Tetronic Star Block Copolymer Micelles: Photophysical

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Tetronic Star Block Copolymer Micelles: Photophysical Characterisation of Microenvironments and Applicability for Tuning Electron Transfer Reactions Papu Samanta, Sonal Rane, Pratap Bahadur, Sharmistha Dutta Choudhury, and Haridas Pal J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b01778 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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The Journal of Physical Chemistry

Tetronic Star Block Copolymer Micelles: Photophysical Characterisation of Microenvironments and Applicability for Tuning Electron Transfer Reactions Papu Samanta,†,¶Sonal Rane,‡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: Detailed photophysical investigations have been carried out using a probe dye, Coumarin-153 (C153), to understand the microenvironments of micelles formed by the newly introduced Tetronic star block copolymers, T1304 and T1307, having the same polypropylene oxide (PPO) block size but different polyethylene oxide (PEO) block sizes. Ground state absorption, steady-state fluorescence and time-resolved fluorescence measurements have been used to estimate the micropolarity, microviscosity and solvation dynamics within the two micelles. To the best of our knowledge this is the first report on these important physicochemical parameters for this new class of the star block copolymer micelles. Our results indicate that T1307 micelle offers a relatively more polar and less viscous microenvironment in the corona region, compared to T1304. The effect of the two micellar systems has subsequently been investigated on the bimolecular photoinduced electron transfer (ET) reactions between coumarin dyes (electron acceptors) and aromatic amines (electron donors). On correlating the energetics and kinetics of the ET reactions, clear Marcus Inversion (MI) behavior is observed in both the micellar media. Interestingly, the ET rates for all the donor-acceptor pairs are much higher in T1307 than in T1304, and the onset of MI also appears at a relatively higher exergenocity (-∆G0) in the former micelle (~0.45 eV for T1307) than the latter (~0.37 eV for T1304). Effect of added NaCl salt studied selectively in T1307 micelle, shows that the ET rate decreases significantly along with a shift in the onset of MI toward lower exergenocity region, so that in the presence of 2 M NaCl the system becomes quite comparable to T1304. Based on the observed results, it is realized that the micropolarity and hence the dynamics of ET process can be tuned very effectively either by changing the constitution of the star block copolymer or by using a suitable additive as a modifier of the micellar microenvironment.


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1. INTRODUCTION Block copolymers are versatile macromolecules with a wide range of applications in pharmaceutics and cosmetic industries.1-5 An important property of block copolymers is their ability to self-assemble spontaneously in aqueous medium to form micellar aggregates. These micelles are useful in targeted drug delivery, catalysis, micro-reactor systems, etc.1-7 Linear block copolymers that consist of repeating blocks of hydrophilic polyethylene oxide (PEO) and hydrophobic polypropylene oxide (PPO) units, i.e., the Pluronic (PEO-PPO-PEO) copolymers, have been quite well investigated with regard to their self-assembly formations.8-12 It is known that beyond a certain temperature (critical micellization temperature; CMT) and concentration (critical micellization concentration; CMC), the hydrophobic PPO blocks of these copolymers associate to form the core of the micelle, while the hydrophilic PEO blocks constitute the external shell or corona of the micelle. Unlike linear Pluronic copolymers, the Tetronic block copolymers have unique star shaped arrangements of their PEO-PPO units. In these star block copolymers, there are four PEO-PPO arms that are attached to a central ethylenediamine unit.13 Although a large number of studies have been carried out with Pluronic block copolymers, studies on Tetronic series of copolymers are relatively limited so far, but gradually gaining attention in recent years, owing to many of their favourable properties.13-17 Because of their branched structure and the presence of the central diamine functionality, micelle formation with Tetronic copolymers is considered to be significantly different than linear Pluronic copolymers. It has been suggested that due to steric constraints among the four PEO-PPO arms, Tetronic copolymers form relatively more hydrated micelles than Pluronics.13-17 Some recent studies have reported that the star shaped Tetronic copolymers provide better opportunities for novel drug delivery systems, mainly because these


