Photochemistry and Photobiology, 2015, 91: 1056–1063

Photophysics and Rotational Dynamics of a Hydrophilic Molecule in a Room Temperature Ionic Liquid Aninda Chatterjee, Banibrata Maity, Sayeed Ashique Ahmed and Debabrata Seth* Department of Chemistry, Indian Institute of Technology Patna, Patna, Bihar, India Received 1 January 2015, accepted 22 May 2015, DOI: 10.1111/php.12472

ABSTRACT We have studied the photophysics and rotational diffusion of hydrophilic solute 7-(N, N0 -diethylamino)coumarin-3-carboxylic acid (7-DCCA) in a room temperature ionic liquid methyltrioctylammonium bis(trifluoromethylsulfonyl) imide ([N1888][NTf2]). Comparison of activation energies of viscous flow and nonradiative decay shows that the photophysical properties of 7-DCCA are not guided by the bulk viscosity of the medium but are dependent on the specific solute solvent interaction and structural heterogeneity of the medium. The rotational relaxation behaviour of 7-DCCA in [N1888][NTf2] shows significant deviation from the Stokes Einstein Debye hydrodynamic model of rotational diffusion. This is indicative of the influence of specific solute solvent interaction on the rotational relaxation behaviour of 7-DCCA. Comparison of activation energy of rotational relaxation with activation energy of viscous flow clearly reinforces our assumption that the structural heterogeneity of the medium and specific solute solvent interaction plays a dominant role on the rotational diffusion instead of bulk viscosity.

INTRODUCTION A microheterogeneous media, which have drawn a great attention in this era, is room temperature ionic liquids (RTILs). Through, more than 100 years this solvent has gradually emerged out as environment friendly green solvent and has got prominence (1– 3). These days, when the consciousness regarding environmental pollution is gaining importance, the use of RTIL as medium is appreciated. The unique features that have made RTILs highly popular to the researchers are: high ionic conductivity, low volatile nature as well as low vapour pressure, non-flammability, greater thermal stability, wide temperature range as a liquid phase, good solvating agent for many organic and inorganic molecules (4,5). Besides these features RTILs provide heterogeneous characteristics (6–10). Several photophysical studies have been carried out in RTILs (11–22). Among all the popularly used fluorescent probes, the most prominently used probe is substituted amino coumarins. Due to their high environmental sensitivity, they have been used to study the dynamical behaviour of different microheterogeneous medium (11,23–34). Substituted aminocoumarins are mainly noted for their propensity to form twisted intramolecular *Corresponding author email: [email protected] (Debabrata Seth) © 2015 The American Society of Photobiology

charge transfer state (TICT). TICT formation is the main nonradiative decay pathways for the deactivation of the excited state. In this study, we have used 7-(N, N0 -diethylamino)coumarin3-carboxylic acid (7-DCCA) as molecular probe, since it is more hydrophilic than other popularly used aminocoumarin molecules. It provides enough scope for hydrogen bonding with the surrounding medium to affect its spectral behaviour. Very few studies are reported in the literature on the photophysics of the probe 7-DCCA (35–43). Previous study showed that charged probe molecules in RTILs, can provide strong electrostatic interaction with the medium (44–46). Whereas, in our case beside electrostatic interaction, H-bonding interaction is also operative due to the presence of H-bond accepting and donating groups. Previously, several researchers have used the hydrophobic coumarin dyes (24,47) to study the photophysics in RTILs, which have lesser potential to form specific solute solvent type interaction. By using comparatively higher hydrophilic molecule, we have increased the probability of specific solute solvent interaction specifically, H-bonding interaction. We have executed our study in RTIL in order to be affirmative about the effect of structural heterogeneity and specific solute-solvent interaction on the photophysics of relatively hydrophilic and strong H-bond forming dye 7-DCCA. RTILs are quite similar to heterogeneous deep eutectic mixtures containing amide and electrolytes, where decoupling of solute rotation and solvation dynamics from the medium viscosity is possible (48–50). We have applied temperature variation study in order to show that the photophysical processes in these medium are not only guided by viscosity but also by the specific solute-solvent interaction between the solute probe and solvent molecules. In order to further probe the effect of viscosity and specific solute solvent interaction on the photophysics we have chosen the methyltrioctylammonium bis(trifluoromethylsulfonyl) imide ([N1888][NTf2]) as the RTIL. This ammonium cation containing RTIL does not have any hydrogen bond donating centre and at the same time pocesses high viscosity at room temperature. Moreover, this RTIL being aliphatic in nature, its contribution to the emission spectra of probe molecule is small compared to aromatic RTILs. Moreover, this RTIL being highly viscous is expected to restrict the formation TICT state, thereby improving the fluorescence emission properties. Moreover, we also expected that the cationic part of the RTIL, which contains three large octyl chains, would provide less friction to the rotating solute molecule and we expected that the distribution of the dye in the tail domains would prevent it from undergoing specific solute solvent interaction. However, we reported opposite phenomenon in the present work.

