Helical molecular redox actuators with pancake bonds?

In an attempt to design molecular electromechanical actuators with large deformation response, we present here three helicene-like compounds, which offer significant strain above 5 % due to two-electron charge transfer (CT). The shrinking induced by CT is a quantum mechanical orbital effect. A good pi-pi overlap across the helical pitch is critical for this type of actuation. The relevant overlap refers to frontier orbitals that are involved in the CT, and it has some features common with pi-pi stacking pancake bonds; however, these molecules do not represent all aspects of typical pancake bonding. This overlap is accompanied by a change in the bond length alternation pattern indicating significant change in pi-conjugation. Additionally, two further helicene-like molecules included in this study also indicate large electromechanical actuation, but a simple orbital interpretation is not available in those cases.