Probe dependent anomalies in the solvation dynamics of coumarin dyes in dimethyl sulfoxide-glycerol binary solvent: confirming the local environments are different for coumarin dyes.

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Probe dependent anomalies in the solvation dynamics of coumarin dyes in dimethyl sulfoxide– glycerol binary solvent: confirming the local environments are different for coumarin dyes† Somnath Koley, Harveen Kaur and Subhadip Ghosh* The solvation dynamics of coumarin dyes in dimethyl sulfoxide (DMSO)–glycerol (GLY) binary mixtures were studied across the GLY concentrations. Three coumarin dyes with widely different hydrophobicities were used for probing the entire polarity regions of this solvent mixture. Multiple anomalous concentration regions with significantly slow solvation times were detected from all three coumarin dyes. However, their precise positions were found to be probe molecule dependent. The solvation dynamics of the moderately hydrophobic dye coumarin 480 (C480) maintain a plateau region with a similar solvation time (B550 ps) with the increase in GLY concentration until XGLY (the mole fraction of glycerol) reaches 0.5. This plateau region is followed by a sudden slowdown (to B975 ps) on the addition of more GLY to the DMSO–GLY mixture, and then this slow region persists from XGLY B 0.55 to 0.65 (peak at 0.6). On further addition of GLY (XGLY 4 0.7), the solvation dynamics again become slower to B828 ps (at XGLY B 0.8) from B612 ps (at XGLY B 0.7). For very high GLY-content samples (XGLY 4 0.85), the solvation times remain similar on further changes of the GLY concentrations. In contrast to C480, the most hydrophobic dye coumarin 153 (C153) shows a linear increase of solvation time in the DMSO–GLY mixture, from 102 ps (at XGLY B 0.1) to 946 ps (at XGLY B 0.9) with increase in GLY concentration, except for the concentration region, XGLY B 0.45–0.55 (peak at 0.5), where a substantial slowdown of the solvation time is observed. The highly hydrophilic probe coumarin 343 (C343) demonstrates multiple concentration regions (XGLY B 0.05–0.10, 0.25–0.35 and 0.55–0.65) where the solvation dynamics are significantly retarded. The presence of probe dependent anomalies in

Received 7th August 2014, Accepted 26th August 2014

the DMSO–GLY mixture is a clear indication of there being different locations of probe molecules within

DOI: 10.1039/c4cp03525a

several aspects, including the inherit inhomogeneity, intriguing structural transformations in the DMSO–

this solvent mixture. We assume that the slowing-down of the solvation time could be a reflection of GLY mixture, percolation among DMSO molecules and network structure formation, where DMSO:GLY

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complexes contribute to the dynamical features.

1. Introduction Solvent polarity is of great importance in controlling solution phase chemical reactions. The solubility of a nonpolar (or polar) solute in a nonpolar (or polar) solvent can be high, even without much disturbing the solvent clusters.1 However, when a polar (or nonpolar) solute is mixed with a nonpolar (or polar) solvent, the solubility of the solute decreases. The strong associations among the polar solute molecules (or polar solvent molecules) show little tendency to interact with a nonpolar solvent (or nonpolar solute).1 Therefore, the solubility of a School of Chemical Sciences, National Institute of Science Education and Research, Bhubaneswar 751 005, India. E-mail: [email protected]; Tel: +91-674-2306589 † Electronic supplementary information (ESI) available: Experimental section, TRES of coumarin dyes in DMSO–GLY mixtures, rotational anisotropy decays of coumarin dyes in DMSO–GLY mixtures. See DOI: 10.1039/c4cp03525a

22352 | Phys. Chem. Chem. Phys., 2014, 16, 22352--22363

specific substrate in a reaction mixture can be controlled by customizing the solvent polarity on mixing two solvents with broadly different polarities. The binary solvent of a mixture of polar protic and amphiphilic solvent proves to be of great use in chromatographic separation, solubility, elementary chemical reactions, protein folding, and in other numerous biological reactions. The simultaneous presence of hydrophobic and hydrophilic groups in a binary solvent makes its physical properties highly sensitive toward the solvent composition.2–4 Aqueous binary mixtures have received special attention due to their uses as a protein stabilizer, an inhibitor, denaturant, and as an activator.5–7 Binary solvents where DMSO is a cosolvent are also well-studied; their versatile applications are well tested in drug discovery processes, as a cryoprotector, and due to the antagonistic effect on the structure of proteins.8–10 Aqueous DMSO solutions shows a composition dependent non-monotonous dynamical feature.11

