Introduction

Scientific innovations and technological breakthroughs in various sectors require the design and development of novel materials. Many of the existing materials may not satisfy all the fundamental requirements of mankind. This understanding has encouraged researchers to develop hybrid materials that can exhibit properties superior to those of the individual components1,2. As in other research fields, advancements in the area of synthetic hybrid materials have been inspired by the various natural creations observed in the biological world, such as bone (a combination of an organic component, collagen, and an inorganic component, phosphate mineral) and nacre (a mixture of an elastic biopolymer, chitin, and carbonate minerals)3,4. Hybrid materials are usually formed by combining organic and inorganic substances as in the case of the above natural materials; however, in recent times, combinations of different types of polymers, nanomaterials, and carbon allotropes such as CNTs and graphene have also been included in the broad area of composite hybrid materials.

While there are several strategies for preparing hybrid materials, molecular self-assembly using noncovalent interactions5,6,7,8,9,10 is considered a simple and efficient approach to the preparation of hybrid materials11,12,13,14,15,16. The nanoscale and mesoscale ordering of the building blocks in these hybrid materials and the resultant morphological features determine their functional properties, which can be modified by exploiting the reversible and adaptive nature of noncovalent interactions17,18. Different strategies, including complementary interactions, molecular recognition, and templating, have been employed to create self-assembled hybrid materials2. Among these methods, strategies based on π–π interactions are found to be extremely suitable for making hybrid materials in combination with 1D and 2D carbon allotropes and thereby overcome a major drawback associated with solubility19. Thus, the development of hybrid materials from CNTs and graphene using noncovalent functionalization has emerged as an interesting strategy that retains the original electronic properties with minimum structural defects. In this review, we provide an overview and perspective on hybrid materials of 1D and 2D carbon allotropes and synthetic π-systems (Fig. 1) with some illustrative examples.

Fig. 1
figure 1

Pictorial representation showing the utilization of CNT/graphene and π-system hybrid materials in some representative applications

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Graphene-based hybrid materials

Graphene is a 2D layered material with extended π-conjugation and unique electronic properties20,21,22,23. The extended honeycomb structure of graphene formed through covalent bonding between sp2-hybridized carbon atoms gives an exceptional strength-to-weight ratio superior to that of metals and metal-based hybrid materials. Furthermore, the zero electronic band gap in graphene leads to excellent electron mobility and hence make graphene an alternative to the existing semiconductor materials in electronic devices. Graphene derivatives such as GO and RGO possess an atomically thin-layered structure similar to that of graphene but differ in the percentage of oxygen functionalities and sp2-hybridized carbon atoms in their basal plane. RGO, which possesses minimal oxygen functionalities and numerous sp2-hybridized carbon atoms in its basal plane, shows electrical properties superior to those of GO. Graphene and its derivatives along with self-assembled organic molecules are applied to the formation of hybrid materials with enhanced electrical and mechanical properties. Both GO and RGO have been successfully incorporated into a variety of supramolecular materials such as gels and assemblies of π-conjugated molecules and polymers12,14,24,25,26.

Graphene and its derivatives with supramolecular gelators

The interaction of graphene with π-conjugated molecular gelators is a straightforward strategy towards the preparation of hybrid nanostructures. These hybrid gels are mechanically stable and solution processable. There are a few examples of graphene-containing supramolecular gels with improved properties27,28,29,30,31,32. For instance, it has been shown that graphene could be used as a nucleating surface to reduce the critical gelator concentration of 1 (Fig. 2a)27. Rheological studies have revealed that the rigidity of the hybrid gel (storage modulus, G′) increased sevenfold compared to that of the native gel (Table 1). Hybrid gels prepared by the interaction of GO with conducting polymeric gels have been reported to function as sensors30. A hybrid gel composed of GO and polypyrrole (2) prepared by the in situ polymerization of pyrrole in the presence of an aqueous solution of GO (Fig. 2b) has been applied as an ammonia gas sensor owing to its excellent electrical conductivity and electrochemical properties. Upon exposure to ammonia gas (800 ppm) for 600 s, the aerogel prepared by lyophilizing the hydrogel showed a 40% increase in resistance, whereas the corresponding electropolymerized air-dried films displayed marginal changes in resistance (Table 1). This difference in sensing ability was attributed to the intact microstructures of the aerogel that allowed the efficient diffusion of ammonia.

