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A Strategy for the Co-Assembly of Co-Axial Nanotube-Polymer Hybrids. Mingyang Ji, Mahesh B Dawadi, Alexandria R LaSalla, Yuan Sun, David A Modarelli, and Jon Robert Parquette Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02245 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 18, 2017

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A Strategy for the Co-Assembly of Co-Axial Nanotube-Polymer Hybrids Mingyang Ji,† Mahesh B. Dawadi,‡ Alexandria R. LaSalla,† Yuan Sun,† David A. Modarelli,‡* and Jon R. Parquette†* †

Department of Chemistry, The Ohio State University, 100 W. 18th Ave. Columbus, Ohio 43210

‡

Department of Chemistry and The Center for Laser and Optical Spectroscopy, Knight Chemical

Laboratory, The University of Akron, Akron, Ohio 44325-3601

ACS Paragon Plus Environment

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ABSTRACT: Nanostructured materials having multiple, discrete domains of sorted components are particularly important to create efficient optoelectronics. The construction of multicomponent nanostructures from self-assembled components is exceptionally challenging due to the propensity of noncovalent materials to undergo structural reorganization in the presence of excipient polymers. This work demonstrates that polymer-nanotube composites comprised of a self-assembled nanotube wrapped with two conjugated polymers could be assembled using a layer-by-layer approach. The polymer-nanotube nanostructures arrange polymer layers coaxially on the nanotube surface. Femtosecond transient absorption (TA) studies indicated that the polymer-nanotube composites undergo photoinduced charge separation upon excitation of the NDI chromophore within the nanotube.

INTRODUCTION In nature, biological function emerges from the interplay of the ordered arrangement of multiple functional components. Self-assembly offers a convenient, albeit often empirical, strategy to fabricate functional materials with nanoscale order.1 Nanostructured materials having multiple, discrete domains of sorted components are particularly important to create efficient optoelectronics. For these applications, segregated, bicontinuous arrays of donor and acceptor chromophores are needed to optimize charge separation and transport, and are generally difficult to create with precision.2-17 Whereas the design of single-component nanomaterials has been well-established, the self-assembly of composite structures from multiple, discrete building blocks is especially difficult due to the tendency of multi-component systems to self-sort.18-21 Strategies to create multi-functional nanostructures have relied on covalent22-25 or noncovalent10 pre-linkage of separate components within a single building block. This approach is limited by


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the need for multi-step synthetic approaches required to achieve the desired monomer, which when combined with the heuristic nature of nanostructure design, results in relatively slow progress. Alternatively, the co-assembly of separate functional components together into one hierarchically ordered nanostructure represents a nearly ideal strategy.26-27 However, the tendency of these processes to experience molecular self-sorting and pathway complexity,28-30 resulting from the interplay of kinetic and thermodynamic effects,31 greatly decreases the success of de novo designs, and severely hampers efforts to create functional systems by this strategy. Carbon nanotubes (CNTs) are often coated with polymers to enhance solubility, prevent bundling, and to impart specific electronic and mechanical properties.32-36 However, in contrast to solid CNTs, creating polymer-nanostructure composites from self-assembled nanostructures is hampered by the potential for structural reorganization in the presence of the excipient polymer.37 The self-assembly of covalently linked polymer-peptide conjugates has been shown to afford polymer-nanotube composites,38-40 but this strategy hinges on the ability of the modified peptide monomer to assemble into the desired nanostructure. Recently, the assembly of a hexabenzocoronene amphiphile into multiwall nanotubes was postulated to occur via a stepwise mechanism involving “coil-on-tube” intermediates, wherein radial growth takes place by progressive wrapping of a fully formed nanotube by coiled ribbons.41 These “coil-on-tube” intermediates imply the potential to co-assemble hybrid nanotubes via a sequential process using separate components. Thus, we became interested in assembling multicomponent composites from self-assembled nanotubes using a layer-by-layer approach.42-43 Herein, we describe a strategy42-43 to assemble multilayer polymer-nanotube composites by layer-by-layer polymer deposition onto a self-assembled nanotube (Fig.1).


