solvation results to refine the nature of the species responsible for the strong deactivation. © 2013 American Chemical Society

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Received: July 31, 2013 Revised: August 29, 2013 Published: September 3, 2013 9189

dx.doi.org/10.1021/jp4076433 | J. Phys. Chem. A 2013, 117, 9189−9195

The Journal of Physical Chemistry A

Article

Figure 2. Fluorescence spectra of 4 μM solutions of 1 in acetonitrile/methanol mixtures.

cybotactic regions around the solute. The total system response by the indicator is the superposition of those of the three solvated species. This model, when applied to the PRODAN system above, differs from that of Scarlata and Zurawsky in that it considers both the singly and doubly H-bonded complexes as distinct emitting species. In this paper the preferential solvation model is used to analyze the fluorescence properties of two carbonyl-twisted PRODAN derivatives 1 and 2. It will show that the behavior of the singly and doubly H-bonded fluorophores is quite different.

Data Treatment. The electronic noise was subtracted from the raw emission data, and the abscissa scale was converted to wavenumbers before subsequent mathematical treatment. Emission intensity values were determined through numerical integration of the intensity vs cm−1 data, I(ṽ), between 26 300 cm−1 (380 nm) and 13 300 cm−1 (750 nm). The emission center of mass (ṽCM) was determined from eq 3.

EXPERIMENTAL SECTION Compounds 1 and 2 were prepared previously and sublimed under vacuum before use.21,22 Solvents used for photophysical characterization were spectrophotometric grade. Fluorescence emission data were collected using a fiber optic system with 366 nm LED light source and a high sensitivity Ocean Optics Maya CCD detector in a chamber thermostatted at 23 °C. Absorption spectra were obtained from the same fiber optic system with a miniature deuterium/tungsten light source. Data Collection. Solutions of identical concentrations of the fluorophore were made by diluting 20 μL of a stock solution of the fluorophore (∼5 mg/10 mL toluene) to 10 mL with the two solvents of interest. Two sets of emission data were acquired for each binary mixture study. The first set begins with a 2.0 mL sample of the solution in the less polar solvent. To this sample 11−24 aliquots of the solution in the more polar solvent were sequentially added, and the emission spectrum was recorded after mixing for 1 min. In the second set, the initial sample was 2.0 mL of the solution in the more polar solvent, and 3−4 aliquots of the solution in the less polar solvent were added. The relative molar absorptivities in a series of binary mixtures (0−100% in 10% increments) were determined using the method of standard additions.

Two sets of spectral values (Y) were calculated: the relative quantum yield, Qrel, and the product of the center-of-mass and the relative quantum yield, ṽCM·Qrel.10 Relative quantum yield values were calculated using the integrated emission intensities (I), the relative molar absorptivities (ε), and refractive indices (η) in eq 4. The relative molar absorptivities of the mixtures were estimated using the best-fit third-order polynomial to the plot of relative ε determined for the set of evenly spaced binary mixtures (0−100% in 10% increments) vs mole fraction. The refractive indices of the mixtures were calculated using the Gladestone−Dale equation23 using density data24 for known solvent compositions.

vCM ̃ =

■

∫ I(v )̃ ·v d̃ v ̃ ∫ I(v )d ̃ ṽ

Q rel = Iadj/(Iadj)max

(3)

where

Iadj = I ·

εmax η2 · 2 ε ηmin

(4)

In the Rosés and Bosch approach the spectral values (Y) are related to the equilibrium constants through eq 5 where Γ is the fractional change in the spectral values from those in the less polar solvent (S1). Γ= 9190

f2/1 x 2 + f12/1 rx(1 − x) Y1 − Y = Y1 − Y2 (1 − x)2 + f2/1 x 2 + f12/1 x(1 − x)

(5)



Article

Figure 3. Fluorescence spectra of 4 μM solutions of 1 in isopropanol/methanol mixtures.

