Spectroscopic Characterization of the Charge-Separated Zwitterionic State Responsible for the Excimer-like Emission of a Luminescent Diradical.

Luminescent organic radicals often exhibit a red-shifted, broad, and structureless emission band when assembled in a host matrix. This emission has been attributed to pairs of neighboring photoexcited and ground-state radicals forming an excimer. However, the electronic character of this "excimer-like" (EX) state has not been definitively clarified through experiments. Here we present an experimental characterization of this EX state using a spatially confined luminescent diradical as a model for radical dimers. Comprehensive spectroscopic investigations revealed that the EX state of the diradical is a zwitterionic (ZI) state with a closed-shell singlet electronic configuration generated by intramolecular symmetry-breaking radical-to-radical charge transfer. In combination with previous magnetoluminescence studies, we experimentally show that the formation of the ZI state is specific to the singlet spin state. Our results enable a detailed understanding of the photophysics and photochemistry of radical multichromophoric systems for future spin photonics applications.