Computational studies of 4-nitrophenyl- and 2-benzothiazolyl-substituted formazans and tetrazolium salts

Formazans and tetrazolium salts are two structurally related classes of organic compounds that form redox pairs with each other. They have broad application in biomedical research, and as specialty dyes, metal ligands and colorimetric indicators (e.g., for the dosimetry of ionizing radiation). The recent research interest has been focused primarily on formazans, including computational studies of their properties via density functional theory (DFT) and time-dependent DFT (TD-DFT). However, there are, to our knowledge, no computational studies of tetrazolium salts exist. To remedy this situation, we present a comparative DFT and TD-DFT study of eleven pairs of triaryl-substituted formazans and tetrazolium salts, containing phenyl, 4-nitrophenyl, 4-bromophenyl, 2-benzothiazolyl, 6-methoxy-2-benzothiazolyl and 1-naphthyl substituents. Following geometric optimization and selection of the most stable conformer (for formazans), we proceeded to examine the frontier molecular orbitals, global reactivity descriptors, molecular electrostatic potential maps and local reactivity, as well as vertical excitation energies and electronic spectra of the studied materials. Finally, through a comparison of reactivity and electronic spectra, we present a framework for the design of formazan tetrazolium salt pairs that are suited for specific colorimetric applications.