Fig. 2
figure 2

a Molecular structure of gelator 1. b Chemical structure of polymer 2 and photograph of the free-standing 2/GO hybrid hydrogel. (Reprinted with permission from ref. 30 © 2011 The Royal Society of Chemistry). c Chemical structures of the bi-functional molecules (3a, b) and IL (4)

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Table 1 Comparison of different graphene-based hybrid materials, their synthesis procedure, enhanced properties and applications
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Recently, it has been demonstrated that the ionic liquids (ILs) 3a, b having pyrene moieties (Fig. 2c) can disperse GO in imidazolium-based ILs through π–π and cation–π interactions31. The high-thermal stability and boiling point of ILs allow the thermal reduction of GO to RGO upon heating to 150 °C and the conversion of the dispersion into a gel (Table 1). The IL 4 (Fig. 2c) without pyrene failed to disperse GO even after sonication for 2 h, corroborating the role of pyrene. Similarly, the counter anions also played a crucial role in the formation and stability of the hybrid gels. The bi-functional molecule 3a and IL 4 with a larger counter anion, PF6−, showed better gelation than 3b with BF4− due to the formation of strong cation–π interactions with thermally reduced GO. In addition to small aromatic molecules, linear π-conjugated molecules have also been used to disperse exfoliated graphene sheets in a gel medium. An oligo(p-phenylenevinylene) (OPV)-based gelator, 5, led to the exfoliation of RGO in nonpolar solvents via noncovalent interactions (Fig. 3a)32. The RGO sheets were found to be highly dispersed due to the presence of self-assembled fibers of 5 on their surface and formed gels at higher concentrations of 5 (Fig. 3b). The exfoliated RGO hybrid exhibited a large surface area and better bulk conductivity than the gel medium alone, leading to its utilization in energy storage applications (Table 1).

Fig. 3
figure 3

a Schematic representation of the formation of the 5/RGO hybrid by the self-assembly of 5 (laser illumination shows the Tyndall effect from dispersed graphene sheets in toluene). b Photograph of the native and hybrid gels of 5 with varying RGO contents in toluene. (Reprinted with permission from ref. 32 © 2017 American Chemical Society)

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Molecular recognition has been widely used to prepare self-assembled hybrids of carbon nanomaterials through complementary host–guest interactions33. Usually, the host molecules attach to the surface of the carbon materials through covalent or noncovalent functionalization. For example, a hybrid gel-based on GO covalently functionalized with β-cyclodextrin (β-CD) (6/graphene hybrid) and an azobenzene-appended poly(dimethyl acrylamide)-b-poly(N-isopropylacrylamide) (PDMA-b-PNIPAM) block copolymer, 7, was prepared via host–guest interactions (Fig. 4)34. PNIPAM, 8, (Fig. 4a) well known for its lower critical solution temperature (LCST), is soluble in water at room temperature (~32 °C) due to H-bonding interactions, which break at higher temperature, resulting in phase separation of the polymer from water. During the self-assembly of 6/graphene and 7 at higher temperature, sol-to-gel transformation occurred via the formation of cross-linked network structures. On the other hand, the gel-to-sol transition occurred at 30–37 °C via the complete collapse of the cross-linked structure (Fig. 4b; Table 1). Additionally, the PNIPAM/GO hybrid hydrogel was prepared by γ-irradiation-aided polymerization of a GO-containing solution of the corresponding monomer35. A swelling–deswelling response was observed for the nanohybrid gel upon irradiation with a near-infrared laser due to the combined effects of the LCST of 8 and the photothermal characteristics of GO. This hybrid hydrogel was successfully used to regulate liquid flow in micro-fluidic devices.

