Light-driven, artificial rotary molecular motors have captured the imagination of many chemists and materials scientists with their successful application in a plethora of areas, ranging from catalysis to smart materials and surface science.[1] Overcrowded alkene-based motors are designed to perform a unidirectional rotary motion around the central double bond axle following photoexcitation.[2] In order to achieve a 180° rotation, these compounds first undergo a photochemical E/Z isomerization to reach a metastable isomer from which they relax to a second stable isomer via thermal helix inversion. Repeating this sequence completes a full 360° rotation leading back to the starting isomer. Combining this basic property with additional functionalities is key for unlocking many exciting opportunities for more advanced applications by, for example, allowing us to readily detect these motors in-situ, or adapt the kind of motion they perform.

Here we present two 2^nd generation rotary molecular motors which, in addition to their rotation, also perform a second kind of photostimulated motion. In both cases, decorating the motor core with specific substituents was crucial for introducing these additional functionalities. Our experimental results are backed up by in-depth computational studies, revealing the motors’ fundamental modes of operation.

The first example is a Pd complex of a diphosphine substituted motor whose PdCl₂ moiety undergoes an oscillation which is coupled to the motor’s rotation.[3] Due to the plane of the PdCl₂ group being oriented at an almost 90° angle relative to the lower motor half, rotation of the upper half brings the two into close contact, pushing the Pd centre to the other side. The activation barriers along the trajectory of this induced oscillation are small compared to the rate limiting step of the thermal helix inversion, so the speed of rotation remains unaffected.

The second motor is substituted with electron withdrawing cyano groups in the lower and an electron donating amine in the upper half leading to the formation of a push-pull system.[4] One consequence of this is a pronounced redshift of the absorption spectrum of this compound allowing its rotation to be powered with low-energy orange light. This is coupled with a distinct solvent dependence of the absorption spectrum with the lowest-energy absorption maximum being red-shifted by 49 nm going from pentane to DMSO. In addition, we observe a change in the photostimulated motion from the unidirectional rotation of a 2^nd generation molecular motor to a back and forth-type switching motion upon increasing solvent polarity. This is due to a lowering of the activation barrier of the thermal E/Z isomerization of the metastable isomer, reversing the initial photochemical E/Z isomerization. Suppressing the push-pull effect via protonation of the amine leads to recovery of rotary motion in polar solvents.

In short, we present two artificial rotary molecular motors capable of performing two distinct mechanical tasks following photoexcitation. In the first example, an oscillation at the motor’s periphery is coupled to the rotation of the motor core, while in the second example the compound’s mode of operation can be toggled between that of a unidirectional motor and that of a back and forth-type switch by changing its chemical environment. These principles can be exploited for the design of more complex molecular machines.