Molecular Devices DOI: 10.1002/ange.200600683 A Chemically Switchable Molecular Pinwheel** Owain P. H. Vaughan, FedericoJ. Williams, Nick Bampos, and Richard M. Lambert* The bottom-up fabrication of nanoscopic devices such as gears, [1] ratchets, [2] turnstiles, [3] switches, [4] and elevators [5] continues to attract much attention. The interest in molecular rotors in solution, [6] inside crystals, [7–9] and in the gas phase [10] has recently been extended to surface-mounted rotors; [11–13] most recently, a light-driven molecular rotor anchored to a gold surface has been demonstrated, [13] and a recent compre- hensive review of artificial molecular rotors is available. [14] Derivatized porphyrins are versatile building blocks for the creation of many different types of assemblies, including supramolecular structures. [15] When adsorbed on surfaces, they can be imaged with scanning tunneling microscopy (STM). Copper tetra-(3,5-di-tert-butylphenyl)porphyrin (Cu– TBPP) adsorbed on Cu(100) was the first case in which single- molecule manipulation at room temperature [16] and confor- mational recognition were achieved by means of STM. [17] In subsequent work with Cu–TBPP, Moresco et al. [18] were able to observe tip-induced conformational changes in individual di-tert-butyl phenyl (tBP) groups through height changes in the STM images. More recently, a metal-free TBPP function- alized with cyanophenyl terminal groups and adsorbed on Au(111) was used to form molecular assemblies including tetramers and one-dimensional wires; the structures of these assemblies were controlled by purposeful synthetic design of the monomer molecule. [19] Photoexcited molecular rotors have been observed in solution, [20] and the rotation of individual adsorbed molecules has been induced by manip- ulation with an STM tip; [11] however, reports of surface- mounted rotors are extremely sparse. [14] We describe herein a self-assembly method for switching on the rotation of adsorbed porphyrin molecules by mounting them on an appropriately functionalized ligand, one end of which binds to the surface, the other to the “hub” of the porphyrin. This was achieved by using zinc tetra-(3,5-di-tert-butylphenyl)por- phyrin [21] (Zn–TBPP; Scheme 1a), whose structure is such that interaction between the macrocycle component and the Ag(100) surface is minimized. As a result of steric repulsion, the four tBP groups at the meso positions of this porphyrin lie essentially orthogonal to the plane of the macrocycle. [17] These tBP “legs” decouple the molecular p-electron system from the silver surface and provide the principal (relatively weak) interaction between the molecule and the metal surface. The result is an adsorbate that is sufficiently strongly bound so as to prevent unwanted diffusion, while at the same time is able to rotate in the plane should the molecule–surface interaction somehow be weakened sufficiently. Zn–TBPP was deposited by evaporation under ultrahigh vacuum (source temperature 573 K) onto an atomically clean single-crystal Ag(100) surface held at 298 K. Individual Zn– TBPP molecules were distributed across the surface by annealing (523 K for 60 mins) and subsequently characterized by STM. Figure 1 shows images obtained for various degrees of coverage of the surface with Zn–TBPP. The individual molecules are imaged as four bright lobes, [21] each of which corresponds to one of the four tBP groups, and each molecule (circled in white) exhibits an apparent diameter of approx- imately 25 . Note that this adsorption geometry corresponds to the orientation of the porphyrin ring parallel to the Ag surface. At low surface coverages (Figure 1a and b), the molecules tend to form 2D islands on the terraces of the Ag surface but they exhibit no preferred orientation with respect to the underlying metal lattice, despite the fourfold rotational symmetry of both the porphyrin and the Ag(100) surface— this observation highlights the weak coupling between the molecule and the surface. At the highest coverage densities (approximately one monolayer = 1 ML), new features appeared (circled in black) that have approximately three times the apparent height of the flat-lying porphyrin mole- cules (Figure 1d). These features have an ellipsoidal aspect (ca. 17   ca. 7 ) with the major axis approximately equal to the width, and the minor axis approximately equal to the height of Zn–TBPP. This strongly suggests that these ellip- soidal objects are Zn–TBPP molecules tilted on edge and the flat !tilted transition is driven by interaction with neighbor- ing molecules. Such coverage-dependent molecular reorien- tations are well known for simpler systems, for example, pyridine adsorbed on Ag(111). [22] In the present case, this Scheme 1. a) Chemical structure of zinc tetra-(3,5-di-tert-butylphenyl)- porphyrin (Zn–TBPP). b) Chemical structure of 4-methoxypyridine, a ligand known to interact with Zn metalloporphyrins through the N atom. [*] O. P. H. Vaughan, Dr. F. J. Williams, Dr. N. Bampos, Professor R. M. Lambert Department of Chemistry University of Cambridge Cambridge CB21EW (UK) Fax: (+ 44)1223-336-362 E-mail: rml1@cam.ac.uk [**] O.P.H.V. acknowledges financial support from the UK Engineering and Physical Sciences Research Council. F.J.W. acknowledges the award of an Early Career Fellowship by the Leverhulme Trust. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Angewandte Chemie 3863 Angew. Chem. 2006, 118, 3863 –3865 # 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim