Photoinduced Electron Transfer in a Room Temperature Ionic Liquid 1

ARTICLE pubs.acs.org/JPCB

Photoinduced Electron Transfer in a Room Temperature Ionic Liquid 1-Butyl-3-methylimidazolium Octyl Sulfate Micelle: A Temperature Dependent Study Souravi Sarkar, Sarthak Mandal, Rajib Pramanik, Chiranjib Ghatak, Vishal Govind Rao, and Nilmoni Sarkar* Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, WB, India

bS Supporting Information ABSTRACT: The effect of temperature on the dynamics of photoinduced electron transfer (PET) between different coumarin dyes and N,N-dimethyl aniline in a room temperature ionic liquid 1-butyl-3-methylimidazolium octyl sulfate ([C4mim] [C8SO4]) micelle have been investigated using steady-state and time-resolved fluorescence quenching measurements at four different temperatures: 208, 298, 308, and 318 K. The quenching rates (kqTR) of the PET process in this micellar system are found to be lower than the PET rate in sodium dodecyl sulfate and Triton-X 100 micelle and almost comparable to the dodecyl trimethyl ammonium bromide and cetyl trimethyl ammonium bromide micelle due to larger donoracceptor separation in the micellar phase. The temperature dependent PET rates are well correlated with the Arrhenius type of correlation for all the coumarin dyes. Marcus type of inversion in PET rates has been observed at relatively lower exergonicity, and the correlation plots gradually move upward with the increase of temperature.

1. INTRODUCTION Room temperature ionic liquids (RTILs) have now generated great interest for their unique features such as low vapor pressure, high thermal stability, high ionic conductivity, favorable solvating properties for a range of polar and nonpolar compounds, and a wide liquidous temperature range.1For these unique characteristics the RTILs are receiving great attention as novel media in chemical organic synthesis, homogeneous catalysis, electrochemistry, separation technique, etc. They have an effective potential to act as environmentally benign solvent media over the conventional organic solvents.1hl The RTILs are mainly constituted of ions as bulky organic cations and inorganic anions, and they are liquid at ambient pressure and temperature. By changing the anion or alkyl chain of the cation, a wide variety of properties like hydrophobicity, viscosity, density, and solvation can be changed. For this reason, ionic liquids have been termed as designer solvents. The RTILs having the unsymmetrical 1,3-dialkylimidazolium cation with the anions [PF6], [BF4], [CF3SO3], and [(CF3SO2)2N] are now used mostly in various fields of research work. Recently several photophysical, theoretical, and ultrafast spectroscopic studies in RTILs have been reported in the literature.24 The self-aggregation behavior of RTILs in aqueous solution has recently attracted great interest due to structural similarities of RTILs with ionic surfactants. The surface activity of long-chain imidazolium RTILs is superior to conventional ionic surfactants with the same hydrocarbon chain length. Some groups have reported the characterization of RTILs containing micelle and microemulsions which have experienced great interest.57 Recently, Miskolczy et al. showed that 1-butyl-3-methylimidazolium r 2011 American Chemical Society

octyl sulfate ([C4mim][C8SO4]) can form micelles in aqueous solution.7a Some reports are available in the literature related to solvation dynamics in the RTIL containing micellar media.2i,j,4d Photoinduced electron transfer (PET) from donor to acceptor as one of the important reactions occurring in chemistry and biology has become a subject of great interest over the past decades to understand the decay dynamics and quenching of the excited state fluorophores. PET reactions are usually studied in polar solvents, frequently in acetonitrile,8 and also the investigations on PET processes are mostly performed either in neat solvent where the solvent acts as a donor or under diffusive conditions where the solvent is noninteracting and the reactants have to diffuse before PET takes place. But it is very interesting to study PET in organized media such as micelles, reverse micelles, vesicles, and cyclodextrins9,10 because these systems resemble many biological and chemical systems in nature and their local properties like polarity, viscosity, and pH are immensely different in these type of heterogeneous media from those in homogeneous media. Only very limited work on PET has been performed in micelles.9 The micellar solution is a useful medium which has drawn considerable interest as it maintains the photoinduced charge separation because of their geometry and their multiphase character. PET in a micellar media gives information about the nature of the micellar environment. The micelles are composed of hydrocarbon-like core and contain polar surfactant headgroups. Since RTILs are polar liquids similar to that Received: February 21, 2011 Revised: April 11, 2011 Published: April 28, 2011 6100

