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Huber, Ludwig A.; Mayer, Peter; Dube, Henry (2018): Photoisomerization of Mono-Arylated Indigo and Water-Induced Acceleration of Thermal cis-to-trans Isomerization. In: Chemphotochem, Vol. 2, No. 6: pp. 458-464
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Substituted indigo derivatives undergo photoisomerization of the central double bond if both nitrogen atoms are functionalized. Indigo itself however does not photoisomerize because of a competing and highly efficient excited-state proton transfer. In this work, we show that also mono-arylated indigo undergoes photoisomerization despite still possessing one nitrogen-bound proton and the likely presence of a competing intramolecular excited-state proton transfer. The two different isomers exhibit strongly different absorptions and therefore can be distinguished by the naked eye. Different to diaryl-substituted indigo, thermal decay of the metastable cisisomers of mono-arylated derivatives can be greatly accelerated by small amounts of water (by more than a factor of 300 for measured half-lives). Such tunability is of high interest for applications that require quick and autonomous switching-off of photoresponsive trigger units.