Effect of Ionic Liquids as Co-Surfactants on Photoinduced Electron

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Effect of Ionic Liquids as Co-Surfactants on Photoinduced Electron Transfer in Tetronic Micelles Papu Samanta, Pritesh Halder, Pratap Bahadur, Sharmistha Dutta Choudhury, and Haridas Pal J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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The Journal of Physical Chemistry

Effect of Ionic Liquids as Co-Surfactants on Photoinduced Electron Transfer in Tetronic Micelles Papu Samanta,†,¶ Pritesh Halder,‡ Pratap Bahadur,§ Sharmistha Dutta Choudhury,‡,¶,* and Haridas Pal‡,¶,* †Integrated

Fuel Fabrication Facility and ‡Radiation & Photochemistry Division, Bhabha Atomic

Research Centre, Mumbai 400 085, India ¶Homi

Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai 400094,

India §Department

of Chemistry, Veer Narmad South Gujarat University, Surat 395007, India

ACS Paragon Plus Environment

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ABSTRACT: This study investigates the role of varying alkyl chain lengths of a series of surface active 1-alkyl-3-methylimidazolium tetrafluoroborate ([CnMIm][BF4], n=4, 6 and 10) ionic liquids (IL) as co-surfactants, in modifying the micellar characteristics of a tetronic starblock copolymer, T1304, and the consequent effects on bimolecular photoinduced electron transfer (PET) reactions carried out in these T1304-IL mixed micellar systems. Using coumarin 153 as the probe dye, and following ground state absorption, steady-state fluorescence, and timeresolved emission measurements, the micropolarity, microviscosity and solvent relaxation dynamics in the micellar palisade layer have been revealed both in pure T1304 and in T1304-IL systems. With increasing alkyl chain length of the ILs, the palisade layer of the micelles gradually becomes more polar and less viscous, suggesting better incorporation of the longer alkyl chain length ILs as co-surfactants, into the T1304 micelles. The bimolecular PET reactions, involving 7-aminocoumarins as acceptors and N,N-dimethylaniline as donor, are considerably modulated in T1304 micelles by the presence of the ILs; the effect being more prominent for ILs with longer alkyl chain lengths. In all the micellar systems, correlations of the ET kinetics with the reaction exergonicity (G0) show clear Marcus Inversion (MI) behavior where onsets of MI invariably appear at significantly lower exergonicities, suggesting the involvement of twodimensional ET mechanism. Interestingly, the Marcus correlations display significant variations, namely, enhanced reaction rates and gradual shift in the onset of MI towards higher exergonicity, as longer alkyl chain length ILs are sequentially introduced as co-surfactants. From the observed results, it is convincingly realized that 1-alkyl-3-methylimidazolium based ILs can be used satisfactorily as co-surfactants in tetronic star-block copolymer solutions to modulate PET reactions very significantly for their better utilizations in suitable applied areas.


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1.

INTRODUCTION

Electron transfer (ET) is the most fundamental reaction occurring in chemistry and biology.1-5 ET is also directly involved in the harnessing of solar energy.3-5 Understanding the dynamics and tunability of ET under different conditions is of immense importance to bring out practical applications of these reactions. According to the revolutionary ET theory proposed by R. A. Marcus, the ET rate is expressed explicitly as,6-11 k et 

  (G 0   ) 2 2 2 Vel (4k B T ) 1 / 2 exp  4 k B T 

  

(1)

Here ħ is the reduced Planck’s constant, Vel is the electronic coupling between reactant and product states, kB is the Boltzmann constant, T is the absolute temperature, G0 is the free energy change and  is the total reorganization energy, i.e. the sum of intramolecular (i) and solvent (s) reorganization energies,   i  s . Though estimation of i is not easy (requires complex transient Raman study), s can be estimated satisfactorily using the relation,6-11

s 

e2 2

 1 1 2  1       1  2  rA rD r  n    

(2)

Here e is the electronic charge, rA and rD are the radii of the acceptor (A) and donor (D) molecules (assumed to be effective spheres), r is the centre to centre distance between A and D while n and  are the refractive index and static dielectric constant of the solvent, respectively. From eq 1, it is clear that free energy of activation (G*) for ET is a quadratic function of G0 and  as,

G

*

G 

0



4



2

(3)


