Novel moisture‐sensitive binary films that exhibit reversible change in fluorescent color have been created using a fluorescent amorphous molecular material, 4‐[bis(4‐methylphenyl)amino]benzylideneaniline (BMBZA), with organic acids such as pentafluorobenzoic acid and benzoic acid. Orange fluorescent color of the film under ambient dry atmosphere was found to change drastically to greenish blue under wet atmosphere. Response speed of such color change was quite fast and it responded well to exhaled breath. It was suggested that such change in fluorescent color was due to switching the excited species between BMBZA and its protonated iminium cation, and that the acidity level of the organic acids was an important factor for exhibiting such reversible changes in fluorescent colors. In addition, excited‐state intermolecular proton transfer was suggested to contribute the orange fluorescence under dry atmosphere.

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