Spiropyran and merocyanine adsorption on an ionic insulator surface

Spiropyran (SP) is a photochromic, three-dimensional molecular switch which can be converted to Merocyanine (MC), a planar isomer of Spiropyran where the central C-O bond has been broken. The adsorption behavior of various SP/MC species adsorbed on a NaCl substrate has been investigated using a combination of density functional theory and molecular mechanics calculations. The calculations show a nearly flat adsorption geometry of merocyanine trans-conformers with higher adsorption energies than their respective cis-conformers or the spiropyran isomer itself. In a next step to study the onset of film growth of these SP/MC molecules, two identical isomers were placed in high symmetry orientations with respect to each other governed by the underlying substrate and its symmetry. In these cases, dimers can form in parallel and anti-parallel orientation to each other with slightly different inter-molecular binding energies. Similar onset of film growth by means of dimer formation has been experimentally observed in merocyanine molecules adsorbed on a noble metal substrate.