Fig. 4
figure 4

a Chemical structures of amino-functionalized β-CD 6, azobenzene-appended block copolymer 7 and PNIPAM 8. b Schematic representation of the formation of a hybrid hydrogel from 6/graphene and 7. (Reprinted with permission from ref. 34 © 2011 American Chemical Society)

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Graphene and its derivatives with aromatic molecules

Aromatic molecules such as pyrene36,37,38, anthracene39, coronene40, perylene41, porphyrin42,43,44, and their derivatives have been used to prepare hybrid materials of graphene and its derivatives. The direct exfoliation of pristine graphene sheets from graphite has been demonstrated using 1-pyrene carboxylic acid, 9 (Fig. 5a), in a methanol and water mixture36. The functionalized graphene sheets found applications in chemical sensing and as an ultracapacitor after deposition on a nanoporous membrane. The chemical sensor exhibited better selectivity towards ethanol vapor than to water vapor, air, and carbon dioxide (Table 1).

Fig. 5
figure 5

a Chemical structures of 9–11. b Schematic representation showing the exfoliation of graphene sheets by 10

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An aromatic amphiphile, 10 (Fig. 5a), consisting of planar pyrene units and hydrophilic dendrons has also been used for the selective dispersion of larger 2D graphene sheets from graphite powder in water38. The pyrene moieties in 10 adopted a 2D sheet-like arrangement on the surface of graphene and enhanced the effective π–π interactions (Fig. 5b). Simultaneously, the oligoether moiety of 10 provided a hydrophilic environment around the hydrophobic π-surface of graphene, thereby ensuring the efficient dispersion of graphene in water at a concentration of up to 1.5 mg mL−1. The 2D sheet-like conformation of the amphiphile hindered its effective interactions with the curved surface of single-walled nanotubes (SWNTs), leading to its selectivity towards graphene. A coronene carboxylate, 11 (Fig. 5a), has also been used for the noncovalent functionalization and solubilization of graphene from graphite in water and showed better interactions with graphite than with GO40. The strong charge transfer and π–π interactions between negatively charged 11 and graphite produced stable dispersions of exfoliated graphene sheets in water.

The noncovalent functionalization of RGO with aromatic donor and acceptor molecules has resulted in hybrid materials with tunable electronic properties. For example, the interaction of the sodium salt of 1-pyrene sulfonic acid, 12 (donor), and the disodium salt of the bis-benzenesulfonic acid, 13 (acceptor, Fig. 6), with graphene-produced water-soluble hybrid materials41. While the aromatic molecules exhibited strong π-interactions with graphene sheets, the negatively charged molecules prevented aggregation, leading to a stable dispersion of the hybrid material in solution. Upon thermal reduction at higher temperatures in the presence of Ar and H2, these hybrid materials exhibited conductivity values of 1149 and 1314 S cm−1, respectively, whereas pristine RGO exhibited a much lower value of 517 S cm−1. The observed enhancement in conductivity of the hybrid materials is attributed to the better graphitization and deoxygenation of the basal plane of RGO. Furthermore, these hybrid materials showed a better power conversion efficiency than pristine RGO when used as an electrode material in bulk-heterojunction solar cells (Table 1).

Fig 6
figure 6

Chemical structures of 12–16 and schematic representation of the flattening effect of 14 on the surface of graphene

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The water-soluble cationic porphyrin 14 (Fig. 6)42 and the sulfonate-functionalized and quaternary ammonium group-functionalized porphyrins 15a and 15b, respectively (Fig. 6)43, were used to prepare hybrid materials with RGO. The electrostatic and π–π interactions between positively charged 14 and negatively charged graphene sheets resulted in the flattening of 14, leading to enhancements in the π-conjugation and electron-withdrawing effects (Fig. 6). The 14/RGO hybrid was used for rapid and selective detection of Cd2+ in aqueous media via accelerated coordination reaction between 14 and Cd2+ in the presence of RGO sheets (Table 1). While 15a self-assembled on the surface of graphene via π–π interactions, the electrostatic repulsion between the negative charges ensured the stability of the aqueous dispersion. A thin film of this hybrid material exhibited an 80% optical transparency and an electrical resistance of 5 KΩ m−1. In another report, a hybrid material composed of graphene and an anionic phthalocyanine, 16, (Fig. 6) in water was employed as a photosensitizer and a photothermal therapeutic agent, which showed excellent anticancer efficacy (Table 1)44.

