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. 2016 Sep 23;2(9):e1600142.
doi: 10.1126/sciadv.1600142. eCollection 2016 Sep.

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

Vishnu Sukumaran Nair  1 , Rahul Dev Mukhopadhyay  1 , Akinori Saeki  2 , Shu Seki  3 , Ayyappanpillai Ajayaghosh  1
Affiliations

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

Vishnu Sukumaran Nair et al. Sci Adv. .

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; crystallization; fullerene; gels; p-n heterojunctions; photoconductivity; self-sorting.

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Figures

Fig. 1
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
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).
Fig. 3
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.
Fig. 4
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).
Fig. 5
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.
Fig. 6
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).
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