Thioxanthylium Cations: Highly Reversible Hydrochromic Mate-rials with Tunable Color and Moisture Sensitivity

Hydrochromic materials, which change color in response to moisture, are highly sought-after due to some unique applications such as moisture detection, humidity monitoring, anti-counterfeiting, and sweat-pore mapping. We herein report a new thioxanthene-9-ols/thioxanthylium cations hydrochromic system. The hydrochromic thioxanthylium cations can be generated/regenerated by exposing a chemically stable thioxanthene-9-ol to trifluoroacetic acid (TFA). The intensely colored thioxanthylium cations can be chemically reverted to the colorless thioxanthene-9-ol. Interestingly, the conversion between thioxanthene-9-ol and its cation species was found to be highly reversible in most cases (up to 50 activation-deactivation cycles). The color and moisture-sensitivity of thioxanthylium cations were also validated to be tunable via functionalization of the thioxanthene core with different aliphatic and aromatic groups at its 9 or 2,7-positions. Finally, paper probes were prepared using solution-immersion and inkjet-printing techniques to demonstrate the system's potential applications in humidity sensing and anti-counterfeiting

Automated summary

This research presents a new hydrochromic system using thioxanthene-9-ols and thioxanthylium cations. The color change between thioxanthene-9-ol and its cation species is highly reversible, allowing for multiple activation-deactivation cycles. The color and moisture-sensitivity of the thioxanthylium cations can be tuned by functionalizing the thioxanthene core. This system shows potential applications in humidity sensing and anti-counterfeiting, as demonstrated by paper probes.