Reaction dynamics of excited radical ions revealed by femtosecond laser flash photolysis

Herein, we review studies on the dynamics of excited radical ions. The enhanced reactivities of excited radical ions are recognized by many researchers based on product analysis studies conducted around 1980. For cases involving fluorescent excited radical ions, lifetime measurements provide information on excited states, and they sometimes lead to confusion owing to emissive byproducts resulting from their higher reactivities. Nanosecond laser flash photolysis studies provide evidence of the reactions of excited radical ions, and analyses of their kinetics reveal properties such as their excited-state lifetimes despite an indirect manner. Recent femtosecond laser flash photolysis studies provide detailed pictures of excited radical ions, though such studies are few. Furthermore, studies on dyad or triad systems, including excited radical ions, show electron transfer dynamics and disclose characteristics of excited radical ions that differ from those in neutral states. Larger electronic coupling and smaller dumping factor are reported as important characteristics of electron transfer systems of excited radical ions. These systematic studies on excited radical ions demonstrate their suitability for applications.