Article pubs.acs.org/JPCB

Probe Dependent Solvation Dynamics Study in a Microscopically Immiscible Dimethyl Sulfoxide−Glycerol Binary Solvent Harveen Kaur,† Somnath Koley,† and Subhadip Ghosh* School of Chemical Sciences, National Institute of Science Education and Research, Bhubaneswar 751 005, India S Supporting Information *

ABSTRACT: Excited state dipole solvation of three coumarin dyes with different hydrophobicities was studied in DMSO−glycerol binary solvent. The solvation times obtained from the three dyes are remarkably different. The highly hydrophilic dye coumarin 343 (C343) exhibits the slowest solvation time (>12 ns) among all the dyes we used. This is in contrast to the most hydrophobic dye coumarin 153 (C153), where the solvated state is reached just within ∼104 ps. However, the moderately hydrophobic dye coumarin 480 (C480) demonstrates an intermediate (∼396 ps) solvation time. Unprecedented slowdown of solvation time of C343 is probably due to the slow diffusion of solvent molecules in the glycerol-rich first solvation shell followed by hydrogen bond rearrangements around the solute dipole. On the other hand, fast solvation of hydrophobic dye C153 is most likely caused by the fast reorganization dynamics of hydrophobic −CH3 groups of DMSO or the carbon backbone of the glycerol molecule around the solute dipole. Interestingly, a remarkable probe dependency in solvation dynamics was not observed in the case of DMSO−water binary solvent or in a neat solvent isopropanol. Probe dependent solvation in a DMSO−glycerol mixture is attributed to the microscopic phase segregation and different locations of coumarin dyes within this binary solvent.



been well established in several experimental studies.1−8 Levinger and her co-workers investigated the role of dipole and quadrupolar contributions to the excited dipole solvation taking place in an acetronitrile−benzene binary mixture.2 Given the linear spectral Stokes shift with Onsager’s polarity field, they ruled out the existence of any preferential solvation within this solvent mixture.2 However, they observed the fastest solvation response time speeds up drastically on addition of a small amount of acetonitrile to the benzene solvent, which is a clear indication of preferential solvation in the fastest initial response which they assumed due to dipolar−quadrupolar interactions.2 Castner and his co-workers studied preferential solvation of an electronically excited dipole in an aqueous 1propanol binary mixture. The nonlinear relationship between steady state and temporal spectral parameters with a change in the mole fraction of propanol (XPrOH) indicating the fluctuating collective interaction between cosolvents next to the excited dipole is very much different from the bulk which they assigned mainly due to the solvent cluster formation within the low propanol content solvents (XPrOH ∼ 0.15−0.25).3 The mole ratio of nonpolar/polar (Xnp/Xp) solvent in the bulk and next to the excited dipole (Ynp/Yp) can be correlated by the following equation.4

INTRODUCTION Solvent polarity plays a crucial role in establishing equilibrium in a polar chemical reaction. Customizing of solvent polarity is often made by mixing of two solvents with broadly different polarities. On mixing of two solvents, new solvent−solvent interaction produces unique properties which are absent within the pure solvents.1−8 Polarity customization is important in chromatographic separation, solubility, elementary chemical reactions, protein folding, and other numerous biological reactions. Most of the binary mixtures exhibit a nonlinear deviation of equilibrium solvent properties (polarity, viscosity, dielectric constant, etc.) with a linear change of the mole fraction (Xi) of one of its solvent components. In an ideal solvent mixture, the equilibrium property (A) can be determined from their mole weighted combination of the same in its cosolvents, i.e., A = X1A1 + X2A2, where Xi is the mole fraction of the ith solvent component. Solvation dynamics in a binary solvent are generally accompanied by an ultraslow component which is not present in pure solvents.1−3 The origin of this ultraslow solvation component has been a subject of debate among several recent reports. In an early work, Mukherjee et al. reported an ultraslow solvation time in a dioxane−water mixture which changes monotonously nearly three times from 1430 to 570 ps with a change in water mole fraction (Xw) from 0.22 to 0.50. They proposed several tentative reasons for the origin of this ultraslow component, including translational diffusion of polar solvent molecules into the first solvation shell of the excited solute dipole, cluster formation, and excited state hydrogen-bond dynamics.1 The existence of a slow solvation component in binary mixtures has © XXXX American Chemical Society

(Ynp/Yp) = (X np/X p)e−Z

(1)

Received: February 26, 2014 Revised: May 6, 2014

A

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Z is the preferential solvation index and can be calculated from the following equation

Z=

2 1 CMμ ΔFN,P 4πε0 2δRTr 6

static inhomogeneity is an inherit property of binary solvents. Probe dependent solvation is more prominent in a microscopically inhomogeneous solvent mixture. A highly polar probe interacts more with the polar region of the binary mixture, and solvent correlation from this dye will provide the solvent relaxation time in the polar region only. On the contrary, a weakly polar probe molecule will be solvated by a relatively less polar solvent. Therefore, probe dependency can only be observed when a spatial distribution of the dielectric constant is present within the medium.

