**3.2. Energy transfer**

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

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

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

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

(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

the Fermi level of the graphene material (0 V *vs* NHE); the resulting energy gap hence provides

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-

sufficient driving force for the charge-transfer process.[7, 82]

(TMPyP)\* and a subsequent longer-living porphyrin radical cation of

(TMPyP)\* is -0.29 V *vs* normal hydrogen electrode (NHE), which is lower than

(TMPyP)\* to the graphene film is feasible because the oxidation

state by second-order kinetic processes.[78]

100 Graphene - New Trends and Developments

of graphene and photoactive molecules.

measurements.

**3.1. Charge transfer**

singlet excited state of 1

potential of the 1

electron injection from the 1

Graphene exhibits metallic behavior in many respects, in particular, graphene is shown to be a good exciton sink due to the highly efficient nonradiative energy transfer from the nearby fluorescent units through dipole−dipole coupling, which is also known as Forster-type resonant energy transfer (FRET).[85-89] Recently, FRET has been employed for interpreting the energy interaction of graphene combined with photoactive materials such as semiconduc‐ tor nanoparticles and dyes (Figure 6).

Theoretical and experimental studies have disclosed efficient energy transfer to graphene and the process was found to be useful in identifying graphene sheets both on substrates and in solution.[90] Sebastian *et al*. studied the distance dependence of the rate of reso‐ nance energy transfer from the excited state of a dye (pyrene and nile blue) to the π skeleton of graphene.[91] Using the tight-binding model for the system and the Dirac cone approx‐ imation, the analytic expression for the rate of energy transfer from an electronically excited dye to graphene was obtained. It was found that graphene is a very efficient quencher of the electronically excited states and that the rate is proportional to *d*-4 (*d* is distance). Koppens *et al.* measured Rhodamine emitter lifetimes as a function of Rhodamine–graphene distance d, and found agreement with a universal scaling law, governed by the fine-structure constant. The observed emitter decay rate is enhanced 90 times (energy-transfer efficiency of ∼99%) with respect to the decay in vacuum at distances d ≈ 5 nm.[88] Zhao *et al.* reported their study on employing ethidium bromide (EB) as a model for constructing an inexpen‐ sive and label-free biosensor to improve the sensitivity performance of GO–DNA-based sensors. Experiment results indicated that the fluorescence of EB was quenched by GO in the process of long-range resonance energy transfer.[92]

**Figure 6.** Nonradiative decay by dipolar coupling to electron–hole pair transition in the graphene surface and to a low‐ er extent through the emission of radiation.
