sally208 2007-11-23 09:00
Shedding light on an acid switch
[size=2][color=red]UV-Vis absorption spectroscopy has been used to shed light on a novel supramolecular control mechanism in which a nanoscale molecular machine is driven by changes in pH.[/color][/size] The molecular machine acts as a switch, flipping from one photoactive state to another in response to light by exploiting a proton transfer process in the same solution.
According to Alberto Credi of the University of Bologna, Italy, "Most artificial molecular machines reported so far are powered by exoergonic chemical reactions, typically acid-base reactions." He points out that attempts to build a modular light-driven device have usually relied on integrating photochemical functions with mechanically switchable systems. This he concedes has proved difficult. Now, Credi and his colleagues have developed an important general principle for the operation of standalone pH-driven molecular machines with light.
"We achieved this goal by coupling two molecular switches, namely, a pH-controllable molecular machine and a photochromic compound whose acidic strength can be modulated by light irradiation," Credi told SpectroscopyNOW, "They work together in the same solution and communicate with one another by photoinduced intermolecular proton transfer." In principle, this strategy allows light-driven operation to be implemented for any kind of pH-driven molecular device without modifying the structure of the device, provided that a suitable photoacid is employed.
The molecular machine itself is a calix[6]arene-bipyridinium pseudorotaxane and the switching process involves the threading and dethreading of this structure in solution. A merocyanine dye, which acts as a photoacid, endows the system with its response to visible light depending on whether the pH is high or low. "The coupling between the two molecular switches, that communicate with one another by intermolecular proton transfer, provides a general principle for the operation of acid-base controllable molecular machines with light," Credi and colleagues explain.
The team is now investigating the exact details of the switching mechanism, Credi told me,?and attempting to improve performance. They are also applying this strategy to operate more complex pH-controllable nanomachines, such as a recently investigated "molecular elevator" (Science, 2004, 303, 1845). Photoinduced proton transfer between molecular switches is also being employed by Credi and coworkers for constructing all-optical molecular logic devices.
[b]Related links:[/b]
[list][*][url=http://dx.doi.org/10.1021/ja0753851][color=#0000ff]J Am Chem Soc, 2007, in press [/color][/url][/list]
[b][url=http://www.spectroscopynow.com/coi/cda/detail.cda?id=567&type=Feature&chId=0&page=1][color=#0000ff]Article by David Bradley[/color][/url][/b]