In the wide range of materials that are characterised by broad, relatively featureless optical spectra, the absorption of light in the ultravioletâvisible wavelength region is typically followed by rapid internal processes of dissipation and degradation of the acquired energy, the latter ultimately to be manifest in the form of heat. In more complex materialsâthose comprising a variety of light-absorbing atomic or molecular components (chromophores) with optically well characterised absorption and fluorescence bandsâthe absorption of light is commonly followed by a spatial translation of the absorbed electromagnetic radiation between different, though usually closely separated, chromophores. The process takes place well before the completion of any thermal degradation in such materials. This primary relocation of the acquired electronic energy, immediately following photo-excitation, is accomplished by a mechanism that has become known as resonance energy transfer (RET). (At an earlier stage in the development of these ideas, the term âresonanceâ was used to signify that no molecular vibrations were excited; however, such usage is now known to be relevant to few systems and has largely fallen into abeyance.) An alternative designation for the process is electronic energy transfer (EET); both terms are widely used, and in each case, the first letter of the acronym serves as a distinction from electron transfer.
In complex multichromophore materials, the singular properties of RET allow the flow of energy to exhibit a directed character. Because the process operates most efficiently between near-neighbor chromophores, the resonance propagation of energy through such a system generally takes the form of a series of short steps; an alternative process involving fewer long steps proves considerably less favourable. In suitably designed materials, the pattern of energy flow following optical absorption is thus determined by a sequence of transfer steps, beginning and ending at chromophores that differ chemically, or, if the chromophores are structurally equivalent, through local modifications in energy level structure reflecting the influence of their electronic environment. Hence, individual chromophores that act in the capacity of excitation acceptors can subsequently adopt the role of donors. This effect contributes to a crucial, property-determining characteristic for the channeling of electronic excitation in photosynthetic systems; the same principles are emulated in synthetic energy harvesting systems such as the fractal polymers known as dendrimers.
The observation and applications of RET extend well beyond the technology of light harvesting, as will be demonstrated in later sections of this chapter. The phenomenon has an important function in the operation of organic light-emitting diodes (OLEDs) and luminescence detectors; in crystalline solids and glasses doped with transition metal ions, mechanisms based on RET are also engaged for laser frequency conversion. In the fields of optical communications and computation, several optical switching and logic gate devices are founded on the same principle. As we shall see, those possibilities have been considerably extended by a recent discovery that electron spin can be transferred along with the energy. In the realm of molecular biology, the determination of protein structures and the characterisation of dynamical processes are furthered by studies of the transfer of energy between intrinsic or âtagâ chromophores; other ultrasensitive molecular imaging applications are again based on the same underlying principle. Further applications include energy transfer systems designed to act as analyte-specific sensors and as sensitisers for photodynamic therapy. Last but not least, RET provides a rich ground for exploring the fundamental issues that arise from the nanoscale interplay of electromagnetism and quantum mechanics.