Polypeptide-Assisted Organization of π-Conjugated Polymers into

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Polypeptide-assisted organization of #-conjugated polymers into responsive, soft 3D networks Cornelia Rosu, Christopher J Tassone, Ping-Hsun Chu, Paul L. Balding, Andrew Gorman, Jeff L. Hernandez, Michael Hawkridge, Anirban Roy, Ioan I Negulescu, Paul S. Russo, and Elsa Reichmanis Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 08 Jun 2017 Downloaded from http://pubs.acs.org on June 8, 2017

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Polypeptide-assisted organization of π-conjugated polymers into responsive, soft 3D networks Cornelia Rosu†,‡,ǁ*, Christopher J. Tassone#, Ping-Hsun Chu‡, Paul L. Balding§,&, Andrew Gorman†, Jeff L. Hernandez§, Michael Hawkridge£, Anirban Roy¶, Ioan I. Negulescu┴, Paul S. Russo†,§,& and Elsa Reichmanis‡,§,†,&* †

School of Materials Science and Engineering, ‡School of Chemical and Biomolecular Engineering, §School of Chemistry & and Biochemistry, Georgia Tech Polymer Network, GTPN, Georgia Institute of Technology, Atlanta, GA 30332, United States # Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Stanford, CA 94025, United States £ PANalytical, 117 Flanders Rd, Westborough, MA 01851, United States ¶ Anasys Instruments, 325 Chapala Street, Santa Barbara, CA 93101, United States ┴ Department of Textile Apparel Design & Merchandising, Louisiana State University, LA 70803, United States Supporting Information Placeholder ABSTRACT: The ability of poly(γ-benzyl-L-glutamate) (PBLG) to self-assemble into thermoreversible 3D networks was used to enhance semiconducting polymer assembly into π-π stacked structures. Confinement of poly(3-hexylthiophene) (P3HT) within the physically crosslinked α-helical PBLG bundles afforded ordered structures of J-type signature. The long-range order of the Jaggregates was attributed to intense interactions between the two polymers. Static and in situ X-ray scattering revealed that, in time, P3HT and PBLG synergistically interacted to form a hybrid crystal structure. The hybrid, responsive gel reversibly switched its photophysical properties providing a platform for the future development of bioelectronics, sensors for biomedical applications and monitoring of food quality.

Natural and synthetic polypeptides are known to assemble into supramolecular structures1 that often generate self-supported gels.2 These gels find many applications, especially in biomedicine as platforms for cell growth and drug delivery.3 The synthetic homologs are more appealing because their structure can be tailored to promote self-assembly in a wide range of solvents.4, 5 Poly(γ-benzyl-L-glutamate), (PBLG) is a biocompatible homopolypeptide6 that self-assembles in toluene, a weakly helixpromoting solvent, and generates thermoreversible gels above a critical concentration.7 While there is debate as to mechanism,7-9 it is generally accepted that PBLG aggregates end-to-end even in hot toluene solution.10 With a decrease in temperature, the entangled PBLG helices start to interact with each other through the benzyl side chains11 and pack into bundles. Further aggregation facilitates physically cross-linked 3D networks.9, 10 The benzyl side chain substituents are an appealing structural feature: their aromatic nature can facilitate interactions with π-conjugated systems but, to date, this feature has not been exploited to assist organization of conjugated polymers. Here, we report the proficiency of PBLG to enhance the assembly of poly(3-hexylthiophene) (P3HT) into π-π stacked structures. The hybrid 3D networks were thermoreversible, as demonstrated by associated changes in photophysical properties. UV-vis and Xray scattering revealed that intense interactions between PBLG and P3HT influence PBLG helix packing, as well as P3HT π-π

