Acidichromism in the Langmuir-Blodgett films of novel photochromic spiropyran and spirooxazine derivatives

Two novel photochromic spiropyran (SP) and spirooxazine (SO) derivatives without substituted long alkyl chains were synthesized. They can form stable Langmuir monolayers at the air-water interface and can be transferred to solid substrates successfully through vertical dipping as Z-type Langmuir-Blodgett (LB) films. LB films of both of the compounds showed typical photochromism. An additional acidichromism was found for the LB films. The LB film of SO turned red upon exposure to HCl gas. Consecutive exposure to NH₃ gas caused the film to turn blue. The red colour and blue colour can be changed reversibly upon alternate exposure to HCl and NH₃ gas. For the LB film of the SP derivative, due to the protection of the spiropyran by the substituted group, the acidichromism was not obvious in the initial exposure to the HCl and NH₃ gases. However, after several repeated exposures to HCl and NH₃ gas alternately, the acidichromism became obvious. This paper presents a new route for SO and SP LB films changing from the spiro form to the merocyanine form through acidichromism besides the well-known photochromic route of this system.