(2)

ΔFN,P is the difference of Onsager’s polarity function (F(ε)) [where F(ε) = 2(ε − 1)/(2ε + 1)] among the cosolvents, which is a positive quantity. Details of other parameters have been discussed elsewhere.1,4−8 A positive Z value in a binary mixture is an indication of dielectric enrichment at the solvation shell next to the solute dipole. The extent of local enrichment of a particular cosolvent around the solute dipole depends on its specific interactions (electrostatic interaction, hydrogenbond formation, etc.) with the solute molecule. In most cases, a highly polar solute-dipole attracts the polar solvent to its immediate solvent shell; therefore, preferential solvation is anticipated when a highly polar dye molecule is being solvated in nonpolar/polar binary mixture. The extent of dielectric enrichment next to the solute dipole is quantified by the Zindex. Huppert and his co-worker studied preferential solvation of C153 in hexane/propionitrile binary solvent, where they obtained a very high (∼1.7) Z-index in excited dipole solvation, which is quite natural as highly polar C153 molecules preferably get solvated by polar propionitrile solvent.6 A similar high Zindex was also reported by Krolicki et al. in toluene−methanol binary solvent, which they ascribed to the solute−solvent hydrogen-bond formation followed by structural rearrangements.8 This observation was further supported by a significant IR band shift of the carbonyl group of C153 in toluene solvent on addition of a small amount methanol.8 Among several binary solvents, the DMSO/glycerol (DMSO/GLY) mixture has received special attention for the following reasons. First, the applications of the DMSO/GLY mixture are well-known as a cryo-protective agent and its antagonistic effect on the structure of proteins.9−11 Second, DMSO shares many similar physical properties (i.e., dielectric constant, refractive index, density, etc.) with GLY which provide an advantage to studying viscosity dependent bimolecular processes in DMSO/GLY mixtures, presuming other physical properties remain the same.12−16 In a recent work, Eric Vauthey and his co-workers studied the solvent viscosity effect on the bimolecular PET reaction in DMSO/ GLY binary solvent, assuming the DMSO/GLY mixture can be used as a substituent of isoviscous ionic liquid.16 However, in a very recent work, Bhattacraryya and his co-workers have pointed out from their spatially resolved FCS studies that the DMSO/GLY binary mixture exhibits two microscopically distinguishable solvent regions, one having a viscosity similar to the bulk and the other region exhibits DMSO-like viscosity.17 Although the outcome of FCS measurement is indeed a striking observation for a relatively unexplored DMSO/GLY binary mixture, further studies are warranted for a comprehensive understanding of the microscopic properties of this system. Recent FCS and PET studies have motivated us for further investigating this binary solvent which we believe to be important for understanding bimolecular reactions taking place within this medium. In this report, we have examined the static inhomogeneity in DMSO/GLY binary solvent by measuring the solvation dynamics of three coumarin dyes with considerably different hydrophobicity. Subsequently, we compared it with a well-studied binary solvent, DMSO/water (DMSO/WT), with similar viscosity to verify if the presence of



RESULTS AND DISCUSSION Steady-State Spectra and Preferential Solvation. Absorption and emission spectra of C153, C480, and C343 in DMSO/GLY and DMSO/WT binary solvents have been depicted in Figure 1. Preferential solvation of C153 and excited

Figure 1. Absorption (A, C) and emission (B, D) spectra of coumarin dyes in DMSO/glycerol (η ∼ 3.7 cP, XDMSO = 0.89) mixture (A, B) and DMSO/water (η ∼ 3.7 cP, XDMSO = 0.33) mixture (C, D). Blue, green, and red lines represent the spectra of C480, C153, and C343.

state hydrogen-bond formation with alcohol in alcohol/alkane binary solvent have been established by experimental studies and simulations.5,18 The polar protic solvent GLY has three hydroxyl groups (similar to alcohol) and is an excellent hydrogen-bond donor and acceptor. DMSO on the other hand is a polar aprotic solvent and is a good hydrogen bond acceptor. Most of the coumarin dyes, being hydrogen-bond donors, are expected to form a hydrogen bond with GLY during the solvation process in the DMSO/GLY mixture. Strong hydrogen bond formation and high viscosity make the solvation time longer (∼500−1000 ps) in neat glycerol.19 However, apart from hydrogen-bond donating ability, glycerol has an alkyl backbone which can easily solvate the hydrophobic surfaces. This is in contrast to the case of polar DMSO (ε0 = 50), which is already established as a very good excited dipole solvator; a much faster solvent response (∼3 ps) has been observed within this solvent.20 All coumarin dyes we used here are equally soluble in DMSO and glycerol; therefore, our results are not biased on the basis of solubility. Nonideality of a binary solvent has been investigated by studying Stokes shifts in absorption and emission energies on changing the mole fraction (Xp) of its polar component.21,22 The polarity of an ideal solvent mixture can be expressed by the sum of the mole weighted polarities of its cosolvents. In real case, most of the binary solvents exhibit a clear deviation of the B

