Surface Pressure Dependent Fluorescence Resonance Energy Transfer in Mixed Monolayers of Amphiphilic Coumarin and Texas Red at the Air−Water Interface

Abstract

This paper reports the spectroscopic characteristics of pure N-(Texas Red sulfonyl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine triethylammonium salt (TR) and 3-(2-benzothiaxolyl)-4-cyano-7-(octadecyloxy)coumarin (BCOC) at the air−water interface. Absorption and steady state fluorescence studies of these dyes at the air−water interface reveals the formation of aggregates at the air−water interface. Detailed studies revealed the formation of J-aggregates in the case of TR and both J- and H-aggregates in the case of BCOC. Polarized fluorescence studies confirmed that the dye molecules were highly oriented at the air−water interface. Surface pressure dependent fluorescence emission studies showed extensive quenching of the TR fluorescence that has been attributed to an efficient energy transfer from the monomeric species of TR to their aggregates which are nonfluorescent. In the case of BCOC, fluorescence emission occurs only from the J-aggregates of BCOC as the H-aggregates are nonfluorescent. Mixed films of BCOC and TR with DPPC show unique behavior. Even at very low concentration of TR, the BCOC fluorescence is completely quenched and the sensitized fluorescence from TR dominate, thus indicating efficient energy transfer from BCOC to the TR moieties. With an increase in the surface pressure, the same features are found to predominate and the quenching of TR fluorescence is attributed to the generation of TR aggregates caused as a result of compressing the monolayer. At higher surface pressures, the characteristics of the TR fluorescence are found to disappear and the fluorescence characteristics of BCOC dominate. One explanation of such behavior could be the deactivation of BCOC via the radiative pathway and inefficent energy transfer from the aggregates of BCOC to the aggregates of TR. This study therefore establishes that FRET can be a useful tool for studying aggregation in supramolecular LB assemblies.