Characterization of π-conjugated monolayers of oligo(phenylenevinylene) derivatives by absorption spectroscopy and fluorescence microscopy

Monolayer formation of oligo(phenylenevinylene) derivatives, methyl 4-(2-(4-(2-phenylethenyl)phenyl)ethenyl)benzoate (I) and N,N-dimethyl-4-(2-(4-(2-phenylethenyl)phenyl)ethenyl)benzamide (II), was demonstrated and confirmed by surface pressure-area isotherms and fluorescence microscopy. Stable monolayers were formed in the case of 4-(2-(4-(2-(4-methoxyphenyl)ethenyl)phenyl)ethenyl)benzyltrimethylammonium bromide (III) only through formation of a polyion complex at the interface. Fluorescence microscopy revealed island formation even at large molecular areas in all cases. Absorption spectra of monolayers (I) and (II) were much blue-shifted relative to those in solution, suggesting parallel stacking of the π-conjugated molecules. Mixed monolayers of (I) with stearic acid or dioctadecyldimethylammonium bromide were phase-separated, as indicated by absorption spectra. However, the domain size of the latter mixture was smaller than the resolution of fluorescence microscopy.