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we used to construct F(ε).25 The effect of dielectric nonideality is found to be more severe when two solvents with widely different polarities are mixed. However, in the case of DMSO/ GLY mixtures, owing to a small difference of dielectric constants (ε0,GLY = 43, ε0,DMSO = 50),26 ε of DMSO/GLY mixtures (at different ratios) virtually remains constant with a change in polar mole fraction (XDMSO).12−16 Therefore, we used a linear combination of mole fraction weighted F(ε) of DMSO and GLY which quantifies the polarity in DMSO/GLY mixtures. The dielectric continuum theory predicts solvatochromic shifts in absorption and emission energies within the binary solvents at different polar mole fractions (Xp) should be linear with F(ε). However, in our case, a linear Stokes shift can be observed only up to F(ε) ≤ 0.979 (Xwater ≤ 0.3) in DMSO/ WT binary solvent (Figure 2). In high water content (Xwater > 0.3) samples, relatively large nonlinear bathochromic shifts were observed in both absorption and emission energies of C153 within this mixture. Nevertheless, in DMSO/GLY mixtures, the absorption energy shifted monotonously to the low energy side with an increase in GLY content (i.e., with decreasing solvent polarity). However, emission maxima were seen to follow two linear shifts, first a red shift of emission peak until XGLY ∼ 0.7, followed by a relatively fast but steady blue shift when XGLY is in the range ∼0.7−1.0 (Figure 2). When a small amount of GLY is added to the DMSO solvent, coumarin molecules in DMSO solvent start to form strong hydrogen bonds with the glycerol molecules which makes the red shift in emission spectra. On addition of more GLY (XGLY > 0.7) to the DMSO/GLY solvent, all coumarin molecules are already saturated with hydrogen bonds, where only the polarity governs the spectral shift. GLY is less polar compared to the DMSO, which drives emission energy to the blue side in higher (XGLY > 0.7) GLY content DMSO/GLY samples. This fact confirms other than dipole−dipole stabilization specific interaction like hydrogen-bond formation plays a crucial role in excited state dipole solvation in DMSO/GLY mixtures. The extent of preferential solvation (Zsp) can be measured experimentally from the “non-ideality” ratio, ρexp, which is1

dielectric constant with a change in Xp, which incorrectly predicts the presence of preferential solvation within the binary solvent.4,23 A better appreciation of solvatochromic shifts caused by specific solvation can be obtained by using Onsager’s polarity function [F(ε) = 2(ε − 1)/(2ε + 1)] in the polarity axis instead of Xp. Figure 2 shows the plots of absorption

Figure 2. Shift of absorption and emission maxima of C153 in DMSO/water mixtures (A) and DMSO/glycerol mixtures (B) as a function of the Onsager polarity function, F(ε).

maxima and fluorescence maxima of C153 in binary solvents with a change in Onsager’s polarity function F(ε).24 ε values of DMSO/WT mixtures were obtained from the literature, which

Figure 3. Picosecond decay of C153 (λex = 405 nm, A and C) and C343 (λex = 405 nm, B and D) in DMSO/glycerol (η ∼ 3.7 cP, XDMSO = 0.89) (A, B) and DMSO/water (η ∼ 3.7 cP, XDMSO = 0.33) (C, D) mixtures. Emission wavelengths (λem) are mentioned in the figure. Solid lines are the best fits. C

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∫0

1

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frequencies of TRES at different times mentioned at their subscript. We used three solvatochromic dyes with widely different hydrophobicities which provided the opportunity of studying micro-heterogeneity within the solvent mixture. The highly hydrophobic dye C153 is expected to probe only the hydrophobic region of the solvent. This is in contrast to the highly hydrophilic dye C343 which is likely to investigate only the hydrophilic part of the solvent. C480 with intermediate polarity prefers the moderately polar region of the solvent. The decay of C(t) for the three coumarin dyes in DMSO/WT and DMSO/GLY binary mixtures is depicted in Figure 4. The

(νexp − νlinear,bulk ) dx p Δνp,n

(3)

