Light-induced J-aggregation of merocyanine in Langmuir and Langmuir-Blodgett films

The structures and photoreactions of the Langmuir and LB films of an amphiphilic spiropyran, 1',3'-dihydro-3',3'-dimethyl-6-nitro-1'-octadecyl'-8-(docosanoyloxymethyl)spiro[2H-1-benzopyran-2,2'-(2H)-indole] (SP) are investigated. A chloroform solution of SP at a concentration of 0.1 mM is spread on the water surface. When the Langmuir film of SP is compressed on a subphase at 30degreesC and a phase transition takes place, the monolayer changes, into a mosaic structure where bilayer domains are buried. in the monolayer. When the Langmuir films are illuminated with UV light, SP is isomerized to merocyanine (MC). J-aggregates of MC are formed at high temperatures and at high surface pressures where the bilayer domains are formed. The Langmuir films of SP are transferred as LB films onto solid substrates at 30degreesC. The LB films consist of the bilayer domains with a diameter of 10 to 20 mum and a height of 4 to 5 nm. Three-dimensional structures are present in almost all the bilayer domains. Room-temperature illumination of the LB films with UV light produces the I-aggregation of MC with the development of dendritic structures starting from the three-dimensional structures in the domains. These dendrites consist of the J-aggregates. Molecules are aligned radially in the dendrites. When the LB film is illuminated at 7degreesC, the J-aggregation of MC does not proceed. All these results indicate that the criteria of the J-aggregation of MC in the Langmuir and LB films are the presence of the bilayer domains and the nucleation sites with the mobility of the molecules large enough for the morphological change necessary for the J-aggregation.