*2.3.1. Covalent functionalization of graphene*

Similar to fullerenes and carbon nanotubes (CNTs), functionalization of graphene with different functional groups can open up new routes to hybrid materials that exhibit even more exciting features than graphene itself.[46] The design and feature tuning/altering of transpar‐ ent pristine graphene by integrating a versatile electron-donor system has attracted more attention. However, modification of the flat and rigid structure of graphene is a more chal‐ lenging work than that of the curvature structured fullerenes and carbon tubes because of the

Recently, rational design and efficient strategy for preparation of photoactive graphene have achieved considerable progresses, which are motivated by many potential applications of photoactive graphene. For example, a large number of photoactive graphene with different properties and structures have been designed and synthesized. As shown in Figure 4, photo‐ active moieties and graphene were linked through either covalent functionalization approach, such as amidation reaction, cycloaddition, Suzuki coupling, "click" chemistry, or noncovalent functionalization manner, including π–π interaction, electrostatic interaction, and electrostat‐

necessity to overcome a high-energy barrier.[47, 48]

96 Graphene - New Trends and Developments

**Figure 4.** Chemical functionalization approach for preparation of photoactive graphene.

optical and/or optoelectronic applications may be generated.[49]

Photoactive organic moieties, including small molecules and conjugated polymers have been used to prepare photoactive graphene. The above-mentioned organic photoactive molecules are generally planar, electron-rich, and liable to photochemical electron-transfer process and show remarkably high extinction coefficients in the visible region. It is expected that by combining graphene with photoactive molecules, multifunctional graphene composites for

ic–π interaction.

Recently, combining graphene with photoactive organic functional components, including small molecules and polymers, has attracted widespread attention, and a number of important photoactive moieties have been attached either to edge or the basal plane of graphene surface through covalent functionalization or noncovalent functionalization approaches.

Covalent functionalization of graphene with photoactive small molecules offers key advan‐ tages such as greater stability of the composite materials, and control over the degree of functionalization, and good reproducibility.[50] Meanwhile, conjugated polymers are one of the most successfully exploited classes of materials due to the incredible variety of chemical structures and their relatively low cost, facile processing, and their possible recyclability and applicability as sustainable materials.[51] Employing these organic photoactive molecules to functionalize graphene has grown to become a crucial branch in nanoscience and nanotech‐ nology, which offers significant potential in the development of advanced photoactive graphene materials in numerous and diverse application areas.[52]

**Amidation reaction:** Chen *et al.* reported the first example of covalently attaching porphyrin units onto graphene, and explored the photophysical properties of the graphene composites. [53] The synthesized porphyrin–graphene composites consist of amino-containing porphyrin (TPP-NH2) molecule and GO covalently bonded together *via* amidation reaction. Fourier transform infrared (FTIR) spectroscopic characterization confirmed that the TPP-NH2 molecules had been covalently bonded to the edge of GO by the amide linkage. The linear relationship between the absorption and the concentrations of graphene moiety in the composites indicates good dispersibility of composite. Moreover, the fluorescence quenching of TPP-NH2 was observed, indicating that there is a strong interaction between the excited state of TPP-NH2 and graphene moieties in the composite. Kang *et al.* used arylaminecontaining conjugated polymer TPAPAM to covalently modify GO.[54] The covalent attach‐ ment of TPAPAM onto the GO *via* amide linkage was confirmed by XPS and FTIR spectroscopy. In contrast to fluorescence quenching often observed in luminescence molecule– graphene systems, the steady-state fluorescence spectra showed that electronic interaction between TPAPAM and GO entities resulted in enhancement of the fluorescence intensity of the parent TPAPAM. In addition to the examples provided above, amidation reaction was also applied to link fullerene,[55] phthalocyanine zinc (PcZn),[56] and oligothiophene moieties to the surface of graphene.[57]

**Cycloaddition reaction:** Feringa *et al.* applied one-pot cycloaddition approach to prepare porphyrin derivative functionalized graphene composites.[37] The excited state energy/ electron-transfer processes between graphene and the covalently attached porphyrin mole‐ cules was demonstrated from fluorescence quenching and reduced fluorescence lifetime phenomenon.[58] Guldi *et al.* presented their work on linking photoactive phthalocyanines (Pcs) to graphene surfaces.[50] Covalent functionalization of the fewer-layered graphene with Pcs was achieved through 1,3-dipolar cycloaddition and the esterification reaction yielded Pcsgraphene nanoconjugate.

**Suzuki coupling reaction:** The 2010 Nobel Prize for Chemistry rewarded a family of palladi‐ um-catalyzed coupling reactions for forging carbon–carbon bonds, which have already helped to create new graphene hybrid materials. Ma *et al.* reported covalent functionalization of graphene with polythiophene through Suzuki coupling reaction.[59] A donor–spacer– acceptor triad conjugated polymer containing fluorene, thiophene, and benzothiadazole moieties, which was covalently attached to r-GO *via* Suzuki polymerization procedure.[60] These polymer–graphene composites show excellent solubility in different type of solvents and exhibit superior optical-limiting performance. Moreover, Loh *et al.* applied Heck reaction to synthesize dye molecule functionalized graphene composite.[61] In their work, r-GO was covalently modified by diazonium, followed by the Heck reaction to give a 4-(2-(pyridin-4 yl)vinlyl)phenyl group modified graphene. Considering the high efficiency of the palladium catalyzed C-C coupling reaction, we believe that more and more attention will be paid to the synthesis of photoactive-moieties–graphene hybrid materials.

**"Click" chemistry:** "Click" chemistry has emerged as a useful strategy for rapid and efficient attachment of functional groups to various materials since its reinvention in 2001.[62] In previous works, "click" chemistry has succeeded in linkage of various functional groups onto CNTs and fullerenes.[63, 64] Zhang *et al.* reported a facile approach for covalently attaching various photoactive organic molecules onto graphene surfaces *via* "click" chemistry.[65] Kaminska *et al.* presented a one-step protocol for simultaneous reduction and functionalization of GO with a dopamine derivative bearing an azide function. The chemical reactivity of the azide moieties was demonstrated by a post-functionalization with ethynylferrocene using "click" chemistry.[66] Salvio *et al.* treated GO suspension with sodium azide, and the obtained azido derivative can be used to functionalize the graphene oxide with long alkyl chains through a "click" chemistry approach. This functionalization results in the exfoliation of this material in organic solvent.[67] Salavagione *et al.* reported the preparation of polyfluorenemodified graphene by azide–alkyne "click" coupling.[68]
