Spectral response to protonation of 4-(4-(4-(didodecylamino)phenylazo)phenyl)butyric acid in its Langmuir-Blodgett monolayer

4-(4-(4-(Didodecylamino)phenylazo)phenyl acid is synthesized, and its Langm nir-Blodgett (LB) monolayer is successfully fabricated. The drastic changes in electronic spectra of this compound are found in acidic solution and in LB monolayer upon exposure to HCl vapor (mainly the decrease in absorption at 416 nm and great increase at 540 nm). The UV-visible and infrared spect ra reveal a mechanism of protonation to either azo group or amino group. Th is phenomenon has been suggested to have its prospective application in fun damental interfacial materials for pH sensors. (C) 2000 Academic Press.