The subscripts 1 and 2 refer to the values in the pure aprotic (or less polar, protic) and protic (or more polar) solvents, respectively. The x-variable is the mole fraction of the more polar component. The parameter r is the ratio (Y1 − Y12)/(Y1 − Y2) where Y12 is the spectral value for the pure I(S12) species. Both sets of fractional changes, ΓQ and ΓCM•Q, are governed by the same equilibrium constants f 2/1 and f12/1. The two sets of data are combined to determine f 2/1 as follows. Taking the difference between the solvatochromic values eliminates the squared term in the numerator (eq 6): ΓCM·Q − ΓQ = where

f2/1 =

Q 12 = Q 1(1 − rQ ) + Q 2rQ

f12/1 Δrx(1 − x) (1 − x) + f2/1 x 2 + f12/1 x(1 − x)

dx

=

■

(6)

vCM(12) = (vCM ̃ ̃ ·Q )12 /Q 12

(11)

RESULTS AND DISCUSSION The fluorescence behavior of 1 in acetonitrile/methanol solutions is not explained by only two solvated fluorophores. An isoemissive point is evident at very low methanol concentrations but disappears when the methanol fraction exceeds 1% (Figure 2). At the low methanol concentrations, the emission maximum shifts to the red with increasing methanol concentration. While the peak intensity decreases, the peak width increases, and as a result the integrated intensity remains roughly constant. After the isoemissive point disappears, the integrated intensity decreases rapidly. Thus, the presence of methanol results in at least two distinct methanol-solvated fluorophores in addition to the acetonitrile-solvated fluorophore. The fluorescence behavior of 1 in isopropanol/methanol does not show an isoemissive point (Figure 3). Both the integrated intensity and the emission center-of-mass show

[x 2(1 − f2/1 ) − 2x + 1](f12/1 Δr ) [(1 − x)2 + f2/1 x 2 + f12/1 x(1 − x)]2 (7)

The difference function ΓQ − ΓCM·Q in eq 5 has a maximum value when the quadratic term equals zero (eq 8). x 2(1 − f2/1 ) − 2x + 1 = 0

(10)

The emission center-of-mass for the I(S12) fluorophore is determined from (ṽCM·Q)12 and Q12 (eq 11)

The derivative with respect to mole fraction gives a quadratic term in the numerator (eq 7) d(ΓCM·Q − ΓQ )

(9)

The remaining parameters, f12/1 and r, in eq 4 are determined through nonlinear least-squares fitting for both ΓCM·Q and ΓQ. The relative quantum yield of the I(S12) fluorophore is obtained from rQ (eq 10).

2

Δr = rCM·Q − rQ

1 2 − +1 2 x x

(8)

The mole fraction where the maximum occurs is determined by fitting six points around the maximum to a third-order polynomial function, taking its derivative, setting it to zero, and solving for the mole fraction. The mole fraction then determines the value for f 2/1 (eq 9) 9191



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regularly spaced changes as the fraction of methanol increases. The overall changes in both values are similar to the changes that occur in acetonitrile/methanol mixtures. As reported previously, the Stokes shift and the relative emission intensity for 1 in acetonitrile are very similar to those in isopropanol, despite the fact that one solvent is protic and the other is not.9,22 The changes in the spectral features can be quantified through two calculated values: the emission center-of-mass (eq 3) and the relative quantum yield (eq 4). While the latter can be related to the proportion of each fluorescing species, it is not appropriate to use just the center-of-mass in cases when the quantum yield changes significantly. Here the net shift depends not only on the amount of each component but also on how each contributes to the overall intensity. Instead, the product of the center-of-mass and the relative quantum yield is used (Y).10 Plots of the fractional change in these values (Γ, eq 5) vs methanol mole fraction are shown in Figures 4 and 6 for 1 in

Figure 5. Magnification of the plots of ΓCM·Q (◇) and ΓQ (□) in Figure 3 between 0 and 10% mole fraction methanol. The line with unit slope is shown for comparison.

