**1. Introduction**

Non-radiative energy transfer (ET) is one of the most fundamental processes at the nano‐ scale. It is associated with funnelling excitation energy between molecules, quantum dots or other nanostructures [1]—in most cases—via dipole-dipole interaction. Such a scheme evolved

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for instance in natural photosynthesis [2] for efficient capturing and transport of the sun‐ light energy, and has been recently implemented in artificial light-harvesting assemblies [3]. The efficiency of ET depends on the spectral properties of a donor and an acceptor,their mutual orientation, as well as separation between them [4]. In particular, the distance dependence of the ET efficiency, which for localized dipoles scales with d−6, has been exploited as a useful tool to measure lengths at the nanoscale, both statically and dynamically. In particular, the energy transfer has been considered an attractive way to control light harvesting and bio(sensing) [5–7]. As a result, various strategies had to be devised to fabricate hybrid nanostructures with well-defined morphology required for controlling the energy transfer efficiency. Among the most feasible, one can find robust nanolayers, either polymer or dielectric, deposited on a surface of nanoparticles or a substrate [8, 9], and more flexible linkers based for instance on DNA strands or biotin-streptavidin conjugation [10, 11].

Another critical parameter that influences the interaction between two dipole moments in the context of the energy transfer is the relation of their spectral properties. Namely, as shown in **Figure 1**, the absorption of one of the molecules (acceptor) has to overlap with the emission of the second molecule (donor). The larger the overlap, the higher the efficiency of the energy transfer. The final parameter that has to be considered in energy transfer geometry is the mutual orientation of the dipole moments of a donor and an acceptor. All these factors are included in the equation shown in **Figure 1**. It is important to note that the equation de‐ scribes the case where both donors and acceptors are classical dipole moments.

**Figure 1.** Schematic representation of the energy transfer between two dipole moments of a donor (green) and acceptor (red). The energy transfer is possible only when the emission of the donor overlaps with the absorption of the acceptor.

Optical spectroscopy, and in particular fluorescence spectroscopy, provides a variety of tools to probe the energy transfer in hybrid nanostructures. It stems from the fact that the emer‐ gence energy transferresults in the decrease of the emission intensity of a donor at the expense emission intensity of an acceptor. This is in fact the most straightforward consequence energy transfer between two nanostructures. In addition to the intensity flow between donors and acceptors, another signature of the energy transfer is a shortening of the fluorescence decay time of the donor. Indeed, the energy transfer can be considered as a new channel for nonradiative recombination from the point of view of the donor, and as such it should result in shortening of the lifetime.

The purpose of this chapter is to review recent research carried out on hybrid nanostruc‐ tures composed of graphene and graphene derivatives and naturally evolved photosynthet‐ ic complexes. Our aim is to emphasize effects that are not readily available when studying classical emitters, such as organic dyes or semiconductor nanocrystals, which have spectral‐ ly limited absorption and emission, as in contrast to these, photosynthetic complexes feature absorption that spans over the whole visible spectral range. They are also pigment-protein complexes, with chlorophyll molecules protected from the environment. Thus, the interac‐ tion between photosynthetic complexes and graphene is not immediate. However, before we describe the results obtained for photosynthetic complexes coupled with graphene, we review several key results reported for organic dyes and semiconductor nanocrystals on graphene. This is important to illustrate basic mechanisms and processes that take part in such hybrid architectures.
