Mechanized Molecules

The surface science results entail to a great extent the formation of ordered arrays. We continued our study of ordered monolayers and multilayer films of fumaric rotaxane on metal substrates grown by sublimation in ultra high vacuum and characterized by XPS and HREELS, in view of validating the structural models developed from theoretical calculations. We could demonstrate that the adsorption properties of fumaric rotaxane are similar on Au and Ag, i.e. the molecule chemisorbs through interaction of three amide functions with the substrate. However, the absence of a surface component in the O1s XPS spectrum of fumaric rotaxane/Ag(111) and the presence of such a component for fumaric rotaxane/Au(111) clearly confirmed the weaker interaction with Ag compared to Au as deduced from the theoretical calculation. Moreover we were able to show that it was the macrocycle which was responsible for the order in the monolayers. In fact, thread monolayers were not ordered, macrocycle monolayers showed the highest degree of order and fumaric rotaxane/Au(111) showed the best order at coverages slightly below monolayer but were still well ordered when the monolayer was reached. STM studies of these surfaces in ultra high vacuum did not give good results because the molecules interacted very strongly with the scanning tip and were displaced at room temperature during the scan. However, STM studies in tetradecane macrocycle monolayers on Au(111) prepared ex situ by sublimation in ultra high vacuum, gave very interesting results: it was possible to generate an ordered layer also by adsorption in an electric field. In fact, we found that the molecules desorbed from the gold surface when the tetradecane drop was deposited so that only the clean gold surface was seen by STM but with time they re-adsorbed under the influence of the electric field between the tip and the surface.

Photoswitchable surface using molecular machines: A new technique has been developed which is able to move micolitre droplets across a surface using rotaxane molecules as molecular motors, one of the most ambitious goals of the project. This could have applications in Lab-on-a-chip technologies. The results of the research will be made available to the general scientific community through publicationin major peer-reviewed journals and presentation at national and international conferences. The participating groups will also make the results available in an informal way to the many scientists who visit their laboratories. Three major publications have resulted from this project namely, - Nature Materials, (2005), 4, 704-710; - Angewandte Chemie Int. Ed., (2005), 44, 2-7; - Science, (2004), 306, 1532-1537. A total of 13 publications and 16 conference presentations/workshops have been produced by the MechMol consortium as a result of this project.

A single layer of light-driven molecular shuttles attached to a self-assembled monolayer of thiols on gold is able to transport microlitre droplets of diiodomethane along the surface - a mechanical macroscopic response from a mechanical molecular event. In order to demonstrate macroscopic movement of an object across a surface through mechanical motion at the molecular level, we created a photo-responsive surface based on switchable fluorinated molecular shuttles. For this we developed a protocol for grafting light-switchable rotaxanes with the fluoroalkane region exposed (E-1) onto a SAM of 11-MUA on Au(111) deposited onto either glass or mica to create a polarophobic surface, E-1.11-MUA.Au(111) which could be switched by light to polarophilic. Photoemission spectroscopy and scanning tunnelling microscopy data showed that the grafting was successful and were consistent with the molecular shuttles lying parallel to the Au surface. Both measurements gave evidence for a functionalisation yield of 2-3% of the alkanethiols, which, given the dimensions of the rotaxane, corresponds to a rather dense packing of the molecules. The mechanism is that the macrocycle movement of the shuttle is used to either conceal or reveal a fluoroalkane (Teflon-like) residue, which dramatically alters the surface tension of the droplet. Not only can this transport a droplet across a flat surface but it can also be used to the transport of a droplet against gravity, elevating it 1 mm up a 12° slope.

One of the crucial points to be tested was that the surface covered with light-switchable rotaxanes with the fluoroalkane region exposed (E-1) onto a SAM of 11-MUA on Au(111)/mica resisted UV illumination without degradation. For this purpose we exposed the surface to UV light for different periods of time and looked for signs of degradation by monitoring the oxidation of sulphur. Oxidation of sulphur gives rise to the appearance of an additional component in the S2p photoemission signal. We could show that no oxidized sulphur was present neither on the freshly prepared sample, nor after 5 minutes of UV irradiation. After 10 minutes of UV irradiation the slight change in background around 168.5eV (oxidized sulphur) hinted at a commencement of sulphur, however only after 15 minutes a small component characteristic of oxidized sulphur could be identified with confidence. This meant that all experiments concerning property changes of the surface in response to UV light and consequent movement of a macroscopic object had to be performed within this time span. In order to understand the interactions leading to macroscopic motions through the exploitation of rotaxane-based molecular machines the Bologna group used a phenomenological approach to obtain the rate of movement of the drop, focussing on the simulation of the interface between the rotaxanes and the drop. Experiments showed that the thermodynamics of interaction is subtler than expected. Indeed, the experimental contact angles, that according to Young law reflect the thermodynamics of the three-phase system, do not provide a simple explanation for the drop motion.

A series of rotaxanes has been synthesized which contain different imide and bisimide units. Other rotaxanes have been synthesized which contain a new type of station, which made it possible to have better control of the relative energies involved in shuttling processes. New rotaxanes have been synthesized which contain chromophores absorbing in the visible spectral range, and which have a strong fluorescence with a large sensitivity for the medium in which the molecule is contained.

