*2.6.3 Nanoscale electronic energy transfer*

Additional electrical energy emitted by an optical transition occurred due to nanoscopic interaction dynamics may be transmitted from one center (atom, ion, or molecule) to the next, typically on a nanoscopic level, though long-range transfer of energy is also possible. This transfer of electrical energy does not require the flow of electrons, but rather the transfer of excess energy. As a result, one center of this mechanism has excess energy in an excited electronic state and serves as an energy donor by passing the excitation to an acceptor, resulting in an electron in the energy donor group becoming excited and returning to the ground state, whereas an electron in the energy acceptor group becomes excited. Exciton migration occurs as a result of interactions between energetically related centers, either coherently by a series of closely spaced levels creating an exciton band or by hopping an electron– hole pair from one core to another incoherently [6]. Fluorescence resonance energy transfer (FRET) is also another form of energy transfer that happens when two different kinds of molecules interact. Fluorescence from the energy acceptor can be detected by optically exciting a molecule to a higher electronic state, and this type of transition is frequently observed with two fluorescent centers separated by a few nanometers. FRET is a widely used bioimaging technique for probing nanoscale interactions between cellular components, such as protein–protein interactions [7]. Throughout this context, one protein may be identified with a fluorescent dye which, once electronically excited by light, behaves as an energy donor. If two proteins are well within 1–10 nm of one another, the other protein is identified as an energy acceptor, and will absorb energy when the two proteins are within this nanoscopic range of distances. This type of energy transfer occurs often in a dipole– dipole interaction with a distance dependency between the energy donor and acceptor. To maximize FRET activity, the donor's emission spectrum and acceptor's absorption spectrum may have significant spectral overlap [6].

*Nanophotonics: Fundamentals, Challenges, Future Prospects and Applied Applications DOI: http://dx.doi.org/10.5772/intechopen.98601*
