Protoporphyrin IX zinc(II) organization at the air/water interface and its Langmuir-Blodgett films

Protoporphyrin IX zinc(II) (ZnPP) and its mixtures with N-hexadecylethylenediamine, hexadecyltrimethylammonium bromide, and didodecyldimethylammonium. bromide in a molar ratio of 1:2 were used to investigate the Langmuir monolayer and Langmuir-Blodgett (LB) film behaviors. The well-defined Langmuir monolayers were formed at the air/water interface for ZnPP alone and its hybrids with surfactants. While the ultraviolet-visible spectrum of ZnPP alone indicated the formation of an H-aggregate in the LB film from the blue shift of the Soret band, the introduction of surfactants into the ZnPP LB film reduced the pi-pi interaction between ZnPP macrocycles. Linear dichroism revealed that the average orientation angle of the protoporphyrin macrocycle against the substrate surface was in the range of 39.5-44.5degrees in a ZnPP LB film and three ZnPP/surfactant LB films. Fourier transform infrared spectra of the LB films suggested that the alkyl chains of the surfactants take a disordered gauche conformation and porphyrin carboxylic acids partly converted to carboxylates with the introduction of surfactants. The atomic force microscopic results further confirmed the formation of homogeneous LB monolayers. These results demonstrate that there is preferable mutual miscibility in ZnPP/surfactant hybrids, and no isolated phase or domain formed by the surfactants exists in the hybrid LB films.