Charge separation, charge recombination, long-lived charge transfer state formation and intersystem crossing in organic electron donor/acceptor dyads

Photo-induced charge separation (CS) and charge recombination (CR) are crucial for molecular systems used in artificial photosynthesis, photovoltaics, and photocatalysis, and these processes are also important for fundamental photochemical studies. This review article focuses on the recent developments in CS/CR, long-lived charge transfer (CT) state formation, and CR-induced intersystem crossing (ISC) in organic molecular systems, ranging from discussions on the molecular structural features dictating the kinetics and thermodynamics of CS/CR and electron spin transport, as well as the spectroscopies used for the characterization of the CS/CR and the CR-induced ISC. Emphases are put on the molecular design rationales, from the point of view of organic chemists, to achieve fast and efficient CS but slow CR, as such to access the long-lived CT state. The roles of the electron spin in controlling the CS, CR and formation of the long-lived CT state are discussed with exemplars. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy for characterization of the charge and electron spin transport in the CS, CR and ISC processes is introduced.