Light- and Redox-Gated Molecular Brakes Consisting of a Pentiptycene Rotor and an Indole Pad

Two photochemically and electrochemically active alkenes 3Me and 3An containing pentiptycene and indole groups have been synthesized and investigated as light and/or redox-gated molecular brakes. The pentiptycene group functions as the four-bladed rotor, the indole group as the brake pad, and the vinylene group as the switch module. The E configuration corresponds to the brake-off state, in which the rotation of the rotor is free with a rotation rate of 10(8)-10(9) at ambient temperature according to DFT calculations. The Z configuration corresponds to the brake-on state, in which the rotation rate is decreased to 10(1)-10(2), depending on the N-substituent of indole, according to line-shape analysis of variable temperature C-13 NMR spectra. The overall braking effect reaches a factor of 10(6)-10(8). While the combined photochemical E Z and electrochemical Z E switching has a higher capacity than the two-way photochemical switching in the case of 3Me, the switching capacity are comparable for the two methods in 3An. The results also show that photochemical E-Z isomerization is much more reliable than the electrochemical counterpart, as the stability of the redox intermediates plays a critical role in determining the robustness of the molecular brakes via electrochemical switching.