The bidirectional interconversion between ketone and enol structures of 4-cyclopentene-1,3-dione derivatives was realized by photoswitching of fused dithienylethene using UV and visible light. A loss of antiaromaticity offered the driving force for light-triggered enolization and was supported by theoretical studies. Solvent and substituent effects provided additional means for regulating photoswitchable keto-enol tautomerism. Moreover, a significant change of acidity was revealed with light-induced keto-to-enol conversion, enabling control over base-catalyzed Michael addition.
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