External infrared reflection absorption spectroscopy was used to study the carboxylate-counterion interactions and changes in these interactions during the photopolymerization of a long-chain diacetylene monocarboxylic acid, 10,12-pentacosadiynoic acid (DA), at air-water interfaces in the presence of divalent metal ions, Ba2+ (pH 7.7), Cd2+ (pH 6.8), and Pb2+ (pH 6.0). Upon reducing the molecular area (0.80 --> 0.18 nm(2)/molecule), the DA monolayer on each subphase exhibited discrete frequency changes of an IR band due to a carboxylate antisymmetric stretching vibration (v(as)(COO-)), indicating discrete changes in the coordination and/or association states of the carboxylate,groups. Ab initio molecular orbital calculations were applied to confirm the empirical relationships between the frequency differences of the asymmetric and symmetric stretching bands of the carboxylate group and the modes of coordination of the group to metal ions. On the basis of the relationships, it was proposed that, when the molecular area is reduced, the carboxylate group in the DA monolayer on the Ba2+ subphase changes its coordination mode from a bridging state to a bidentate one, while the carboxylate group in the DA monolayer on the Cd2+ subphase keeps a bidentate coordination state. It was also. suggested that the DA monolayer on the Pb2+ subphase exhibits a coordination change from a bridging to bidentate state upon compression. IR spectral changes in the v(as)(COO-) and v(s)(COO-) regions observed during UV-irradiation-induced polymerization of the DA monolayers were similar to the IR spectral changes observed by compressing the monolayers without irradiation. The results indicated that the polymerization induces more densely packed states of the carboxylate groups in the monolayers.