stacking. PBLG self-assembly into thermoreversible gels represents a route to effect the organization of conjugated polymers into long-range ordered structures, providing a foundation for the design of biocompatible and highly responsive hybrid materials. First, we evaluated the gel thermoreversible behavior. PBLG/toluene and P3HT-PBLG/toluene were able to switch reversibly between solution and gel states as a function of temperature (Scheme 1). Mixtures containing P3HT (0.16 wt%) and PBLG (0.5 wt%) became soluble in toluene upon heating at 80 °C. Unassisted cooling of S1 to ambient temperature induced gelation, and the fresh gel (G1f) was orange. Aging the gel for 30 days (G1a) led to a color change from orange to dark purple, which is reminiscent of changes observed during P3HT aggregation. Reheating G1a afforded the solution, S2; which upon cooling yielded G2f, followed by G2a upon aging. The PBLG gel exhibited two distinct endothermic events during the first DSC heating mode (Figure S1, Supporting Information): one at 14.2 °C (∆H= 25.2 mW per gram of gel (g-1)) and another broad event centered at 38.8 °C (∆H = 27 mW g-1). Because the parent PBLG gel was isotropic, the observed endothermic events were associated with either melting of weak crosslinks/toluene-PBLG associations or, more likely, with hysteresis between gelation and gel melting. In the P3HT-PBLG case, these thermal events were less well defined; however, transitions were discerned at 16.2 °C (∆H = 14.8 mW g-1) and 31.6 °C (∆H= 15.5 mW·g-1). The observed attenuation suggests that the two polymers interacted. Because P3HT/toluene solution does not exhibit transitions in this temperature regime,12 these events most likely emanate from changes in the PBLG network.13

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SCHEME 1. Physical Appearance and Schematic Illustration of P3HT-PBLG/Toluene 3D Network Assembly

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Figure 1. Static UV-vis profiles of 0.5 wt% PBLG/0.16 wt% P3HT/toluene gels recorded during the two cycles outlined in Figure 1, plus a control showing P3HT/toluene. Solutions and fresh gels (A); aged P3HT/toluene solution and aged gels (B); in situ UV-vis traces gathered over a time span of 50 min while cooling from the solution state S1 (80°C) to the gel state G1f (C).

The non-reversing heat flow curves revealed only a single endothermic event centered at 48 °C for the PBLG/toluene gel and at 47.3 °C for P3HT-PBLG/toluene gel. These signals were assigned to the melting of the two systems. UV-vis spectroscopy (Figure 1), provided insight into the photophysical response of the P3HTPBLG/toluene gels. Figure 1A presents spectra of fresh solutions: P3HT is ‘well solubilized’ and exhibits the expected absorbance centered at 454 nm (2.73 eV). PBLG/toluene gel does not exhibit absorption signals in these regions (see Supporting Information, Figure S2A). In the fresh gel (G1f, G2f), two new low-energy bands emerged at 617 nm (2.01 eV) and 566 nm (2.19 eV), corresponding to the P3HT 0-0 and 0-1 electronic transitions, respectively, suggesting the presence of ordered P3HT structures. The higher energy absorbance (454 nm) representative of amorphous content was still visible. Hysteresis was not observed in the ‘fresh’ spectra, demonstrating reversible photophysical behavior. Upon aging (30 days), distinctly different spectral characteristics materialized (Figure 1B): the 454 nm band intensity decreased dramatically, a new absorbance peak appeared at 527 nm (2.35

eV), and the vibronic bands at 617 nm and 566 nm significantly increased in magnitude, and became dominant. After the second aging cycle (G2a), the 454 nm absorbance was markedly suppressed; the vibronic band intensities appeared comparable to those of G1a, suggesting that aging enhanced P3HT chain ordering. Additionally, a small shoulder at 1.7 eV (720 nm) emerged in G2a. Conceivably over time, P3HT became doped, perhaps due to charge transfer interactions with the aromatic PBLG benzyl side chains, leading to polaron absorption.14 In the absence of PBLG, aged P3HT/toluene solution spectra exhibited the expected behavior (Figure 1B). The signal centered at 720 nm seen in G2a was not observed. The presence of this low-energy shoulder solely in the hybrid gel points to intense interactions between the side chains of PBLG and the P3HT thiophene rings. The emergence of the two low-energy bands with the dominant 0-0 transition is believed to originate from enhanced intrachain planarization and suggests J-aggregate formation.15

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Figure 2. In situ SAXS results of parent PBLG, G1f and G1f-G1a (A; inset shows a synchrotron SAXS envelope); 1D diffractograms from static GIWAXS (B); in situ GIWAXS (C); AFM images of dry PBLG/toluene gel (D), dry G1F (E) and dry G1a (F).

