Hydrolysis of Benzopyrylium Dyes – An Application of the Concept of Chemical Hardness

A systematic study on the relationship between the substitution pattern, the pK_a values, and spectral properties of flavylium cations and the respective–(CH₂)₂–bridged analoga (5,6-dihydrobenzoc]xanthylium cations) is given in order to find rules concerning their spectral behaviour and their chemical reactivity. Our results show that the concept of chemical hardness can explain the different reactivity of 4′- and 7-substituted flavylium ions against HO⁻. The pK_a values of these species, which can act as a measure of reactivity with respect to a nucleophile, correlate linearly with the absolute hardness calculated from half the energy gap between the frontier orbitals. Since the longest-wavelength UV/VIS absorption maximum is mainly determined by the HOMO-LUMO transition, an analogous linear correlation is obtained between the spectral 0–0 transition and the pK_a value. Deviations from these correlations are assumed to be due to steric effects.