Photophysical and Electrochemical Characterization of a Helical Viologen, N,N′-Dimethyl-5,10-diaza[5]helicene

The first helical viologen (4,4'-bipyridinium salt) has been prepared and characterized. Its reduction to the radical cation at -0.22 V vs SCE makes it the most easily reduced redox-active helicene known. It exhibits absorption at 397 nm for the S-1 -> S-0 transition, and it is luminescent allowing measurement of both its singlet (59.3 +/- 0.1 kcal/mol) and triplet (54 +/- 1 kcal/mol) energies. In contrast to neutral helicenes, it is not aromatic pi-stacked in the crystal and has a shortest interdication distance of 4.977 angstrom. Its racemization barrier is calculated to be a sensitive function of its redox state.