The UV spectra of the diazonium coupling products of 1 and
3 provide additional evidence that such compounds have the
hydrazone structure (C) rather than (A) or (B). Most of the dyes
show three main absorption bands in the region 240–390 nm
(2445–2455 nm “–*”, 2710–2950 “n–*” related to pyridine
moiety and 3445–390 “n–*” assigned to hydrazo center). These
bands are referred to as A, B and C, starting from the long wavelength
side. The data indicate that absorption maxima in the
solvents examined almost coincide. This suggests that such a compound
is not in a tautomeric equilibrium, but exists in one form.
The relatively small differences in max may be due to the general
solvent effect [18].