MONOLAYER BEHAVIORS OF 4-OCTADECANOXYCINNAMIC ACID AND THE PHOTODIMERIZATION KINETICS IN ITS LANGMUIR-BLODGETT-FILMS

pi-A isotherms and Brewster angle microscopy (BAM) images of 4-octadec anoxycinnamic acid (OCA) monolayers spread from different concentratio ns of the solutions indicated that the OCA monolayers depended on the concentrations. The dilution of the spreading solutions could have eff ectively weakened the intermolecular aggregation in the OCA monolayers . The OCA Langmuir-Blodgett (LB) films were successfully deposited on hydrophilic substrates by selecting a suitable concentration of 1.0 x 10(-4) mol dm(-3) at 18 +/- 1 degrees C. The LR and UV spectra indicat ed that the OCA molecules in LB films can dimerize under UV light irra diation with a second-order process by use of spectrophotometry. The s econd-order rate constant k obtained from the absorbance changes at 25 7 nm is 2.98 x 10(-2) mol(-1) dm(3) s(-1). X-ray diffraction patterns of the OCA LB films are almost the same before and after irradiation, suggesting that the photodimerization only occurred in each monomolecu lar layer and did not change the periodic structure of the OCA LB film s. The syn head-to-head photoproduct is very characteristic and defini te due to the ordered reaction environment provided by the OCA LB film s. A possible reaction mechanism has been proposed.