Photophysical and Electrochemical Characterization of a Helical Viologen, <i>N</i>,<i>N</i>′‑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 S1 ← S0 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 π-stacked in the crystal and has a shortest interdication distance of 4.977 Å. Its racemization barrier is calculated to be a sensitive function of its redox state.