**3. Energy transfer and charge transfer**

**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

**"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 polyfluorene-

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

**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.*

graphene hybrid materials generally have good solubility in polar solvents.

synthesis of photoactive-moieties–graphene hybrid materials.

98 Graphene - New Trends and Developments

modified graphene by azide–alkyne "click" coupling.[68]

*2.3.2. Noncovalent functionalization of graphene*

It is believed that electron-transfer and energy-transfer processes between photoactive chromophores and graphene are the two fundamentally important processes responsible for the photophysical processes of photoactive graphene.

Generally, each decay step of an excited photoactive molecule is characterized by its own rate constant and each excited state is characterized by its lifetime. In solution, when the intramo‐ lecular deactivation processes are not too fast, that is, when the lifetime of the excited state is sufficiently long, an excited photoactive molecule may have a chance to encounter graphene. In such a case, some specific interaction can occur, leading to the deactivation of the excited state by second-order kinetic processes.[78]

Disentangling the detailed charge-transfer and energy-transfer dynamics in photoactive graphene composite is essential for a full understanding of their photophysical properties, and is expected to open new avenues for their unique and specific applications.[79] For this purpose, characterization of charge- and energy-transfer rates is essential. Although the investigations on the properties of the excited states and excitons are very challenging, this generally involves delicate fluorescence lifetime and transient absorption spectroscopic measurements.
