Dynamics of Solvent Response in Methanol−Chloroform BinarySolvent Mixture: A Case of Synergistic SolvationShradhey Gupta, Shahnawaz Rafiq, and Pratik Sen*

Department of Chemistry Indian Institute of Technology Kanpur, Kanpur, UP, PIN-208016, India

*S Supporting Information

ABSTRACT: Steady-state absorption, emission, and femto-second transient absorption spectroscopies were used toascertain the static and dynamic nature of the solvent responseof methanol−chloroform binary solvent mixtures of differentstoichiometric ratios using 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) as the probemolecule. The appearance of synergistic solvation behaviorin the steady-state absorption measurements can be explainedin terms of solvent−solvent interactions through an extendedhydrogen-bonding network. The disappearance of suchsynergistic behavior in the excited state of the DCM dye wasrecently proposed by us to be due to the weak nature of theintermolecular interactions present in binary solvent mixtures (J. Phys. Chem. B 2012, 116, 1345). It was anticipated andsubsequently confirmed by the dynamics of the solvent response that the disruption of the weak interactive solvent network isthe main reason for the absence of the synergism in the excited state. As expected, we observed the slowest dynamics for themixture with XMeOH = 0.45, with an average solvation time of 12.03 ps, which is much higher than the values for the pure bulkcounterparts (⟨τs⟩Methanol = 4.32 ps and ⟨τs⟩Chloroform = 1.32 ps). The unprecedented slowing of solvation for DCM is probably dueto the rigid interactive methanol-chloroform solvent system in the first solvation shell, followed by solvent rearrangementsaround the solute dipole. Overall interactions present within the methanol-chloroform binary solvent mixture furnish clearevidence of solvent association through weak hydrogen bonding.

1. INTRODUCTION

The importance of binary solvent mixtures in chemical,physical, and biochemical processes has been extensivelydiscussed in the literature.1−10 Often, the physical propertiesof mixed solvents modify the pathways of chemical trans-formations by altering the energies of participating reactantspecies, and the solvent-dependent characteristics of anintermediate state determine the type of product formedduring a chemical reaction.1,4,6,7 The features of mixed solventsthat determine the course of the reaction and the rates oftransformations are the dynamical response of the solvent andthe local structure of the solvent molecules participating in theformation of solvation shells. Revealing the dynamicalresponses of mixed solvent systems has been a challenge inthe past;10−21 however, recently, a great deal of attention hasbeen focused on establishing the role of solvation in chemicaland biochemical reactions.1−10 The dynamical characteristics ofone solution phase can thus enhance or retard the rate andyield of a chemical transformation as compared to anothersolution phase depending on the dynamical responses and localstructures of the solvent systems.10−13,22−27 In essence, thedynamical features of a solvent system at the molecular leveldetermine the rate of fluctuations of solvent molecules, whichsubsequently determines how rapidly and instantaneouslysolvent molecules respond to an external perturbation.10−13

The time scale of the solvent response will thus depend on therate of breakage of the initial solvent structure and the re-establishment of a new one. As a result, the existence of variousinteractions among solvent molecules and the structuralevolution of an interactive solvent system are significant factorsthat will anticipate the overall solvation dynamics and the rateof intermolecular energy flow.10−13,28,29

The different modes of solvation of solvatochromicmolecules have been widely investigated by various ultrafasttechniques such as femtosecond or picosecond time-resolvedfluorescence spectroscopy,16,14,18,19,21,30−33 femtosecondpump−probe spectroscopy,5,11−13 two-dimensional IR spec-troscopy,5 dielectric relaxation spectroscopy,14 and 1H nuclearOverhauser enhancement spectroscopy (NOESY).34,35 Addi-tionally, much work has also been carried out using moleculardynamics simulations5,16,36−39 and ab initio calculations.14,20,37

Fayer and co-workers5 measured the solvation dynamics ofphenol-OD in pure and mixed solvents by ultrafast two-dimensional IR and IR pump−probe techniques. They noticeda difference in the slow dynamics for pure solvents and theirmixtures, which they ascribed to the different natures of the first

