**3.1. Charge transfer**

Excited-state photophysical processes between graphene and photoactive moieties have been of much importance because of their relevance to optoelectronic and photo-energy conversion applications.[80] In a considerable number of cases, the phenomena of graphene to quench fluorescence of aromatic molecules is shown to be associated with photo-induced electrontransfer process, and can be conveniently verified by the fluorescence decay and time-resolved transient absorption spectroscopic characterizations. These measurement results provide quantitative insights, both kinetically and spectroscopically, into the nature of the interactions of graphene and photoactive molecules.

Kamat *et al.* reported the excited electron-transfer interaction between the photo-excited porphyrin and graphene (Figure 5).[81] In their work, cationic 5,10,15,20-tetrakis (1-methyl-4 pyridinio)porphyrin tetra (p-toluenesulfonate), noted as TMPyP, was employed to noncova‐ lent functionalization of graphene (Kamat *et al., 2010*). Upon complexation with graphene, the fluorescence lifetime of porphyrin was significantly reduced from 5 ns to 1 ns. Moreover, the femtosecond transient absorption measurements confirmed the formation of a short-lived singlet excited state of 1 (TMPyP)\* and a subsequent longer-living porphyrin radical cation of (TMPyP)⋅+ with an absorption maximum around 515 nm, which clearly indicated the occur‐ rence of electron-transfer process between TMPyP and graphene. Furthermore, it inferred that electron injection from the 1 (TMPyP)\* to the graphene film is feasible because the oxidation potential of the 1 (TMPyP)\* is -0.29 V *vs* normal hydrogen electrode (NHE), which is lower than the Fermi level of the graphene material (0 V *vs* NHE); the resulting energy gap hence provides sufficient driving force for the charge-transfer process.[7, 82]

Malig and coworkers reported the transient absorption characterization studies on the interactions of zinc phthalocyanines (ZnPc) oligomer–graphene composite both in the ground state and excited state. The experiment results confirmed that the nature of these interactions is electron transfer from ZnPc to graphene, both in the ground and in the excited state, affords an electron-transfer product that survives for several hundred picoseconds.[83] More inter‐ estingly, by combining with characterization results of the steady-state and femtosecond time-

**Figure 5.** Photo-excited TMPyP molecules undergo charge-transfer interaction with r-GO (Kamat *et al., ACS Nano* 2010, 4, 6697-6706).

resolved spectroscopy, Mohammed *et al*. found that the charge-transfer process at the porphyrin–graphene carboxylate interfaces could be tuned from zero to very sufficient and ultrafast by changing the electronic structure of the meso unit and the redox properties of the porphyrin cavity.[84]