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micelles have a slower rate of dissociation and thus allow retention of loaded drugs for longer periods.18,19 Further, the presence of the central diamine moiety introduces pH sensitivity to the Tetronic micelles as an additional quality, which is quite advantageous to realize their stimuli responsive behaviour.16 An important aspect of micellar assemblies is their ability to modulate chemical reactions, both in terms of mechanism and dynamics, as compared to those occurring in homogeneous media. This modification happens because micelles can provide a confined microenvironment for the reacting species.20-22 In this regard, it is interesting to investigate the characteristics of the microenvironments of Tetronic micelles as nano reactor systems, and to elucidate their effects in modulating specific chemical reactions. As in the case of the linear Pluronic copolymer series, the sizes of the PEO and PPO blocks can be varied quite largely using the series of Tetronic copolymers, providing an opportunity to alter the micellar microenvironments and hence to modulate the mechanism and dynamics of the chemical reactions as desired. In the present study, we have characterized the microenvironments of two micellar assemblies formed by the structurally related star block copolymers, Tetronic 1304 (T1304) and Tetronic 1307 (T1307), using various photophysical measurements of a probe dye. The chemical structures of the two Tetronic copolymers are given in Chart 1. While both T1304 and T1307 copolymers have the same PPO block length, their PEO block sizes are considerably different.2325

Accordingly, we anticipate that the microenvironments of the micellar assemblies formed by

the two Tetronic copolymers would be substantially different. Using the fluorescent probe dye, coumarin-153 (C153), as a reporter for the micellar microenvironments,26-30 we have estimated the micropolarity, microviscosity and solvation dynamics in the corona region of these Tetronic micelles, following ground state absorption, steady-state emission and time-resolved


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fluorescence measurements. To the best of our knowledge, there is no literature report so far on the estimations of these important physicochemical parameters for the studied star block copolymer micelles, especially following such photophysical measurements. Chart 1. Chemical Structures of T1304 and T1307 Star Block Copolymers.

HO(EO)24(PO)27

(PO)27(EO)24OH N-CH2-CH2-N

HO(EO)24(PO)27

(PO)27(EO)24OH

(T1304) HO(EO)61(PO)27

(PO)27(EO)61OH N-CH2-CH2-N

HO(EO)61(PO)27

(PO)27(EO)61OH

(T1307) Having characterized the micellar microenvironments of T1304 and T1307 copolymers, we then investigated the effectiveness of these assemblies in modulating the kinetics and energetics of bimolecular photoinduced electron transfer (PET) reactions. For this, we have used a series of coumarin dyes as the electron acceptors and N,N-dimethylaniline (DMAN) as well as N,Ndimethyl-p-toluidine (DMPT) as the electron donors, to have a large number of donor-acceptor combinations that enable us to cover a wide range of reaction exergonicities. Electron transfer (ET) processes have been the topic of intense research interests for many decades owing to their direct involvement in solar energy harvesting, photovoltaics, photocatalysis, etc.31-35 Importantly, many biological processes also involve ET reactions between selective electron donor-acceptor units, distinctively organised in molecular assemblies, accomplishing the defined outcomes in living organisms. In 1956, professor R. A. Marcus put forward his revolutionary theory on outer sphere ET reactions, incorporating the role of nuclear and solvent reorganizations in determining the


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dynamics of such reactions in solution.36-40 Marcus ET theory eventually provided a unique expression for the rate constant of ET, given as,

k et =

 − (∆G 0 + λ ) 2 2π 2 Vel (4πλk B T ) −1 / 2 exp h 4λk B T 

  

