INFRARED REFLECTION ABSORPTION SPECTROSCOPIC STUDY ON THE PHOTODIMERIZATION PROCESS OF A STILBAZOLIUM CATION EMBEDDED IN LANGMUIR-BLODGETT-FILMS

Infrared reflection absorption (IRA) spectroscopy was applied to photodimerization process induced by irradiation (at 340 nm) of N-(1-octadocyl)-4-stilbazolium cations (C18S) incorporated in the Langmuir-Blodgett films of perdeuterated arachidic acid (CD3(CD2)18COOH, A). The IRA spectra proved that the main process proceeds through cyclobutane ring formation of the stilbazolium cations in a syn head-to-head arrangement. The extent and rate of photodimerization were followed by measuring the intensity of an olefinic C=C stretching vibration of C18S, changing the incorporation ratio (C18S:A) and the state of the matrix molecule, A (acid form or dissociated form). The extent and rate of photodimerization in the matrix with the acid form does not depend on the incorporation ratio ((1:4) and (1:9)); the result indicates that the C18S molecules assemble with each other, forming a phase separated from the matrix. The rate and extent of photodimerization in the matrix with the dissociated form are much lower than those observed in the matrix with the acid form, suggesting more dispersed distribution of C18S in the former matrix. The photodimerization consists of two steps of second-order processes; the first process (in the irradiation period of 0 to ca. 45 s) proceeds much faster than the second one (in the period of 45 to ca. 180 s). These kinetic data correspond to the time course of orientation changes of the alkyl chains of C18S, which was elucidated by the IRA spectra in CH3 and CH2 stretching vibration regions. In the first process the stilbazolium cations, which are favorably stacked for dimerization, readily form cyclobutane rings, causing a rapid change in the orientation. This orientation change results in a repulsive interaction between the alkyl chains and the matrix molecules. A slow change in the orientation observed for the second step was interpreted as a relaxation process of the repulsive interaction.