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Published October 26, 2005 | public
Journal Article

Molecular Dynamics Simulation of Amphiphilic Bistable [2]Rotaxane Langmuir Monolayers at the Air/Water Interface


Bistable [2]rotaxanes 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) (CBPQT^(4+)), between a monopyrrolotetrathiafulvalene (mpTTF) unit and a 1,5-dioxynaphthalene (DNP) unit located along a dumbbell component. The most stable state of the rotaxane (CBPQT^(4+)@mpTTF) is that in which the CBPQT^(4+) ring encircles the mpTTF unit, but a second less favored metastable co-conformation with the CBPQT^(4+) ring surrounding the DNP (CBPQT^(4+)@DNP) can be formed experimentally. For both co-conformations of an amphiphilic bistable [2]rotaxane, 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 per molecule increases the thickness of the LM decreases while the molecular tilt increases. Both co-conformations led to similar LM thicknesses 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 CBPQT^(4+) ring is encircling the mpTTF or the DNP unit. However, the conformation of the dumbbell does depend on the location of the CBPQT^(4+) 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 the surface pressure−area isotherms for each co-conformation in which the CBPQT^(4+)@mpTTF form has smaller surface tension and therefore larger surface pressure than the CBPQT^(4+)@DNP at the same packing area, differences that decreases with increasing area per molecule, which is verified experimentally.

Additional Information

© 2005 American Chemical Society. Received 13 May 2005. Published online 27 September 2005. Published in print 1 October 2005. The computational work was initiated with support by the National Science Foundation (NIRT, W.A.G.). The collaboration was supported by the Microelectronics Advanced Research Corporation (MARCO, J.F.S.) and its Focus Centers on Functional Engineered NanoArchitectonics (FENA) and Materials Structures and Devices, the Moletronics Program of the Defense Advanced Research Projects Agency (DARPA, J.F.S. and J.R.H.), the Center for Nanoscale Innovation for Defense (CNID, J.F.S.), and the MARCO Materials Structures and Devices Focus Center (J.R.H.). In addition, the facilities of the MSC (W.A.G.) were supported by ONR-DURIP, ARO-DURIP, IBM (SUR), and the Beckman Institute.

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