Response of strongly fluorescent carbazole-based benzoxazole derivatives to external force and acidic vapors

A carbazole-based D-pi-A benzoxazole derivative was found to self-assemble into long fibers, and its emission increased because of the presence of J-aggregates. These fibers exhibited a distinctive response to volatile acid vapors. The vapor of strong acids such as hydrochloric acid or trifluoroacetic acid destroyed the molecular packing within the fiber and produced a fluorescent color change. However, the fibers did not react to the vapor of weak acids such as acetic acid (HOAc). In addition, the fibrous film exhibited isothermally reversible piezo-fluorochromism. Its blue fluorescence was converted to blue-green under a mechanical force and was spontaneously restored at room temperature. More importantly, the response of the fibrous film to HOAc vapor could be controlled by a mechanical stimulus. The colorless ground film readily absorbed HOAc vapor and emitted an orange fluorescence. Furthermore, such colored films were not self-healing, but returned to colorless upon heating, with a blue emission. Acetic acid vapor, may thus, selectively act as a stabilizer and developer to retain the information imparted by a mechanical force. These results show that the response of organic nanofibers to stimuli may be adjusted and controlled by a mechanical stimulus, and vice versa.