Received: December 2, 2014Revised: January 16, 2015Published: January 21, 2015

Article

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solvent shells (around the solute) in solvents of differentcompositions and also justified by molecular dynamicssimulations. Recently, Zhang et al.14 tested mixtures of anionic liquid and water by dielectric and ultrafast time-resolvedfluorescence techniques and examined the utility of thedielectric solvation model. They found that the solventresponse functions predicted by the dielectric continuummodel were similar to the spectral response functions measuredwith coumarin-153. The only difference they observed was thatthe measured spectral response functions was ∼7 times slowerthan predicted by the dielectric model.14 Agmon20 discussedthe dynamics of preferential solvation by introducing thetranslational diffusion origin of preferential solvation, which isessentially an extension of the Smoluchowski model.In practice, many interactive mixed solvent systems induce

shifts in the steady-state spectra of solvatochromic molecules,and the shifts continue with increasing concentration of themore polar solvent in the binary solvent mixture.40−45 Whensolvatochromic molecules are solvated preferably by the morepolar solvent molecules, this selective solvation is termedpreferential solvation. Some interactive solvent mixtures, on theother hand, induce large shifts in the steady-state spectra ascompared to those in the bulk counterparts, and the observedlarger shifts are caused by the solvation of probe molecules bythe combined components of the solvents in the binarymixture. This nonselective solvation is called synergism.Recently, we noticed synergetic behavior in MeOH−CHCl3

46

and MeOH-dichloromethane binary solvent mixtures that westudied by spectroscopic techniques, 1H NMR analysis,electron-transfer dynamics, and further justified by analyticalmodeling.46 We proposed that the existence of a weakhydrogen-bonded network between hydrogen-bond-accepting(e.g., MeOH) and -donating (e.g., CHCl3, dichloromethane)solvents is mainly responsible for the observed synergism. Themiscibility of alcohols in such weakly interactive solvents leadsto distinct hydrogen-bond networking and, hence, couldpresent a different solvent response. However, to the best ofour knowledge, the solvation dynamics in MeOH−CHCl3binary solvent mixtures has not been investigated. In thepresent study, we address the dynamical characteristics ofMeOH−CHCl3 binary solvent mixtures through femtosecondpump−probe measurements.

2. EXPERIMENTAL SECTION2.1. Materials. High-performance-liquid-chromatography-

(HPLC-) grade chloroform (CHCl3) was purchased fromMerck (Mumbai, India). UV-grade methanol (MeOH) waspurchased from Spectrochem (Mumbai, India) and used afterbeing dried. All care was taken to avoid moisture contaminationduring the experiments. All of the experiments were performedat room temperature. 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) dye was purchasedfrom Exciton, Inc. (Dayton, OH), and used as received.2.2. Methods. 2.2.1. Steady-State Measurements. Steady-

state absorption and fluorescence spectra were measured in acommercial spectrophotometer (UV-2450, Shimadzu Corpo-ration, Kyoto, Japan) and spectrofluorimeter (Fluoromax-4,Jobin-Yvon, Edison, NJ), respectively.2.2.2. Femtosecond Transient Absorption Measurements.

All femtosecond transient absorption measurements wereperformed in a commercially available spectrometer (Femto-Frame-II, IB Photonics, Sofia, Bulgaria). The laser system usedfor transient absorption experiments consisted of a mode-

locked Ti:sapphire femtosecond oscillator (Mai Tai SP,Spectra-Physics, Mountain View, CA) and a Ti:sapphireregenerative amplifier (Spitfire Pro XP, Spectra-Physics)pumped by a 20-W Q-switched Nd:YLF laser (Empower,Spectra-Physics). The regenerative amplifier generated 50-fspulses, centered at 800 nm, at a 1-kHz repetition rate with anenergy of ∼4 mJ per pulse. Briefly, the fundamental beam wassplit into two beams by means of a beam splitter, with themajor proportion being used to generate the pump pulses.Frequency doubling was performed in a 0.2-mm β-bariumborate (BBO) crystal. The fundamental (800 nm) and thesecond harmonic beam (400 nm) were collected collinearly inanother 0.2-mm BBO crystal to generate the third harmoniclight (266 nm). A small portion of the fundamental light wasallowed to pass through a computer-controlled motorized delaystage and was used to generate a white-light continuum with aspectral range of 450−770 nm by focusing it onto a 0.3-mmsapphire crystal. All measurements were performed with apump energy of ca. 1 μJ, and the polarization of the pump wasset at the magic angle (54.7°) with respect to the probe beam.The probe was delayed in time relative to the pump pulse usingan optical delay line providing a maximum time window of 2.0ns. The probe beam after transmission through a sample cellwas focused onto a 200-μm optic fiber cable and was thendispersed by means of a polychromator onto a CCD camera.

