Dynamic Solvent Control of a Reaction in Ionic Deep Eutectic Solvents: Time-Resolved Fluorescence Measurements of Reactive and Nonreactive Dynamics in (Choline Chloride + Urea) Melts.

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The Journal of Physical Chemistry

Dynamic Solvent Control of a Reaction in Ionic Deep Eutectic Solvents: Time Resolved Fluorescence Measurements of Reactive and Non-reactive Dynamics in (Choline Chloride+ Urea) Melts Anuradha Das and Ranjit Biswas∗ Chemical, Biological and Macromolecular Sciences, S. N. Bose National Centre for Basic Sciences, J. D. Block, Sec.III, Salt Lake, Kolkata 700 098, West Bengal, India Abstract Dynamic fluorescence anisotropy and Stokes shift measurements of [f choline chloride+ (1-f) urea)] deep eutectic solvents at f= 0.33 and 0.40 have been carried out using a dipolar solute, coumarin 153 (C153), in the temperature range, 298 ≤ T K ≤ 333 . Subsequently, measured time-dependent solvent response is utilised to investigate the dynamic solvent control on the measured rates of

photo-excited intramolecular charge transfer (ICT) reactions of two

molecules, 4-(1-azetidinyl)benzonitrile (P4C) and 4-(1-pyrrolidinyl)benzonitrile (P5C) occurring in these media. Measured average reaction timescales ( τ rxn ) exhibit the following dependence on average solvation times scales ( τ s ): τ rxn ∝ τ s

α

with α =0.5 and 0.35 for

P4C and P5C, respectively. Such a strong dynamic solvent control of τ rxn , particularly for P4C,

is different from earlier observations with these ICT molecules

in conventional

molecular solvents. Excitation wavelength dependent fluorescence emissions of C153 and trans-2-[4-(dimethylamino)styryl]-benzothiazole (DMASBT), which differ widely in average fluorescence lifetimes ( τ life ), suggest presence of substantial spatial heterogeneity in these systems. Dynamic heterogeneity is reflected via the following fractional viscosity ( η ) dependences of τ s and τ r ( τ r being solute’s average rotation time): τ x ∝ (η T ) with p

0.7 ≤ p ≤ 0.9 . Different correlations between τ s and τ r emerge at different temperature regimes, indicating variable frictional coupling at low and high temperatures. Estimated dynamic Stokes shifts in these media vary between ~1200 cm-1 and ~1600 cm-1, more than 50% of which possess a timescale much faster than the temporal resolution (~75 ps) employed in these measurements. Estimated activation energy for η is closer to that for τ r

∗

Address for correspondence: [email protected]; Phone: +91 33 2335 5706; Fax: +91 33 23353477 1 ACS Paragon Plus Environment


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than that for τ s , suggesting τ s being more decoupled from η than τ r .

Keywords: Deep eutectic solvents, Choline chloride, Urea, Time-resolved fluorescence measurements, Intramolecular charge transfer reactions, Dynamic solvent control

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I. Introduction During the past few decades, attempts have been made repeatedly for finding environmentfriendly solvents or solvent systems that can have large scale industrial applications. Ionic liquids1,2 and deep eutectic solvents (DESs)3-10 are two liquid systems that are now increasingly finding industrial applications as alternative reaction media to common organic solvents. DESs have become the more-preferred alternatives primarily due to less toxicity, easy synthesis protocol and transportation.3-10 When biodegradability couples with costeffectiveness and reduced toxicity of this class of solvents,3-5,11-13 DESs surmount the barrier for being considered as ‘green solvents’. DESs are easily prepared by mixing two or more compounds at a certain mole ratio and heating to convert the solid mixture into a stable liquid phase at a temperature much lower than the individual melting temperatures of the constituents.8 Extensive inter-species H-bonding and gain in entropy upon melting are the main factors that drive the depression of freezing points, producing liquids that may even sometime show properties reminiscent of supercooled liquids near glass transition.14-16

There exists a wide window of choice for constituents resulting in the formation of neutral and ionic DESs.5 For example, neutral DESs can be obtained by mixing amides like acetamide and urea,17 whereas the ionic ones can be obtained by choosing electrolytes as one of the components. A new set of ionic DESs can be prepared by mixing a quaternary ammonium salt like choline chloride with any of the following species: amides, salts, acids, sugars, and alcohols.4,5 This provides a unique and easy handle for changing important medium properties, such as, polarity, viscosity and conductivity, which play key roles in deciding rate of a reaction occurring in liquid phase.18-24 In addition, freedom of choice for constituents makes solvent engineering easy with DESs as a reaction medium can be tailored with much less effort to drive a reaction for an intended product. Acetamide based ionic DESs have been already investigated via picosecond-resolved fluorescence measurements.1417,25,26

These measurements have reconfirmed the microheterogeneous nature of these amide-

based DESs which have been suggested earlier by several different studies employing nuclear magnetic

resonance,27

viscoelastic,28,29

ultrasonic28,29

and

dielectric

relaxation30,31

measurements. Subsequent time-resolved fluorescence studies and atomistic simulations16,26 revealed that several of these systems are dynamically heterogeneous, extent of which depends upon the identity of the electrolyte used. The signature of dynamic heterogeneities in 3 ACS Paragon Plus Environment


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the acetamide-based DESs was mainly the observation of fractional viscosity dependence of the measured solvation and rotation rates of dissolved solute probes. Decoupling of probe solvation and rotation from medium viscosity has also been observed in several ionic liquids.32-34 Interestingly, similar study involving acetamide and urea showed that creation of liquid phase via mere depression of freezing points does not ensure development of heterogeneity in DESs in general.17 Dependence of ion-amide interaction on the identity of the cation and anion has been recently investigated via femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) measurements.35

Towards the beginning of twenty-first century,8,9 choline chloride based DESs had been introduced and were accepted as green solvent systems.11,36 This is because choline chloride, being similar to vitamin B,37,38 is both biocompatible and biodegradable.11,36,39,40 Choline chloride has been an important constituent in the animal and poultry food industry41,42 as well as in health drinks.43,44 Phase diagrams for several choline chloride based DESs have been studied along with transport properties, providing a temperature range for subsequent use.5,810,45

The most widely studied system of choline chloride based DESs is that of with urea

which forms liquid at 285 K fertilizers46,47

in the ratio of 1:2.8 Urea is one of the most important

and a strong protein denaturant.48,49 SCHEME 1 provides the chemical

structures for both choline chloride and urea. The melting point of choline chloride is 575 K8 and urea is 406 K,8 and the corresponding DESs are found to be moderately polar.50 Molecular dynamics simulation study of choline chloride and urea system has shown that the co-ordination number of urea around chloride ion is greater than that for choline cation.51 In addition, the H-bonding preference of the cis-H and trans-H are also investigated by infra-red measurements.51 Theoretical treatments for investigating H-bond mechanism and lifetimes have also been attempted.52 NMR studies have been performed to explore the coupling between viscosity and particle diffusion.53 However, no study has been carried out so far exploring the interrelationship between dynamic solvent response and rate of a reaction occurring in these DESs. In addition, characterizations of heterogeneity aspect and solutemedium coupling have not been performed. We focus here on these aspects of choline chloride/urea deep eutectics where temperature dependent measurements of solute-centred reactive and non-reactive dynamics have been executed.


