Rapid and Efficient Electrochemical Actuation in a Flexible Perylene

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Rapid and Efficient Electrochemical Actuation in a Flexible Perylene Bisimide Dimer Samaresh Samanta, Narottam Mukhopadhyay, and Debangshu Chaudhuri Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04077 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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Chemistry of Materials

Rapid and Efficient Electrochemical Actuation in a Flexible Perylene Bisimide Dimer Samaresh Samanta,a,‡ Narottam Mukhopadhyay,a,‡ and Debangshu Chaudhuri*a,b Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741246, India. a

Centre for Advanced Functional Materials (CAFM), Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741246, India. b

ABSTRACT: The ability to reversibly alter molecular structure using an electrochemical stimulus is an area of great contemporary interest, where molecular design plays a key role in determining the efficiency and speed of the process. While the choice of redox active center decides the threshold potential and energy efficiency of the process, selecting a backbone structure with sufficiently low barrier to deformation ensures a fast response. We present a flexible perylene bisimide (PBI) dimer that fulfills these criteria remarkably well, and exhibits a rapid and very large conformational change at a much lower applied voltage than similar systems. A detailed investigation using cyclic voltammetry, spectroelectrochemistry and EPR spectroscopy elucidates the mechanism of the actuation process.

To elicit a reversible mechanical activity in response to an external stimulus is at the heart of molecular actuation. In this context, electrochemical stimulation is a particularly desirable trigger as it produces little or no byproduct, and has a direct relevance to the design of artificial muscles1 and biomechanical applications.2 In traditional conducting polymers based electrochemical actuators, mechanical activity results from the diffusion of ions and solvent molecules in and out of the bulk polymer phase.3 Limited by the rate and reversibility of the diffusion process, such actuators typically have long response times and limited work cycles. In contrast, actuation brought about by a change in the molecular conformation is anticipated to be inherently faster. While this idea has led to the development of several molecular actuators that accomplish a variety of motions such as sliding,4 rotation,5 cavity expansion,6 pivoting7 and hingemotion,8-10 there are also certain limitations. In addition to a faster response, an efficient electrochemical actuator should also be able to produce large deformations at relatively low operating voltages, demonstrate stability across the entire electrochemical window of operation, and offer the possibility of building macroscopic actuators11 through a bottom-up assembly process. With these objectives in mind, we chose PBI as the building block for the design of a molecular actuator. Owing to its electron deficiency, PBI has a fairly low reduction potential,12 and an exceptional chemical/electrochemical stability that ensures a reversible redox behaviour.13 Moreover, the fact that the ability to form stable pi-stacks depends on the redox state of PBI, can be exploited to design efficient actuating structures. Despite these obvious advantages, PBI based electrochemical actuators are surprisingly rare, with the

only notable exception of a bay-substituted PBI cyclophane dimer that shows a redox-driven ring expansion/contraction,14 and a pivoting molecular actuator based on naphthalene diimide.7 In this work, we present electrochemical actuation in a PBI folda-dimer, 2PBI (Figure 1a). In the folded conformation, the two PBI units are cofacially held by an intramolecular pi-stacking interaction. We envisaged that reducing the neutral PBI units to the radical anion state could introduce a strong coulombic repulsion that will trigger dimer unfolding, resulting in a very large conformational change. As our subsequent results show, such a large structural response in 2PBI is not only very fast, but is also achieved at a much lower potential than many of the reported systems. An investigation of redox-triggered conformation change begins with the folded conformer of 2PBI. For this purpose, it is absolutely crucial to select a solvent that promotes intramolecular pi-stacking (folding) in 2PBI, but dissuades any intermolecular interaction leading to aggregation or self-assembly. In an earlier work, we reported competing self-assembly pathways of 2PBI in a strongly aggregating solvent, such as methylcyclohexane.15 An investigation into the electronic and structural aspects of solvent induced 2PBI folding allowed us to identify the appropriate solvent medium. In CHCl3, 2PBI exists predominantly in the open form, as is evident from the close resemblance of its optical absorption spectrum with that of an isolated PBI chromophore (Ref-PBI): 0-0, 0-1 and 0-2 vibronic bands appear at 528, 490 and 460 nm, respectively (Figure 1b and S1). In contrast, the dimer spectrum in DMSO is characterized by a marked hypochromism, the extent of which is different for the 0-0 and 0-1 vibronic bands. The corresponding decrease in the absorbance ratio (A0-0/A0-1) from 1.6 (in CHCl3) to 0.76 (in

