Credit: © 2008 Wiley

Lewis acids are used in a wide variety of reactions, not only as independent reagents but also as catalysts and co-catalysts. An in situ method to externally control the electron-accepting properties of a Lewis acid would give greater control over their reactions and now, Neil Branda and co-workers at Simon Fraser University in Canada have used1 a photoresponsive molecular switch to do just this.

The switch was created by incorporating a 1,3,2-dioxaborole system into a well-known dithienylethene-based architecture. The compound exists as two isomers — with different Lewis acidities — that can reversibly photoisomerize through a ring-opening/closing reaction initiated by light of a specific wavelength. The 1,3,2-dioxaborole system in the ring-open form has reduced Lewis acidity because of its conjugation, which means that the p-orbital on the boron is partially occupied by delocalized electrons. After conversion to the ring-closed form, the borate group loses its aromatic character through cross-conjugation and a redistribution of the π-electrons, reducing the electron density at the boron centre and improving its electron-accepting properties.

Branda and co-workers assessed the Lewis acidity of the photoisomers by measuring their binding affinities to pyridine — a Lewis base — using 1H NMR spectroscopy, revealing the ring-closed form to be significantly more acidic. Moreover, the molecule retains its photoswitching activity even when pyridine is bound to the more acidic isomer, enabling it to be released in a controlled fashion by exposing the system to light of the appropriate wavelength.