Graphene and its derivatives with π-conjugated polymers

π-conjugated polymers are known to interact with graphene through noncovalent interactions. Hybrid materials of RGO and π-conjugated polymers such as poly(p-phenylenevinylene)45, poly(p-phenyleneethynylene)46, polyaniline47,48,49, and polypyrrole50 have found applications in solar cells as well as in optoelectronic devices such as field-effect transistors and sensors.

A hybrid material of the polymer 17 (Fig. 7) and RGO has been reported for studying the mechanism of energy transfer between donor and acceptor systems via time-resolved fluorescence dynamics45. Polymer 17 showed a fluorescence lifetime of 200 ps that, however, showed a faster decay in the presence of 50% RGO due to energy transfer from the polymer to RGO. A hybrid of a conjugated polyelectrolyte, 18 (Fig. 7), and graphene was reported to exhibit a low resistance of 30 KΩ and found applications in optoelectronic devices (Table 1)46.

Fig 7
figure 7

Chemical structures of the π-conjugated polymers (17–22) used for the preparation of hybrid materials with GO/RGO

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A hybrid composed of GO and polyaniline 19 (Fig. 7) was prepared via mild oxidation of aniline with a mixture of H2O2, HCl, and FeCl3·6H2O in the presence of various amounts of GO47. The charge storage capacity of 19 increased considerably with a minimum weight % of GO. A similar kind of hybrid material was also prepared by in situ polymerization of aniline in the presence of GO followed by reduction using hydrazine hydrate48. Morphological studies revealed the presence of nanofibrous structures of 19 on the surface of the graphene sheets. The 19/RGO hybrid exhibited a higher conductivity and charge storage capacity than 19. A sulfonated polyaniline, 20 (Fig. 7), was reported to form a highly water-soluble (>1 mg mL−1) electroactive hybrid material with RGO that exhibited a conductivity of 0.3 S m−1 in the film state49.

Hybrid materials of polypyrrole 2 and GO were prepared by emulsion polymerization of pyrrole in the presence of multi-layered graphite oxide (an oxidized form of bulk graphite)50. The π–π interaction-mediated self-assembly of 2 on the surface of the unoxidized domains of GO in the multi-layered graphite oxide led to delamination of the latter into individual GO sheets in water. The electrical conductivity of the 2/GO hybrid material was reported to be 5 S cm−1, whereas 2 and GO exhibited low conductivity values of 0.94 and 1 × 10−6 S cm−1, respectively (Table 1).

To exploit the excellent electronic properties of graphene as an acceptor in bulk-heterojunction organic solar cells, a few graphene-based hybrid materials with poly(3-hexylthiophene) (P3HT, 21)51,52 and poly(3,4-ethylenedioxy-thiophene) (PEDOT, 22) (Fig. 7) were investigated53,54. For instance, a hybrid material composed of 21 and functionalized graphene (0–15%) was tested for solar cell applications52. Photovoltaic devices fabricated with this hybrid material exhibited a power conversion efficiency of 1.1% with an open-circuit voltage of 0.72 V (Table 1). Hybrid materials composed of RGO and 22 were used as transparent electrode materials for optoelectronic devices53. A hybrid film of 22/RGO exhibited an electrical conductivity of ~0.2 S cm−1 and an 88% transmittance in the range 400–1800 nm. The hybrid materials of RGO with 22 as double-layered (22/graphene) and triple-layered (graphene/22/graphene) films (~32 nm thick) exhibited conductivity values of 13 and 12 S cm−1, respectively54. Furthermore, free-standing films made of these hybrid materials were 91% optically transparent and exhibited a sixfold enhancement in the mechanical strength relative to that of the pristine films (Table 1). These exceptionally conductive, transparent, and free-standing films with superior mechanical strength were explored for potential applications in optoelectronics and sensing. All the details of the synthetic routes, properties and applications of graphene-based hybrid materials are summarized in Table 1.

Carbon nanotube-based hybrid materials

CNTs are an allotrope of carbon with a 1D nanotubular structure and are broadly classified as SWNTs, double-walled, and multi-walled nanotubes (MWNTs)11,23. The electrical conductivity of CNTs ranges from metallic to semiconducting based on the diameter and the rolling angle, which also impart chirality in the tubes11. CNTs have excellent chemical, mechanical, and electronic properties that make them promising for various applications. The noncovalent functionalization of CNTs with aromatic molecules and π-conjugated polymers has been extensively investigated11,12,13,14,55,56,57,58,59,60,61,62. The surface modification of CNTs through noncovalent interactions improves their dispersion and solution processability, rendering them suitable for device applications.

