Torsional Impacts on Quaterthiophene Segments Confined within

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Torsional impacts on quaterthiophene segments confined within peptidic nanostructures Tejaswini S. Kale, Herdeline Ann M Ardoña, Alyssa Ertel, and John D. Tovar Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03708 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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Torsional impacts on quaterthiophene segments confined within peptidic nanostructures Tejaswini S. Kale,†, ⊥ Herdeline Ann M. Ardoña,†, ‡, #, ⊥ Alyssa Ertel‡, ○ and John D. Tovar*,†, ‡, §

†Department

of Chemistry, Krieger School of Arts and Sciences, Johns Hopkins University,

3400 N. Charles St., Baltimore, MD 21218 ‡

Institute for NanoBioTechnology, Johns Hopkins University, 3400 N. Charles St., Baltimore,

MD 21218 USA §Department

of Materials Science and Engineering, Whiting School of Engineering, Johns

Hopkins University, 3400 N. Charles St., Baltimore, MD 21218 USA.

Current Address: #

Disease Biophysics Group, John A. Paulson School of Engineering and Applied Sciences,

Wyss Institute for Biologically Inspired Engineering, Harvard University, 29 Oxford St., Cambridge, MA 02138 USA ○

Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States

Corresponding author email: *[email protected]

⊥

T. S. K. and H. A. M. A. contributed equally.

ACS Paragon Plus Environment

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ABSTRACT The co-assembly behavior of peptide-π-peptide and peptide-alkyl-peptide triblock molecules that form 1-D nanostructures under acidic, aqueous environments is dependent on the peptide sequence and on the torsional constraints imposed within the nanomaterial volume. While a hydrophilic tripeptide sequence (Asp-Asp-Asp, DDD-) sequence previously promoted isolation/dilution of minority pi-electron components in the matrix of aliphatic peptides, a β-sheet promoting sequence (Asp-Val-Val, DVV-) led to blocks of the two components distributed within larger 1-D selfassembled nanostructures. Furthermore, the torsional restrictions exerted on the oligoaromatic πelectron unit by the self-assembly process can lead to changes in its conformation (e.g. planarity), which has ramifications on its functionality within the peptide matrix. Here, we study this impact on thiophene based π-electron units with inherently different geometries, viz. relatively planar 2,2':5',2'':5'',2'''-quaterthiophene

(OT4)

and

3'',4'-dimethyl-2,2':5',2'':5'',2'''-quaterthiophene

(OT4dM), which is twisted at the core bithiophene unit due to the presence of the two methyl groups. These peptides were co-assembled at 5 and 20 mol% with peptide-n-decyl-peptide triblock molecules and the resultant assemblies were studied using UV-vis absorption, photoluminescence and circular dichroism spectroscopies. We found that torsional restriction in dimethylated quaterthiophene units can impact the stacking behavior of these 1-D peptide nanoassemblies and have consequences on their photophysical properties. Additionally, these insights help in the understanding of the dependence of optoelectronic properties of these materials on both the intrinsic conformation of π-units and the geometric constraints imposed by their immediate local environment under aqueous conditions.


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INTRODUCTION The molecular-level organization of π-segments in conjugated materials plays a crucial role in their charge transport and photophysical properties. Fluctuations in local packing and distribution of torsion angles due to conformational freedom in flexible molecules or chains can lead to energetic disorder which affects transport.1 Considering their applications to biology, several π-conjugated systems have been used as fluorescent probes such as for membrane imaging,2 but the effect of condensed lipid environments on the polarization and planarization of these conjugated probes are rarely discussed.3-4 Among the known π-oligomers, the torsional flexibility of oligothiophenes have been one of the most extensively studied,5-7 and several of these reports have utilized methyl substitutions to perturb the π-system planarization.8-9 The rotation around the α-α’ single bonds between thiophene rings has a small energetic cost, making oligothiophenes quite sensitive to torsional perturbation.2 Oligothiophenes with zero, one, or two adjacent methyl groups in the β-positions are planar in their excited state, but in the ground state, the central dihedral angle considerably varies.8 Based on the crystal structure of 5,4',3'',5'''-tetramethyl-2,2':5',2'':5'',2'''-quaterthiophene, the dihedral angle between the interior thiophene rings was found to be 19.8º even as a majority of the molecules were found to adopt the anti-anti-anti orientation of the thiophene rings.10-11 The adjacent outer and inner rings were found to be more coplanar, dihedral angles between the mean planes being 2.1º and 5.3º for the two pairs. Different alkyl-substituted oligothiophenes displaying varied torsional conformations on the Au (111) surface were also previously studied, showing that cis–trans torsional disorder may not be a significant source of electronic disorder for oligothiophenes.12 In separate studies on the torsional motion about the central dihedral angles of oligothiophene, it was interestingly found that non-zero degrees of charge transport were predicted


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even at 90° twists and that the twisting potentials are highly dependent on chain length.13-14 These studies provided a better understanding on the correlation of torsional flexibility of oligothiophenes and their resulting electronic properties. When the interchain interactions were considered for crystalline sexithiophene, the neighboring chains induce an individual flexible unit to adopt a planar configuration with high energetic cost for torsional defects.15 This shows that the neighboring chains also play an important role in determining the final geometric conformation of packed oligoaromatic chains, and not just their intrinsic flexibility as an individual unit. Considering the emerging use of π-oligomers for bioelectronic applications, several groups have utilized self-assembling biomolecules to direct hierarchical ordering and act as a scaffold to establish better π-π interactions. Such strategies present diverse opportunities to functionalize the electronically-active systems with bioactive groups, as well as to impart more biologically relevant physical properties (i.e., softer mechanical properties of hydrogels).16-21 Despite the extensive literature on π-segments appended to biomolecule assemblies and their applications, the effect of spatial constraints imposed by the assemblies on the conformation of individual π-units and dihedral angles between the aromatic units are rarely discussed. In this study, we report selfassembling peptide-π-peptide triblock units in completely aqueous environments whereby we study the torsional impacts of the confinement of π-units within peptidic nanostructures on their photophysical properties while minimizing inter-chromophore exciton coupling (Figure 1). Peptides bearing n-decyl chains (C10) as the core unit of triblock monomers were co-assembled with peptide-π-peptide units in order to provide an electronically inert environment that can still confine π-segments within a one-dimensional nanostructure. Previously, we reported on the behavior of oligo(p-phenylenevinylene) (OPV3) containing peptides co-assembled with the inert C10 containing peptides.22 In this case, the more hydrophilic


