Research ArticleCRYSTALLIZATION

A π-gel scaffold for assembling fullerene to photoconducting supramolecular rods

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Science Advances  23 Sep 2016:
Vol. 2, no. 9, e1600142
DOI: 10.1126/sciadv.1600142

Abstract

Nonequilibrium self-assembly of molecules holds a huge prospect as a tool for obtaining new-generation materials for future applications. Crystallization of neutral molecules within a supramolecular gel matrix is one example in which two nonequilibrium processes occur orthogonal to each other. On the other hand, electronically interacting donor-acceptor two-component systems are expected to form phase-miscible hybrid systems. Contrary to the expectation, we report the behavior of a π-gel, derived from oligo(p-phenylenevinylene), OPVA, as a scaffold for the phase separation and crystallization of fullerene (C60) to supramolecular rods with increased transient photoconductivity (φƩμmax = 2.4 × 10−4 cm2 V−1 s−1). The C60 supramolecular rods in the π-gel medium exhibited high photocurrent in comparison to C60 loaded in a non–π-gel medium. This finding provides an opportunity for large-scale preparation of micrometer-sized photoconducting rods of fullerenes for device application.

Keywords
  • Self-assembly
  • fullerene
  • gels
  • crystallization
  • p-n heterojunctions
  • self-sorting
  • photoconductivity

INTRODUCTION

Systems produced by equilibrium self-assembly represent a state of energetic minima and persist for a long time due to their thermodynamic stability. These systems are inert in working on their surroundings and require an external energy source to undergo a structural transformation (1, 2). On the other hand, self-assembly of molecules under nonequilibrium conditions can lead to complex architectures with fascinatingly dynamic and adaptive properties (3, 4). Both crystallization and gelation of organic molecules are known to occur under conditions that are far from equilibrium (1, 2). Specifically, gelation follows a nucleation growth mechanism driven by the kinetically controlled supersaturation gradient reasonably similar to the crystallization process (1, 5). The nucleation process leads to the formation of one-dimensional (1D) structures with high aspect ratio, which further assemble into a 3D network, better known as self-assembled fibrillar networks that encapsulate the solvent in which they form, resulting in gelation (6, 7). Gel phase crystallization has therefore drawn incredible attention to gain essential understanding of two simultaneously occurring orthogonal self-assembly processes (1, 8). Supramolecular gel fibers can even act as active nucleation substrates for the crystallization of organic molecules (9, 10). Weak and flexible behavior of supramolecular assembly, especially in low–molecular weight gelators, helps easy recovery of crystals from the gel matrix. Depending on the degree of interaction between the gelator and the crystallizing unit, mesoscopic compartmentalization through self-sorting or intimately mixed nanoscale phase-segregated heteroassembly can be the two possible extreme ends in these multicomponent systems (11). In systems that have a weak interaction between the two principal components, these nonequilibrium processes influence or modify the outcome of the assemblies to undergo nano- to mesoscale compartmentalization without losing the distinct identity of the individual components.

If such a hybrid assembly is composed of an electron donor and acceptor, it can give rise to bulk heterojunctions at varied length (nano- to meso-) scales, which are important for efficient exciton migration toward respective electrodes in an organic optoelectronic device (12, 13). Among various electron-rich π-conjugated small molecular gelators, oligo(p-phenylenevinylene)s (OPVs) occupy a unique position because of their excellent optoelectronic properties (1416). Proper functionalization of OPV molecules results in splendid supramolecular nanostructures that can function as scaffolds for exciton diffusion and energy transfer (15). The quest for new molecules and materials has also drawn the attention of material chemists toward the interaction of electron-deficient molecules, such as fullerenes (C60 and C70), carbon nanotubes, and graphene, with OPVs (1520). These studies have resulted in new hybrid materials of nanoscale to mesoscale morphology having donor-acceptor heterojunctions, which play a major role in the development of a variety of organic electronic devices (2124). In this context, fullerenes have been playing a major role as electron-accepting materials. Depending on the donor medium and experimental conditions, fullerenes tend to aggregate differently to generate nanostructures of diverse shape and dimensions (2529). All these fullerene nanostructures retain their inherent optoelectronic properties, hence having significant fundamental applications as those of solar cells, field effect transistors, and superconducting materials (26, 2932). Therefore, in the search for new hybrid materials, fullerenes and π-conjugated systems remain as an attractive choice for scientists (3337). π-Conjugated systems, particularly π-gels, have a strong tendency of interacting with C60 (14). Interaction of C60 and C60 derivatives with gels and other media is known in the literature (22, 3844). For example, crystallization of C60 in bis-urea–based gels forms C60 rods (40). However, the electronic properties of such hybrid gels are not explored. Similarly, the interaction of a π-system with a C60 derivative functionalized with an imidazole moiety was reported (38). Note that functionalized C60 is inferior with respect to electronic properties. In contrast, crystallization of pristine C60 has more relevance and continues to be a challenge. Therefore, preparation of hybrid materials comprising pristine C60 with π-systems, insights on their mode of interaction, morphological features of the resulting composites, and their electronic properties are of great interest.