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Figure 1. Solvent induced folding. (a) Structures of the dimer 2PBI and the reference monomer, Ref-PBI. In CHCl3, 2PBI exists in the open conformation, and folds spontaneously into a pi-stacked H-aggregate in DMSO. (b) Optical absorption spectra of 2PBI (1 μM) in open (black) and folded (red) states. (c) 1H-NMR spectra of 2PBI in CHCl3 and DMSO (~14 μM) shows differential chemical shifts of the aromatic protons, color coded in 1(a). Solvent signals are marked as ()

DMSO) is a direct evidence of an intramolecular H-type exciton coupling.16 A review of several flexible17-19 as well as rigid20-22 PBI dimers reveals a range of A0-0/A0-1 values, from 0.8 to 0.9, thereby indicating that the folded state of 2PBI supports a fairly strong exciton coupling between the two PBI chromophores. It is worth noting that the propensity of intermolecular pi-stacking between PBI units is generally very weak in DMSO, as is obvious from the concentration independent absorption spectra of 2PBI as well as Ref-PBI in DMSO (Figure S2). An independent validation of the open and folded 2PBI conformations in CHCl3 and DMSO, respectively, is obtained from 1H-NMR spectroscopy. Figure 1c presents a comparison of the aromatic region of 1H-NMR spectra in CDCl3 and d6-DMSO. A comparison of the 1H,1H-2D COSY NMR spectra in the two solvents is presented in Figure S3 and S4. In CDCl3, the spectrum is fairly straightforward and the peaks can be reliably assigned. The perylene protons together appear as three well-resolved and one broadened doublets, between 8.35-8.7 ppm. The aromatic protons of the electron-rich m-xylylene group appear at 7.77 (Ha, singlet), 7.44 (Hb, doublet) and 7.31 ppm (Hc, triplet). In DMSO, the perylene protons are significantly up-field shifted to 7.58-7.79 ppm, a clear indication of increased shielding in the pi-stacked folded state.17,23 However, the most interesting change is seen in the

aromatic protons of the m-xylylene bridge, which now collectively appear between 7.2-7.44 ppm. With the help of 2D-COSY NMR spectra (Figure S3), we conclude that Ha exhibits the largest up-field shift ( = 0.41 ppm). Hb and Hc protons on the other hand show a very modest change in the chemical shift. We take this as a direct proof of the folded conformation, in which the Ha proton is wedged between the two PBI units and is likely to be the most shielded by the aromatic ring current. Having characterized the open and the folded forms, we now elucidate the conformational lability of 2PBI and the sensitivity of its folding equilibrium to a redox stimulus. Cyclic voltammetry demonstrates how each of the 2PBI conformers responds to a stepwise electrochemical reduction.14,24,25 Once again, the experiments were performed in CHCl3 as well as in DMSO.26 In order to distinguish the effect of 2PBI folding from any solvent polarity dependent effects, we also compared the results against cyclic voltammograms (CV) of Ref-PBI in the two solvents. In CHCl3, the CV of 2PBI is similar to that of Ref-PBI (Figure 2a), as expected for the non-interacting open form of the dimer. A pair of one-electron reduction waves appear at E1/2(1) = 0.635 V and E1/2(2) = 0.822 V, corresponding to the formation of [PBI]˙ radical anion and [PBI]2 dianion, respectively. In