To study molecular motors in the gas phase, we employed laser desorption in combination with supersonic expansions to obtain isolated, vibrationally and rotationally cold molecules. We have shown that it is possible to seed these supramolecular systems with molecular weight above 1,000 intact into a supersonic beam expansion. To fully investigate the structural and spectroscopic properties of these molecular systems, a combination of high-resolution spectroscopic techniques have been used, REMPI to obtain the mass-selected vibrational resolved excitation spectra, UV-UV hole-burning to identify the different conformers and IR Ion-Dip spectroscopy to characterize these conformers. From these experiments we have been able to determine the influence of the macrocycle on the conformational phase space accessible to the thread.

The rotaxane synthesized by the Edinburgh group used in these experiments was grafted onto a carboxylic acid-terminated self-assembled monolayer. Its macrocycle can be translocated from one position (station) on the thread to a second site through exposure to UV light and this movement conceal fluoroalkane residues and thereby modify surface tension. When a drop was deposited on this surface, one could change the wettability of the surface in front of the drop by light-induced movement of the macrocycles and hence induce drop movement. Evidence for this is shown in the lateral photographs of light-driven directional transport of a 1.25µl diiodomethane drop along the surface of a E-1.11-MUA/Au(111) substrate on mica. The direction of transportation was controlled by irradiation with a perpendicular beam of 240-400 nm light focused on one side of the drop and the adjacent surface. - Before irradiation (pristine E-1.11-MUA.Au(111)), contact angle 35 ± 2?. Immediately after this image was taken, the UV light beam positioned as indicated was switched on. - After 900 s irradiation, contact angle 13 +/- 2° (illuminated side), 15 +/- 2° (non-illuminated side). The diiodomethane drop has spread from the high E/Z-1 ratio area in the direction of the low E/Z-1 ratio region, increasing the total wetted area, and is about to be transported. - After 1010 s irradiation (just after transport), contact angle 13 +/- 2. - After 1110 s irradiation (at the photostationary state), contact angle 12 +/- 2.

Methods have been developed to enable attachment of rotaxanes to various surfaces, such as self-assembled monolayers on silicon, terminated with carboxylic acids, and on glass. We have adapted the synthesis procedures for covalent attachment of alkenes to Si(111) or Si(110). When simple alkenes were used, densely packed monolayers could be easily formed, in agreement with the literature. In order to obtain a carboxylic acid terminated monolayer, procedures were followed in which alkene esters (R=Me or R =CH2CF3) were attached to the surface and subsequently hydrolyzed. In this case, it was found that the published procedures do not work well. Complete hydrolysis of all ester groups could not be accomplished without damaging the monolayer, as demonstrated by XPS and contact angle measurements in collaboration with the Groningen group. For the purpose of attaching rotaxanes using hydrogen bonding to the COOH-groups it is, luckily, sufficient to hydrolyse only a part of the ester groups. Furthermore, procedures were established to immobilize rotaxanes on glass, starting with the condensation of aminopropyltriethoxysilane to the SiOH groups on the surface. This method may be less attractive for creating monolayers because it is known to produce less ordered materials than the procedure described above, but for immobilization of single rotaxane molecular switches it will be particularly suitable. Methods for detecting time-resolved fluorescence of rotaxane monolayers have been established. New rotaxanes have become available which can be attached to surfaces, and measurements will be carried out in the near future. The effect of thickness of the alkane spacer on the photophysical properties was investigated for the rotaxane that was studied previously on a self-assembled monolayer of mercaptoundecanoic acid on gold (F. Cecchet, P. Rudolf, S. Rapino, M. Margotti, F. Paolucci, J. Baggerman, A. M. Brouwer, E. R. Kay, J. K. Y. Wong, D. A. Leigh, J. Phys. Chem. B 2004, 108, 15192.). Surprisingly, the same molecule on a SAM of hexadecanoic acid showed a spectrum that was blue-shifted compared with that on the thinner C11 spacer, and similar to that in solution. Apparently, when the SAM is thin enough, the metal not only influences the fluorescence intensity, but also the fluorescence spectrum.

We successfully developed a system in which the translocation of a ring along a peptide-based thread, induced by changes in the nature of the local environment, could be used to switch the fluorescence of a rotaxane on or off. Remarkably, the system not only works in solution but also in polymer films where patterns visible to the naked eye can be generated solely through controlled submolecular motion. A polymer film INHIBIT logic gate based on a combination of control of submolecular positioning and chemical modification (protonation) was also demonstrated.

- Rotaxanes have been designed and synthesized in which the shuttling process can be monitored by means of fluorescence spectroscopy. This allowed us to observe shuttling in individual molecules and in thin films of rotaxanes. - To be able to study the structural and dynamic properties of molecular motors under actual working conditions in solution, we performed studies in which we employed (femtosecond) two-dimensional (2D) vibrational spectroscopy to probe the conformation and conformational heterogeneity of a rotaxane composed of a benzylic amide-based ring hydrogen-bonded onto a succinamide-based thread. Both the ring and the thread contain carbonyl groups, and by determining the coupling between the stretching modes of these groups from the 2D-IR spectrum, the solution conformation of the ring-thread system have been investigated. From the diagonal peaks in the 2D spectrum, we found that the carbonyl stretching bands of the thread were inhomogeneously broadened, which suggested that there existed a distribution of ring-thread hydrogen-bond lengths. From the position of the cross peak, we found evidence that the hydrogen-bond strength and the coupling strength were correlated. The results demonstrated the feasibility of a time-resolved 2D-IR experiment in which the motion in a molecular device was followed on a subpicosecond time scale. Molecular dynamics calculations in this respect formed an important input for the analysis.

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