dx.doi.org/10.1021/jp201702x | J. Phys. Chem. B 2011, 115, 6100–6110

The Journal of Physical Chemistry B of PET in common micellar media the RTIL micelle is also useful for performing the PET reaction. There are some reports in the literature on the photophysical studies in RTIL containing micelles.2i,j,4c,7g7j So it will be very interesting to investigate PET in RTIL containing micelles as PET in this type of heterogeneous media can provide information about the kinetics and mechanisms of various chemical reactions for practical applications. Recently we have investigated PET between coumarin dyes and N,N-dimethyl aniline in a RLIL containing microemulsion and observed a Marcus type of inversion in the electron transfer rate vs free energy correlation curve.10j The conventional ET theory was originally proposed by Marcus,11and thereafter further modifications on quantum-mechanical aspects have been incorporated into this. The simplified form of the rate constant of ET is given by " # ðΔG0 þ λÞ2 kET ¼ ν exp ð1Þ 4λkB T where υ is the frequency of the motion in the reactant potential well, λ is the composite reorganization energy, given as (λsþ λi), λs being the solvent reorganization energy and λi being the intramolecular reorganization energy, ΔG0 is the free energy change for the overall process, and kB is the Boltzmann constant. The most important feature of the above expression is that with increase in ΔG0 the rate constant initially increases, reaches a maximum at ΔG0 = λ, and then decreases when ΔG0 > λ. The theoretical and experimental investigations have been carried out on the dynamical aspect of electron transfer in different neat solvents like CH3CN and CH3OH.8 The use of ionic liquid as a solvent media now holds a great deal of interest for PET study. Recently Falvey12a,b and coworker have studied PET reaction in two RTILs and observed that the PET reaction rate follows the RehmWeller behavior. Margulis et al. investigated the excited state intramolecular electron transfer reaction of crystal violet lactone (CVL) in the room temperature ionic liquid (RTIL) N-propyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [Pr31þ][Tf2N] using molecular dynamics simulation.12c Moreover, recently Lynden-Bell12d,e has investigated some simulation studies of model systems of two ionic liquids and compared with acetonitrile to judge the applicability of Marcus theory in ionic liquids. The free energy and dynamics of PET reaction in a RTIL and in supercritical water and the reaction dynamics and kinetics of adiabatic electron transfer in a RTIL have been well studied by Kim et al.12g,h Recently, Wasielewski and coworker have reported the intramolecular electron transfer within a covalent, fixed-distance donoracceptor molecule in an ionic liquid and Li et al. have reported hindered intramolecular electron transfer in a room temperature ionic liquid.12i,j We have also investigated PET in a protic RTIL12k and observed saturation in the PET correlation curve like other homogeneous and neat solvents reported previously.8 In this study we have focused our attention on the PET processes in a micelle composed of an RTIL, 1-butyl-3-methylimidazolium octyl sulfate [C4mim][C8SO4], due to limited studies on the PET in this type of ionic liquid containing micellar system, and compared it with other micellar systems. Here the electron transfer in this RTIL micelle has been carried out at four different temperatures between the excited state coumarin dyes and N,N-dimethyl aniline (DMA) in [C4mim][C8SO4] micellar solution using both steady-state and time-resolved fluorescence quenching measurements to find out how the rate and the dynamics are changing with variation of temperatures. We have studied the quenching dynamics at four different temperatures

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Scheme 1. Chemical Structures of the Coumarin Dyes, N, N-Dimethyl Aniline, and RTIL

with the addition of DMA, and we have tried to find out the distinct features of this system compared to the other micellar systems of common surfactant like SDS. The critical micellar concentration (cmc) of this micelle is 0.031 M. Our group has already investigated the solvent and rotational relaxation of C-153 in this RTIL (1-butyl-3-methylimidazolium octyl sulfate) containing micelle.4c Previously, PET was compared with the solvation time as we know that the solvation dynamics and PET are competitive processes. Yoshihara et al.13a have reported that the ET rate is faster than solvation dynamics in neat donor solvents. Barbara et al.13b have reported that the ultrafast PET in betaine dyes is completely decoupled from solvation. Bagchi et al. have reported interplay between ultrafast polar solvation and vibrational dynamics in electron transfer reactions, the role of high-frequency vibrational modes, and also the effects of ultrafast solvation on the rate of adiabatic outer-sphere electron transfer reactions.13c,d Moreover, we would like to compare the PET rate in this RTIL micelle with other conventional micelles and also the variation of the PET rate with temperature. Here the RTIL is behaving as the surfactant molecule, forming micelle in water like other micelle forming anionic surfactant, and is quite rare among the commonly used RTILs.

2. EXPERIMENTAL SECTION The coumarin dyes were obtained from Exciton (laser grade) and used as received. N,N-Dimethyl aniline (DMA) was obtained from Aldrich Chemical and distilled under reduced pressure before use. The ionic liquid 1-butyl-3-methylimidazolium octyl sulfate [C4mim][C8SO4] was obtained from Fluka and purified according to the literature procedure. The chemical structure of the coumarin dyes, DMA, and the ionic liquid are shown in Scheme 1. Triply distilled Milli-Q water was used to prepare all the micellar solutions, and the concentrations of the ionic liquid in solutions were all about five times the critical micelle concentration (cmc). The final concentration of the coumarins in solution for experiment was kept at ∼106 M. All the coumarins were initially dissolved into methanol and transferred to a vial. Then micellar solution was added to the vial 6101

dx.doi.org/10.1021/jp201702x |J. Phys. Chem. B 2011, 115, 6100–6110

The Journal of Physical Chemistry B

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Table 1. Absorption and Emission Maxima of Different Coumarin Dyes in Pure Water and in [C4mim][C8SO4] Micellar Solution at 5 cmc Concentration 0 λmax (nm) abs

0 λmax (nm) emi

C151 in water

364

492

C-151in [C4mim][C8SO4] micelle

382

492

C-152 in water

402

526

coumarins

C-152 in [C4mim][C8SO4] micelle

406

521

C-152A in water C-152A in [C4mim][C8SO4] micelle

411 412

527 515

C-153 in water

434

549

C153 in [C4mim][C8SO4] micelle

433

542

C-480 in water

392

489

C-480 in [C4mim][C8SO4] micelle

393

484

Figure 1. Surface tension vs log(C) plot at 288 K temperature.

