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

A 4′-hydroxyflavylium bearing a julolidine (2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizine) substituent in position 7 to minimize the amine rotation was characterized. Similarly to the 7-(N,N-diethylamino)-4′-hydroxyflavylium, the compound does not show trans-cis chalcone photoisomerization in water and the fluorescence emission quantum yield of its flavylium cation (Φ_f = 0.004) is 2.5 times lower than the non-rigid analog. These results seem to exclude a Twisted Intramolecular Charge Transfer (TICT) model to explain the lack of photoisomerization of the chalcones and the small quantum yield of fluorescence emission in these compounds. 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.