Microstructure and Optical Properties of Mixed Monolayers Containing a J-band Forming Cyanine Dye and Various Cosurfactants

Mixed monolayers of stearic acid and the dye 1-methyl-1'-octadecyl-2,2'-cyanine iodide (S120) at the air-water interface and on a solid support are studied by fluorescence microscopy and spectroscopy, by electron microscopy and diffraction and by synchroton X-ray reflection. Rotationally symmetric arrangements of needle-like J-aggregates observed by fluorescence microscopy can also be observed by electron microscopy.

Electron diffraction again shows rotational symmetry of the crystallographic axes within domains of dimensions 100 ωm. The sharpness of the electron diffraction lines indicates a high degree of positional order. Yet structures at the centre and near the periphery of a domain differ in that the lattice constants are slightly larger in the latter case.X-ray reflection measurements demonstrate and quantify the rearrangement of the chromophore groups during pressure-induced growth of a J-band, forming either in a 1:2 S120:stearic acid mixture or purely from S120 with no discernible difference in the fluorescence spectrum.The fluorescence emission and excitation spectra depend strongly on the temperature, which can be understood within a model considering thermally assisted detrapping of excitons.

A molecular picture of the nature of these traps, which are distinguished by a very large Stokes shift, will be given in conjunction with studies of exciton-surface plasmon interactions.