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The anisotropy decay is fitted by the stretched exponential function (52,53):

MATERIALS AND METHODS Material. 7-DCCA (Scheme 1) was purchased from Sigma–Aldrich and used as received. Methyltrioctylammonium bis(trifluoromethylsulfonyl) imide ([N1888][NTf2]), (Scheme 1) high purity grade, were purchased from Sigma–Aldrich. Sample preparation. From the stock solution of 7-DCCA in methanol required amount of aliquot was taken out by a microliter syringe in a quartz cuvette and dried under vacuum. Then sufficient volume of [N1888][NTf2] was added to the cuvette and allowed sufficient time for complete solubilisation of the dye in each of the solvent. The concentration of dye is maintained at ~3 9 106 (M). Methods. Ground state absorption measurements were carried out by using UV–Vis spectrophotometer (Model: UV-2550; Shimadzu). The steady-state fluorescence emission spectral studies were carried out using Fluoromax-4P spectrofluorometer (Horiba Jobin Yvon). For absorption and fluorescence measurement the path length of the used quartz cuvette is 1 cm. The fluorescence quantum yields of 7-DCCA in different solvent medium was measured using the fluorescence quantum yield of Coumarin 480 in water solution (/r = 0.66) as reference (51), by using the following equation:

/f ¼ /r

Is Ar n2s Ir As n2r

ð1Þ

where, s and r stand for the sample and reference respectively. The quantum yield calculation was done using excitation wavelength at 405 nm. Here I stands for the integrated area under the fluorescence curve, A stands for the absorbance of the sample at excitation wavelength and n stands for the refractive index of the medium. The fluorescence time resolved decays were collected by using a picosecond time-correlated single-photon counting (TCSPC) technique. We have used a time-resolved fluorescence spectrophotometer from Edinburgh Instruments (model: LifeSpec-II, UK). We have used picoseconds diode laser with excitation wavelength at 405 nm. The full width at half maximum (FWHM) of our system is ~ 90 ps. The fluorescence transients were detected at magic angle (54.7°) polarization using Hamamatsu MCP PMT (3809U) as a detector. The decays were analysed using F-900 decay analysis software. The fluorescence anisotropy decay (r(t)) was measured by using the same instrument. The following equation was used to obtain r(t).

rðtÞ ¼

III ðtÞ  GI? ðtÞ III ðtÞ þ 2GI? ðtÞ

1057

ð2Þ

where, the emission intensities at parallel (III) and perpendicular (I┴) polarizations were collected alternatively by fixing the time for both the decays. We have used motorised polarizers to collect the parallel and perpendicular decays. The parallel (III) and perpendicular fluorescence decays (I┴) are measured with respect to the polarisation of the excitation source. G is the correction factor for the detector sensitivity to the polarization direction of the emission. F-900 software was used to analyze the anisotropy decays.

(
r ðtÞ ¼ r0 exp 

brot )

t

ð3Þ

srot

where, r0 is the limiting anisotropy, brot represents a fitting parameter. hsrot i was calculated using the following equation (52,53):

hsrot i ¼


 srot 1 C brot brot

ð4Þ

For time-resolved emission studies, the temperature was varied from 278 to 323 K by using peltier-controlled cuvette holders from Quantum Northwest (Model: TLC-50). For steady state measurements the temperature was varied by using Jeiotech refrigerated bath circulator (Model: RW0525G).

RESULTS AND DISCUSSION Steady state absorption, emission and time resolved fluorescence studies The steady state absorption maximum of 7-DCCA in [N1888] [NTf2] is shown in Table 1 and Fig. 1. The fluorescence emission spectra of 7-DCCA in [N1888][NTf2] and the emission spectra of neat [N1888][NTf2] is shown in Fig. 2 and Fig. S1. It shows very little contribution of RTIL on the emission spectra of 7-DCCA, although we have constructed the emission and absorption spectra of 7-DCCA in [N1888][NTf2] by subtracting the contribution due to neat RTILs. With the gradual rise of temperature, the fluorescence quantum yield gradually decreases (Table 1). This is due to the fact that increase of temperature decreases the viscosity of the medium, and it increases the nonradiative decay by twisting of the diethylamino group. In [N1888][NTf2] the FWHM of the emission spectra of 7-DCCA gradually decreases with the gradual increase of temperature (Fig. S2). This behaviour in [N1888][NTf2] demonstrates that, in this system the emission properties are not entirely driven by the viscosity of the medium. In RTIL, the spectra are affected by electrostatic interaction. Similar type of interaction was reported in case of Coumarin 153 in RTIL (16). H-bonding interaction between 7-DCCA and [NTf2] is also expected to be operative. These types of interactions in RTIL are expected to cause the change of FWHM of emission band with the increase

Table 1. The photophysical parameters of 7-DCCA in [N1888][NTf2] and variation of viscosity of [N1888][NTf2] with temperature. Sr no 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Scheme 1. Schematic representation of 7-DCCA and [N1888][NTf2].

System 7-DCCA in [N1888] [NTf2]

Temperature (K)

Viscosity (cP) (g)

kabs (nm)

kemi (nm)

/f

< sf > (ns)

278 283 288 293 298 303 308 313 318 323

2762 1781 1149 828 597 412 294 221 164 127

432

460 461 461 461 462 462 462 463 463 463

0.70 0.66 0.52 0.50 0.45 0.37 0.31 0.28 0.25 0.21

2.35 2.23 1.87 1.78 1.69 1.57 1.40 1.24 1.07 0.93

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0.4 278 K

Fluorescence Intensity

7-DCCA in [N1888][NTf2]

Absorbance

0.3

0.2

0.1

1000

100

323 K

IRF 10

0.0 400

450

500

Wavelength (nm) Figure 1. The steady state absorption spectra of 7-DCCA in [N1888] [NTf2] at 298 K.