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peptide had a propensity toward isolating the minority OPV3 containing peptide in the majority C10 containing peptide matrix, while the peptide with higher tendency of β-sheet formation led to self-sorting of the two peptides in the co-assembled nanostructures. The local environments in the assemblies of the two peptide sequences were distinctly different leading to drastic variations in the photophysics of the OPV3 units within these peptide assemblies. Since the OPV3 unit is a planar, rigid moiety with two constituent double bonds restricting rotation within the molecule, only the free rotation of the phenyl rings provides possibilities for varying conformation within the molecularly dissolved or self-assembled state. We were further intrigued by the effects of internalizing flexible π-units with varying degrees of planarity and greater freedom of rotation on their photophysical properties. Here, we specifically study quaterthiophene (OT4) due to its widely understood properties and is known as nearly planar with anti-orientation of the thiophene rings, and its β-methylated derivative at the interior thiophene rings (OT4dM).8,

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Although a previous study on methyl-

substituted quaterthiophenes in tetradecane at low temperature (i.e., aggregation is minimized) showed that planar equilibrium in ground (S0) and first excited state (S1) is afforded despite the steric hindrance, it is known that at ambient temperature, such symmetry can break due to the thermal population of torsional modes leading to a non-planar S0 geometry.8 We also expect that the methyl substituents in OT4dM provides steric hindrance in π-stacking within peptidic nanostructures in aqueous solutions and can therefore influence exciton coupling and the assembly behavior of π-conjugated peptide units. Moreover, the use of quaterthiophene-peptide assemblies as the active layer in a field-effect transistor have been successfully reported,23-25 and therefore merits further understanding of structure-property correlation. Our goal in the study presented herein is to expand our previous findings from oligo(p-phenylenevinylene) peptides to


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quaterthiophenes, focusing on a new element of torsional restriction. We have previously reported that torsional flexibility has a significant influence on the self-assembly process of these selfassembling peptides, and therefore on the mechanical properties of the fibers that they form.26 Beyond showing such mechanical effects, here we provide insights on the torsional impacts of constraining π-segments within nanostructures, which will be useful for understanding the photophysical effects of interfacing these materials with biological membranes and other related lipid-like environments.

EXPERIMENTAL SECTION Peptide synthesis. Peptides were prepared according to our previous reports. Full synthetic details for both the peptides and the chromophores used herein can be found in the Supporting Information. UV-Vis absorption and photoluminescence (PL) spectroscopy. UV-Vis absorption spectra were obtained using a Varian Cary 50 Bio UV-Vis spectrophotometer. We recorded the photoluminescence spectra of peptide solutions using a PTi Photon Technology International Fluorometer (QuantaMaster 40) with a 75-W Ushio Xenon short arc lamp and operated with Felix32 Version 1.2 software (Build 56). Samples for spectroscopic measurements were prepared by diluting a 1 mg/mL stock peptide solution to the appropriate concentration in Millipore water to achieve an optical density near 0.1. The pH was then adjusted by adding 10 μL of 1M KOH (basic, pH 10) followed by 20 μL of 1M HCl (acidic, pH 1). The excitation wavelengths used correspond to the λmax observed in the absorption spectra of the peptides. The fluorescence lifetime decay measurements were performed using a Q-25 Lifetime add-on for the same QuantaMaster PTi instrument as the steady-state measurements (equivalent to a TM-200 TimeMaster LED


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system; lifetime range, 200 ps to 1 µs). The samples were excited with a PTi-L375 Class 1 LED source (375 nm). The instrument response function (IRF) was recorded with respect to a dilute LUDOX (Aldrich) AS-40 colloidal silica suspension in water. Fluorescence lifetime exponential fits were all executed using Felix32 Version 1.2. Circular dichroism (CD). CD spectra were obtained using an AVIV Circular Dichroism Spectrometer Model 420. Spectroscopic samples were prepared by diluting the peptide solution to the appropriate concentration in Millipore water. The pH was then adjusted by adding 10 μL of 1M KOH (basic, pH 10) followed by 20 μL of 1M HCl (acidic, pH 1). Fourier transform-infrared (FT-IR) spectroscopy. All data were obtained on dry peptide samples using a Thermo Scientific Nicolet Nexus 670 FT-IR spectrometer. The samples were prepared by lyophilizing acidic peptide solutions. Reverse-phase HPLC: Samples for purification using HPLC were prepared from lyophilized peptide solids dissolved in Millipore water. A small amount of ammonium hydroxide was added to adjust the pH to 8. Purification and analysis was performed using an Agilent SD1 PrepStar System with a Phenomenex C8 column (Luna 5 μm, 250x21.20mm and 250x4.60mm). An ammonium formate aqueous buffer (1% (v/v), pH 8) and acetonitrile were used as the mobile phase. Transmission electron microscopy (TEM). Imaging was performed on a Philips EM 420 transmission electron microscope equipped with an SIS Megaview III CCD digital camera at an accelerating voltage of 100 kV. The samples were prepared by pipetting a drop of 1 mg/mL solution of assembled peptide in water onto 200 mesh copper grids coated with Formvar film (Electron Microscopy Sciences) and adsorbed for 5 min at room temperature. The grid was washed


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with DI water and the excess solution was wicked off by touching the side of the grid to filter paper. The sample was then stained with a 2% uranyl acetate solution, washed with DI water and excess moisture was wicked off. The grid was allowed to dry in air before imaging.

RESULTS AND DISCUSSION Similar to our previous molecular design,22 aspartic acid (Asp, D) was used as the terminal amino acid of the peptide-π-peptide monomers to impart pH sensitivity. In particular, this monomer design leads to essentially dissolved peptides under basic conditions (pH 10), due to Coulombic repulsion among negatively charged carboxylate units, and assembled structures under acidic conditions (pH 1) at concentrations as low as 0.1 mg/mL (0.1 mM) when these ionizable groups become protonated. Either valine (Val, V), which has a high propensity towards β-sheet formation27 and provides a hydrophobic environment, or aspartic acid (Asp, D) which provides an entirely hydrophilic backbone, were used as the two interior amino acids. All peptides (Figure 1) were produced using solid-phase synthesis, followed by on-resin dimerization via amidation or Pd-catalyzed cross coupling, according to our previous reports.24, 28 Details of the synthesis of new peptides can be found in the Supporting Information.


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Figure 1. Molecular structures of π-conjugated peptides used in this study and in Ref. 22.

