Electron spin dynamics in excited state photochemistry: recent development in the study of the intersystem crossing and charge transfer in organic compounds

Electron spin dynamics are crucial to photochemical and photophysical processes. However, to large extent it is neglected in routine photochemistry studies. Herein we summarized the recent development of the electron spin dynamics in organic molecular systems. The electron spin selective of the intersystem crossing (ISC) and charge separation (CS) and charge recombination (CR) of the organic molecular system are discussed, including ISC of the compounds with twisted - conjugation frameworks and CR-induced ISC in compact orthogonal electron donor-acceptor dyads. We found that the electron spin polarization (ESP) of the triplet state formed in these systems is highly dependent on the molecular structure and geometry. The zero-field-splitting (ZFS) D and E parameters of the triplet state of series chromophores determined with the time-resolved electron paramagnetic resonance (TREPR) spectroscopy are presented. Some unanswered questions in the related areas are raised, which may inspire further theoretical investigations. The examples demonstrate that the study of the electron spin dynamics is not only important in fundamental photochemistry to attain in-depth understanding of the ISC and the charge transfer processes, but also useful for designing new efficient organic molecular materials for applications in photodynamic therapy, organic light-emitting diodes and photon upconversion, etc.