Evidence against the Twisted Intramolecular Charge Transfer (TICT) model in 7-aminoflavylium derivatives

The julolidine substituent is however slightly more efficient to prevent the hydration of the flavylium cation than the non-rigid amines. In both compounds hydration takes place only at basic pH values through the attack of the hydroxyl in position 2 of the quinoidal base. The chalcone forms of the julolidine derivative are unstable and decompose after several hours. The rate of flavylium formation from trans-chalcone is slow but increases directly with the mole fraction of protonated trans-chalcone for 1 < pH < 3 and at higher proton concentrations it is proportional to [H3O+] suggesting the existence of proton catalysis for the isomerization in the chalcone form. This catalytic effect of the proton is also observed in the compound 4′,7-dihydroxyflavylium, showing that it is a more general phenomenon not related to the protonation of the amine substituent and not reported so far.