3. RESULT AND DISCUSSION3.1. Steady-State Absorption and Emission Spectra in

MeOH−CHCl3 Binary Solvent Mixture. The steady-stateabsorption and fluorescence spectra of 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM, Scheme1) were recorded in MeOH−CHCl3 binary solvent mixtures

with different mole fractions of MeOH. Panels a and b ofFigure 1 show the absorption and fluorescence spectra,respectively, of DCM dye in MeOH−CHCl3 mixtures ofdifferent compositions. The absorption maxima of DCM dye inneat MeOH and CHCl3 are at 466.6 and 469.2 nm,respectively. In the solvent mixture of 0.10 mole fractionMeOH, the absorption maximum changed to 471.4 nm. As themole fraction of MeOH in the MeOH−CHCl3 solvent mixtureincreased from 0.10 to 0.45, the absorption maximum of DCMdye exhibited a monotonic red shift from 471.4 to 475.3 nm.Further increasing the mole fraction of MeOH caused ahypsochromic shift of the absorption maximum from 474.8 nm(XMeOH = 0.5) to 470 nm (XMeOH = 0.9). The variation of theabsorption maximum of DCM dye as a function of XMeOH ispresented in Figure 1a. This figure clearly shows that theabsorption maximum of DCM dye depends strongly on thecomposition of the solvent mixture. To represent thestabilization of the DCM dye in the corresponding binarymixture, the concept of molar electronic transition energy, ET

Scheme 1. Structure of 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) Used in ThisStudy

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(kcal mol−1), was used, whose relationship to the absorptionmaximum (λmax, nm) is given by46−49

λ=E 28591/T max (1)

For an ideal mixture, one expects that any equilibriumproperty in a binary liquid (e.g., solvation energy, composition)will be a weighted average of the values in the two solvents.Thus, in an ideal case, where all solvent−solvent interactionsare assumed to be the same, the dye is characterized by amaximum value of the molar electronic transition energy givenby46,49,50

= +E X E X E(ideal)T S1 T(S1) S2 T(S2) (2)

where XS1 and XS2 are the mole fractions of solvents 1 and 2,respectively, and ET(S1) and ET(S2) are the ET values of the dyeor indicator solute in pure solvents 1 and 2, respectively. Thisequation corresponds to the ideal behavior of a binary solventmixture, and the variation of ET as a function of the molefraction of either component should always be linear. However,in practice, the variation is barely linear, which suggests theexistence of some specific and selective interactions betweenthe dye molecule and one of the components of the solventmixture; this is termed preferential solvation. In addition,deviations from the ideal behavior might also appear from theinteraction of the combined solvent counterparts in the binarysolvent mixture, exhibiting a synergistic effect. In terms of molartransition energy, ET, which provides a direct measure ofstabilization energy, the variation is expressed as a function ofXMeOH in the MeOH−CHCl3 binary mixture (see Figure 2a). Itcan be seen that the maximum stabilization in terms of

solvation was obtained at ca. 0.45 mole fraction of MeOH, afterconsidering the ideal behavior of the dye in the binary mixturein accordance with eq 2 (see Figure 2c). The efficient solvationimposed by the mixed solvent system implies the prevalence ofmutual solvation around the solvatochromic probe molecule,and such a phenomenon is recognized as synergism.46 Thisconfirms that the MeOH−CHCl3 solvent mixture exhibits adifferent solvation behavior compared to those of the puresolvents and that the mutual interaction appears to be strongerthan the interactions of the pure bulk counterparts. However,such a peculiar variation was not observed in the emissionmaxima of DCM dye in MeOH−CHCl3 solvent mixtures;rather, the maxima displayed a monotonic red shift from 567.5to 623.5 nm (see Figures 1b and 2b).In our previous study,46 the appearance of similar features in

MeOH−CHCl3 solvent mixtures using different types of probemolecules (coumarins and 4-aminophthalimide) vividly sug-gested the existence of a more polar environment in the mixedsolvent system. This inherently facilitates distinct synergisticsolvation behavior as compared to that in the pure solvents.However, the observed synergistic solvation disappeared in theexcited state. It was proposed46 that the disappearance ofsynergism in the excited state is most probably due to theperturbations induced in the solvation shell by the large dipolemoment in the excited state, resulting in the breakage of thesolvent structure. This allows selective solvation of the probemolecules by the more polar component of the binary solventmixture. In the present situation, because the excited-statedipole moment of DCM dye is very large, μe = 26.3 D,51 theinteractive solvent structure no longer remains intact, and as a

Figure 1. (a) Normalized absorption and (b) emission spectra of DCM dye in MeOH−CHCl3 mixtures with different mole fractions of MeOH(XMeOH = 0.00, 0.30, 0.45, 0.60, 0.80, 1.00). The excitation wavelength was 450 nm.

