Different J-aggregates of an amphiphilic cyanine dye in monolayers at the air-water interface

J-Aggregates of the amphiphilic symmetrical cyanine dye 5,5'-6,6'-tetrachloro-1,1'-diethyl-3,3'-dioctadecylbenzimidazolocarbocyanine toluenesulfonate are formed in monolayers of the pure dye at the air-water interface as well as in mixtures with the coaggregates hexadecane (HD) and methyl octadecanoate (MOD), respectively, at a molar ratio of dye/coaggregate of 1:1. The monolayers have been characterized by surface pressure-area and surface potential-area isotherms as well as reflection spectroscopy. The position of the J-aggregate band that is red-shifted with respect to the monomer band depends on the kind of coaggregate. This in the case of symmetrical cyanines surprising phenomenon was analyzed by applying the extended dipole model according to the model of two-dimensional "brickstone" structure assuming that the coaggregate MOD in contrast to HD modifies chromophore packing.