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Accordingly, this relation predicts that ket follows an inverse parabolic behavior with reaction exergonicity (-G0), displaying the famous Marcus Inversion (MI) behavior, i.e. ket increases with -G0 at lower exergonicity region (-G0 < ; normal Marcus region), reaches a maximum value at -G0 =  where G* = 0 (barrierless condition) and then decreases again with -G0 at higher exergonicity region (-G0 > ; Marcus Inversion region). Although MI behavior has been documented for many intramolecular ET and charge recombination (CR) reactions, similar behavior is still elusive for bimolecular photoinduced ET (PET) reactions because in polar low viscosity solvents, the observed reaction rate (kobs or kq, the quenching constant) is largely controlled by the slower rate of diffusion (kd) of the reactants, causing kq to saturate at kd and masking the otherwise much faster intrinsic ET rate at the higher exergonicity region.10-18 To suppress the influence of reactant diffusion on bimolecular PET several studies have been carried out in constrained reaction media like micelles, reverse micelles, etc, where entanglement of reactants in the surfactant assemblies makes their diffusion extremely slow and thereby enforces the reactions to occur mostly under non-diffusing condition, involving mainly those donor-acceptor pairs that already pre-exist in the reaction sphere at the moment of photoexcitation.19-28 Due to high microviscosity in the constrained media, the solvent reorganization around the reacting species is also quite slow such that solvent relaxation remains incomplete during ET reaction. In ET reactions, the major contribution in  comes from the solvent reorganization energy, s.9-11,17,20 Considering incomplete solvent relaxation in constrained media, the effect of s on G* is expected to be reduced substantially, as envisaged form two-dimensional ET (2DET) model.29-32 Accordingly, in these cases the onset of MI is expected to shift significantly towards lower exergonicity, making it conducive to observe MI behavior easily. In fact, in the literature several studies have been reported on bimolecular PET


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in various constrained media where MI has been demonstrated clearly with onset of MI appearing at reasonably lower exergonicities than expected following conventional 1DET theory.19,26,28,33 In a recent study, we have investigated bimolecular PET in two tetronic copolymer micelles, Tetronic 1304 (T1304) and Tetronic 1307 (T1307), comparing the results with respect to their differential micellar microenvironments.33 Tetronics are a new class of star-block copolymers having immense research interests in recent times due to their stimuli responsive micellar properties, useful in pharmaceuticals and drug delivery systems.34-39 Our study revealed that the palisade layer of T1307 micelle being more hydrated, polar and less viscous, the PET rates in this micelle are significantly higher and the onset of MI appears at a relatively higher exergonicity as compared to T1304 micelle.33 Further, it was observed that addition of NaCl causes significant dehydration of T1307 micelle, causing ET kinetics in the presence of 2M NaCl to become quite similar to that in T1304 micelle.33 Realizing that micellar characteristics are very important in tuning bimolecular PET reactions we were encouraged to study the role of ionic liquids (IL) of varying alkyl chain lengths in their cations as the co-surfactants in modulating microenvironment of T1304 micelle, and consequently to understand the influence of these ILs in tuning the bimolecular PET reactions in the so formed mixed micellar systems. The ILs are environmentally benign molten electrolytes, considered as “green solvents”, having unique properties like, low vapor pressure, non-flammability, high ionic mobility, and excellent chemical stability.40,41 ILs that possess hydrophobic alkyl chains at their cations show properties similar to conventional surfactants, behaving as surface active ILs (SAILs).42,43 Being biologically friendly, SAILs have better prospects as co-surfactants than common surfactants to modulate characteristics of co-polymer micelles.27,44-46 However, only few studies have been


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reported so far on the use of ILs as co-surfactants, focusing mainly on the changes in the micellar size and aggregation number.47-49 To the best of our knowledge, changes in the physicochemical characteristics of micelles, such as, micropolarity, microviscosity, solvent relaxation dynamics and so on, and their consequent effect on bimolecular PET reactions in Tetronic-IL mixed micelles have not been attempted so far. Since the necessary structural parameters of T1304-IL mixed micelles using 1-alkyl-3-methylimidazolium tetrafluoroborate ([CnMIm][BF4]) series of ILs are readily available in the literature,49 we have selected these assemblies to study PET reactions involving a series of coumarin dyes as electron acceptors and N,N-dimethylaniline (DMAN) as electron donor. Physicochemical characteristics of the studied mixed micellar systems, which are essential in correlating PET results in these media, have also been carried out using different photophysical measurements. In the present study, we have used three [CnMIm][BF4] type of ILs, having n values equal to 4, 6 and 10. Our main aim in this study is to understand how the varying alkyl chain lengths of the ILs influence the physicochemical characteristics of the T1304-IL mixed micellar systems and consequently to reveal the modulations that take place in the kinetics and energetics of PET reactions investigated in the aforesaid mixed micellar media, especially in regard to the observation of the MI behavior.

2.

