Multistate of chemical species of 2,6-Bis(arylidene)cyclohexanones. On the role of chalcone and spiro species

The similarity between 2,6-Bis(2-hydroxyarylidene)cyclohexanones and 2′-hydroxychalcones in generating respectively styrylflavylium and flavylium cations was explored. The complex pH and light dependent multistate of species of the asymmetric of 2,6-Bis(2-hydroxyarylidene)cyclohexanone bearing a diethyl amine in position 7 of the flavylium cation was studied by pH jumps, monitored by UV–vis spectrophotometry, stopped flow, and NMR. The possible structural isomers corresponding to the migration of the amine substituent at the benzopyrylium to ring B of the flavylium cation via chalcone, or spiro species were not detected. However, cis and trans isomers from the pendent arm take place and a photochromic system involving flavylium cation switching from red to blue was characterized. In acidic medium the flavylium cation and the hemiketal protonated at the amine substituent are the equilibrium species. In moderately acidic solutions the spiro is the most stable species and in very basic medium the ionized trans-chalcone.