νexp is the emission energy in a binary mixture at polar solvent mole fraction xp and νlinear,bulk (=Xpνp + Xnpνnp) representing the same in an ideal solvent mixture. The difference in emission frequencies of the excited dipole in polar and nonpolar solvents is quantified by Δνp,n. For ground state calculation of coumarin dye, one uses absorption frequencies (instead of emission frequencies) in the above equation. The experimentally measured nonideality ratio, ρexp (=ρps + ρni), contains both preferential solvation (ρps) and dielectric nonideality (ρni). ρni was obtained from experimentally determined ε values as described elsewhere.27 In a DMSO/WT binary mixture, ρni is calculated from reported values of ε and has been estimated to be 0.27.25 Owing to the small difference in ε between DMSO and GLY, the dielectric nonideality in the DMSO/GLY mixture is assumed to be zero.16 Kaufmann and his co-workers have shown Zps has ∼3.2 times higher magnitude than ρni (when ρni < 1).27 Using this linear relationship, we obtained Zps values of excited state C153 in DMSO/WT and DMSO/GLY of −0.63 and −4.13, respectively. A similar negative Zps value in a dioxane/water mixture was also reported for excited state solvation of C153.7 The total nonideality, ρexp (∼0.081), in DMSO/WT is much smaller than dielectric nonideality, ρni (∼0.27), which is a clear indication that none of the C153 dipoles is contributing any additional nonideality to the binary mixture. Similar observation was also obtained in DMSO/GLY binary solvent. This observation rules out the possibility of dielectric enrichment (i.e., preferential solvation) next to the excited solute dipole. A very high negative magnitude of Zps in DMSO/GLY is quite unnatural. The dielectric enrichment model has been successfully employed on a mixture of two widely different polarity solvents; however, in our case, DMSO and GLY exhibit similar dielectric constants. In addition to this, C153 forms a strong hydrogen bond with glycerol in a DMSO/ GLY mixture which shows a strong bathochromic shift in emission spectra of C153 in a DMSO/GLY mixture with a decrease in solvent polarity (i.e, increasing XGLY until ∼0.7). This anomalous behavior of DMSO/GLY mixtures could be the reason for the high negative Zps. Time Resolved Fluorescence Stokes Shift of Coumarin Dyes in DMSO/WT and DMSO/GLY Binary Solvents. Figure 3 shows florescence transients of coumarin dyes in DMSO/GLY (XDMSO = 0.89) and isoviscous DMSO/WT (XDMSO = 0.33) binary solvents. The lifetime at blue wavelengths consists of only decay components; i.e., in DMSO/GLY at 480 nm, the fluorescence lifetime of C153 can be fitted with a biexponential decay function with components of 66 ps (39%) and 4980 ps (61%), respectively. This is in contrast to the case of red end wavelength 640 nm, where a 5000 ps (107%) decay is preceded by a distinct growth component of 390 ps (−7%) (Figure 3A). The decay at the blue end emission and rise at the red end emission is a clear signature of excited dipole solvation taking place inside the solvent medium. Lifetime fitting parameters of coumarin dye in binary solvent were utilized to construct time-resolved emission spectra (TRES), as described by Marancelli and Fleming (Figures S1 and S2, Supporting Information).28 The time evolution of solvation is described by a correlation function, C(t) [=(νt − ν∞)/(ν0 + ν∞)], describing the emission peak shift of TRES with the passage of time (Figures S1 and S2, Supporting Information). νt, ν0, and ν∞ are the peak

Figure 4. Decay of the solvent correlation function C(t) of C153 (λex = 405 nm), C480 (λex = 375 nm), and C343 (λex = 445 nm) in (A) DMSO/glycerol mixture (η ∼ 3.7 cP, XDMSO = 0.89) and (B) DMSO/ water mixture (η ∼ 3.7 cP, XDMSO = 0.33). The solid line represents the best fit. The inset of part A shows the decay of C(t) of C343 at initial time.

solvation times obtained from the fitting of C(t) of three coumarin dyes in DMSO/WT are not remarkably different. The average solvation time (⟨τ⟩solv) changes nearly 3 times from 54 ps with C153 to 144 ps with C343 (Table 1) within this solvent mixture. This is in contrast to the case of the isoviscous DMSO/GLY mixture (XDMSO ∼ 0.89), where the average solvation time changes over 100 times (or more) from 104 ps with C153 to more than 12 ns in the case of C343. In DMSO/GLY, C343 exhibits anomalously slow dynamics with the solvent correlation curve decaying only ∼5−10% of its total amplitude within the excited state lifetime of C343. The C(t) of which can be fitted with an initial fast decay of 85 ps (5%), followed by a very slow never converging component (>12 ns). On the other hand, for C153 in the same solvent mixture (DMSO/GLY), the solvation dynamics are found to be completed within the sub-nanosecond time region which is accompanied by a fast 90 ps (93%) component and a relatively slow 288 ps (0.07%) component (Table 1). In comparison to C153, C480 exhibits ∼4 times slower average solvation time (396 ps) in the same DMSO/GLY mixture (XDMSO = 0.89) and is only ∼2 times slower (∼116 ps with C480, 54 ps with C153) in the isoviscous DMSO/WT (XDMSO = 0.33) mixture. The microscopic origin of the ultraslow solvation component (>12 ns) of C343 in the DMSO/GLY mixture is not clear to us. For further investigation on the origin of this component, we studied the solvation dynamics of C343 in neat glycerol (Figure S3A, Supporting Information) as well as in neat DMSO. D

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Table 1. Decay Parameters of C(t) of Coumarin Dyes in Different Solvent Mediums decay parameters of C(t) probe

mediums

Δνobsa (ν0) (cm−1)

C343 (λex = 445 nm)

DMSO/GLY DMSO/WT GLY isopropc DMSO/GLY DMSO/WT GLY isopropc DMSO/GLY DMSO/WT GLY isopropc

- (20390) 163(20633) 575(21010) 90 (20716) 175 (21931) 190 (21430) 736 (21501) 390 (22242) 220 (19034) 275 (18760) 500 (19020) 200 (19350)