lifetimes, it is not likely that a complete equilibrium has been established. In particular, the rate of H-bond cleavage for the newly formed H-bonds must be much slower than the rate of fluorescence. Inoue and Yatsuhashi determined this rate with 2aminoanthraquinone, a related aminocarbonyl fluorophore, and found that it was negligible compared to the rate of H-bond formation.25 Nevertheless, the above mathematical treatment of preferential solvation remains valid. Because the cybotactic region undergoes extensive reorganization upon photoexcitation, the probability of forming a particular cybotactic region around the relaxed excited state depends on the relative rates of formation of each. This analysis assumes that the concentration of fluorophore is constant under conditions of steady-state irradiation and that the bulk solvent concentrations do not change. In this case, the proportion of the three different solvated species is determined by their relative rates of formation. The mathematical treatment of this scheme still results in eq 5, but the f values are relative rates of formation rather than equilibrium constants. Specifically, f12/1 = k12/k1 and f 2/1 = k2/k1 where the k values are rate constants for formation of each of the solvated species. It is known that PRODAN shows some preferential solvation in the ground state. As such, the calculated f values are only upper bounds. While f values are reported in this paper, the conclusions that are reached are not based on them. The preferential solvation of excited PRODAN and derivatives is already well established. Recent experimental evidence with PRODAN not only supports the idea of extensive reorganization of the cybotactic region but also suggests that any preferential solvation of the ground state is mostly irrelevant. Multivariate photokinetic results3 indicate that after photoexcitation the solvent sphere reorganizes by first breaking the H-bond to the dimethylamino group, then by reorienting the solvent molecules around the excited state dipole, and finally by forming H-bonds to the carbonyl group. Variable temperature results using LAURDAN, a derivative of PRODAN, in cold ethanol show that the formation of H-bonds can be frozen out.26 If H-bonding to the ground state were significant, then the relaxed, H-bonded fluorophore should form quickly, and it should not be possible to freeze out H-bond formation to the excited state while still allowing for the solvent dipoles to reorganize. Finding the best fit to the Γ plots can potentially provide the two f values and the photophysical characteristics of the mixed-

Figure 4. Plots of ΓCM·Q (◇), ΓQ (□), and ΓCM·Q − ΓQ (Δ) vs mole fraction methanol for 4 μM solutions of 1 in acetonitrile/methanol mixtures.

acetonitrile/methanol and isopropanol/methanol, respectively. Both figures show plots with significant positive deviations from the line with unit slope. Such deviations are evidence for preferential solvation. A blowup of Figure 4 before 10 mol % methanol is shown in Figure 5. The expanded view shows that the initial slopes of the plots are significantly less positive than the later slopes. In fact, the initial slope for ΓQ is slightly negative. This behavior is not seen with 2 in Figure 6. In both figures the ΓCM·Q and ΓQ plots are not coincident. The plots of ΓCM·Q − ΓQ are included in Figures 4 and 6 using the secondary vertical axis. These difference plots show well-defined maxima. In acetonitrile/methanol the maximum is around 13 mol % methanol, whereas in isopropanol/methanol it is around 46%. Both the existence of infection points in the case of 1 and the lack of coincidence for the Γ plots in both 1 and 2 indicate the existence of several solvated fluorescent species. Compound 2 behaves just like compound 1. The analogous spectra and plots are shown in the Supporting Information (Figures S1−S5). In the preferential solvation model, the shapes of the fractional change plots are governed by the two preferential solvation parameters, f12/1 and f 2/1, and the solvatochromic parameters of species with a mixed solvent shell, I(S12), through eq 5. The model casts the f values in terms of equilibrium constants. Since the singlet excited states have short 9192



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Figure 6. Plots of ΓCM·Q (◇), ΓQ (□), and ΓCM·Q − ΓQ (Δ) vs mole fraction methanol for 4 μM solutions of 1 in isopropanol/methanol mixtures.

from the free rotation about the carbonyl−aryl bond available in 2, but not in 1. The carbonyl oxygen is more sterically accessible when the carbonyl group is rotated out of the plane of the naphthalene ring. The results with binary protic mixture (isopropanol and methanol) give other insights. There is very little preferential interaction for one solvent over the other. The fact that the preference is larger in 2 than in 1 could well be due to steric hindrance again. Isopropanol is a larger molecule than methanol, and 2, bearing a t-butyl group, should show stronger sensitivity toward steric effects. The lack of preferential solvation in protic solvents suggests that H-bond-donating ability of the solvent is not a factor in determining which Hbonds are formed with the excited fluorophore. One the other hand, the H-bond-donating ability of the solvent very much affects the deactivation of the doubly H-bonded species. Only the doubly H-bonded species with methanol is deactivated significantly. The species with a single H-bond to methanol and another to isopropanol is hardly quenched. The formation of a doubly H-bonded excited state is supported by calculations for PRODAN in water.27 Reasonable reproduction of the emission maximum requires incorporation of both the specific Hbonding interactions and solvent field effects.28 An example having specific H-bonding interactions but with dramatically different local fields is found in lipid bilayers.29 Quenching by proton transfer is implicated in calculations that use hydronium ion to model solvent effects in aqueous solution.30 Finally, differential quenching by singly and doubly H-bonded fluorophores has been reported by Inoue and co-workers for aminoanthraquinones and aminofluorenones.25,31,32 In the present case, quenching requires twisting about the carbonyl−aryl C−C bond and becomes significant over a very narrow range of H-bond-donating ability.

