As seen in Table 1, the UV–vis absorption spectra of all the synthesized dyes especially dyes 3a, 3d and 3e, showed a weak band at
about 320 nm, assigned to the azo (S1) tautomeric form and a strong
band at about 510 nm, which was assigned to hydrazone (S2 and/or
S3) tautomeric forms (Scheme 3). However, the shape and intensity of
both forms in all the dyes depend on the solvent–solute interactions
and solvent nature as well as on the substituent type. As shown in
Table 1, the UV–vis absorption spectra of the dyes3b and 3c in basic
aprotic solvents such as DMF (~ 10−5 M) showed a main band at
about 500 nm, assigned to the hydrazone tautomeric form. However, in
protic and chlorinated solvents, the two main bands at about 330 nm
(azo) and 500 nm (hydrazone) were observed, assigned to the tautomeric azo-hydrazo equilibria. It was observed that, all the synthesized dyes
exist in the azo-hydrazone tautomeric forms in all the employed solvents. In addition, an increase in the polarity of studied solvents has
caused a slightly positive bathochromic shift in the absorption maxima
of prepared dyes. For example, in the absorption spectrum of dye 3a,
the λ max is shifted from λmax = 490 nm to λmax = 524 nm (Δλmax =
34 nm) by changing the solvent from diethyl ether (π = 0.27) to
DMSO (π = 1.00).