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At least two anomalous regions are observed with significant deviations in most of their physical properties: one at B30–40 mole percentage DMSO, and another at a lower concentration range, B10–15%.12–15 Among the various binary mixtures, DMSO in protic solvents have been studied rigorously in theoretical and experimental research.12–20 In a recent simulation, Bagchi and coworkers studied the anomalous behavior of protein (lysozyme) structure fluctuations in aqueous DMSO solvent.13 They observed that at 10–15 mole% DMSO, native lysozyme transforms to a partially unfolded state. Further increases of the DMSO concentration (to 15–20 mole%) leads to a collapsed state, before it transforms into a completely unfolded state at very high DMSO concentrations.13 Using molecular dynamics simulation with a polarizable force field, Head-Gordon and coworkers studied the molecular response of hydration water at the peptide (N-acetyl-leucine-methylamide, NALMA) surface in the presence of DMSO and GLY.6 They observed an increase of hydration number in the glycerol solution, while the same decreases in the neat DMSO solution. Interestingly, in the presence of both the co-solvents, the water dynamics around NALMA are suppressed ¨gelein et al. experimentally detercompared to neat water.6 Go mined the effect of GLY and DMSO on the phase behavior of lysozyme.16 When GLY and DMSO are added, they observed experimentally that the fluid-solid transition was shifted to the lower temperature. Their observation was also supported by theoretical calculations based on the thermodynamic perturbation theory and on the Derjaguin–Landau–Verwey–Overbeek model.16 In an experimental UV circular dichroism study, Bhattacharjya et al. observed, at low DMSO concentrations (o6 mole%), an initiation of transition from the native state to a partially unfolded state of lysozyme, which was essentially subsequently completed at 18 mole% of DMSO concentration.17 They interpreted this observation as a potential consequence of the preferential binding of organic co-solvent to the hydrophobic residues of lysozyme.17 Solvation dynamics in binary solvents have received a considerable amount of interest over the last few decades. Solvation dynamics in binary solvents are generally accompanied with an ultraslow component whose precise origin is not clear.3,4,21,22 Several tentative reasons on the origin of this ultraslow component have been proposed, including the translational diffusion of the polar solvent molecules into the first solvation shell around the excited solute dipole, cluster formation, excited state hydrogen-bond dynamics, structural transformations, or percolation among the solvent molecules, to name a few.12,13,21,22 The emergence of the ultraslow component is ubiquitous among all types of binary solvents. Levinger and coworkers observed quadrupole-assisted dipole solvation taking place in a nonaqueous acetronitrile–benzene binary mixture.3 They found that, on the addition of an insignificant amount of acetonitrile to the benzene, a favorable dipole–quadrupole interaction was generated that causes a dramatic speed up of the fastest solvation component.3 Solvation dynamics in an aqueous-propanol mixture follow a nonmonotonous relationship between the temporal progress of solvation and the isopropanol mole fraction in the mixture.4


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Castner and coworkers assigned this anomalous solvation behavior in an aqueous propanol mixture mainly to solvent cluster formation within the low propanol content solvent mixtures (XPrOH B 0.15–0.25).4 Huppert and coworkers explained the extent of the preferential solvation of C153 in hexane–propionitrile binary solvent, by using a preferential solvation index (Z-index). The positive Z value (B1.7) in their study indicated the presence of preferential solvation, i.e., dielectric enrichment in the immediate vicinity of the solute dipole.23 Among several binary solvents, DMSO–GLY mixtures at correct ratios have emerged as an important candidate for their uses: as a medium to study the viscosity effect on bimolecular processes,19,20 in the partial disruption of protein structure,10 and as a cryo-protective agent.9–11 In a recent work, Bhattacharyya and coworkers studied spatially resolved FCS of coumarin dyes in DMSO–GLY binary mixtures. They observed two microscopically distinguishable environments within the mixture: one neat DMSO-like region, while the other is bulk like.18 Nevertheless, the DMSO–water binary mixture is well-studied in literature; however, only a few reports are available for DMSO–GLY mixture, though its uses have been proven to be equally important as the DMSO–water mixture.15,24–30 In a very recent work, we observed that the solvation dynamics of a hydrophilic dye (coumarin 343) in DMSO–GLY (XDMSO B 0.89) mixture is completed with a much slower timescale (412 ns) compared to the solvation time (B100 ps) obtained from a hydrophobic dye coumarin 153.31 As a continuation of this work, in the present report we study the solvation dynamics of coumarin dyes (differing their hydrophobicities, Scheme 1) in DMSO–GLY binary mixtures as a function of GLY concentration. Our findings provide a significant contribution to this relatively unexplored binary solvent. Interestingly, our observations show that although DMSO and GLY share several similar physical properties (such as dielectric constant, refractive index, and density), their mixtures at different ratios produce multiple anomalous concentration regions, where the solvation dynamics are significantly slow.

2. Results and discussion 2.1 Steady state spectra and time resolved fluorescence Stokes shifts of coumarin dyes in DMSO–GLY binary mixtures The emission spectra of coumarin dyes in DMSO–GLY mixtures at different ratios are depicted in Fig. 1. In all the cases, the hydrogen bond formation between the coumarin dyes and GLY causes an emission peak shift (to the red side) in the DMSO– GLY mixture with increases in the GLY concentration until XGLY reaches B0.7. In the very high GLY content samples (XGLY 4 0.7), all the coumarin dyes are already hydrogen bonded with GLY molecules and the blue shifts of the emission peaks of C153 and C343 are initiated by lowering the polarity within the high GLY content samples (eGLY = 43, eDMSO = 50).32 However, the emission maximum of C480 remains the same within the higher GLY content samples (XGLY Z 0.7). The magnitude of the emission energy shift is a maximum in the case of C480, where we observed lmax em changes over 26 nm from

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Scheme 1

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Chemical structures of coumarin dyes.

Fig. 1 Emission spectra of coumarin dyes in DMSO–GLY binary mixture with XGLY = 0 (black), 0.2 (blue), 0.4 (green), 0.6 (red), 0.8 (dark yellow) and 1.0 (wine), respectively. Absorption ( ) and emission ( ) peak wavelengths as a function of XGLY are depicted in the inset.