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The glass transition temperature ( Tg ) of the DES (choline chloride+ urea) is ~200 K (Fig. S1) which is similar to what we have already observed for acetamide based DESs.16,17 Our measurements have been carried out in the temperature range, 293 ≤ T K ≤ 333 , which is far above the Tg . We consider excited state solvation and rotation dynamics of the dissolved dipolar probe (C153 in the present case) as representatives of non-reactive solute-centered dynamics. On the other hand, photo-induced excited state intramolecular charge transfer (ICT) reactions of 4-(1-azetidinyl)benzonitrile (P4C) and 4-(1-pyrrolidinyl)benzonitrile (P5C) (SCHEME 1) have been considered as representatives for reactive dynamics. ICT reactions in these molecules have previously been studied in conventional molecular solvents54 and electrolyte solutions19-21 and interpreted in terms of twisted intramolecular charge-transfer (TICT) mechanism (SCHEME 2) although alternative mechanism55-57 was available. The conversion between the locally excited (LE) and charge transferred (CT) states of these molecules involves a barrier of ~5kBT54 which is considered as an example of low barrier reaction. SCHEME 3 presents the transformation of the LE state to CT state via TICT process on photo-illumination. The LE state is naturally favoured in low polarity condition while the more polar CT state is further stabilized via increased dipole-dipole interaction. The rate of conversion is therefore expected to be affected by both solvent dependence of reaction barrier and coupling of solvent dynamics to reaction timescale.23,24,58 Temperature can also affect the reaction via changing the polarity and viscosity of the medium.59 Probes used here for fluorescence studies are C153 ( τ life ~3-5ns)60 and trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT) ( τ life ~0.5ns)15,61 (SCHEME 1).



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SCHEME 1: R

R= N

N

C P4C

P5C

N CF3

S

N

O

N

N

O

DMASBT

C153

O

CH3 OH Cl N

C

CH3

NH2

NH2 CH3

Choline chloride

Urea


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e N

Donor

SCHEME 2:

N

Acceptor

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N

N

CT

LE

SCHEME 3:

S0 = Ground state

S1 = Excited state

II. Experimental section (i) Materials Choline chloride ( ≥ 99%, Sigma Aldrich) and urea ( ≥ 99%, SRL) were vacuum dried at room temperature for ~48 hours before use. C153 (laser grade, Exciton) and DMASBT62 were used



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as received. P4C and P5C were synthesized and purified using the method reported earlier.20,63

(ii) Sample Preparation Choline chloride and urea were weighed as required in a sample vial of 20ml and heated gently at ~340K until it was completely molten. Once fully molten, the heat source was removed and the resultant liquid was allowed to cool down to room temperature slowly. Subsequently, 3ml of this room-temperature molten mixture was transferred to a transparent quartz cuvette (path length= 1 cm). The concentration of the probe (C153 or DMASBT) was maintained at ~10-5 M. A temperature controller (Julabo, model-F32) was connected to the cuvette chamber for temperature equilibration ( ± 0.5 K). Sample preparations and measurements were done in a tightly humidity controlled environment. Few samples were bubbled with dry nitrogen gas but produced results no different from those obtained for unbubbled samples.

(iii) Data collection and analysis Steady state absorption spectra and fluorescence spectra were collected with a UV-visible spectrophotometer (UV-2450, Shimadzu) and a Fluoromax-3, Jobin-Yvon (Horiba) ﬂuorimeter, respectively. Steady state fluorescence spectra were collected using 2 nm slits at both the excitation and emission ends. Time resolved fluorescence measurements were carried out with a time correlated single photon counting set-up (LifeSpec-ps) from Edinburgh Instruments (Livingston), U.K. A laser excitation source of 409 nm was used to excite C153 using an excitation slit of 2 nm. The full width at half maximum (FWHM) of instrument response function (IRF) of the used LASER was ~ 75 ps. To maintain the experimental temperature, a Peltier Heater (LFI-3751) was used for steady state fluorescence measurements. Spectral data for steady state measurements and time resolved studies were collected and analysed by methods described elsewhere.14,60,64-66 Solvation response function [ S( t ) ] was then constructed as follows:67

S( t ) = {ν( t ) − ν(∞)} /{ν(0) − ν(∞)}

,

(1)

where ν (0) , ν (t ) and ν (∞ ) denote frequency (usually peak) for the reconstructed emission spectrum respectively at t = 0 (that is, immediately after excitation), at any given instant ( t ), 8 ACS Paragon Plus Environment

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and at a sufficiently long time ( t = ∞ ). Integration of the multi-exponential fits to S (t ) gives us average solvation time ( < τ s > ) as: ∞

∞

0

0

〈τ s 〉 = ∫ dtS (t ) = ∫ dt[∑i a i exp(−t / τ i )] = ∑i a iτ i ,

Where

∑i ai = 1 ,

(2)

and a i and τ i respectively denote the amplitude and time constant

associated with the i-th component of the total decay. Fluorescence intensity decays for time resolved anisotropy measurements were collected at the peak wavelength of the steady state fluorescence spectra of C153. Three types of decay of each sample and temperature have been collected - vertically ( I para ), horizontally ( I perp ) and magic angle polarized emission with respect to the polarization of the excitation light. The anisotropy analysis was performed via the standard protocol described in earlier works.68-70 Time resolved fluorescence anisotropy, r ( t ) was then constructed as follows:

r(t) =

Ipara(t) −GIperp(t) Ipara(t) +2GIperp(t)

,

(3)

The G-factor, obtained via tail matching, was 1.15 ± 0.05. The larger time constant of the magic angle decay fit was used for the subsequent fits of parallel and perpendicular decays. r ( t ) decays so constructed were found to fit to bi-exponential functions of time of the following form:

r(t) = r(0)[a1 exp(−t / τ1 ) + a 2 exp(−t / τ2 )] ,

(4)

where a1 + a 2 = 1 , and τ 1 and τ 2 are the two time constants associated with amplitudes

a1 and a 2 respectively. The initial anisotropy, r(0), was fixed at 0.376 for71 all the samples used here. The average rotational correlation time, τ r , was obtained via integrating the normalized function, r (t ) r(0) , as, ∞

2

0

i =1

τ r = ∫ dt ∑ a i exp[− t τ i ] = a 1τ1 + a 2 τ 2 ,

(5)



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Steady state absorption, emission, and time-resolved fluorescence measurements and the subsequent analysis of the data of P4C and P5C were carried out following methods described elsewhere.20,21 Concentrations of the ICT molecules (P4C and P5C) in solutions were always maintained at ≤10-5M. Collected steady state fluorescence spectra of these ICT molecules were deconvoluted into separate bands following the procedure described earlier20,54,72 to get peak frequency, widths and areas under the LE and CT bands for further analyses. Errors associated with the peak frequency and width are typically ±150 cm−1 and ±200 cm−1, respectively. Time resolved fluorescence intensity decays for ICT molecules were measured using a laser diode coupled to the TCSPC set-up, producing excitation light of ~300 nm (IRF was ~400 ps). Peak frequencies and widths for spectra of C153 and DMASBT were obtained following the procedure described elsewhere.73,74

Densities (ρ) of these DES samples were measured by an automated temperature-controlled density-cum-sound velocity analyzer (Anton Paar, model DSA 5000) at required temperatures ( ± 0.5K).75 Viscosities (η) of the DES systems were measured by AMVn automated microviscometer from Anton Paar (falling ball method). Refractive indices are measured by Rudolph J357 Automated refractometer. Glass transition temperature (Tg) shown in Fig. S1 for this system was measured by Density Scanning Calorimter (DSC Q200). Measured densities, viscosity coefficients and refractive indices are summarized in Table S2.