Carbon nanotubes and supramolecular gelators

The solution processability of CNTs can be enhanced by incorporating them into supramolecular gel matrices. For instance, the phenylacetylene polymer 23 (Fig. 8a) interacted with HiPco-SWNTs in toluene to form a hybrid gel63. The wrapping and unwrapping of 23 over the SWNT surface could be controlled by an electrochemical redox response (Fig. 8b). This gelation process did not occur with CoMoCAT-SWNTs due to the inability of 23 to adhere to the surface of SWNTs having a small diameter. This selective interaction of 23 with SWNTs was exploited to chirally enrich a mixture of SWNTs with multiple chiral structures (Table 2).

Fig 8
figure 8

a Chemical structure of polymer 23. b Stimuli-responsive wrapping and unwrapping of 23 over the SWNT surface

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Table 2 Comparison of various CNT-based hybrid materials, their synthesis procedure, enhanced properties and applications
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OPV-based gelators have been shown to interact strongly with SWNTs and MWNTs through π-stacking interactions to form stable hybrid gels in nonpolar solvents55. The interaction of OPVs with CNTs leads to debundling of the nanotubes, and the specific arrangement of the debundled SWNTs/MWNTs and 24 reinforces the supramolecular fibrous structures (Fig. 9). This strong π–π interaction of OPVs with SWNTs was exploited to facilitate the self-assembly of a chiral π-gelator, 24, resulting in the amplification of chirality even at low concentrations64. The OPV 24 with amplified chirality in the presence of SWNTs showed distinct morphological features, such as coaxially aligned carbon nanocables in which each SWNTs were helically wrapped with the gelator (Table 2).

Fig 9: Chemical structure of OPV-based gelator 24 and a schematic representation of the formation of coaxially aligned carbon nanocables of 24/SWNTs with amplified supramolecular chirality.
figure 9

(Reprinted with permission from ref. 64 © 2016 Wiley-VCH)

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Host–guest interactions between photoresponsive polymers containing azobenzene moieties and SWNTs have been successfully utilized to form stimuli-responsive hybrid hydrogels65,66. In another report, a supramolecular hybrid gel composed of a squaraine derivative, 25, and SWNTs was prepared through ultrasonic irradiation (Fig. 10)67. Upon sonication, the self-assembly of 25 resulted in the formation of entangled fibrous structures leading to gelation. Furthermore, the rate of nucleation and growth of the self-assembled structures could be enhanced by introducing heterogeneous nucleation sites by adding very small amounts of SWNTs to an n-butanol solution of 25 at concentrations less than the critical gelation concentration (Table 2). Morphological analyses revealed that the ultrasound-induced self-assembly of 25 led to crystalline fibrous structures, whereas in the presence of SWNTs, the same process led to a nanotape-like morphology (Fig. 10b). Intermolecular H-bonding, π-stacking, and hydrophobic interactions were responsible for the sonication-induced formation of crystalline fibers of 25. However, the heterogeneous nucleation and growth of 25 on the SWNT surface via π-stacking and H-bonding resulted in a less crystalline nanotape-like morphology.

Fig 10
figure 10

a Chemical structure of the squaraine dye-derived gelator 25. b Photographs of 25 in n-butanol with 0.1 mg of SWNTs after sonicating for 5 min. Transmission electron microscopy images of (i) 25 gel and (ii) 25/SWNT gel (n-butanol, 2 mM). (Reprinted with permission from ref. 67 © 2013 Wiley-VCH)

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CNTs and π-conjugated supramolecules

