Influence of the Molecular Structure on the Lateral Distribution of Xanthene Dyes in Langmuir−Blodgett Films

Steady-state and time-resolved fluorescence experiments suggest an important difference in the lateral distribution of two rhodamine dyes, differing by one nitrogen substituent, upon incorporation in Langmuir−Blodgett (LB) films. While the fluorescence decay curves of both dyes could be analyzed as a stretched exponential, the decay parameters of the NH derivative suggest that the fluorescence is, over the complete emission band, due to a single species, probably the monomer. The concentration dependence of the decay parameters suggests furthermore that the molecules of the NH derivative are distributed homogeneously over the Langmuir−Blodgett film. The fluorescence decay parameters of the N-ethyl derivative suggest on the other hand a distribution of two phases with a different dye concentration. For the latter molecule the wavelength dependence of the decay parameters suggests the presence of several emitting species.