A molecular rotor based on N-heterocyclic carbenes (NHCs) has been rationally designed following theoretical predictions, experimentally realized, and characterized. Utilizing the structural tunability of NHCs, a computational screening protocol was first applied to identify NHCs with asymmetric rotational potentials on a surface as a prerequisite for unidirectional molecular rotors. Suitable candidates were then synthesized and studied using scanning tunneling microscopy/spectroscopy (STM/STS), analytical theoretical models, and molecular dynamics simulations. For our best NHC rotor featuring a mesityl N substituent on one side and a chiral naphthylethyl substituent on the other, unidirectional rotation is driven by inelastic tunneling of electrons from the NHC to the STM tip. While electrons preferentially tunnel through the mesityl N substituent, the chiral naphthylethyl substituent controls the directionality. Such NHC-based surface rotors open up new possibilities for the design and construction of functionalized molecular systems with high catalytic applicability and superior stability compared with other classes of molecular rotors.
Keywords: N-heterocyclic carbenes; inelastically tunneling electrons; molecular dynamics simulations; scanning tunneling microscopy/spectroscopy; unidirectional rotation.