Acidichromism and supramolecular chirality of tetrakis(4-sulfonatophenyl)porphyrin in organized molecular films

An anionic porphyrin, tetrakis(4-sulfonatophenyl)porphyrin (TPPS), was assembled into complex organized molecular films with an L-glutamic acid derivative (L-GluC 18) or octadecylamine (ODA) through in situ complex formation at the air/water interface. TPPS could form J-aggregates when deposited from the pH 3.1 subphase or exist as H-dimers from the pH 5.1 subphase. The complex films showed acidichromism (i.e., reversible color changes upon alternate exposure to acidic and basic gases). Despite the initial color, the film turned into a wheat color, and TPPS aggregated into an H-dimer when exposed to ammonia gas. When treated with dry HCl gas, the film became light yellow, and the H-dimer changed into the protonated species. Upon subsequent exposure to water vapor, the film became yellow, and TPPS formed a J-aggregate in the film. Three states including the H-dimers, protonated species, and J-aggregates in the films could be switched by exposing the film to NH3, dry HCl, and subsequently water vapor, respectively. This process can be repeated many times. When TPPS formed the J-aggregate in the complex film, the film showed circular dichroism in the TPPS Sorel band regardless Of L-GluC 18 or ODA. However, the supramolecular chirality changed and disappeared upon exposing the film to the NH3 and HCl gases. A linear and helical stacking of porphyrin in the film was proposed to explain such phenomena.