Langmuir-Blodgett films of hexamolybdate and naphthylamine prepared by two different approaches: Synthesis, characterization, and materials properties

Two novel covalently-linked organic-inorganic hybrid compounds [(n-C ₄H ₉) ₄N] ₂[Mo ₆O ₁₈{≡N-(2-Np)}] (NA-Mo6) and [(n-C ₄H ₉) ₄N] ₂[Mo ₆O ₁₇{≡N- (1-Np)} ₂] (NA2-Mo6) (2-Np = 2-naphthyl; 1-Np = 1-naphthyl) have been prepared by the reaction of 2-naphthylamine (2-NpNH ₂) or 1-naphthylamine (1-NpNH ₂) with [(n-C ₄H ₉) ₄N] ₂[Mo ₆O ₁₉] (Mo6) in the presence of N,N′-dicyclohexylcarbodiimide (DCC) in anhydrous acetonitrile, and have been characterized by ¹H NMR, IR, UV/Vis, and fluorescence spectra as well as X-ray crystallography. Structural analyses reveal the formation of self-assembled infinite 1-D and 3-D supramolecular frameworks. The UV/Vis spectra of NA-Mo6 and NA2-Mo6 show that the lowest energy electronic transfers occur at 362 and 394 nm, respectively, which are dramatically red-shifted when compared to that of Mo6. The luminescence studies show that Mo6 can notably quench the emission of naphthylamine. Monolayer and multilayer films of NA-Mo6 and NA2-Mo6 were fabricated on different substrates by the Langmuir-Blodgett (LB) technique using H ₂O as the subphase. For comparison, another two types of organic-inorganic hybrid films composed of noncovalently-linked 1-NpNH ₂/Mo6 and 2-NpNH ₂/Mo6 systems were also deposited onto the same substrates under the same conditions by the LB technique using an aqueous solution of Mo6 as the subphase. The as-prepared organic-inorganic hybrid films were characterized by π-A isotherms, UV/Vis absorption spectra, photoluminescence spectra, and scanning tunneling microscopy. The results indicate that stable monolayers can be formed at the air-water interface for NA-Mo6 and NA2-Mo6, and at the air-Mo6 solution interface for 1-NpNH ₂ and 2-NpNH ₂. The four LB films containing NA-Mo6, NA2-Mo6, 1-NpNH ₂/Mo6, and 2-NpNH ₂/Mo6 display interesting electrical conductivity.