In addition, the 0-0/0-1 transition ratio observed in fresh gels is 4.2 and 3.5, respectively, which supports the presence of J-like structures. Upon aging, the gels exhibited a decrease in 0-0 band intensity, and the 0-0/0-1 ratio approached unity (1,8 and 1.6 for G1a and G2a, respectively).16,17 In situ UV-vis showed that the 00 transition occurred first during the transition from S1 to G1f (Figure 1 C). The P3HT-PBLG/toluene gels displayed rapid (~5s) photophysical response due to P3HT structural changes (Figure S2C, Supporting Information). Concomitantly, the gel color gradually changed. UV-vis analysis demonstrated that within the thermoreversible gel network, interactions between P3HT and PBLG enhanced semiconducting polymer ordering. As confirmed by development of the vibronic signals, P3HT chains became more planarized and ultimately afforded highly ordered structures. Raman spectroscopy also supports these findings (Figure S3, Supporting Information). The excess small-angle X-ray scattering (SAXS) envelope above the solvent of PBLG/toluene gel (Figure 2A, inset) reflects rod-like behavior (see Supporting Information). The length of the cylinder exceeded that measurable using the available q range. The q-4 Porod slope reflects a well-defined interface between the fibers and the voids of the gel. The broad peak at d = 1.9 nm was attributed to the distance between the PBLG helices, identical to that found for fibrillar PBLG gels by Cohen.18 In G1f q-1 and q-4 behaviors were not observed (Figure 2A, inset). Interpretation of these observations is complicated by the presence of two polymers, both of which contribute almost the same electron density. One scenario consistent with the results points to interactions between the P3HT chains and bundled PBLG rods leading to the disappearance of the sharp interface observed in the PBLG/toluene gel. The packing distance of the helices identified at d = 1.7 nm lies slightly lower than that of the parent gel (1.9 nm) suggesting more closely packed structures. An unusual feature is the appearance of a broad peak corresponding to a dspacing of 16.5 nm. P3HT/PBLG structural development was investigated during the transition from G1f to G1a by in situ SAXS (Figure 2A). A 2D Guinier analysis showed that the cross sectional radius, Rc, increased from 5.7 ± 0.03 nm to 9.2 ± 0.015 nm (8h) then decreased, ultimately reaching a plateau at 7.4 ± 0.012 nm (24h 57h). Despite the complications of a mixed polymer system, the

