Light-Driven Dual Rotary Molecular Motors and Beyond

Conspectus Biological molecular machines, such as ATP synthase, kinesin, and the bacterial flagellar motor, demonstrate how coordinated nanoscale rotary motion can drive complex tasks with remarkable efficiency. These natural systems inspire the questions of how multiple nanoscale rotors might be synchronized or differentiated within a single artificial molecule and what new types of motion could emerge from such interactions. Light-driven rotary motion lies at the core of some of the most advanced artificial molecular machines. Overcrowded-alkene motors have been instrumental in revealing how sequential photochemical excitation and thermal relaxation can produce continuous unidirectional rotation, though so far almost exclusively in single-rotor architectures. Extending this concept to dual rotary molecular motors, in which two rotary elements are embedded within a single framework, opens opportunities to study emergent behavior, coupled motion, and new modes of directional control inaccessible to isolated rotors. In this Account, we summarize our progress in the development of symmetric and mixed light-driven dual motors, the mechanistic insights gained, and the opportunities these systems create for next-generation molecular machines. Our efforts began with third-generation fluorene-based motors, created by fusing two overcrowded-alkene rotors into a compact meso architecture. By balancing steric demand at the pseudoasymmetric center with synthetic accessibility of the dual rotor structures, we developed motor scaffolds enabling the systematic exploration of substitution effects on unidirectionality and rotary frequency. These studies established that a pseudoasymmetric center suffices to impose directionality on both rotors and that steric tuning at the core strongly modulates the thermal helix inversion (THI) barrier, thereby affecting the overall speed. Ultrafast spectroscopy further revealed that the photochemical E / Z isomerization proceeds through solvent-sensitive excited-state pathways analogous to those in single-rotor motors. To address practical limitations and gain deeper mechanistic access, we introduced a second family of dual motors based on oxindole rotors. Their intrinsic rotor asymmetry and a strategically placed fluorine nucleus allowed direct, rotor-resolved observation of all involved stable, single-metastable, and double-metastable states. These studies uncovered a fundamentally new feature of multirotor systems: coupled rotary motion, manifested by an accessible double-metastable intermediate and unprecedented THI relaxation pathways─a first glimpse of collective behavior in synthetic multimotor systems. Building on this foundation, we recently created mixed-rotor motors containing two distinct oxindole-based rotors. This additional desymmetrization reinstates point chirality and, importantly, produces a photochemical rotor bias, where one rotor is preferentially photoactivated. A single molecule can therefore sustain two distinct unidirectional rotational frequencies, a capability unmatched in biological or synthetic molecular machinery. The rotor bias depends on substitution, solvent, and irradiation wavelength, offering new avenues for selective control of rotational behavior. Together, these advances establish dual rotary motors as a versatile platform for interrogating coupled motion, asymmetric photochemistry, and multifrequency rotation. Looking ahead, expanding mixed-rotor designs, integrating different rotor types, and creating systems with three or more coupled rotors will open pathways toward molecular assemblies in which complex motion emerges from simple design rules. Such systems bring us closer to realizing programmable, cooperative nanoscale rotary motion in molecular machines, materials, interfaces, and synthetic biological settings.