Effect of hydroxyl substitution in squaraine dyes on their aggregation in Langmuir-Blodgett films

In this paper we study the effect of hydroxyl substituents at the benzene ring of the squaraine dye chromophore and different alkyl substituents at the N atom on the aggregation state of dyes in Langmuir-Blodgett (LB) films. Squaraine dyes were used as follows: dye I, [2-(4-dipropylaminophenyl)-4-(4-dipropyl-iminocyclohexa-2,5-dieny lidene)-3-oxocyclobut-1-enolate]; dye II, [2-(2-hydroxyl-4-diethylaminophenyl-4-(2-hydroxyl-4-diethyliminocyclohexa-2,5-dienylidene)-3-oxocyclobut-1-enolate]; and dye III, [2-(2-hydroxyl-4-ethyl, stearylaminophenyl)-4-(2-hydroxyl-4-ethyl, stearyliminocyclohexa-2,5-dienylidene)-3-oxocyclobut-1-enolate]. Surface pressure-area isotherm studies showed that the molecules of squaraine dyes I, II and III may be residing vertically along its short axis on the water surface with an area of 0.75 nm(2), standing at the water surface on the faces of smallest area of 0.27 nm(2) and residing vertically along its short axis on the water surface with an area of 0.72 nm(2) respectively. Stable LB films can be obtained by mixing dye with arachidic acid (C-20) The aggregation of the squaraine dyes in these mixed monolayers were studied by reflection spectra of surface and that of their LB films by absorption spectroscopy. Spectral data showed that the aggregation states of dyes I, II and III in mixed monolayers and LB films not only are different from their solution absorption spectra but also are different from each other.