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Chemistry of Materials DMSO, CV of the monomeric Ref-PBI once again bears the characteristics of a

coupled folded conformation, in which the reduced dianion state can delocalize over both the PBI units, thus effectively reducing e-e repulsion.14,24,25 The DPV peak at 0.67 V is broader and nearly twice as intense as the other two peaks, thus accounting for poorly resolved second and third reduction steps. That these two reduction events occur at very similar potentials indicates that twoelectron reduction of 2PBI triggers the onset of a chemical (structural) change. Considering that the folded 2PBI is held together by weak pi-stacking interaction, it is quite conceivable that coulombic repulsion in the dianion, [2PBI]2 leads to the unfolding of the intramolecular pi-stack. Scan-rate () dependent measurements provide a deeper insight into the intervening conformational change and the resultant irreversibility of the CV.27 Figure 3a presents a series of CVs recorded at gradually increasing scan rates, from 0.1 to 2.5 Vs-1. A cursory examination reveals nonidentical shifts of the three cathodic peaks. The first peak (Ec1) corresponding to the reduction of 2PBI to [2PBI]˙ shows a negligible shift, and the corresponding peak current rises linearly with 1/2 (Figure S6). In contrast, the second (Ec2) and third (Ec3) cathodic peaks shift gradually

Figure 2. CV of Ref-PBI and 2PBI (~10 μM) at 0.1 V/s scan rate vs. saturated calomel electrode in (a) dry CHCl3/1 mM TBAP at 25 C, and (b) dry DMSO/1 mM TBAP, at 70 C.26 Corresponding differential pulse voltammograms are presented at the bottom of each panel; dotted lines mark the peaks in the differential current.

reversible redox system with a pair of one-electron reduction features appearing at 0.445 and 0.729 V (Figure 2b). An overall shift in the CV of Ref-PBI to more positive potentials points to a greater stabilization of the reduced anionic species in polar DMSO. Besides a similar polarity dependent shift, the CV of 2PBI in DMSO is marked by starkly different voltammetric responses generated by the forward and reverse potential sweeps, an indication that a stepwise electrochemical reduction triggers a chemical/structural change in the folded dimer.14 A strongly coupled folded dimer can undergo a complete electrochemical reduction in four distinct, sequential steps: 2PBI → [2PBI]˙ → [2PBI]2 → [2PBI3˙ → [2PBI]4. In the forward sweep, we can identify three cathodic peaks at 0.477, 0.673 and 0.847 V, respectively. The exact sequence of reduction events is better resolved in differential pulse voltammetry (DPV), wherein three differential current peaks appear at 0.425, 0.67, and 0.82 V. We note that the first cathodic peak of the folded dimer is 20 mV shifted towards more positive potential than the Ref-PBI in DMSO, suggesting a greater stabilization of the radical anion state of the interacting dimer. Also, a smaller separation between the first two cathodic peaks in folded 2PBI (Ec1 Ec2 = 245 mV vs. 284 mV in Ref-PBI) is consistent with a strongly



Figure 3. Scan-rate dependent (a) CV of 2PBI in DMSO, and (b) cathodic peak shifts, relative to the CV for 0.1 V/s confirms the irreversible nature of second and subsequent reduction steps. (c) A schematic representation of the mechanism of electrochemical actuation of 2PBI.

Figure 4. (a) UV-visible-NIR spectroelectrochemistry of Ref-PBI in DMSO. Spectra at selected potentials are highlighted in color to identify the dominant species in solution; colored arrows mark the characteristic absorption peaks of PBI chromophore in neutral (purple), radical anion (cyan) and dianion (red) states. (b) dA/dV curves for the three redox species of Ref-PBI; the inset shows the corresponding changes in absorbance (A) values. For 2PBI in DMSO, spectra measured at different potentials and dA/dV curves are presented in (c) and (d), respectively.

