Photophysics and Rotational Diffusion of Hydrophilic Molecule in

Article pubs.acs.org/JPCB

Photophysics and Rotational Diffusion of Hydrophilic Molecule in Polymer and Polyols Aninda Chatterjee, Banibrata Maity, Sayeed Ashique Ahmed, and Debabrata Seth* Department of Chemistry, Indian Institute of Technology Patna, Patna 800013, Bihar, India S Supporting Information *

ABSTRACT: In this work we report the photophysics and rotational diffusion of a hydrophilic solute 7-(N,N′-diethylamino)coumarin-3-carboxylic acid (7-DCCA) in four protic solvents: poly(ethylene glycol), ethylene glycol, tetraethylene glycol, and glycerol, with variation of temperature. The cumulative effect of polarity, viscosity, and structural features of these solvents, as well as specific solute−solvent interaction on the photophysical properties of 7-DCCA was discussed. We observed significant differences in both steady-state and time-resolved emission properties. Estimation of activation energy of viscous flow and activation energy of nonradiative decay reinforce our assumption of a cumulative effect. It was observed that, in all solvents, H-bonding interactions are mainly responsible for changing the spectral properties. Study of rotational relaxation behavior demonstrates superstick boundary condition to be operative in ethylene glycol. It is due to the H-bonding interaction between 7-DCCA and ethylene glycol. Similarly, stick boundary condition is followed in case of tetraethylene glycol at 278 K and further from 293 K. Convergence to the stick boundary is observed in case of poly(ethylene glycol). These changes can be attributed to the change in structural organization in both poly(ethylene glycol) and tetraethylene glycol.

1. INTRODUCTION Studies on different physical and photophysical properties have been extended beyond the neat solvents to different polymer melts, polyols, starlike amphiphilic macromolecules, and several microheterogeneous media.1−4 These media have attracted enough attention to several researchers, because of their inhomogeneous nature. Several intermolecular interactions prevailing in them, orientation of molecules, and intramolecular conformations are significantly different from the normal and simple solutions. These features of the media caused different behavior of dynamics compared to that in simple solutions. Polymer liquids and melts exhibit divergent dynamical properties compared to the simple molecular liquids. This is due to different local motions, including constitutional repeat units, segments, and side groups, the motion of entire polymer, and cooperative motions.5−9 These different dynamical behaviors have made these polymer liquids and melts attractive to the researchers. Beside polymer liquids and polymer melts several associated liquids such as glycerol, ethylene glycol, etc. have been a subject of considerable discussion for a long time. Several works are reported in the literature on the relaxation behavior of different associated liquids.10−13 Meier et al. had studied the intermolecular relaxation in glycerol using the field cycling 1H NMR relaxometry dilution experiments and interpreted the relaxation to be contributed both by translational and rotational motions.13 They also compared their results on 1H NMR relaxometry and dielectric spectroscopy (DS) for various viscous liquids.14 © 2014 American Chemical Society

Shirota and Segawa reported the solvation dynamics in liquid poly(ethylene glycol)s with varying molecular weight using coumarin 153 (C153) as probe15 and also compared the feature of solvation dynamics in poly(ethylene glycol)s with deuterated poly(ethylene glycol)s.16 The spectral properties of molecular rotor were used to find out the molecular weight dependence of viscosity in polymer melts.17 Among all the popularly used fluorescent probes, the most prominently used probe is substituted aminocoumarins. Due to their high environmental sensitivity, they have been used to study the dynamical behavior in different microheterogeneous media.18,19 In this study, we have used 7-(N,N′-diethylamino)coumarin3-carboxylic acid (7-DCCA) as molecular probe, since it is more hydrophilic than other popularly used aminocoumarin molecules. It provides enough scope of hydrogen bonding with surrounding media to modulate its spectral behavior. Very few studies are reported in the literature on the photophysics of the probe 7-DCCA.20−28 In this article, we have showed the effect of the microheterogeneous nature of the medium on the photophysical behavior of 7-DCCA. We have showed the effect of varying chain length of polymeric material poly(ethylene glycol) (PEG), tetraethylene glycol (TEG) on the rotational relaxation dynamics of 7-DCCA. We report how the specific solute−solvent interaction affects the rotational relaxation Received: May 16, 2014 Revised: October 10, 2014 Published: October 13, 2014 12680

dx.doi.org/10.1021/jp506786m | J. Phys. Chem. B 2014, 118, 12680−12691

The Journal of Physical Chemistry B

Article

studies were carried out using a Fluoromax-4P spectrofluorometer (Horiba Jobin Yvon). For absorption and fluorescence measurements, the path length of the used quartz cuvette is 1 cm. The fluorescence quantum yields of 7-DCCA in different solvent media were measured using the fluorescence quantum yield of coumarin 480 in water solution (ϕr = 0.66) as reference,29 by using the following equation:

behavior of the molecule. This will help to explore the complicacy arising due to different kinds of specific solute− solvent interaction, which affects the spectral properties of an H-bond forming molecule. In this study we are trying to explore the cumulative effect of polarity, viscosity, and structural heterogeneity of the medium and also specific solute−solvent interaction in associative protic solvents (ethylene glycol, glycerol), polymer, and polymer-like melts (poly(ethylene glycol), tetraethylene glycol). We have applied temperature variation study in order to show that the photophysical processes in these media are not only guided by viscosity but also by the specific solute−solvent interaction between the solute probe and solvent molecules. We have used ultraviolet−visible absorption spectroscopy, steady-state fluorescence emission spectroscopy, and time-resolved emission spectroscopy to study the photophysics of 7-DCCA. To measure the rotational diffusion dynamics we have used timeresolved fluorescence anisotropy measurements.

ϕf = ϕr

IsA r ns 2 IrA snr 2

(1)

where s and r stand for the sample and reference, respectively. Here, I stands for the integrated area under the fluorescence curve, A is the absorbance of the sample at the excitation wavelength, and n is 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, U.K.). We have used a picoseconds diode laser with excitation wavelength at 405 nm. The full width at half-maximum (fwhm) of our system is ∼80 ps. The fluorescence transients were detected at magic angle (54.7°) polarization using a Hamamatsu MCP PMT (3809U) as a detector. The decays were analyzed using F-900 decay analysis software. The fluorescence anisotropy decays (r(t)) were measured by using the same instrument. The following equation was used to obtain r(t).

