From spontaneously formed aggregates to J-aggregates of photochromic spiropyran

Nitrospiropyrans form spontaneously aggregates in aliphatic solvents at room temperature. During solvent evaporation and simultaneous exposition of photochromic solution to UV irradiation, blue crystalline “particles” are abundantly formed within several seconds without any decomposition side effect. Their size can reach dimensions equal to tens of microns depending on the concentration or may form continuous film depending on the substrate surface pre-treatment. Pure solid samples of merocyanine isomer were investigated using UV–VIS, fluorescence and IR spectroscopy. It was found that the structure of aggregate was influenced by the interactions induced by the presence of the nitro-group in spiropyran. The deposition method applied in our studies consisting in fast evaporation of photochromic solution during irradiation allows one to selectively control the formation of molecular assemblies by changing a solvent, substituent on spiropyran molecule, preparation conditions (i.e., temperature, concentration) or substrate. Since merocyanine isomer exhibits the appropriate molecular symmetry, such aggregates can be used as NLO material. Its thermal stability, good optical properties and the ease of formation suggest that the films can be used for optical memory.