Luminescence Studies on Formycin, Its Aglycone, and Their N‐methyl Derivatives: Tautomerism, Sites of Protonation and Phototautomerism

A detailed study has been made of the luminescence spectra of 3‐β‐d‐ribofuranosyl‐7‐amino‐pyrazolo(4,3‐d)pyrimidine (formycin A), 3‐propyl‐7‐aminopyrazolo(4,3‐d)pyrimidine (7APP), and their various N‐methyl derivatives, at room temperature and in methanol‐water glasses at 77 K. Comparisons of the foregoing, together with the observed dependence of the emission spectra of formycin and 7APP on excitation wavelength, demonstrated that these consist of two tautomeric species, N(1)H and N(2)H, both of which emit at 300 and 77 K. The two tautomers may be distinguished by the location of the emission maxima, especially for phosphorescence, and quantum yields for emission. Comparisons of the emission spectra of the protonated forms of 7APP and its N‐methyl derivatives showed that the fluorescence of the cations of 7APP and its N,‐ and N2‐methyl derivatives originates from the forms protonated on N(4). By contrast, the forms protonated on N(6) contribute appreciably to the phosphorescence at 77 K. On the basis of the emission spectra at 77 K, it is concluded that the major tautomeric form of the formycin cation is N(1)H,N(4)H+, but there is also some contribution by the form N(2)H,N(4)H+. In acid medium at room temperature, there is photodissociation of a proton from the pyrazole ring of the formycin cation. This leads to formation in the state S! of the tautomeric species N(4)H, which does not exist in the ground state. This conclusion, similar to that previously reported for the analogous isomeric 4‐aminopyrazolo(3,4‐d)pyrimidines, is derived from a comparison of the fluorescence spectra of the cations of formycin and N4‐methylformycin, which exhibit two bands at 375 and 440 nm, the latter corresponding to the emission of the neutral form of N,i‐methylformycin. The proposed mechanism of phototautomerization is supported by a study of solvent and salt effects.