**2.3. Chemical functionalization**

cial charge transfer is necessary for the improvement of the conversion efficiency from

In recent years, there has been a growing interest in graphene functionalized with photoactive units owing to their significance in both fundamental research and practical applications. Recent research results have demonstrated that chemical functionalization of graphene with

Graphene can be functionalized at the basal plane and the edges.[32] On the basal plane, sp2 hybridization of carbon leads to a strong covalent bonding, as well as delocalization of the π electrons. The sheet edges are considered the most reactive sites,[33] and the dangling bonds at edge sites of graphene are highly reactive to guest atoms or molecules. In addition, the functional molecules attached onto the basal plane of graphene lead to modification of the π– π conjugation and thus the physical and chemical properties and the electron density distri‐ bution. There are generally three major purposes for chemically functionalizing graphene: enabling its solution processing, tuning its energy level and gap, as well as providing photo‐

A major obstacle in the synthesis and processing of bulk-quantity graphene sheets is the preparation of monolayer graphene and its insolubility, with the latter being responsible for poor handling and manipulation during graphene processing.[34, 35] Graphene has a strong tendency to cluster together into aggregation, caused by the electrostatic forces and the strong π–π interaction between individual graphene flakes, which make further manipulation and device fabrication using graphene difficult.[36] For this reason, covalent and noncovalent manners for modification of graphene have been developed for exfoliation and dispersion of graphene.[37] Besides, by derivatizing graphene with different moieties, the solubility of graphene can be tuned to suit varied solvents needed for different applications. For example, chemically grafted CH2OH-terminated regioregular poly(3-hexylthiophene) (P3HT) onto carboxylic groups of graphene oxide (GO) *via* amidation reaction; the resultant P3HT-grafted GO sheets are soluble in common organic solvents, which facilitates the structure/property

Experiment and theory studies demonstrated that semiconductor graphene (p-type and ntype) can be obtained by modifying graphene with organic semiconductor molecules (electron-acceptor and electron-donor), which provides a simple and nondestructive way of tuning the Femi level and controlling the charge carriers concentration of graphene.[39] As shown in Figure 3, for the p-type graphene, the Fermi level shifts upward relative to the Dirac point when the electron-acceptor coverage increases. In contrast, for the n-type

characterization and the device fabrication by solution processing.[38]

optoelectronic and photo-energy to electricity.

94 Graphene - New Trends and Developments

photoactive moieties is a necessity to harvest its full potential.[8]

**2. Rational design of photoactive graphene**

active functionalities.[8]

**2.2. Energy level tuning**

**2.1. Processability**

The chemistry of graphene is a powerful route to tailor its properties through introduction of various chemical functional groups to graphene.[32] For most applications, graphene need to be integrated with other functional materials, and the modification of graphene *via* chemical approach holds promise for tuning the electronic and optical properties of graphene, control‐ ling interfaces with other materials, and tailoring surface chemical reactivity.[43] Therefore, functionalization of graphene with various functional components is considered to be crucial for graphene processing.[44, 45]

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

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‐ ic–π interaction.

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

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