Fluorescence Intensity

278 K

323 K

450

500

550

600

650

Wavelength (nm) Figure 2. The steady state emission spectra of 7-DCCA in [N1888][NTf2] with the variation of temperature.

of temperature. When studying the steady state emission spectra in RTIL we are considering the system as a heterogeneous ensemble of emitters due to the different physical interactions with the different local environments. Therefore, dye in different environment exhibit different spectral properties. In case of inhomogeneous or hetereogeneous medium where the microenvironments of the fluorophores differ, the relaxation behaviour is distinctly different as the solvent sheaths of individual fluorophores respond with different rates to the changes of local electric field (54). When we are increasing the temperature, the emitting species with higher ability to form TICT state in those particular environments, quenches faster than those with comparatively less propensity to form TICT state. The emission spectra of 7-DCCA in [N1888][NTf2] represent overall emission spectral feature from all the emitters distributed in different environment in the microheterogeneous medium. This concept has been supported by literature (55). Therefore, when the emitters, with higher nonradiative decay rate constant quenches, the population of contributing species to the steady state emission spectra decreases. This causes the decrease in bandwidth of the steady

2

4

6

8

10

Time (ns) Figure 3. The time resolved emission spectra of 7-DCCA in [N1888] [NTf2] with variation of temperature.

state emission spectra with the rise of temperature in RTIL. The quantum yield values (/f) of 7-DCCA in [N1888][NTf2] are tabulated in the Table 1. /f values of 7-DCCA decrease in a regular way, with the gradual increase in temperature. The increase of temperature causes lowering of the viscosity of the medium and hence facilitates the formation of TICT state. This causes the decrease of /f with the rise of temperature. We have studied the time resolved fluorescence emission spectra and found that the average fluorescence decay time gradually decreases with the increase in temperature, as shown in Fig. 3 and Table 1. The negligible contribution of the neat [N1888][NTf2] to the emission decays of 7-DCCA in [N1888] [NTf2] at three different temperatures are shown in Fig. S3. All the time resolved decays are fitted by triexponential function. It is observed that for 7-DCCA in [N1888][NTf2], the fast and intermediate components gradually decrease in their timescale value whereas, their relative amplitude values gradually increases (Table S1). Only for the slow component the time scale of the component remains almost same whereas, the weight percentage of this particular component gradually decreases. This indicates the absence of any aggregates of 7DCCA in this RTIL. Effect of temperature on the viscosity In order to investigate the effect of temperature on viscosity we have measured the viscosity of the medium at different temperatures varying from 278 to 323 K. The activation energies of viscous flow in all the solvents can be calculated using Arrhenius equation: ln g ¼ ln ga þ

Eg RT

ð5Þ

where, ga represents the limiting solvent viscosity at infinite temperature and Eg stands for the activation energy of viscous flow. Using equation 5 we have calculated that the activation energies of viscous flow for [N1888][NTf2] is 50.5 (1.0) kJM1 K1. The activation energy of viscous flow for [N1888][NTf2] is higher compared to imidazolium, pyrrolidinium, sulfonium containing RTILs reported earlier (56).

Photochemistry and Photobiology, 2015, 91 0.4

1.1

298 K

1.0

313 K 0.3

0.9 0.8

r (t)

1/< f > (ns-1)

1059

0.7

0.2

0.6 0.1

0.5

323 K

0.4 0.0031 0.0032 0.0033 0.0034

0.0

0.0035 0.0036

0

5

-1

10

15

Time (ns)

1/T (K ) Figure 4. Arrhenius plots for the determination of the activation energy of non radiative decay of 7-DCCA in [N1888][NTf2].

Figure 5. The variation of rotational relaxation decay of 7-DCCA in [N1888][NTf2] with temperature.

We have calculated the activation energy of non-radiative decay process in order to understand whether, the viscosity of the media influences the nonradiative decay. We have used the modified Arrhenius equation in order to calculate the activation energy of nonradiative decay (57):

rescence decay time in the entire solvent medium especially in the low temperature range. Even, at 323 K the rotational relaxation time is almost 6 times higher than the fluorescence decay time. This is mainly due to the presence of nanostructure domains in [N1888][NTf2]. Presence of such nanostructure domains in RTIL with long hydrophobic tail has been described by Wang and Voth. They described the tail aggregation and domain diffusion mechanisms, which can create such nanostructure domain (6,7). In order to understand the effect of specific solute-solvent interaction, structural heterogeneity and viscosity of the medium on the solute rotational relaxation dynamics we have determined the activation energy of rotational relaxation using the average rotational correlation time (Table 2, Fig. S4). It shows a regular trend of decreasing with the temperature. We have determined the activation energy of rotational relaxation using the following equation:


 1 1 Enr ¼ þ A exp  RT \sf[ s0

ð6Þ

where Enr represents the activation energy of nonradiative decay process, A is the pre exponential factor and s0 is the intrinsic lifetime. By plotting reciprocal of lifetime value against reciprocal of temperature we have obtained the activation energy of nonradiative decay process. This nonlinear fitting (Fig. 4) provides the activation energies of nonradiative decay 37.5 ð4:9ÞkJM1 K1 . This significant difference between Eg and Enr signifies the fact that the nonradiative decay pathway is not entirely guided by the bulk viscosity of the medium. Rather, location of the molecule in RTIL and specific solute solvent interaction play prominent role in excited state deactivation. As, the molecular probe 7-DCCA undergoes the nonrdiative decay through the TICT state production by twisting of NEt2 along the C-N bond, we can expect from the Enr value and its comparison with Eg that the immediate surrounding or microenvironment of the probe is enough lubricated to facilitate the production of TICT state. This effect of microenvironment on the p  p* charge transfer spectra in a molecular rotor has been previously supported by Lu et al. (58). Moreover, specific solute solvent interaction may also promote the TICT state production. Rotational relaxation dynamics of 7-DCCA The time resolved fluorescence anisotropy decays of 7-DCCA in [N1888][NTf2] show that the average rotational relaxation time of the dye gradually decreases with the increase in temperature (Fig. 5). This is obvious since with gradual rise of temperature the bulk viscosity of the solvent medium gradually decreases, thereby facilitating the rotational relaxation of the dye. However, in our present system we have observed that the rotational relaxation time of 7-DCCA is significantly higher than the fluo-

hsrot i ¼


 VfCg0 Erot exp : kB T RT

ð7Þ

The activation energy of rotational relaxation is 65.4 ( 8.6) kJM1K1. This value is higher than the activation energy of viscous flow of bulk viscosity (Eg). This shows that the rotational relaxation of 7-DCCA is not only driven by the bulk viscosity of the medium but also guided by the specific solute solvent interaction. Moreover, these indicate stronger hindrance experienced by the solute 7-DCCA from its immediate

Table 2. The rotational relaxation time of 7-DCCA in [N1888][NTf2] at different temperatures and activation energy of rotational diffusion. Sr no 1. 2. 3. 4. 5. 6.