III. Results and discussions (i) Steady state spectroscopic results Fig. 1 displays the absorption and emission spectra of C153 dissolved in [f choline chloride+ (1-f) urea] at f =0.33 and compares with those in 1-pentanol. Among many solvents, we found that the spectral properties of C153 in this deep eutectic solvent are close to those in 1pentanol (static dielectric constant, ε 0 ~14).60 Note the emission spectra nearly overlap on each other while the absorption spectra are slightly shifted. Peak frequencies and widths (full widths at half maxima) for these spectra are provided in the insets. Similar spectral widths and peak frequencies suggest very similar solute-medium interactions in both the systems under comparison. In this context we would like to mention that (acetamide + electrolyte) DESs14-17 were found to be more polar than the present DES as the corresponding spectra resembled those in formamide ( ε 0 ~110).76 Fig. 2 shows the temperature dependent spectral features of 10 ACS Paragon Plus Environment

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C153 in [f choline chloride+ (1-f) urea] at two different choline chloride mole fractions, f. Note the absorption peak frequencies (ν abs ) are nearly insensitive to the solution temperature, while emission peak frequencies (ν em ) register a total red-shift of ~450 cm-1 upon raising the temperature. The temperature-induced red-shift in ν em may suggest enhanced solute-medium interaction at higher temperatures. Interestingly, increase of temperature is known to reduce

ε 0 ,77-79 and such an observation has been made

for imidazolium ionic liquids and

conventional solvents.60,80 However, emission frequencies obtained after extrapolating the time-resolved emission spectra to t = ∞ (that is, ν (∞) ) do not show any such shift with temperature, excluding thereby the aspect of temperature-enhanced solution polarity. In addition, ν (∞) are uniformly red-shifted compared to steady state fluorescence emission frequencies. Therefore, the relatively blue-shifted steady state emission frequencies at lower temperatures are due to incomplete solvent relaxation because of high viscosity ( 80 ≤ η cP ≤ 1250 ) in the temperature range studied, ( 298 ≤ T K ≤ 333 ). Spectral widths shown in the middle panel also do not show any dependence on temperature and resemble to those for spectra in 1-pentanol. A comparison between the measured dynamic Stokes’ shift ( ∆ν t ) and the relative steady state Stokes’ shift ( ∆∆ν ) values, shown in the lower panel, also support this view of incomplete solvent relaxation due to large solution viscosity. Note here that ∆ν t and ∆∆ν are quite close at the lowest temperature studied (~298 K), suggesting presence of extremely slow relaxation at this temperature that remains undetected in the 20 ns time-window employed in the present measurements.

While discussing Fig. 1, we had mentioned that the system of (choline chloride+ urea) resembles the spectral characteristics of 1-pentanol for which ε 0 ~14. We have estimated

ε 0 of these DESs using the correlation developed earlier between emission frequencies and dielectric field factors using data for C153 emission in a number of conventional molecular solvents.73 For these DESs, temperature dependent refractive indices ( n ) and emission frequencies (ν em ) were measured. Temperature dependent ε 0 values thus estimated are tabulated in Table S3 which indicates an increase in ε 0 with temperature. This increase in ε 0 with temperature is a direct reflection of the red-shift in ν em due to faster solvent relaxation at higher temperature, and does not suggest temperature-induced enhancement of solution polarity. 11 ACS Paragon Plus Environment


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Note the value ( ε 0 =14) at near room temperature is very close to what has been reported earlier from a qualitative comparison between emission frequencies (ν em ) using a different fluorescent solute.50 However, ε 0 values estimated from ν em this way are expected to differ from direct measurements via dielectric relaxation spectroscopy because ν em is influenced by both dipole-dipole and dipole-ion interactions between the dipolar probe (C153 here) and the surrounding environment in such ionic deep eutectics.

Next, we investigate the spatial heterogeneity aspect of these DESs via monitoring the excitation wavelength (λ exc. ) dependence of fluorescence emissions of two probes, C153 and DMASBT. Note τ life

for DMASBT is much shorter61 than that for C153,69 and hence faster

density fluctuations are expected to be reported better26 by DMASBT. Fig. 3 displays λ exc. dependence of ν em and Γem for these probes dissolved in choline chloride/urea DES at f= 0.33 and ~298K. Fig. S4 shows the chosen λ exc. for these two probes on their absorption spectra. Note ν em red shifts by ~530 cm-1 for DMASBT and 340 cm-1 for C153 as λ exc. moves from the highest (340 nm) to the lowest (480 nm) energies. Also, this red-shift accompanies spectral narrowing (see the lower panel of Fig. 3). Larger λ exc. -induced spectral shift for DMASBT suggests, as before,15,26 a certain population of microheterogeneous domains formed in (choline chloride+ urea) DES fluctuate/interconvert at a timescale faster than the average lifetime for C153. Fig. S5 shows the excitation wavelength dependence of fluorescence emission wavelength maxima and widths for C153 and DMASBT. C153 shows a shift of ~12 nm whereas DMASBT shows ~14 nm. The change in width is ~2.4 nm for C153 and ~3.8 nm for DMASBT. Temperature dependence of these λ exc. -induced shifts in

ν em and Γem are presented in Fig. 4 for these two solutes in this DES at f=0.33. Note the overall decrease of these spectral parameters (ν em and Γem ) with temperature suggests lower viscosity at higher temperature allows greater solution homogeneity and facilitates relaxation. This trend remains the same at higher f as well (see Fig. S6). Interestingly, the excitation wavelength dependence of ν em and Γem is

weaker in these choline chloride/urea deep

eutectics than that found in amide/electrolyte systems,15,16 suggesting that the extent of heterogeneity in ionic deep eutectics can depend both on the identity of the ions and that of 12 ACS Paragon Plus Environment

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the non-ionic component. Deep eutectics composed of acetamide and urea, in contrast, showed no excitation wavelength dependence for these steady state spectral features.17

