The effect of pH on the electronic absorption, fluorescence and phosphorescence spectra of purines and pyrimidines. Determination of the lowest excited singlet and triplet state ionization constants

Abstract

Room-temperature electronic absorption, fluorescence emission spectra, and low-temperature (77 K) phosphorescence spectra of sixteen purines and pyrimidines have been determined in aqueous solutions of different pH (range : — 7.6 — 14). Significant shifts of the long-wavelength absorption maxima and of the short-wavelength fluorescence maxima, respectively from 1,500 to 4,000 cm-1 and from 400 to 21,000 cm-1 , occur upon protonation and deprotonation. pH-related shifts of the O-O band of phosphorescence range between 100 and 2,000 cm-1 , according to the compound. Lowest excited singlet (pKS a ) and triplet-state ionization constants (pKT a ), have been calculated from these spectral shifts. pKS a and pKT a values are found to be significantly different from the corresponding ground-state pKa values, differences ranging respectively from 0.2 to 10 and from 0.2 to 4.5 log units. In the equilibria between cation and neutral species, the basicity of most of the purines and pyrimidines is increased in the excited singlet and triplet-state compared to the ground-state. In the equilibria between neutral and anion species, all purines and pyrimidines are more acidic in the excited singlet-state. The same behaviour is observed in the triplet-state, except for 6-mercaptopurine, uric acid, uracil and 5-fluorouracil which are significantly more basic, probably because of the increase of p-electron density at the NH group, in the triplet-state compared to the ground-state. The substituent effects upon the pKS a and pKT a are discussed.