Two forms of stilbazolium merocyanine on Langmuir-Blodgett monolayers

Abstract

Monolayers of stilbazolium merocyanine were prepared at the air-water interface, and were transferred onto a hydrophilic quartz surface by the vertical dipping method. The molecular forms in the films were studied for two types of merocyanine: A) 1-(6′-hydroxyhexyl)-4[(3,5-di- t-butyl-4-oxocyclohexane-2,5-dienylidene)ethylidene]-1,4-dihydropyridine and B) 1-(hexadecyl)-4[(3,5-di- t-butyl-4-oxocyclohexane-2,5-dienylidene)ethylidene]-1,4-dihydropyridine both with reference to the pH of the buffer. The protonated form gave a more stable monolayer film than the free-base form. Also the molecules of protonated merocyanine did not show degradation, but the free-base was more fragile. Both forms could be transferred onto a hydrophilic substrate building a multilayer structure. Using UV-Vis absorption spectroscopy for one monolayer on quartz, the orientation of molecules relative to the film surface was found to depend on the form of merocyanine. The polarized components of the absorbance spectra indicate that the free-base form of merocyanine is preferentially oriented parallel to the quartz substrate, in contrast to the protonated form which has a more out-of-plane orientation.