(ii) Dynamic Stokes’ Shift Measurements The intensity decays collected at the blue edge of the emission spectrum (steady state) show decay component that fit to a tri-exponential function of time. On the other hand, intensity decays at the red edge fit to a bi-exponential, showing both a rise and decay components. Representative decays for C153 dissolved in these DESs with fitted parameters and residuals are shown in Fig. 5. These representative decays indicate presence of dynamic Stokes’ shift in these media as well as solvent relaxation timescales in picoseconds and nanoseconds. Dynamical solvent response faster than picosecond is also expected because collective intermolecular modes formed via extensive inter-species H-bonding can participate, as in other amide and H-bonded systems,81-87 in ultrafast medium reorganization in these DESs. Next we show in Fig. 6 the time-resolved emission spectra (TRES) of C153 in this system at f =0.33 and ~298K. Interestingly, the steady state emission spectrum is blue shifted even with respect to the time-resolved emission spectrum at t=0, suggesting a large missing component which has a timescale much faster than the IRF of the detection system employed. Moreover, the emission spectrum at τ = ∞ is ~900 cm-1 red-shifted with respect to the steady state emission spectrum at this composition and temperature. Lower panel of Fig. 6 shows the time evolution of widths at three different temperatures. The initial increase of Γ(t ) followed by decrease to a plateau value with time is a common feature observed for a variety of complex systems that include neat solvents,60 ionic liquids88-90 and DESs.14-16 Note also that Γ(t ) at

τ = ∞ is similar to Γem obtained from steady state measurements. The overall initial change in Γ(t ) is within ~300-400 cm-1 and amounts to a small percentage (~10%) of the steady state emisison spectral width ( Γem ~ 3000 cm-1). Because such a small change in the time dependent width was observed earlier in diverse systems60,88-90 and attributed no critical importance while interpreting measured solvation timescales, we follow the same tradition and refrain from attaching any particular significance of this aspect on the measured solvation timescales for the present systems.



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t t Table 1 summarizes the temperature-dependent observed ( ∆ν obs ) and estimated67 ( ∆ν est )

dynamic Stokes’ shift magnitudes and missing amplitudes these DESs at different temperatures. Note these shift magnitudes are less than those obtained with the same solute (C153) in (acetamide + Na/KSCN)14 and (acetamide +LiBr/NO3)15 deep eutectics. This is probably due to the presence of urea whose ε 0 is much less91 than that of acetamide.31,92 Clearly, more than 50% of the total dynamic shift has been missed in the present measurements. It may be noted that the missing percentage is considerable even though the viscosity of the solvent is fairly high. This is because the system has choline ions and urea molecules, both of which can contribute to solvation energy relaxation via collective H-bond librations involving –OH and –NH groups.81-84,87,93 Further studies using more sophisticated detection techniques and simulations are therefore required to explore the full solvation response in these DESs.

Fig. 7 shows representative S(t) decays at three different temperatures for this DES at at f= 0.33. Clearly, the faster decay at higher temperature is due to the decrease in solution viscosity (see Table S2). Bi-exponential fit parameters required to describe the temperature dependent S(t) decays at f= 0.33 and 0.4 are summarized in Table 2. For all temperatures, the decays are characterized by a faster component (~40-50%) with time constant ( τ1 ) ~50 - 300 ps and a slower one (~60-50%) with timescale ( τ 2 ) ~200 - 2500 ps. τ s , the average solvation time, ranges between ~0.2 ns and ~1.5 ns in this temperature range. With a rise in temperature, both τ1 and τ 2 become faster, suggesting solvent structural relaxation being their origin.93 For example, upon raising the solution temperature from ~298K to ~333K, the viscosity at f= 0.33 reduces from 1008 cP to 80 cP. Thus, approximately an order of magnitude reduction in viscosity accounts for similar shortening of τ 2 - from 2.2 ns to 0.2 ns. Similar is the case at f= 0.40. However, the structural relaxation associated with this slowest component may not involve purely hydrodynamic diffusive relaxation. Stokes-Einstein (SE) relation predicts ~435 ns as diffusion time scale ( σ 2 / D ) for choline cation at ~298K (using

σ = 2r = 2 × 3.29Å53, η = 1008 cP). Similarly, SE timescales for urea or chloride ions at a given temperature have been found to be much longer than the corresponding τ 2 measured in the present experiments. Therefore, it is clear that particle movements very different from


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hydrodynamic diffusion, such as, jumps are involved in the slow solvent rearrangement at the long time.53,94

Fig. 8 explores more quantitatively the viscosity coupling of the temperature dependent solvation response in this these media by showing

< τ s > as a function of temperature-

reduced viscosity ( η / T ) in a double logarithmic fashion. Lines going through the data suggest the following viscosity dependence: < τ s > ∝ (η / T ) p , where p=0.81 and 0.71 for f=0.33 and 0.40, respectively. Such a fractional viscosity dependence indicates dynamic heterogeneity in the medium, and may result from non-hydrodynamic centre-of-mass movements. Similar results have also been found for (acetamide+ electrolyte) deep eutectics,14-16,25 and resurrection from fraction to near unity value for p upon replacement of electrolytes by urea to these ionic acetamide deep eutectics.17 We have also estimated the activation energies, E a ( τ s ) , associated with solvation process in these media and this is shown in Fig. S7. Data in Fig. S7 indicates Arrhenius-type temperature dependence for average solvation rates ( τ s

−1

), and produces ~42 kJ/mole and ~51 kJ/mole as E a ( τ s ) at

f=0.4 and 0.33, respectively. This may be compared with corresponding E a (η) values which are respectively 57 kJ/mole and 59 kJ/mole (see Fig. S8, supporting information). This disparity between E a ( τ s ) and E a (η) provides further support in favour of the viscosity decoupling of particle translation that governs the solvation response at long time.

(iii) Dynamic Anisotropy Measurements Table 3 lists the bi-exponential fit parameters required to describe the r ( t ) decays measured using C153 in these DESs. Representative fits to temperature dependent anisotropy decays at f=0.33 and 0.4 are shown in Fig. S9 (supporting information). As the fit parameters indicate, r ( t ) decays are characterized by a slower time scale ranging from ~3 to ~40 ns contributing 70-80% of the total decay, and a component with much faster time constant in ~20 ps range. Here also the slower time constant decreases with increase in temperature and follows the viscosity trend. Note slower time constants at lower temperatures are ~7-8 times longer than the τ life

of the probe C153. Similar observation has also been found for C153 in ionic

liquids at lower temperature with relaxation time scales as sluggish as ~100 ns95. Activation 15 ACS Paragon Plus Environment


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energies associated with the average rates of solute rotation ( τ r