2. MATERIALS AND METHODS 2.1. Materials. 7-DCCA was purchased from Sigma-Aldrich and used as received. Poly(ethylene glycol) (average Mn = 400) (PEG) and tetraethylene glycol (TEG) were purchased from Sigma-Aldrich and used as received. Glycerol (ACS grade) and ethylene glycol (EG) were purchased from RANKEM, India and CDH, India, respectively, and used without further purification. The structures of all chemical used are shown in Scheme 1.

r (t ) =

Scheme 1. Structure of 7-DCCA and Different Solvents

I (t ) − GI⊥(t ) I (t ) + 2GI⊥(t )

(2)

where the emission intensities at parallel (I∥) and perpendicular (I⊥) polarizations were collected alternatively by fixing the time for both the decays. We have used motorized polarizers to collect the parallel and perpendicular decays. G is the correction factor for the detector sensitivity to the polarization direction of the emission. A similar method was used to measure the G factor. G factor is represented by the following equation: G=

SV S I I = V ⊥ = HV SH SH I⊥ IHH

(3)

IHV represents intensity of horizontally polarized excitation and vertically polarized emission. Similarly, IHH represents intensity of horizontally polarized excitation and horizontally polarized emission. SV and SH are the sensitivities of the emission channel for vertically and horizontally polarized light. The G factor is measured by applying horizontally polarized excitation. Both polarizers are oriented perpendicular to the polarization of the excitation. F-900 software was used to analyze the anisotropy decays. The overall anisotropy decay is fitted by the stretched exponential function:30

2.2. Sample Preparation. From the stock solution of 7DCCA in methanol the required amount of aliquot was taken out by a microliter syringe in a quartz cuvette and dried under vacuum. Then sufficient volume of each solvent was added to the cuvette, and sufficient time was allowed for complete solubilization of the dye in each of the solvents. The concentration of dye in each experiment is maintained at ∼3 × 10−6 M. 2.3. Methods. Ground-state absorption measurements were done by using a UV−vis spectrophotometer (model UV-2550, Shimadzu). The steady-state fluorescence emission spectral

βrot ⎫ ⎧ ⎪ ⎛ t ⎞ ⎪ r(t ) = r0 exp⎨−⎜ ⎟ ⎬ ⎪ ⎪ ⎩ ⎝ τrot ⎠ ⎭

(4)

where r0 is the limiting anisotropy and βrot represents a fitting parameter. ⟨τrot⟩ was calculated using the following equation:30 ⟨τrot⟩ = 12681

τrot ⎛ 1 ⎞ Γ⎜⎜ ⎟⎟ βrot ⎝ βrot ⎠

(5)



Article

Table 1. Photophysical Parameters of 7-DCCA in PEG, EG, Glycerol, and TEG and Viscosities of Solvents at Different Temperatures Sr no. 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. a

system 7-DCCA in PEG

7-DCCA in EG

7-DCCA in glycerol

7-DCCA in TEG

temp (K)

viscosity (cP) (η)

λmax abs (nm)

278 283 288 293 298 303 308 313 318 323 278 283 288 293 298 303 308 313 318 323 278 283 288 293 298 303 308 313 318 323 278 283 288 293 298 303 308 313 318 323

374 250 180 129 97 74 57 44 35 28 44 33 25 20 17 12 10 8 7 6 6735 3727 2115 1306 852 550 360 250 174 124 178 136 97 67 49 38 30 22 18 15

391

408

431

428

λmax emi (nm)

ϕf

⟨τf⟩ (ns)a

450 450 450 450 450 450 450 450 450 465 465 465 465 465 465 465 465 465 465 473 473 474 475 475 475 476 476 476 476 470 470 470 470 470 470 470 470 470 470

0.89 0.81 0.79 0.75 0.75 0.75 0.72 0.69 0.68 0.39 0.33 0.30 0.28 0.24 0.21 0.19 0.18 0.16 0.15 0.44 0.44 0.41 0.32 0.27 0.23 0.19 0.16 0.13 0.11 0.28 0.23 0.19 0.16 0.14 0.12 0.11 0.10 0.09 0.08

2.55 2.45 2.38 2.26 2.20 2.12 2.06 1.97 1.92 1.18 1.05 0.94 0.86 0.76 0.68 0.61 0.56 0.52 0.47 2.30 2.05 1.72 1.54 1.27 1.02 0.83 0.66 0.56 0.46 1.06 0.89 0.75 0.65 0.58 0.49 0.46 0.40 0.36 0.39

⟨τf⟩ = a1τ1 + a2τ2 + a3τ3.

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 other measurements the temperature was controlled by using a Jeiotech refrigerated bath circulator (model RW0525G). The viscosities of the solutions were measured by using a Brookfield cone/plate viscometer (model DV-II+Pro).

3. RESULTS 3.1. Steady-State Absorption and Emission Spectral Studies. 3.1.1. Steady-State Absorption Spectra. The steadystate absorption spectra of 7-DCCA in PEG, TEG, EG, and glycerol are shown in Table 1. The absorption maxima of 7DCCA in PEG is highly blue-shifted compared to that of EG and TEG as shown in Figure 1. All these three glycols have two polar hydroxyl groups per molecule. However, in case of PEG, the ethylene oxide chain is much more hydrophobic and less

Figure 1. Absorption spectra of 7-DCCA in PEG, EG, glycerol, and TEG at 298 K.

12682



Article

Figure 2. Steady-state emission spectra of 7-DCCA in (a) PEG, (b) EG, (c) glycerol, and (d) TEG with the variation of temperature. The interval of the temperature variations for the fluorescence spectra is 5 K.

follow any regular trend on polarity of the medium. This clearly indicates that, besides the polarity, specific solute−solvent interaction plays a prominent role in deciding the anomalous spectral behavior of 7-DCCA. In case of polymeric and polymer-like solvent media (PEG and TEG), where the number of free hydroxyl groups remains the same, the change in absorption maxima is mainly guided by the ethylene oxide chain length, which controls the availability of hydroxyl group at the two ends of the polymer chain to interact with 7-DCCA. In the case of protic nonpolymeric media (EG and glycerol) the large red shift in absorption spectra is mainly guided by the availability of the hydroxyl group. On going from EG to glycerol the number of OH groups increases, thereby increasing the hydrogen-bonding network between 7-DCCA and solvent medium. Due to this reason the absorption spectra are significantly blue-shifted in EG compared to glycerol. 3.1.2. Steady-State Fluorescence Emission Study. The steady-state fluorescence spectra (Figure 2) show a common feature in all solvent media. With the gradual rise of temperature, the fluorescence quantum yield gradually decreases (Table 1). This is due to the fact that a rise of temperature decreases the viscosity of the medium and it increases the nonradiative decay by twisting the diethylamino group of 7-DCCA. For every system, the fwhm gradually increases with the increase in temperature (Supporting Information Figure S1). In our present experiment, the relation between fwhm and viscosity of the solvent medium does not follow any regular trend. Therefore, we must consider specific solute−solvent interaction to interpret the emission spectral behavior of 7DCCA, beside viscosity and polarity of the medium. The

