Transmittance of electronic effects in the fluorescein molecule: nitro and amino groups in the phthalic acid residue

This paper is aimed to estimate the transmittance of electronic effects in fluorescein molecule between phthalic acid residue and xanthene moiety. Despite almost orthogonal orientation latter with respect rest molecule, some influence substituents this 9-aryl ring on dissociation hydroxyl group hydroxyxanthene cannot be ruled out. In order reveal (possible) effect, we blocked carboxylic via esterification. The reason using dimethyl sulfoxide as solvent was high Hammett’s “rho” constant for phenolic group. pKa values eight methyl or ethyl esters 3'-, 4'-, 5'-nitro amino fluoresceins were determined benzoate salicylate buffer solutions spectrophotometric method. For nitro derivatives, constants cationic forms dyes also diluted p-toluenesulfonic acid. study reveals increasing by 0.2–0.3 units case while decreases pKas 0.2–0.8. Also, position substituent importance; 5'-substituents display least influence. addition, eosin (2,4,5,7-tetrabromofluorescein) clarify alkyl absorption maxima molar absorptivities anions, well DMSO coincide within margin errors.