INFRARED SPECTRAL STUDY OF THE PHOTODIMERIZATION PROCESS OF ENYL-4-(4-(N-OCTADECYLPYRIDINIUMYL))-1,3-BUTADIENE EMBEDDED IN LANGMUIR-BLODGETT-FILMS

Infrared reflection absorption and infrared absorption spectroscopies were applied to study photochemical reactions of Langmuir-Blodgett fil ms of a mixture of enyl-4-(4-(N-octadecylpyridiniumyl))-1,3-butadiene (C18B) and fully deuterated arachidic acid (DA) with the mixing ratio of 1:4 (C18B:DA). The results proved that, on irradiation of the LB fi lms at 370 nm, C18B undergoes a dimerization, resulting in the formati on of a cyclobutane ring in the matrices of DA in both the acid and sa lt forms. The extent and rate of dimerization were followed by measuri ng the intensity of an olefinic C=C stretching vibration of C18B, prov ing that the reaction consists of two steps of second-order processes; the first process (in the irradiation time of 0 to ca. 60 s) proceeds much faster than the second process (60-300 s). The final extent of t he dimerization in the matrix with the acid form (ca. 95%) is much lar ger than the extent in the matrix with the salt form (ca. 65%); the re sult suggests that C18B in the former matrix exists in a phase separat ed state, while C18B in the latter matrix exists in a more dispersed s tate. As the extent of dimerization in the matrix with the acid form i ncreases from 0 to 60%, the average tilt angle of the pyridiniumyl gro up of C18B (i.e., the angle between the long axis of the group and the surface normal of a substrate) regularly decreases from 77 to 74 degr ees. On the other hand, the tilt angle in the matrix with the salt for m remains virtually at a constant value of 74-73 degrees. Presumably, a stacking state in the matrix with the acid form allows most of the C 18B molecules to change orientation favorable for the dimerization, re sulting in the regular decrease in the tilt angle. On the other hand, a steric repulsion between C18B and DA in the matrix with the salt for m inhibits such an orientation change during the dimerization. This ma y be another factor which explains the lower extent of the dimerizatio n in the matrix with: the salt form compared to that in the matrix wit h the acid form.