after removing the methanol under vacuum. The solutions were transferred in the quartz cuvette for both steady-state and timeresolved fluorescence quenching experiment with the gradual addition of DMA. During addition of DMA, each and every time the solution was allowed to mix thoroughly so that the quencher molecules got sufficient time to enter into the micelles. The total coumarin concentrations in the solutions were kept lower than the concentration of the micelle used. The experimental condition maintained was that the probability of a single micelle containing more than one coumarin molecule is negligible but the concentration of DMA varied over a wide range and the total study has been carried out at five times the cmc value of the micelle. For each probe the experiment was performed at four different temperatures: 208, 288, 308, and 318 K. The absorption and fluorescence spectra were measured using a Shimadzu (model no. UV-1601) spectrophotometer and a Spex-Fluorolog-3 (model no. FL3-11) spectrofluorometer. The fluorescence spectra were corrected for the spectral sensitivity of the instrument. For steady-state experiments, all samples were excited at 410 nm. The time-resolved fluorescence setup is described in detail in our earlier publications.10 Briefly, the samples were excited at 408 nm using a picosecond diode laser (IBH, Nanoled), and the signals were collected at magic angle (54.7) polarization using a Hamamatsu microchannel plate photomultiplier tube (3809U). The instrument response function of our setup is ∼90 ps. The same setup was used for anisotropy measurements. For the anisotropy decays, we used a motorized polarizer in the emission side. The emission intensities at parallel (I//) and perpendicular (I^) polarizations were collected alternatively until a certain peak difference between parallel (I//) and perpendicular (I^) decay was reached. The analysis of the data was done using IBH DAS, version 6, decay analysis software. The same software was also used to analyze the anisotropy data. The dynamic light scattering (DLS) measurements have been carried out by using a Malvern Nano ZS instrument employing a 4 mW HeNe laser (λ = 632.8 nm) and equipped with a thermostatic sample chamber. The viscosity measurement of the micelle has been carried out at different temperatures using a Brookfield DV-II viscometer. The cyclic voltammetric (CV) measurements were carried out in a CH (model 620A) instrument to measure the oxidation and reduction potential of the donor and acceptors in this [C4mim]

Figure 2. (a) Absorption spectra of (i) C151, (ii) C480, (iii) C152, (iv) C152A, and (v) C153 in [C4mim][C8SO4] micelle and (vi) neat [C4mim][C8SO4] ionic liquid. (b) Steady-state fluorescence spectra of (i) C480, (ii) C151, (iii) C152A, (iv) C152, and (v) C153 in [C4mim][C8SO4] micelle.

[C8SO4] micellar solution at the ground state. The solution of the coumarins in micellar media containing 0.1 mol dm3 potassium chlorides has been used as supporting electrolyte, and the N2 gas has been purged for several minutes. The 6102



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reduction potential values reported in this work were calibrated with respect to the saturated calomel electrode (SCE). The oxidation potential value was also measured by using cyclic CV measurement in the micellar solution. The cmc values of micellar solution at four different temperatures were determined by the measurement of surface tension using an GBX3S instrument. In Figure 1 we have shown the cmc plot measured at 288 K.

3. RESULTS 3. 1. Steady-State Absorption and Emission Measurements. The absorption and emission maxima for all the coumar-

in dyes in pure water and in [C4mim][C8SO4] micellar solution at about five times the cmc are tabulated in Table 1. During the change from pure water to the [C4mim][C8SO4] micellar solution, we have not observed any significant change in the absoption spectra for the other coumarin dyes except C-151. The absoption maximum of C-151 in water is 364 nm which is redshifted by almost 18 nm from that in micellar solution. The red shift is attributed to the groundstate interaction between C-151 molecule and the micellar medium, i.e., there is hydrogen bonding between the NH2 group of C-151 and the polar headgroup of the ionic liquid. However, the emission maximum of C-151 does not change from pure water to micellar solution. But there is significant blue shift in the emission maxima for other probes compared to pure water. The blue shifts in the emission spectra confirm that the probe molecules are moving from polar aqueous phase to the relatively nonpolar surface of the micelles. The normalized absorption and emission spectra of all the dyes and RTIL are shown in the Figure 2a and b, respectively. We also observed that the steady-state absorption and emission peak positions do not change with temperature. DMA is sparingly soluble in water, but it is soluble in [C4mim][C8SO4] micelles. From steady-state measurements, it is seen that there is no change in the shape of absorption and emission spectra with the addition of DMA, so the possibility of exciplex formation is ruled out. In our experiment, the total [[C4mim][C8SO4] concentration was kept at about five times of its cmc value (0.031 M at 298 K), and at this concentration the effective micelle concentration is 1.305 103 M, which is very high compared to the concentration of coumarins, so it is expected that almost all the dye molecules will reside in the micellar phase and only a very few micelles will occupy probe molecules because the coumarin dyes used in this study are almost insoluble in water but they have significant solubility in micellar solution. The coumarin dyes used in this study are reasonably polar in nature due to their intramolecular charge transfer (ICT) character, and they should prefer to reside preferably at the micellar Stern layer rather than going into the nonpolar micellar core. 3. 2. Time-Resolved Anisotropy Measurements. Timeresolved anisotropy measurements have been carried out for determination of the location of the probe molecule in the micellar environment. The expression used to calculate the anisotropy measurement is as follows rðtÞ ¼