Due to their extended π-surfaces, polyaromatic molecules such as naphthalene, anthracene, pyrene, and perylene have been used extensively to prepare hybrid materials with CNTs11,12,13,14,68,69,70,71. Supramolecular systems of these molecules have been exploited in the preparation of hybrid materials with CNTs. For instance, a supramolecular SWNT hybrid (28) composed of a ruthenium complex tethered to β-CD, 26, and a pyrene-functionalized adamantane, 27, were prepared by mixing the three components (26, 27, and SWNTs) in the solid state (Fig. 11)72. The finely ground solid was then dispersed in 0.01 M NaOH by sonication, and the interactions between the components were studied by various analytical techniques. The appearance of van Hove singularities in the absorption spectra indicated that the dispersion of SWNTs in water occurred via π–π interactions between the pyrene of 27 and the SWNTs. The fluorescence intensity of ruthenium complex 26 was quenched due to the photoinduced charge transfer process arising from the close proximity of the SWNTs and 26. The cationic supramolecular hybrid 28 was further functionalized with DNA and used for non-viral gene delivery applications (Table 2).

Fig 11
figure 11

Preparation of supramolecular hybrid 28 from host (26) and guest (27) molecules functionalized with a ruthenium complex and pyrene, respectively

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A water-soluble perylene diimide (PDI) dye, 29, functionalized with Newkome-type dendritic moieties (Fig. 12) was also used to disperse SWNTs in water73,74. The polar carboxyl groups led to enhanced water solubility, and the nonpolar perylene moiety facilitated the dispersion of SWNTs. Absorption spectroscopy revealed that 0.01 wt% of 29 was enough to stabilize the dispersion. The fluorescence of molecule 29 was quenched in the hybrid, indicating the possibility of photoinduced electron/energy transfer from 29 to the SWNTs. The tweezer-shaped molecule 30 with two PDI units (Fig. 12) facilitated the sorting of SWNTs based on their diameters75. This molecule was not only used to disperse SWNTs but also to enrich (up to 70%) the as-produced HiPco-SWNTs having a diameter of 0.8 nm (Table 2). Furthermore, the PDI-based amphiphiles 31 and 32 (Fig. 12) were also found to disperse SWNTs in organic and aqueous media76. Detailed morphological studies revealed the monolayer absorption of 31 and 32 on the SWNTs in chloroform, and a helical assembly of 31 was formed over the SWNTs in water due to hydrophobic and π-stacking interactions. Transient absorption spectroscopy indicated that a rapid photoinduced electron transfer occurs from SWNTs to 31 upon excitation. The composite of 31/SWNTs forms a porous supramolecular membrane, which was used to separate 2–6 nm-sized gold nanoparticles from a mixture having different sizes (Table 2). In addition, the hybrid materials of 31/SWNTs were utilized to prepare bucky paper for electronic applications.

Fig 12
figure 12

Chemical structures of the PDI-based molecules 29–32 used for the preparation of hybrid materials with SWNTs

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Furthermore, OPV 33 (Fig. 13a) was reported to interact with SWNTs and MWNTs to form uniform dispersions in nonpolar solvents such as n-hexane and n-decane77. Molecule 33 self-assembled over the π-surface of the MWNTs, leading to a wax-like coating (Fig. 13b). Morphological analysis of the hybrid material revealed the presence of nanoscopic and microscopic architectures, as seen in natural superhydrophobic surfaces like lotus leaves. The hybrid material exhibited a water contact angle of 165° with self-cleaning properties (Fig. 13c, d) and was stable under acidic and basic conditions (Table 2).

Fig 13
figure 13

a Chemical structure of OPV 33. b Schematic representation showing the OPV molecules self-assembled on a SWNT with their alkyl chains projected outwards. c Contact angle and d anti-wetting properties of the 33/MWNTs hybrid coated on a glass plate. (Reprinted with permission from ref. 77 © 2008 Wiley-VCH)

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The reversible noncovalent functionalization of MWNTs was demonstrated using oligo(p-phenyleneethynylene)s (OPEs) (34a, b and 35) with complementary H-bonding moieties (Fig. 14a)78. The π–π and CH–π interactions present between the MWNTs and OPEs resulted in wrapping of the H-bonded supramolecular polymer over the graphitic surface of the MWNTs, leading to the solubility of the MWNTs in nonpolar solvents. The reversibility of MWNTs solubilization was investigated by adding polar/protic solvents that perturbed the H-bonding interactions between the OPE molecules via partial detachment of the wrapped molecules followed by the precipitation of MWNTs. In addition to chemical stimuli, the role of light in the reversible solubilization of nanotubes was also studied by introducing light-sensitive azobenzenes into the OPE backbone of 36 (Fig. 14b)79. The proper orientation of the OPE molecules for better interactions with the MWNTs was achieved by the cis/trans-isomerization of the azobenzenes attached to the molecule, eventually leading to the iterative solubilization and precipitation of MWNTs (Table 2).