To visualize the assembled nanostructures, we recorded TEM images representing nanomaterials present in the solutions analyzed during the photophysical investigations (Figure S1-S5). Similar to our previous reports on quaterthiophene units symmetrically appended with peptides, all OT4 and OTdM peptides involved in this study formed 1-D nanostructures upon acidification, under all the dilution conditions with C10 peptides used. For DDD peptide sequence with both OT4 and OT4dM π-electron units, nanostructures with shorter aspect ratios and uniform widths of ca. 30 nm were observed. These appear to resemble hollow nanotubes whereas both the pure DVV-OT4 and -OT4dM show nanostructures with smaller widths and higher aspect ratios. Upon dilution with the respective C10 peptides, the nanostructure widths in all co-mixtures are dominated by that of the C10 majority components. Overall, 1-D nanostructure formation is maintained throughout the dilution series used for all the photophysical investigations for both the


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DDD- and DVV- peptide series. The observed differences between the aspect ratios formed by the two peptide sequences suggests that the difference in torsional angle between OT4 and OT4dM only results in a subtle variation in their assembly behavior at the micron scale. Therefore, any dramatic changes in the photophysical properties discussed below can be attributed to a more localized difference in geometric conformation of the chromophores studied here. In our previous investigations,29-30 we associated the shifts in photophysical properties to the delocalization that arises from the exciton coupling among the internal π-electron groups, whereby the π-π stacking distances and conformation of the cores can be influenced by the surrounding peptide conduits. It is expected that the nature of assemblies induced within the peptidic environments presents a close packed internal matrix that might influence the optoelectronic properties of chromophore ensembles, for example, by restricting the torsional freedom among the aromatic groups that make up the core π-electron segments. We previously found that diluting the optoelectronic peptide nanostructures with peptides containing a C10 core (Figure 1a-b) results in statistically minimizing core π-π intermolecular interactions but provide a sufficiently rigid matrix to influence torsional dynamics.22 This dilution yields two-component mixed assemblies that can result in two separate “homoassemblies” peptides with π-electron core and those with C10 (Figure 2, i), self-sorted “aggregates” of π-conjugated peptides stacked together with C10 peptide blocks (Figure 2, ii), or isolated units of the minority component stacked well dispersed within each nanostructure (Figure 2, iii).31 The preference between these possible outcomes is dependent on the nature of the local field imparted by the peptide matrix,22 similar to the influence of local fields found in analogous peptide systems.32 In the case of the peptidic assemblies studied herein, we expect to form kinetically-trapped structures due to the abrupt acidification of the mixtures, wherein co-assembled peptides with


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electronically “inert” or “active” central cores randomly coexist in each 1-D nanostructure. There have been limited studies on multicomponent supramolecular systems with π-units that have demonstrated perfect self-sorting into separate structures33 via programming the sequential assembly of each of the components and random mixing into co-assemblies34 by encouraging mutual interactions of the individual components based on molecular design. Although the kinetic and thermodynamic control in favoring co-assembled or perfectly self-sorted structures in multicomponent mixtures of supramolecular systems is still a challenge in aqueous systems, it is well supported that pathways towards kinetic products that result in random component mixing are more often favored for larger π-systems under aqueous conditions due to hydrophobic effects.35-36 It is therefore most plausible that each co-assembled nanostructure with peptides bearing aliphatic cores as the major components results in peptidic π-electron units distributed as either “isolated” or “aggregated” within the alkyl peptide matrix assemblies (Figure 2, ii and iii). If the π-electron units in the co-assembled nanostructures are isolated within the alkylated peptides (Figure 2, ii), then we expect the photophysical properties of the ensemble to reflect that of an essentially dissolved π-electron unit. If the system prefers to maintain the aggregation between πelectron units, then the photophysical properties should be reminiscent of the exciton-coupled homo-assemblies of pure π-conjugated peptides (Figure 2, iii). In our previous work with OPV3 peptides (Figure 1b) diluted with C10 peptides (Figure 1a), we found that DDD- peptides resulted in isolated OPV3 units within co-assemblies while DVV- peptides preferred aggregation.22 Here, we describe the results of our studies on the effects of varying the geometries of the π-electron units on the assembly behavior of isolating DDD- and aggregating DVV- peptides, with the intent of understanding the behavior of individual π-electron units within distinct peptide ensembles. Particularly, we study the effects of torsional restriction due to the nature of the core (OT4 vs.


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twisted OT4dM, due to methyl groups) on the photophysical properties of the assembled π-electron units.

Figure 2. Schematic illustration of the possible outcomes of peptide co-assembly that lead to 1-D nanostructural geometries (red ■ = quaterthiophene units; — = alkyl core; and ∼ = peptide matrix).

We then did spectroscopic investigations for the basic (dissolved) and acidic (assembled) solutions of OT4, OT4dM, C10 peptides and their respective co-assemblies (at 5 and 20% DDDand DVV- OT4/OT4dM in C10) to measure the π-π* transitions associated with the π-electron units contained within the peptides. These profiles are considered as ensemble averages of all local chromophore orientations within the self-assembled nanostructures of different polydispersities. In order to correlate the observed photophysical properties to the geometric configuration of πelectron units, we considered that oligomeric pi-conjugated molecules usually have structureless


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absorption and emission spectra due to free rotation about linking single bonds while rigid molecules (or molecules in rigid or viscous environments) often show distinct vibronic coupling in their absorption and emission spectra.37 Thus, any change observed in the photophysical properties after C10 dilution suggests a consequential spatial constraint due to the hydrophobic alkyl chains in the immediate environment of the chromophore. More importantly, we hypothesize that the methyl groups in OT4dM core can present some steric perturbation on the inherent πstacking of OT4 units even with peptides appended on both sides of the chromophore. It also is important to note that the observed photophysical changes, as a result of dilution, are highly concentration dependent.22 Considering that water, peptides and the aliphatic chains comprising the nanomaterial matrix solvate each individual π-electron unit, this observation in the changes in photophysical properties of peptide solutions could be due to the differences of the density of surrounding peptides that contribute a local field and structural constraints to the chromophore core. In addition to comparing the “homo-assemblies” with the OT4/OT4dM and C10 peptide “co-assemblies”, we recorded the spectral profiles of “separately assembled” solutions, whereby OT4/OT4dM and C10 peptides where separately acidified and then mixed. As in our previous report,22 we expect that co-assembled solutions contains peptide nanostructures with both OT4/OT4dM and C10 peptide monomers coexisting within one stack (Figure 2, ii or iii), whereas the separately assembled solutions primarily have OT4/OT4dM and C10 peptide assemblies in different stacks (Figure 2, i) or coexisting within one stack as blocks (Figure 2, ii). We have recorded the spectra for “separate assemblies” to serve as a control system to set a spectral comparison for π-conjugated peptides that are aggregated within a C10 environment (Figure S6S7). Using these samples, we can also assess whether the pre-assembled OT4/OT4dM peptides will further undergo significant conformational changes within the stack upon the addition of