I== ðtÞ GI^ ðtÞ I== ðtÞ þ 2GI^ ðtÞ

ð2Þ

where I//(t) and I^(t) are the fluorescence decays polarized parallel and perpendicular to the polarization of the excitation light, respectively. G is the correction factor for detector

sensitivity to the polarization direction of the emission. The emission intensities at parallel (I//) and perpendicular (I^) polarizations were collected alternatively until a certain peak difference between parallel (I//) decay and perpendicular (I^) decay was reached. The absorption and emission spectra can give a qualitative idea regarding the location of the probe molecules. This can be more accurately predicted by the time-resolved fluorescence anisotropy. Our system is a ionic liquidwater micellar system; for this reason we have compared the fitted results of anisotropy decays for all the dyes in pure water and in [C4mim][C8SO4] micelle and also the rotational relaxation time measured at four different temperatures. The fitted results of average rotational relaxation time for all the coumarin dyes in pure water and in [C4mim][C8SO4] micellar solution at different temperatures are summarized in Table 2. The representative decays of time-resolved fluorescence anisotropy plots in micellar solution at different temperatures for C-151 and C-480 molecules are given in Figure 1a and b in the Supporting Information. It shows that with the increase of temperature, the rotational relaxation time of the probe molecules gradually decreases. The rotational motions of all the probes are slower compared to pure water. It strongly suggests that probe molecules are residing at the micellar surface. The anisotropy decays for all the coumarins in pure water are single exponential, but in micellar solution the anisotropy decays of all the coumarins are fitted with a biexponential function. Various model such as the wobbling-ina-cone and two-step model have been used to explain the rotational relaxation dynamics in micelles.14a,b 3. 3. Quenching Study Using Steady-State and TimeResolved Fluorescence Measurements. We have investigated the fluorescence quenching experiments by gradual increase of the DMA concentration in the solution of coumarin dyes in [C4mim][C8SO4] micellar solution at different temperatures. It is observed that with addition of DMA the fluorescence intensity of coumarin dyes is continuously quenched in the case of both steady-state as well as time-resolved measurements (Figures 3a, b and 4a, b). The time-resolved decays were fitted to a biexponential function, and the average lifetimes (τav) of the dye molecule in the presence of different amine concentrations are calculated using the following relation τav ¼ Æτæ ¼ a1 τ1 þ a2 τ2

ð3Þ

where τ1 and τ2 are the two fluorescence lifetime components and a1 and a2 are their relative amplitudes. The fluorescence-quenching constant is determined by well-known SternVolmer equation as I0 τ0 ¼ 1 þ KSV ½Qeff ¼ 1 þ kq τ0 ½Qeff ¼ I τ

ð4Þ

where I0 and I are the steady-state fluorescence intensity and τ0 and τ are the fluorescence lifetimes of the coumarin dyes in the absence and in the presence of the quencher, KSV is the SternVolmer constant, and [Qeff] is the effective concentration of the quencher in the micellar Stern layer. The quencher molecules are mainly solubilized in the micellar phase, which is in fact expected due to insolubility of the amine molecules in water so that the effective concentration of the quencher will be much higher than the total concentration used in the solution. The absorption and NMR spectra of these quenchers in micellar solution that have been reported are vastly different from that of the pure water. It indicates that the donor molecules are residing in the Stern layer of the micelles.14c,d We have considered the [C4mim][C8SO4] micelles to be spherical. The average radius of the micelle is 1.4 nm (14 Å) and 6103



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Table 2. Average Rotational Relaxation Time (Æτræ) of Coumarin Dyes in Pure Water and in [C4mim][C8SO4] Micelle at Different Temperatures and the Analytical Rotational Parameters of the Dye Molecules in [C4mim][C8SO4] Micelle [C4mim][C8SO4] micelle Æτræ (ns) coumarins

water Æτræ (ns)

288 K

298 K

308 K

S

DL (106 cm2 s1)

τD (ns)

C-152A

0.120

0.535

0.382

0.301

0.66

6.87

0.51

C-152

0.133

0.458

0.346

0.294

0.55

7.31

0.48

C-151 C-153

0.096 0.100

0.585 0.584

0.468 0.428

0.325 0.355

0.95 0.64

2.11 3.39

1.66 1.03

C-480

0.125

0.450

0.371

0.297

0.87

3.47

1.01

Figure 3. Steady-state fluorescence quenching of C-153 with the gradual addition of DMA (shown by arrow) in [C4mim][C8SO4] micelle at (a) 288 K and (b) 308 K temperatures.

is already reported in our earlier study on solvation dynamics in this micelle.14f In the case of ionic micelles (cetyl trimethylammonium bromide, CTAB, and sodium dodecyl sulfate, SDS), the Stern layer is ∼69 Å as is already reported in the literature.14g The aggregation behavior of aqueous solutions of ionic liquids has been already reported in the literature.14h The shapes and sizes of aggregates of the ionic liquid based on the 1-alkyl-3 methylimidazolium cation have been investigated by small angle neutron scattering (SANS) measurements, and it is observed that the aggregates are nearspherical.16c From this SANS data we have taken the thickness of the micellar Stern layer to be ∼6 Å in our case, which gives the volume

Figure 4. Time-resolved fluorescence spectra of C153 in [C4mim][C8SO4] micelle by gradual addition of different concentrations of DMA at (a) 288 K and (b) 308 K temperatures.

of the Stern layer to be about 6.672 dm3 per mole of the micelle. Since almost all the aromatic amines reside in the micellar Stern layer, the effective concentration of the quencher amine (DMA) molecule is calculated as ½Qeff ¼

Nagg ½Qt 6:672½S cmc

ð5Þ

where Nagg is the average aggregation number for [C4mim] [C8SO4] micelle and Nagg ∼ 44,7h S is the total ionic liquid concentration in [C4mim][C8SO4] micelle (155 mM), cmc is the 6104



