Hysteretic Control of Near-infrared Transparency Using a Liquescent Radical Cation

A liquescent dihydrophenazine radical cation, 1^.+ ⋅NTf₂ ^- , showed drastic changes in near-infrared (near-IR) transparency and opaqueness through hysteretic phase transitions with no measurable degradation of the compound even under aerated conditions. During the heating and slow cooling process (0.5 K min^-1 ), its electronic and magnetic properties were altered clearly and repeatedly changed between solid and liquid states. The liquid state was transparent to near-IR light (940 nm), but the solid state was opaque, despite both samples exhibiting a similar green color under room light. Additionally, the liquid state was changed to a glass state under a fast cooling process (2-10 K min^-1 ). UV/Vis/near-IR and electron spin-resonance spectroscopy revealed that these drastic changes were attributable to the dynamic dissociation and association of a π-dimer structure for 1^.+ accompanying with the solid-liquid phase transitions even under the neat conditions.