*2.3.2. Noncovalent functionalization of graphene*

Noncovalent functionalization strategy is advantageous in the preservation of the properties of the graphene, while weak forces between absorbed molecules and graphene may lower the load transfer in the composite, and as a result of free molecules and molecules adsorbed on graphene exist in equilibrium in the solution. Moreover, photoactive small molecules are commonly planar in structure and electron-rich; these advantages promote the interaction between the small molecules and graphene *via* π–π stacking, electrostatics interactions, and electrostatic–π interactions, as illustrated in Figure 4. Meanwhile, conjugated polyelectrolyte with highly electron-delocalized backbones and ionic side chains are water-soluble, fluores‐ cent, rigid-rod polymers, which thereby combine the electronic properties of conjugated polymers with the electrostatic behavior of electrolytes. The conjugated polyelectrolytes and graphene hybrid materials generally have good solubility in polar solvents.

**Via π–π stacking:** Highly aromatic molecules may assemble themselves onto graphene surface *via* π–π stacking interaction. The π–π stacking interaction between aromatic skeleton of graphene and conjugated planar molecules afford synergistic binding interactions. Loh *et al.*

observed that the perylene wire could be coated on graphene surface to form a hybrid system. Such type of synergistic interaction between organic nanostructures and graphene affords a novel route to synthesis of hybrid materials with new properties and novel functions.[69] Mai *et al.* synthesized methyl blue functionalized graphene which exhibited excellent solubility and stability in water due to the photoactive molecules that were noncovalently stacked onto the basal plane of graphene.[70]

**Via electrostatic interaction:** GO and r-GO in aqueous dispersion is expected could act like a two-dimensional conjugated polyelectrolyte because of they are negatively charged, on which the cationic aromatic derivatives can be assembled through electrostatic and π–π stacking interactions. Shi *et al.* reported the supramolecular assembly and complexation of r-GO sheets with cationic porphyrin derivative in aqueous media.[71] In the UV-visible spectra, the Soret band of prophyrin showed a large bathochromic shift (37 nm) after linking on r-GO sheets. This phenomenon was attributed to the structure isomerization of porphyrin, which was caused by twist of their cationic methylpyridinium groups. For the structure of the unstrained porphyrin derivative, four cationic methylpyridinium moieties are nearly perpendicular to the plane of porphyrin because of the strong steric hindrance.[72] When the pyridinium groups rotate toward the coplanar conformation with respect to the flattened porphyrin ring, the π conjugation and electron-withdrawing effect of porphyrin will be enhanced.[73] Zhang and coworkers employed a specially designed anionic CPE to stabilize r-GO during the hydrazinemediated reduction in aqueous solution.[74] The conjugated-polyelectrolyte (CPE) function‐ alized r-GO sheets show good solubility in a variety of polar solvents, because of the presence of double bonds along the backbone-endowed PFVSO3 with a preferred coplanar backbone geometry, which matched the flat shape of r-GO and thus further enhanced the π–π interac‐ tions.

**Via electrostatic–π interaction:** The chemistry community now recognizes the electrostatic– π interaction as a major force for molecular recognition.[75] Jia *et al.* presented a novel molecular strategy based on cationic dye Rhodamine B (RhB) to functionalize graphene *via* in situ reduction.[76] RhB was designed to prevent the aggregation of graphene when it was modified to graphene surface through cation–π and π–π interaction. Zhang *et al.* reported the design and synthesis of a novel amphiphilic graphene composite by using amphiphilic coil– rod–coil conjugated triblock copolymer (PEG-OPE) as stabilizer to improve the dispersibility of r-GO. The rational designed PEG-OPE is composed of one lipophilic π-conjugated oligomer and two hydrophilic PEG coils. The conjugated rigid-rod backbone of PEG-OPE prefers to adsorb onto the basal plane of r-GO *via* strong π–π interaction; whilst the lipophilic side chains and two hydrophilic coils of the backbone would fly away from the surface of r-GO to form an amphiphilic outer layer, consequently facilitating the dispersion of modified r-GO common solvents.[77]
