Spectral Inspections on Molecular Configurations of Nile Blue A Adsorbed on the Elementary Clay Sheets

Studies on the configuration of dye molecules are of great importance in revealing origins of the electronic bands as well as understanding their transitions. In this work, we utilized dye molecules named Nile blue A, which are a type of oxazine dyes, to study the molecular configurations when they are transferred from solutions to a solid surface. The Langmuir-Blodgett (LB) technique was employed to construct such an interface where the interaction between the dye molecules and solid supports can be pursued. Hybrid films were prepared via the LB depositions, and the dye molecules were assembled on the elementary clay sheets (laponite, saponite). The configuration of Nb reflected by the molecular orientation, packing density, phase behavior, and variances of the surface tension has been derived. The ex situ spectroscopy characterizations such as UV-vis absorption, fluorescence emission, and excitation spectra were carried out on these LB films to reveal the fact that the adsorbed Nb molecules are mainly assembled in two types of configurations. Adsorbed state I was found to be achieved at high concentrations (1-10 ppm) of clay dispersions and low surface pressure (∼5 mN/m). In this state the anionic oxazine rings of Nb are adsorbed on clay sheets sharing a large lift-off area. This configuration gives allowable fluorescence (λ = 550 nm). Lower clay concentration (<1 ppm) and high surface pressure (10-30 mN/m) yield the adsorbed state II in which the oxazine chromophores were arranged in a side-by-side style, and the dye molecules stand perpendicularly to the clay sheets. This conformation exhibits no photoluminescence.