MATERIALS AND METHODS

The Tetronic T1304 was a gift from BASF Corp., Parsippany, NJ, USA, and used without further purification. The [CnMIm][BF4] series of ILs with 1-butyl (n=4; [BMIm][BF4]), 1-hexyl (n=6; [HMIm][BF4]) and 1-decyl (n=10; [DMIm][BF4]) substituents were obtained from Prof. Anil Kumar at the CSIR-National Chemical Laboratory, Pune, India, and used as received. Synthesis and characterizations of these ILs have been reported earlier.50 All experimental solutions in the


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present study were freshly prepared and thus the decomposition of [BF4] anion in the experimental solutions was assumed to be quite insignificant.51 Moreover, since the coumarin dyes used in the present study are not prototropic in nature, their photophysics are not expected to be influenced much in the presence of a small amount of HF in the solutions if produced by any decomposition of [BF4] anion in the aqueous environments. Laser-grade coumarin-153 (C153), coumarin-152 (C152), coumarin-481 (C481), coumarin-522 (C522) and coumarin-151 (C151) dyes were obtained from Exciton and used as received. N,N-Dimethylaniline (DMAN) was obtained from Sigma and was vacuum distilled just before use. Chemical structures of T1304, ILs, coumarin dyes and DMAN quencher used in this study are presented in Chart 1. Nanopure water, having conductivity ket(T1304-[HMIm][BF4]) > ket(T1304-[BMIm][BF4]) > ket(T1304). Present observation thus suggests that there is a clear possibility of tuning bimolecular PET reactions, not only in terms of shifting the onset of MI along the exergonicity scale but also to tune the effective ET rates substantially, by modulating the mocroenvironment of a Tetronics micelle through the use of ILs of varying alkyl chain lengths as the co-surfactants to form the mixed-micellar systems as the reaction media.

7.

CONCLUSION

Bimolecular PET reactions between a series of 7-aminocoumarin dyes as electron acceptors and DMAN as electron donor have been investigated using T1304 micelle and T1304-IL mixed micellar assemblies as constrained reaction media, following both SS and TR fluorescence quenching studies. Presence of 1-alkyl-3-methylimidazolium tetrafluoroborate ([CnMIm][BF4]) ILs as co-surfactants influences both the energetics and kinetics of the studied PET reactions in T1304-IL mixed micellar systems. While Stern-Volmer (SV) plots from SS fluorescence quenching show positive deviation from linearity, indicating substantial transient quenching contributions, the SV plots from TR fluorescence quenching show expected linearity, indicating negligible transient quenching contributions. Accordingly, TR quenching data were used to estimate the bimolecular quenching constants (kq(TR)) for the donor- acceptor pairs in the studied micellar media. Microenvironments of the studied micelles have also been investigated following


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SS fluorescence and TR fluorescence anisotropy and solvent relation dynamics studies using coumarin 153 as the probe. It is found that ILs with longer alkyl chains affect the micellar microenvironments more in terms of increased hydration and reduced micropolarity. It is proposed that ILs with longer alkyl chains are incorporated/solubilized in larger numbers into T1304 micelles, and accordingly, the presence of more number of cationic head groups of the ILs leads to higher hydration for the palisade layer of the mixed micelles formed. Correlations of kq(TR) values with reaction exergonicities (∆G0) in different micelles show the expected MI behavior, where the onsets of MI evidently arise at much lower exergonicities than expected following conventional 1DET mechanism. The observed results suggest the involvement of 2DET mechanism in the present systems, apparently due to incomplete solvent relaxation during the progress of ET reaction. In T1304-IL mixed micelles, it is further observed that with the increasing alkyl chain lengths of the ILs the kq(TR) values gradually increase and the onsets of MI systematically shift towards higher exergonicities, ascribed to the effect of gradually increased hydration and reduced micropolarity of the palisade layer in the mixed micellar systems. PET results in the studied micelles clearly suggest that the constrained micellar environments enforce the ET to effectively occur under 2DET mechanism than conventional 1DET mechanism. The results further suggest that bimolecular ET reactions can be tuned to a reasonable extent, both in terms of observed ET rates and onsets of MI, just by using a series of ILs with varying alkyl chain lengths as the co-surfactants, which might find usefulness in some practical applications.

ASSOCIATED CONTENT Supporting Information: Absorption spectra of probe dye C153; Essential micellar parameters; Note on estimation of CMC and relevant figures; Note on calculation of electrochemical


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parameters for acceptors and donor in micellar media and relevant table; Estimation of zeta potentials for the studied micelles.

AUTHOR INFORMATION Corresponding Authors: * E-mail: [email protected] (SDC) & [email protected] (HP); Telephone: 91-22-25590296; Fax: 91-22-25505151 NOTES The authors declare no competing financial interest.

ACKNOWLEDMENTS The authors gratefully acknowledge the generous support provided by the host institution, Bhabha Atomic Research Centre, Mumbai, during the course of the present work. The authors are also thankful to Shri. A. C. Bagchi, Head, IF3, BARC, and Dr. P. D. Naik, Head, RPCD, BARC, for their constant encouragement and support. The work was supported by the Department of Atomic Energy (DAE) under project XII-N-R&D-02.02.

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TOC Graphic (3)

22

lnkq(TR)

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


(2) 3

21 -0.6

T1304-long chain IL

2 1 -0.4

G0 (eV)

-0.2

T1304-small chain IL

T1304

(1)


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Effect of Ionic Liquids as Co-Surfactants on Photoinduced Electron

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