REVERSIBLE LIGHT-INDUCED MORPHOLOGICAL CHANGE IN LANGMUIR-BLODGETT-FILMS

An amphiphilic anionic azobenzene derivative, soluble in water, formed a monolayer on an aqueous subphase containing a water-soluble polycat ion. The monolayers were transferred onto solid substrate by the Langm uir-Blodgett (LB) technique. XPS measurements showed that ion exchange reaction proceeded almost completely at the air-water interface and t hat the ratio of the monomer unit of the polycation to the azobenzene was almost unity. UV/vis absorption and IR measurements indicated that the azobenzene photoisomerized reversibly in the LB films on alternat e illumination with UV and vis light. Furthermore, a reversible morpho logical change induced by light was observed in the LB films with AFM. Before illumination, the surface of the single-layer LB film was very smooth with a surface undulation of less than 1 nm. On illumination w ith UV light, however, a number of hills, with the height of ca. 5 nm and the diameter of the base of ca. 100 nm, appeared on the film surfa ce. These structures almost disappeared on illumination with vis light . This indicates that the widely accepted assumption that photoisomeri zation should not change the two-dimensional structures significantly does not hold in the present case.