The organic gel medium has already been reported as a useful scaffold for the crystallization of polymorphic pharmaceutical molecules (7, 45, 46). Encouraged by these studies, we attempted to understand the behavior of C60 in an OPV-based π-gel matrix. The outcome of this study reveals parallel gelation and crystallization process of OPVs and C60, respectively, leading to phase separation and crystallization of C60 to supramolecular rods with high photoconductivity.

RESULTS

Synthesis and characterization of hybrid gel assembly

π-Gelators have a strong affinity toward carbon allotropes, such as C60, carbon nanotubes, and graphene (44, 47). Gelators of extended π-systems, such as OPV gelators, are expected to interact with C60 in aliphatic and aromatic hydrocarbon solvents (22, 40, 44). With the objective of preparing hybrid π-gels with improved electronic properties, we studied the interaction of C60 in different ratios with the π-gelator OPVA in toluene (Fig. 1). It is observed that the gel melting temperature (Tgel), which is a direct measure of gel stability, increases with the addition of each fullerene equivalent (Fig. 2A). Enhancement in the Tgel values indicates that the colloidal assemblies in the hybrid gels are sufficiently stronger than the bare OPVA gelator. It was observed that the rate of increase in gel strength was much faster up to 2 eq of C60, wherein the gel strength increases with a steeper slope (1.25). From 2 to 6 eq of C60 addition, the rate of increase in gel melting temperature is almost reduced to half (slope, 0.625) before getting saturated (Fig. 2A). To compare the mechanical strength of the OPVA and the OPVA/C60 (1:6) hybrid gel, we performed rheological measurements. The rheological response as a function of the angular frequency at a fixed strain for OPVA gels with and without C60 is illustrated in Fig. 2B. Both gels showed a plateau region when the angular frequency was varied from 100 to 1 rad s−1. Both gels show a substantial elastic response, and the G′ values are greater than the G″ values over the entire range of frequencies. By the addition of fullerene, the value of G′ was found to increase by an order of magnitude in comparison to OPVA. The ratio of G′ and G″ is higher for the hybrid gel than that for the OPVA gel, indicating the better robustness of the former. The quasi-solid nature of the composite gel is higher (tan δ = G″/G′ = 0.43 to 0.45) than that of the OPVA gel (tan δ = 0.75 to 0.77) (48). The complex viscosity (η*) of the hybrid gel was also found to be higher when compared to the OPVA gel.

Fig. 1 Molecular structures and hybrid gel preparation.

The structures of molecules used in this study and thermoreversible gelation of OPVA with and without fullerene (C60).

Fig. 2 Improved stability of hybrid gel and aggregates.

(A) Tgel as a function of fullerene concentration. (B) Rheological data of OPVA (5 × 10−4 M) and OPVA/C60 (1:6) showing a change in the complex viscosity (ƞ*), storage modulus (G′), and loss modulus (G″) with angular frequency (ω). (C) Variable temperature absorption spectra of OPVA/C60 (1:6) in toluene (concentration of OPVA, 1 × 10−5 M). Inset shows variation of fraction of aggregates (αagg) with temperature (T). (D) Extent of emission quenching in OPVA/C60 hybrid in different states (all emission spectra have been obtained by exciting at 440 nm and normalized for comparison).