polar than the two polar terminal hydroxyl groups. In the case of TEG, the length of ethylene oxide chain is much smaller than that of PEG. On going from TEG to PEG the chain length of ethylene oxide chain increases and the absorption peak position blue-shifted from 428 nm in TEG to 391 nm in PEG. We have calculated the volume of hydroxyl group and molecule by using the method introduced by Edward.31 In case of PEG, TEG, and EG the total volume of two hydroxyl groups per molecule is 26.8 Å3. The total volume of three hydroxyl groups in glycerol is 40.2 Å3. In PEG, the long hydrophobic chain of ethylene oxide wraps the 7-DCCA molecule. On the other hand, this polymeric solvent provides less opportunity for the hydrogen bonding with the terminal hydroxyl group, since the volume fraction of the hydroxyl group in PEG is very small. However, in case of TEG, the volume fraction of the hydroxyl group is almost 2 times higher than that of PEG. So, in PEG hydrogenbond formation ability decreases compared to TEG. Also due to more hydrophobic and less polar environment blue shift in absorption spectra of 7-DCCA was observed in PEG compared to TEG. We have found that the absorption maximum of 7DCCA in EG appeared at an intermediate wavelength between that of PEG and TEG. EG is polar compared to TEG and PEG. Dielectric constants of TEG and PEG are 15.7 and 14.1, respectively, whereas that of EG is 37.7. The absorption maximum of 7-DCCA in EG appears at 408 nm. This indicates that the shifts of the absorption maximum not entirely due to the change of polarity of the medium but also due to the specific solute−solvent interaction between 7-DCCA and solvent medium. Moreover, in case of glycerol (dielectric constant is 42.5 at 298 K), the absorption maximum appears at 431 nm. Therefore, the change of absorption maxima does not 12683



Article

Figure 3. Time-resolved emission spectra of 7-DCCA in (a) PEG, (b) EG, (c) glycerol, and (d) TEG with the variation of temperature. The interval of the temperature variations for the fluorescence transients is 5 K.

difference of polarity of EG and glycerol is too small33 to affect the shift of emission maxima of 7-DCCA. The quantum yield values (ϕf) of 7-DCCA is different media are tabulated in Table 1. The ϕf values of 7-DCCA have no regular correlation with the viscosity and polarity of the media. First let us consider the case of 7-DCCA in TEG and PEG. The longer hydrophobic ethylene oxide chain in PEG compared to TEG causes higher ϕf value. The viscosity of glycerol is about 50 times higher than that of EG at room temperature. However, ϕf of 7-DCCA in glycerol is 0.27, whereas that in EG is 0.24. Similar result is also observed at low temperature. This shows that, beside viscosity, the presence of −OH groups affects the quantum yield values. So, from the steady-state spectral studies we can have a primitive idea about the effect of structural features of the media on the photophysical properties of 7-DCCA. 3.2. Time-Resolved Fluorescence Study. We have measured the time-resolved fluorescence emission spectra and found that in all solvent systems the average fluorescence decay time gradually decreases with the increase in temperature, as shown in Figure 3 and Table 1. All the time-resolved decays are fitted by multiexponential function. We have found highest decay time of 7-DCCA in PEG as tabulated in Table 1. Decrease of poly(ethylene oxide) chain length in TEG causes decrease in fluorescence decay time from 2.26 ns in PEG to 0.58 ns in TEG, at 298 K. However, to our surprise, we observed that the fluorescence decay time of 7-DCCA in EG is in between that of PEG and TEG. So the photophysical properties of 7-DCCA in EG are influenced differently than PEG and TEG. Here, neither bulk viscosity nor polarity plays any prominent role in determining the photophysical properties. In case of 7-DCCA in PEG and TEG, it is quite clear that

emission peak of 7-DCCA in PEG was observed at 450 nm. This blue shift in emission maximum is mainly due to the long hydrophobic chain of PEG. This provides a nonpolar environment around the dye molecule. Because of decrease of the ethylene oxide chain length, the emission spectra undergo significant red shift to 470 nm in TEG. In case of PEG and TEG the number of free hydroxyl groups at the two ends of the chain remains the same. Here, the volume fraction of the terminal hydroxyl group is responsible for red-shifted emission on going from PEG to TEG. Moreover, in case of TEG the possibility of H-bonding is more pronounced than that in PEG. This is further substantiated by taking the FTIR spectra of 7-DCCA in PEG and TEG. Here, the carbonyl peak shows substantial shifts toward lower wavenumber in TEG (Supporting Information Figure S2). Both PEG and TEG are made of ethylene oxide chain, which is derived from EG. The similarity between PEG, TEG, and EG is that all these three molecules have only two terminal OH groups to form an H-bond. As the polarity of ethylene glycol is higher than that of PEG and TEG, so we could expect that the emission peak of 7-DCCA in EG to be red-shifted, if the polarity would be the only driving factor. We have found that the emission maximum of 7-DCCA in EG is in between those of PEG and TEG. This shows that, beside polarity and viscosity, structural features of the medium are responsible for unusual spectral shift. EG has high ability to form a H-bond. They tend to form intermolecular H-bonding to provide a polymer-like structural array.32 This causes the emission maxima to be in the intermediate position of TEG and PEG. Again increase in proticity on going from EG to glycerol has profound effect on the spectral properties of 7-DCCA, since the 12684



