Photoresponsive ion‐pairing assemblies

Abstract This review provides an overview of the π‐electronic ion pairs and assemblies that demonstrate photoresponsive behaviors through photoisomerization and photoinduced electron transfer. Various ion pairs arise from the incorporation of ionic substituents to photoisomerizable π‐electronic systems, such as azobenzene and diarylethene moieties, and their association with counterions. Conversely, numerous ion pairs, made up of positively and negatively charged π‐electronic systems serving as electron acceptor and donor units, respectively, are prepared through ion‐pair metathesis. In both scenarios, the design and synthesis of charged π‐electronic systems play a crucial role in ion‐pairing assemblies. Moreover, the structures and properties of these assembled entities are influenced by the inclusion of accompanying counterions. These ion pairs display photoresponsive actions, including phase transitions through photoisomerization and the emergence of excited‐state radical pairs due to photoinduced electron transfer. Notably, π‐stacked ion pairs ( π‐sips ) composed of charged porphyrins transform into π‐stacked radical pairs ( π‐srps ) upon photoexcitation. Multiple components, as oppositely charged π‐electronic systems associated by noncovalent interactions, possessing varying electronic states and traits are suitable for switching behaviors in response to external stimuli.