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July 24, 2008

Organic Chemistry

Light-Triggered Base

Radiation induces reversible cis-trans isomerization

Carmen Drahl

BASE ON DEMAND Light-mediated double-bond isomerization reversibly activates an organic base.
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By employing a light-responsive molecular shield, Stefan Hecht and coworkers at Humboldt University, in Berlin, have developed an organic base that can be reversibly switched on and off on demand (Angew. Chem. Int. Ed. 2008, 47, 5968). The controllable process could lead to new applications in chemical surface patterning, the researchers suggest.

Hecht's team used piperidine, a nitrogen-containing six-membered ring, as the starting point for their reversible base. By adding a substituent to the nitrogen, they effectively locked its basic lone pair of electrons into place. They then incorporated a photoisomerizable azobenzene tether tipped with a bulky aromatic ring.

Short-wavelength light (365 nm) causes the azobenzene to adopt a cis form that leaves the nitrogen's basic lone pair sterically accessible (right). Long-wavelength light beyond 400 nm regenerates the trans form, which blocks access to the lone pair, rendering the base inactive (left). The research complements recent work directed by Neil R. Branda of Simon Fraser University, in Burnaby, British Columbia, which used light to tune acidity by altering a Lewis acid's electronic properties (Angew. Chem. Int. Ed. 2008, 47, 5034).

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