Article

external forces exerted by the electric field during the poling process. The much stronger internal electric field forces the dipolar dyes not to orient in the given external electric field.35 In our case the electron-deficient part and electron-rich part of the dye approach from the opposite direction on the top of each other and maximize the electrostatic interaction between them. This is the main driving force of the aggregate formation. Aggregate formation of 7-DCCA in few solvents have been reported by Liu et al.28 In our case aggregate formation of 7DCCA is only observed in TEG. This indicates that the complex structural feature in the polymer meltlike solvent TEG causes the formation of aggregate in this particular medium. Similarly, the structural features and specific solute−solvent interaction prevents the formation of aggregates in other solvent media studied by us. 3.3. Effect of Temperature on the Viscosity. In order to understand the effect of temperature on viscosity we have measured the viscosity of the medium at different temperatures varying from 278 to 323 K. The interval of the temperature variations is 5 K. In this entire range of temperature the viscosity of four solvents exhibit Arrhenius-like behavior. The activation energies of viscous flow for all solvents are calculated using Arrhenius equation:

decrease in chain length causes the decrease in fluorescence decay time. This is due to the change in ethylene oxide chain length from PEG to TEG, causing the increase in micropolarity and decrease in microviscosity around the dye molecule. This causes the increase in the propensity to undergo charge transfer followed by twisting mechanism to produce a twisted intramolecular charge-transfer (TICT) state, hence, decrease in fluorescence decay time. Similar observation was reported by Lee and Lee.34 In our present study we have found that the fluorescence decay time in EG is significantly higher than that in TEG and smaller than that in PEG. This is just opposite to the observation of Lee and Lee34 where it was demonstrated that the fluorescence decay time of hemicyanine dyes gradually increases with the gradual increase in chain length on going from EG to 8EG. In our case on going from EG to TEG the fluorescence decay time decreases and then further increases in PEG. This proves our previous assumption that the photophysics of 7-DCCA in EG, PEG, and TEG is guided mainly by specific solute−solvent interaction. Since EG is known to form polymer-like structure by using H-bonding network, this provides restricted microenvironment around the dye, to restrict the formation of the TICT state. Again, on changing the solvent medium to glycerol, it is found that the fluorescence decay time is higher than that in TEG and EG but shorter than in PEG. Probably, the polarity effect coupled with specific solute−solvent interaction such as H-bonding between dye and medium cause the increase of fluorescence decay time of 7DCCA. The emission decays of 7-DCCA in PEG and EG are fitted biexponentially as shown in Supporting Information Figure S3, parts a and b. In case of 7-DCCA in PEG, both the components of emission decay decreases almost regularly with the increase of temperature. However, in EG, the fast component of emission decays remains unchanged with increase of temperature. On the other hand, the slow component decreases in a regular pattern. The regular decreasing pattern of both the components with increasing temperature (in PEG) and only slow component (in EG) is indicative of absence of any kind of aggregated form of 7-DCCA in these solvents. In case of glycerol, the decays are fitted biexponentially up to 298 K. Above 298 K, the decays are triexponential in nature (Supporting Information Table S1 and Figure S3c). This result is also consistent with the presence of structural groups in the associated liquids. This indicates that the immediate surroundings of the dye are changing with increasing temperature. The most surprising feature is observed in TEG. In this medium, we have observed triexponential decays in the entire temperature range studied (Supporting Information Table S1 and Figure S3d). Here, we have observed that both fast and intermediate components decrease in time scale with increase in temperature, whereas the slow component increases in time scale with the increase of temperature. Moreover, the weight percentage of the slow component decreases with the gradual rise of temperature. This indicates the presence of aggregation of 7DCCA in TEG. With gradual rise of temperature the aggregates start to break down. This causes the increase in time scale of the slow component. Moreover, this dissolution of the aggregated form of dye causes the decrease of the weight percentage of this aggregated dye. This behavior is absent in other solvent media. 7-DCCA possesses high dipole moment. It was reported in the literature that the dipolar dyes are prone to form aggregates in an antiparallel fashion to maximize the internal electrostatic interaction.35 This counterbalances the

ln η = ln ηa +

Eη RT

(6)

where ηa stands for the limiting solvent viscosity at infinite temperature and Eη represents the activation energy of viscous flow. Using eq 6 we have found that the activation energies of viscous flow are 40.0 (± 0.3), 32.0 (± 2), 62.0 (± 0.7), and 40.0 (± 1) kJ M−1 K−1 for PEG, EG, glycerol, and TEG, respectively (Figure 4). Here, fact is that although the viscosity of PEG is higher than that of TEG, the activation energy of viscous flow of both the media is almost the same.

Figure 4. Arrhenius plots for the variation of viscosity with 1/T for PEG, EG, glycerol, and TEG.

We have found out the activation energy of nonradiative decay process in order to understand whether the viscosity of the media influences the nonradiative decay. We have used the modified Arrhenius equation to find out the activation energy of nonradiative decay:36 ⎛ E ⎞ 1 1 = + A exp⎜ − nr ⎟ ⎝ RT ⎠ τ τ0 12685

(7)



Article

Figure 5. Arrhenius plots for the determination of the activation energy of nonradiative decay of 7-DCCA in (a) PEG, (b) EG, (c) glycerol, and (d) TEG.

where Enr is the activation energy of the nonradiative decay process, A is the pre-exponential factor, and τ0 is the intrinsic lifetime. By plotting inverse of average lifetime (1/τ ) value against inverse of temperature (1/T) we obtained the activation energy of nonradiative decay process. This nonlinear fitting (Figure 5) provides the activation energies of nonradiative decay 7.1 (± 2.7), 16.3 (± 1.5), 43.5 (± 2), and 23.0 (± 4.9) kJ M−1 K−1 for PEG, EG, glycerol, and TEG, respectively. These values are less than the activation energy of viscous flow. This indicates that, in these systems, the nonradiative decay pathway is not fully guided by bulk viscosity of the medium. This confirms our previous assumption that in these systems the nonradiative decay mechanism is mainly controlled by the specific solute−solvent interaction. Due to these specific solute−solvent interactions, 7-DCCA faced different microviscosity than bulk viscosity. 3.4. Rotational Relaxation Dynamics of 7-DCCA. The time-resolved fluorescence anisotropy decays of 7-DCCA in each solvent medium show that the rotational correlation time of the dye gradually decreases with the increase in temperature (Supporting Information Figure S4, Table S2). 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. We have fitted all the anisotropy decays by a stretched exponential function. We have observed that at 298 K the rotational correlation times of 7-DCCA in the solvent media follows the order EG < TEG < PEG. The variation of rotational correlation time in different solvents at 298 K is represented in Figure 6, and the rotational correlation times are tabulated in Table 2. In glycerol, the rotational relaxation of the dye is completely hindered as shown in Figure 7. Therefore, we have not been

Figure 6. Time-resolved fluorescence anisotropy decays (in logarithm scale) of 7-DCCA in PEG, TEG, and EG at 298 K.

Table 2. Rotational Correlation Time of 7-DCCA in PEG, EG, Glycerol, and TEG at 298 K Sr no. 1. 2. 3. 4.

system 7-DCCA 7-DCCA 7-DCCA 7-DCCA

in in in in

PEG EG glycerol TEG

temp (K) 298

r0

⟨τrot⟩ (ns)

0.321 7.58 0.341 1.45 restricted rotation 0.367 4.40

able to find out the rotational correlation time of 7-DCCA in glycerol at any temperature. Similar type of feature of hindered rotational diffusion was also observed for 7-DCCA in PEG at low temperatures, mainly from 278 to 293 K. However, in our present system we have observed that the rotational correlation time of 7-DCCA is significantly higher than the fluorescence decay time in the entire solvent media. 12686



Article

Figure 7. Time-resolved fluorescence anisotropy decays of 7-DCCA in glycerol at (a) 278 K and (b) 323 K, showing hindered rotation of the dye molecule in this solvent medium.