(1)

where h is the reduced Plank constant, Vel is the electronic coupling parameter representing the interaction strength between the reactant (R) and product (P) states and is determined by the extent of overlapping of electronic wave functions of the reacting species involved in the ET process, kB is the Boltzmann constant, T is the absolute temperature, ∆G0 is the free energy change associated with the ET reaction and λ is the total reorganization energy related to the ET reaction, which in effect is the sum of intramolecular (λi) and solvent (λs) reorganization energies, i.e.,

λ = λi + λ s

(2)

The most important feature that emerges from eq 1 is the predicted parabolic nature of the ET rate constant (ket) with the changing reaction exergonicity (-∆G0). Thus, following eq 1, the ET rate will increase in an asymptotic manner on increasing the -∆G0 value at the lower exergonicity region, called the normal Marcus region. Subsequently, the ET system will reach a situation where -∆G0 becomes equal to λ such that the exponential term in eq 1 becomes equal to unity, and thus, the ET rate attains its maximum value, representing the barrierless condition for ET. Beyond this exergonicity, the ET rate will start decreasing with increasing -∆G0 value, leading to the appearance of the intriguing Marcus inversion (MI) region for the ET reactions, a famous behaviour predicted from Marcus outer sphere ET theory. In conventional low viscosity and polar homogeneous solvents, the MI behaviour mostly remains obscured for bimolecular ET reactions.40-47 In these solvent media, the donor-acceptor pairs are required to diffuse together and come within the reaction sphere to form the precursor


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or encounter complexes, which is a prerequisite for the occurrence of ET from the donor to the acceptor unit. Under the situations where the intrinsic ET rate becomes faster than the diffusional rate of formation of the encounter complexes (as often encountered for bimolecular ET reactions at higher -∆G0 values), the ET reaction takes place almost instantaneously upon encounter complex formation. Thus, in these cases, the observed reaction rate effectively becomes controlled by the bimolecular diffusional rate, embedding the intrinsic ET rate completely. Accordingly, at higher -∆G0 values, the observed ET rates show a kind of saturation at the bimolecular diffusion controlled rate, kd. Unlike low viscosity conventional solvents, in constrained microheterogeneous media, e.g., in micelles, reverse micelles or vesicles, the reactant molecules are strongly entangled by the surfactant chains, and as a result, their diffusional encounter is drastically retarded. Accordingly, in these cases, the bimolecular ET reactions necessarily occur under a non-diffusing reaction condition, involving only the pre-existing near-neighbour donor-acceptor pairs present within the reaction spheres.48-53 Thus, the mechanism and kinetics of bimolecular ET reactions in these constrained media are expected to follow a distinctly different behavior than in conventional homogeneous solvents. Present results show that the non-diffusing reaction condition is in fact realized for the bimolecular ET reactions studied in the two Tetronic micellar media. It is observed that the T1304 and T1307 micelles act as nanoreactors, entrapping the electron donor and acceptor molecules within their confined structures and thereby leading to the unique modulations in the bimolecular ET reactions that are not encountered in homogeneous media. Moreover, the differences in the microenvironments of the two Tetronic micelles lead to different extents of modulations on the ET rates. Observed results further demonstrate that apart from employing different copolymer micelles, one can also change the micellar properties and


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consequently the ET rates in a single Tetronic micelle just by using an external additive, e.g., NaCl, acting as a modifier for the micellar microenvironments.

2. MATERIALS AND METHODS The star block copolymers, T1304 and T1307 (cf. Chart 1) were received as a gift from BASF Corp., Parsippany, NJ, USA. The laser-grade coumarin dyes, coumarin-153 (C153), coumarin152 (C152), coumarin-481 (C481) and coumarin-151 (C151), were obtained from Exciton, USA, and were used as received. The amines, N,N-dimethylaniline (DMAN) and N,N-dimethyl-ptoulidine (DMPT) were obtained from Sigma, and were vacuum distilled just before use. Molecular structures of the coumarin dyes, DMAN and DMPT are provided in Chart 2. Nanopure water, having a conductivity of

Tetronic Star Block Copolymer Micelles: Photophysical

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