453 nm (at XGLY = 0) to 479 nm (XGLY B 0.8). This is in contrast to the hydrophilic dye C343, which registered a maximum of only a 7 nm shift of emission peak on changing the GLY


concentration from 0 mole% (lmax em B 484 nm) to 60 mole% (lmax em B 491 nm). The hydrophobic dye C153 displays a moderate red shift (B11 nm) of the emission peak, from 530 nm


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(at 0% GLY) to 541 (at 70% GLY). In the inset of Fig. 1, we show the emission and absorption peak wavelengths of coumarin dyes as a function of GLY concentration in GLY–DMSO mixtures. In summary, the emission peak wavelengths (lmax em ) of all the coumarin dyes show a gradual red shift with increases in the GLY concentration, and reach a maximum at B60–70 mole% GLY concentration. This red shift is followed by a 2–5 nm blue shift of the lmax em of C153 and C343 on further increases in the GLY concentration in the DMSO–GLY mixture. However, the lmax em of C480 remains unchanged within the higher GLY content samples. The absorption spectra of coumarin dyes show 8–13 nm red shifts with increases in the GLY concentration in the DMSO– GLY mixture. C480 demonstrates the maximum shift of absorption energy by 13 nm, which changes monotonically from 388 nm (at XGLY = 0) to 401 nm (at XGLY = 1). While C153 and C343 display a maximum shift of 9 nm (427 nm to 436 nm) and 8 nm (433 nm to 441 nm) absorption peak shifts with changing the GLY concentrations from 0 to 1 mole fraction (for C153) and 0 to 0.6 mole fraction (for C343), respectively. The preferential solvation of a coumarin dye by alcohol in an alcohol/alkane binary solvent is a common phenomenon, which is initiated by the hydrogen-bond formation between alcohol and the probe molecules.33,34 GLY molecules have three hydroxyl groups, which are strongly involved in hydrogen bond formation with dye molecules in DMSO–GLY mixtures. Apart from the hydrogen bond forming ability, GLY molecules possess a hydrophobic backbone that can easily bind with any hydrophobic surface. Solvation dynamics in neat glycerol is

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hydrogen bond assisted and thus requires local rearrangements of GLY molecules around the excited dipole of the solute molecule. The slow relaxation of GLY molecules within neat GLY makes the entire solvation time much slower (B500–1000 ps) compared to the bulk water (Bfew ps).35–37 In contrast to the GLY, DMSO is known as a fast solvent. The typical solvation time within this solvent is only B3 ps.38 In this work, the solvation dynamics of coumarin dyes in DMSO–GLY mixtures at different ratios were studied, utilizing a diode laser with limited time resolution (B70 ps), although this meant that a part of the solvation dynamics which falls in the ultrafast region (o30 ps) cannot be detected by our setup. The solvation time was obtained by employing the fluorescence time-dependent Stokes shift (TDSS) technique, which directly quantifies the time evolution of the solvation energy of the excited dipole. TDSS was obtained from the steady state emission spectrum and the time-resolved parameters were reconstructed as described by Maroncelli and Fleming.39 Fig. 2 shows the fluorescence lifetime decays of C480 and C343 in DMSO–GLY binary mixtures at 0.1 and 0.6 mole fraction of GLY, respectively. The lifetime transients at the blue wavelengths were adequately fitted with only decay components, i.e., at XGLY B 0.6, the lifetime decay of C480 at the extreme blue-end (440 nm) consists of 580 ps (47%) and 4640 ps (53%) decay components. This is in contrast to the redend emission (560 nm), where a 4790 ps decay is preceded by a distinct growth component of 860 ps. The blue-end emission is obtained from the unsolvated state, where the excited state population can be either directly funneled to the ground state

Fig. 2 Picosecond lifetime decay of C343 (lex = 405 nm, A & B), and C480 (lex = 375 nm, C & D) in DMSO–GLY binary mixtures at XGLY B 0.1 (A & C) and 0.6 (B & D). Emission wavelengths (lem) are mentioned in the figure. The solid lines are the best fits. Lifetime decays were measured up to 15 ns, only the initial portions are given in the figure.



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(as a blue emission) or converted to a solvated state before it actually reaches the ground state. On the other hand, a redend emission originates from the solvated state, where the population enrichment (‘‘growth’’) occurs at an initial time, due to excited state solvation, which is followed by an exponential decay at a longer time. Decay at the blue end and growth at the red end emissions are a clear indication of solvation dynamics taking place within the medium. The time progression of the solvation process is described by a correlation function, C(t) [=(nt nN)/(n0 nN)], constructed from the emission energies of the excited dipole immediately after excitation [i.e., at time zero (n0)], at a particular time t (nt), and after a long time (nN) when the solvation process has come to its end.39 Time-dependent emission energies were obtained from constructing the time dependent emission spectra (TRES) as described in the literature.39 Fig. S1 (in ESI†) shows the TRES of C480 and C343 in DMSO–GLY binary mixtures at different ratios (XGLY B 0.1 and 0.6). The solvation correlation functions [C(t)] of coumarin dyes at different GLY content DMSO–GLY samples are depicted in Fig. 3 and 4. The C(t) curves obtained from C153 and C343 were adequately fitted with a bi-exponential decay function [eqn (1), b = 1]. However for C480, the initial time evolution of the solvent correlation function is fitted with an exponential decay, and the latter part becomes diffusion assisted, which can be better appreciated by introducing a stretched exponential fitting parameter within the fitting function (eqn (1)).40 The non-exponential decay of C(t) of C480 at a longer time might be a reflection of multiple relaxation channels, which is quite possible as at a longer time, the moderately hydrophobic dye C480 starts diffusing and experiences heterogeneous micro-domains across the DMSO–GLY mixture. This conjecture is further supported by the large magnitude (B26 nm) of the emission peak shift of C480 with changing the GLY concentration in the DMSO–GLY mixture, indicating that different polar environments are being experienced by C480. In the case of the other two dyes: C153 (highly hydrophobic) and C343 (highly hydrophilic), diffusion of the excited dipole is unlikely, which is why a simple bi-exponential decay function can successfully fit the solvent correlation curves obtained from these two dyes. The uses of a similar stretched exponential function has been employed in a recent simulation study of the solvation dynamics in DMSO–water mixture by Bagchi and coworkers.15 They found a hybrid fitting function with a relatively slow stretched exponential decay, which was preceded by an initial Gaussian type decay in the ultrafast time region, was required for an adequate fitting of the solvation correlation curves obtained from the DMSO–water mixtures.15 The initial sub-100 fs Gaussian type decays in the solvation dynamics were revealed in experimental studies on several fast liquids, including water, DMSO, and acetonitrile.36,38 In our study, owing to the limited time resolution, we can only detect the latter part (430 ps) of the solvation dynamics. Therefore, incorporation of Gaussian function, which accounts for sub-100 fs dynamics, is meaningless in our study. Thus, we used a combination of a fast exponential