C480 (λex = 375 nm)

C153 (λex = 405 nm)

a

−1 b

τ1b (a1) (ps) 85 144 248 71 65 116 820 73 90 54 1100 64

(0.05) (1.0) (0.62) (0.94) (0.33) (1.0) (1.0) (1.0) (0.93) (1.0) (1.0) (0.79)

τ2b (a2) (ps)

⟨τ⟩solv (ps)

>12000 (0.95) 1400 (0.38) 386 (0.06) 560 (0.67)

288 (0.07)

691 (0.21)

144 686 90 396 116 820 73 104 54 1100 195

±15 cm . ±10%. Isopropanol. c

of ∼2.0 Å. This is about the same magnitude as the thickness of a solvation shell.1 Although these assumptions can fairly explain the anomaly in dipole solvation of C343 in the DMSO/GLY mixture, further studies like neutron diffraction, two-dimensional infrared (2D-IR), optical heterodyne detected optical Kerr effect (OHD-OKE), and simulations are warranted for a comprehensive understanding. In the next paragraph, we will discuss the probable reasons for slow solvation dynamics in the other binary mixture DMSO/WT, which is one of the most popular and well-studied solvent mixtures. Most of the physical properties of DMSO/WT binary solvent exhibit a remarkable deviation from its ideal behavior at a composition with a mole fraction of DMSO ∼0.33. In an early work, Skaf and his co-workers studied MD simulation of solvation dynamics of C153 in DMSO/WT mixtures across the entire composition range, where they observed the most sluggish solvation at XDMSO ∼ 0.32, which was found to be ∼7 ps, much slower compared to ∼0.45 ps (or 0.9 ps) in neat water (or in neat DMSO).31 Similar anomalies in dipolar solvation in a DMSO/WT mixture have also been established by Bagchi and his co-worker in their recent simulation study.32 They observed a remarkable slowdown in solvation dynamics around 10−20% and 35−50% mole percentage of DMSO.32 According to them, emergence of a slowdown in the DMSO/WT binary mixture can be due to the percolation among DMSO molecules cooperated by water molecules. This phenomenon has been successfully employed in their simulation study, where slowdown in solvation was observed as structural transformations from 1DMSO:2H2O to 1DMSO:1H2O and 1H 2 O:2DMSO at relatively low DMSO concentration (XDMSO = 0.15). At relatively high DMSO concentration (XDMSO = 0.4−0.5%), slowdown in solvation dynamics is due to the formation of network structure where 1DMSO:1H2O and 2DMSO:1H2O are the dominating structures in the network. The presence of a similar network in DMSO/WT has also been established in neutron diffraction experiments, simulations, and polarization selective infrared pump−probe studies.33−37 From 2D-IR and OHD-OKE studies, Fayer and his co-workers have found well-defined water−DMSO hydrogen bond association slows the spectral diffusion by 4 times in the intermediate concentration rage of DMSO in comparison with the cosolvent rich samples.37 In summary, all of these studies have established the presence of strong association among water and DMSO molecules inside the DMSO/WT mixture, which leads the solvation dynamics to the slower time regime. Another striking

Interestingly none of them shows an ultraslow component in their time propagation of the solvent response function [C(t)]; rather, solvation in neat DMSO was found to be ultrafast (few ps)29 and went much faster than the temporal resolution (IRF ∼ 70 ps) of our setup. However, solvation dynamics probed by C343 in highly viscous (η ∼ 1412 cP) neat glycerol is quite slow (∼700 ps) but still much faster than that (>12 ns) observed in a low viscous (η ∼ 3.7 cP) DMSO/GLY mixture. The origin of the ultraslow component of C343 in DMSO/ GLY may be due to the hydrogen bond formation between the −COOH group of C343 with the DMSO−GLY complex. Recently, Dutt et al. have revealed ∼2 times slower decay of the rotational correlation function [r(t)] of C343 compared to C334 in neat DMSO. C334 has an identical structure to C343, except C334 has a −COCH3 group at the 3-position instead of −COOH in C343. They have explained their observation by assuming the presence of a strong hydrogen bond between the −COOH group of C343 with the sulfoxide group of the DMSO, which leads to solvation dynamics at a slower time scale. The existence of similar slow rotational dynamics of C343 is also evident in our binary solvent mixtures (to be discussed in a latter section). Nevertheless, this inadequate information could not explain why C343 in neat GLY is solvated at >10 times faster time scale compared to DMSO/GLY, where hydrogen-bond formation of C343 with a highly viscous glycerol molecule is equally probable as in the DMSO/GLY mixture. However, a sluggish rotational relaxation of C343 in all solvent mixtures is an indication of strong hydrogen bond formation with solvent molecules, which leads to a preferential solvation of C343 by the sulfoxide (or hydroxide) group of DMSO (or glycerol). Solvent relaxation of the DMSO/GLY mixture around the C343 dipole would require rupturing of hydrogen bonds predominately formed with glycerol molecules in the ground state and followed by a diffusion of the polar solvent DMSO to the first solvation shell.5 Diffusion is the slow step of solvation dynamics here. As emergence of the slow component of C343 is evident only in the DMSO/GLY binary solvent, it is reasonable to assume slow diffusion of a glycerol molecule from a glycerol dense (first solvation layer around the solute molecule) region to a DMSO rich bulk region could be the most probable reason for the dramatic slowdown in dipole solvation of C343 within this binary mixture. According to the relation ⟨Z2⟩ = 2Dt, and using a diffusion coefficient of glycerol of 1.77 × 10−12 m2 s−1 (∼1412 times slower than water),30 we obtain in a 12 ns time scale that a molecule can diffuse a length E

