NANO LETTERS
Surface-Mounted Molecular Rotors with Variable Functional Groups and Rotation Radii
2009 Vol. 9, No. 12 4387-4391
Dingyong Zhong,† Tobias Blo¨mker,‡ Katrin Wedeking,‡ Lifeng Chi,*,† Gerhard Erker,*,‡ and Harald Fuchs†,§ Physikalisches Institut, UniVersita¨t Mu¨nster, Wilhelm-Klemm-Strasse 10, 48149 Mu¨nster, Germany, Center for Nanotechnology (CeNTech), UniVersita¨t Mu¨nster, Heisenbergstrasse 11, 48149 Mu¨nster, Germany, Organisch-Chemisches Institut, UniVersita¨t Mu¨nster, Corresstrasse 40, 48149 Mu¨nster, Germany, and Institut fu¨r Nanotechnologie, Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany Received August 17, 2009; Revised Manuscript Received September 28, 2009
ABSTRACT A strategy for designing and activating surface-mounted molecular rotors with variable rotation radii and functional groups is proposed and demonstrated. The key point of the strategy is to separate the anchor and the rotating functional group from each other by using a connector of adjustable length. The three independent parts of the molecule are responsible for different functions to support the rotating movement of the molecule as a whole. In this way, one can easily change each part to obtain molecular rotors with different sizes, anchors, and functional rotating groups.
Molecular machines, which convert external chemical, electric, or optical energies into controlled mechanical movements at molecular levels, play an important role in many biological processes.1-5 Molecular rotors are a type of molecular machine that can rotate with respect to the environment or involves one part that rotates with respect to another part.6,7 Compared with molecular rotors in solutions or in solids, molecular rotors anchored on surfaces by strong molecule-substrate interactions or confined in a supramolecular assembly8-23 have the advantages of being easily accessible by external fields15,19 and addressable by surface analysis methods,14,18,23 as well as being easily organized due to the reduced dimensions.13,17 Here we propose a concept for designing surface-mounted molecular rotors containing two end groups connected by a linear linker. One of the end groups interacts strongly with substrate surfaces, serving as anchor or stator, while the other group weakly interacts with the substrate, which allows the rotor to undergo a thermally activated rotating movement around the anchor (Figure 1a). In this way, the rotation radius is adjustable by simply changing the length of the connector, and it is possible to choose different anchors and rotating * To whom correspondence should be addressed. E-mail: (L.F.C.)
[email protected]; (G.E.)
[email protected]. † Physikalisches Institut, Center for Nanotechnology (CeNTech), Universita¨t Mu¨nster. ‡ Organisch-Chemisches Institut, Universita¨t Mu¨nster. § Institut fu¨r Nanotechnologie. 10.1021/nl902670k CCC: $40.75 Published on Web 10/07/2009
2009 American Chemical Society
groups, so that the rotors may be used to perform specific functions. Candidates to realize this concept are diferrocene derivatives Fc-(CH2)n-Fc (diFc-n), which contain two ferrocene (Fc) groups bridged by an oligoethylene chain.24 Ferrocene is an electrochemically active molecule that is widely used in industry for the generation of Fe catalysts as fuel additives for better fuel burning and has been recently become intriguing in nanoelectronics for its capablilities to perform reversible switching, stochastic conductance variation, negative differential resistance, and so forth.25-28 On metal surfaces such as Cu(110), ferrocene has low binding energies (