In order to probe the rotational relaxation studies in more detail, we have analyzed our data by using the Stokes− Einstein−Debye (SED) hydrodynamic model of rotational diffusion. This shows that the rotational correlation time (τSED r ) of a noninteracting solute in a solvent of viscosity η is given by the following equation:

For TEG, at 323 K the rotational correlation time is almost 3 times higher than the fluorescence decay time. Nanostructure domains are expected to exist for TEG. Moreover, the presence of some aggregated 7-DCCA molecule may have caused the higher rotational correlation time in TEG than the fluorescence decay time. In case of associated liquids like glycerol and ethylene glycol, the presence of specific regions of order or structural groups was described by McDuffie and Litovitz.11 These caused restriction over the rotational relaxation of 7DCCA in EG at low temperature and in glycerol at the entire temperature range from 278 to 323 K. In our present system, we have used a strategy to understand the effect of specific solute−solvent interaction and viscosity of media on the rotational relaxation of solute in the solvent media. We have determined the activation energy of rotational relaxation using the rotational correlation time (Supporting Information Table S2, Figure S5). It shows a regular trend of decreasing with temperature. We have determined the activation energy of rotational relaxation by using the following equation: ⟨τrot⟩ =

VfCη0 kT

⎛ E ⎞ exp⎜ − rot ⎟ ⎝ RT ⎠

τrSED =

VηfC kBT

(9)

where V stands for molecular volume or van der Waals volume; f and C are the shape factor and friction coefficients, respectively. kB and T stand for Boltzmann’s constant and temperature, respectively. f accounts for the nonspherical nature of the solute molecule and was first introduced by Perrin.38 For a spherical particle the value of f is 1, whereas for an asymmetric ellipsoid its value is greater than 1. We have treated 7-DCCA as an oblate ellipsoid, and its radii along the three axes are calculated as a (radius along the semimajor axis) = 6.8 Å, b (radius along the semiminor axis) = 3.2 Å, c (width of the molecule) = 2.7 Å. 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.39 When the size of the rotating particle or molecule is much higher than that of 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 (Table 3).

(8)

where the prefactor V, f, C, η0 stand for van der Waals volume, shape factor, friction coefficients, and infinite high-temperature viscosity, respectively. k and T stand for Boltzmann’s constant and temperature, respectively. For PEG, the activation energy of rotational relaxation is 38.0 (± 1.4) kJ M−1 K−1. This value is smaller than the activation energy of viscous flow. This shows that the rotational relaxation of 7-DCCA is not entirely driven by the bulk viscosity of the media. In EG, the activation energy of rotational relaxation of 7-DCCA is not very much different from the activation energy of viscous slow. This indicates significant influence of bulk viscosity of the medium, as well as influence of dielectric friction and specific solute−solvent interaction. The influence of dielectric friction has been described later. This coupled effect of both hydrodynamic friction arising due to bulk viscosity as well as dielectric friction causes the strong hindrance to solute rotation. Strong specific solute−solvent interaction, specially H-bonding interaction, also plays a prominent role.37 In TEG, the activation energy of rotational relaxation of solute molecule is significantly lower than the activation energy of viscous flow (Eη). These indicate comparatively lower hindrance experienced by the solute from its immediate surrounding.

Table 3. Values of Cobs, CGW, CDKS, Crot, and Cslip for 7-DCCA in PEG, EG, and TEG at 298 K Sr no.

system

Cobs (av)

CGW

CDKS

Crot (av)

1. 2. 3.

7-DCCA in PEG 7-DCCA in EG 7-DCCA in TEG

0.894 1.120 1.013

0.144 0.250 0.178

0.079 0.171 0.099

0.810 1.242 0.902

Cslip 0.203

The variation of rotational correlation time (⟨τrot⟩) with η/T is shown in Figure 8, with respective stick and slip boundary conditions. Here we have plotted the observed rotational correlation times along with calculated τstick and τslip against η/T values. We have fitted the observed rotational correlation time ⟨τrot⟩ against η/T using (Supporting Information Figure S6) the following formula described by Mali et al.:40,41 12687



Article

Figure 8. Plot of rotational correlation times of 7-DCCA against η/T in (a) PEG, (b) EG, and (c) TEG. Theoretically calculated reorientation times using SED theory with slip (lower dotted line) and stick (upper dashed line) boundary conditions are shown in the figure.

⎛ η ⎞p ⟨τrot⟩ = A⎜ ⎟ ⎝T ⎠

288 K, the rotational correlation time again becomes almost comparable to τstick, and this phenomenon continues up to 323 K. Following of the stick hydrodynamics after 288 K is due to the specific solute−solvent interaction, mainly strong Hbonding interaction. In our present case we have found out the values of Cobs and observed that in PEG Cobs lies in between Cstick and Cslip but close to Cstick (Table 3). In case of TEG, the Cobs value is almost same as Cstick (Table 3). In case of EG, the Cobs value is greater than 1. This suggests the superstick boundary condition. This can be explained by using “solventberg” model or Nee−Zwanzig dielectric friction.43,44 The “solventberg” model considers the specific solute−solvent interaction that causes the solvent molecules of non-negligible size to the solute particle to anchor on the solute. This causes the increase of effective volume of the rotating solute. Nee− Zwanzig dielectric friction mechanism considers the electrostatic torque between a rotating dipolar solute and the reactive field of the surrounding dielectric cavity and can be represented as follows:

(10)

7-DCCA in PEG ⎛ η ⎞0.98 ± 0.03 ⟨τrot⟩ = (22.55 ± 1.00)⎜ ⎟ ⎝T ⎠ (N = 5, R = 0.996)

7-DCCA in EG ⎛ η ⎞0.93 ± 0.3 ⟨τrot⟩ = (23.45 ± 2.50)⎜ ⎟ ⎝T ⎠ (N = 10, R = 0.993)

7-DCCA in TEG ⎛ η ⎞1.01 ± 0.05 ⟨τrot⟩ = (25.64 ± 1.34)⎜ ⎟ ⎝T ⎠ (N = 10, R = 0.993)

