Dimer adsorption and identification for various spiropyran-based molecular switches using Molecular Mechanics calculations

Abstract

In this study we have systematically investigated the adsorption geometries and energies of various merocyanine/spiropyran molecules on NaCl. The molecules are closed-ring spiropyran molecules with various side groups which can be transferred into open-ring merocyanine molecules. These isomers act as molecular switches which can be triggered by different external stimuli. The underlying substrate for this study is a two-layer film of the ionic insulator sodium chloride (NaCl). Adsorption of the spiropyran molecules is mostly driven by a competition between the sets of aromatic rings being able to find configurations close to the surface which leads overall to more preferable adsorption energies. On the other hand, since the merocyanine isomer as T-conformers have a generally flat geometry, they will be able to orient themselves nearly parallel to the substrate. This leads to higher binding energies for T-conformer compared to C-conformers, which due to steric effects cannot adsorb in a flat configuration. The beginning of film growth for these molecules starts with dimer formation which can be in parallel or anti-parallel geometries. In general, the anti-parallel configurations lead to higher binding energies when oppositely charged moieties of the molecules can interact through Coulomb forces. Comparing the calculated dimer configurations to experimentally observed dimers allows us to identify the particular conformers for each merocyanine molecule which might be energetically more favorable upon adsorption on the substrate. For two molecules with similar side groups, namely benzo and naphtho merocyanine, the so-called CTT and TTC conformers form dimers with highest binding energies, whereas for nitro and methoxy merocyanine, which have a common nitro side group, the dimers with highest binding energies consist of CTC and TTT conformers.