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electronically-inert C10 nanostructures and if this brings any further changes in their photophysics. A summary of the solution formulations for all the samples/dilution conditions used for spectral measurements can be found in the Supporting Information (Table S1). For the basic solutions (~ pH 10) of DDD- and DVV- OT4 and OT4dM peptides (dashed lines in Figures 3a, 3c, 4a and 4c), we observed a blue-shift in the absorption spectrum of essentially dissolved OT4dM peptides as compared to OT4 peptides (λmax, DDD-OT4, 408 nm; DVV-OT4, 420 nm; DDD-OT4dM, 376 nm; DVV-OT4dM, 375 nm). This initially confirms that the methyl groups attached to the quaterthiophene units perturb the planarity of the π-system that effectively breaks the π-conjugation of individual thiophene rings.8, 38 After acidification, all four peptides show a decrease in absorption intensity after of the peptide solutions due to the assembly. Particularly, the absorption intensity decreased and the λmax slightly red-shifted to 413 nm and 384 nm in the case of DDD-OT4 and -OT4dM, respectively (Figure 3). Such red-shifts in absorption spectra of quaterthiophene molecules have been previously observed and attributed to planarization of the chromophore units upon confinement in space simultaneous with exciton coupling between π-units within the assemblies.2, 9, 39 On the other hand, the DVV-OT4 homoassemblies showed bimodal absorption profiles, with one of the peaks being considerably blueshifted (350 nm) with respect to the peak of the basic, unassembled DVV-OT4 (Figure 4a). This is indicative of a mixed population of H-like assembly arrangements within DVV-OT4 nanostructures. These λmax values were previously observed in the spectral profiles of OT4conjugated peptides of varying sizes of side chains. Interestingly, the absorption intensity of DVVOT4dM decreased upon acidification but only has a slight red shift (380 nm) from the λmax of the basic solution, which also suggests the perturbation of close π-stacking due to the methyl groups in the quaterthiophene (Figure 4c). In both DVV-OT4 and OT4dM cases, it is also notable that a


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shoulder in ~480 nm exists, which was also a spectral feature observed in the previously reported OT4-conjugated peptides with bulky, hydrophobic side chains.29 For the absorption spectra of all OT4 and OT4dM peptides, we did not see a significant difference between the spectra of coassembled and separately assembled solutions (Figure S6).

Figure 3. UV-vis absorption spectra of DDD-OT4 (a, b) and DDD-OT4dM (c, d) peptides. Solutions were prepared as (a, c) homoassemblies and (b, d) co-assemblies. For the twocomponent solutions, [C10]= 33 µM was kept constant between dilutions. The concentration of OT4/ OT4dM is the same as that in the 20 mol% dilution (ca. 8 µM). All spectra were recorded at room temperature (ca. 25 °C).


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Figure 4. UV-vis absorption spectra of DVV-OT4 (a, b) and DVV-OT4dM (c, d) peptides. Solutions were prepared as (a, c) homoassemblies and (b, d) co-assemblies. For the twocomponent solutions, [C10]= 33 µM was kept constant between dilutions. The concentration of OT4/ OT4dM is the same as that in the 20 mol% dilution (ca. 8 µM). All spectra were recorded at room temperature (ca. 25 °C).

Broad, featureless emission spectra under the essentially dissolved solutions were observed for DDD-OT4 and DDD-OT4dM (λmax, 516 nm and 522 nm, respectively), as well as for both DVV-OT4 and DVV-OT4dM which had peaks centered at 509 and 521 nm, respectively (Figure 5 and 6). Dimethylation of the core quaterthiophene unit is known to predominantly affect the absorption properties, whereas the chromophore adopts a planar excited state conformation therefore resulting in an emission profile resembling that of the unfunctionalized α,ωquaterthiophene (possibly accompanied with a lower quantum yield).38 Upon acidification, the


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spectrum red-shifted to 539 nm in the case of DDD-OT4 and slightly blue-shifted in the case of DDD-OT4dM to 519 nm, while both maintained broad featureless peaks. In contrast, acidification of DVV-OT4 and DVV-OT4dM resulted in more structured spectra with bimodal bands between 510 to 550 nm region that were more prominent in the case of OT4dM peptide. This bimodal nature was previously observed for quaterthiophenes appended with peptides that have hydrophobic side chains adjacent to the π-electron core,29 supporting the local field influence of the surrounding peptide moieties to their emission profile. In both DDD- and DVV- cases, the assembled solutions under acidic conditions for OT4dM showed less quenching from their basic counterparts than the OT4 peptides. This further suggests that the OT4dM peptides, due to the methyl perturbation, do not stack as efficiently as the unfunctionalized OT4 peptides do. In addition, the observed red-shift in emission peak for assembled OT4dM as compared to OT4 peptides can be associated to slight π-donor property of methyl substituents. Similar to our previous reports,22, 29 despite the similarity in chromophore content, we have also observed in this library of peptides that changing the sequence of the peptide backbone resulted in dramatic changes in the absorption and photoluminescence spectral profiles of DDD(Fig. 3 and 5) and DVV- (Fig. 4 and 6) peptides. We attribute this to the global influence of local fields imparted by the different solvation environments, which includes the peptide moieties surrounding the π-electron units. We also note that the distinct absorption peaks at ~275 nm can be associated to local carboxamide linkages between the π-electron units and peptides. This higher energy transition is also affected by the same intermolecular interactions and local field effects that affect the π-π* exciton coupling amongst the π-electron cores,41 which due to the high degree of variability in the local orientations of transition dipole couplings, also lead to high variability in the intensities of the ~275 nm peak across all the absorption spectra reported above.


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Co-assembly of these peptides with C10-containing peptides was carried out to understand the effects of varying chromophore geometries on the properties of the chromophores within peptide ensembles. Each peptide sequence was previously reported to elicit different co-assembly behavior when OPV3 peptides were co-mixed with C10 peptides, whereby DDD- peptides lead to isolated units within C10 assemblies while DVV- peptides prefer aggregation even at lower molar ratios.22 As was observed in the case of the planar OPV3 chromophore in DDD- peptides,22 both OT4 and OT4dM chromophore units appear to be isolated at low molar ratios in the DDDC10 matrices. This preference towards isolation was supported by the minimal blue-shift in the absorption spectra with dilution (415 nm for 20 mol% and 413 nm at 5 mol% DDD-OT4), as well as the minimal red-shift in the PL spectra (531 nm for 20 mol% and 519 nm at 5 mol% DDDOT4) of acidic assemblies with respect to the spectra of basic solutions. The absorption and PL spectral peaks of 5 mol% DDD-OT4/ DDD-C10 peptide coassembly resemble that of the corresponding DDD-OT4 alkaline solution. We note that this is not the case for the separately assembled control for 5 mol% DDD-OT4 dilution (529 nm, Figure S7a), further verifying the preference of DDD- peptides towards isolation when acidified together with C10 peptides. This is consistent with the observation in the case of previously reported DDD-OPV3 peptide,22 suggesting a similar behavior involving isolation of minority, DDD-OT4 peptide, within the predominantly DDD-C10 peptidic stacks. In the case of DDD-OT4dM/ DDD-C10 co-assembled nanostructures, no significant spectral shifts were observed in either the absorption or the emission spectra with dilution. In addition, the PL spectral peak of 5 and 20 mol% DDD-OT4dM (both coassembled and separately assembled cases) aligns with that of molecularly dissolved peptide mixture under basic conditions. These spectral signatures further support the isolation of DDDOT4dM units within C10 peptides, showing less preference towards aggregation as brought by