−1

) at these compositions

have been estimated (see Fig. S10, supporting information) and they are ~53 kJ/mole and ~62 kJ/mole at f= 0.33 and 0.4, respectively. Note these E a ( τ r ) values are much closer to E a (η) than E a ( τ s ) values are, suggesting environmental coupling is more extensive for solute rotation than solute solvation whose long-time rate is dictated by centre-of-mass diffusion. This observation of rotation being more coupled to viscous friction than translation in these DESs is qualitatively similar to results for deeply supercooled liquids near glass transition, and interpreted as evidences for translation-rotation decoupling.96,97 The coupling to the medium viscosity and validity of Stokes-Einstein-Debye (SED) relation for solute rotation is tested next in Fig. 9 where temperature dependent average rotational times ( τ r ) are shown as a function of temperature- reduced viscosity ( η / T ) in a doublelogarithmic plot. SED predictions from the relation, τ r = (Vη k B T ) f s C , are also shown for a comparison. Note for SED calculations, shape factor, f s =1.71, and C =0.24 (slip) and 1(stick)80 have been used along with solute volume, V = 246 Å3. In addition, average times obtained by fitting 100%, 90%, 80% and 70% of the collected r (t ) decays are shown to examine the fidelity of the long time constants obtained in our fits. As the figure suggests, there is not much difference in the array of the four considerations. Obviously, measured times are better predicted by slip hydrodynamics than the stick counter-part. A fit of these data to τ r ∝ (η T ) leads to an average value, p ~0.87. This value for the fraction power, p

p, is larger than that found for η / T dependence of τ s , and suggests a sort of rotationtranslation decoupling. Fig. S11 (supporting information) shows the validity of SED model and viscosity coupling of

τr

at f =0.40. The conjecture that the higher the viscosity the

greater is the translation-rotation decoupling is examined for these DESs in Fig. 10 by showing temperature dependent τ r as a function of τ s Lines going through the data obeying the relation,

in a double logarithmic fashion.

τr ∝ τs

α

, clearly indicate, via their

slopes, different dependence on τ s of τ r at high and low temperature regimes. This can be understood if we consider that both τ s

and τ r

provide a measure of the dynamical

friction experienced by a particle while moving through a medium, and in motion it couples differently at different temperatures producing different values of α . Interestingly, when data 16 ACS Paragon Plus Environment

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for conventional molecular liquids60 and ionic liquids71,80,98 are considered together, a single correlation with α =0.65 is obtained (see Fig. S12, supporting information). This underlies the difference in solute-medium interactions among these different kinds of media.

(iv) Dynamic Solvent Control of a Reaction: Coupling between Average Reaction and Solvation Timescales Next we explore how dynamic solvent response affects the timescale of photo-induced excited state intra-molecular charge transfer reaction in P4C and P5C in these DESs. Results from steady state absorption and fluorescence spectroscopic measurements are provided in the Fig. S13 and Fig. S14 (supporting information). These studies indicate rise in solution temperature favours formation of CT state and induces spectral redshift. However, these apparent ‘polarity effects’ on LE → CT conversion in these molecules are arising from the better stabilization of the CT state by more rapid solvent rearrangement upon rising the medium temperature. For the investigation of reaction kinetics in these DESs, time-resolved emission intensity decays were collected at the peak wavelengths of the LE and CT emission bands of emission spectra of these ICT molecules. As found in conventional molecular solvents21,54 and electrolyte solutions,18 intensity decays (both at LE and CT wavelengths) of P4C in the temperature range studied have been found to be bi-exponential and those for P5C tri-exponential. Unconstrained fits to these decays have produced fast LE decay times equal or nearly equal to the CT rise times, identifying the reaction timescale ( τ rxn ) associated with the LE → CT conversion in these ICT molecules. In addition, our measured τ rxn for these ICT molecules in several common solvents18,21 compare well with those measured earlier with much sharper (~25 ps) time resolution.54

Fig. S15 (supporting information) provides a representative example where intensity decays, fits through them, residuals and fit parameters are shown for P4C in [f choline chloride + (1f) urea] DES at f=0.33. Table S16 (supporting information) summarizes LE and CT fit parameters for both P4C and P5C in this DES at a few representative temperatures. τ rxn has been calculated by using the following general formula:54



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n −1

∑a τ

i i

τ rxn =

i =1 n −1

∑a

,

(6)

i

i =1

where a i and τ i are the fractional amplitudes and time constants observed in n-exponential fits. Intensity decays for P4C and P5C in this deep eutectics at f=0.4 were, as observed at f=0.33, found to fit to bi- and tri-exponentials respectively, differing only in reaction times due to the difference in solution viscosities. Because of this qualitative similarity, we had refrained from presenting the relevant data at f=0.4, and considered f=0.33 as a representative composition for investigating the dynamic solvent effects on reaction times in this system.

Fig. 11 depicts the temperature dependence of τ rxn chloride+ (1-f) urea]

for both P4C and P5C in [f choline

DES at f=0.33 where average reaction rates, τ rxn

−1

, are shown as a

function of ( RT ) −1 , R being the universal gas constant. Clearly, the measured rates show Arrhenius-type temperature dependence with activation energies, E a ( τ rxn ) , ~26 and ~18 kJ/mole for P4C and P5C, respectively. These activation energies are similar to what have been found earlier for P4C and P5C in low polar solvents, such as, ethyl acetate and tetrahydrofuran.54 This estimation is based on the observation that ICT reactions in this DES is barrier dominated because viscosity has a secondary control on the measured τ rxn . For example, the ratios between τ rxn at ~293 K and ~333 K (that is, τ rxn

293

τ rxn

333

) are ~3

and ~2 for P4C and P5C respectively (see Table S16) whereas the corresponding viscosity ratio is ~18 (see Table S2). In such a scenario qualitatively correct information regarding the dynamic solvent control of τ rxn in this DES may be extracted.

Fig. 12 presents the results on dynamic solvent effects on these ICT reactions where measured

τ rxn are shown as a function of experimental τ s in a log-log plot. The medium is in [f choline chloride+ (1-f) urea] DES at f=0.33. Since a twisting is assumed to be involved in the LE → CT conversion of these ICT molecules,54,56,63,99,100 τ rxn

is likely to couple to τ s

which is a measure of sort for dynamic friction exerted by the medium. Indeed this is the case here as fits to the data obeying the relation, τ rxn = a (< τ s >) b , provide b = 0.52 and 0.35 for 18 ACS Paragon Plus Environment

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P4C and P5C. This result is different from what has observed earlier with these molecules in conventional molecular solvents54 on two aspects. First, in molecular solvents ICT reaction in P4C did not show any dependence on τ s

while P5C showed a moderate dependence.54

Second, the τ s dependence for τ rxn in P4C in this DES is larger than that for P5C, which is opposite to the findings in conventional molecular solvents.54 This is because relatively slower reaction time compels the reactive mode in P4C to experience more solvent fluctuations (and thus greater dynamic friction).23,24. This in turn allows greater dynamic solvent control on τ rxn , producing stronger dependence on τ s .