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Fig. 3 (A) Decay of solvent correlation function C(t) of C480 (lex = 375 nm) in DMSO–glycerol mixtures with XGLY = 0 (grey B, exponential decay with time component of 3 ps as obtained from ref. 37) 0.01 (red &), 0.6 (green J), 0.7 (blue n), and 0.8 (black ,). (B) Nonideal behavior of b parameter (upper panel) and average solvation time (htsolvi, lower panel) with increases of XGLY in DMSO–GLY binary mixture. At least two anomalous concentration regions are detected: the first, has a prominent peak at XGLY B 0.6, and the second is a relatively weak peak at XGLY B 0.8, where the solvation dynamics become significantly slow. Deviations of b from 1 indicates the nonexponential solvation dynamics (i.e., microscopic heterogeneity). Low b values at anomalous regions indicate the medium becomes more heterogeneous. Red dashed circles showing b and htsolvi are highly correlated at those two said XGLY regions.

decay, together with the stretched exponential function at a longer time, which could reasonably account for the entire solvation dynamics obtained from C480 in DMSO–GLY mixtures (eqn (1)). b

C(t) = a1e(t/t1) + a2e(t/t2)

(1)

The average solvent relaxation time was calculated by adding the contributions from the weighted time constants of the fast (t1) and slow (t2) relaxing components as follows: htsi = a1t1 + a2t2

(2)


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Fig. 4 (A) Decay of solvent correlation function C(t) of (A) C153 (lex = 405 nm) and (B) C343 (lex = 405 nm) in DMSO–glycerol mixtures with XGLY = 0 (greyB, exponential decay with a time component of 3 ps as obtained from ref. 37), 0.05 (grey ), 0.2 (red &), 0.5 (green J), 0.6 (blue n), 0.9 (cyan x), and 1.0 (black ,). (C–D) Average solvation time (htsolvi) of C153 (C) and C343 (D), as a function of XGLY in DMSO–GLY binary mixture. Multiple anomalous concentration regions are detected for C343, with peaks at XGLY B 0.05, 0.3, and 0.6, respectively. However, in the case of C153, only one anomalous concentration region was found (peak at XGLY B 0.5). We found b = 1 gives a reasonably good fitting for these two dyes. Solvation time (B3 ps) at XGLY = 0 obtained from ref. 37.

In our study, we first tried a simple bi-exponential fitting function (b = 1) to fit the experimentally obtained C(t) curves for all the coumarin dyes, but this turned out to be satisfactory only for C153 and C343 (Fig. 4). In the case of C480, especially at the longer time region, the fitted line clearly deviates from the actual decay profile. However, the best fitting condition can be achieved only after modifying the bi-exponential fitting equation to a hybrid function, namely, by combining a stretched exponential function of time with a fast exponential decay function (eqn (1), Fig. 4). The solvation dynamics in neat DMSO is completely ultrafast (B3 ps), but become more than two orders of magnitude slower on the addition of an insignificant amount of GLY (XGLY B 0.1).38 The magnitude of slowdown is at the maximum in the case of C343 (htsi B 16 700 ps) and minimum for C153 (htsi B 102 ps, Table 1, Fig. 4). Several probe dependent anomalous concentration regions are detected within this solvent mixture, where the solvation dynamics become significantly slow. Among all the coumarin dyes studied here, C343 reported the maximum deviation (410-fold) in solvation time in the DMSO–GLY mixtures from B23 030 ps (at XGLY B 0.05) to B2130 ps (at XGLY B 0.8), while C480 shows only B2-fold deviation between the fastest (B476 ps at XGLY B 0.2) and slowest (B974 ps at XGLY B 0.6) solvation time across the entire GLY concentration range within the DMSO–GLY mixtures (Table 1, Fig. 3 and 4). Not only in their magnitudes (i.e., between the fastest and slowest solvation times)


are they probe dependent, but also the precise positions of these anomalous concentration regions are not the same for every probe molecule. For instance, C153 exhibits an almost linear increase in the average solvation time (htsi B 102 ps to 946 ps) with increases in the GLY concentrations (XGLY B 0.1 to 0.9) within the solvent mixture; except in one concentration region, XGLY B 0.45–0.55, where the solvation time deviates non-monotonically to a slower dynamics (B745 ps, Fig. 4). This is in contrast to the most hydrophilic dye C343, where the solvent response progresses nonmonotonically on the gradual addition of GLY to the DMSO–GLY mixture. At least three (instead of one with C153) slow dynamics regions are detected, with two prominent slowdowns centred at XGLY B0.05, 0.3 and one relatively weak slowdown centred at XGLY B 0.6 (Fig. 4). In the case of the moderately hydrophobic dye C480, solvent relaxation in the DMSO–GLY binary solvent is found to be non-exponential and diffusion assisted at longer times. Such a nonexponential behaviour of solvent relaxation could be a reflection of the non-Markovian character of the friction present within the relaxing solvent molecules, as well as the dynamical heterogeneity inside the solvent mixture.12,43 The b value (eqn (1)) bears the nonideality character of the solvent mixture, which fluctuates in the range of 0.46–1.2 (XGLY B 0.6–0.1) in the DMSO–GLY mixtures when C480 is used as a solvatochromic probe (Table 1). The dynamical deviation of the average solvation times (htsolvi) and changes in the b values with changing the XGLY can identify the anomalous ranges simultaneously in a single frame (Fig. 3B).