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rearrangements of hydrophilic groups as required during the solvation of C343 dipole. Fluorescence Anisotropy Decay of Coumarin Dyes in Binary Mixtures. The fluorescence anisotropy decay (Figure 5) was recorded at ∼20 nm blue side of the emission peak in all

feature of this (DMSO/WT) binary mixture, the difference of solvation times between hydrophobic (obtained from C153) and hydrophilic (obtained from C343) regions of the DMSO/ WT mixture, is significantly less (∼3 times) compared to that (>100 times) observed in the case of the DMSO/GLY mixture. Is such a small difference in the DMSO/WT mixture due to the microscopic inhomogeneity within the solvent mixture, or does it arise from the nonideality in excited state charge distributions among the probe molecules? This question was successfully addressed by studying probe dependent solvation dynamics in neat solvents (homogeneous), such as glycerol and isopropanol (Figure S3, Supporting Information). We could not study the probe dependency in the other neat solvents water and DMSO, as experimental studies revealed that several low viscous polar solvents like water, DMSO, and acetonitrile demonstrate a relatively slower decay (∼few ps) which is preceded by a first initial sub-100 fs decay (Gaussian function); none of these two components can be detected in our system (IRF ∼ 70 ps).38 In neat GLY, the solvent correlation function of C153 can be adequately fitted with a single-exponential decay function with an ∼1.1 ns component, which is ∼1.6 times slower compared to the solvation time (∼0.7 ns) observed in the case of C343. Solvation times obtained from different probe molecules in isopropanol solvent show similar small deviations, where the maximum deviation is ∼2.7 times detected between the solvation time of C153 and C480 (Figure S3B, Supporting Information). Therefore, the ∼3 times difference in solvation time from C153 to C343 in the DMSO/WT mixture may be due to the nonideal excited state charge distributions within the dye molecules or nonideality in their shapes and sizes. In order to explain slow solvation in neat GLY, we may recall the Nandi−Bagchi model which proposed a dynamic exchange between completely immobilized “bound” water at the protein surface with the “free” water in the bulk. At a high binding energy, the slow component (τslow) is given by39 ⎛k T ⎞ ⎛ −ΔG ⎞ ⎟ τslow −1 = k bf = ⎜ B ⎟ exp⎜ ⎝ RT ⎠ ⎝ h ⎠

Figure 5. Decay of the rotational correlation function r(t) of C153 (λex = 405 nm), C480 (λex = 375 nm), and C343 (λex = 445 nm) in (A) DMSO/water mixture (η ∼ 3.7 cP, XDMSO = 0.33) and (B) DMSO/ glycerol mixture (η ∼ 3.7 cP, XDMSO = 0.89). Solid lines (C153, green line; C480, blue line; C343, red line) represent the best fit.

mediums. We found a biexponential fitting curve adequately fits the experimental rotational correlation function (eq 5).

(4)

⎡ ⎛ t ⎞ ⎛ t ⎞⎤ r(t ) = r0⎢a exp⎜ − ⎟ + (1 − a) exp⎜ − ⎟⎥ ⎢⎣ ⎝ τslow ⎠ ⎝ τfast ⎠⎥⎦

kbf denotes the bound-to-free rate constant, and ΔG is the activation energy of solvation dynamics. From eq 4, ΔG values corresponding to ∼700 ps (C343 in glycerol) and 1100 ps (C153 in glycerol) solvation components are −4.9 and −5.2 kcal mol−1, which are in the order of hydrogen bond formation energy. Therefore, the slow step in solvation dynamics in neat glycerol may be the rupturing of hydrogen bonds around the probe molecule. The solvation dynamics of C153 in binary mixtures reports the fastest solvation time, and C343 exhibits the slowest solvation; however, in neat GLY, crossover is observed with the slowest solvation by C153 and the fastest with C343. The latter case is expected, as C153 probes the dry (hydrophobic) region within the solvent where solvent relaxation is restricted. On the contrary, C343 resides in the hydrophilic mobile region of the solvent where solvent molecules can rearrange easily around the excited solute dipole. Probable reasons for the crossover may be due to the amphiphilic character of DMSO; the >SO group of DMSO forms strong hydrogen bonds with water (or glycerol) hydrogen atoms, while methyl groups aggregate to form a hydrophobic region.40 The hydrophobic dye C153 in binary solvents is solvated by the local rearrangement of these weakly coupled methyl groups, which is quite faster compared to the rupturing of hydrogen bonds and followed by

