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Switches based on submolecular movement
RESEARCH NEWS
Spaced out colloid arrays FABRICATION & PROCESSING Non-close-packed (ncp) arrays of colloidal spheres on solid substrates have potential applications in optics, photonics, sensing, surface patterning and other fields. Optical tweezers, laser beams, and atomic force microscopy have been used to place spheres one-by-one in desired locations, however serial positioning makes these processes relatively slow. Templating methods enable the rapid, simultaneous placement of large numbers of spheres in two-dimensional structures, but generating the templates can be expensive. Now, Bai Yang and coworkers at Jilin University in China have used a simple soft lithography method to transfer a ncp array of colloidal spheres onto a substrate (Yan et al., J. Am. Chem. Soc. (2005) 27 (21), 7688). Silica spheres are first assembled into hexagonal-close-packed (hcp) arrays on a Si wafer. A single layer of the hcp silica spheres is transferred to the surface of a polydimethylsiloxane (PDMS) stamp. The solvent-swelling behavior of PDMS is then used to adjust the silica sphere lattice spacing, creating a two-dimensional ncp array from the hcp array. The PDMS stamp is swollen by addition of an acetone solution of toluene. The process is dependent on the concentration of toluene, and the lattice spacing can be adjusted from 1.00 to 1.49 times the sphere diameter. The modified array is transferred to a substrate surface that is then spin-coated with a ~200 nm film of poly(vinyl alcohol). More than 95% of the silica spheres are transferred onto the polymer-coated substrate. The ncp structure is mechanically stable because the spheres sink into the polymer film, anchoring them in place. John K. Borchardt
Switches based on submolecular movement POLYMERS
David A. Leigh and coworkers at the UK universities of Edinburgh, St. Andrews, and Warwick, Amsterdam in the Netherlands, and Università degli Studi di Bologna in Italy have developed a polymer film molecular switch, or logic gate, based on controlling submolecular positioning without making or breaking chemical bonds (Leigh et al., Angew. Chem. Int. Ed. (2005), doi 10.1002/anie.200500101). The researchers used a rotaxane molecule consisting of a macrocyclic ring threaded on a chain that has two end groups to prevent it from slipping free of the ring. Rotaxanes provide a mechanism for nanoscale molecular switches in which the relative positions of the interlocked components can be changed in response to an externally applied input. The change in position can vary physical properties such as conductivity, circular dichroism, and fluorescence, so providing a molecular switch. Leigh and colleagues describe a system in which changes in the local environment induce movement of the macrocyclic ring along the thread. This movement switches the fluorescence of the rotaxane ‘on’ or ‘off’. The system works both in solution and in polymer films, where controlled submolecular motion upon light irradiation generates patterns visible to the naked eye.
(Top) Structure of the rotaxane molecule. (Bottom) Fluorescent switching in rotaxane polymer films. (© 2005 Wiley-VCH.)
The translational isomerism of the macrocyclic ring and thread components of the rotaxane is controlled to either permit or prevent fluorescence quenching by intercomponent electron transfer. No chemical bonds are formed or broken in this process, so the optical response is a result of changes in the relative positions of the macrocycle and the thread. Thus, a visible response results from a purely mechanical submolecular event. This behavior has potential applications in sensing and security applications. Electrochemistry, temperature change, pH change, or covalent-bond formation could also be used to induce this process as well as light. John K. Borchardt
Construction sites for polyaromatic structures POLYMERS Polyaromatic compounds have potential for organic light-emitting diodes (OLEDs), photovoltaics, and transistors. However, one drawback is their tendency to form aggregates that exhibit undesirable exciplex emissions, unwanted crystallization, and limited solubility or excessive viscosity. Attaching polyaromatics to star or dendrimer structures increases their solubility and reduces their tendency to aggregate, but syntheses can be long and costly and the resulting products often have limited thermal stability. Researchers at the University of Michigan, Ann Arbor and Kyoto University have reported a new method of synthesizing polyaromatics using cubic octasilsesquioxanes that overcomes these disadvantages (Brick et al., Macromolecules (2005) 38 (11), 4655; Macromolecules (2005) 38 (11), 4661).