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Paper Decay parameters (at 24 1C) of C(t) of coumarin dyes in DMSO–glycerol binary mixtures at different mole fractions of glycerol (XGLY)


Decay parameters of C(t) Probe

XGLY

Dnobsa (n0) (cm1)

t1b (a1) (ps)

C153 (lex = 405 nm)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0c

210 225 291 323 270 350 325 354 367 500

(19 030) (18 916) (18 938) (18 860) (18 766) (18 792) (18 765) (18 770) (18 845) (19 020)

88 160 195 256 420 395 435 680 574 1100

(0.92) (0.97) (0.45) (0.60) (0.92) (0.97) (0.95) (0.98) (0.10) (0.5)

257 720 195 256 4485 4850 4850 5200 988 1100

(0.08) (0.03) (0.55) (0.40) (0.08) (0.03) (0.05) (0.02) (0.90) (0.5)

1 1 1 1 1 1 1 1 1 1

C480 (lex = 375 nm)

0.1 0.2 0.3 0.4 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1.0c

271 325 335 380 354 279 258 432 368 465 486 526 533 530 736

(20 946) (21 900) (21 700) (21 590) (21 520) (21 385) (21 325) (21 436) (21 378) (21 375) (21 367) (21 424) (21 471) (21 410) (21 500)

75 102 465 450 480 572 627 466 55 328 700 44 45 39 820

(0.2) (0.12) (0.92) (0.86) (0.75) (0.7) (0.55) (0.5) (0.21) (0.4) (0.84) (0.04) (0.11) (0.3) (0.5)

628 527 1400 1400 1350 2000 1400 1300 760 1064 1500 733 755 1059 820

(0.8) (0.88) (0.08) (0.15) (0.25) (0.3) (0.45) (0.5) (0.79) (0.6) (0.16) (0.96) (0.89) (0.7) (0.5)

1.12 0.95 0.8 0.77 0.7 0.64 0.46 0.59 0.78 0.75 0.73 0.85 1 0.92 0.94

C343 (lex = 405 nm)

0.05 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0c

143 135 127 203 176 190 300 245 274 300 575

(20 687) (20 690) (20 703) (20 716) (20 560) (20 750) (20 682) (20 680) (20 800) (20 930) (21 010)

370 82 144 231 144 284 417 413 467 480 248

(0.08) (0.28) (0.29) (0.25) (0.42) (0.67) (0.55) (0.71) (0.80) (0.5) (0.62)

25 000 23 000 20 585 23 260 16 344 9300 12 700 7183 8610 5000 1400

(0.92) (0.72) (0.71) (0.75) (0.58) (0.33) (0.45) (0.29) (0.20) (0.5) (0.38)

1 1 1 1 1 1 1 1 1 1 1

a

15 cm1.

b

t2b (a2) (ps)

b

htisolv (ps) 102 177 195 256 745 529 656 770 946 1100 517 476 540 582 698 924 974 883 612 769 828 707 677 753 820 23 030 16 700 14 657 17 502 9540 3260 5945 2376 2130 2740 686

10%. c Ref. 31.

Any deviation of b values from unity represents the nonexponentiality of the dynamical process.15 Non-exponentiality is a reflection of the heterogeneity of the solvent mixture. One can also study the solvent inhomogeneity in terms of b values as a function of XGLY, which provides the same feature that we came across from the dynamical deviation of htsolvi (Fig. 3B). Two anomalous ranges of solvation dynamics of C480 in DMSO– GLY mixtures (across all the GLY concentrations) are captured together by low b values, as well as by long htsolvi values. The solvation dynamics in bulk DMSO is ultrafast (B3 ps).38 However, the htsolvi in the DMSO–GLY binary mixture is at least a few orders of magnitude slower compared to that in neat DMSO. The effect of GLY on the solvation time of C343 in DMSO is much more drastic compared to the other dyes. The solvation times of coumarin dyes in neat DMSO are too fast (o30 ps) to be detected by our setup with limited time resolution (B70 ps). However, the addition of a small amount of GLY to the neat DMSO makes the entire solvent relaxation process substantially slow. The microscopic origin of ‘‘slow solvation’’ in binary mixtures has been the subject of much debate for the last few decades. The addition of


an insignificant amount of GLY to the neat DMSO abruptly slows down the solvation dynamics of C343 to a longer time (htsolvi B 23 ns at XGLY B 0.05; Table 1). C480 and C153 rather show a much faster solvation dynamics in DMSO–GLY mixtures across the entire GLY concentration range. At this stage, it might be worth mentioning that the 23 ns solvation components of C343 were obtained only after truncating the C(t) function at 12 ns (Fig. 4B). Certainly, if we had considered C(t) for a longer time window (i.e., until C(t) touches the time axis with a zero slope, like for C153 and C480), we could have detected even slower dynamics.31 The lifetimes of coumarin dyes are 4–5 ns; therefore, extrapolating the C(t) beyond 12 ns is meaningless. However, the truncated C(t) of C343 is significant in a qualitative sense, especially when comparing the solvation times of C343 in DMSO–GLY mixtures at different GLY concentrations. Similar analyses with nonconverged C(t) curves are also reported by other groups.15,44 The microscopic origin of the 23 ns solvation time of C343 in DMSO–GLY mixture (XGLY B 0.05) is not clear to us. Even in a highly viscous neat GLY solvent, C343 is solvated at a much


