The kinetics and thermodynamics of the 2,6-bis(2-hydroxybenzilidene)cyclohexanone chemical reactions network was studied at different pH values using NMR, UV-vis, continuous irradiation, and flash photolysis. The chemical behavior of the system partially resembles anthocyanins and their analogue compounds. 2,6-Bis(2-hydroxybenzilidene)cydohexanone exhibits a slow color change from yellow to red styrylflavylium under extreme acidic conditions. The rate constant for this process (5 x 10(-5) s(-1)) is pH independent and controlled by the cis-trans isomerization barrier. However, the interesting feature is the appearance of the colorless compound, 7,8-dihydro-6H-chromeno[3,2-d]xanthene, isolated from solutions of acid to neutral range, characterized by H-1 NMR and single crystal X-ray diffraction. Light absorption by 2,6-bis(2-hydroxybenzilidene)cyclohexanone solutions immediately after preparation exclusively results in cis-isomer as photoproduct, which via hemiketal formation yields (i) red styrylflavylium by dehydration under extremely acidic solutions (pH < 1) and (ii) colorless 7,8-dihydro-6H-chromeno[3,2-4canthene by cyclization in solutions of acid to neutral range.
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Moro, A. J., Pana, A-M., Cseh, L., Costisor, O., Parola, A. J. D., Cunha-Silva, L., Puttreddy, R., Rissanen, K., & Pina, F. J. D. S. (2014). Chemistry and Photochemistry of 2,6-Bis(2-hydroxybenzilidene)cyclohexanone. Journal of Physical Chemistry A, 118(32), 6208-6215. https://doi.org/10.1021/jp505533b