Building pHotoswitches

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Science  01 Aug 2008:
Vol. 321, Issue 5889, pp. 612
DOI: 10.1126/science.321.5889.612a

Some compounds exhibit significant changes in inherent acidity upon electronic excitation. However, the excited states in these photoacids tend to be too short-lived for practical use in tuning bimolecular reactivity. Two research teams have extended the scope of phototunable reactivity by building molecules in which light absorption at different wavelengths switches the structure reversibly between two stable isomers of differing acidities. Lemieux et al. prepared a boronate derivative in which the boron is initially conjugated in a planar ring with six π electrons from oxygen and olefin groups, and so has comparatively low Lewis acidity. Ultraviolet (UV) irradiation links two thiophene rings pendant to the olefin, disrupting the conjugation geometry and thereby raising the Lewis acidity to increase the pyridine binding constant from undetectable to ∼7000. Blue light cleaves the thiophene linkage and restores the inert geometry. At the opposite end of the pH scale, Peters et al. relied on sterics rather than electronics to tune the basicity of a piperidine derivative. A pendant azobenzene group blocks the basic piperidine nitrogen with a bulky aryl or tert-butyl substituent in the trans geometry, but rotates this blocking group out of the way upon UV-induced isomerization to the cis geometry. — JSY

Angew. Chem. Int. Ed. 47, 5034; 10.1002/anie.200802050 (2008).

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