Photoisomerization of Azobenzene: A Quantitative Force Field-based Implementation and Simulation of Assemblies with Layered Silicates in Comparison with Experiment

Hendrik Heinz, The University of Akron

Abstract

Classical MD simulation is a suitable tool to investigate trans-cis isomerizations as a function of various molecular environments and conditions (temperature, pressure). A temporary modification of the C–N=N–C torsion potential in azobenzene is presented to model quantitatively the input of photon energy. The relative energies of the trans and the cis isomer, the excited state energy, the thermal conversion barrier, and the time scale of the conversion (1 ps) are accurately accounted for in this force field-based implementation, using a standard three-term torsion potential as available in the polymer consistent force field or in OPLS-AA. MD simulations were carried out for layered silicates with attached photoactive azobenzene-containing surfactants, and reversible changes up to 2.8 Å (14%) in basal plane spacing between montmorillonite layers have been identified upon trans-cis isomerization. The best responses are indicated for (4,4'-pheylazophenyl)diammonium ions and (4-phenylazophenyl)ammonium ions in pillar-like, upright orientation on the mineral surface. Experimentally, optical switching of more than 1 Å has still remained challenging without support from excess amounts of solvent [Okada et al. J. Mater. Chem. 2005, 15, 987], and the simulation helps explain the orientation of the azobenzene units relative to the surface and the reorganisation in the interlayer space upon excitation in the context of UV/VIS absorption measurements and X-Ray diffraction data.