Novel metal coordinations in the monolayers of an amino-acid-derived Schiff base at the air-water interface and Langmuir-Blodgett films

The metal coordinations in the monolayers of an amino-acid-derived Schiff base, 4-(4-dodecyloxy)-2-hydroxyberizylideneamino)benzoic acid (DSA), at the air-water interface and Langmuir-Blodgett (LB) films have been investigated using surface pressure-area isotherms, infrared reflection absorption spectroscopy (IRRAS), FTIR (transmission and reflection absorption), and UV-vis spectroscopy, respectively. A combination of the three modes of infrared spectroscopy is employed to study interfacial metal coordinations on different substrates (water, CaF2, and gold). The DSA molecules form a condensed monolayer on pure water, in which the Schiff base segments take a nonplanar conformation and their long axes are oriented almost perpendicular to the water surface. The C=N stretching bands are assigned for the monolayers on the H2O-based subphases. In the presence of Ca2+ and Zn2+ in the subphases, the monolayers are expanded to a certain extent. Ca2+ and Zn2+ ions only coordinate to the carboxylate oxygen atoms in the fashion of chelating bidentate coordination but not to the imino nitrogen atoms or phenolate oxygen atoms of the salicylideneamino moieties. The long axes of the Schiff base segments are preferentially oriented along the monolayer normals. The twist angle for the nonplanar conformations of the Schiff base segments is considered to increase in the case of Ca2+ and decrease in the case of Zn2+ in comparison with that on pure water. In the presence of Cu2+, the monolayer is expanded and the alkyl chains are disordered. Cu2+ ions coordinate not only to the carboxylate groups in the fashion of bridging bidentate coordination but also to the salicylideneamino moieties. The Schiff base segments are significantly tilted away from the monolayer normal.