growth of ordered elongated P3HT structures is supported by the increase in scattering intensity (see Supporting Information for details). Effects associated with a highly interactive system (over dilute limit) and polydisperse aggregates could cause mismatching between the simulated and experimental data at low q, confirmed by the observed leveling of Rc values. The long-range order and structure of the dried gels was examined by grazing-incidence wide-angle X-ray scattering (GIWAXS). The PBLG/toluene gel exhibited the characteristic 100, 200 and -130/-230 diffraction peaks at 1.46, 0.73 and 0.42 nm, respectively (Figure 2B)19 designated as form B.20 The peak associated with the pitch of the PBLG α-helical secondary conformation was evident at d = 0.5 nm. The expected P3HT diffraction signatures were identified in the hybrid P3HT/PBLG/toluene gel at correlation distances d = 1.6 nm (d100, alkyl chain packing), d = 0.81 nm (d200) and d = 0.38 nm (d010, π-π stacking) (Table 1, Supporting Information). The latter reflection became more intense in G1a and G2a, suggesting that P3HT structures have either fewer defects or increased crystallinity. The size of the gel crystalline domains was in the range of 50 to 60 nm, which is in between that of the PBLG/toluene gel (41.2 ± 0.12 nm) and a drop-cast P3HT film (84.5 ± 1.00 nm). Notably, the 100 reflection (d100 = 1.46 nm) attributed to the distance between the centers of adjacent helices in the polypeptide gel disappeared in the hybrid P3HTPBLG/toluene gels (Figure 2B). In situ GIWAXS studies performed during the gel drying process (see Movies 1 ,2, 3, Supporting Information) revealed that the most dominant structural changes were associated with the d100 reflection (Figure 2C). The d100 spacing was centered at 1.6 nm and leveled at 1.46 nm in the PBLG/toluene gel, while in the hybrid gels the decrease was less noticeable (from 1.66 nm and 1.68 to 1.57 nm in G1f and G1a, respectively), assuming almost equal scattering from P3HT and PBLG. Because the PBLG dried gel reflection at 1.42 nm was not apparent in the hybrid, a distinct PBLG phase is probably absent. Either PBLG was unable to crystallize due to interaction with P3HT, or it fits within the P3HT structure with its d100 spacing ~ 1.6 nm. In-depth X-ray investigations are necessary to fully elucidate crystalline packing of the hybrid gels. These findings corroborate the SAXS data and confirm that P3HT interfered with normal PBLG crystallization. Moreover, the

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GIWAXS data showed that, as a result of interactions between the polymers, P3HT and PBLG formed a stable hybrid crystal lattice whose ordering accommodates the packing of the two polymer components. Atomic force microscopy (AFM) showed that the dried PBLG gel consists of physically cross-linked polydisperse bundles of 2 8 aggregated fibers. G1f presented additional short, most likely P3HT, fiber-like structures (white arrows) (Figure 2E) that filled the interstices between PBLG bundles (Figure S6, Supporting Information). 21 In contrast, G1a (Figure 2F), suggests that P3HT and PBLG cooperatively “merged” into a single structure. This process is reversible and reproducible (Figure S6, Supporting Information). Confocal microscopy supports the AFM results: P3HT resembled G1f and G1a (see Figure S7, Supporting Information). AFM-IR confirmed that the presence of the π-conjugated polymer chains does not prevent the α-helical secondary conformation of PBLG (Figure S6C, Supporting Information). Moreover, the absorption peaks associated with the P3HT thiophene repeat units (~1450 cm-1, Figure S6D Supporting Information) were invariably found near those of the PBLG α-helix (1653 cm-1 and 1550 cm-1) in all hybrid gels. The results discussed hitherto indicate that PBLG and P3HT interact to form responsive, interconnected networks. One plausible mechanistic interpretation of the findings is illustrated in Scheme 1. In hot solution, well-solvated P3HT chains coexist with end-toend aggregated PBLG helices. During cooling, P3HT chains interact with the benzyl side-chains of the developing PBLG bundles leading to π-π associations. During this process, P3HT chains may undergo conformational changes whereby the thiophene ring orients perpendicular to the PBLG benzyl group and the thiophene sulfur atom points toward the center of the six-carbon aromatic cycle.22 The chains may then start to extend and wrap around the PBLG helices and physically crosslinked bundles. P3HT chain extension may initiate ordering and π-π stacking events necessary for P3HT aggregation. Concurrently, crystals may nucleate selectively at the surface of the PBLG fibers that may also serve as pinning sites. A helical turn contains 3.6 repeat units.23 Slow and selective crystallization under confinement conditions may also favor induced fractionation24 and self-poisoning25 of P3HT polymer chains during self-ordering. While the precise mechanism requires further elucidation, phase separation of the polymer components would most likely have been observed upon solution gelation if no interactions were present. Additionally, the crystal packing associated with each phase would be discerned in the 2D GIWAXS spectra. Recall, however, the static and dynamic GIWAXS/SAXS results presented above demonstrated that the P3HT/PBLG/toluene system is highly interactive. Further, predominantly J-aggregate structures were observed along with a low-energy shoulder (720 nm) which is indicative of polaron exchange. Note also that intrachain charge transport requires effective extension of the P3HT backbone, a feature difficult to achieve in P3HT/toluene systems, as reported by Grey and co-workers in their extensive studies.21 Undoubtedly, confinement alone, provided in this case by the PBLG network, might also induce alignment and organization of P3HT, as was demonstrated for the semiconductor encapsulated with the protein, Cerato Ulmin.26 In the present case, the occurrence and dominance of J-aggregation in the P3HT/PBLG/toluene gels, along with doping/polaron exchange underscore that confinement alone does not account for the observed results, rather other structural features compete and drive the organization of P3HT into ordered structures. Moreover, the hybrid packing of P3HT and PBLG polymers identified in the gels support the concept that intense interactions between the PBLG benzyl side chains and P3HT thiophene rings occur. Thus, analysis of the data presented in this study, combined with previous reports26 strongly points to a highly interactive hybrid gel, especially when aged, whereby structural