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EXPERIMENTAL SECTION Preparation of Polymer-Nanotube Composites. Freeze-dried NDI-Bola (5.5 mg) was added to HPLC-grade water (500 µL), and the mixture was sonicated until the solid dissolved. After incubating the NDI-Bola solution at 20 mM for 24 h, an aqueous solution of PCT-SO3Na (10 mM, based on repeat unit) was added to NDI-Bola (20 mM) in molar ratios (NDI-Bola/PCTSO3Na) of 10:1, 10:2 and 10:4, resulting in the immediate formation of self-supporting hydrogels. After incubation at 20°C for 24 h, the NDI-Bola/PCT-SO3Na hydrogels were diluted with water (4.5 mL) and centrifuged at 5000 rpm for 15 min. The resultant supernatants were decanted, and the pellet of the NDI-Bola/PCT-SO3Na composite was collected. The pellets were then re-dispersed in water (2 mL) and then treated with PFQ-Br (10 mM) to prepare NDIBola/PCT-SO3Na/PFQ-Br (10:2:2 and 10:2:4) composites. The resultant composites were dispersed in water (4.5 mL), then isolated after pelleting by centrifugation (5000 rpm) RESULTS AND DISCUSSION Previously, we demonstrated that a naphthalenediimide-lysine bolaamphiphile (NDI-Bola) assembled in water via the initial formation of monolayer rings that progressively stacked into well-defined nanotubes with diameters of 12±1 nm.44-45 Surfactants and ionic polymers are wellknown to co-assemble into a wide variety of structures that are often difficult to predict a priori.46 A combination of electrostatic interactions, hydrophobic effects and entropic changes due to counterion release lead to strong association, and occasional phase separation, between the polymer and surfactant.47 Given the well-defined structure of the NDI-Bola nanotubes,45 we reasoned that an oppositely charged, water-soluble polymer might bind to its surface via similar effects. This approach relies on a preformed NDI-Bola nanotube to interact more strongly with the polyelectrolyte than the NDI-Bola monomer, based on the potential for a multivalent


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interaction between the two components. Accordingly, dissolving NDI-Bola (pI 7.6, Fig. S1) in pure water (20 mM) produced a transparent solution with a pH of 6.7. To accommodate the positively charged surface of the nanotube at this pH, we explored the binding of a negatively charged, p-conjugated polymer, poly[2,6-(4,4-bis-sodium butanylsulfonate-4H-cyclopenta-[2,1b;3,4-b’]-dithiophene) alt-1,4-phenylene] (PCT-SO3Na, (MW = 7800, PDI = 1.08).48 The NDIBola nanotube, pre-assembled into nanotubes (20 mM, 24 h) and pelleted by ultracentrifugation, was treated with the polymer in molar ratios (NDI-Bola/PCT-SO3Na) of 10:1, 10:2 and 10:4, resulting in immediate formation of transparent hydrogels. After 24 h, the hydrogels were centrifuged and the pellet and supernatant were analyzed by UV, TEM and AFM (Figs. 2 and 3). Evidence for the co-assembly of the nanotube and the polymer in the centrifuged pellets obtained from NDI-Bola/PCT-SO3Na was apparent in the UV-Vis spectra (Fig. 2a). In TFE, the monomeric form of NDI-Bola exhibited peaks at 342, 359, 380 (Band I) and 234 nm (Band II). Intermolecular NDI p-p stacking within the NDI-Bola nanotubes in water produces characteristic red-shifts in both bands, due to J-type packing.44, 49 UV-Vis spectra of NDIBola/PCT-SO3Na (10:2) pellets dissolved in water showed red-shifting of Band I (9 nm) and Band II (15 nm), indicative of NDI- Bola self-assembly, as well as a broad absorption in the 420–600 nm region resulting from PCT-SO3Na. In contrast, the spectrum of the supernatant displayed low intensity peaks that were characteristic of the monomolecular form of the NDIBola, as well as a slightly blue-shifted absorption attributed to PCT-SO3Na (Fig. 4). Under these centrifuge conditions (5000 rpm), neither the isolated polymer nor the NDI-Bola monomer forms a pellet. It is noteworthy that these conditions also do not pellet the NDI-Bola nanotubes. Thus, the majority of the polymer forms a complex with the NDI-Bola nanotubes, leaving only trace amounts of NDI-Bola monomer and PCT-SO3Na in the supernatant. The 10:1 NDI-Bola/PCT-


