Self-Assembled Double-Stranded Homotopic Helical Complexes of a Hexadentate Dihydrazone: Selective Detection of Picric Acid and Biological Applications

Finding reliable luminous metal complexes to detect nitroaromatic-based explosives, particularly picric acid, has long been a promising area of study. Here, we report the self-assembly of saturated homotopic bimetallic double-stranded Co(III), Zn(II), and Ni(II) complexes from a dihydrazone ligand, 1,10-bis(2-benzoylpyridine) adipic acid dihydrazone (H2L). The double-helix structures have been comprehensively confirmed. All the compounds were found to crystallize in the triclinic P1̅ space group, while a polymorph of the ligand in the monoclinic lattice with the P21/c space group was also identified. The dihydrazone-based helicates were exclusively formed with respective metal perchlorates; however, the double helix of Co(III) resulted from deprotonation on one of the two hydrazone moieties of both the ligand units. As a result, in contrast to the ligand and the Zn(II) and Ni(II) complexes, the Co(III) complex is found to exhibit excellent fluorescence emission. Moreover, the fluorescence emission was found selectively quenched in the presence of explosive picric acid over a range of 5 × 10–7 M to 10 × 10–5 M, with an impressive detection limit of 27 nM. UV–visible absorption and fluorescence lifetime titrations of picric acid with the probe indicate that the quenching effect is likely due to the formation of a ground-state charge transfer complex and an inner filter effect. Furthermore, the compounds were evaluated for their in vitro antibacterial activity, which again reveals the efficacy of the cobalt complex and makes it a bioprobe. The compounds were also screened for in vitro anticancer activity against A549 lung adenocarcinoma cells and the zinc complex demonstrated maximum efficacy with an IC50 of 41.05 μM.