Hydrochromic xanthylium cations with tuneable colour and high moisture-sensitivity for potential smart packaging

Hydrochromic materials exhibit a colour change in response to water and moisture. Intensely coloured xanthylium cations, generated by acidifying xanthene-9-ol precursors with trifluoroacetic acid (TFA), are found to exhibit dramatic decolourization with high moisture-sensitivity. Mechanistic study confirms that the hydrochromic mechanism of xanthylium cations involves the nucleophilic addition of water molecules at the carbocation centre at 9-position of xanthylium, thus regenerating xanthene-9-ol. This accounts for the high reversibility with up to 50 acid-activation/moisture-deactivation cycles. Furthermore, functionalizing the cations with different aliphatic and aromatic groups (electron rich, electron poor and extended pi-conjugated moieties) at the 9- and 2,7-positions, enables the tuning of colour and moisture sensitivity of this hydrochromic system, by controlling the cations’ frontier molecular orbital energies and optical bandgap, as well as their chemical stability and nucleophilicity. To demonstrate the potential of the hydrochromic system for humidity sensing in smart packaging, optical paper probes and polymer films pre-doped with xanthene-9-ol precursors were prepared. Upon activation with TFA fume, they reveal different colour and decolourization rate in response to atmospheric moisture, ranging from minutes to hours, demonstrating feasibility for smart packaging and other potential applications.