Polymer Engineering Faculty Research


Photoisomerization of azobenzene grafted to layered silicates: simulation and experimental challenges

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Molecular dynamics (MD) simulation has been employed to examine the molecular-level orientation and reorganization of model azobenzene derivatives in the interlayer space of layered silicates upon laser excitation in comparison with existing X-ray diffraction data and UV/vis absorption measurements. MD results show uniform reversible changes in basal plane spacing of montmorillonite up to 2.8 Å (14%) upon trans−cis isomerization of attached photoactive surfactants such as (4,4′-phenylazophenyl)diammonium ions and (4-phenylazophenyl)ammonium ions. Uniform, significant reponses are supported by the presence of cointercalate to compensate changes in interlayer density, by conformational rigidity and upright orientation of the azobenzene-containing surfactants on the mineral surface, and a medium-to-high packing density. Experimentally, Okada et al. have shown nonuniform reversible optical switching of the gallery height for semiflexible surfactants up to 10 Å (41%) in the presence of phenol and uniform reversible optical switching of 0.9 Å (4%) without cointercalates [Okada et al. J. Mater. Chem. 2005, 15, 987−992]. Further experimental data also show the absence of changes in gallery spacing for azobenzene derivatives with attached flexible hydrocarbon chains at low packing density without cointercalates and are explained by simulation. From a methods viewpoint, an approach is introduced to simulate the photoisomerization reaction using classical molecular dynamics, taking quantitatively into account the input of photon energy, the time scale of the isomerization (1 ps), and the relative energies of the trans and of the cis isomer, as well as the thermal conversion barrier. A temporary modification of the C—N═N—C torsion potential describes the input of photon (excitation) energy, which can be applied to common force fields (including PCFF, OPLS-AA, COMPASS, CVFF, AMBER, CHARMM) and facilitates the simulation of the photoisomerization reaction as a function of molecular environment, pressure, temperature, and excitation time.

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Chemistry of Materials





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