System 7-DCCA in [N1888] [NTf2]

Temperature (K)

r0

hsrot i (ns)

brot

298 303 308 313 318 323

0.378 0.375 0.363 0.364 0.358 0.352

48.66 28.93 28.05 18.92 10.29 5.40

0.57 0.68 0.63 0.61 0.74 0.84

Erot (kJM1 K1) 65.4 (8.6)

Aninda Chatterjee et al.

surrounding or microenvironment than that predicted by the bulk viscosity because of very strong friction exerted due to specific solute solvent interaction causing greater microviscosity. Our result clearly demonstrates that in our case the probe 7-DCCA experiences greater microviscosity for the rotational diffusion than predicted from bulk viscosity. This increased microviscosity due to the specific solute solvent interactions makes the solute rotation slower. The diffusional behaviour of dissolved probe molecule is known to be affected by the specific solute-solvent type interaction (59). Prabhu et al. (60) demonstrated that in RTIL the rotational relaxation behaviour of hydrogen bond donating solute is highly affected by the specific solute solvent interaction and is dependent upon the microenvironment of the probe molecule. As our probe molecule 7-DCCA is a hydrogen bond donating solute so it is quite expected that its rotational relaxation behaviour would be dominated by the specific solute solvent interaction between RTIL and solute. Our result is in sharp contrast to the results reported in the literature for other probes in the RTILs (61). Solutes with no H-bond donating ability such as BTBP exhibited activation energy of rotational relaxation, which is almost identical to the activation energy of viscous flow of [bmim][PF6], depicting that solute dynamics is in accordance to the RTIL dynamics (62). In our present experiment, the activation energy of rotational diffusion clearly demonstrates that the solute rotational diffusion is not solely guided by the bulk viscosity of the medium but also by the microenvironment of the probe. In order to probe in more details about the rotational relaxation studies, we have analysed our data by using the Stokes Einstein Debye (SED) hydrodynamic model of rotational diffusion. This shows that the rotational relaxation time (sSED ) of a nonr interacting solute in a solvent of viscosity g is given by the following equation: sSED ¼ r

VgfC kB T

ð8Þ

where, V stands for molecular volume or van der Waal volume. f and C are the shape factor and friction coefficients respectively. kB and T stand for Boltzman’s constant and temperature respectively. f accounts for the non-spherical nature of the solute molecule and was first introduced by Francis Perrin (63). For a spherical particle the value of f is 1, whereas for asymmetric ellipsoid its value is greater than 1. We have treated 7-DCCA as an oblate ellipsoid and its radius along the three axes are calculated as a (radius along the semi major axis) = 6.8
A, b (radius along the semi minor axis) = 3.2
A, c (width of the molecule) = 2.7
A. Here we have calculated the value of f as 1.52. The shape factor C is a boundary condition parameter and its value generally lies between 0 and 1. The value of C is determined by the axial ratio as described by Hu and Zwanzig (64). When the size of the rotating particle or molecule is much higher than the solvent molecule, the stick boundary condition prevails (C = 1). For slip boundary condition, the value of C is less than 1. By using the calculation of Hu and Zwanzig, we have determined the value of C, which is 0.203 and tabulated in Table 3. The variation of rotational relaxation time (\srot [) with (g/T) is shown in Fig. 6 with respective stick and slip boundary conditions. Here we have plotted the observed rotational relaxation times along with calculated sstick and sslip against (g/T) values. We have fitted the observed rotational relaxation time hsrot i

Table 3. The values of Cobs, CGW and CDKS for 7-DCCA in [N1888] [NTf2] at 298 K. System

Cobs

CGW

CDKS

Cslip

7-DCCA in [N1888][NTf2]

0.865

0.122

0.055

0.203

60 Stick boundary

50

(ns)

1060

40 30 Slip boundary

20 Experiment 10 0 0.4

0.8

1.2

1.6

2.0

-1

η /T (cP K ) Figure 6. Plot of average rotational relaxation times of 7-DCCA against (g/T) in [N1888][NTf2]. Theoretically calculated reorientation times using SED theory with slip (lower dotted line) and stick (upper dashed line) boundary conditions are shown in the figure.

against (g/T) using the following formula described in the literature (65,66).  g p hsrot i ¼ A T

ð9Þ

7-DCCA in [N1888][NTf2] hsrot i ¼ ð10:51  0:71Þ

 g 1:030:14 T

ðN ¼ 6; R ¼ 0:95Þ:

The rotational relaxation time of 7-DCCA in [N1888][NTf2] lie in between slip and stick boundary although in some temperature it is very close to stick boundary. In our present case we have found out the values of Cobs and observed that for [N1888][NTf2] Cobs at 298 K lies very close to Cstick (Table 3). Again in case of 7-DCCA in [N1888][NTf2] the value of Cobs is high and remains very close to Cstick. It is observed that at comparatively lower temperature (298 K and 303 K) the rotational correlation times hsrot i reside in between stick and slip boundary although very close to stick boundary. This indicates the presence of strong specific solute solvent interaction. However, with the rise of temperature the rotational relaxation behaviour bears super stick characteristics at two temperatures 308 and 313 K. This superstick behaviour of the rotational relaxation behaviour of 7-DCCA in these two temperature can be explained by using “solventberg” model or NeeZwanzig dielectric friction mechanism (67,68). The “solventberg” model takes into consideration the specific solutesolvent interaction, which makes it possible for the solvent molecules of non-negligible size to the solute particle to anchor on the solute. This inflicts the greater effective volume of the rotating solute particle. NeeZwanzig dielectric friction mechanism takes into account the electrostatic