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are shown in Figure 5ac; all plots are linear in nature. The kTR q values are then determined from the KSV values, knowing the lifetime (τ0) of the coumarin dyes in the absence of the quencher. The kTR q values thus measured for different coumarinamine systems and different temperature are listed in Table 3, and the average fluorescence lifetime of the coumarinamine systems are listed in Table 4. The quenching kinetics for the electron transfer in micellar solution can be realized by determining the average distribution of the quencher around the excited dye in the micellar Stern layer. 3. 4. Dynamic Light Scattering (DLS) Measurement. The measured hydrodynamic diameter (Dh) of [C4mim][C8SO4] micelle at 5 cmc is 2.8 nm. We have not found any significant change in the size of the micelle with variation of temperature. 3. 5. Free Energy Change. The electron transfer rate depends on the free energy change of the system. In the Marcus type of system in the electron transfer rate vs free energy correlation curve the ET rate initially increases with negative change in the free energy gap, reaches a maximum, and then decreases with further negative change in the free energy gap. The last region is called the Marcus inverted region. The free energy change (ΔG0) for a photoinduced electron transfer reaction between electron donor and electron acceptor is given by the RehmWeller equation15a ΔG0 ¼ EðD=Dþ Þ EðA=A Þ EIPS E00

ð6Þ

where E(D/Dþ) and E(A/A) denote the oxidation potential of the donor (DMA) and reduction potential of the acceptors (coumarin dyes), respectively, and were calculated in this micellar system by using cyclic voltametry as listed in Table 3. E00 is the energy difference between S0 and S1 states, which were estimated from the overlap of the normalized absorption and emission spectra of the dyes in the micelle. Since the emission maxima and the absorption maxima of the coumarin dyes in the micelle are almost unaltered with change in temperature, the E00 values were considered to be similar at all the temperatures studied. EIPS denotes the ion pair stabilization energy in the medium given by EIPS ¼ e2 =ε0 R

Figure 5. τ0/τ vs [DMA]eff plot of the coumarin dyes at (a) 288, (b) 298, and (c) 308 K temperatures in [C4mim][C8SO4] micelle.

critical micellar concentration of the micelle (31 mM), and [Qt] is the total amine concentration used in the solution. The SternVolmer plot of I0/I vs [Qeff] and τ0/τ vs [Qeff] for different coumarin molecules with increasing concentration of DMA gives an estimation of KSV. The typical plots for τ0/τ vs [Qeff] for different coumarin molecules at different temperatures

ð7Þ

where e is the charge of the electron and ε0 is the static dielectric constant of the medium. The emission maxima of the C-153, C-152A, C152, C-480 in the [C4mim][C8SO4] micelle were found to be markedly blue-shifted compared to that in water. This suggests that the probe molecules are located in the hydrated Stern layer of the micelle which is less polar compared to bulk water. The observed emission maximum of the coumarin dyes in the [C4mim][C8SO4] micelle is very close to the reported emission maxima of the coumarin dyes in 50% ethanol15b which suggests that the probe molecules in the [C4mim][C8SO4] micelle are facing the microenvironment similar to that of 50% ethanol, and thus the polarity sensed by the probe is very close to the polarity of 50% ethanol. Therefore we have used the dielectric constant of the 50% ethanol ε0 ∼ 37.1 for the calculation of EIPS. R is the distance between the donor and the acceptor and is assumed to be the sum of the radii of the donors and the acceptors. The radii of the donor and acceptor molecules were estimated by following Edward’s volume addition method assuming the molecules to be spherical.15c The calculated ΔG0 values for each coumarinDMA pair in the [C4mim][C8SO4] micelles are summarized in Table 3. We have 6105



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Table 3. Time-Resolved Quenching Constants, Ground-State Redox Potentials of Donor and Acceptors, E00 Values, and ΔG0 for Different CoumarinDMA Systems Studied in [C4mim][C8SO4] Micellar Solution at Different Temperatures 9 1 1 kTR S ) q (10 M

coumarins

E00 (eV)

donor

E(C/C) (V)

E(D/Dþ) (V) 0.855

ΔG0 (eV)

288 K

298 K

308 K

318 K

C-480

DMA

2.814

1.833

0.235

0.103

0.127

0.154

0.282

C-153

DMA

2.517

1.471

0.328

0.284

0.337

0.422

0.490

C-152A C-152

DMA DMA

2.686 2.680

1.449 1.419

0.501 0.519

0.673 0.464

0.886 0.543

1.268 0.620

1.627 0.954

C-151

DMA

2.815

1.367

0.676

0.302

0.331

0.410

0.548

Table 4. Average Lifetimes (Æτfæ) of the Coumarin Dyes in the Absence of Quencher at Different Temperatures and the Activation Energies Obtained from an Arrhenius Type Temperature Dependence in [C4mim][C8SO4] Micelle Æτfæ (ns) coumarins

288 K

298 K

308 K

318 K

Ea (kcal/mol)

C151

5.915

5.842

5.762

5.68

2.660

C-152

0.739

0.595

0.484

0.39

5.811

C-152A

0.517

0.398

0.310

0.25

5.728

C-153

3.395

3.336

3.307

3.30

3.529

C-480

6.172

6.1

6.04

5.96

5.796

plotted the ΔG0 vs the electron transter rate for all the coumarin dyes at different temperatures as shown in Figure 7. The activation energies (Ea) for the fluorescence quenching process for all the coumarinDMA systems were estimated using Arrhenius equation as ln kq ¼ ln A

Ea RT

ð8Þ

In this case the temperature dependent time-resolved quenching constant values (kq) are well correlated with the Arrhenius type of correlation where ln kq varies linearly with 1/T. The activation energies have been calculated for different coumarinDMA pairs from the Arrhenius plots and are listed in Table 4 (Figure 6a).