(5)

Rotational dynamics in a simple solvent can be explained by using Stokes−Einstein−Debye hydrodynamics theory as41 ηV fC (6) kT where f (shape factor) accounts for any shape deviation of the solute dipole from its spherical geometry.42 C is the boundary condition which measures the extent of solute−solvent coupling, the value of which varies from 0 (slip boundary) to 1 (stick boundary).43 In the stick case, the tangential velocity of the solvent vanishes at the surface of the rotator. This is in contrast to the slip case, where the tangential component of the normal stress is zero at the surface of the solute molecule. To calculate the C values (using eq 6), we obtained van der Waals volumes (V) and shape factors (f) of probe molecules directly from Table 2 of ref 45. C480 has a similar structure to C153, except at the 4-position, the CF3 group of C153 is replaced with CH3 in C480. Thus, we can reasonably assume the geometrical parameters of C480 will be similar to C153. The solute−solvent coupling parameter (C) was calculated from eq 6, using τr as the average rotational correlation time (Table 2), τr =

F

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Table 2. Parameters of Anisotropy Decay of Coumarin Dyes in Different Mediums probe

medium

r0

C343 (λex = 445 nm)

DMSO/GLY DMSO/WT DMSOb DMSO/GLY DMSO/WT DMSOb DMSO/GLY DMSO/WT DMSOb

0.34 0.35 0.35 0.34 0.34 0.36 0.34 0.35 0.35

C480 (λex = 375 nm)

C153 (λex = 405 nm)

a

τfasta (afast) (ps) 285 397 302 145 258 145 135 260 150

(0.25) (1.0) (1.0) (0.45) (1.0) (1.0) (0.50) (1.0) (1.0)

τslowa (aslow) (ps)

⟨τ⟩

C

600 (0.75)

521 397 302 241 258 145 222 260 150

1.2 0.91 1.53 0.65 0.70 0.72 0.60 0.70 0.75

270 (0.65)

310 (0.50)

±10%. bFigure S4 (Supporting Information).

and η = 3.7 cP is the viscosity coefficient of the DMSO/GLY and DMSO/WT mixtures. The C values obtained from three coumarin dyes in DMSO/WT mixtures are slightly different (0.70 for C153 and C480; 0.91 for C343), indicating a little stronger solvent−solute interaction is being experienced by C343 compared to the other two dyes within this solvent mixture (Table 2). However, in the case of the DMSO/GLY mixture, significantly scattered C values were observed: C343 strongly interacts with the solvent molecules and appears in the superstick region with a C value higher than unity (C = 1.2).44 The other two dyes experience moderate solvent−solute interaction (C = 0.60 for C153 and 0.65 for C480) in the DMSO/GLY binary mixture. The drastic increase in the C value of C343 in DMSO/GLY could be due to the specific solvent−solute hydrogen bond interactions between the OH (or SO) group of glycerol (or DMSO) with the COOH group of C343. This observation provides an explanation that the dramatic slowing down of the solvation dynamics of C343 in the DMSO/GLY mixture may be due to the strong solvent− solute interaction. The rotational correlation functions [r(t)] of coumarin dyes in DMSO/GLY mixtures (Figure 5) are adequately fitted with a biexponential fitting curve, the fast component (τfast) ∼ 135 ps (for C153) and 285 ps (for C343) is essentially similar to the single-exponential rotational time in neat DMSO (C153, 150 ps; C343, 300 ps; Figure S4, Supporting Information, Table 2). The slow component (τslow) of r(t) for all coumarin dyes is ∼1.8−2.3 times slower compared to τfast (or τrot in DMSO), which is commensurate with the bulk viscosity of the DMSO/GLY mixture (η = 3.7 cP), which is 1.85 times higher than the viscosity of neat DMSO (η = 2 cP). This observation clearly indicates that, in the DMSO/GLY mixture, there are broadly two different environments. The first microenvironment is neat DMSO-like and the second microenvironment has a similar bulk-like viscosity. The existence of different microregions in a DMSO/ GLY mixture has already been established in a recent FCS study by the Bhattacharyya group.17 In this study, they observed collective FCS curves of coumarin dye from different regions of the DMSO/GLY mixture, which they fitted with two distinct diffusion constants, corresponding to diffusion in a neat DMSO-like environment and diffusion in a bulk DMSO/GLYlike environment.17 In the case of the DMSO/WT mixture, rotational dynamics can be described by a single-exponential decay constant. The rotational correlation time of C153 and C480 in the DMSO/ WT (η ∼ 3.7 cP) mixture is ∼1.75 times slower compared to neat DMSO (η ∼ 2 cP), which is reasonable, as viscosity changes over ∼1.85 times from neat DMSO to the DMSO/ GLY mixture. However, C343 exhibits only ∼1.3 times slower