Variable temperature absorption studies in the solution state (1 × 10−5 M, toluene) provided evidence for the influence of C60 on the supramolecular assembly of OPVA (Fig. 2, C and D). Upon aggregation, absorbance at λmax (440 nm) decreases with a notable blue shift for both OPVA as well as OPVA/C60 (1:6). Although this observation indicates the formation of H-type aggregates in both cases (49), there is considerable difference in the thermal stability of the aggregates (see Fig. 2C, inset, and fig. S1A). Melting transition temperature (Tm; temperature at which αagg = 0.50) for hybrid aggregates (Tm = 49°C) is found to be higher than that of the self-assembled aggregates of OPVA (Tm = 38°C). Greater thermal stability of the hybrid aggregates can be attributed to the close packing of the molecules, which is further reflected in the increased photophysical interaction of the π-gelator and C60 from solution to gel to film state (see Fig. 2D and fig. S1).

Concentration-dependent morphological analysis

Transmission electron microscopy (TEM) analysis of OPVA xerogel revealed micrometer-long supramolecular tapes of width varying from 50 to 100 nm (Fig. 3A). TEM images of C60 showed the formation of clusters ranging in size from 100 to 200 nm (Fig. 3B). This observation fairly matches with the atomic force microscopy (AFM) and scanning electron microscopy (SEM) analyses of both the samples (fig. S2). TEM analysis of the hybrid gels with different compositions of OPVA/C60 revealed the formation of C60 clusters and rods on the supramolecular tapes of OPVA at varying concentrations of C60. At the low molar ratio of C60 (1:1 to 1:2), fullerene clusters were formed, which gradually spread out to nucleate the growth of fullerene rods (fig. S3). Upon increasing the concentration of fullerene (OPVA/C60 = 1:4), the growth of 1D fullerene domains was observed (Fig. 3C, yellow boxes). Above a 1:6 molar ratio of C60, micrometer-sized crystalline fullerene rods were exclusively formed with phase-separated OPVA nanofibers (Fig. 3D, white arrows). These supramolecular rods were 1 to 2 μm in length and 125 nm in width and had a high aspect ratio (fig. S4). The restricted mobility of the solvent within the gel medium and the enhanced local concentration of C60 between the interstitial spaces of OPVA assemblies facilitate the nucleation of C60 supramolecular rods. The amide H-bond interaction and the strong π-π interactions between the OPVA molecules prevent the interaction of OPVA with C60, which facilitates a phase separation between the two systems. With the gradual increase in the local supersaturation gradient of C60 near the supramolecular gel fibers, mesoscopic linear domains of C60 start to grow along the interstitial space of the bundled OPVA nanofibers. Crystallization started at this stage will be followed by an elongation process in a nonequilibrium manner to form nanorods until the local supersaturation gradient is depleted and complete mesoscale phase segregation is achieved. However, the substantial increase in the viscosity of OPVA gel with the addition of C60 indicates the interaction of the gelator and the solvent molecules with the C60 microstructures at a macroscopic level. This is clear from the TEM image in Fig. 3D, which reveals the entanglement of fullerene rods with the supramolecular tapes of OPVA (also see fig. S3C).

Fig. 3 TEM analysis of crystallization of C60 within OPVA gel matrix.

(A to C) TEM image of (A) OPVA tapes, (B) C60 clusters, and (C) OPVA/C60 composite in 1:4 ratio. Fullerene domains in the OPVA matrix (yellow boxes) and yellow arrows show the growth direction of domains. (D) OPVA/C60 composite in 1:6 ratio. White arrows show supramolecular tapes of OPVA. Concentration of OPVA, 1 × 10−5 M in toluene.

Fullerene nanorods grown inside the gel medium showed a molecular-level arrangement with well-ordered lattice fringes. The polycrystalline nature of the bulk fullerene (Fig. 4A) is clear from the high-resolution TEM studies and observed fast Fourier transform (FFT) pattern (Fig. 4B, inset). Careful examination of the high-resolution images and FFT patterns further proves the crystallization of fullerene rods inside the supramolecular gel medium (Fig. 4C). These fullerene rods formed in the gel medium exhibited better crystalline nature (Fig. 4D) than the fullerene crystals grown from toluene under ambient conditions. Reconstructed high-resolution TEM (HRTEM) image of the fullerene rods clearly shows crystallized C60 domains (Fig. 4E). The presence of C60 was confirmed by taking a line profile in Fig. 4E, whereby a distance of 0.79 nm was repeatedly observed between regions of same contrast (Fig. 4G). This value fairly matches the diameter (0.71 nm) of a C60 molecule (Fig. 4F) and the HRTEM image of C60 reported in literature (50).

Fig. 4 HRTEM analysis of gel-assisted fullerene assembly.