Figure 2. Variations of the (a) absorption maximum and molar electronic energy ET, (b) peak position of the emission spectrum, and (c) differencein molar electronic transition energy of DCM dye in MeOH−CHCl3 binary solvent mixtures of different compositions as functions of the molefraction of MeOH in the solvent mixture.

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result, no synergism is observed in the excited state. On thecontrary, the magnitude of the ground-state dipole moment (μg= 5.6 D)51 is not sufficient to make any perturbation in thestructure of the solvent mixture, and hence, synergisticsolvation prevails. Thus, the degree of synergistic solvationdepends directly on the magnitude of the dipole moment of thedye molecule used and the strength of the H-bonding networkbetween the interactive solvents present in the solvent mixture.If the disruption of the interactive solvent system upon thecreation of a strong dipole is true, similar behavior should bedetected in the stabilization of the suddenly created dipole as afunction of the mixture composition. To establish the aboveproposition of efficient solvation provided by the solventmixture as compared to the pure solvents, a solvation dynamicsstudy was undertaken to determine the solvation times of thepure solvents and the solvent mixture. Femtosecond transientabsorption measurements of DCM dye were pursued to trackthe time evolution of the stimulated emission band.3.2. Solvation Dynamics Study in MeOH−CHCl3

Solvent Mixture. The time-resolved difference absorbance(ΔA) spectra of DCM dye in MeOH−CHCl3 mixtures withdifferent proportions of MeOH were measured by femtosecondtransient absorption spectroscopy over the wavenumber rangeof 13000−22000 cm−1. Figure 3 shows the differenceabsorption spectra (ΔA) of DCM dye in neat MeOH; neatCHCl3; and MeOH−CHCl3 mixtures with XMeOH = 0.45, 0.70,

and 0.20. In each plot, the difference spectrum correspondingto the negative delay time between the pump and probe beamsis shown as a reference (black). In all five cases, within thecurrent spectral window, two processes can be observed:excited-state absorption (ESA) and stimulated emission (SE).The detailed solvent-dependent transient absorption signalassignment of DCM dye has already been reported in theliterature.52−54 Briefly, in MeOH, the time evolution oftransient absorption signals suggests the involvement of twostate in the excited-state dynamics of DCM dye (Figure 3a).The locally excited (LE) state is characterized by an ESA signalat 20000−22000 cm−1 with a broad SE signal at 14000−19000cm−1. Within 1 ps, a new ESA signal appears on the lower-wavenumber side with a maximum at ca. 19000 cm−1 and isassigned to the charge-transfer (CT) state.52−54 The stimulatedemission signal occurs from both the locally excited state andthe charge-transfer state in polar solvents, whereas in nonpolarsolvents (CHCl3), the SE is from the LE state only (no ESAfrom the CT state can be observed in the transient absorptionspectrum of DCM in CHCl3). Instead of the detailed solvent-dependent excited-state dynamics, the evolution of the SEsignal as a function of solvent mixture will be discussed here.More importantly, this procedure signifies the use of transientabsorption in deducing solvation dynamics. In essence, insteadof constructing the time-resolved spectra as in fluorescencespectroscopy, transient absorption can give the raw time-resolved emission spectra (in the case that the SE signal isprominent and does not overlap with other transientabsorption signals), such that no reconstruction is required. Itis effective in removing any ambiguity caused during the fittingof fluorescence transients and can report features such as time-dependent broadening of the emission spectra exactly. Toobtain the exact SE peak position, each ΔA spectrum was fittedby a log-normal function in the SE spectral region.In MeOH, the first distinct SE signal can be identified at 120

