On‐Demand Control of Short‐Wave Infrared Light Transparency Based on Stimuli‐Responsive Association of Tetrathiafulvalene Radical Cations

The light‐transmissive properties of a solid‐state tetrathiafulvalene radical cation‐bis(trifluoromethanesulfonyl)imide, 1‐C5⋅+ ⋅ NTf2−, underwent instantaneous changes in the short‐wave infrared (SWIR) region (1000–2500 nm) upon exposure to solvent vapor or the application of mechanostress at room temperature. The initial solid state of 1‐C5⋅+ ⋅ NTf2− exhibited strong absorption in the near‐infrared (NIR; 700–1000 nm) and SWIR regions, whereas the absorption in the SWIR region was significantly diminished in the stimulated state induced by dichloromethane vapor. Upon cessation of vapor stimulation, the solid state spontaneously and promptly reverted to its original state, characterized by absorption bands in the NIR/SWIR region. Moreover, the SWIR absorption was absent upon the application of mechanical stress using a steel spatula. The reversal was fast and occurred within 10 s. These changes were visualized using a SWIR imaging camera under 1450‐nm light irradiation. Experimental investigations demonstrated that the transparency to the SWIR light in the solid states was modulated through significant structural transformations of the associated radical cations, with transitions between columnar and isolated π‐dimer structures under ambient and stimulated conditions, respectively. An ensemble consisting of the solid form of the tetrathiafulvalene radical cation and bis(trifluoromethanesulfonyl)imide showed rapid and reversible changes in short‐wave infrared light (>1000 nm) transparency upon weak external stimulation. This behavior originated from the conversion of the association structures of the radical cation from columnar arrangements of π‐dimers into isolated π‐dimers.