The octahedral structure and nanometer size of octasilsesquioxane makes it a potentially useful nanoconstruction site. Since each of the eight phenyl- or vinyl-group substituents on the silsesquioxane cube is reactive, Richard M. Laine and coworkers are able to create three-dimensional, branched aromatic cores for the synthesis of dendrimer-like and/or hyperbranched molecules. The positioning of functional groups at cube corners, the variety of possible groups, and their nanometer size enables the preparation of many nanocomposite/hybrid materials in one, two, or three dimensions, one nanometer at a time. Moreover, the single-crystal silica core provides good thermal and mechanical stability, making these highly branched polyaromatic structures stable to 400°C in air. John K. Borchardt
July/August 2005
21
Spaced out colloid arrays FABRICATION & PROCESSING Non-close-packed (ncp) arrays of colloidal spheres on solid substrates have potential applications in optics, photonics, sensing, surface patterning and other fields. Optical tweezers, laser beams, and atomic force microscopy have been used to place spheres one-by-one in desired locations, however serial positioning makes these processes relatively slow. Templating methods enable the rapid, simultaneous placement of large numbers of spheres in two-dimensional structures, but generating the templates can be expensive. Now, Bai Yang and coworkers at Jilin University in China have used a simple soft lithography method to transfer a ncp array of colloidal spheres onto a substrate (Yan et al., J. Am. Chem. Soc. (2005) 27 (21), 7688). Silica spheres are first assembled into hexagonal-close-packed (hcp) arrays on a Si wafer. A single layer of the hcp silica spheres is transferred to the surface of a polydimethylsiloxane (PDMS) stamp. The solvent-swelling behavior of PDMS is then used to adjust the silica sphere lattice spacing, creating a two-dimensional ncp array from the hcp array. The PDMS stamp is swollen by addition of an acetone solution of toluene. The process is dependent on the concentration of toluene, and the lattice spacing can be adjusted from 1.00 to 1.49 times the sphere diameter. The modified array is transferred to a substrate surface that is then spin-coated with a ~200 nm film of poly(vinyl alcohol). More than 95% of the silica spheres are transferred onto the polymer-coated substrate. The ncp structure is mechanically stable because the spheres sink into the polymer film, anchoring them in place. John K. Borchardt
Switches based on submolecular movement POLYMERS
David A. Leigh and coworkers at the UK universities of Edinburgh, St. Andrews, and Warwick, Amsterdam in the Netherlands, and Università degli Studi di Bologna in Italy have developed a polymer film molecular switch, or logic gate, based on controlling submolecular positioning without making or breaking chemical bonds (Leigh et al., Angew. Chem. Int. Ed. (2005), doi 10.1002/anie.200500101). The researchers used a rotaxane molecule consisting of a macrocyclic ring threaded on a chain that has two end groups to prevent it from slipping free of the ring. Rotaxanes provide a mechanism for nanoscale molecular switches in which the relative positions of the interlocked components can be changed in response to an externally applied input. The change in position can vary physical properties such as conductivity, circular dichroism, and fluorescence, so providing a molecular switch. Leigh and colleagues describe a system in which changes in the local environment induce movement of the macrocyclic ring along the thread. This movement switches the fluorescence of the rotaxane ‘on’ or ‘off’. The system works both in solution and in polymer films, where controlled submolecular motion upon light irradiation generates patterns visible to the naked eye.
(Top) Structure of the rotaxane molecule. (Bottom) Fluorescent switching in rotaxane polymer films. (© 2005 Wiley-VCH.)
The translational isomerism of the macrocyclic ring and thread components of the rotaxane is controlled to either permit or prevent fluorescence quenching by intercomponent electron transfer. No chemical bonds are formed or broken in this process, so the optical response is a result of changes in the relative positions of the macrocycle and the thread. Thus, a visible response results from a purely mechanical submolecular event. This behavior has potential applications in sensing and security applications. Electrochemistry, temperature change, pH change, or covalent-bond formation could also be used to induce this process as well as light. John K. Borchardt
Construction sites for polyaromatic structures POLYMERS Polyaromatic compounds have potential for organic light-emitting diodes (OLEDs), photovoltaics, and transistors. However, one drawback is their tendency to form aggregates that exhibit undesirable exciplex emissions, unwanted crystallization, and limited solubility or excessive viscosity. Attaching polyaromatics to star or dendrimer structures increases their solubility and reduces their tendency to aggregate, but syntheses can be long and costly and the resulting products often have limited thermal stability. Researchers at the University of Michigan, Ann Arbor and Kyoto University have reported a new method of synthesizing polyaromatics using cubic octasilsesquioxanes that overcomes these disadvantages (Brick et al., Macromolecules (2005) 38 (11), 4655; Macromolecules (2005) 38 (11), 4661).
The octahedral structure and nanometer size of octasilsesquioxane makes it a potentially useful nanoconstruction site. Since each of the eight phenyl- or vinyl-group substituents on the silsesquioxane cube is reactive, Richard M. Laine and coworkers are able to create three-dimensional, branched aromatic cores for the synthesis of dendrimer-like and/or hyperbranched molecules. The positioning of functional groups at cube corners, the variety of possible groups, and their nanometer size enables the preparation of many nanocomposite/hybrid materials in one, two, or three dimensions, one nanometer at a time. Moreover, the single-crystal silica core provides good thermal and mechanical stability, making these highly branched polyaromatic structures stable to 400°C in air. John K. Borchardt
July/August 2005
21