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SO3Na resulted in a nearly flat absorption of the supernatant; whereas, the 10:4 ratio produced more monomer and polymer in supernatant, indicating saturation of the nanotube surface with the polymer. The appearance of small amounts of unassembled NDI-Bola monomer in the presence of excess polyelectrolyte may be attributed to a shift in the monomer-nanotube equilibrium induced by a change in the ionic strength of the solution and the potential for interactions between NDI-Bola monomer with unbound polyelectrolyte. Transmission electron microscopy (TEM) imaging of the NDI-Bola/PCT-SO3Na pellet (10:2, n/n) revealed a homogeneous array of uniform nanotubes showing a 6 nm increase in diameter from 12 ± 1 nm, for the parent NDI-Bola nanotube, to 18 ± 1 nm for the polymer-nanotube composite (Figs. 3a/c). The increase in diameter emerged primarily from the larger dimensions of the polymer-nanotube walls, which increased from 2.5 ± 0.5 (NDI-Bola) to 5.5 ± 0.5 nm. The inner diameter of the nanotube maintained the initial dimensions (~7 nm). Under these conditions, the nanotube was uniformly coated by the polymer, resulting in a remarkably smooth exterior surface. Tapping-mode AFM imaging of NDI-Bola/PCT-SO3Na (10:2, n/n) on mica also indicated larger cross-sectional heights (~15 nm), compared with NDI-Bola nanotubes (~9 nm) (Fig. 3b, d). Cross-sectional heights measured by AFM are typically slightly smaller than those determined by TEM, due to the compression of the nanotubes during imaging. TEM imaging of the 10:1 and 10:4 (NDI-Bola/PCT-SO3Na) composites revealed micrometer-long nanotube structures (Fig. S9, S10). However, whereas the 10:1 ratio produced nanotubes with smaller diameters (15 ± 1 nm), the 10:4 ratio afforded nanotubes with the same dimensions as the 10:2 ratio (18 ± 1 nm), consistent with the UV-Vis spectra indicating that the NDI-Bola surface was saturated with the polymer at this ratio. Directly mixing the NDI-Bola monomer, without preforming the nanotube, with PCT-SO3Na resulted in the formation of random aggregates of


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short rods (Fig. S12). This observation confirms the necessity of pre-assembling the NDI-Bola nanotubes prior to introduction of the polyelectrolyte to circumvent strong monomer-polymer interactions that divert the co-assembly process. EDX (energy-dispersive X-ray spectroscopy) of an unstained 10:2 sample provided further direct evidence that the PCT-SO3Na polymer was present on the surface of the nanotubes (Fig. S11).50 Notably, in Figure S11b, two characteristic peaks were detected at 0.38 and 2.31 eV within nanotubes, and were assignable to the nitrogen and sulfur, respectively. The peak at 2.31 eV arose from sulfate groups of PCT-SO3Na. To explore the potential to create a coaxial, multilayer structure, a positively charged polymer, poly (9,9-bis-[6’- (N,N,N,-trimethylammonium) hexyl] fluorene-co-alt-1,4-phenylene) bromide (PFQ-Br, (MW = 100400, PDI = 2.1)) was added as a second polymer layer.51 We reasoned that electrostatic interactions between each layer would partially drive binding on each surface. Accordingly, adding PFQ-Br directly to the parent NDI-Bola nanotube, which was positively charged at pH 6.7, did not result in any apparent coating of the nanotube by UV and TEM analysis (Fig. S13). In contrast, treating the NDI-Bola/PCT-SO3Na construct, which putatively presented a negatively charged surface due to the PCT-SO3Na layer, resulted in the formation of a second polymer layer. Compared with the UV spectrum of NDI-Bola/PCT-SO3Na, NDIBola/PCT-SO3Na/PFQ-Br (10:2:2), pelleted by centrifugation, displayed an increase in the band intensity between 300-400 nm resulting from overlap of the PFQ-Br and NDI bands (Figs. 2a, S7). TEM imaging of the NDI-Bola/PCT-SO3Na/PFQ-Br (10:2:2) composite revealed a uniformly coated nanotube with a further increase in diameter to 21 ± 1 nm (Fig. 3e), and an increase of the AFM cross-sectional height to 17.5 nm (Fig. 3f). The increase in diameter is attributed to a thickening of the nanotube wall from 5.5 nm (for NDI-Bola/PCT-SO3Na) to 7 nm. Quantification