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the twisted and sterically-hindered nature of the OT4dM core, as well as the nature of the hydrophilic DDD-peptide. In the case of mixtures of pre-assembled peptides, DDD-OT4 and DDD-C10 as well as DDD-OT4dM and DDD-C10, a decrease in intensity of absorbance as well as photoluminescence was observed as the concentration of the π-electron unit containing peptide was decreased. However, a small change in spectral position was observed to accompany this change in the case of pre-assembled mixtures of DDD-OT4dM and DDD-C10, suggesting some interactions amongst stacks in solution that would impart electronic communication and hence spectral variation across the dilution series. Considering that these results reaffirm our previous findings that DDD- peptides promote the isolation of pi-conjugated peptides amongst such 1-D assemblies,22 we note that the presented spectra do not ascertain whether the quaterthiophene units are becoming more planarized within the C10 matrix (Figure 3b/d) or if the homo-assemblies themselves are planarizing but not undergoing significant exciton coupling (Figure 3a/c).


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Figure 5. Photoluminescence spectra of DDD-OT4 (a, b) and DDD-OT4dM (c, d) peptides. Solutions were prepared as (a, c) homoassemblies and (b, d) co-assemblies. For the twocomponent solutions, [C10]= 33 µM was kept constant between dilutions. The concentration of OT4/ OT4dM is the same as that in the 20 mol% dilution (ca. 8 µM). All spectra were recorded at room temperature (ca. 25 °C).

In the case of more hydrophobic DVV- systems with higher preference towards β-sheet formation, OT4 and OT4dM (neat 100%) emission spectra showed some vibronic features that resemble those of OT4 units that have been previously conjugated with more hydrophobic peptide units.24 The 5% co-assemblies for both OT4 and OT4dM peptides showed broad and less resolved spectra than the higher co-mixing at 20 mol%, suggesting the absence of significant exciton coupling due to the dilution amongst electronically-inactive C10 units. Comparing the 20% chromophore dilution to the pure solutions investigated, the emission spectra show similar


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features, but the quantum yield is higher with the co-assembly. This relatively reduced quenching in the co-assembled case reflects a decrease in an H-like photophysical behavior, which supports the decrease in resonant effects upon dilution with C10 peptide matrices. We also note that the red shifted nature of DVV-OT4 in the assembled state relative to free peptide suggests better exciton interaction than the DVV-OT4dM case, where the assembled signature falls under the umbrella of the free peptide spectral response. In both cases, the dilution did not result to a huge difference on the photophysical properties of the DVV- peptides. On the other hand, the separately-assembled conditions with DVV-OT4 show a broad PL peak with slightly less resolved vibronic bands than the co-assemblies at 20 mol% while retaining the same features at 5 mol% (Figure S7c). The DVVOT4dM separate assemblies did not show any change in intensity or spectral features as compared to the corresponding co-assembled nanostructures (Figure S7d). This suggests that not all preassembled samples have exactly the same photophysical characteristics as their pure peptide counterparts because the presence of pre-assembled C10 fibers likely imposes different bundling interactions with stacks of π-electron unit containing peptide. Taken together, for the DVV- series that prefers aggregation over isolation, the steady-state absorption and emission data show that blocks of OT4/OT4dM peptides diluted within C10 peptides or quaterthiophene units laterally interacting with other C10 stacks do not impose enough torsional strain to drastically affect the planarity of the chromophore.


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Figure 6. Photoluminescence spectra of DVV-OT4 (a, b) and DVV-OT4dM (c, d) peptides. Solutions were prepared as (a, c) homoassemblies and (b, d) co-assemblies. For the twocomponent solutions, [C10]= 33 µM was kept constant between dilutions. The concentration of OT4/ OT4dM is the same as that in the 20 mol% dilution (ca. 8 µM). All spectra were recorded at room temperature (ca. 25 °C).

Photoluminescence lifetime measurements were also carried out for all four peptides containing OT4 and OT4dM chromophore units, as well as their co-assemblies with the respective C10 peptides at 5, 9 and 20 mol% ratios. As summarized in Table 1, pure DDD-OT4 and DVVOT4 peptides fit a mono-exponential decay model under basic conditions, with lifetimes of 0.43 ns and 0.53 ns, respectively. These values were consistent with those previously reported for essentially dissolved units of OT4 peptides. Under acidic conditions, the PL decay for DDD-OT4 peptide fit a bi-exponential decay profile with components at 0.69 ns and 0.18 ns (τavg = 0.40 ns),


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while DVV-OT4 maintained a mono-exponential decay profile with a lifetime of 0.67 ns. Longer lifetimes under acidic conditions are indicative of electronic delocalization within the peptide nanostructure, in agreement with the steady state H-like aggregate behavior observed in both these peptides. Both co-assembled and separately assembled nanostructures of DDD-OT4 and DDDC10 peptides at 20 mol% DDD-OT4 content showed lifetimes that fit a bi-exponential decay profiles. However, at lower molar ratios (5 and 9 mol%), both these kinds of assemblies reverted to mono-exponential decay profiles, with lifetimes around those observed in the case of molecularly dissolved samples. This suggests that the interaction among the π-electron units in the co-assembled nanostructures is disrupted leading to lifetimes similar to those observed in individual chromophore units. In the case of DVV-OT4 and DVV-C10 co-assembled nanostructures and mixtures of the pre-assembled nanostructures, the lifetime decay profiles remained to be considerably longer average lifetimes as compared to that in the molecularly dissolved state, with a bi-exponential nature for the co-assemblies and mono-exponential for the neat DVV-OT4. These lifetime decay profiles indicate that electronic communication amongst πelectron units is maintained in all of these nanostructures despite the low molar ratios of OT4 containing peptides. This is similar to the observation with DVV-OPV3/ DVV-C10 coassemblies,22 which promoted stacks of each of the constituent peptides within the co-assembled nanostructure rather than isolated units, further indicating that the DVV- peptide sequence has a strong preference towards maintaining an aggregated state even when co-mixed with C10 peptides.


