Orientational structure in hemicyanine self-assembled films studied by absorption spectroscopy and optical second-harmonic generation

Self-assembled monolayers (SAMs) of the hemicyanine dyes, 2-[4-(dimethylamino)styryl]-1-methylpyridinium iodide (MeHEMI) and 4-[4[(dimethylamino)styryl]-1-docosylpyridinium bromide (C22HEMI), were successfully deposited on silica substrates by drawing a substrate from their ethanol solutions. Both hemicyanine SAMs on a hydrophilic substrate generate a strong second-harmonic (SH) light due to their polar ordering. Interferometry of the SH lights revealed that the dimethylamino group is attached to the surface in the C22HEMI SAM, while the pyridinium ion part is attached to the hydrophilic surface in the MeHEMI SAM. These results indicate that the polar orientation of the C22HEMI SAM is strongly influenced by the presence of a long alkyl chain in the chemical structure.