Porphycene protonation: a fast and reversible reaction enabling optical transduction for acid sensing

Nano‐sensor materials, especially if they target biological purposes, require a fine control and tooling of the properties at the molecular scale when intra‐molecular properties have to be exploited, e. g., in optical sensors. By means of optical spectroscopies (namely, spectrophotometry, spectrofluorimetry, and surface differential reflectivity), we unveil that thin films of porphycene show a spectacular chromatic change when exposed to acid vapors (namely, to hydrochloric acid). After the exposure, the original optical properties of porphycene films are recovered in few seconds and without any thermal annealing. In addition, since clear spectroscopic signatures are observed both in absorption and in reflectivity, the porphycene reveals to be suitable even for deposition of films onto opaque substrates. These findings are of significant interest in view of a potential engineering of the molecules and an implementation in devices.