The primary result of UV-Visible photon absorption by complex organic molecules is the population of short-lived
electronic excited states. Transportation of their excitation energy between single molecules, formally mediated by
near-field interactions, may occur between the initial absorption and eventual fluorescence emission events, commonly
on an ultrafast timescale. The routing of energy flow is typically effected by a sequence of pairwise transfer steps over
numerous molecules, rather than a single step over the same overall distance. Directionality emerges when there is
structure in the molecular organisation. For a chemically heterogeneous system with local order, and with suitable
molecular dispositions, automatically unidirectional transfer can be exhibited as the result of a 'spectroscopic gradient'.
However it is also possible to exert control over the directionality of excitation flow by the operation of external
influences. Examples are the application of an electrical or optical stimulus to the system - achieved by the
incorporation of an ancillary polar species, the application of a static electric field or electromagnetic radiation. Most
significantly, based on the latter option, an all-optical method has recently been determined that enables excitation
transportation to be completely switched on or off, such that the energy flow is subject to controllable photoactivated
gating. It is already apparent that this photonic process, termed Optically Controlled Resonance Energy Transfer, has
potentially numerous applications. For example, it represents a new basis for optical transistor action.