Molecular dynamics simulation of amphiphilic bistable (2)rotaxane Langmuir monolayer at air/water interface
S. S. Jang, Y. H. Jang, Y.-H. Kim, W. A. Goddard III, J. W. Choi, J. R. Heath, A. H. Flood, B. W. Laursen, and J. F. Stoddart, J Amer Chem Soc, 127, 14804 (2005).
Bistable 2rotaxanes display controllable switching properties in solution, on surfaces, and in devices. These phenomena are based on the electrochemically- and electrically-driven mechanical shuttling motion of the ring-shaped component, cyclobis(paraquat-p-phenylene) (CBPQT4+), between a monopyrrolotetrathiafulvalene (mpTTF) unit and a 1,5-dioxynaphthalene (DNP) unit located along a dumbbell component. The most stable state of the rotaxane (CBPQT4+@mpTTF) is when the CBPQT4+ ring encircles the mpTTF unit, but a second less favored metastable co-conformation with the CBPQT4+ ring surrounding the DNP (CBPQT4+@DNP) can be formed experimentally. For both its co-conformations of an amphiphilic bistable 2rotaxane, we report here the structure and surface pressure-area isotherm of a Langmuir monolayer (LM) on a water subphase as a function of the area per molecule. These results from atomistic molecular dynamics (MD) studies are validated by comparing with experiments based on similar amphiphilic rotaxanes. For both co-conformations, we found that as the area/molecule increases the thickness of the LM decreases while the molecular tilt increases. Both co-conformations led to similar LM thickness at the same packing area. From the simulated LM systems, we calculated the electron density profiles of the monolayer as a function of area per molecule, which show good agreement with experimental analyses from synchrotron X-ray reflectivity measurements of related systems. Decomposing the overall electron density profiles into component contributions, we found distinct differences in molecular packing in the film depending upon the co- conformation. Thus we find that the necessity of allowing the tetracationic ring to become solvated by water, leads to differences in the structures for the two co-conformations in the LM. At the same packing area, the value of the overall tilt angle does not seem to be sensitive to whether the CBPQT4+ ring is encircling the mpTTF or the DNP unit. However, the conformation of the dumbbell does depend on the location of the CBPQT4+ ring, which is reflected in the segmental tilt angles of the mpTTF and DNP units. Using the Kirkwood-Buff formula in conjunction with MD calculations, we find that the surface pressure-area isotherms for each co-conformation in which the CBPQT4+@mpTTF form has smaller surface tension and therefore larger surface pressure than the CBPQT4+@DNP at the same packing area, differences that decreases with increasing area/molecule, which is verified experimentally.
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