Temperature‐Dependent Dynamics of Push–Pull Rotor Systems Based on Acridinylidene Cyanoacetic Esters

A series of asymmetrically substituted acridinylidene cyanoacetic esters was synthesized and analyzed for dynamic behavior and spectroscopic features. Although formally connected by a C=C double bond, the cyanoacetic ester groups behave as molecular rotors due to the electronic push–pull situation in the system. The donor–acceptor character lowers the rotational barrier and allows controlling the rotational rate by adjusting the electronic contribution of the ester substituent and changing the solvent polarity. Besides rotation, a much faster flipping motion can be assigned to the tricyclic acridinylidene motif, both of which lead to a rapid interconversion between axially chiral conformers of the system. Treatment with acid induces an intense fluorescence of the system, due to protonation of the C=C carbon on the rotor site, concomitant with the formation of a fluorescent acridinium ion. The dynamic, fluorogenic, and electrochemical behavior of three systems is compared with the aid of NMR, UV/Vis and fluorescence measurements, X‐ray structural analysis, cyclic voltammetry, and molecular modelling. Highly dynamic push–pull rotor systems based on acridinylidene cyanoacetic ester derivatives show a rotational motion with a tunable rate around a formal double bond. Reversible protonation of the systems leads to fluorescent acridinium species.