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of the intensity of the dark-field images obtained by tilted-beam transmission electron microscopy (TB-TEM) provided mass-per-length (MPL) values consistent with the increase in mass expected at each stage of the nanotube polymer assembly (Figs. 5, S15-17).52 Using the tobacco mosaic virus as an internal mass calibration standard, MPL values were determined to be 176.8, 297.7 and 350.2 kDa/nm for the NDI-Bola, NDI-Bola/PCT-SO3Na (10:2) and the NDIBola/PCT-SO3Na/PFQ-Br (10:2:2) nanotubes, respectively. It is worthy to note that when the NDI-Bola:polymer ratio exceeded 10:2 for NDI-Bola/PCT-SO3Na or 10:2:2 for NDI-Bola/PCTSO3Na/PFQ-Br, the polymer-nanotube diameters did not increase, but increased bundling of the nanotubes was observed (Fig. S14c). The increased aggregation of the composite nanotubes was likely induced by the presence of partially bound and unbound polymer strands, which enhance the potential for inter-nanotube interactions. The TEM images in Figure 3 revealed the NDI-Bola and NDI-Bola/PCT-SO3Na nanotubes as two white, parallel lines separated by a dark center, due to the hollow tubular interior, which was filled with the negative stain, uranyl acetate. The dimensions of the nanotube interior remained constant (~ 7 nm) for the NDI-Bola nanotube and both polymer-nanotube conjugates, consistent with selective polymer binding on the outer wall surface. Although this central, internal channel was also visible for NDI-Bola/PCT-SO3Na/PFQ-Br, additional smaller channels arose in this structure on either side of the nanotube center (Fig. 3e, inset). The channels were separated by a distance of ~12 nm, suggesting the occurrence of a microphase separation between the NDI-Bola nanotube surface and the two polymer coatings. The presence of these channels may be attributed to the competition between polymer-polymer association and their interaction with the nanotube surface, similar to the behavior of polymer-surfactant mixtures on surfaces.53 Assuming a slight excess of PCT-SO3Na, the first coating would create a slightly negative surface, which


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would then interact with the positively charged PFQ-Br polymer to form the second layer.54 In the absence of salts, oppositely charged polymers strongly associate with a tendency for phaseseparation.47, 55 Thus, the small channels may emerge from a reorganization of the electrostatic interactions that favor binding to the NDI-nanotube surface toward a stronger polymer-polymer interaction, resulting in partial desorption from the nanotube surface. The estimated lateral dimensions of an extended PCT-SO3Na (~12 Å, Fig. S5) monomer would be consistent with multiple layers (~3) of the polymer being deposited on the nanotube surface to achieve a 3 nm increase in wall thickness upon binding PCT-SO3Na. In contrast, addition of the PFQ-Br second layer afforded a 1.5 nm increase in wall thickness. Whereas the thickness of each layer in layerby-layer deposition processes often grows linearly for strong polyelectrolytes, weaker electrostatic interactions may lead to an exponential increase in thickness as layers are added.56-57 This exponential increase in incremental thickness emanates from the formation of a weaker electrostatic complex between layers that leads to diffuse association of the polymer with the multilayer assembly. Computational simulations have indicated that increasing polymer length and stiffness reduces the strength of polymer-carbon nanotube interactions, suggesting that the longer length of PFQ-Br would result in weaker binding to NDI-Bola nanotube surface, compared with PCT-SO3Na.58-59 However, given the identical molar composition of each polymer layer (10:2:2 NDI:PCT-SO3Na:PFQ-Br) and the lateral dimensions of the PFQ-Br monomer (~14 Å), the lower incremental increase in wall thickness upon adding the second polymer layer likely emerges from the stronger electrostatic interaction of PFQ-Br with PCTSO3Na than the corresponding interaction of PCT-SO3Na with the NDI nanotube. Closely spaced and spatially constrained chromophores, such as those contained within NDIBola nanotubes, have previously been shown to form delocalized excited states capable of


