PHOTOREACTIVITY OF 10,12-PENTACOSADIYNOIC ACID MONOLAYERS AND COLOR TRANSITIONS OF THE POLYMERIZED MONOLAYERS ON AN AQUEOUS SUBPHASE

Photoreactivity by UV irradiation of 10,12-pentacosadiynoic acid (PDA) monolayers and color transitions of the polymerized PDA monolayers on an aqueous subphase have been investigated. Different surface pressure-area isotherms could be obtained for the PDA monolayers by changing pH of the subphase, and the PDA monolayers were condensed with increasing pH. The color transition of the monolayers changed depending on the condensation degree when the monolayers were irradiated by UV light under a constant surface pressure. With decreasing molecular area from 29 to 23 angstrom 2, the absorption peak (lambda-max) of the monolayers shifted from 650 to 540 nm and the colors changed from blue to red. The most condensed monolayer at molecular area of 22 angstrom 2 had no absorption peak, indicating that the monolayer did not react at all. This indicates that the polymer was produced by the light irradiation when the PDA molecules were arranged at suitable distances on the subphase. The color transition after photopolymerization occurred reversibly from red to blue when the pH was changed from 6.5 to 5.0 or when the surface pressure was reapplied. This may indicate that the color transition occurs depending on the conformation change of side groups bonded to a conjugated double and triple bond.