to more negative potentials with increasing scan rates (Figure 3b). These observations are consistent with the fact that while the first electron reduction of 2PBI is reversible, its subsequent reduction causes a structural change.27 By recording CV at different switching potentials (Figure S7), we confirm that the unfolding is indeed triggered by the second electron reduction (Figure 3c). Finally, the effect of dimer unfolding is clear from the back oxidation of 2PBI, as the reverse sweep generates only two anodic peaks at 0.664 and 0.402 V, strikingly similar to that observed for Ref-PBI. Once restored to the neutral state, unfolded 2PBI spontaneously folds back into the pi-stacked conformer. In addition to a detailed account of the redox-triggered actuation, cyclic

voltammetry results also deliver two very important facts about the efficiency and the response time of the actuation process. An extensive comparison with previously reported systems4-10,14,28 reveals that electrochemical actuation of 2PBI achieves a much larger conformational change at a much lower applied potential (Table S1), thus underlining the efficiency of our molecular design. Also the irreversibility of CV even at the highest scan rate of 2.5 Vs-1 proves unambiguously that the redox-triggered unfolding of 2PBI is indeed fast, and cannot be outrun by electron transfer events. We performed spectroelectrochemistry to characterize different reduced intermediates of 2PBI, and monitor their interconversion at different stages of


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Chemistry of Materials electrochemical reduction. Figure 4 presents the spectral evolution of the reduced PBI species in DMSO solution, as the applied potential is scanned quasi-statically (0.05 Vs-1). In the case of Ref-PBI, one can clearly identify the neutral, radical anion and dianion forms at different applied potentials, from their characteristic absorption peaks at 490, 713 and 572 nm (Figure 4a), respectively.29 A change in the relative fractions of these three species can be studied by monitoring the changes in their respective absorbance values (A), as a function of the applied potential. It is particularly informative to examine the first derivative (dA/dV) that allows one to identify the potential at which a redox process is most spontaneous. Figure 4b summarizes the results for Ref-PBI reduction. We find that the minima in the dA=490/dV curve at 0.75 V30 is perfectly correlated with the maxima in the dA=713/dV. A similar correlation is also seen at 0.87 V, between the minima in dA=713/dV and the maxima in dA=572/dV curves. This is consistent with the fact that Ref-PBI undergoes a sequential two-step reduction, in which the appearance of one species is nicely correlated with the disappearance of its redox counterpart. In contrast, folded 2PBI in DMSO shows a more complex behavior (Figure 4c). From the plot of dA=490/dV, we can clearly resolve the first two reduction events, appearing as two minima at 0.78 and 0.83 V (Figure 4d). The potential difference between the first and the second reduction steps in 2PBI is considerably less than the same for Ref-PBI (56 vs. 103 mV), in agreement with the CV results discussed earlier. Also, the two dips in the dA=490/dV plot correlate perfectly well with the appearance of two maxima in the dA=712/dV plot. We therefore, conclude that [2PBI]2– largely exists as a pair of covalently bridged radical anions, one localized on each PBI unit. Interestingly, we fail to observe a similar correlation between the disappearance of radical anion species and the appearance of PBI dianion, at higher negative potentials. This is also obvious from comparing the spectra measured between 0.81 and 0.87 V (cyan and green curves in Figure 4c). In this potential range, the relative fractions of both radical anion and dianion species increase simultaneously, resulting in nearly overlapping dA/dV curves. The peak in dA=568/dV plot appears at 0.87 V, merely 40 mV apart from the dA=712/dV peak. That the third reduction of 2PBI begins even before the second reduction is concluded, is consistent with the unfolding of doubly-reduced [2PBI]2–. The stabilization gained as a result of reduced repulsion in the unfolded state allows the subsequent reduction to be achieved at a lower potential. Finally, an independent corroboration of the unfolding of [2PBI]2– dianion was obtained from low temperature (80 K) electron paramagnetic resonance (EPR) spectroscopy. Figure S8a presents EPR spectra of 2PBI in DMSO upon successive addition of a reducing agent, cobaltocene (CoCp2).31 The EPR signal of [2PBI]2– dianion produced in-situ, is twice as intense as that of the singlyreduced [2PBI]˙, suggesting a two-fold increase in the number of unpaired spins. This is strong evidence in