D=

The deviation from the linear behavior is more pronounced in case of 7-DCCA in EG and least in case of PEG and TEG. The deviation from the linear behavior is due to the presence of heterogeneity in the medium.42 This shows that rotational correlation times of 7-DCCA in case of PEG lie in between slip and stick boundary. However, in case of EG the rotational correlation time is even somewhat higher than τstick. In case of 7-DCCA in TEG at 278 K, the rotational correlation time is almost the same as τstick, but then deviates further from stick boundary at 283 and 288 K, although the deviation is not very high except at 288 K. After

kBT 6ηVfC + ζNZ

ζNV =

2 2μ2 (ε∞ + 2) (ε0 − ε∞) τD 3(2ε0 + ε∞) a3

(11)

(12)

where D stands for the diffusion coefficient, ζNZ is the friction coefficient for dielectric friction, μ is the dipole moment of the solute particle, a is the dielectric cavity radius, τD is the dielectric relaxation time, and ε∞ and ε0 are the optical and static dielectric constants. 12688



Article

Considering μ = 10.41 D (obtained from the optimized geometry of 7-DCCA, shown in Supporting Information Figure S7), a = 3.830A, and for EG τD = 92.40 ps,45 ε0 = 41.20, and ε∞ = n2 = 2.05. We have found out an additional contribution of dielectric friction to the total observed friction coefficient is 0.025 in the case of EG. Here, we can expect both the mechanisms to be operative, to provide a superstick boundary condition. Here the specific solute−solvent interaction between the 7-DCCA and EG is mainly H-bonding interaction. Similar kind of observation was observed by Fruchey and Fayer.46 Horng et al. had reported the higher contribution of dielectric friction to the rotational relaxation time of solute molecule in normal alcohol solvents than in polar aprotic solvents.47 For TEG after 288 K at higher temperatures (mainly from 293 K), the rotational correlation time, and hence Cobs, is almost comparable to stick boundary condition. Thus, the transition from stick to intermediate (between stick and slip) boundary to further stick boundary condition is mainly due to the change of structural organization of TEG molecule. This change in structural organization causes the specific solute−solvent interaction between solute 7-DCCA and solvent TEG, thereby increasing the friction between the dye and solvent. In case of PEG, the experimental rotational correlation time converges to stick boundary at higher temperature (318 and 323 K). The profound deviation from the stick boundary condition, which starts mainly from 313 K and continues even at 278 K, signifies the decoupling of rotational relaxation from the medium viscosity. With the lowering of the temperature, PEG starts to become more structured and organized thereby providing lesser scope of specific solute−solvent interaction. This causes the significant deviation from the stick hydrodynamics. Now, the regular decrease of Cobs with increasing the chain length, on going from EG to PEG, can also be explained by considering average Crot (= τstick/τslip) values, which accounts for solute−solvent interaction (Table 3). On going from EG to PEG the specific solute−solvent interaction, especially Hbonding interaction, decreases due to increase in chain length. This causes lower friction experienced by 7-DCCA. It is reported that the size of free space between the solvent molecules increases as the size of solvent molecules increases.48 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 and does not take any specific solute−solvent interaction, such as H-bonding interaction, electrostatic interaction, etc., into consideration. By comparing the Cobs values with the calculated Cslip, we observed that the values are always higher than Cslip. 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 Gierer−Wirtz (GW)48 and Dote− Kivelson−Schwartz (DKS)49 to explain the rotational relaxation behavior of the solute molecule depending on the solvent volume. According to the GW theory, the parameters of the boundary condition can be rationalized in terms of solvent volume (VS) and solute volume (Vp): CGW = σC0 σ=

⎧ ⎫−1 ⎛ Vs ⎞1/3 ⎪ ⎪ 6⎜ V ⎟ ⎪ ⎪ ⎝ p⎠ 1 ⎬ + C0 = ⎨ 4 3 ⎡ ⎪⎡ ⎛ Vs ⎞1/3⎤ ⎛ Vs ⎞1/3⎤ ⎪ ⎢1 + 4⎜ ⎟ ⎥ ⎪ ⎪ ⎢1 + 2⎜ V ⎟ ⎥ ⎝ p⎠ ⎦ ⎝ Vp ⎠ ⎦ ⎭ ⎣ ⎩⎣

By using Edward’s increment method we have calculated the Vs and Vp for PEG, EG, and TEG solvents and 7-DCCA. The boundary condition parameter in this model (CGW) was determined for the entire solvent media (PEG, EG, TEG) and tabulated in Table 3. Although the van der Waals volume of the solvent molecules increases ∼6 times on going from EG to PEG in the order EG < TEG < PEG, the CGW value decreases in the order EG > TEG > PEG by ∼2.00. On the other hand, the Cobs values decreases ∼1.3 times in the same order mentioned above. Moreover, the values of Cobs are higher in every solvent medium than the CGW values. This shows that GW theory cannot explain the rotational dynamics of 7-DCCA in these solvent media. The quasihydrodynamic 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: −1 ⎛ γ⎞ C DKS = ⎜1 + ⎟ ϕ⎠ ⎝

(16)

where ϕ = f Cslip and γ is represented as follows: 2/3 ⎡ ⎤ ΔV ⎢ ⎛ Vp ⎞ 4⎜ ⎟ + 1⎥ γ= ⎥⎦ Vp ⎢⎣ ⎝ Vs ⎠

(17)

where ΔV stands for the free space per solvent molecule for associative liquids. ΔV can be expressed as follows: ΔV = 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 values of CDKS are tabulated in Table 3. The (γ/ϕ) parameter is the ratio of the free space available to its effective rotational volume. Here we have found that the value of CDKS decreases in the order EG > TEG > PEG by 2.2 times. However, the main feature is that the values of CDKS of 7-DCCA in each solvent medium are at least 6 times smaller than the Cobs values. This indicates that the quasihydrodynamic DKS theory cannot explain the rotational relaxation process in the previously mentioned solvent medium. Thus, here we can affirmatively say that the H-bonding interaction between the solute and solvents predominantly affect the rotational relaxation behavior of 7DCCA.

