Monolayers Langmuir–Schaefer films and acidichromism of a nonamphiphilic acetone derivative containing carbazole

Abstract

A carbazole-containing compound, di(3-N-ethylcarbazoylidene) acetone (dNECa), which has no long substituted alkyl chain, was found to form a stable monolayer at the air–water interface when spread from a chloroform solution. The monolayer can be transferred onto solid substrates by the horizontal lifting method to form a uniform ultrathin film (Langmuir–Schaefer film or LS film). Quartz-crystal microbalance measurements of the transferred LS film confirmed monolayer formation at lower surface pressures (10 mN m⁻¹) and the collapse at higher surface pressures (20 mN m⁻¹). The transferred LS film showed a strong color change from yellow to blue when exposed to HCl gas. The film returns to its original yellow color spontaneously in air or on exposure to NH₃ gas. These color changes can be repeated many times in the transferred films by alternative exposure to HCl and NH₃ gases. It is suggested that the color change or acidichromism is due to the keto-enol tautomerization of the compound in the film. Fluorescence spectra have been used to confirm our conclusions.