Adsorption of Sulforhodamine Dyes in Cationic Langmuir−Blodgett Films: Spectroscopic and Structural Studies

Abstract

This paper reports the incorporation of sulforhodamine B (SRB) in the octadecylamine Langmuir−Blodgett (LB) films and the adsorbability of the cationic LB films have been investigated. It was observed that the adsorption process mainly occurred due to the ionic interaction between the cationic amino group of the long-chain amine and the sulfonic group of the SRB dye. Surface pressure−area isotherm studies confirmed the strong interaction between the long-chain amine and the SRB dye and the change of orientation of the molecules were observed with variation of pH of the subphase. Absorption, steady-state and ultrafast time-resolved fluorescence spectroscopic studies suggested the presence of the monomer and dimer of the SRB molecule in the restricted geometry of LB films. Furthermore, the contents of the monomer and dimer depend on the concentration of the dye and the adsorption process. The orientation of the SRB dye molecule in the LB films determined by polarized absorption spectra indicated a lying-flat orientation of the SRB molecule. Structural characteristics of the LB films such as orientation, layer−layer distance, and topography of the film surface have been investigated by FTIR spectroscopy, low-angle X-ray diffraction, and atomic force microscopy (AFM). Our preliminary results imply that the functional molecules in LB films are incorporated with a desired orientation by means of designing the molecular structure of the adsorbates.