4. DISCUSSION In this work, we have investigated the photoinduced electron transfer process between N,N-dimethyl aniline and coumarin dyes in a room temperature ionic liquid forming micellar structure in water. The PET reactions between coumarin dyes and amines have been extensively studied in homogeneous acetonitrile solution.8a,d In homogeneous solution the donor and acceptor are free to move as they can efficiently diffuse to a close contact to form the encounter complex, but in the case of micelles the diffusions of the reactants are highly restricted and they are forced to remain in the thin Stern layer. Thus, the PET rate is much slower in the case of micellar media than that observed in the homogeneous solution. We have compared the PET rate in this ionic liquid micelle system to the PET rate observed in other conventional micellar systems. It is observed that the PET rate in our case is slower compared to that in SDS and TX-100 micelles but almost comparable with the PET rate in DTAB and CTAB micelle reported previously.9a,bIn SDS micelle the smaller size cation (Naþ) results in a lesser steric repulsion

Figure 6. (a) Arrhenius plot for PET rates for different coumarinDMA pairs in [C4mim][C8SO4] micelle. (b) Plot of ln(1/τf 1τf0) vs 1/T for C-152A and C-152 in the [C4mim][C8SO4] micellar system.

and admits the donor and acceptor at close distance by cationπ interaction and also in neutral TX-100 micelles, where no counterion is present to separate the donor and acceptor and the presence of a larger thick palisade layer helps the donor and acceptor to achieve a closer distance. But in the case of [C4mim][C8SO4] micelle the larger size ([C4mim][C8SO4] may be due to the influence of the bigger and more hydrophobic imidazolium counterions, which are more effective in screening the intramicellar electrostatic repulsion among the polar headgroups) of cation leads to a steric hindrance and results in a larger donoracceptor separation as in the case of CTAB and DTAB 6106


The Journal of Physical Chemistry B micelle. The electron transfer rate depends strongly on the electronic coupling matrix element, which is expected to decrease with an increase in the donoracceptor separation (r), and the relation predicts a slight change in the donoracceptor separation which may result in a significant change in the coupling constant due to the exponential relationship.11b,c,14e,14e The PET rate in this ionic liquid containing micellar media has been found to be almost 10 times slower compared to the solvation dynamics in this media.4c With the increase in temperature the PET rate increases for all the probe molecules. The kq values at different temperatures are listed in Table 3. As discussed earlier, due to statistical distribution of the quenchers around the fluorophore in the micelle, the fluorescence decays were expected to follow a nonexponential behavior. In this case the time-resolved decays were fitted with a biexponential function. The individual τ1 and τ2 components of the fluorescence decays cannot be assigned separately to any particular distribution of the donoracceptor pairs in the micelle. So we have used τav to estimate the quenching constants because it gives an average idea which is more meaningful than that of using the individual components for determining fast and slow quenching constants in this micellar system. The average fluorescence lifetimes of the coumarin dyes in [C4mim][C8SO4] micelle in the absence of quencher molecules thus estimated at different temperatures are listed in Table 4. Most interestingly it is seen that the lifetimes of C-152A and C-152 are strongly dependent on temperature but for other dyes remain almost independent of temperature. This experimental fact suggests that efficient nonradiative process is involved in C-152A and C-152 in this micellar system, and this causes internal quenching with temperature. With increase in temperature, the rates of the nonradiative processes increase. From the steady-state measurements we have already seen that the fluorescence emission maxima of the coumarin dyes in [C4mim][C8SO4] are very close to the emission maxima of the dyes in 50% ethanol. From this comparison we can conclude that the micropolarity sensed by the coumarin dyes in the excited state in [C4mim][C8SO4] micellar system is very close to the polarity of 50% ethanol. Again, it is reported that the rate of nonradiative decay process of C-152A in 50% ethanol is 0.22 1010 S1. So we may expect an efficient nonradiative quenching process for C-152A and C-152 in the present micellar system with temperature, which is clear from Figure 6b. The plot of ln (1/τf 1/τf0) vs 1/T, where 1/τf = kf þ knr, is identical to the inverse of lifetime in solvents where nonradiative quenching occurs, and 1/ τf0 = kf, which is set to (0.2 ( 0.03 ns1), is identical to the inverse lifetimes in solvents where no quenching takes place.15d,e It has been reported that the TICT- mediated nonradiative deactivation is a strongly activation-controlled process for C-152 and C-152A, and the estimated activation energy (Ea) from the quenching study of C-152-DMA and C-152A-DMA systems cannot be a measure of the energy barrier for the electron transfer process alone, but should also include a contribution from the activation barrier for the TICT-mediated deactivation process.8e For this reason in this case the activation energy (Ea) for C-152 and C-152A is unusually high as a large contribution from other deactivation process along with ET process is taking place. There is only one report present in the literature for this unusual behavior in SUVs by Pal et al.,10c and they have explained that because of higher local polarity and lower microviscosity in micelles and reverse micelles the TICT process is short-lived due to fast nonradiative decay to the ground

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Figure 7. Plot of ln kq vs ΔG0 for the coumarinDMA system in [C4mim][C8SO4] micelle at different temperatures.

