Effect of Processing Conditions on the Formation of Aggregates and Phase Domains in Monolayers of the Hemicyanine Dye, 4-(4-(Dihexadecylamino)styryl)-N-methylpyridinium Iodide

Abstract

Epifluorescence microscopy and tapping mode AFM measurements were used to study the morphology of Langmuir−Blodgett monolayers of the hemicyanine dye, 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide, I, prepared under conditions of varying spreading solution concentration, compression speed, aging, subphase composition, and deposition pressure. The morphology, which is the result of coexisting liquid-expanded (LE) and liquid-condensed (LC) phases, is very sensitive to film preparation conditions but relatively insensitive to deposition pressure for a given film preparation protocol. Emission images reveal the following: domain size and shape are unaffected by deposition pressure; increasing spreading solution concentration while maintaining a constant number of molecules on the surface results in increased domain size; emission from the LC domains is more highly variegated as compression speed increases; and incorporation of I- in the subphase results in an entirely new morphology, in contrast to Cl- which only reduces domain size slightly. Far-field fluorescence from films of I is nearly constant as a function of pressure, indicating that aggregation occurs long before the compression process, perhaps even persisting from the spreading solution. This suggests that for monolayers of I, unlike monolayers of fatty acids, both aggregates and phase domains are seeded early in the Langmuir process. This is corroborated by the observation of aggregate formation at the air−water interface before compression takes place. A third phase was also observed and shown to be tens of monolayers in thickness and ca. 1 μm in lateral size by atomic force microscopy. This phase is likely associated with local film collapse and the presence of small bright regions in the epifluorescence images.