Double Protonation of a cis-Bipyridoallenophane Detected via Chiral-Sensing Switch: The Role of Ion Pairs.

We prove that the confinement of the conformational space of pyridoallenophanes leads to intense chiroptical responses. Unlike the cyclic dimer [142], single-conformation [141]pyridoallenophanes isomerize under thermal and photochemical conditions. Yet, less-strained [141]-bipyridoallenophanes are stable and are prepared successfully. They, unexpectedly, undergo double protonation as a result of cooperative ion-pairing and hydrogen bonding. The complex formation forces a single configuration of the axis connecting both pyridyl rings recognized by a diagnostic circular dichroism (CD) signal at 330 nm.