solvent species (r). The issue that arises is that these plots can be reasonably fit by parameter sets with widely ranging values. Since there is little difference in the goodness-of-fit for these sets, it is not clear that the “best” fit, as defined by the largest R2 value, is necessarily the proper fit. The approach taken in this paper is to first determine one of the parameters ( f 2/1) by combining the ΓCM·Q and ΓQ plots and then the remaining two parameters (f12/1 and r) by a nonlinear least-squares fit. The f 2/1 values are determined through eq 8 from the bulk methanol mole fractions corresponding to the maxima of the ΔΓ plots. The results of these calculations are shown in Table 1. Table 1. Calculated Preferential Solvation Parameters for 1 and 2 in Acetonitrile/Methanol (A) and Isopropanol/ Methanol (B) Mixturesa

a b

mixture

f12/1

f 2/1

Q12b

Q2

rCMc

1

A

2

A

1

B

2

B

13.1 (0.4) 13.9 (1.8) 1.78 (0.01) 3.0 (0.3)

37.6 (0.7) 121 (16) 1.81 (0.03) 4.6 (0.7)

0.99 (0.01) 1.0 (0.1) 0.82 (0.01) 0.70 (0.04)

0.17 (0.01) 0.10 (0.01) 0.18 (0.01) 0.11 (0.01)

0.97 (0.03) 0.78 (0.03) 0.79 (0.01) 0.79 (0.09)

Parenthetical values are standard deviations of multiple experiments. Q1 ∼ 1.0. crCM = (ṽCM(1) − ṽCM(12))/(ṽCM(1) − v C̃ M(2)).

Parameters extracted from the preferential solvation models offer insight into excited states of 1 and 2 in the presence of protic solvents. The results in acetonitrile/methanol mixtures indicate a strong, specific interaction with methanol. This interaction has been interpreted as H-bonding with the carbonyl oxygen. The relative quantum yields of the singly H-bonded fluorophore offer the biggest surprise. They are just as large as the non-H-bonded fluorophore. In contrast, the doubly H-bonded species has nearly an order of magnitude smaller quantum yield. Yet, the bathochromic shift in the singly H-bonded species is more than three-quarters of that of the doubly H-bonded species (rCM > 0.75). The relative rate of formation of the doubly H-bonded fluorophore for 2 is over three times greater than that for 1. This difference may result

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CONCLUSIONS The preferential solvation model of Rosés and Bosch is applied and augmented to elucidate the quenching behavior of two carbonyl-twisted PRODAN derivatives in protic solvents. Notably, the behavior of the singly H-bonded fluorophore is revealed by this approach. While the singly H-bonded species shows a significant Stokes shift, it is hardly quenched. The strong quenching of these carbonyl-twisted fluorophores in 9193



