HYDROLYSIS OF BENZOPYRYLIUM DYES - AN APPLICATION OF THE CONCEPT OF CHEMICAL HARDNESS

A systematic study on the relationship between the substitution patter n, the pK(a) values, and spectral properties of flavylium cations and the respective-(CH2)(2)-bridged analoga (5,6-dihydrobenzo[c]xanthylium cations) is given in order to find rules concerning their spectral be haviour and their chemical reactivity. Our results show that the conce pt of chemical hardness can explain the different reactivity of 4'- an d 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 n ucleophile, correlate linearly with the absolute hardness calculated f rom half the energy gap between the frontier orbitals. Since the longe st-wavelength UV/VIS absorption maximum is mainly determined by the HO MO-LUMO transition, an analogous linear correlation is obtained betwee n the spectral 0-0 transition and the pK(a) value. Deviations from the se correlations are assumed to be due to steric effects.