A Little Photochemistry and Luminescence
A round-up of the main photochemical principles underlying chemically-driven luminescent switches is presented and their relationships are outlined. Several of these principles relate to electronically excited states involving charge transfer of some description: internal charge transfer (ICT), twisted internal charge transfer (TICT) and charge transfer (CT). The switching behaviour is controlled by charge effects of the chemical inputs once they engage a receptor site. On the other hand, the switching action of metal-centered (MC) or nπ* and ππ* excited states are due to more subtle chemical interactions. Post-excitation processes such as photoinduced electron transfer (PET), excited state intramolecular proton transfer (ESIPT), electronic energy transfer (EET) and association, e.g. excimer formation, also find wide use.