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Table 1. Fluorescence lifetimes of DDD- and DVV-OT4 and solutions co-mixed with C10 peptides. (λexc= 375; λem= 520 nm for pH 10 and 540 nm for pH 1 samples). Full PL decay profiles are shown in Figures S8-S9. τ/ ns mol% OT4 5% DDD9% DDD20% DDD100% DDD5% DVV9% DVV20% DVV100% DVV-

Dissolved 0.44 0.46 0.43

Co-assembled τ (%) 0.41 (100) 0.49 (100) 1.88 (95); 0.38 (5)

τavg 0.41 0.49 0.45

Separately assembled τ (%) τavg 0.45 (100) 0.45 0.49 (100) 0.49 0.69 (43); 0.18 (57) 0.40

0.43

-

-

0.69 (43); 0.18 (57)

0.40

0.51 0.52 0.52

6.48 (2.2); 0.58 (97.8) 6.05 (2.3); 0.57 (97.7) 4.24 (1.5); 0.63 (98.5)

0.71 0.70 0.68

3.93 (3.3); 0.58 (96.7) 1.56 (10.4); 0.44 (89.6) 5.77 (0.95); 0.59 (99.5)

0.69 0.56 0.62

0.53

-

-

0.67 (100)

0.67

In the case of DDD-OT4dM peptide assemblies and co-assemblies with DDD-C10 peptide, no significant contributions from longer lifetime components were observed. The neat peptide solution had a mono-exponential photoluminescence decay with a lifetime of 0.40 ns in the molecularly dissolved state, which changed to a bi-exponential decay upon assembly with lifetime components of 6.83 ns (0.5%) and 0.35 ns (99.5%), and an average lifetime of 0.38 ns. The higher contribution of a shorter lifetime component similar to that observed in the molecularly dissolved state suggests that electronic delocalization is minimal in these nanostructures due to the isolated behavior of DDD-OT4dM assembled within DDD-C10, which is in agreement with the steady state photophysical observations where there are no significant spectral position differences between the two pH conditions tested. Upon co-assembling the peptide with DDD-C10 peptide at different molar ratios, the lifetime at 20 mol% DDD-OT4dM was similar to that of the pure assembled DDD-OT4dM peptide whereas further dilution led to a mono-exponential decay with lifetimes similar to those of the respective molecularly dissolved states. The inherent twist in the chromophore along with the impact of peptide sequence on the geometry of the assembly is likely


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to have a significant impact on this behavior. For DVV-OT4dM, a mono-exponential decay with a lifetime of 0.38 ns was observed in the molecularly dissolved state, which changed to a biexponential decay with an average lifetime of 0.49 ns in the acidic state. Upon co-assembling with DVV-C10 peptide, the decay maintained a bi-exponential profile throughout the dilution series, but with components of shorter lifetime values than that in the DVV-OT4 case. These PL lifetime data further strengthens the argument that the perturbation in assembly due to the methylation of quaterthiophene also evidently effects the electronic communication between DVV- π peptides that has a strong propensity to aggregate even under diluted conditions with C10 peptides. Considering both of the lifetime data for DDD- and DVV-OT4 assemblies and co-assemblies, more diluted DDD-OT4 units in C10 matrix leads towards a more monomeric lifetime profile while DVV-OT4 co-assemblies maintain significant contribution from excimeric components that lead to longer lifetimes. In the case of OT4dM peptides, DDD-OT4dM co-assemblies still show a molecular lifetime behavior but the DVV-OT4dM co-assemblies maintain a shorter lifetime that can be better associated to a tighter exciton coupling within the nanostructures. Table 2. Fluorescence lifetimes of DDD- and DVV-OT4dM and solutions mixed with C10 peptides. (λexc= 375; λem= 520 nm for pH 10 and 540 nm for pH 1 samples). Full PL decay profiles are shown in Figures S8-S9. τ/ ns

mol% OT4dM

Dissolved

5% DDD9% DDD20% DDD100% DDD5% DVV9% DVV20% DVV100% DVV-

0.38 0.39 0.40 0.40 0.39 0.39 0.41 0.38

Co-assembled τ (%) 0.41 (100) 0.40 (100) 6.67 (0.5); 0.41 (99.5) 0.85 (26.4); 0.40 (73.6) 3.20 (0.9); 0.53 (99.1) 0.62 (52.4); 0.17 (47.6) -

τavg 0.41 0.40 0.44 0.52 0.55 0.41 -


Separately assembled τ (%) τavg 0.42 (100) 0.42 0.40 (100) 0.40 4.11(0.5); 0.41 (99.5) 0.41 6.83 (0.5); 0.35 (99.5) 0.38 0.41 (81.1); 0.88 (18.9) 0.50 0.24 (62.2); 0.69 (37.8) 0.41 0.50 (91.6); 1.04 (8.4) 0.54 0.44 (95.9); 1.7 (4.1) 0.49

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Combining the steady-state data with the lifetime measurements discussed above provides a broader insight on the torsional impact of a hydrophobic, electronically-inert environment to πelectron unit containing peptides discussed herein. The thiophene units in OT4 and OT4dM peptides with single bond connectivities between aromatic groups are seemingly more affected by the new solvation environment created by C10 peptides around the π-electron stacks via lateral interactions of the peptide stacks than mere deplanarization upon dilution. At room temperature (Figure 7), we also recorded the circular dichroism spectra of our peptides where we found that the exciton coupling between OT4/OT4dM π-electron units create a weak bisignate the Cotton effect and the signals in the higher energy region are primarily dominated by the ‘β-sheet’-like imprints due to the C10 peptides when co-assembled. These higher energy CD bands are consistent with the presence of β-sheet amide IR bands from lyophilized acidic samples (Figure S10). Similar to previous reports,22, 29 the peptides studied here have broad high energy minima which are redshifted by ~20 nm from an ideal β-sheet signals, indicating some twisting within the motif and some random coil character. The less sharp higher energy peaks for OT4/ OT4dM peptides as compared to the C10 peptides could be due to the diverse chiral electronic environments created by the polydisperse supramolecular peptide assemblies. With the exception of 5 mol% DDD-OT4 and -OT4dM, these chiral environments create a consistent direction of handedness for the chromophores even under diluted conditions as shown by the evident bisignate signal within the region of OT4/ OT4dM absorption. The absence of bisignate signal within the region of absorption of quaterthiophene units in both 5 mol% DDD-OT4 and -OT4dM solutions indicate minimal exciton coupling between the π-electron units. This is consistent with the observations from steady-state photophysical data and time-resolved PL data that these peptides have a strong preference towards an isolated behavior within C10 nanostructures—hence, weaker electronic


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communication between π-units. Interestingly, the 20 mol% DDD-OT4dM co-assembly also do not show a bisignate signal within the region of quaterthiophene absorption. Because the photophysical data discussed above shows that there is some extent of aggregation, the CD profile of 20 mol% DDD-OT4dM co-assemblies is more indicative that the C10 peptides do not constrain the OT4dM units in a perfectly chiral environment as its homoassembly equivalent.

