Intersystem crossing after photoexcitation

Photoinduced electron transfer (PET Scheme 6.2) is a mild and versatile method to generate radical ion pairs in solution," exploiting the substantially enhanced oxidizing or reducing power of acceptors or donors upon photoexcitation. The excited state can be quenched by electron transfer (Eq. 7) before (aromatic hydrocarbons) or after intersystem crossing to the triplet state (ketones, quinones). The resulting radical ion pairs have limited lifetimes they readily undergo intersystem ... 

We focused here on triplet-state reactions occurring after photoexcitation. We have not treated the ISC event rigorously, and believe, subject to the evidences provided above, that the ISC event itself is of minor interest in these systems. However, there exist systems where the ISC itself plays an important role. Recent advances in method developments have provided a tool for treating intersystem crossing events. Worthwhile to mention is the work of Gonzalez (SHARC) [71] and Thiel and coworkers [72] in treating internal conversion (IC) and ISC events on the same footing. 

The study of the triplet dynamics in fully conjugated polymer hosts turned out to be difficult, because the intrinsic phosphorescence is barely observed from these polymers. The main reason is the lack of spin-orbit coupling in these carbon-based conjugated polymers, resulting in low intersystem-crossing (ISC) rates. Consequently, the polymer triplet state is only weakly populated after photoexcitation and the quantum yield of phosphorescence is low. 

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