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undergoing rapid energy migration.60 The emission spectra of NDI-Bola in water is characterized by a relatively strong and concentration-dependent low energy emission band at ~450-525 nm (Figs. 2b, S8). In contrast, only the molecularly dissolved NDI-Bola is observed in TFE (lem 388 and 410 nm). The addition of low concentrations of PCT-SO3Na to the NDI-Bola nanotube (i.e., in a 10:1 or 10:2 ratio of NDI-Bola/PCT-SO3Na) completely quenches the low energy NDI-Bola nanotube emission band, leaving only the weak monomer NDI-Bola (388 and 410 nm) and polymer (~520 nm) emission bands. At a 10:4 ratio of NDI-Bola/PCT-SO3Na, a new low-energy emission band attributed to an aggregated polymer is observed at ~575 nm, and the polymer band at 520 nm disappears (Fig. S8). The quenching of the NDI-Bola emission, taken with the TEM and AFM results, suggests formation of a co-axially aligned NDI-Bola/PCT-SO3Na bilayer that undergoes either photoinduced electron-transfer (to form NDI-Bola-./PCT+.) or energytransfer (to form NDI-Bola/PCT*). Electron-transfer is calculated to be exergonic by ca. -2.17 V for NDI-Bola excitation, and by ca. -1.10 V for PCT-SO3Na excitation (see SI for details). PCTSO3Na has a non-negligible absorption in the 330–400 nm range, and the small emission observed in these emission experiments is likely the result of direct excitation of the polymer that is not in direct contact with the nanotube. Femtosecond TA experiments on an aqueous solution of PCT-SO3Na with 400 nm excitation resulted in a biexponential decay at the PCT stimulated emission at ~530 nm (tem ~870 fs (61%) and ~24 ps (39%)) (Fig. S22). Similar excitation of an aqueous solution of the NDI-Bola nanotubes, also at 400 nm, resulted in the appearance of absorption bands at ~460, 490 and 630 nm, all of which are characteristic of the NDI-Bola delocalized excited state23 and which were fit to biexponential decays with short-lived (~680 fs, 77%) and longer-lived (~195 ps, 23%) lifetime components (Figs. S23, S24). Excitation of the NDI-Bola/PCT-SO3Na layered nanotube at 400


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nm resulted in the formation within the laser pulse of an absorption band at ~460 nm and an emission at ~530 nm (Fig. 6), corresponding to the NDI-Bola excited state absorption and the PCT-SO3Na stimulated emission, respectively.61 The PCT-SO3Na stimulated emission band decayed with a rate constant (~370 fs (70%) and ~20 ps (30%)) that was nearly identical to the polymer by itself (Figs. S21, S22). The NDI-Bola 460 nm band, on the other hand, decayed with tNDI* ~0.90 ps and was simultaneously replaced by new absorption bands at 580 and 620 nm (Fig. S20).62 Both bands subsequently decayed with lifetimes of t ~20 ps. The absorption band at 620 nm has been shown to result from the NDI-Bola radical anion,23 while the 580 nm band has been attributed to the PCT-SO3Na radical cation.63 These results suggest electron-transfer in the coaxial NDI-Bola/PCT-SO3Na nanotube occurs on the ultrafast time-scale from the PCT-SO3Na polymer to the electronically excited NDI-Bola, forming the charge-separated coaxial nanotube (i.e., NDI-Bola.-/PCT-SO3Na.+) with a rate constant of kET ~1 ´ 1012 s-1. Charge-recombination subsequently occurs with kCR ~5 ´ 1010 s-1. Energy transfer in this experiment was ruled out since the emission of PCT-SO3Na* with NDI-Bola excitation was identical to that of the polymer alone.

CONCLUSION Although self-assembly offers an efficient strategy to create multicomponent arrays of segregated chromophores, it is often exceptionally challenging to create discrete nanomaterials comprised of precisely positioned components using noncovalent methods. This work demonstrates that multidomain charge-separating composites can be assembled using a layer-bylayer strategy to uniformly coat the surface of a self-assembled nanotube with a series of conjugated polymers. The resulting polymer-nanotube nanostructures arrange the donor and


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acceptor chromophores in layered arrays. Femtosecond TA experiments indicate that excitation of the NDI chromophore of the NDI-Bola nanotube induces PCTàNDI electron-transfer with kET/kCR ratios that are comparable to nanotubes formed by bolaamphiphiles containing covalently linked NDI and porphyrin chromophores.23,

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The self-assembly of multicomponent

nanomaterials using this strategy may enable a diverse range of applications that require multiple components for optimal function.


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Figure 1. Structures of NDI-Bola nanotubes, conjugated polyelectrolytes PCT-SO3Na and PFQBr. NDI-Bola nanotubes assembled via the progressive stacking of initially formed NDI-Bola monolayer rings. The NDI-Bola nanotubes were then sequentially wrapped with two oppositely charged polyelectrolytes, PCT-SO3Na and PFQ-Br, to give NDI-Bola/PCT-SO3Na and NDIBola/PCT-SO3Na/PFQ-Br nanotubes.