Fig 14
figure 14

Chemical structures of a 34a, 34b, and 35. b Structure and schematic representation showing the hypothetical interaction of a light-sensitive OPE, 36, in (i) trans (ii) cis orientations with the MWNT surface

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A triphenylene-based ionic liquid crystalline (LC) molecule (37) was capable of dispersing SWNTs in a columnar LC phase (Fig. 15a)80. The dispersed SWNTs forced the LC columns to orient in the macroscopic length scale. The LC columns and dispersed SWNTs adopted three different orientations with respect to each other: random, homeotropic, and horizontal (Fig. 15b). The alignment of the SWNTs was found to be retained for a long period, which makes the SWNT–LC hybrid materials potential candidates for anisotropic electrical conductors (Table 2).

Fig 15
figure 15

a Chemical structure of triphenylene-based ionic LC molecule 37. b Polarizing optical microscopy images of the 37/SWNTs hybrid (i) before and (ii) after shearing. (Reprinted with permission from ref. 80 © 2012 Wiley-VCH)

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CNTs and π-conjugated polymers

π-conjugated polymers interact strongly with CNTs due to their extended π-surfaces and large area of contact between them81. A large number of polymers such as poly(p-phenylenevinylene) (PPV), poly(p-/m-phenyleneethynylene) (PPE), poly(9,9-dialkylfluorene), and poly(3-alkylthiophene) have been reported to form self-assembled hybrids with CNTs81,82,83,84. Polymer 38 (Fig. 16a) formed hybrid materials with SWNTs via helical wrapping of individual or bundles of nanotubes85. The formation of the fibrous network structure by wrapping of the bundles containing a few SWNTs rather than the aggregation of individually wrapped SWNTs was confirmed by microscopic analysis. Theoretical studies predicted the parallel arrangement of 38 over the π-surface of the bundled SWNTs through strong supramolecular interactions. Polymers 39a and 39b (Fig. 16a) were used to prepare hybrid materials of polypseudorotaxanes with SWNTs86. These hybrid materials may find applications as molecular actuators and switches with improved properties (Table 2).

Fig 16
figure 16

Chemical structures of a PPV (38 and 39a, b), and b PPE (40–44) polymers and a schematic representation of a SWNT helically wrapped with 40

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Similar to PPVs, PPEs have also been found to facilitate the dispersion of SWNTs in both aqueous and organic solvents. For example, the size-selective dispersion of SWNTs of a particular chirality was achieved using polymer 40 (Fig. 16b)87. Absorption and fluorescence studies showed that the aqueous dispersion of 40/SWNTs exclusively contained small-diameter nanotubes such as (6, 5: d = 0.757 nm) and (8, 3: d = 0.782 nm), whereas the large diameter ones ((7, 5), (8, 4), and (7, 6); d = 0.829, 0.840, and 0.895 nm) were removed from the as-synthesized SWNTs samples by centrifugation. The rotation around the –C≡C– bonds of the phenyleneethynylene unit of 40 played a crucial role in the helical wrapping of SWNTs having a definite chirality. Theoretical calculations indicated that the size of the cavity formed via the helical arrangement of 40 was most suited to SWNTs with diameters of 0.757–0.782 nm (Table 2). A series of PPEs, 41–43 (Fig. 16b), were used for dispersing as-synthesized SWNTs in organic solvents through noncovalent interactions88. Polymer 41 showed a poor dispersion ability towards raw SWNTs, whereas polymers 42 and 43 with longer conjugation lengths afforded stable dispersions in organic solvents. The appearance of van Hove singularities in the absorption spectra and the quenching of the fluorescence of 42 and 43 indicated that the dispersion of the debundled SWNTs occurred via a self-assembly process. A water-soluble polymer, 44 (Fig. 16b), was also utilized to disperse SWNTs in an aqueous medium89. Absorption spectroscopy and atomic force microscopy analysis confirmed the individualization of the SWNTs and the helical self-assembly of 44 over the π-surface of the SWNTs.