Figure 7. Circular dichroism spectra of DDD- (a, b) and DVV- (c, d) peptides with OT4 (a, c) and OT4dM (b, d) core. The spectra for both homoassembled and co-assembled (5 and 20 mol%) solutions are shown. For the co-assemblies, [C10]= 33 µM was kept constant between dilutions. The concentration of OT4/ OT4dM is the same as that in the 20 mol% dilution (ca. 8 µM). All spectra were recorded at room temperature (ca. 25 °C).

To further investigate the dynamic changes in the π-π interactions of OT4 and OT4dM peptides, we recorded the absorption, PL and circular dichroism profiles of these peptides and their co-assemblies with C10 peptides at 5° and 60°C and compared them with the room-temperature


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data (Figure S11-S13). Under all these conditions, nanostructures still form as confirmed by TEM (Figure S2-S4). Although it is not surprising that photophysical properties at the temperature minimum (5°C) were not significantly different from the room temperature data, increasing the temperature of the peptide solutions enables thermal annealing that promotes internal chromophore and peptide reorganization and enhances the magnitude of the observed local chirality.42 This is shown by the particularly higher intensity bisignate CD signal for DDD-OT4dM, as well as the more resolved vibronic bands observed in the absorption and PL profiles of DVV-OT4dM assemblies and co-assemblies with a stronger bisignate CD signal at 60°C (Figure S13f, h). These observations verify that we can both utilize molecular design (peptide and chromophore functionalization) and temperature control to systematically modulate the stacking behavior, and thus electronic communication between π-systems even under physiologically-relevant aqueous environments.

CONCLUSIONS Rational control over the electronic properties of supramolecular assemblies, although often challenging, is key towards ultimately engineering a desired function in the resulting nanomaterials. Here, we studied how torsional constraints imposed on the conformation of central π-units in 1-D peptide-π-peptide nanoassemblies can affect their overall photophysical properties. In particular, peptides conjugated to π-electron units with single bond connectivities (OT4, OT4dM) between the aromatic units were used to investigate the torsional impacts of chromophore deplanarization on their photophysical properties. Two different peptide sequences with previously studied aggregation behavior (DDD-, which prefers isolation, and DVV-, which prefers aggregation), were each used to compare the photophysical properties of OT4 and OT4dM units


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confined within peptidic nanostructures. In both isolating DDD- and aggregating DVV- peptides, we observed that the twisted conformation of OT4dM core due to the methyl groups decreased the propensity of closely stacked π-electron units, therefore leading their acidic solutions to have photophysical properties that are less reminiscent of strong exciton coupling due to H-aggregation. This was the case even for OT4dM peptides that were geometrically constrained as co-assemblies with peptide-decyl-peptide molecules. The time-resolved PL data further supports the electronic perturbation brought by the methylation of quaterthiophene when homo-assembled or when coassembled with C10 peptides. By changing the thermal conditions of the solutions where the nanomaterial assemblies exist, OT4dM peptides were able to access conformations that allowed for better exciton coupling as exhibited in the spectral profiles of their assemblies and coassemblies. Taken together, we demonstrated that functional groups that present steric hindrance against a more efficient π-stacking can be used as a way to modulate the absorption and emission behavior of π-conjugated nanoassemblies. Additionally, the consistent observation regarding the preference of hydrophilic peptides towards an isolated behavior and of hydrophobic towards aggregated behavior when co-assembled with lipid-like peptides, similar to a previous report on OPV3-bearing peptides, showed how peptide sequence can be used to tune the co-assembly behavior of peptide nanostructures. Although peptide-centered hydrogen bonding networks and πelectron quadrupolar interactions dominate the assembly landscape, we show here how subtle torsional constraints can further tune electronic couplings and can therefore play a significant role in the overall optoelectronic properties of functional nanostructures when interfaced with aqueous biological environments.


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ACKNOWLEDGMENTS We thank Johns Hopkins University, and the National Science Foundation (previously from the DMR Biomaterials (1407493) and currently from DMR DMREF (1728947)) for financial support. H. A. M. A. thanks the generous support from Howard Hughes Medical Institute (International Student Research Fellowship) and Schlumberger Foundation (Faculty for the Future Fellowship). We also thank the Center for Molecular Biophysics (JHU) for the use of circular dichroism spectrometer.

