Effects of aliphatic tails on monolayer structures of hemicyanine dyes at the air/water interface as studied by in situ SHG measurements and surface pressure-area isotherms

Abstract

Langmuir monolayer assemblies of three kinds of hemicyanine (aminostyryl-pyridinium) dyes having aliphatic tail(s) at different positions of dye skeleton were studied by in situ second harmonic generation (SHG) measurements, also with surface pressure-molecular area (π-A) isotherms. The dye having an aliphatic tail at the nitrogen atom of the aminostyryl moiety afforded stable condensed phases at higher surface pressures than ∼30 mN m −1, whether the layer was diluted with arachidic acid (AA) or not. In the case of the dye having an aliphatic tail at the nitrogen atom of the pyridinium moiety, stable monolayers were formed when the dye fraction was lower than 0.6. As to the dye having two aliphatic tails at the both nitrogen atoms, expanded phases were prominent when the dye fraction was larger than 0.5 and condensed phases appeared at a substantially diluted condition (0.2 dye fraction). It was suggested that the aliphatic tail bound to the amino group was favorable to form the ordered monolayer structure, while that bound to the pyridinium moiety was unfavorable to dense and robust packing. All dyes exhibit similar monolayer structures only at a substantially diluted condition (dye: AA=0.2:0.8).