Non-conventional Electronic Spin Conversions in the Excited States of Organic Materials: Spectroscopic Studies on Reverse Intersystem Crossing and Singlet Exciton Fission

As the demands for diverse and complex functionality, the development of photo-functional molecular materials is advanced. In molecules without heavy metals,the electronic excited states can be largely classified into luminescent singlet excited states and non-luminescent triplet excited states. The ability to transition between these two states with vastly different properties through intra/intermolecular electron spin conversion,coupled with the diversity of molecular design, provides a unique advantage in designing photo-functions. Thus, understanding electron spin conversion in molecular systems is fundamentally important from both fundamental science and application perspectives. However, electronic functions are heavily modulated by vibrational degrees of freedom inherent in molecules. Also, study of their photo-functions requires extensive use of time-resolved spectroscopy using ultrafast lasers for capturing the short-lived excited states. Here, several attempts to disentangle the complex molecular excited state dynamics based on ultrafast spectroscopy were introduced. In particular, reverse intersystem crossing and singlet fission are the main topics.