(A to D) HRTEM images of (A and B) bulk C60 and (C and D) C60 rods grown inside the gel medium. Insets show respective FFT patterns of the images. (E) Processed image of (D) using the Gatan Microscopy Suite software. (F) Magnified image of the area in yellow box of (E) showing closely packed fullerenes in yellow circles. Molecular dimensions of C60 is represented. (G) Line profile corresponding to the orange line drawn in image (E).

Structural elucidation from x-ray diffraction experiments

Wide-angle x-ray scattering (WAXS) measurement was conducted on bulk C60, OPVA xerogel, and the hybrid xerogel (Fig. 5A). The diffraction peaks obtained in bulk C60 matched well with the standard face-centered cubic (fcc) crystal phase [Joint Committee on Powder Diffraction Standards (JCPDS) card no. 00-043-0995] (51). The results also confirmed the complete absence of hexagonal close-packed (hcp) structure (fig. S5) (52). Diffraction peaks with d-spacing of 8.4, 4.4, 3.5, and 3.4 Å are indexed as (111), (311), (222), and (331) planes, respectively, from the fcc lattice. These peaks were unaltered in the hybrid xerogel, justifying a retained fcc packing of the fullerene rods, which is important for better charge transport due to close packing of the C60 molecules (53). The noticeable diffraction peaks observed in the case of OPVA xerogel includes a broad shoulder corresponding to a d-spacing of 32.1 Å, a sharp peak corresponding to 15.7 Å, and a broad, intense peak corresponding to an average d-spacing value of 7.47 Å. Deconvolution of the third peak gives us access to three diffraction peaks, which contribute to its broadness. Deconvoluted peaks 1, 2, and 3 correspond to a d-spacing value of 8.48, 7.47, and 6.74 Å, respectively (Fig. 5B). These peaks can be assigned to a mixture of syn- and anti-oriented catemers of OPVA and match well with the distance between the molecular stacks, that is, 32.1 and 15.3 Å along the width and length of the tape justifying the short- (broad shoulder) and the long-range (sharp peak) order, respectively (49). The presence of both syn- and anti-oriented catemers in OPVA tapes results in the variation of the distance between the planes, which contain the amide functionality in the alternately stacked OPVA molecules. This explains the broadening of the peak at 7.47 Å. In the hybrid gel, the first peak corresponds to a d-spacing of 36 Å, followed by a hump at 15.9 Å, and the third peak corresponds to a d-spacing of 8.51 Å. From our morphological investigations, we observed that on the addition of C60, the OPVA tapes are chopped down into thinner fibers. A probable explanation for this observation may be that, in the hybrid gel, compartmentalization of C60 occurs within syn-oriented OPVA catemer domains, the width of which corresponds to the molecular length of OPVA (36 Å). The reduction in the number of anti-oriented catemers joining the syn-oriented catemer stacks in the hybrid gel results in the thinning of the OPVA tapes. The abundance of syn-oriented OPVA catemers in the thin hybrid gel fibers results in a uniform distance between the planes that contain the amide functionality (8.52 Å) between alternately stacked OPVA molecules. This is a probable explanation for the gradual shifting and sharpening of the peak (corresponding to a d-spacing of 8.51 Å) in the hybrid gel (Fig. 5, C and D). The molecular distances have been calculated on the basis of the crystal structure of acetanilide and benzanilide, which crystallize in the anti- and syn-oriented catemer, respectively (5456).

Fig. 5 WAXS analysis to elucidate the plausible mechanism of orthogonal self-assembly.

(A) WAXS pattern of OPVA (pink), OPVA/C60 (1:6) (violet), and C60 (black). (B) Deconvoluted spectra of the x-ray diffraction (XRD) peak of OPVA at 7.47 Å. (C) Schematic representation of anti- and syn-oriented catemer formation in OPVA assembly, leading to the formation of tapes. (D) Plausible scheme of formation of C60 rods in OPVA/C60 composites.