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faster timescale (B686 ps). The origin of the ultraslow component of C343 in DMSO–GLY could be a reflection of hydrogen bond formation between the –COOH group of C343 with DMSO–GLY complexes within the binary mixture. In a recent study, Dutt and coworkers observed that the hydrogen bond formation between C343 and DMSO causes B2-fold slower rotation of C343 in neat DMSO compared to the rotational time obtained from a molecule with similar dimensions (with C343) but not hydrogen bonded to the DMSO solvent.41 This observation provides an explanation towards the exceptionally slow solvation dynamics of C343 in DMSO–GLY mixture, which could be a reflection of extensive hydrogen formation between C343 and the solvent molecules. Nevertheless, this piece of information could not provide a satisfactory explanation to the question as to why the solvent relaxation in neat GLY is faster than the DMSO–GLY mixtures, even though the probability of hydrogen bond formation of C343 with GLY is the same as with the DMSO–GLY mixture. The solvent relaxation around the excited state dipole involves a rupturing of the hydrogen bonds predominantly formed between C343 with the GLY molecules in the first solvation shell and is followed by an exchange of less polar GLY molecules from the GLY-rich first solvation shell with the more polar DMSO molecules in the periphery of the first solvation shell.33 Using the relation, hZ2i = 2Dt, and the diffusion coefficient of glycerol, which is 1.77 1012 m2 s1 (B1400-fold lower than water),42 we find that in a 23 ns timescale, a molecule can diffuse a length of B2.8 Å, which is about the thickness of a solvation shell.2 Another useful model proposed by Nandi-Bagchi, which shows a possible origin of the slow solvation component in the binary mixture, could be a reflection of the dynamical exchange of ‘‘bound’’ and ‘‘free’’ solvent molecules around the excited dipole.43 The ‘‘bound’’ molecules are solvent molecules that are completely immobilized and attached to a surface by hydrogen bonds, while ‘‘free’’ molecules are the bulk solvent molecules. At a high binding energy, the slow component (tslow) can be approximated as follows:43

tslow1 ¼ kbf ¼

kB T DG exp h RT

(3)

where DG is the activation energy for the solvent relaxation process, and kbf represents the bound-to-free rate constant. From eqn (3), DG values corresponding to B250–5000 ps (tslow of C153 and C480 in DMSO–GLY mixtures, Table 1) fall in the range of 4.35 to 6.12 kcal mol1, which are of the order of hydrogen bond formation energy.31 In summary, the origin of the slow solvation dynamics in the binary mixture could be a reflection of several processes, including dynamical exchange among the co-solvent molecules, diffusion of the solvent molecules in the first solvation shell, hydrogen bond rearrangements, the non-Markovian character of the friction present within the relaxing solvent molecules and dynamical heterogeneity inside the solvent mixture.


2.2 Fluorescence anisotropy decay of coumarin dyes in DMSO–GLY binary mixtures The fluorescence anisotropy decays of coumarin dyes in DMSO– GLY mixtures were recorded across the entire concentration range of GLY (XGLY B 0 to 1) at B20 nm in the blue side of their respective emission peaks (Fig. S2 in ESI†). A bi-exponential fitting function (eqn (4)) adequately fitted the experimental rotational correlation decay curves. t t rðtÞ ¼ r0 a exp þ ð1 aÞ exp (4) tslow tfast The rotational dynamics in common solvents were found to be single exponentials, and could be explained by using the Stokes–Einstein–Debye (SED) hydrodynamics theory as follows:45,46 tr ¼

ZV fC kT

(5)

In the above equation, the shape factor f compensates for any shape deviations (from its spherical geometry) of the solute dipole.46 Apart from the other parameters, the solute solvent interaction affects the rotational dynamics of the excited dipole and is accounted for by the solute solvent coupling parameter (C). The value of C varies from zero, for the slip boundary condition, to one for the stick boundary condition.47 In the stick boundary condition, the tangential velocity of the solvent molecules at the surface of the rotating dipole vanishes, while in the slip boundary condition, the tangential component of the normal stress is zero at the surface of the solute molecule. The shape factors ( f ) and the van der Waals volumes (V) of C153 and C343 were obtained directly from Table 2 of ref. 41. For C480, we assumed f and V would remain the same as for C153, which is reasonable as C480 has a similar structure to C153, except the 4-position –CH3 group of C480 is replaced by the –CF3 group in C153. Using the values of f, V, viscosity coefficient (Z) and trot as the average rotational correlation time in eqn (5), we calculated the C values of coumarin dyes in DMSO–GLY mixtures across all GLY concentrations (Table 2). Fig. 5 shows the log–log plots of the average rotational relaxation time (htroti) as a function of the viscosity of the medium (Z) within the slip and stick boundary lines.48–50 The slip (or stick) boundary line was obtained by connecting two trot values at the complete slip (or stick) boundary condition in DMSO and GLY, respectively. The trot at the complete slip (or stick) condition was calculated using eqn (5) in neat DMSO (Z B 2 cP) and in neat GLY (Z B 1400 cP) for a particular coumarin dye using Cslip (or Cslick), V, and f values from literature.41,49 From Fig. 5 (and Table 2), it is evident that except for C343, solute solvent coupling parameters (C) remain within the two boundary lines across the entire XGLY range in the DMSO–GLY mixtures. In the case of C343, at low GLY concentrations (XGLY o 0.2), the C values appear at the super stick region (above the stick boundary line) with values of more than unity. A high C value is a clear manifestation of a high solvent–solute interaction that hinders the rotation of the excited dipole. This observation is analogous to the anomalous slowdown of the solvation time (B23–14 ns)


