A Chemically Switchable Molecular Pinwheel

Abstract

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][8][9] and in the gas phase [10] has recently been extended to surface-mounted rotors; [11][12][13] most recently, a light-driven molecular rotor anchored to a gold surface has been demonstrated, [13] and a recent comprehensive 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 singlemolecule manipulation at room temperature [16] and conformational 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 functionalized 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 manipulation with an STM tip; [11] however, reports of surfacemounted 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)porphyrin [21] (Zn-TBPP; Scheme 1 a), 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 obtai...

Deprotection, Tethering, and Activation of a One-Legged Metalloporphyrin on a Chemically Active Metal Surface: NEXAFS, Synchrotron XPS, and STM Study of [SAc]P−Mn(III)Cl on Ag(100)

“…2,3,7 Supramolecular arrays are of interest with respect to the study of energy transfer and photosynthetic mechanisms 7,8 while two-dimensional porphyrin assemblies have attracted attention for possible application as chemical sensors, in molecular electronics, or as chemically switchable 2D rotors. 3,9,10 Related to this, we recently showed that a porphyrin-functionalized silver surface exhibits ligand binding and unbinding reactions characteristic of the free metalloporphyrin 11 and showed that a well-chosen ligand can strongly alter the dynamics of adsorbed porphyrins. 10 Porphyrins have been attached to oxide surfaces for use as sensors by means of organophosphonate linkers.…”

“…3,9,10 Related to this, we recently showed that a porphyrin-functionalized silver surface exhibits ligand binding and unbinding reactions characteristic of the free metalloporphyrin 11 and showed that a well-chosen ligand can strongly alter the dynamics of adsorbed porphyrins. 10 Porphyrins have been attached to oxide surfaces for use as sensors by means of organophosphonate linkers. 12 Our interest, however, lies in attaching metalloporphyrins to chemically active metal surfaces and we have reported a viable route for de-protecting and covalently attaching Mn porphyrin molecules to silver surfaces by means of four thio tethers.…”

Colloquium: Time-resolved scanning tunneling microscopy

“…Since the late 1980s several systems that exhibit thermally induced molecular rotation have been reported ͑Alvey et al, 1987;Mo, 1993;Gimzewski et al, 1998;Stipe et al, 1998b;Rao et al, 2003͒. The rotation of porphyrins has been studied quite extensively with STM ͑Hersam et al, 2000;Stöhr et al, 2001;Rao et al, 2004;Iancu and Hla, 2006;Vaughan et al, 2006;Ye et al, 2006;Wintjes et al, 2007͒. Other appealing systems are thioethers ͑Baber et al, 2008͒ and tetra-tert-butyl zinc phthalocyanine ͑Gao et al, 2008͒ on Au surfaces.…”

Deprotection, Tethering, and Activation of a Catalytically Active Metalloporphyrin to a Chemically Active Metal Surface: [SAc]4P−Mn(III)Cl on Ag(100)

“…12 Such weakly bound systems are unlikely to be sufficiently robust for catalytic applications, however, especially in the presence of a solvent where resistance to leaching is an essential attribute.…”