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Figure 2. (a) UV-Vis spectra of NDI-Bola in TFE (dashed black) and water (solid black), NDIBola/PCT-SO3Na (10:2) in water (red), and NDI-Bola/PCT-SO3Na/PFQ-Br (10:2:2) in water (blue). The concentration of NDI-Bola in all samples was 250 μM. (b) Fluorescence spectra of NDI-Bola in TFE (dashed black) and water (solid black), NDI-Bola/PCT-SO3Na (10:2) in water (red), and NDI-Bola/PCT-SO3Na/PFQ-Br (10:2:2) in water (blue). Except for NDI-Bola in TFE (100 μM), the concentration of NDI-Bola in all samples was 2 mM. All samples were excited at 330 nm.


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Figure 3. TEM and tapping-mode AFM images of (a, b) NDI-Bola, (c, d) NDI-Bola/PCT-SO3Na (10:2, n/n), and (e, f) NDI-Bola/PCT-SO3Na/PFQ-Br (10:2:2, n/n/n) in water (carbon-coated copper grid). The concentration of NDI-Bola in all samples was 2 mM. TEM insets: Highresolution images of individual nanotubes. Parts of the additional smaller channels in NDIBola/PCT-SO3Na/PFQ-Br (10:2:2, n/n/n) are labeled in cyan. AFM insets: Section analysis showing uniform heights of corresponding nanotubes. NDI-Bola pre-assembled into nanotubes within 24 h. After treatment with polymers and incubating for 24 h, the resulting polymernanotube composites were centrifuged (5000 rpm) and redispersed in water for TEM and AFM tests.


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Figure 4. UV-Vis spectra of NDI-Bola/PCT-SO3Na (10:1), (10:2), and (10:4). For comparison, NDI-Bola (250 μM, dashed black) and supernatants (black) of corresponding NDI-Bola/PCTSO3Na composites after centrifugation (5000 rpm for 15 min) are shown in each figure. Compared with supernatants, the absorption spectrum of corresponding NDI-Bola/PCT-SO3Na composite showed redshifts in Band I (389 nm) and Band II (249 nm), together with the broad absorption between 420-600 nm due to PCT-SO3Na. These redshifts indicate centrifugation separates monomolecular NDI-Bola from polymer-nanotube hybrids.


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Figure 5. MPL histograms extracted from TB-TEM images and solid curves fit to Gaussian functions. NDI-Bola nanotubes fit one Gaussian peak at 176.8 kDa/nm, with 33.0 kDa/nm full width at half maximum (FWHM). NDI-Bola/PCT-SO3Na (10:2) nanotubes fit one Gaussian peak at 297.7 kDa/nm, with 44.7 kDa/nm FWHM. NDI-Bola/PCT-SO3Na/PFQ-Br (10:2:2) nanotubes fit one Gaussian peak at 350.2 kDa/nm, with 56.3 kDa/nm FWHM.


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Figure 6. Transient absorption spectra of an aqueous solution of NDI-Bola/PCT-SO3Na (10:2) at various time-delays following a 40 fs excitation pulse.


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ASSOCIATED CONTENT Supporting Information The following files are available free of charge. TEM, AFM, UV, emission, MPL, elemental analysis STEM and NMR data of NDI-Bola and NDI-Bola/polymer composites (PDF) AUTHOR INFORMATION Corresponding Author *Jon R. Parquette, Department of Chemistry, The Ohio State University, 100 W. 18th Ave. Columbus, Ohio 43210. Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by the National Science Foundation (CHE-1412295 and CHE1412362) and, in part, based upon work supported by the U. S. Army Research Laboratory and the U. S. Army Research Office under contract/grant number W911NF1410305. ACKNOWLEDGMENT We acknowledge the technical assistance and usage of the AFM facility at the Ohio State Surface Analysis Lab; and the Ohio State Center for Chemical and Biophysical Dynamics (CCBD) for their assistance with the transient absorption experiments. The authors also thank Dr. Jojo Joseph with help with the femtosecond experiments and Prof. Elvin Alemán,


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Guadalupe Calvillo and Christopher Reed from California State University, Stanislaus for performing the CV measurements used in this work. REFERENCES 1.

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