Charge transport properties

Because the fullerene supramolecular rods are expected to be photoconducting, we conducted flash photolysis time-resolved microwave conductivity (FP-TRMC) measurement, which is an electrodeless technique for evaluating intrinsic photoconductivity of the material with minimum trapping effects (57, 58). The quantification of intrinsic photoconductivity in terms of φΣμ values is shown in Fig. 6A, where φ is the charge-carrier generation quantum yield upon photoexcitation and Σμ represents the sum of charge-carrier mobilities. The maximum value (φΣμmax) usually signifies the intrinsic short-range charge-carrier mobility. The measurements showed a φΣμmax value of 2.0 × 10−5 and 2.5 × 10−5 cm2 V−1 s−1 for OPVA/C60 blends with a molar ratio of 1:2 and 1:4, respectively. The φΣμmax increases by ~22-fold to a value of 2.4 × 10−4 cm2 V−1 s−1 (Fig. 6B) for a 1:6 blend ratio of OPVA/C60, as compared to that of OPVA xerogel (φΣμmax = 1.1 × 10−5 cm2 V−1 s−1). The sudden increase found between OPVA/C60 = 1:6 and OPVA/C60 = 1:4 is probably due to the increase in Σμ because of better electron percolation pathways in the extended fullerene nanorod (59).

Fig. 6 Photoconductivity from the coassembled hybrid nanostructures.

(A) FP-TRMC data for OPVA with different ratio of C60. Graph is zoomed to show the variation of conductivity in the lower ratio of C60. (B) Secondary plot showing the variation of the φ∑μ value for OPVA with different ratios of C60. The value increases 22-fold in the case of OPVA/C60 (1:6) blend. (C) I-V characteristics of gelators and their respective hybrid samples (at the bias voltage of 20 V). Inset shows the device configuration. (D) Comparison of photocurrent generation from C60 in two different gel media (concentration of gelator, 1 × 10−5 M in toluene). Upon addition of 20 eq of C60, the photocurrent generated is ~104 times in OPVA, whereas a 190-fold increase is found in the non–π-gelator (2).

Furthermore, the electrical transport in the fullerene rods is studied by photocurrent measurement of spin-coated xerogel samples on organic field effect transistor platform (Fig. 6C) (53, 60, 61). Current-voltage (I-V) characteristics illustrate considerable enhancement in the photocurrent with increases in C60 content in OPVA/C60 blends. Random aggregates formed at the low concentration of C60 do not conduct well because of nanoscale discontinuity combined with dimensional limitations, which is already demonstrated in morphological studies. As the ratio of fullerene increases, photoresponse of the composite enhances hinting toward the importance of nano- to mesoscale phase segregation and formation of heterojunctions in the transport properties. Advantages of long-range mesoscale phase segregation and molecular space confinement in the fullerene rods facilitate the efficient charge transport at 6 eq of fullerene and above.

Supramolecular gel-assisted growth of fullerene rods is an interesting example of gel phase crystallization. Gelation and crystallization are two orthogonal processes, which occur at different rates. The outcome and properties of the resultant nanostructures from these orthogonal assemblies depend on effective phase separation, which in turn depends on the weak and slow interaction between gelator and crystallizing substrate in this multicomponent assembly. Therefore, to justify the effect of the π-gel medium, OPVA was replaced by n-decanamide (2), which is a non–π-conjugated gelator (fig. S6). When compared to the OPVA/C60 system, the hybrid gel obtained from 2/C60 showed low photoconductivity (Fig. 6, C and D), although the formation of nanoscale C60 rods is reported in case of gelators without a π-backbone (40). Reversible photoresponse of the fullerene nanostructures from both gels is illustrated in the photocurrent time profile (fig. S7). It is observed that excess loading of C60 is required in the gel matrix of 2 to generate a substantial photoresponse. Therefore, it can be said that more than acting as a medium for crystallization, the supramolecular gel matrix of OPVA plays the role of an active material, which connects nanorods with each other, having continuous charge carrier pathway in micrometer dimensions. In contrast to the π-gel medium, 2 provides an insulating environment that reduces electrical transportability.

DISCUSSION

The supramolecular tapes of the OPVA π-gelator are found to be excellent scaffolds for the crystallization of C60 as supramolecular rods. Phase separation and orthogonal self-assembly of OPVA and C60 occur within the gel medium, resulting in hybrid gels comprising crystalline fullerene rods. The strong π-π interaction and H-bonding between the OPVA molecules prevent OPVA-C60 interaction, thus facilitating the phase separation between the two. Morphological studies established the concentration dependency of the supramolecular rod formation. The supramolecular gel medium slows down the rate of crystallization of C60 molecules. Crystallization at a slower rate results in molecular-level ordering, which leads to higher crystallinity of the supramolecular fullerene rods. The C60 rods formed in the π-gel medium exhibited relatively high photocurrent generation when compared to C60 dispersed in a non–π-gel medium. Formation of donor-acceptor heterojunctions in nanoscale level is proved by transient photoconductivity measurements. Transient conductivity depends on the composition ratio of fullerene, which in turn also affects the nano- to mesoscale morphology of this orthogonal self-assembling system. This study opens up the possibility of exploring a variety of π-gels as scaffolds for the assembly of carbon allotropes and their use in optoelectronic applications.