rotational time in the DMSO/WT mixture compared to neat DMSO, which is not reasonable with their viscosity change (∼1.85 times), indicating dielectric friction is more drastic in neat DMSO.45 Fluorescence Correlation Spectroscopy Study of Diffusion in a DMSO/WT Mixture. The Bhattacharyya group has already reported the presence of microscopic phase segregation in the DMSO/GLY mixture.17 The dimensions of these domains are too big (>300 nm) to be spatially resolved by a confocal microscope. Phase segregation was apparent in two distinctly different diffusion coefficients (Dt) from a single coumarin dye as observed in a spatially resolved FCS study within a DMSO/GLY mixture.17 Interestingly, the fast Dt resembles the Dt in neat DMSO and remains insensitive with the medium viscosity (i.e., change in DMSO:GLY ratios). On the contrary, a slow diffusion coefficient (Dt) is a direct reflection of bulk viscosity and highly sensitive to the medium viscosity (i.e., DMSO:GLY ratios).17 We studied similar spatially resolved FCS on the DMSO/WT binary system and compared it to the data obtained from the DMSO/GLY mixture, as reported by the Bhattacharyya group.17 In our study, the distribution of Dt values of a coumarin dye in DMSO/WT is quite broad and a single diffusion component adequately fitted the FCS curves (Figure 6 and Figure S5, Supporting Information). Dt values obtained for these dyes fall in the range 320 ± 70 μm2/S. The slight differences in average Dt values obtained from individual dyes may be due to their slight differences in size or some specific dye dependent solvent−solute interactions. A similar distribution of single Dt values has also been observed in neat solvent (i.e., DMSO).17

Figure 6. Distribution of Dt values of coumarin dyes in DMSO/water mixture (η ∼ 3.7 cP, XDMSO = 0.33). G

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ACKNOWLEDGMENTS S.G. thanks Ramanujan fellowship grant (SR/S2/RJN-36/ 2012), DST, India, for the support to carry out this work. We also thank NISER for providing grant and instrumental facilities.

Therefore, DMSO/WT can be seen as a neat solvent formed by two highly interacting solvents, where every DMSO molecule forms a complex with water molecules. This is in contrast to the DMSO/GLY mixture where a population of DMSO molecules is unwilling to interact with GLY molecules, which renders the microscopic heterogeneity within the medium.





CONCLUSIONS A very popular but oversimplified assumption, “owing to their many similar physical properties (dielectric constant, refractive index, density etc.) of DMSO and GLY, the DMSO/GLY mixtures at correct ratios would provide an opportunity to study the influence of viscosity on bimolecular reaction process without changing the other physical parameters”, might have rendered an incorrect understanding of a reaction mechanism in several studies.12−16 In this study, we have established that an isoviscous DMSO/GLY mixture can neither be a substituent of a neat liquid (i.e., isopropanol, ionic liquid, etc.) nor be a substituent of another binary mixture, like DMSO/WT. In a DMSO/GLY mixture, three widely different polar dyes experience remarkably different solvent environments. Apart from our solvation dynamics study, a recent spatially resolved FCS study of diffusion in a DMSO/GLY mixture has also revealed the presence of microscopic (>300 nm) nanodomains with widely varying viscosity. Owing to the similar refractive indices of DMSO and GLY, a phase segregated DMSO/GLY mixture cannot be identified from neat solvent by the bare eye. On the other hand, FCS and solvation dynamics studies on DMSO/WT unambiguously confirm no phase segregation within the mixture. FCS study directly shows the appearance of similar nanodomains (like in the DMSO/GLY mixture) is absent in the case of DMSO/WT, at least in the microscopic scale length (∼300 nm, spatial resolution of microscope). Slight probe dependency in solvation dynamics in the DMSO/WT mixture is not due to the inherit inhomogeneity within the mixture; rather, the nonidealty in excited state charge distribution among dye molecules could be the reason. A similar probe dependency was also observed in neat solvent (homogeneous) like GLY and isopropanol. A high interaction among the cosolvents has made the DMSO/WT binary mixture as good as like a neat solvent. On the other hand, noncooperative association of DMSO with GLY renders phase segregation within the DMSO/GLY mixture, no way which can behave like a neat solvent.



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ASSOCIATED CONTENT

S Supporting Information *

Experimental section, TRES in DMSO/WT and DMSO/GLY of all coumarin dyes, C(t) in neat GLY and isopropanol, r(t) in neat DMSO, and FCS curves in DMSO/WT. This material is available free of charge via the Internet at http://pubs.acs.org.



Article

AUTHOR INFORMATION

Corresponding Author

*Phone: +91-674-2306589. E-mail: [email protected] Author Contributions †

Both authors contributed equally.

Notes

The authors declare no competing financial interest. H

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Article

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dx.doi.org/10.1021/jp502003x | J. Phys. Chem. B XXXX, XXX, XXX−XXX

Probe Dependent Solvation Dynamics Study in a Microscopically Immiscible Dimethyl Sulfoxide-Glycerol Binary Solvent.

Excited state dipole solvation of three coumarin dyes with different hydrophobicities was studied in DMSO-glycerol binary solvent. The solvation times...
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