DFT and experimental study of N,N′-bis(3′-carboxy,4′-aminophenyl)-1,4-quinonediimine, a carboxyl substituted aniline trimer

Density functional calculations were performed on N,N′-bis(3′-carboxy,4′-aminophenyl)-1,4-quinonediimine, a carboxylic acid substituted aniline trimer. Results of the calculations were compared to experimental properties of the herein synthesized trimer, as well as to the properties of the anthranilic acid/aniline co-polymer reported in the literature. The calculated LUMO levels for isomers of the title compound range from −4.45 to −5.05 eV. The calculated electron affinities range from 75.93 kcal mol−1 to 89.04 kcal mol−1 (3.29–3.86 eV). Both the LUMO levels and electron affinities are greatest in magnitude for the anti, syn isomer. The HOMO levels, on the other hand, range from −5.32 eV (for the trans, trans isomer) to −5.36 eV (syn, syn inner). In acetonitrile solvent, the zwitterionic form is calculated to be energetically preferred to the non-zwitterion. Experimental UV–vis and near-IR studies in acetonitrile and ethanol show little evidence for zwitterion formation, but those in water show strong evidence. The predicted electronic transitions for the non-zwitterion in acetonitrile solvent correspond closely to those seen at 533 and 416 nm. The zwitterion present in solvent corresponds to a trimer with the capability to “self-dope”, suggesting that the trimer would be effective at corrosion inhibition in the emeraldine base form, unlike other trimers which are only effective in the emeraldine salt form. This effectiveness in the emeraldine base form would enable the material to be utilized in corrosion inhibition applications in alkaline environments where standard oligo- and polyanilines fail.