IV. Conclusion In summary, the ionic deep eutectics made of choline chloride and urea studied here indicates moderate presence of both spatial and temporal heterogeneities, and, depending upon temperature, very long solute solvation and rotation timescales. Solvation and rotation times in these DESs do not follow a single correlation for the temperature range studied and reflects differing coupling to the medium friction. Dynamic Stokes shifts are quite large (~1000-1500 cm-1) in these media and possess an ultrafast component. Reactive dynamics show stronger coupling to density fluctuations in these DESs than found earlier in conventional molecular solvents although reaction activation energies compare well with those obtained for these reactions in molecular solvents. Estimated activation energies for solute rotation follow those for viscosity better than activation energies for solute solvation, reflecting rotation-translation decoupling of sort. It would be interesting to explore the nature of viscosity decoupling of

τ rxn for these ICT molecules in these DESs by employing non-Markovian rate theory.101-103 Measurements of complete Stokes shift dynamics in these media and relating to that to measured dielectric relaxation data are warranted for a complete characterization of solvent response and subsequent smarter use in chemical industries.

Supporting Information DSC scans showing glass transition temperature, temperature dependent density, refractive index and viscosity coefficient, estimated static dielectric constant, spectral features, Arrhenius-type plots for rotation, solvation and viscosity data, representative anisotropy decays and fits, a brief discussion on intramolecular charge transfer reaction and steady state 19 ACS Paragon Plus Environment


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spectral features of the two ICT molecules (P4C and P5C) used, representative kinetic decays, and fit parameters to these decays are provided. This material is available free of charge via the Internet at http://pubs.acs.org.


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81. Kashyap, H. K.; Pradhan, T.; Biswas, R. Limiting Ionic Conductivity and Solvation Dynamics in Formamide. J. Chem. Phys. 2006, 125, 174506/1-10. 82. Roy, S.; Bagchi, B. Solvation Dynamics in Liquid Water - a Novel Interplay Between Librational and Diffusive Modes. J. Chem. Phys. 1993, 99, 9938-9943. 83. Biswas, R.; Nandi, N.; Bagchi, B. Solvation Dynamics in Monohydroxy Alcohols: Agreement Between Theory and Different Experiments. J. Phys. Chem. B 1997, 101, 29682979. 84. Daschakraborty, S.; Pal, T.; Biswas, R. Stokes Shift Dynamics of Ionic Liquids: Solute Probe Dependence, and Effects of Self-Motion, Dielectric Relaxation Frequency Window, and Collective Intermolecular Solvent Modes. J. Chem. Phys. 2013, 139, 164503/1-12. 85. Kindt, J. T.; Schmuttenmaer, C. A. Far-Infrared Dielectric Properties of Polar Liquids Probed by Femtosecond Terahertz Pulse Spectroscopy. J. Phys. Chem. 1996, 100, 1037310379. 86. Chang, Y. J.; Castner, E. W. Femtosecond Dynamics of Hydrogen-Bonding Solvents. Formamide and N-Methylformamide in Acetonitrile, DMF, and Water. J. Chem. Phys. 1993, 99, 113-125. 87. Biswas, R.; Bagchi, B. Solvation Dynamics in Slow, Viscous Liquids: Application to Amides. J. Phys. Chem. 1996, 100, 1238-1245. 88. Khara, D. C.; Paul, A.; Santhosh, K.; Samanta, A. Excited State Dynamics of 9,9 'Bianthryl in Room Temperature Ionic Liquids as Revealed by Picosecond Time-Resolved Fluorescence Study. J. Chem. Sci. 2009, 121, 309-315. 89. Paul, A.; Samanta, A. Photoinduced Electron Transfer Reaction in Room Temperature Ionic Liquids: A Combined Laser Flash Photolysis and Fluorescence Study. J. Phys. Chem. B 2007, 111, 1957-1962. 90. Jin, H.; Li, X.; Maroncelli, M. Heterogeneous Solute Dynamics in Room Temperature Ionic Liquids. J. Phys. Chem. B 2007, 111, 13473-13478. 91. http://deltacnt.com/99-00032.htm. . 92. Kerridge, D. H. The Chemistry of Molten Acetamide and Acetamide Complexes. Chem. Soc. Rev. 1988, 17, 181-227.


Page 29 of 45

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93. Bagchi, B.; Biswas, R. Polar and Nonpolar Solvation Dynamics, Ion Diffusion, and Vibrational Relaxation: Role of Biphasic Solvent Response in Chemical Dynamics. Adv. Chem. Phys. 1999, 109, 207-433. 94. Das, S.; Biswas, R.; Mukherjee, B. Reorientational Jump Dynamics and Its Connections to Hydrogen Bond Relaxation in Molten Acetamide: An All-Atom Molecular Dynamics Simulation Study. J. Phys. Chem. B 2015, 119, 274-283. 95. Funston, A. M.; Fadeeva, T. A.; Wishart, J. F.; Castner, E. W. Fluorescence Probing of Temperature-Dependent Dynamics and Friction in Ionic Liquid Local Environments. J. Phys. Chem. B 2007, 111, 4963-4977. 96. Ediger, M. D. Spatially Heterogeneous Dynamics in Supercooled Liquids. Annu. Rev. Phys. Chem. 2000, 51, 99-128. 97. Huang, W.; Richert, R. Dielectric Study of Probe Rotation in Viscous Liquids. Philos. Mag. 2007, 87, 371-382. 98. Zhang, X. X.; Liang, M.; Ernsting, N. P.; Maroncelli, M. Complete Solvation Response of Coumarin 153 in Ionic Liquids. J. Phys. Chem. B 2013, 117, 4291-4304. 99. Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures. Chem. Rev. 2003, 103, 3899-4031. 100. Lippert, E.; Rettig, W.; Bonacickoutecky, V.; Heisel, F.; Miehe, J. A. Photophysics of Internal Twisting. Adv. Chem. Phys. 1987, 68, 1-173. 101. Grote, R. F.; Hynes, J. T. The Stable States Picture of Chemical Reactions. II. Rate Constants for Condensed and Gas Phase Reaction Models. J. Chem. Phys. 1980, 73, 27152732. 102. Biswas, R.; Bagchi, B. Activated Barrier Crossing Dynamics in Slow, Viscous Liquids. J. Chem. Phys. 1996, 105, 7543-7549. 103. Hynes, J. T. Chemical Reaction Dynamics in Solution. Ann. Rev. Phys. Chem. 1985, 36, 573-597.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 45

Table 1: Dynamic Stokes’ shift magnitudes for C153 in [f choline chloride+ (1-f) urea] DESs at different temperatures for f =0.33 and 0.40 T (K)

Estimated Shift

Observed Shift

Missed

t ∆ν est (103 cm-1)

t (103 cm-1) ∆ν obs

%

f =0.33 298

1.477

0.585

60

303

1.411

0.669

52

308

1.408

0.640

54

313

1.348

0.605

55

323

1.360

0.626

54

333

1.272

0.676

47

f =0.40 298

1.457

0.695

52

303

1.528

0.7

54

308

1.533

0.699

54

313

1.557

0.648

58

323

1.587

0.832

47

333

1.550

0.749

52


Page 31 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2: Fit parameters for S(t) measured using C153 in [f choline chloride+ (1-f) urea] DESs at different temperatures for f =0.33 and 0.40.a