state. In this case of ionic liquid micellar system, this similar process has been observed. We have plotted the electron transfer rate constant kq with the free energy change (ΔG0) of the system, and all the plots of ln kq vs free energy change (ΔG0) at different temperatures revealed the same feature. There was not any significant change in the shape of the plots with the increase in temperature, and the plot gradually move upward in direction with increase in temperature. In these plots, initially PET rate increases with change in the free energy, reaches a maximum, and then falls off; i.e., in the present case, we observed retardation in the electron transfer rate at the higher free energy region (Figure 7) as predicted by Marcus ET theory.11a There are several reports by other groups including our group on PET in these type of microheterogeneous media, where the Marcus type of inversion has been found due to the retardation of solvent motion and confinement of reactant molecules.9,10 The steady-state absorption spectra of C-151 (Table 1) show a large red shift on going from water to the micellar medium, while the other coumarin molecules do not show any significant shift in the absorption spectra; the emission spectra remain unshifted for C-151, while for the other coumarin molecules a certain blue shift is observed. This fact suggests that C-151 most probably forms ground-state complex with the surfactant molecules. C-151 is a hydrophilic probe so the dipole moment increases on excitation, and it migrates toward the more polar region and also C-151 has a free NH2 group and it may be hydrogen bonded to the surfactant molecules. This indicates that C-151 experience a more confined environment compared to other coumarin dyes, which leads to the decrease in the electron transfer rate at the higher free energy region which apparently looks like an inverted region. The Marcus inverted region can be observed in unimolecular electron transfer reactions, where the donor and acceptors are separated by the rigid spacers or for back electron transfer where the close contact ion pairs are formed.16ac The Marcus inverted region in the bimolecular reaction has been established by a few groups.16d,e Recently Gopidas and co-workers showed that the Marcus inverted region could be obtained in a series of donors and acceptors when these are hydrogen bonded.17 There are several examples of the PET in micellar media like PET in CTAB micelle, DTAB micelle, SDS micelle, and TX-100 micelle for similar donoracceptor pairs, where the inverted region has been observed. Several groups have considered the 6107



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role of diffusion in the PET in the micelle. The diffusion coefficient of the coumarin dyes in this micellar solution can be obtained from rotation relaxation dynamics of the dyes in the micelle. We have measured the fluorescence anisotropy of all the dyes at four different temperatures in the micellar media. In the micelle the rotational relaxation value of the probe molecules is much slower than that in water and indicates that the rotation of the probe molecules is hindered in the micelle; i.e., the probe molecules are facing a rigid environment in the micelle, which suggests that the probe molecules are residing at the micellar surface. In the micellar solution the anisotropy decays of all the coumarin dyes were fitted with biexponential function, but in case of water it is single exponential. Our previous study on the solvation dynamics in this [C4mim][C8SO4] micelle using C-153 as probe molecules explained about the several rotational parameters using the two-step and wobbling in a cone models, which showed that the observed biexponential nature of rotational is due to the different types of rotational motion of the probe molecules in the micelle.14a,b,g The probe molecules are not stationary within the micellar surface so there is a finite possibility of translational diffusion of the probe molecules in the micelles. The different coumarin dyes which we have used in our experiment have different sizes, so the diffusion of the different coumarin dyes in the micellar solution should not be the same and the translational diffusion coefficient may vary. According to the two-step model, the observed slow rotational relaxation contains contribution from the overall rotation of the micelles (τm) and the lateral diffusion of the probe in the micelles (τD). The value of τm can be calculated using the StokesEinsteinDebye relation as τm ¼

4πηrm3 3kT

ð9Þ

where η is the viscosity of water, rm is the hydrodynamic radius of the micelles, and k and T are the Boltzmann constant and absolute temperature, respectively. The hydrodynamic radius of [C4mim] [C8SO4] micelle in water is observed as 1.4 nm, which is determined from the DLS measurement reported previously.4c The translational diffusion coefficient (DL) is defined as DL ¼

rh 2 6τD

ð10Þ

We have calculated the τm, τD, and DL in [C4mim][C8SO4] micelle for different coumarin dyes used in our experiment. We have also calculated the order parameters (S) from the following equation to know the exact location of the probe in the micelle pffiffiffiffiffiffi ð11Þ S ¼ a2r The magnitude of S is a measure of spatial restriction and has values from 0 (unrestricted motion) to 1 (completely restricted motions). The very high value of S implies that the probe molecules are facing a more confined geometry in the micelle. In this case the C-151 molecule has the highest order parameter value. The different parameters τD, DL, and S accounting for the two-step model for all the coumarins are tabulated in Table 4. We have plotted translational diffusion coefficient (DL) for the different probes against the free energy change of the system (Figure 8) and observed that the translational diffusion coefficient (DL) of different probes shows almost the same trend as that of the Marcus correlation plot of the PET rate. Since the translational diffusion coefficient (DL) of the probes is changing in the presence of DMA, it clearly shows that we cannot ignore this in PET reaction in the present system.

Figure 8. Plot of DL vs ΔG0 for different probes in [C4mim][C8SO4] micellar system at 298 K.

