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 s elf-assembled hydrocarbon monolayer are examined by molecular dynamics comp uter simulations and are compared with the same chromophore undergoing the same electronic transitions at the water/nonane interface. Two different se lf-assembled monolayers are considered, One is made of only C18H38 molecule s giving rise to a smooth surface, and one is made of a 1:1 random mixture of C18H38 and C22H46 molecules giving rise to a rough surface. Different ch oices of the chromophore charge distribution and its location at the interf ace are considered and provide insight into different microscopic factors w hich influence the electronic line shape and the water dynamic response. We find that the electronic spectrum of the chromophore at the interface betw een water and a self-assembled hydrocarbon monolayer is very similar to the spectrum calculated when the same chromophore is located at the water/nona ne interface, with variations which are consistent with the structure of th e interface and, in particular, the degree of exposure of the chromophore t o interfacial water molecules. The same observation applies to the water dy namic response, with the exception that slow components of water dynamics a t the normal liquid/liquid interface (which can be shown to be due to micro scopic capillary waves) are missing at the water/self-assembled monolayer i nterface.