T (K)

a1

τ 1 (ps)

a2

τ 2 (ps)

τ s (ps)

f = 0.33 298

0.46

294

0.54

2230

1339

303

0.44

278

0.56

2000

1242

308

0.40

204

0.60

1250

834

313

0.38

154

0.62

769

535

323

0.41

130

0.59

496

345

333

0.38

60

0.62

233

166

f = 0.40 298

0.39

196

0.61

2500

1588

303

0.38

217

0.62

2000

1316

308

0.36

167

0.64

1325

910

313

0.31

181

0.69

833

631

323

0.41

45

0.59

666

399

333

0.52

96

0.48

500

285

a) Fitted time constants are better than ± 10% of the reported values (estimated based on limited data sets).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 45

Table 3: Parameters obtained from Biexponential Fits to r(t) of C153 [f choline chloride+ (1-

f) urea] DESs at different temperaturesa

T (K)

η (cP)

χ2

a1

τ1

a2

(ps)

τ2

τr

(ns)

(ns)

f = 0.33 298

1008

1.09

0.28

18

0.72

38.53

27.92

303

631

1.06

0.30

18

0.70

24.91

17.45

308

411

1.07

0.41

19

0.59

13.96

8.29

313

278

1.19

0.32

16

0.68

9.54

6.51

318

195

1.08

0.27

15

0.73

6.96

5.12

323

141

1.12

0.21

12

0.79

5.58

4.40

333

80

1.16

0.18

14

0.82

3.39

2.78

f = 0.40 298

1244

1.09

28

18

72

38.53

38.9

303

932

1.06

30

18

70

24.91

22.83

308

671

1.07

41

19

59

13.96

11.16

313

451

1.19

32

16

68

9.54

8.48

323

214

1.08

27

15

73

6.96

3.77

333

118

1.12

21

12

79

5.58

2.91

a)Time constants are better than ± 5% of the reported values ( based on limited data sets).


Page 33 of 45

1.0

Normalised Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8 0.6 0.4

P-OH fCC=0.33

0.2

P-OH DES

υem

Γem

18.61 18.59

2.96 2.99

P-OH DES

υabs

Γ abs

23.92 23.58

4.36 4.40

0.0 16

18

20

22 3

24

26

28

-1

ν (10 cm ) Fig. 1: Comparison between steady state absorption and emission spectra (color coded) of C153 at ~298 K in [f choline chloride+ (1-f) urea] DES at f= 0.33, and 1-pentanol (P-OH). Tables in the insets summarize the values of frequency (ν abs ,ν em ) and width (fwhm,

Γabs , Γem ).



26

f CC = 0.33

ν (103cm-1)

fCC = 0.40

24

abs

20

em

18

ν∞

Γ (103cm-1)

16 5.6 4.8

abs

4.0 3.2

em

2.4 1.2

Stokes' Shift (103cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 45

∆∆ ν 1.0

0.8

∆ν t

0.6

0.4

300

310 320 330 Temperature (K)

340

Fig. 2: Spectral parameters at two different fraction of ‘f’ in [f Choline chloride+ (1-f) Urea] (color coded) are shown here. Temperature dependence of absorption and emission frequencies (ν ) are shown in the upper panel, the full width at half maxima ( Γ ) in the middle panel and both steady state and time resolved Stokes’ shift are shown in the lower panel. The two shifts are defined as mixture hep tan e mixture hep tan e ∆∆ν = [ν abs. − ν em. ] − [ν abs. − ν em. ] = ∆ν − ∆ν and

∆ν t = [ν . (t = 0) . − ν (t = ∞)] by the Fee-Maroncelli method (Ref. 67) and ∆ν hep tan e is -1 measured as 4230 cm . Uncertainties (based on at most three independent measurements) associated with ν and Γ are ± 200 cm-1 and ± 150 cm-1; those for ∆∆ν and ∆ν t are ± 200 cm-1. mixture


Page 35 of 45

νem (103 cm-1)

19.6 fCC = 0.33 T ~ 298K

19.2 DMASBT

18.8 C153

18.4 3.2 Γem (103 cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.1 3.0 2.9 2.8 2.7 2.6 320

360

400

440

480

520

λexc (nm) Fig. 3: Excitation wavelengths (λexc) dependence of emission peak frequencies (νem) and fullwidths-at-half-maxima (fwhm, Гem) for DMASBT (circles) and C153 (triangles) in [f choline chloride+ (1-f) urea] DES at f= 0.33 and ~298 K.



700

-1

∆νem (cm )

C153 DMASBT

fCC=0.33

600 500 400 300 200 100 500 400 -1

∆Γem (cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 45

300 200 100 0 290

300

310

320

330

340

350

Temperature (K) Fig. 4: Temperature dependent total differences (∆x) in ν em and Γem for f= 0.33 in [f choline chloride+ (1-f) urea] DES. Note ∆x(T) = x(T, λexc, b) − x(T, λexc, r), x being ν em or Γem . λexc, b and λexc,

r

are the shortest (bluest) and longest (most red) wavelengths used for solute

excitation.


Page 37 of 45

λ (nm )

Normalised Counts

1.0

470 640

a3 τ3 a1 τ1 a2 τ2 χ2 1.03 0.577 139 0.279 764 0.144 3691 1.15 -0.55 1011 1.55 3982

0.8

T ~298 K fCC= 0.33

0.6 0.4

640nm

470nm

0.2 IRF

0.0 4

640nm

2 0

Residual

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-2 -4 4

470nm

2 0 -2 -4 2000

4000

6000

8000 10000 12000 14000 16000

Time (ps) Fig. 5: Representative fluorescence intensity decays at blue (470 nm) and red (640 nm) wavelengths for C153 in choline chloride/urea DES at f= 0.33 (T ~298 K). Circles denote the experimental data and solid lines fits through them. Instrument response function (IRF) is also shown in the same figure. The respective residuals (color coded) are presented in the bottom panels. Fit parameters are shown in the inset of the upper panel. Time constants ( τ i ) are in the unit of picosecond. The goodness-of-fit parameters ( χ 2 ) in these two wavelengths are 1.03 and 1.15, respectively.



Normalised Intensity

1.0 0.8

t (ps)

T ~298 K fCC= 0.33

0.6

0 100 1000 7500

τ∞

0.4 Steady state

0.2

τ0

0.0 16

18 20 Frequency (cm-1)

22

3.6 ~298 K ~313 K ~333 K

3.4 Γt(103cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 45

3.2 3.0 2.8 2.6 2.4 0

2000

4000

6000

8000

Time (ps)

Fig. 6: Synthesized time resolved emission spectra (TRES) of C153 at different times from the experimentally obtained decays at f = 0.33 in [f choline chloride+ (1-f) urea] (T ~298 K). TRES shown in the upper panel are at the following times: 0 ps (red), 100 ps (green), 1000 ps (deep green) and 7500 ps(blue). The steady state emission spectrum of the same is shown by broken line. Note that t=7500 ps is also the τ ∞ spectrum. The lower panel shows the variation of widths of the time-resolved spectra at three representative temperatures.


