Electron Spin Dynamics of the Intersystem Crossing of Triplet Photosensitizers That Show Strong Absorption of Visible Light and Long-Lived Triplet States
Zhijia Wang, Xue Zhang, Jianzhang Zhao
Abstract
Intersystem crossing (ISC) is one of the fundamental photophysical processes. Achieving efficient ISC is critical in the design of triplet photosensitizers and also in the study of photochemistry. Most of the previous studies on ISC and the triplet state are focused on the kinetics of the ISC, the quantum yield, or the lifetime of the triplet states, with femtosecond or nanosecond transient absorption spectroscopic methods. However, another fundamental feature of ISC, i.e., the electron spin selectivity, or the electron spin polarization (ESP) of the triplet state, was much less studied, especially for the newly developed triplet photosensitizers. These features characterize the non-Boltzmann populated sublevels of the T1 state. Herein we summarized the recent progress on the study of the electron spin selectivity of ISC and the ESP of the T1 state of the triplet photosensitizers that show strong absorption of visible light and a long lifetime of the triplet state. The electron spin selectivity of the compact orthogonal electron donor–acceptor dyads (spin–orbit charge transfer ISC mechanism) and compounds with twisted π-conjugation molecular structures, studied with the time-resolved electron paramagnetic resonance (TREPR) spectroscopy, are discussed.