Spatial and spectral heterogeneity in fluorescence from monolayers of 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide

Far-field fluorescence of Langmuir-Blodgett (LB) films of the hemicyanine d ye, 4-(4-(dihexadecylamino)-styryl)-N-methylpyridinium iodide, I, is spectr ally heterogeneous, displaying bands assigned to monomer and various aggreg ated species in both excitation and emission. LB films of I visualized by e pifluorescence microscopy were also observed to be spatially heterogeneous. Under the preparation conditions used here the morphology consists of coex isting liquid expanded (LE) and liquid condensed (LC) phases. LC domains di splay distinct spatial structure. They are ca. 5-20 mum in lateral size, di splay a multilobed shape, and are surrounded by LE phase. The characteristi c aggregation behavior of these stilbazolium-based hemicyanines allowed the ir electronic spectra to be used to provide the contrast in fluorescence mi croscopy of LB films of I. Emission images thus reveal the aggregation-indu ced electronic structure of the molecules composing the domains. The degree of internal structure of the LC domains was observed to vary with excitati on wavelength with the richest internal structure being observed with UV ex citation of the fully developed W-aggregate. Correlation of film and soluti on fluorescence spectra suggest that the LC domains are composed of dimers, trimers, and larger n-mers up to the fully extended aggregate, while the L E phase is predominantly populated by monomers with some fully extended agg regates.