Page 39 of 45

1.0 ~298 K ~313 K ~333 K

0.8 0.6 S(t)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4 0.2 fCC= 0.33 0.0 10

100

1000

10000

Time (ps)

Fig. 7: Representative decays of solvation response function, S(t), measured using C153 at f= 0.33 in [f choline chloride+ (1-f) urea] DESs at three different temperatures.



3.6

log [ /ps]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 45

f CC= 0.33; p= 0.81 f CC= 0.40; p= 0.71

3.2

2.8

2.4

Log < τ s >= A + pLog[η / T ] 2.0 -2.8

-2.6

-2.4

-2.2

-2.0

-1.8

-1.6

-1.4

-1.2

log [η (Poise) / T (K)] Fig. 8: Medium viscosity (η) dependence of average solvation time, τ s , for C153 in [f choline chloride+ (1-f) urea] DESs. Circles represent data at f=0.33, triangles f=0.40. Lines through

the

data

are

obtained

from

fits

to

the

following

expression:

Log τ s = A + pLog (η / T ) where A = 1.39 (f=0.33) and 1.18 (f =0.40). Representative error bars are calculated from at most three independent measurements.


Page 41 of 45

5.0

log [ /ps]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fCC= 0.33

4.5

Stick

4.0 p

3.5

100 % 90% 80% 70%

Slip 3.0 -2.8

-2.6

-2.4

-2.2

-2.0

-1.8

: : : :

0.83 0.86 0.88 0.89

-1.6

-1.4

log [η (Poise) / T (K)] Fig. 9: Viscosity coupling of rotation times ( τ r ) for C153 in [f choline chloride+ (1-f) urea] (f=0.33) and comparison to hydrodynamic stick and slip predictions. Temperature dependent measured rotation times (color codes are for fittings with different range as explained in the text) are shown as a function of temperature-reduced viscosity (η T ) in a log-log fashion. As in Fig. 8, representative error bars are calculated from at most three independent measurements.



5.0 4.5

log (/ps)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 45

log =-2.65+2.55log log=1.99+0.63log

4.0 3.5 3.0 f= 0.33 f= 0.40

2.5 2.0 2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

log (/ps) Fig. 10: Correlation between experimental average rotational times ( τ r ) and solvation times ( τ s ) for C153 in [f choline chloride+ (1-f) urea] DESs at f=0.33 (circles) and f=0.40 (triangles). Lines going through the symbols represent fits to the relation shown in the panel.


Page 43 of 45

-4.4 P4C ; Ea = 26.03 kJ mol-1

ln[1/ (ps)]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


P5C ; Ea = 18.42 kJ mol-1

-4.8 -5.2 -5.6 -6.0 -6.4 3.5

3.6

3.7

3.8

3.9

-1 1/R T (10-4/Jmol K-1)

Fig. 11: Arrhenius plot of ln(1 / τ rxn

4.0

4.1

) vs 1/RT for reaction time of P4C and P5C in [f choline

chloride+(1-f) Urea] at f= 0.33. Solid lines are the linear fits through the data points. Circles represent inverse of reaction times for P4C, and triangles for P5C. Activation energies are shown as legends.



2.6

log [/ ps]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 45

P4C ; a = 0.96 , b = 0.52 P5C ; a = 1.03 , b = 0.35

2.4 2.2 2.0 1.8 2.0

log < τ rxn >= a + b log < τ s >

2.2

2.4

2.6

2.8

3.0

3.2

log [/ ps]

Fig. 12: Correlation between τ rxn and τ s for ICT reactions of P4C and P5C in [f choline chloride+(1-f) Urea] DES at f= 0.33.


Page 45 of 45

Table-of-Content (TOC) Graphic for the article entitled “Dynamic Solvent Control of a Reaction in Ionic Deep Eutectic Solvents: Time Resolved Fluorescence Measurements of Reactive and Non-reactive Dynamics in (Choline Chloride+ Urea) melts” by A. Das, and R. Biswas.

loglog [/ rxnps]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.6 2.6 2.4 2.4 2.2 2.2 2.0 2.0 1.8 1.82.0 2.0

P4C ; a = 0.96 , b = 0.52 P5C ; a = 1.03 , b = 0.35 P4C ; a = 0.96 , b = 0.52 P5C ; a = 1.03 , b = 0.35

log < τ rxn >= a + b log < τ s > log < τ rxn >= a + b log < τ s >

2.2 2.2

2.4 2.6 2.8 2.4log [/ 2.8 ps] s

3.0 3.0

3.2 3.2

log [/ ps]


Electrochemical fabrication of nanoporous copper films in choline chloride-urea deep eutectic solvent.

Self-aggregation of sodium dodecyl sulfate within (choline chloride + urea) deep eutectic solvent.

How polar are choline chloride-based deep eutectic solvents?

Density relaxation and particle motion characteristics in a non-ionic deep eutectic solvent (acetamide + urea): time-resolved fluorescence measurements and all-atom molecular dynamics simulations.

Evidence of self-aggregation of cationic surfactants in a choline chloride+glycerol deep eutectic solvent.

Evaluation of toxicity and biodegradability of choline chloride based deep eutectic solvents.

Choline chloride-thiourea, a deep eutectic solvent for the production of chitin nanofibers.

How a protein can remain stable in a solvent with high content of urea: insights from molecular dynamics simulation of Candida antarctica lipase B in urea : choline chloride deep eutectic solvent.

Improving agar electrospinnability with choline-based deep eutectic solvents.

Ionic liquids and deep eutectic solvents in natural products research: mixtures of solids as extraction solvents.

Electrochemical synthesis of copper nanoparticles using cuprous oxide as a precursor in choline chloride-urea deep eutectic solvent: nucleation and growth mechanism.

Selected issues related to the toxicity of ionic liquids and deep eutectic solvents--a review.

EXAFS study into the speciation of metal salts dissolved in ionic liquids and deep eutectic solvents.

Functionalization of graphene using deep eutectic solvents.

Magnetic graphene oxide modified with choline chloride-based deep eutectic solvent for the solid-phase extraction of protein.

Deep eutectic solvents (DESs) and their applications.

Natural deep eutectic solvents: cytotoxic profile.

Deep eutectic solvents as efficient solvent system for the extraction of κ-carrageenan from Kappaphycus alvarezii.

Aqueous Ionic Liquids and Deep Eutectic Solvents for Cellulosic Biomass Pretreatment and Saccharification.

Deep eutectic solvents in countercurrent and centrifugal partition chromatography.

urea deep eutectic solvent-modified silica and an examination of the ion exchange properties of modified silica as a Lewis adduct.

Recent progress on deep eutectic solvents in biocatalysis.

Low-frequency collective dynamics in deep eutectic solvents of acetamide and electrolytes: a femtosecond Raman-induced Kerr effect spectroscopic study.

Deep eutectic solvents as novel extraction media for protein partitioning.