favour of dimer unfolding, such that each PBI unit is reduced to a radical anion state, with no magnetic interaction between them. In absence of unfolding, the [2PBI]2– dianion is expected to be EPR silent due to a strong pairing of electron spins.24 The paramagnetic nature of unfolded [2PBI]2– is further confirmed from the 1H-NMR spectrum (Figure S8b) that shows no signal from the aromatic protons of the two PBI units.32 The only signal is from the protons of the oxidized cobaltocenium cation, appearing as a singlet peak at 5.74 ppm. In conclusion, we have demonstrated a fast and efficient electrochemical actuation in a flexible bichromophoric system, 2PBI. Using cyclic volatammetry and spectroelectrochemistry, we investigated the mechanism of electrochemical actuation. In the neutral state, 2PBI assumes an intramolecularly pi-stacked folded conformation, which unfolds upon two-electron reduction, resulting in a large conformation change. A strongly electron deficient nature of PBI ensures that the reduction and the subsequent structural change in 2PBI is accomplished at a very low potential, and the flexibility offered by the m-xylylene bridge enables a fast structural response to the electrochemical stimulus. Making the transition from a solution based actuation to more realistic solvent-free actuators may be realized by employing solid-state33 or ionic liquid electrolytes34. Furthermore, macroscopic actuation can also be achieved by making use of PBI motif’s pi-stacking ability. In its selfassembled state, a redox-triggered unfolding of a few 2PBI molecules may elicit a large structural change in the aggregate structure.35 Alternately, one can also dope 2PBI as an active material in a foreign host, such as a gel or a liquid crystalline matrix. Changing the conformation of the embedded 2PBI molecules can potentially alter the arrangement of neighbouring host molecules, resulting in an amplified macroscopic actuation, similar to the work of Ferringa and coworkers.36 Finally, such multichromophoric systems, where the nature of interchromophoric interactions can be reversibly altered upon the application of an external stimulus may also find use in the design of responsive organic optoelectronic materials.

ASSOCIATED CONTENT

Supporting Information Experimental methods, additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] ‡ These authors contributed equally to this work.

Funding Sources DST-SERB Project: EMR/2014/000223

ACKNOWLEDGMENT



The authors gratefully acknowledge IISER Kolkata and Department of Science and Technology (DST), India (Project: EMR/2014/000223) for financial support. SS and NM acknowledge UGC for scholarship.

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2PBI at 25 and 70 C are identical, as confirmed by optical absorption spectroscopy (Figure S5). Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications. John Wiley and Sons: New York, 2001. We compared the threshold potential for electrochemical actuation of 2PBI with 23 reported molecular systems (see Table S1), and the corresponding references are provided therein. La Porte, N. T.; Martinez, J. F.; Hedström, Rudshteyn, B.; Phelan, B. T.; Mauck, C. M.; Young, R. M.; Batista, V. S.; Wasielewski, M. R. Photoinduced Electron Transfer from Rylenediimide Radical Anions and Dianions to Re(bpy)(CO)3 using Red and Near-infrared Light. Chem. Sci. 2017, 8, 3821-3831. In all spectroelectrochemistry experiments, reduction is typically achieved at a higher negative potential compared to that in cyclic voltammetry experiments. This may be attributed to a higher overpotential involved with the reduction of bulk sample in spectroelectrochemistry, as opposed to a very small sample volume being reduced in CV. The lack of a well-defined hyperfine structure in the EPR spectra of reduced [2PBI] species is likely due to the presence of multiple, chemically distinct H (4 nos.) and N (2 nos.) atoms of the two PBI units, each with a slightly


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Rapid and Efficient Electrochemical Actuation in a Flexible Perylene

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