The area per molecule of 10,12-pentacosadiynoic acid (DA) was smaller on a pure water subphase than on an aqueous subphase containing a water-soluble polymer, polyallylamine (PAA), at a concentration of the monomer unit of PAA of 0.01 mM. The transmission IR spectra of the LB films of DA transferred from a PAA-containing subphase showed the presence of two species of DA in the LB films, the one in a deprotonated form and the other in the free acid form. The XPS spectra showed that a half of DA molecules formed salt with PAA and that the other half was in the free acid form in the LB films. When the films were irradiated with UV light, two absorption bands appeared at 540 nm (red phase) and 630 nm (blue phase) simultaneously. Upon prolonged irradiation, only the intensities of the two bands increased gradually without noticeable changes in the band positions and the intensity ratio of the two bands. Before irradiation, the films were two-dimensional except for some three-dimensional crystalline structures on the surface. On irradiation of UV light, these three-dimensional structures developed and new three-dimensional crystalline structures also appeared, resulting in three-dimensional structures with several tens of μm in length, a few μm in width and 6 nm in height. Fluorescence microscopy indicated that the red phase was confined within the three-dimensional structures.
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