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12. Kislitsyn, D. A.; Taber, B. N.; Gervasi, C. F.; Zhang, L.; Mannsfeld, S. C. B.; Prell, J. S.; Briseno, A. L.; Nazin, G. V., Oligothiophene wires: impact of torsional conformation on the electronic structure. Phys. Chem. Chem. Phys. 2016, 18 (6), 4842-4849. 13. Darling, S. B.; Sternberg, M., Importance of Side Chains and Backbone Length in Defect Modeling of Poly(3-alkylthiophenes). J. Phys. Chem. B 2009, 113 (18), 6215-6218. 14. Darling, S. B., Isolating the Effect of Torsional Defects on Mobility and Band Gap in Conjugated Polymers. J. Phys. Chem. B 2008, 112 (30), 8891-8895. 15. Vujanovich, E. C.; Bloom, J. W. G.; Wheeler, S. E., Impact of Neighboring Chains on Torsional Defects in Oligothiophenes. J. Phys. Chem. A 2012, 116 (11), 2997-3003. 16. Kim, S. H.; Parquette, J. R., A model for the controlled assembly of semiconductor peptides. Nanoscale 2012, 4 (22), 6940-6947. 17. Jatsch, A.; Schillinger, E. K.; Schmid, S.; Bauerle, P., Biomolecule assisted self-assembly of π-conjugated oligomers. J. Mater. Chem. 2010, 20 (18), 3563-3578. 18. Tovar, J. D., Supramolecular Construction of Optoelectronic Biomaterials. Acc. Chem. Res. 2013, 46 (7), 1527-1537. 19. Schillinger, E.-K.; Mena-Osteritz, E.; Hentschel, J.; Börner, H. G.; Bäuerle, P., Oligothiophene Versus β-Sheet Peptide: Synthesis and Self-Assembly of an Organic Semiconductor-Peptide Hybrid. Adv. Mater. 2009, 21, 1562–1567. 20. Stone, D. A.; Hsu, L.; Stupp, S. I., Self-Assembling Quinquethiophene-Oligopeptide Hydrogelators. Soft Matter 2009, 5, 1990-1993. 21. Liyanage, W.; Ardona, H. A.; Mao, H. Q.; Tovar, J. D., Cross-Linking Approaches to Tuning the Mechanical Properties of Peptide pi-Electron Hydrogels. Bioconjug Chem 2017, 28 (3), 751-759. 22. Ardona, H. A. M.; Kale, T. S.; Ertel, A.; Tovar, J. D., Nonresonant and Local Field Effects in Peptidic Nanostructures Bearing Oligo(p-phenylenevinylene) Units. Langmuir 2017, 33 (30), 7435-7445. 23. Besar, K.; Ardoña, H. A. M.; Tovar, J. D.; Katz, H. E., Demonstration of Hole Transport and Voltage Equilibration in Self-Assembled π-Conjugated Peptide Nanostructures Using FieldEffect Transistor Architectures. ACS Nano 2015, 9 (12), 12401-12409. 24. Sanders, A. M.; Dawidczyk, T. J.; Katz, H. E.; Tovar, J. D., Peptide-Based Supramolecular Semiconductor Nanomaterials via Pd-Catalyzed Solid-Phase “Dimerizations”. ACS Macro Lett. 2012, 1 (11), 1326-1329. 25. Wall, B. D.; Diegelmann, S. R.; Zhang, S.; Dawidczyk, T. J.; Wilson, W. L.; Katz, H. E.; Mao, H.-Q.; Tovar, J. D., Aligned Macroscopic Domains of Optoelectronic Nanostructures Prepared via Shear-Flow Assembly of Peptide Hydrogels. Adv. Mater. 2011, 23, 5009-5014. 26. Zhou, Y.; Li, B.; Li, S.; Ardona, H. A. M.; Wilson, W. L.; Tovar, J. D.; Schroeder, C. M., Concentration-Driven Assembly and Sol-Gel Transition of pi-Conjugated Oligopeptides. ACS Cent Sci 2017, 3 (9), 986-994. 27. Kim, C. A.; Berg, J. M., Thermodynamic β -sheet propensities measured using a zincfinger host peptide. Nature 1993, 362 (6417), 267-270. 28. Vadehra, G. S.; Wall, B. D.; Diegelmann, S. R.; Tovar, J. D., On-resin Dimerization Incorporates a Diverse Array of pi-conjugated Functionalities within Aqueous Self-Assembling Peptide Backbones. Chem. Commun. 2010, 46, 3947-3949. 29. Ardoña, H. A. M.; Besar, K.; Togninalli, M.; Katz, H. E.; Tovar, J. D., Sequence-dependent mechanical, photophysical and electrical properties of pi-conjugated peptide hydrogelators. Journal of Materials Chemistry C 2015, 3 (25), 6505-6514.


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30. Wall, B. D.; Zacca, A. E.; Sanders, A. M.; Wilson, W. L.; Ferguson, A. L.; Tovar, J. D., Supramolecular polymorphism: tunable electronic interactions within pi-conjugated peptide nanostructures dictated by primary amino acid sequence. Langmuir 2014, 30 (20), 5946-56. 31. Safont-Sempere, M. M.; Fernández, G.; Würthner, F., Self-Sorting Phenomena in Complex Supramolecular Systems. Chem. Rev. 2011, 111 (9), 5784-5814. 32. Ghadiri, M. R.; Granja, J. R.; Milligan, R. A.; McRee, D. E.; Khazanovich, N., Selfassembling Organic Nanotubes Based on a Cyclic Peptide Architecture. Nature 1993, 366 (6453), 324-327. 33. Morris, K. L.; Chen, L.; Raeburn, J.; Sellick, O. R.; Cotanda, P.; Paul, A.; Griffiths, P. C.; King, S. M.; O’Reilly, R. K.; Serpell, L. C.; Adams, D. J., Chemically programmed self-sorting of gelator networks. Nat. Commun. 2013, 4, 1480. 34. Görl, D.; Zhang, X.; Stepanenko, V.; Würthner, F., Supramolecular block copolymers by kinetically controlled co-self-assembly of planar and core-twisted perylene bisimides. Nat. Commun. 2015, 6, 7009. 35. Weissman, H.; Rybtchinski, B., Noncovalent self-assembly in aqueous medium: Mechanistic insights from time-resolved cryogenic electron microscopy. Curr. Opin. Colloid Interface Sci. 2012, 17 (6), 330-342. 36. Rest, C.; Mayoral, M. J.; Fernandez, G., Aqueous Self-Sorting in Extended Supramolecular Aggregates. Int. J. Mol. Sci. 2013, 14, 1541-1565. 37. Nijegorodov, N. I.; Downey, W. S., The Influence of Planarity and Rigidity on the Absorption and Fluorescence Parameters and Intersystem Crossing Rate Constant in Aromatic Molecules. J. Phys. Chem. 1994, 98 (22), 5639-5643. 38. DiCésare, N.; Belletête, M.; Garcia, E. R.; Leclerc, M.; Durocher, G., Intermolecular Interactions in Conjugated Oligothiophenes. 3. Optical and Photophysical Properties of Quaterthiophene and Substituted Quaterthiophenes in Various Environments. J. Phys. Chem. A 1999, 103 (20), 3864-3875. 39. Doval, D.; Matile, S., Increasingly twisted push-pull oligothiophenes and their planarization in confined space. Org. Biomol. Chem. 2013, 11 (43), 7467-7471. 40. Ardoña, H. A. M.; Besar, K.; Togninalli, M.; Katz, H. E.; Tovar, J. D., Sequence-dependent mechanical, photophysical and electrical properties of pi-conjugated peptide hydrogelators. J. Mater. Chem. C 2015, 3 (25), 6505-6514. 41. Wall, B. D.; Zhou, Y.; Mei, S.; Ardona, H. A.; Ferguson, A. L.; Tovar, J. D., Variation of formal hydrogen-bonding networks within electronically delocalized pi-conjugated oligopeptide nanostructures. Langmuir 2014, 30 (38), 11375-85. 42. Ardona, H. A. M.; Tovar, J. D., Energy transfer within responsive pi-conjugated coassembled peptide-based nanostructures in aqueous environments. Chem Sci 2015, 6 (2), 14741484.


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Torsional Impacts on Quaterthiophene Segments Confined within

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