Photochemistry and Photobiology, 2015, 91 torque between a rotating dipolar solute and the reactive field of the surrounding dielectric cavity. The specific interactions, which can be operative between 7-DCCA and RTIL, are predominantly the electrostatic interaction and H-bonding interaction between 7DCCA and [NTf2] anion. After 313 K, the rotational relaxation behaviour further remains in between the stick and slip boundaries. This change of rotational relaxation behaviour of 7-DCCA from intermediate to superstick to further intermediate condition clearly demonstrates the change of structural organisation. The change of structural organisation causes the change of specific solute solvent type interaction between 7-DCCA and RTIL. As the temperature goes down, the nanostructural domains start to appear much prominently causing less friction due to less probability of specific solute solvent interaction. With the increase of temperature as the nanostructural aggregates starts to smear out the probability of specific interaction increases. Further, increase in temperature even causes the weakening of the specific solute solvent interaction. The high value of average Cobs (0.865), which is close to stick boundary condition (Cstick = 1), further confirms strong specific solute-solvent interaction between the dye 7-DCCA and RTIL. It is reported that the size of free space between the solvent molecules increases as the size of solvent molecules increases (69). This causes lower coupling of solute rotation with the solvent medium. According to SED theory the rotational diffusion of a solute molecule in solvent medium is entirely governed by the bulk viscosity of the medium. SED theory does not take any specific solute-solvent interaction into consideration. So the presence of Cobs in the vicinity of Cstick in case of [N1888][NTf2] is also due to the presence of structural heterogeneity of the medium and strong specific solute solvent interaction. By comparing the Cobs values with the calculated Cslip we have observed that the values are always higher than Cslip to a significant extent and remains always in the close proximity of Cstick. The SED theory, which only considers the solute volume, is inadequate to explain the observed anomaly. Therefore, we have used the quasihydrodynamic theories such as GiererWirtz (GW) (69) and DoteKivelsonSchwartz (DKS) (70) to explain the rotational relaxation behaviour of the solute molecule depending on the solvent volume. According to the GW theory, the parameters of the boundary condition can be rationalised in terms of solvent volume (VS) and probe solute volume (Vp): CGW ¼ rC0

r¼ 8 > > >
> Vs > = 6 Vp 1 þ C0 ¼      1=3 4  1=3 3 > : > > > > > : 1 þ 2 VVps ; 1 þ 4 VVps

ð10Þ

ð11Þ

ð12Þ

By using Edward’s increment method we have calculated Vs and Vp for [N1888][NTf2] and 7-DCCA. The boundary condition parameter in this model CGW was determined for [N1888][NTf2] and is tabulated in Table 3. The values of Cobs are higher in than

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the CGW values. This clearly shows that GiererWirtz theory cannot explain the rotational dynamics of 7-DCCA in this medium. The quasihydrodynamic DoteKivelsonSchwartz (DKS) theory does not take into account the free volume of the solvent but also the cavities created by the solvent surrounding the solute during the calculation of boundary conditions. The boundary condition parameter, CDKS, is represented as follows:
CDKS ¼

1þ

 c 1 /

ð13Þ

where, / = fCslip and c is represented as follows: "
2 # Vp 3 DV c¼ 4 þ1 Vp Vs

ð14Þ

where, DV stands for the free space per solvent molecule for associative liquids. DV can be expressed as follows: DV ¼ Vm  Vs : Here Vm stands for the ratio of solvent molar volume with respect to the Avogadro’s number. It is dependent on solvent density. Solvent density changes with changing temperature. Therefore, we have measured Vm and CDKS values at 298 K. The value of CDKS is tabulated in Table 3. c=/ parameter is the ratio of the free space available to its effective rotational volume. The main observation is that the values of CDKS of 7-DCCA is ~ 16 times smaller than the Cobs values. This indicates that the quasihydrodynamic DoteKivelsonSchwartz (DKS) theory cannot explain the rotational relaxation process in this solvent medium. Thus here we can affirmatively say that the presence of H-bonding interaction between the solute and solvents as well as electrostatic interaction between solute and RTIL predominantly affect the rotational relaxation behaviour of 7-DCCA. The location of the probe molecule in [N1888][NTf2] is also play important role.

CONCLUSION We have studied the photophysics and rotational diffusion of 7DCCA in a RTIL [N1888][NTf2]. Determination of both activation energy of viscous flow and activation energy of nonradiative decay clearly demonstrates that the non-radiative decay process of 7-DCCA molecule is not solely guided by bulk viscosity of the medium but by the specific solute-solvent interaction and the structural heterogeneity of the medium. The rotational relaxation of 7-DCCA in [N1888][NTf2] lies in between stick and slip boundary limits at some temperature. From this study we can affirmatively say that the specific solute solvent interaction specifically H-bonding interaction between the solute and solvents as well as electrostatic interaction between solute and solvent in RTIL predominantly affect the rotational relaxation behaviour of 7-DCCA. The location of the probe molecule in the [N1888] [NTf2] also plays an important role. Acknowledgements—All the authors are thankful to Indian Institute of Technology Patna, India for the research facilities. B.M. and S.A.A are thankful to IIT Patna for research fellowships. A.C. is thankful to CSIR, New Delhi for research fellowships.

