Optically distinguishable electronic spin isomers of a stable organic diradical

Herein, we demonstrate a model of electronic spin isomers, the electronic counterpart of nuclear spin isomers, by employing a through-space conjugated stable organic diradical. We investigated a benzotriazinyl radical dimer linked via triptycene skeleton. The diradical has a small singlet–triplet energy gap of -3.0 kJ/mol, indicating ca. 1:1 coexistence of the two spin states at room temperature. The diradical at room temperature shows characteristic near-IR absorption bands around 500–900 nm, which is absent for the corresponding mono-radical subunit. Variable temperature measurement revealed that the absorbance of the NIR band depends on the abundance of the singlet state, and we assigned the NIR band as the singlet-specific absorption band. It enables photoexciting one of the two spin states coexisting in thermal equilibrium. Ultrafast transient spectroscopy disclosed that the two spin-states independently follow qualitatively different excited-state dynamics. Namely, the singlet excited state undergoes intramolecular symmetry-breaking charge transfer, and the triplet excited state goes exciton fusion to form a monomer-like excited state. These results demonstrate optically distinguishable spin isomers.