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(14) Bosch, E.; Rived, F.; Rosés, M. Solute−Solvent and Solvent− Solvent Interactions in Binary Solvent Mixtures. Part 4. Preferential Solvation of Solvatochromic Indicators in Mixtures of 2-Methylpropan-2-ol with Hexane, Benzene, Propan-2-ol, Ethanol, and Methanol. J. Chem. Soc., Perkin Trans. 2 1996, 2177−2184. (15) Ràfols, C.; Rosés, M.; Bosch, E. Solute−Solvent and Solvent− Solvent Interactions in Binary Solvent Mixtures. Part 5. Preferential Solvation of Solvatochromic Indicators in Mixtures of Propan-2-ol with Hexane, Benzene, Ethanol and Methanol. J. Chem. Soc., Perkin Trans. 2 1997, 243−248. (16) Rosés, M.; Buhvestov, U.; Ràfols, C.; Rived, F.; Bosch, E. Solute−Solvent and Solvent−Solvent Interactions in Binary Solvent Mixtures. Part 6. A Quantitative Measurement of the Enhancement of the Water Structure in 2-Methylpropan-2-ol−Water and Propan-2-ol− Water Mixtures by Solvatochromic Indicators. J. Chem. Soc., Perkin Trans. 2 1997, 1341−1348. (17) Buhvestov, U.; Rived, F.; Ràfols, C.; Bosch, E.; Rosés, M. Solute−Solvent and Solvent−Solvent Interactions in Binary Solvent Mixtures. Part 7. Comparison of the Enhancement of the Water Structure in Alcohol−Water Mixtures Measured by Solvatochromic Indicators. J. Phys. Org. Chem. 1998, 11, 185−192. (18) Herodes, K.; Leito, I.; Koppel, I.; Rosés, M. Solute−Solvent and Solvent−Solvent Interactions in Binary Solvent Mixtures. Part 8. The ET (30) Polarity of Binary Mixtures of Formamides with Hydroxylic Solvents. J. Phys. Org. Chem. 1999, 12, 109−115. (19) Skwierczynski, R. D.; Connors, K. A. Solvent Effects on Chemical Processes. Part 7. Quantitative Description of the Composition Dependence of the Solvent Polarity Measure ET (30) in Binary Aqueous−Organic Solvent Mixtures. J. Chem. Soc., Perkin Trans. 1994, 2, 467−472. (20) Reichardt, C.; Welton, T. In Solvents and Solvent Effects in Organic Chemistry; Wiley-VCH Verlag GmbH: Weinheim, Germany, 2011. (21) Naughton, H. R.; Abelt, C. J. Local Solvent Acidities in βCyclodextrin Complexes with PRODAN Derivatives. J. Phys. Chem. B 2013, 117, 3323−3327. (22) Everett, R. K.; Nguyen, H. A. A.; Abelt, C. J. Does PRODAN Possess an O-TICT Excited State? Synthesis and Properties of Two Constrained Derivatives. J. Phys. Chem. A 2010, 114, 4946−4950. (23) Herráez, J. V.; Belda, R. Refractive Indices, Densities, and Excess Molar Volumes of Monoalcohols in Water. J. Solution Chem. 2006, 35, 1315−1328. (24) Nikam, P. S.; Shirsat, L. N.; Hasan, M. Density and Viscosity Studies of Binary Mixtures of Acetonitrile with Methanol, Ethanol, Propan-1-ol, Propan-2-ol, Butan-1-ol, 2-Methylpropan-1-ol, and 2Methylpropan-2-ol at (298.15, 303.15, 308.15, and 313.15) K. J. Chem. Eng. Data 1998, 43, 732−737. (25) Yatsuhashi, T.; Inoue, H. Molecular Mechanism of Radiationless Deactivation of Aminoanthraquinones through Intermolecular Hydrogen-Bonding Interaction with Alcohols and Hydroperoxides. J. Phys. Chem. A 1997, 101, 8166−8173. (26) Viard, M.; Gallay, J.; Vincent, M.; Meyer, O.; Robert, B.; Paternostre, M. Laurdan Solvatochromism: Solvent Dielectric Relaxation and Intramolecular Excited-State Reaction. Biophys. J. 1997, 73, 2221−2234. (27) Mennucci, B.; Caricato, M.; Ingrosso, F.; Cappelli, C.; Cammi, R.; Tomasi, J.; Scalmani, G.; Frisch, M. J. How the Environment Controls Absorption and Fluorescence Spectra of PRODAN: A Quantum-Mechanical Study in Homogeneous and Heterogeneous Media. J. Phys. Chem. B 2008, 112, 414−423. (28) Marini, A.; Muñoz-Losa, A.; Biancardi, A.; Mennucci, B. What is Solvatochromism? J. Phys. Chem. B 2010, 114, 17128−17135. (29) Parisio, G.; Marini, A.; Biancardi, A.; Ferrarini, A.; Bennucci, B. Polarity-Sensitive Fluorescent Probes in Lipid Bilayers: Bridging Spectroscopic Behavior and Microenvironmental Properties. J. Phys. Chem. B 2011, 115, 9980−9989. (30) Artukhov, V. Y.; Zharkova, O. M.; Morozova, J. P. Features of Absorption and Fluorescence Spectra of Prodan. Spectrochim. Acta, Part A 2007, 68, 36−42.

good H-bond-donating protic solvents dervives from the doubly H-bonded species.

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ASSOCIATED CONTENT

S Supporting Information *

Analogous figures (S1−S5) for compound 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for support of this research.

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REFERENCES

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Preferential Solvation in Carbonyl-Twisted PRODAN Derivatives

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