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PCCP Table 2

Paper Parameters of anisotropy decay (at 24 1C) of coumarin dyes in DMSO–GLY binary mixtures at different mole fractions of glycerol (XGLY)


r0

Coumarin dye

tfasta (1 a) (ps)

tslowa (a) (ps) — — —

htroti/Z (ps/cP)

Crot

150 145 302

75 72 150

0.75 0.72 1.53

415 (0.17) 722 (0.06) 538 (0.78)

203 211 470

60 62 138

0.60 0.62 1.24

175 (0.26) 125 (0.35) 300 (0.38)

392 (0.74) 375 (0.65) 770 (0.62)

336 286 590

59 50 103

0.59 0.50 0.93

C153 C480 C343

160 (0.12) 120 (0.20) 320 (0.11)

567 (0.88) 580 (0.80) 1060 (0.89)

520 490 980

51 48 97

0.51 0.48 0.87

0.35 0.33 0.35

C153 C480 C343

200 (0.07) 215 (0.10) 300 (0.12)

1010 (0.93) 985 (0.90) 1850 (0.88)

950 910 1670

51 49 90

0.51 0.49 0.81

31.4

0.35 0.33 0.35

C153 C480 C343

140 (0.05) 150 (0.20) 310 (0.05)

1610 (0.95) 1510 (0.80) 2800 (0.95)

1540 1230 2675

49 39 85

0.48 0.39 0.76

70.8

0.35 0.32 0.35

C153 C480 C343

160 (0.20) 155 (0.06) 300 (0.02)

2780 (0.80) 3060 (0.94) 4740 (0.98)

2250 2880 4650

31 41 66

0.32 0.40 0.59

Z (cP)

XGLY b

0

2.0

0.35 0.35 0.35

C153 C480 C343

150 (1.0) 145 (1.0) 302 (1.0)

0.1

3.4

0.34 0.33 0.36

C153 C480 C343

159 (0.83) 179 (0.94) 220 (0.22)

0.2

5.7

0.34 0.33 0.35

C153 C480 C343

0.3

10.1

0.36 0.33 0.35

0.4

18.5

0.5

0.6

htroti (ps)

0.7

119

0.35 0.34 0.35

C153 C480 C343

150 (0.01) 140 (0.06) 290 (0.03)

3890 (0.99) 4210 (0.94) 6050 (0.97)

3850 3965 5880

32 33 49

0.32 0.33 0.44

0.8

275

0.36 0.33 0.35

C153 C480 C343

160 (0.06) 150 (0.20) 360 (0.03)

7550 (0.94) 7900 (0.80) 12 500 (0.97)

7100 6350 12 130

26 23 44

0.26 0.23 0.40

0.9

510

0.35 0.33 0.35

C153 C480 C343

160 (0.10) 165 (0.01) 300 (0.03)

10 130 (0.90) 14 050 (0.99) 20 680 (0.97)

9130 13 910 20 070

18 27 39

0.17 0.27 0.35

1.0

1400

0.35 0.34 0.35

C153 C480 C343

36 000 (1.0) 34 220 (1.0) 46 500 (1.0)

— — —

36 000 34 220 46 500

26 24 33

0.26 0.24 0.30

a

10%.

b

Ref. 31.

within this region (XGLY B 0.05–0.2, Table 1). Except in this region, the overall trends of C values are the same for all coumarin dyes, the C value decreases monotonically from the near stick boundary values (at low XGLY) to near the slip boundary values (at high XGLY). Similar observations were obtained when we plotted the viscosity normalized average rotational time (htroti/Z) as a function of GLY concentration in DMSO–GLY binary mixtures. The value of htroti/Z decreases with increases in GLY concentration in the binary mixtures. For instance, the viscosity normalized average rotational time of C153 decreases almost monotonically over B4.2-fold, from 75 ps in neat DMSO to 18 ps in 90 mole% GLY DMSO–GLY mixture (Table 2). The same (htroti/Z) changes a maximum of B4.2-fold for C343, from 138 ps (at XGLY B 0.1) in the DMSO–GLY mixture to 33 ps in neat GLY (XGLY = 1). On increasing the GLY concentration in the DMSO–GLY binary mixture, the small DMSO molecules are replaced by large GLY molecules. Solvents of small molecules (like DMSO and water) offer good packing inside the solvent cluster compared to the solvents of large molecules (like GLY).


