Direct trapping excitation energy transfer from rhodamine B to crystal violet in Langmuir-Blodgett monolayer and stacked multilayer films

Abstract

Picosecond time-resolved fluorescence decays of N,N′-dioctadecyl rhodamine B (DORB) have been measured in order to examine both the morphology of chromophores and the excitation energy transfer in restricted geometries of Langmuir-Blodgett (LB) monolayer and stacked multilayer films which contain both DORB and octadecyl crystal violet (ODCV). Fluorescence decays of DORB deposited in the LB films are affected by ODCV, as a result of direct trapping excitation energy transfer from DORB to ODCV. The decay analysis shows that ODCV in the LB monolayer film distributes near uniformly at low concentrations below 6 mol% and that ODCV distributes with a fractal-like structure with the Hausdorff fractal dimension of 1.4 at high concentrations above 6 mol%. Thus, the morphology of ODCV in the LB monolayer films shows a kind of phase transition at around 6 mol%. The efficiency of the energy transfer from DORB to ODCV in LB monolayer films is much lower than that in solution or on vesicle surfaces. Fluorescence decay which is characterized by the two-dimensional system for the LB monolayer films is seen to approach the one characterized by the three-dimensional system, as the number of stacking layers increases.