BIDIMENSIONAL DISTRIBUTION OF A CYANINE DYE IN LANGMUIR-BLODGETT (LB)MONOLAYERS STUDIED BY TIME-RESOLVED AND SPATIALLY-RESOLVED FLUORESCENCE

The excitation energy transfer from monomers to dimers of N,N'-dioctad ecyl thiacyanine incorporated in Langmuir-Blodgett (LB) films of Cd-ar achidate or dipalmitoyl phosphatidic acid was studied as a function of the dye concentration, ranging from 0.5 to 6 mol %, to obtain insight in the two-dimensional distribution of the dye. The details of the en ergy transfer were determined by the global analysis of picosecond tim e-resolved fluorescence decays recorded on a single mixed monolayer. T hose studies were correlated with scanning confocal fluorescence micro scopy measurements. From the spectral decomposition of the absorption spectra, the fraction of molecules present as dimers or aggregates amo unts to 0.2 and is constant over the dye concentration range studied. The spatially unresolved data are supported by the confocal experiment s which show that the dye distribution in the LB films studied is inho mogeneous and suggests the formation of two separate phases: strongly fluorescing regions of high dye concentration, containing a mixture of monomers and dimers participating in the energy transfer process, are surrounded by regions of low fluorescence intensity and of small dye concentration containing almost exclusively dimers. Although in the Cd -arachidate matrix the fraction of dimers not participating in the ene rgy transfer process amounts to 0.135 +/- 0.015 over the concentration range studied, this amount can be considered negligible in the dipalm itoyl phosphatidic acid matrix.