fs, and the shape of the subsequent SE signals at longer timesimplies that there is no overlapping of the other transientsignals in the region of maximum emission. The SE signal at120 fs is primarily from the LE state (νmax = 17023 cm−1). Withthe advancement of time, a contribution from CT states alsoappears, and finally, the SE signal results exclusively from theCT state (Figure 3a). With time, there is a continuous shift inthe SE maximum toward lower wavenumbers, and this accountsfor the time-dependent dynamic Stokes shift (TDSS) of theexcited state. Initially, the dynamic Stokes shift is very rapid,and then it slows until 107 ps (νmax = 15550 cm−1) in methanol.In CHCl3 (Figure 3b), as mentioned above, the SE signaloccurs predominantly from the LE state, and thus, the observeddynamic Stokes shift is essentially because of the solvation ofthe LE state. In CHCl3, the first distinct SE signal can beobserved at around 120 fs (νmax = 17750 cm−1) followed by acontinuous shift toward lower wavenumber until reaches asteady value at ca. 30 ps (νmax = 16730 cm−1) (Figure 3b). Inthe MeOH−CHCl3 solvent mixture with XMeOH = 0.45 (Figure3c), the evolution of the transient absorption signal with timesuggests initial decay of the LE state into the CT state. The SEsignal occurs from both the LE and CT states, with the firstdistinct signal observed at 120 fs (νmax = 17284) cm−1. There isan initial fast evolution followed by a slow dynamic Stokes shiftthat appears to reach a steady value at ca. 135 ps (νmax = 16230cm−1).

Figure 3. Time-resolved or transient absorbance spectra of DCM dyein (a) pure MeOH; (b) pure CHCl3; and (c−e) MeOH−CHCl3binary solvent mixtures with XMeOH = (c) 0.45, (d) 0.70, and (e) 0.20at different time periods. The black solid line in each case representsthe spectrum before the pump pulse (i.e., at time zero).

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To extract information from the time-dependent variation ofthe SE signals of the transient absorption spectra, we plotted asa function of time the correlation function C(t), given by52−60

ν νν ν

= − ∞− ∞

C tt

( )( ) ( )(0) ( ) (3)

where ν(0), ν(t), and ν(∞) are the peak wavenumbers of theSE band at time zero, at intermediate times, and at infinite time,respectively. The correlation functions C(t) of DCM dye inpure methanol, pure chloroform, and MeOH−CHCl3 solventmixtures with various proportions of MeOH are shown isFigure 4. The correlation function of DCM in each solvent was

either fitted biexponentially or triexponentially (see Table 1).From the fitting parameters, the average solvation time (τs) wasobtained. The τs value in pure MeOH was found to be 4.32 ps;that in pure CHCl3, 1.32 ps; and that in the mixture with XMeOH= 0.45, 12.03 ps. The average solvation times in both puresolvents in solvent mixtures with and various proportions ofMeOH are reported in Table 1, and the variation is depicted inFigure 5. As can be observed, the XMeOH = 0.45 mixture offersconsiderably slower solvation of the excited state than either ofthe pure counterparts. This result is in concordance with thevariation of the steady-state absorption maximum, wherein theabsorption maximum is red-shifted in the XMeOH = 0.45 mixturecompared to that in either of the pure solvents. The possiblereason for the slower solvation offered by the XMeOH = 0.45mixture might be the rigid solvent cage, which restricts itselffrom undergoing immediate reshuffling once the molecule is

promoted to the excited state. In other words, in the groundstate of DCM, the solvent cage will have some arrangementaround the molecule, and once the molecule is promoted to theexcited state by irradiation, immediate charge redistribution willrequire a simultaneous change in the structure of the solventcage. The time scale of the restructuring of the solvent cagedetermines the solvation time, and it depends on how fast thesolvent molecules within the cage can break their oldarrangement and re-establish new interactions according tothe requirements of the excited state. Slower solvationessentially means that the solvent molecules within thesolvation shell are kept in place by means of intermolecularinteractions and, hence, respond slowly to the changing dipolemoment of DCM in the excited state. This explanationemphasizes the existence of an extended network involvingweak hydrogen-bond formation between MeOH and CHCl3 inthe binary solvent mixture. Consequently, one can expect thatthe binary solvent mixture is well-organized compared to thebulk solvent counterparts and that the maximum organizationprevails at XMeOH = 0.45. These experimental results clearlysuggest the significant role of the enhancement in polarity andthe formation of an extended weak hydrogen-bond network(which was established previously in ref 46) in MeOH−CHCl3binary solvent mixtures in determining the dynamics ofsolvation. The present solvation study supports the propositionof enhanced intermolecular hydrogen-bonding interactions andpolarity of the binary solvent mixture and, hence, synergism.Note that the initial part of the solvation dynamics can be

missed even in the present femtosecond setup with a timeresolution of 120 fs. The magnitude of the missed solvationtime can be calculated as follows.58−60 The emission frequencyat zero time in a polar medium, νem

p (0), can be determinedusing the equation

ν ν ν ν= − −(0) ( )emp

absp

absnp

emnp

(4)

where νabsp represents the steady-state frequency of absorption

of DCM dye in the solvents or solvent mixtures under study.