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SUPPORTING INFORMATION Additional Supporting Information may be found in the online version of this article: Table S1. The fluorescence lifetime components of 7-DCCA in [N1888][NTf2] at different temperature. Figure S1. The steady state fluorescence emission spectra of 7-DCCA in [N1888][NTf2], and neat [N1888][NTf2] at 278 K. Figure S2. Variation of FWHM of 7-DCCA in [N1888][NTf2] with variation of temperature. Figure S3. The fluorescence emission decays of 7-DCCA in [N1888][NTf2] and neat [N1888][NTf2] under same condition i.e. same slit width, emission wavelength, cuvette, temperature and same time for collecting decays at (a) 278 K, (b) 298 K and (c) 323 K. Figure S4. The Arrhenius plots for the determination of the activation energy of rotational diffusion of 7-DCCA in [N1888] [NTf2].

18.

19. 20.

21.

22.

23. 24.

REFERENCES 1. Ray, P. C. and J. N. Rakshit (1911) CLXVII.—Nitrites of the alkylammonium bases: ethylammonium nitrite, dimethylammonium nitrite, and trimethylammonium nitrite. J. Chem. Soc. Trans. 99, 1470–1475. 2. Walden, P. (1914) Bull. Acad. Imper. Sci. (St. Petersburg) 1800. 3. Plechkova, N. V. and K. R. Seddon (2008) Applications of ionic liquids in the chemical industry. Chem. Soc. Rev. 37, 123–150. 4. Buszewski, B. and S. A. Studzin0 ska (2008) A review of ionic liquids in chromatographic and electromigration techniques. Chromatographia 68, 1–10. 5. Wilkes, J. S. A. (2002) short history of ionic liquids—From molten salts to neoteric solvents. Green Chem. 4, 73–80. 6. Wang, Y. and G. A. Voth (2005) Unique spatial heterogeneity in ionic liquids. J. Am. Chem. Soc. 127, 12192–12193. 7. Wang, Y. and G. A. Voth (2006) Tail aggregation and domain diffusion in ionic liquids. J. Phys. Chem. B 110, 18601–18608. 8. Canongia Lopes, J. N. A. and A. A. H. Pa0 dua (2006) Nanostructural organization in ionic liquids. J. Phys. Chem. B 110, 3330–3335. 9. Atkin, R. and G. G. Warr (2008) The smallest amphiphiles: nanostructure in protic room-temperature ionic liquids with short alkyl groups. J. Phys. Chem. B 112, 4164–4166. 10. Macchiagodena, M., F. Ramondo, A. Triolo, L. Gontrani and R. Caminiti (2012) Liquid structure of 1-ethyl-3-methylimidazolium alkyl sulfates by X-ray scattering and molecular dynamics. J. Phys. Chem. B 116, 13448–13458. 11. Chakrabarty, D., P. Hazra, A. Chakraborty, D. Seth and N. Sarkar (2003) Dynamics of solvent relaxation in room temperature ionic liquids. Chem. Phys. Lett. 381, 697–704. 12. Gangamallaiah, V. and G. B. Dutt (2013) Influence of the organized structure of 1-alkyl-3-methylimidazolium-based ionic liquids on the rotational diffusion of an ionic solute. J. Phys. Chem. B 117, 9973– 9979. 13. Karmakar, R. and A. Samanta (2002) Solvation dynamics of coumarin-153 in a room-temperature ionic liquid. J. Phys. Chem. A 106, 4447–4452. 14. Samanta, A. (2006) Dynamic stokes shift and excitation wavelength dependent fluorescence of dipolar molecules in room temperature ionic liquids. J. Phys. Chem. B 110, 13704–13716. 15. Zhang, X. X., M. Liang, N. P. Ernsting and M. Maroncelli (2013) Conductivity and solvation dynamics in ionic liquids. J. Phys. Chem. Lett. 4, 1205–1210. 16. Jin, H., X. Li and M. Maroncelli (2007) Heterogeneous solute dynamics in room temperature ionic liquids. J. Phys. Chem. B 111, 13473–13478. 17. Khara, D. C., J. P. Kumar, N. Mondal and A. Samanta (2013) Effect of the alkyl chain length on the rotational dynamics of nonpolar and

25.

26.

27.

28. 29.

30. 31.

32. 33. 34. 35.

36.

37.