MATERIALS AND METHODS

Synthesis: General procedures

Unless otherwise stated, all organic starting materials and reagents were purchased from commercial suppliers and used without further purification. Amide-functionalized oligo(p-phenylenevinylene)–based gelator (OPVA) was synthesized according to the reported procedure and characterized by standard methods (49).

Gelation studies

The compound was taken in a sealed glass vial with a known volume of solvent. The glass vial was heated until the compound dissolved. Gel formation occurred when the hot solution was allowed to cool and was confirmed by the incapability of the content to flow when inverting the glass vial. Repeated heating and cooling of the solution confirmed the thermal reversibility of the supramolecular gel. Tgel was determined by the dropping ball method (49).

Rheology experiments

The rheological properties of the OPVA and the hybrid gels with C60 were measured using a Physica Modular Compact (MCR 150) stress-controlled rheometer from Anton Paar with a cone-and-plate geometry (CP 50-1). A parallel plate sensor 50 mm in diameter was used to maintain a gap size of 0.1 mm. Gels in toluene were transferred to the peltier, and the plate was covered properly to avoid the solvent evaporation. Dynamic oscillatory mode was used for the measurements, keeping a constant strain amplitude (γ) of 1%, and the angular frequency was varied from 1 to 100 rads−1.

Electronic spectral measurements

A Shimadzu UV-3101 PC NIR scanning spectrophotometer was used to record the electronic absorption spectra, and the emission spectra were recorded on a SPEX Fluorolog FL-1039 spectrofluorimeter. All optical measurements were carried out using 0.1- or 1-cm cuvettes with a thermistor directly attached to the wall of the cuvette holder for controlling the temperature.

Morphological analysis

An NTEGRA (NT-MDT) operating with a tapping mode regime was used to record AFM images under ambient conditions. Microfabricated TiN cantilever tips (NSG10) with a resonance frequency of 299 kHz and a spring constant of 20 to 80 N m−1 were used. AFM section analysis was done offline. Samples for the imaging were prepared by drop-casting toluene solution of OPVA, C60, and OPVA/C60 onto a freshly cleaved mica sheet at the required concentrations under ambient conditions and were dried under vacuum. TEM measurements were carried out using FEI (Tecnai G2 30 S-TWIN) with an accelerating voltage of 100 kV. Samples were prepared by drop-casting toluene solutions of OPVA, C60, and OPVA/C60 onto carbon-coated copper grids at the required concentrations under ambient conditions. The samples were dried under vacuum. TEM images were obtained without staining. Inverse FFT reconstruction of the HRTEM images was done using the software program Digital Micrograph (Gatan Inc.), following a reported procedure (62). FFT of the experimentally obtained image was initially taken, followed by appropriate mask filtering to remove the spatial frequency of the diffractogram. Finally, the inverse FFT gave rise to a reconstructed HRTEM image. SEM images were obtained with a Zeiss EVO 18 cryo-SEM Special Edn with variable pressure detector working at 20 to 30 kV. Samples were prepared from toluene solutions of appropriate concentration by drop-casting onto a freshly cleaved mica surface.

XRD analysis

Samples for the XRD studies were prepared by transferring respective gels onto aluminum foil and dried slowly to evaporate the solvent. Finally, all the samples kept under vacuum to ensure complete removal of solvent. X-ray diffractogram of the dried films were recorded on a Xeuss SAXS/WAXS system using a Genix microsource from Xenocs operated at 50 kV and 0.6 mA. The Cu Kα radiation (λ = 1.54 Å) was collimated with a FOX 2D mirror and two pairs of scatterless slits from Xenocs. The 2D patterns were recorded on a Mar345 image plate and processed using the Fit2D software. All measurements were made in the transmission mode. The sample-to-detector distance was calibrated with silver behenate standard, and it was found to be 214.5 mm. Deconvolution of XRD data was performed using the Fityk 0.9.8 software (63).

