Temperature effect on photochromic reaction in Langmuir-Blodgett films of amphiphilic spiropyran and their morphological changes

The effect of temperature on photochromic reaction of amphiphilic spiropyran, 1',3'-dihydro-3',3'-dimethyl-6-nitro-1'-octadecyl-8-(docosanoyloxyme thyl)spiro [2H-1-benzopyran-2,2'-(2H)-indole] (SP1822), was investigated in single-component LB films. The morphological changes accompanying the photochromic reaction of SP1822 in the LB films were measured by atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM). Many circular domains with a width of 10-20 mum and a height of 4-5 nm were observed in as-deposited LB films before irradiation. Surface structures on circular domain did not change accompanying isomerization of SP1822 to open-colored photomerocyanine (PMC) on irradiation with UV light at room temperature, but the SNOM image reveals that the isomerization to PMC occurs into the circular domains. After reaching the photostationary state of PMC, further addition of heat at 50 degreesC during UV irradiation induces J-aggregate of PMC. The AFM shows that many cone-shaped structures with average width of 0.6 mum and average height of 40 nm grow up from the circular domain. We have clarified that the heating of the sample at 50 degreesC during UV irradiation is necessary for the PMC to form J-aggregates.