Global Analysis of the Fluorescence Decays of N,N‘-Dioctadecyl Rhodamine B in Langmuir−Blodgett Films of Diacylphosphatidic Acids

Abstract

Langmuir−Blodgett multilayer films were prepared from N,N‘-dioctadecyl rhodamine B (RB18) diluted in dioleoylphosphatidic acid (DOPA) or dipalmitoylphosphatidic acid (DPPA) with various mixing ratios. Surface pressure−area isotherms of monolayers of the two matrices were recorded. Absorption and fluorescence spectra and fluorescence decays of the multilayer samples of RB18 were further examined. The fluorescence decay curves of RB18 in the multilayer assemblies, determined using time-correlated single photon counting, were analyzed in the framework of several models leading to nonexponential decays: (1) multiexponential decay; (2) Gaussian distribution of rate constants; (3) stretched-exponential decay, characteristic of Forster energy transfer in self-similar fractal-like structures with a fractal dimensionality of <2; and (4) Forster energy transfer in a two-dimensional system accompanied by an unquenched decay. Using the global analysis method, only the decay law for Forster energy transfer in two dim...