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. Temperature‐dependent hysteretic changes in near‐infrared transparency were achieved for a liquescent bis(trifluoromethanesulfonyl)imide‐(dihydrophenazine radical cation) salt. These hysteretic changes were attributed to dynamic dissociation and association of a dimer structure for the radical cations, which accompanied the solid–liquid phase transitions even under neat conditions with no measurable degradation of the compound.