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Two pairs of 1: 2 neutral trans mononuclear transition-metal (M = Ni(II) and Cu(II)) complexes of pyridine-2,4-dione and quinoline-2,4-dione based heterocyclic dyes have been structurally and spectrally characterized and compared herein. X-ray single-crystal diffraction analyses of four complexes, namely trans-[Ni(La)2(DMF)2] (1), trans-[Cu(La)2(DMF)2] (2), trans-[Ni(Lb)2(DMF)2] (3) and trans-[Cu(Lb)2(DMF)2] (4), reveal that they have the same trans configuration between the bidentate chelating dianionic ligands and two axially coordinated DMF molecules. Furthermore, a transformation from the hydrazone to azo configuration has been observed for both bidentate chelating ligands La(-) and Lb(-) after metal-ion complexation. More importantly, the simultaneous DSC/TG-MS-FTIR method has been used to explore the thermal stability of four neutral metal-complex dyes 1-4, where the two axially coordinated DMF molecules in Ni(II) and Cu(II) complexes exhibit distinguishable decomposition behavior because of their different M-O bond lengths originating from the Jahn-Teller distortions.

A rigid trans pyridine-4-thiol coordinated dinuclear zinc(II) macrocyclic complex trans-[Zn2L(py-4-SH)2](ClO4)2 (1) is introduced for the first time to fabricate self-assembled nanocomposite films with gold nanoparticles (Au-NPs) covering 1 μm gap gold electrodes. Utilizing the same self-assembled fabrication technique but different concentrations of dithiol 1 and Au-NPs in chloroform, a series of molecular-scale electronic devices 2–5 have been obtained exhibiting typical temperature-dependent (8–300 K) semiconducting I–V characteristics on the order of μA, nA and pA in the temperature range of 8–300 K. The transformation from an insulating dinuclear zinc(II) macrocyclic complex to a semiconducting nanocomposite thin film is thereby achieved by means of the Au–S bonded contacts between dithiol end-capped molecules and Au-NPs as well as the unique charge separation and electron transportation abilities of Au-NPs. From the classical Arrhenius plots, it is concluded that the temperature-independent tunneling current between Au-NPs in the 1 × 1 μm2 region dominates the conductance below 100 K. However, the tunneling current is overwhelmed by the temperature-dependent thermal excitation current with the increase of temperature and the hopping conductance finally governs the electron transportation among the molecules.

The azo-hydrazone tautomerism of two pyridine-2,6-dione based Disperse Yellow dyes has been achieved by pH control and metal-ion complexation, respectively, which is evidenced by UV-visible spectra using pH-titration, (1)H NMR and X-ray single-crystal diffraction techniques for two dyes and one neutral dinuclear dye-metal complex. pH-titration experiments under strong and weak acidic conditions (HCl and HOAc) as well as strong and weak alkaline conditions (NaOH and ammonia) demonstrate that there is an equilibrium between the azo (HL(1-A) and HL(2-A)) and hydrazone (HL(1-H) and HL(2-H)) tautomers for two dyes in solution but the hydrazone form is dominant under conventional conditions. The hydrazone proton is also observed in the (1)H NMR spectra of HL(1-H) and HL(2-H) which can be verified by the hydrogen-deuterium exchange and the presence of cooperative six-membered intramolecular hydrogen rings involving the hydrazone proton in their X-ray single-crystal structures. Moreover, the azo-hydrazone tautomerism is evidenced by the formation of a novel neutral dinuclear dye-metal complex Cu(2)(L(2-A))(4), where all the ligands are in the azo form and two types of coordination modes are present for four L(2-A) ligands. Namely, the side two ligands serve as the bidentate capping ligands, while the middle ones act as the quadridentate bridging ligands linking adjacent Cu(II) centers in a reverse fashion.