4. CONCLUSION In this work, we have reported the cumulative effect of polarity, viscosity, specific solute−solvent interaction, and structural feature of solvent medium on the photophysical properties of 7DCCA. Determination of both activation energy of viscous flow and activation energy of nonradiative decay clearly demonstrates that for the entire solvent media the nonradiative decay process of 7-DCCA molecule is not solely guided by bulk

(13)

1 ⎛ V ⎞1/3 1 + 6⎜ Vs ⎟ C0 ⎝ p⎠

(15)

(14) 12689



Article

Amphiphilic Starlike Macromolecules. J. Phys. Chem. B 2002, 106, 7463−7468. (5) (a) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (b) de Gennes, P. D. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, NY, 1979. (6) Doi, M.; Edwards, S. F. The Theory of Polymer Dynamics; Oxford University Press: Oxford, U.K., 1986. (7) Matsuoka, S. Relaxation Phenomena in Polymers; Carl Hanser Verlag: Munich, Germany, 1992. (8) Strobl, G. R. The Physics of Polymers; Springer: Berlin, Germany, 1997. (9) Davidson, D. W.; Cole, R. H. Dielectric Relaxation in Glycerol, Propylene Glycol, and nPropanol. J. Chem. Phys. 1951, 19, 1484− 1490. (10) Cole, R. H.; Davidson, D. W. High Frequency Dispersion in nPropanol. J. Chem. Phys. 1952, 20, 1389−1391. (11) McDuffie, G. E., Jr.; Litovitz, T. A. Dielectric Relaxation in Associated Liquids. J. Chem. Phys. 1962, 37, 1699−1705. (12) Davidson, D. W.; Cole, R. H. Dielectric Relaxation in Glycerine. J. Chem. Phys. 1950, 18, 1417−1418. (13) Meier, R.; Kruk, D.; Gmeiner, J.; Rössler, E. A. Intermolecular Relaxation in Glycerol as Revealed by Field Cycling 1H NMR Relaxometry Dilution Experiments. J. Chem. Phys. 2012, 136, 034508/ 1−8. (14) Meier, R.; Kahlau, R.; Kruk, D.; Rössler, E. A. Comparative Studies of the Dynamics in Viscous Liquids by Means of Dielectric Spectroscopy and Field Cycling NMR. J. Phys. Chem. A 2010, 114, 7847−7855. (15) Shirota, H.; Segawa, H. Time-Resolved Fluorescence Study on Liquid Oligo(ethylene oxide)s: Coumarin 153 in Poly(ethylene glycol)s and Crown Ethers. J. Phys. Chem. A 2003, 107, 3719−3727. (16) Shirota, H.; Segawa, H. Deuterium Substitution Study on Solvation Dynamics of Liquid poly(ethylene glycol)s. Chem. Phys. 2004, 306, 43−50. (17) Zhu, D.; Haidekker, M. A.; Lee, J. S.; Won, Y. Y.; Lee, J. C.-M. Application of Molecular Rotors to the Determination of the Molecular Weight Dependence of Viscosity in Polymer Melts. Macromolecules 2007, 40, 7730−7732. (18) Zhang, L.; Yin, Q.; Su, J.; Wu, Q. Local Polarity and Microviscosity of the Interior of Dendritic Polyethylene Amphiphiles. Macromolecules 2011, 44, 6885−6890. (19) Sahu, P. K.; Das, S. K.; Sarkar, M. Fluorescence Response of a Dipolar Organic Solute in a Dicationic Ionic Liquid (IL): Is the Behavior of Dicationic IL Different from that of Usual Monocationic IL? Phys. Chem. Chem. Phys. 2014, 16, 12918−12928. (20) Ramakrishna, G.; Ghosh, H. N. Efficient Electron Injection from Twisted Intramolecular Charge Transfer (TICT) State of 7Diethylamino Coumarin 3-Carboxylic Acid (D-1421) Dye to TiO2 Nanoparticle. J. Phys. Chem. A 2002, 106, 2545−2553. (21) Zhang, H.; Yu, T.; Zhao, Y.; Fan, D.; Chen, L.; Qiu, Y.; Qian, L.; Zhang, K.; Yang, C. Crystal Structure and Photoluminescence of 7(N,N′-diethylamino)-Coumarin-3-Carboxylic Acid. Spectrochim. Acta, Part A 2008, 69, 1136−1139. (22) Tablet, C.; Matei, I.; Pincu, E.; Meltzer, V.; Hillebrand, M. Spectroscopic and Thermodynamic Studies of 7-diethylaminoCoumarin-3-Carboxylic Acid in Interaction with β- and 2-hydroxypropyl-β-Cyclodextrins. J. Mol. Liq. 2012, 168, 47−53. (23) Chatterjee, A.; Maity, B.; Seth, D. 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. 2013, 15, 1894−1906. (24) Chatterjee, A.; Seth, D. Photophysical Properties of 7(diethylamino)Coumarin-3-Carboxylic Acid in the Nanocage of Cyclodextrins and in Different Solvents and Solvent Mixtures. Photochem. Photobiol. 2013, 89, 280−293. (25) Chatterjee, A.; Maity, B.; Seth, D. Influence of Double Confinement on Photophysics of 7-(diethylamino)Coumarin-3Carboxylic Acid in Water/AOT/Isooctane Reverse Micelles. RSC Adv. 2014, 4, 13989−14000.

viscosity of the media. It is depended on the specific solute− solvent interaction and the structural heterogeneity of the media. The rotational relaxation dynamics showed significant deviation from the SED hydrodynamic model of rotational diffusion. The rotational relaxation of 7-DCCA in PEG lies in between stick and slip boundary limits and that of 7-DCCA in TEG changes from stick to intermediate to further stick. However, in case of EG the rotational relaxation of 7-DCCA follows superstick boundary condition. This indicates the specific solute−solvent interaction, especially H-bonding interaction, together with dielectric friction mechanism to be operative. H-bonding interaction between the dye and PEG and TEG is expected to cause such kind of behavior. However, at higher temperature the rotational relaxation behavior in case TEG follows stick boundary condition, whereas in PEG, rotational relaxation approaches the stick boundary condition. This is due to change in structural organization of both TEG and PEG. Determination of activation energy of rotational relaxation and comparison with activation energy of viscous flow demonstrates that rotational relaxation is not entirely influenced by bulk viscosity of the medium but also by the specific solute−solvent interaction and structural features of the media.

■

ASSOCIATED CONTENT

S Supporting Information *

The components of time-resolved fluorescence emission decays and rotational relaxation decays and the (i) variation of fwhm in different solvents, (ii) FTIR spectra of 7-DCCA in PEG and TEG, (iii) variation of the fluorescence lifetime components of 7-DCCA with temperature, (iv) variation of rotational correlation time of 7-DCCA in PEG, EG, and TEG with temperature, (v) Arrhenius plots for the determination of the activation energy for rotational diffusion, and (vi) optimized structure of 7-DCCA in the ground state. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 91-612-2277383. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS All authors are thankful to IIT Patna, India for research facilities. A.C. is thankful to CSIR, New Delhi for a research fellowship. B.M. and S.A.A are thankful to IIT Patna for research fellowships.