A lightweight, transparent, and highly conductive thin film for optoelectronic applications was prepared from a hybrid material comprising P3HT, 21 (Fig. 7) and SWNTs (Table 2)90. The thickness of the film could be tuned by adjusting the dispersion concentration of the hybrid material. Furthermore, the hybrid films could be transferred to any substrate, and an additional layer of 21 could also be added to these conductive films by electropolymerization of 3-hexylthiophene. A regio-regular P3HT polymer, 45 (Fig. 17), was found to be suitable for dispersing SWNTs in organic solvents such as chloroform, THF and o-dichlorobenzene91. Interestingly, the hybrid 45/SWNTs exhibited a nematic lyotropic LC phase, whereas the polymer 45 and the SWNTs alone did not show any lyotropic LC properties. The appearance of birefringence in the hybrid material was reported to depend upon the SWNT concentration. The nematic lyotropic LC phase of the hybrid could have resulted from the alignment of the individualized SWNTs at high concentrations.

Fig. 17
figure 17

Chemical structures of the regio-regular P3HT polymer (45), the fluorene-based polymers (46-48) and poly(phenylene acetylene), 49, used for the preparation of hybrid materials with CNTs

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Fluorene-based polymers such as poly(9,9-dialkylfluorene) (46) and poly(9,9-dialkylfluorene-co-3-alkylthiophene) (47, Fig. 17) have been reported to disperse SWNTs in organic solvents (up to 40%)83. The dispersions were stable for several weeks, even after the removal of the free polymer. The conductivities of the hybrid materials were comparable to that of pristine SWNTs but better than those of the polymers alone. In addition, the fluorene-based copolymer was used to separate chiral SWNTs through its selective diameter-dependent interaction with a mixture of SWNTs. By introducing π-conjugated molecules with a defined chiral phase, such as R-/S-chiral binaphthyl groups, 48a, b (Fig. 17), an enantiomeric pair of SWNTs could successfully be separated (Table 2)92.

A pyrene-tethered poly(phenylene acetylene) polymer, 49 (Fig. 17), was shown to disperse MWNTs in solvents such as CHCl3 and THF93. The higher solubility of the hybrid material was attributed to the π–π interaction of the MWNTs with the π-conjugated polymer as well as the extended π-surface of the pyrene moiety. The hybrid materials exhibited bi-polar behavior in photovoltaic devices due to the uniform mixing of n-type MWNTs and p-type polymers in addition to photoinduced charge transfer between them (Table 2). All the details of the synthetic routes, properties and applications of carbon nanotube-based hybrid materials are summarized in Table 2.

Conclusions

The various reports discussed in this review reiterate the importance of functional hybrid materials consisting of π-conjugated organic small molecules/polymers and gelators with CNTs/graphene/graphene derivatives in a wide range of applications. A large number of π-conjugated systems have been exploited for the preparation of hybrid materials with intriguing optical and electronic properties. The insights gained from these studies over the years have helped researchers to design new materials with improved properties. For example, small aromatic molecules such as pyrene, perylene, naphthalene, and anthracene with appropriate functional groups have been designed and utilized in the preparation of self-assembled hybrid materials with CNTs and graphene/graphene derivatives. Moreover, the incorporation of these carbon nanomaterials in gel media improves the physical and mechanical properties of the hybrid systems relative to those of the individual counterparts. The extent of improvement in the material properties depends mainly on the strength of the noncovalent interactions between the carbon nanomaterials and the molecules that undergo self-assembly. The preparation of these hybrid materials via supramolecular self-assembly has several advantages, such as facilitating the solution processability of carbon nanomaterials while preserving their inherent electronic properties. Hybrid materials composed of conducting polymers such as polyaniline, polypyrrole, and polythiophene have potential for application in novel light-harvesting systems, solar cells, and organic field-effect transistors. Thus, the noncovalent functionalization of carbon allotropes with aromatic molecules/polymers and gelators is a promising strategy for realizing hybrid materials with potential applications. However, a breakthrough in the practical use of these hybrid materials is yet to come, and this unmet need will further drive research in this area.