CHEMISTRY: Still Twisted After All These Years

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Science  08 Mar 2002:
Vol. 295, Issue 5561, pp. 1797b
DOI: 10.1126/science.295.5561.1797b

A long-standing puzzle in photochemistry has been the nature of the fluorescence from p-dimethylaminobenzonitrile and numerous related compounds in polar solvents. In 1962, Lippert reported two emission peaks for this compound: the expected vertically excited state and an anomalous red-shifted feature. In Grabowski's twisted internal charge transfer model, the red-shifted peak originates from a charge-transfer state in which the amino group twists to stabilize a positive charge. Direct verification for this structural change has been elusive. Dobkowski et al. have prepared a less symmetric analog that shows syn-anti isomerization and anomalous emission when photoexcited at low temperatures (-90°C) in methanol, but no isomerization and only normal emission when the experiment was repeated in nonpolar tetrahydrofuran. The structural assignments were made by nuclear magnetic resonance. — PDS

J. Am. Chem. Soc., 10.1021/ja012326t (2002).

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