For bimolecular electron transfer reaction under diffusive conditions,18 the donoracceptor are freely diffusing in the medium, the ET does not take place at a fixed donoracceptor separation and occurs over a wide range of distance, and the diffusion is the rate-determining step that restricts the electron transfer rate to the diffusion rate at the higher free energy region. These types of micellar systems are not completely rigid; there should be a possibility of finite diffusion, and the diffusion plays a major role in governing the electron transfer kinetics as the diffusional rate is somewhat higher that the PET rate. The bimolecular quenching constants under diffusive conditions are well correlated within the framework of conventional Marcus ET theory.11a,b For a particular donoracceptor pair, the different diffusional rate of the latter is responsible for the difference in the electron transfer rate. It is indicated that the ET reaction under nondiffusive conditions occurs with much faster rates than those under diffusive conditions.9e Now if the diffusion rate is very slow compared to the electron transfer rate in that condition we can ignore the diffusion process in electron transfer rate. In this case of [C4mim][C8SO4] micelle, the diffusion rate calculated from the StokesEinstein equation (kdiff ∼ 1.1 109) is almost comparable to or a somewhat higher amount than the observed PET rate (kq∼0.9 109) at 298 K. So here contribution of the diffusion process may have a role in governing the PET kinetics. Due to the heterogeneity of the system, the diffusion of the different coumarin molecules should not be the same due to the differing strength in the binding of different coumarin molecules in the micellar phase, and it is clear from Figure 7 that the PET rate is highly retarded for C-151. This is the probable reason for getting an inversion in the correlation of the free energy change with the PET rate, and thus it can be concluded that in this micellar media the PET dynamics is diffusion-controlled.

5. CONCLUSION In this study the PET in the [C4mim][C8SO4] micelle has been investigated using the coumarinDMA system by using steady-state and time-resolved fluorescence quenching measurements at four different temperatures. Here the dynamic part of the quenching is specially determined from the time-resolved measurements. When the quenching rates (kTR q ) is plotted 6108


The Journal of Physical Chemistry B against free energy change (ΔG0), an inversion is observed in the Marcus predicted correlation plots like other micellar system studied previously. The plots at four different temperatures revealed the same feature. The main interesting aspect in this present study is that the onset of the Marcus inversion in the electron transfer rate appears at exergonicity, which is much lower in comparison with the total reorganization energy in the systems. The rates of PET process for the different coumarin dyes in this micellar system are found to be lower than in SDS and TX-100 micellar system and almost comparable to the PET rate in DTAB and CTAB micelles. In this case of [C4mim] [C8SO4] micelle, the diffusion rate (kdiff ∼ 1.1 109) is almost comparable to or a somewhat higher amount than the observed PET rate (kq ∼ 0.9 109). The translational diffusion coefficient (DL) calculated from the two-step model for fluorescence anisotropy decay in this micellar system is plotted against free energy change of the system which shows the same feature as observed in the Marcus correlation plot of the PET rate. The temperature dependent time-resolved quenching constant values (kq) are well correlated with the Arrhenius type of correlation where ln kq varies linearly with 1/T.

’ ASSOCIATED CONTENT

bS

Supporting Information. Representative decays of timeresolved fluorescence anisotropy measurements plots. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected]; fax 91-3222-255303.

’ ACKNOWLEDGMENT N.S. is thankful to the Council of Scientific and Industrial Research (CSIR) and the Board of Research in Nuclear Sciences (BRNS), Government of India for generous research grants. S. M., V.G.R, C.G, and R.P are thankful to CSIR for research fellowship. S.S is thankful to BRNS for a SRF. ’ REFERENCES (1) (a) Haumann, M.; Riisager, A. Chem. Rev. 2008, 108, 1474. (b) Parvulescu, V. I.; Hardacre, C. Chem. Rev. 2007, 107, 2615. (c) Lee, S.-g. Chem. Commun. 2006, 1049. (d) Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792. (e) Anderson, J. L.; Armstrong, D. W. Anal. Chem. 2003, 75, 4851. (f) Baker, G. A.; Baker, S. N.; Pandey, S.; Bright, F. V. Analyst 2005, 130, 800. (g) Baker, G. A.; Baker, S. N. Aust. J. Chem. 2005, 58, 174. (h) Ding, J.; Welton, T.; Armstrong, D. W. Anal. Chem. 2004, 76, 6819. (i) Welton, T. Chem. Rev. 1999, 99, 2071. (j) Welton, T. Coord. Chem. Rev. 2004, 248, 2459. (k) Hagiwara, R.; Ito, Y. J. Fluorine Chem. 2000, 105, 221. (l) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. Rev. 2002, 102, 3667. (2) (a) Karmakar, R.; Samanta, A. J. Phys. Chem. A 2002, 106, 4447. (b) Mandal, P. K.; Samanta, A. J. Phys. Chem. B 2005, 109, 15172. (c) Samanta, A. J. Phys. Chem. B 2006, 110, 13704. (d) Paul, A.; Samanta, A. J. Phys. Chem. B 2007, 111, 4724. (e) Arzhantsev, S.; Jin, H.; Baker, G. A.; Maroncelli, M. J. Phys. Chem. B 2007, 111, 4978. (f) Jin, H.; Baker, G. A.; Arzhantsev, S.; Dong, J.; Maroncelli, M. J. Phys. Chem. B 2007, 111, 7291. (g) Jin, H.; Li, X.; Maroncelli, M. J. Phys. Chem. B 2007, 111, 13473. (h) Chowdhury, P. K.; Halder, M.; Sanders, L.; Calhoun, T.; Anderson, J. L.; Armstrong, D. W.; Song, X.; Petrich, J. W. J. Phys. Chem. B 2004, 108, 10245. (i) Mukherjee, P.; Crank, J. A.; Halder, M.; Armstrong,

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Photoinduced Electron Transfer in a Room Temperature Ionic Liquid 1

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