Self-Assembled Thin Solid Films of Dioctadecyldimethylammonium Cinnamate Lamella Units That Control the Photostationary State of E−Z Photoisomerizations

UV illumination of dioctadecyldimethylammonium cinnamates (C18 trans- cin and C18 cis -cin) was found to result in the E − Z photoisomerization of the cinnamate moieties in self-assembling multilamella films. In the present case, a common photostationary state (pss), which can usually be observed in solution, was not seen in the photolysis of the self-assembled material films of C18 trans- cin and C18 cis -cin having a bimolecular (bilayer)-unit lamellar structure. Both the E - and Z -isomers are difficult to isomerize and exhibit different E / Z ratios, 73:27 and 10:90, respectively, even under prolonged UV irradiation. However, the same E / Z ratio (15:85) could be attained at the pss from either isomer in a monolayer-unit structure. The E / Z ratios were found to change abruptly upon heat treatment at around 55 °C before photoisomerization, implying a thermal structural change in the films. Analysis of X-ray diffraction, differential scanning calorimetry, and polarized IR and UV−vis spectroscopic investigations have indicated that the solid films have a characteristic multilamellar structure consisting of bilayer units when they are cast on the silica glass without heat treatment above 55 °C. Moreover, the solid films preheated to 80 °C were composed of an interdigitated monolayer unit having a loose packing structure. This result is in contrast to the bilayer-unit structure of the solid films where E − Z photoisomerization of both isomers is suppressed.