Organic Monolayers as Mimics of Liquid/Liquid Interfaces: Molecular Dynamics Study of Electronic Spectra and Solvent Dynamics

The electronic spectra and the solvation dynamics following the electronic transition of a chromophore attached to the interface between water and a self-assembled hydrocarbon monolayer are examined by molecular dynamics computer simulations and are compared with the same chromophore undergoing the same electronic transitions at the water/nonane interface. Two different self-assembled monolayers are considered. One is made of only C 18 H 38 molecules giving rise to a smooth surface, and one is made of a 1:1 random mixture of C 18 H 38 and C 22 H 46 molecules giving rise to a rough surface. Different choices of the chromophore charge distribution and its location at the interface are considered and provide insight into different microscopic factors which influence the electronic line shape and the water dynamic response. We find that the electronic spectrum of the chromophore at the interface between water and a self-assembled hydrocarbon monolayer is very similar to the spectrum calculated when the same chromophore is located at the water/nonane interface, with variations which are consistent with the structure of the interface and, in particular, the degree of exposure of the chromophore to interfacial water molecules. The same observation applies to the water dynamic response, with the exception that slow components of water dynamics at the normal liquid/liquid interface (which can be shown to be due to microscopic capillary waves) are missing at the water/self-assembled monolayer interface.