dipolar solutes in a series of N-alkyl-N-methylmorpholinium ionic liquids. J. Phys. Chem. B 117, 5156–5164. Chowdhury, P. K., M. Halder, L. Sanders, T. Calhoun, J. L. Anderson, D. W. Armstrong, X. Song and J. W. Petrich (2004) Dynamic solvation in room-temperature ionic liquids. J. Phys. Chem. B 108, 10245–10255. Chatterjee, A., B. Maity and D. Seth (2013) Torsional dynamics of Thioflavin-T in room temperature ionic liquids: an effect of heterogeneity of the medium. ChemPhysChem 14, 3400–3409. Khara, D. C. and A. Samanta (2012) Fluorescence response of coumarin-153 in N-alkyl-N-methylmorpholinium ionic liquids: are these media more structured than the imidazolium ionic liquids? J. Phys. Chem. B 116, 13430–13438. Patra, S. and A. Samanta (2012) Microheterogeneity of some imidazolium ionic liquids as revealed by fluorescence correlation spectroscopy and lifetime studies. J. Phys. Chem. B 116, 12275– 12283. Santhosh, K. and A. Samanta (2010) Modulation of the excited state intramolecular electron transfer reaction and dual fluorescence of crystal violet lactone in room temperature ionic liquids. J. Phys. Chem. B 114, 9195–9200. Zhang, L., Q. Yin, J. Su and Q. Wu (2011) Local polarity and microviscosity of the interior of dendritic polyethylene amphiphiles. Macromolecules 44, 6885–6890. Seth, D., S. Sarkar and N. Sarkar (2008) Solvent and rotational relaxation of Coumarin 153 in a protic ionic dimethylethanolammonium formate. J. Phys. Chem. B 112, 2629–2636. Gustavsson, T., L. Cassara, S. Marguet, G. Gurzadyan, P. van der Meulen, S. Pommereta and J.-C. Mialocqa (2003) Rotational diffusion of the 7-diethylamino-4-methylcoumarin C1 dye molecule in polar protic and aprotic solvents. Photochem. Photobiol. Sci. 2, 329–341. Funston, A. M., T. A. Fadeeva, J. F. Wishart and E. W. Jr Castner (2007) Fluorescence probing of temperature-dependent dynamics and friction in ionic liquid local environments. J. Phys. Chem. B 111, 4963–4977. Annapureddy, H. V. R., Z. Hu, J. Xia and C. J. Margulis (2008) How does water affect the dynamics of the room-temperature ionic liquid 1-hexyl-3-methylimidazolium hexafluorophosphate and the fluorescence spectroscopy of coumarin-153 when dissolved in it? J. Phys. Chem. B 112, 1770–1776. Kashyap, H. K. and R. Biswas (2010) Stokes shift dynamics in ionic liquids: temperature dependence. J. Phys. Chem. B 114, 16811. Lee, H. Y., J. B. Issa, S. S. Isied, E. W. Jr Castner, Y. Pan, C. L. Hussey, K. Soon Lee and J. F. Wishart (2012) A comparison of electron-transfer dynamics in ionic liquids and neutral solvents. J. Phys. Chem. C 116, 5197–5208. Castner, E. W. Jr, J. F. Wishart and H. Shirota (2007) Intermolecular dynamics, interactions, and solvation in ionic liquids. Acc. Chem. Res. 40, 1217–1227. Sarma, N., J. M. Borah, S. Mahiuddin, H. A. R. Gazi, B. Guchhait and R. Biswas (2011) Influence of chain length of alcohols on Stokes’ shift dynamics in catanionic vesicles. J. Phys. Chem. B 115, 9040–9049. Lang, B., G. Angulo and E. Vauthey (2006) Ultrafast solvation dynamics of coumarin 153 in imidazolium-based ionic liquids. J. Phys. Chem. A 110, 7028–7034. Zhang, X. X., M. Liang, N. P. Ernsting and M. Maroncelli (2013) Complete solvation response of coumarin 153 in ionic liquids. J. Phys. Chem. B 117, 4291–4304. Kaintz, A., G. Baker, A. Benesi and M. Maroncelli (2013) Solute diffusion in ionic liquids, NMR measurements and comparisons to conventional solvents. J. Phys. Chem. B 117, 11697–11708. Ramakrishna, G. and H. N. Ghosh (2002) Efficient electron injection from twisted intramolecular charge transfer (TICT) state of 7-diethylamino coumarin 3-carboxylic acid (D-1421) dye to TiO2 nanoparticle. J. Phys. Chem. A 106, 2545–2553. Zhang, H., T. Yu, Y. Zhao, D. Fan, L. Chen, Y. Qiu, L. Qian, K. Zhang and C. Yang (2008) Crystal structure and photoluminescence of 7-(N, N0 -diethylamino)-coumarin-3-carboxylic acid. Spectrochim Acta A 69, 1136–1139. Tablet, C., I. Matei, E. Pincu, V. Meltzer and M. Hillebrand (2012) Spectroscopic and thermodynamic studies of 7-diethylamino-coumarin-3-carboxylic acid in interaction with b- and 2-hydroxypropyl-bcyclodextrins. J. Mol. Liq. 168, 47–53.

Photochemistry and Photobiology, 2015, 91 38. Chatterjee, A., B. Maity and D. Seth (2013) The photophysics of 7(N, N’-diethylamino)coumarin-3-carboxylic acid in water/AOT/isooctane reverse micelles: an excitation wavelength dependent study. Phys. Chem. Chem. Phys. 15, 1894–1906. 39. Chatterjee, A. and D. Seth (2013) Photophysical properties of 7-(diethylamino)coumarin-3-carboxylic acid in the nanocage of cyclodextrins and in different solvents and solvent mixtures. Photochem. Photobiol. 89, 280–293. 40. Chatterjee, A., B. Maity and D. Seth (2014) Influence of double confinement on photophysics of 7-(diethylamino)coumarin-3-carboxylic acid in water/AOT/isooctane reverse micelles. RSC Adv. 4, 13989–14000. 41. Inal, S., J. D. K€ olsch, F. Sellrie, J. A. Schenk, E. Wischerhoff, A. Laschewsky and D. Neher (2013) A water-soluble fluorescent polymer as a dual colour sensor for temperature and a specific protein. J. Mater. Chem. B 1, 6373–6381. 42. Chatterjee, A., B. Maity and D. Seth (2014) Supramolecular interaction between a hydrophilic coumarin dye and macrocyclic hosts: spectroscopic and calorimetric study. J. Phys. Chem. B 118, 9768–9781. 43. Liu, X., J. M. Cole, P. C. Y. Chow, L. Zhang, Y. Tan and T. Zhao (2014) Dye aggregation and complex formation effects in 7-(diethylamino)-coumarin-3-carboxylic acid. J. Phys. Chem. C 118, 13042–13051. 44. Guo, J., K. S. Han, S. M. Mahurin, G. A. Baker, P. C. Hillesheim, S. Dai, E. W. Hagaman and R. W. Shaw (2012) Rotational and translational dynamics of Rhodamine 6G in a pyrrolidinium ionic liquid: a combined time-resolved fluorescence anisotropy decay and NMR study. J. Phys. Chem. B 116, 7883–7890. 45. Guo, J., S. M. Mahurin, G. A. Baker, P. C. Hillesheim, S. Dai and R. W. Shaw (2014) Influence of solute charge and pyrrolidinium ionic liquid alkyl chain length on probe rotational reorientation dynamics. J. Phys. Chem. B 118, 1088–1096. 46. Werner, J. H., S. N. Baker and G. A. Baker (2003) Fluorescence correlation spectroscopic studies of diffusion within the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate. Analyst 128, 786–789. 47. Maroncelli, M., X. X. Zhang, M. Liang, D. Roy and N. P. Ernsting (2012) Measurements of the complete solvation response of coumarin 153 in ionic liquids and the accuracy of simple dielectric continuum predictions. Faraday Discuss. 154, 409–424. 48. Guchhait, B., H. A. R. Gazi, H. K. Kashyap and R. Biswas (2010) Fluorescence spectroscopic studies of (acetamide + sodium/potassium thiocyanates)molten mixtures: composition and temperature dependence. J. Phys. Chem. B 114, 5066–5081. 49. Al Rasid Gazi, H., B. Guchhait, S. Daschakraborty and R. Biswas (2011) Heterogeneity and viscosity decoupling in (acetamide + electrolyte) molten mixtures. Chem. Phys. Lett. 501, 358–363. 50. Pal, T. and R. Biswas (2011) Heterogeneity and viscosity decoupling in (acetamide + electrolyte) molten mixtures: a model simulation study. Chem. Phys. Lett. 517, 180–185. 51. Jones, G. II, W. R. Jackson, C. Y. Choi and W. R. Bergmark (1985) Solvent effects on emission yield and lifetime for coumarin laser dyes. Requirements for a rotatory decay mechanism. J. Phys. Chem. 89, 294–300. 52. Ito, N., S. Arzhantsev and M. Maroncelli (2004) The probe dependence of solvation dynamics and rotation in the ionic liquid 1-butyl3-methyl-imidazolium hexafluorophosphate. Chem. Phys. Lett. 396, 83–91.