Figure 4. Solvation response function C(t) of DCM dye in MeOH−CHCl3 binary solvent mixtures with different MeOH mole fractionsXMeOH = (○) 0.00, (□) 0.20, (◊) 0.45, (▽) 0.70, and (△) 1.00.Symbols denote the experimental values of C(t), and correspondingsolid lines represent the best-fit exponential decays.

Table 1. Relevant Parameters of the Solvent Response Functions of MeOH−CHCl3 at Room Temperaturea

XMeOH τ1 (ps) τ2 (ps) τ3 (ps) b1 b2 b3 a0 (cm−1) a∞ (cm−1) ⟨τs⟩

b (ps) C(t) missed (%)

0.00 2.17 0.10 − 0.59 0.41 − 17750 16726 1.32 300.20 0.22 1.59 47.47 0.35 0.56 0.09 17696 16602 5.24 240.45 0.53 7.33 53.47 0.53 0.29 0.18 17416 16186 12.03 290.70 0.37 6.02 23.93 0.45 0.35 0.20 17314 15793 7.06 311.00 0.40 3.41 10.94 0.37 0.36 0.27 17022 15554 4.32 47

aSolvation response function calculated as C(t) = ∑ibi exp(−t/τi) with ν̃(t) = (a0 − a∞)C(t) + a∞.bAverage relaxation time ⟨τ⟩ calculated as Σibiτi.

Figure 5. Variation of the average solvation time as a function of theMeOH mole fraction in MeOH−CHCl3 binary solvent mixtures.

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νabsnp and νem

np represent the steady-state frequencies of absorptionand emission, respectively, of DCM dye in a nonpolar solvent(in the present study, cyclohexane). The percentage ofsolvation missed can be calculated as

ν νν ν

= −− ∞− ∞

×⎡⎣⎢

⎤⎦⎥solvation missed (%) 1

(0) ( )(0) ( )

100abs abs

emp

abs

(5)

The magnitude of the percentage of solvation missed inMeOH−CHCl3 mixtures with different mole fractions ofMeOH was found to be around 30% and is included inTable 1.

4. CONCLUSIONSIn the present work, we re-established the synergistic behaviorof MeOH−CHCl3 binary solvent mixture using the solvato-chromic probe molecule 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) and also explained thedifference between the ground-state and excited-state responsesusing femtosecond transient absorption spectroscopy. Theunique mixed-solvent environment provides a higher degree ofsolvent stabilization energy to the ground state of DCMcompared to the bulk counterparts, and the maximum occursnear XMeOH = 0.45, as proposed in our earlier work.46 Thedisappearance of synergism in the excited state was investigatedthrough a transient absorption study, in which we observed thatthe trend of solvent response time follows variations similar tothose found in the ground-state absorption study. It wasconcluded that the perturbations induced in the solvation shellas a result of the large dipole moment in the excited state resultin the breaking of the solvent structure. The overall interactionspresent within the MeOH−CHCl3 binary solvent mixtureprovide clear evidence of the formation of an interactive solventsystem through a weak hydrogen-bond network. Themystifying feature (higher polarity and weak hydrogenbonding) and inherent complexity present a clear necessityfor further theoretical understanding, as well as moleculardynamics and ab initio quantum chemical studies, which willprovide a more transparent picture of this system in the future.

■ ASSOCIATED CONTENT*S Supporting InformationVariation of the absorption maximum and difference in molarelectronic transition energy of DCM dye in MeOH−CHCl3binary solvent mixtures of different compositions as a functionof the MeOH mole fraction. Absorption and emission spectraof DCM dye in cylcohexane. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: psen@iitk.ac.in.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis paper is dedicated to Professor Kankan Bhattacharyya forhis seminal contributions in solvation dynamics studies. S.G.and S.R. thank CSIR (Council of Scientific and IndustrialResearch, New Delhi, India) for awarding fellowships. Thiswork was financially supported by the Science and EngineeringResearch Board, Department of Science and Technology,

Government of India (Project SR/S1/PC-08/2011) and IITKanpur.

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