title: Molecular dynamics simulation of amphiphilic bistable [2]rotaxane langmuir monolayers at the air/water interface.
abstract: 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.
authors.affiliation: Materials and Process Simulation Center, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: Seung Soon
authors.initials: SS
authors.lastname: Jang
authors.forename: Yun Hee
authors.initials: YH
authors.lastname: Jang
authors.forename: Yong-Hoon
authors.initials: YH
authors.lastname: Kim
authors.forename: William A
authors.initials: WA
authors.lastname: Goddard
authors.forename: Jang Wook
authors.initials: JW
authors.lastname: Choi
authors.forename: James R
authors.initials: JR
authors.lastname: Heath
authors.forename: Bo W
authors.initials: BW
authors.lastname: Laursen
authors.forename: Amar H
authors.initials: AH
authors.lastname: Flood
authors.forename: J Fraser
authors.initials: JF
authors.lastname: Stoddart
authors.forename: Kasper
authors.initials: K
authors.lastname: Nørgaard
authors.forename: Thomas
authors.initials: T
authors.lastname: Bjørnholm
mesh_terms: D000388:Air; D003198:Computer Simulation; D018360:Crystallography, X-Ray; D008567:Membranes, Artificial; D008956:Models, Chemical; D008958:Models, Molecular; D043862:Rotaxanes / Q000737:chemistry*; D013499:Surface Properties; D014867:Water / Q000737:chemistry
chemical_list: D008567:Membranes, Artificial; D043862:Rotaxanes; D014867:Water