FP-TRMC studies

The third harmonic generation (355 nm) of a Nd:YAG laser (5- to 8-ns pulse duration, Spectra Physics GCR-130; incident photon density to a sample was set to 9.1 × 1015 cm−2) was used as an excitation source for TRMC. The transient photoconductivity (Δσ) was measured by TRMC using a 3-mW X-band (~9.1 GHz) microwave. The obtained transient photoconductivity (Δσ) was converted to the product of the quantum yield (ϕ) and the sum of charge carrier mobilities, (Σμ = μ+ + μ), by φΣμ = Δσ (eI0Flight)−1, where e, I0, and Flight are the unit charge of a single electron, the incident photon density of excitation laser (in m−2), and the correction (or filling) factor (in m−1), respectively. The Flight was calculated by taking into consideration the geometry and optical properties of the sample, such as the size, laser cross section, and absorption of the excitation laser. The details of the system were previously reported (58). All experiments were carried out at room temperature. Samples were prepared by drop-casting toluene solutions of OPVA and OPVA/C60 on quartz plates and then drying under vacuum.

Photocurrent measurements

Devices for photocurrent measurement were fabricated on the organic field effect transistors (OFET) platform with bottom gate/bottom contact configuration. For gate electrode and gate dielectric layer, a heavily doped n-type Si wafer and dry oxidized SiO2 were used, respectively (capacitance of 11 nF cm−2) (64). Thickness of the dielectric was 300 nm, with surface roughness less than 0.1 nm. Gold electrodes were used as both source and drain. Photolithography technique has been used to deposit gold electrodes. After sequential washing with water, deionized water, and ethanol, substrates were rinsed with acetone. Surface modification of all substrates was done with n-trichloro(octadecyl)silane, followed by cleaning with n-hexane, CHCl3, and acetone. The system was kept under vacuum for 60 min, and the substrates were washed with ethanol. OPVA, OPVA/C60, 2, and 2/C60 were dissolved in toluene, heated, and cooled to obtain a gel in individual cases. The gels were further diluted with toluene and spin-coated, as mentioned above, with modified silicon substrate at 2000 rpm for 30 s. Samples were kept under vacuum overnight at room temperature to ensure complete removal of the solvent. Photocurrent measurement of the devices was carried under ambient conditions, using a Keithley Model 4200 SCS semiconductor parameter analyzer.

SUPPLEMENTARY MATERIALS

Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/9/e1600142/DC1

fig. S1. Photophysical studies of OPVA and OPVA/C60 (1:6) hybrid assembly.

fig. S2. Morphological characterization of OPVA, C60, OPVA/C60 (1:6) hybrid.

fig. S3. Nucleation and growth of C60 supramolecular rods within the OPVA gel matrix.

fig. S4. Size distribution of C60 supramolecular rods based on TEM analysis.

fig. S5. XRD analysis of C60.

fig. S6. Effect of gelator on photocurrent generation.

fig. S7. On/off switching of photocurrent.

References (51, 52)

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REFERENCES AND NOTES

Acknowledgments: We acknowledge K. K. Kartha [Council of Scientific and Industrial Research–National Institute for Interdisciplinary Science and Technology (CSIR-NIIST)] for help in the synthesis during the initial stages of this research; S. Prasanthkumar and V. C. Nair (CSIR-NIIST) for help in carrying out FP-TRMC studies; D. Zhang, G. Zhang, and Z. Liu (Key Laboratory of Organic Solids, Institute of Chemistry the Chinese Academy of Sciences, China) for general help during the photocurrent measurement; J. D. Sudha and K. Mohan of CSIR-NIIST for the rheological and TEM measurements, respectively. Funding: A.A. is grateful to the Department of Science and Technology (DST-SERB), Government of India, for a J. C. Bose Fellowship (SERB order no. SB/S2/JCB-11/2014, dated 9 June 2015). V.S.N. and R.D.M. are thankful to the CSIR, Government of India, for research fellowships. Author contributions: V.S.N. undertook the synthesis of molecules, and V.S.N. and R.D.M. performed studies. A.S and S.S. supervised the FP-TRMC studies. A.A., V.S.N., and R.D.M. analyzed the data, discussed the results, and wrote and commented on the manuscript. All authors discussed the results and commented on the manuscript. A.A. was responsible for the overall project concept, direction, and coordination. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from A.A.
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