Electronic spectra of dipolar solutes at liquid/liquid interfaces: Effect of interface structure and polarity

Abstract

Molecular dynamics computer simulations are used to elucidate the role of solvent polarity and interface structure in determining the electronic absorption and fluorescence line shapes for model dipolar solutes at the interface between water and one of four different organic liquids. The different organic liquids represent a range of molecular structure and polarity: 1-octanol, 1,2-dichloroethane, n-nonane, and carbon tetrachloride. The solute is represented by two rigidly connected Lennard-Jones spheres. The different electronic states correspond to different charges on the two Lennard-Jones centers. In each interfacial system, different choices of solute charge distribution and solute location relative to the interface (including the bulk region) are considered and provide insight into different microscopic factors that influence the electronic line shape. For the water/1,2-dichloroethane and water/CCl4 interfaces, all of the calculations are repeated while the interface is externally constrained to be smooth in order to investigate the role of surface roughness. The calculated electronic line shapes are Gaussians whose peak positions reflect solvent polarity, interface structure, and probe location. Their widths are in general agreement with the prediction of linear response theory. Although continuum electrostatic models predict qualitatively correct behavior, they miss interesting interfacial effects.