Needlestick‐Stimulation‐Induced Conversion of Short‐Wave Infrared‐Light Transparency Using a Liquescent Radical Anion

Abstract A liquescent salt consisting of a 7,7,8,8‐tetracyanquinodimethane (TCNQ) radical anion and a tetra‐n‐decylammonium ion, 1⁺•TCNQ^•−, exhibits rapid changes in the short‐wave infrared (SWIR) light transparency at 1000–1400 nm upon the application of a one‐shot needlestick‐stimulus. Radical anion salt 1⁺•TCNQ^•− transforms from a blue solid to a green liquid at 90 °C without decomposition under aerated conditions, and remains in the liquid state upon cooling to 70 °C. After applying pressure with a needlestick on a cover glass at 70 °C, the liquid transforms rapidly into the solid state over a timescale of seconds across a centimeter scale of area. Along with the liquid–solid transition, the SWIR‐light transparency at 1200 nm completely switches from the “on” to the “off” states. Experimental results, such as electronic spectra and crystal structure analysis, indicates that the SWIR‐light absorption in the solid state is due to the existence of a slipped‐stacking π‐dimer structure for TCNQ^•−. The rapid rearrangement is induced by the formation of the π‐dimer structures from the monomers of TCNQ^•− and the subsequent generations of the solid‐state seed.