1063

53. Ito, N., S. Arzhantsev, M. Heitz and M. Maroncelli (2004) Solvation dynamics and rotation of Coumarin 153 in alkylphosphonium ionic liquids. J. Phys. Chem. B 108, 5771–5777. 54. Jurkiewicz, P., J. Sykora, A. Olzynska, J. Humpolĭckova and M. Hof (2005) Solvent relaxation in phospholipid bilayers: principles and recent applications. J. Fluoresc. 15, 883–894. 55. D’Amico, M., F. Messina, M. Cannas, M. Leone and R. Boscaino (2008) Homogeneous and inhomogeneous contributions to the luminescence linewidth of point defects in amorphous solids: quantitative assessment based on time-resolved emission spectroscopy. Phys. Rev. B 78, 014203-1-8. 56. Okoturo, O. O. and T. J. VanderNoot (2004) Temperature dependence of viscosity for room temperature ionic liquids. J. Electroanal. Chem. 568, 167–181. 57. Owens, P. and A. G. Ryder (2011) Low temperature fluorescence studies of crude petroleum oils. Energy Fuels 25, 5022–5032. 58. Lu, J., C. L. Liotta and C. A. Eckert (2003) Spectroscopically probing microscopic solvent properties of room-temperature ionic liquids with the addition of carbon dioxide. J. Phys. Chem. A 107, 3995–4000. 59. Sarkar, A., M. Ali, G. A. Baker, S. Y. Tetin, Q. Ruan and S. Pandey (2009) Multiprobe spectroscopic investigation of molecular-level behavior within aqueous 1-butyl-3-methylimidazolium tetrafluoroborate. J. Phys. Chem. B 113, 3088–3098. 60. Prabhu, S. R. and G. B. Dutt (2015) Effect of low viscous nondipolar solvent on the rotational diffusion of structurally similar nondipolar solutes in an ionic liquid. J. Phys. Chem. B 119, 2019–2025. 61. Gangamallaiah, V. and G. B. Dutt (2014) Influence of the organized structure of 1–alkyl-3-methylimidazolium tetrafluoroborates on the rotational diffusion of structurally similar nondipolar solutes. J. Phys. Chem. B 118, 13711–13717. 62. Baker, S. N., G. A. Baker, M. A. Kane and F. V. Bright (2001) The cybotactic region surrounding fluorescent probes dissolved in 1butyl-3-methylimidazolium hexafluorophosphate: effects of temperature and added carbon dioxide. J. Phys. Chem. B 105, 9663–9668. 63. Perrin, F. (1934) Mouvement Brownien D’un ellipsoide (I). Dispersion dlelectrique pour des molecules ellipsoidales. J. Phys. Radium 5, 497–511. 64. Hu, C. M. and R. Zwanzig (1974) Rotational friction coefficients for spheroids with slipping boundary condition. J. Chem. Phys. 60, 4354–4357. 65. Mali, K. S., G. B. Dutt and T. Mukherjee (2005) Do organic solutes experience specific interactions with ionic liquids? J. Chem. Phys. 123, 174504–174507. 66. Mali, K. S., G. B. Dutt and T. Mukherjee (2008) Rotational diffusion of a nonpolar and a dipolar solute in 1-butyl-3-methylimidazolium hexafluorophosphate and glycerol: interplay of size effects and specific interactions. J. Chem. Phys. 128, 054504–054509. 67. Spears, K. G. and L. E. Cramer (1978) Rotational diffusion in aprotic and protic solvents. Chem. Phys. 30, 1–8. 68. Nee, T. W. and R. Zwanzig (1970) Theory of dielectric relaxation in polar liquids. J. Chem. Phys. 52, 6353–6363. 69. Gierer, A. and K. Wirtz (1953) Molekulare theorie der mikroreibung. Z. Naturforsch., A: Phys. Sci. 8, 532–538. 70. Dote, J. L., D. Kivelson and R. N. Schwartrt (1981) Molecular quasi-hydrodynamic free-space model for molecular rotational relaxation in liquids. J. Phys. Chem. 85, 2169–2180.