Rational Design of a Metal-Free Organic Radical Cation Exhibiting Mid-IR π–π* Transition Based on the Exchange Interaction

Mid-IR (2.5-25 μm) responsive dyes, characterized by their ability to produce excitons across the electronic transition to the vibrational energy region, hold significant potential for various applications. However, the commonly used highly π-extended chromophores are inevitably too large for practical synthesis and handling. We herein demonstrate a novel molecular design strategy to control the optical band gap in the near- and mid-IR region based on small chromophores. Namely, the presented radical cation generated from a phenanthrene-based Blatter-type diradical less than 600 Da exhibits π-π* electronic transition reaching the mid-IR region to ca. 2000 cm^-1 (5000 nm). The mid-IR responsive chromophore was stable in solution under ambient conditions for 1 week without any observable degradation. The chromophore was designed through our "oxidation of diradical" strategy by taking advantage of a small interaction between two pseudodegenerated SOMOs of organic diradicals. EPR spectroscopy confirmed the delocalized charge and spin nature of the mid-IR responsive radical cation, supporting the assignments of the mid-IR band to a π-π* transition due to the delocalized SOMO and SUMO.