Electron and energy transfer mechanisms for fluorescence modulation with photochromic switches

The electronic and structural changes associated with photochromic transformations can be exploited to modulate the emission intensity of fluorescent compounds on the basis of electron and energy transfer processes. Indeed, fluorescent and photochromic components can be joined covalently or noncovalently into molecular or supramolecular assemblies respectively and the emission of one component can be regulated by operating the other with optical stimulations. In fact, the photoinduced and reversible interconversion of the colorless and colored states of the photochromic component are accompanied by significant changes in absorption. These changes alter the degree overlap between the absorption band of a photochrome and the emission band of a fluorophore and, thus, activate or prevent the transfer of energy from the latter to the former. Here is presented a survey of diverse fluorophore–photochrome conjugates, macromolecular constructs, supramolecular assemblies, photoswitchable nanoparticles and multilayer arrays which operate successfully with optical inputs.