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features ‘join’ the polymers together. Confocal microscopy and AFM support the concept that ordered P3HT structures appeared to be anchored to the PBLG network like lichens to tree branches. In conclusion, self-assembly of PBLG in toluene enabled organization of P3HT into thermoreversible networks. Association with the physically cross-linked PBLG rodlike chains facilitated growth of P3HT aggregates, as reflected by the emergence of 0-0 and 0-1 vibronic transitions. Physical interactions between P3HT and PBLG, as demonstrated by SAXS and GIWAXS, led to the formation of a hybrid crystal structure. Notably, the PBLG polypeptide was an attractive tool to rapidly organize P3HT into predominantly crystalline J-aggregates, a feature difficult to achieve by P3HT alone. The results presented here offer new opportunities to harness the propensity of polypeptides to assemble into interconnected fibrillary structures to control the organization of optoelectronic polymers, specifically semiconducting polymers, into stable, large area/volume, ordered structures. The thermal and photophysical responsive nature of the P3HT-PBLG/toluene gels examined here provides a foundation for the development of sensors for biomedical applications and food quality monitoring. The presence of the bioderived and biocompatible polypeptide also offers prospects for using hybrid gels as active materials in bioelectronics as well as wearable/implantable electronic devices. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Movie 1, 2 and 3: Time-lapse of 2D GIWAXS diffractograms showing the evolution of PBLG/toluene, G1f and G1a gels. Additional details on experimental procedures, X-ray, Raman, UV-vis, AFM-IR, confocal microscopy and rheology measurements. AUTHOR INFORMATION Corresponding Author Cornelia Rosu; Email: [email protected] Elsa Reichmanis; Email: [email protected]

Author Contributions CR developed the idea, designed the experiments, analyzed UV-vis, GIWAXS, AFM, DSC/MDSC, rheology data, CJT performed in situ GIWAXS experiments, JH did static GIWAXS investigations, AG acquired static SAXS measurements and analyzed the data, MH did in situ SAXS, PLB performed confocal and SIM measurements, PHC did AFM investigations, AR performed AFM-IR experiments, IIN performed rheology and DSC/MDSC measurements, CR did POM and polarized microRaman spectroscopy investigations, CR, PHC and JF recorded static and in situ UV-vis traces. CR, PSR and ER interpreted the data and edited the manuscript. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors appreciate support from National Science Foundation grants DMR 1505105 (PSR, CR), DMR 1609058 (ER, CR, PSR, PHC) and IGERT Traineeship (DGE-1069138) (JH); and the Hightower Family Fund (PSR, CR) and the Brook Byers Institute for Sustainable Systems (ER, PHC). They value access to the Stanford Synchrotron Radiation Lightsource, SLAC National

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Accelerator Laboratory facilities (supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515). Discussions with Prof. Mohan Srinisavarao, Dr. Jinxin Fu and Prof. Jean-Luc Brédas and Mr. Ian Pelse (Georgia Tech) are also deeply appreciated.

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