■

REFERENCES

(1) Virgili, T.; Forni, A.; Cariati, E.; Pasini, D.; Botta, C. Direct Evidence of Torsional Motion in an Aggregation-Induced Emissive Chromophore. J. Phys. Chem. C 2013, 117, 27161−27166. (2) Samanta, S.; Roccatano, D. Interaction of Curcumin with PEO− PPO−PEO Block Copolymers: A Molecular Dynamics Study. J. Phys. Chem. B 2013, 117, 3250−3257. (3) Mensah, E. A.; Gonzalez, W. G.; Bussé, D. R.; Captain, B.; Miksovska, J.; Wilson, J. N. Emission Switching of 4,6-Diphenylpyrimidones: Solvent and Solid State Effects. J. Phys. Chem. A 2012, 116, 8671−8677. (4) Frauchiger, L.; Shirota, H.; Uhrich, K. E.; Castner, E. W., Jr. Dynamic Fluorescence Probing of the Local Environments within 12690



Article

(48) Gierer, A.; Wirtz, K. Molekulare Theorie der Mikroreibung. Z. Naturforsch., A: Phys. Sci. 1953, 8, 532−538. (49) Dote, J. L.; Kivelson, D.; Schwartz, R. N. A Molecular QuasiHydrodynamic Free-Space Model for Molecular Rotational Relaxation in Liquids. J. Phys. Chem. 1981, 85, 2169−2180.

(26) Chatterjee, A.; Maity, B.; Seth, D. Supramolecular Interaction between a Hydrophilic Coumarin Dye and Macrocyclic Hosts: Spectroscopic and Calorimetric Study. J. Phys. Chem. B 2014, 118, 9768−9781. (27) Inal, S.; Kölsch, J. D.; Sellrie, F.; Schenk, J. A.; Wischerhoff, E.; Laschewsky, A.; Neher, D. A Water-Soluble Fluorescent Polymer as a Dual Colour Sensor for Temperature and a Specific Protein. J. Mater. Chem. B 2013, 1, 6373−6381. (28) Liu, X.; Cole, J. M.; Chow, P. C. Y.; Zhang, L.; Tan, Y.; Zhao, T. Dye Aggregation and Complex Formation Effects in 7-(Diethylamino)-coumarin-3-carboxylic Acid. J. Phys. Chem. C 2014, 118, 13042− 13051. (29) Jones, G., II; Jackson, W. R.; Choi, C. Y.; Bergmark, W. R. Solvent Effects on Emission Yield and Lifetime for Coumarin Laser Dyes. Requirements for a Rotatory Decay Mechanism. J. Phys. Chem. 1985, 89, 294−300. (30) Ito, N.; Arzhantsev, S.; Maroncelli, M. The Probe Dependence of Solvation Dynamics and Rotation in the Ionic Liquid 1-Butyl-3Methyl-Imidazolium Hexafluorophosphate. Chem. Phys. Lett. 2004, 396, 83−91. (31) Edward, J. T. Molecular Volumes and the Stokes−Einstein Equation. J. Chem. Educ. 1970, 47, 261−270. (32) Naik, L. R.; Math, N. N. Photophysical Properties of 8-Hydroxy Quinoline. Indian J. Pure Appl. Phys. 2005, 43, 743−749. (33) Reichardt, C. Solvatochromic Dyes as Solvent Polarity Indicators. Chem. Rev. 1994, 94, 2319−2358. (34) Lee, Y.; Lee, M. Volume Increment Effect on the Photoisomerization of Hemicyanine Dyes in Oligo(ethylene glycol)s. J. Phys. Chem. A 2013, 117, 12878−12883. (35) Würthner, F.; Yao, S. Dipolar Dye Aggregates: A Problem for Nonlinear Optics, but a Chance for Supramolecular Chemistry. Angew. Chem., Int. Ed. 2000, 39, 1978−1981. (36) Owens, P.; Ryder, A. G. Low Temperature Fluorescence Studies of Crude Petroleum Oils. Energy Fuels 2011, 25, 5022−5032. (37) Dutt, G. B.; Doraiswamy, S.; Periasamy, N.; Venkataraman, B. Rotational Reorientation Dynamics of Polar Dye Molecular Probes by Picosecond Laser Spectroscopic Technique. J. Chem. Phys. 1990, 93, 8498−8513. (38) Perrin, F. Mouvement Brownien D’un Ellipsoide (I). Dispersion Dlelectrique Pour Des Molecules Ellipsoidales. J. Phys. Radium 1934, 5, 497−511. (39) Hu, C. M.; Zwanzig, R. Rotational Friction Coefficients for Spheroids with Slipping Boundary Condition. J. Chem. Phys. 1974, 60, 4354−4357. (40) Mali, K. S.; Dutt, G. B.; Mukherjee, T. Do Organic Solutes Experience Specific Interactions with Ionic Liquids? J. Chem. Phys. 2005, 123, 174504−174507. (41) Mali, K. S.; Dutt, G. B.; Mukherjee, T. 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. 2008, 128, 054504−054509. (42) Guchhait, B.; Daschakraborty, S.; Biswas, R. Medium Decoupling of Dynamics at Temperatures ∼100 K Above GlassTransition Temperature: A Case Study with (Acetamide + Lithium Bromide/Nitrate) Melts. J. Chem. Phys. 2012, 136, 174503/1−16. (43) Spears, K. G.; Cramer, L. E. Rotational Diffusion in Aprotic and Protic Solvents. Chem. Phys. 1978, 30, 1−8. (44) Nee, T. W.; Zwanzig, R. Theory of Dielectric Relaxation in Polar Liquids. J. Chem. Phys. 1970, 52, 6353−6363. (45) Sengwa, R. J. Comparative Dielectric Study of Mono, Di and Trihydric Alcohols. Indian J. Pure Appl. Phys. 2003, 41, 295−300. (46) Fruchey, K.; Fayer, M. D. Dynamics in Organic Ionic Liquids in Distinct Regions Using Charged and Uncharged Orientational Relaxation Probes. J. Phys. Chem. B 2010, 114, 2840−2845. (47) Horng, M. L.; Gardecki, J. A.; Maroncelli, M. Rotational Dynamics of Coumarin 153: Time-Dependent Friction, Dielectric Friction, and Other Nonhydrodynamic Effects. J. Phys. Chem. A 1997, 101, 1030−1047. 12691


Photophysics and Rotational Diffusion of Hydrophilic Molecule in

Recommend Documents