Zwitterionic Open‐Shell Singlet Diradical with Solvent‐Dependent Singlet–Triplet Energy Gap

A zwitterionic open‐shell singlet diradical composed of electron‐donating anion, anthroxide, and electron‐accepting cation, phenazinium, was designed and synthesized. The diradical has a small HOMO–LUMO energy gap and shows near‐infrared absorption. Temperature‐dependent ESR measurements confirmed its small energy gap between ground singlet and thermally‐excited triplet states (ΔEST). The ΔEST of the diradical was changed by solvents (toluene, 2‐MeTHF, and EtOH). A zwitterionic open‐shell singlet diradical composed of electron‐donating anion, anthroxide, and electron‐accepting cation, phenazinium, was designed and synthesized. The diradical has a small HOMO–LUMO energy gap and shows near‐infrared absorption. Its small energy gap between ground singlet and thermally‐excited triplet states, which was changed by solvents, was experimentally confirmed.