The functionalization of perylene bisimide by squaric acid afforded halochromic dyes with pronounced acidity. The dye senses solvent polarity, pH and humidity by means of pronounced changes in absorption spectra and thin film color through the intramolecular charge transfer between perylene and cyclobutene cores initiated by protonation/deprotonation.
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Maeda et al. (2015) studied this question.
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