for dye 3d with electron-withdrawing group (R_CN) changed at a pH
between 6.0 (λmax = 486 nm) to 7.0 (λmax = 524 nm), while no significant change occurred in pH less than 6 and pH greater than 7. This effect has been attributed to the dissociation of the phenolic proton
(OH) of the dye 3d (λmax = 484 nm) into the conjugate base (λmax =
526 nm) in solutions between pH 6.0 and 7.0. It was also observed
that the absorption spectra (the pH values) of the dye 3e with
electron-donating group (R_OEt) are not sensitive to acidic, neutral
and basic solutions (Table 2 and Fig. 4). As expected, the donor–acceptor
polarization of the dye 3d is increased by introducing an electron
accepting group (CN) in the diazo component, caused bathochromic
shifts relative to similar dye 3e including an electron-donating group
(OEt). These findings indicated that the synthesized dyes with
electron-withdrawing groups in the diazo component were sensitive
to the alkalinity of the medium. Therefore, the compound 3d can be
used at pH range of 6–7 as new pH indicator.