In the GLY-rich DMSO–GLY binary mixtures there are many spaces inside the solvent clusters where the dipole can easily rotate. This could be the reason why an excited dipole exhibits a faster rotational relaxation time (viscosity normalized) with increases in the GLY concentration in the DMSO–GLY mixture.50 Another interesting observation we noted here is the bi-exponential nature of the rotational dynamics of all the coumarin dyes in the DMSO– GLY mixtures across the entire GLY concentrations. The fast component, which is found to be similar with the rotation time in neat DMSO, remains insensitive to the changes in GLY concentration in DMSO–GLY mixtures. The other component (slow) is highly correlated to the bulk viscosity, and changes linearly with linear changes in the GLY concentration in DMSO–GLY mixtures. We may recall here a recent work of spatially resolved FCS by Bhattacharyya’s group, where they reported the presence of a microscopic phase segregation in DMSO–GLY mixtures.18 They observed that two drastically different environments coexisted within this solvent mixture: one with a viscosity coefficient similar


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PCCP

Fig. 5 The left panel shows the plots of viscosity normalized average rotational times (htroti/Z) of coumarin dyes in DMSO–GLY mixtures as a function of XGLY. The right panel show the log–log plots of the average rotational relaxation times (htroti) of coumarin dyes in DMSO–GLY mixtures (across all the GLY concentrations) as a function of viscosity (Z) of the medium within the slip and stick boundary lines.

to the bulk DMSO, and other that shows a bulk-like viscosity. Our rotational dynamics study also confirms the fact that a large population of DMSO molecules refuse to interact with GLY molecules in DMSO–GLY mixtures, and thus creates neat DMSO-like domains within the solvent clusters of DMSO–GLY mixtures. In a recent study from our group, we reported the single exponential nature of the decay of the rotational correlation function of coumarin dyes in a binary mixture of DMSO and water.31 Therefore, in comparison with the DMSO–GLY mixture, where the microscopic phase segregation is evident, DMSO–water can be treated as a ‘‘good’’ binary solvent, where every DMSO molecule forms complex(s) with water molecule(s) and behaves like a single solvent. In summary, for all coumarin dyes, the rotational correlation decays in DMSO–GLY mixtures are best fitted by a bi-exponential fitting function with a DMSO-like time component. The viscosity normalized average rotational time becomes faster with increases in the GLY concentration in the DMSO–GLY mixture. In low GLY concentration regions (XGLY B 0.05–0.2), the solvent–solute interactions increase drastically, and the rotational dynamics of C153 and C480 almost reach the stick boundary limit. This scenario is even more drastic for C343, where we found super stick dynamics within this concentration region of GLY (XGLY B 0.05–0.1) in DMSO–GLY mixtures.

3. Conclusion A solvation dynamics study of three coumarin dyes in DMSO– GLY mixtures (across all the ratios) shows multiple anomalous concentration regions, where the solvation dynamics are


significantly slow. The most interesting observation is that the precise locations of these ‘‘slow regions’’ in DMSO–GLY mixtures are dye dependent. For instance, the most hydrophilic dye C343 experiences the maximum slowdown of solvation time (B23–14 ns) within the low GLY content (XGLY B 0.05–0.2) DMSO–GLY binary mixtures. This is in contrast to the other two dyes: C153 (most hydrophobic) and C480 (moderately hydrophobic), where we observed a substantial slowdown of the solvation dynamics at relatively high GLY concentrations; XGLY B 0.5 (for C153, htsolvi B 745 ps) and 0.6 (for C480, htsolvi B 974 ps). The overall feature of the average solvation time vs. XGLY curve is also dye dependent. C153 captures only one anomaly (peak at XGLY B 0.5) within the DMSO–GLY mixtures across the entire GLY concentration range, while C343 and C480 detected at least three (peaks at XGLY B 0.05, 0.3 and 0.6) and two (peaks at XGLY B 0.6 and 0.8) anomalous concentration regions in the same binary mixtures. One can assume that in anomalous concentration regions, inhomogeneity (or nonideality) of the solvent mixture reaches its maximum. Therefore, what particular ratios of the mixtures of DMSO and GLY will produce a inhomogeneous mixture depends on the hydrophobicity of the dye molecule. The hydrophilic dye C343, which appreciates only the hydrophilic domains within the solvent mixture, finds DMSO–GLY mixtures are mostly inhomogeneous when GLY concentrations are low (XGLY B 0.05–0.3). If were to use the most hydrophobic dye C153 to study the hydrophobic domains within the DMSO–GLY mixtures, we would find that the hydrophobic domains are mostly homogeneous across the entire GLY concentration range, except for the region XGLY B 0.45–0.55, where the hydrophobic domains reach maximum nonideality.


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PCCP

The solvation dynamics, studied with a moderately hydrophobic dye C480, were found to be diffusion assisted at longer times. C480 utilizes the advantage of being a moderately polar dye, and thus able to diffuse across the domains with widely varying polarities. The two dyes, C153 and C343, remain strictly confined within the hydrophobic and hydrophilic regions only, and a simple bi-exponential fitting function was able to adequately fit the solvent correlation curves obtained from these two dyes in DMSO–GLY mixtures. However, for C480 in DMSO–GLY mixtures, only a stretched exponential fitting function could adequately fit the solvent correlation functions. The deviation of the stretched exponential parameter (b) from unity implies the nonexponentiality of the solvation dynamics.15 The values of b remain highly correlated with the average solvation times in anomalous regions, indicating that the slowing down of the solvation dynamics at the anomalous regions is due to the nonexponential nature of the dynamics. The non-exponential dynamics of the DMSO–GLY mixture is a reflection of the nonideality or inhomogeneity of the medium. In summary, the rotational and solvation dynamics of coumarin dyes in DMSO– GLY mixtures unambiguously establish the presence of microdomains with widely varying polarities within this mixture. The existence of dye specific anomalous regions within the DMSO– GLY mixtures confirms the fact that at particular compositions of DMSO and GLY that produce DMSO–GLY mixtures with inhomogeneous polar domains inside the solvent clusters, the same compositions may not produce the same inhomogeneous nonpolar domains at the same time and vice versa.

Acknowledgements 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 contingency grant and instrumentation facilities

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