Supramolecular Chemistry

Anchored but Unrestricted: Molecular Rotation in Nanocage Cavities

Context

Artificial light-driven molecular motors represent a class of systems capable of transforming photon energy into directional mechanical motion at the molecular scale. While extensive efforts have focused on integrating such motors into surfaces, polymers, frameworks, and membranes, their operation within discrete, fully enclosed supramolecular cavities remains largely underdeveloped. Confined environments are of particular interest because they introduce spatial constraints that may fundamentally alter reaction pathways, dynamics, and mechanical output. Metal–organic nanocages provide rigid yet tunable confined spaces with well-defined internal functionalities, offering a platform to explore how molecular motion behaves under nanoconfinement and how mechanical activity can be retained or modulated within host–guest assemblies.

What's New

This study demonstrates that a second-generation light-driven molecular motor can undergo a complete unidirectional 360° rotational cycle while remaining encapsulated inside a metal–organic nanocage. Unlike conventional host–guest systems that rely primarily on steric complementarity, confinement in this system is achieved through a specific, directional hydrogen-bond interaction between a functionalized motor side chain and internal cage residues. As a result, stable encapsulation is maintained despite substantial conformational changes of the motor during its photochemical and thermal steps, while the cage interior retains sufficient free volume to avoid mechanical hindrance.

Why It Matters

The ability to preserve directional rotary motion under spatial confinement represents a key step toward functional molecular machinery operating in complex environments. This work shows that confinement does not inherently suppress motor function, provided that the interaction between host and guest is both selective and mechanically permissive. These findings establish design principles for constructing supramolecular systems in which molecular motion can occur without loss of efficiency, enabling future strategies for transmitting motion, storing mechanical information, or coupling multiple dynamic components within confined architectures.

Limitations & Open Questions

The system exhibits photoinduced degradation in the presence of oxygen, necessitating rigorous deoxygenation protocols. Stable encapsulation depends critically on the presence of a terminal carboxylic acid anchoring group, limiting immediate generalization to other motor structures. Observations are restricted to solution-phase behavior; collective or solid-state effects are not addressed. Long-term operational stability beyond multiple cycles under continuous irradiation remains to be fully quantified.

Key Takeaways

Key Takeaways Directional molecular rotation can persist under strict spatial confinement Anchoring interactions can replace size-based host–guest recognition Motor dynamics remain largely unchanged relative to bulk solution Confined systems can tolerate large internal geometric fluctuations

How It Works

Light excitation triggers isomerization of the overcrowded alkene core of the molecular motor, producing a metastable configuration. Subsequent thermal relaxation induces helix inversion, enforcing directional rotation. Within the nanocage, the motor is retained by a hydrogen-bond interaction between its flexible side chain and inward-facing cage functionalities, which constrains translational motion without restricting rotational degrees of freedom.

Practical Notes

Effective confinement requires a balance between binding strength and internal free volume. Oxygen exclusion is essential to prevent photochemical side reactions. The host framework must be photochemically stable under irradiation. Functional group placement on the motor is critical for selective anchoring.

Open Questions

How does confinement influence rotational kinetics over extended timescales? Can multiple motors be simultaneously hosted and mechanically coupled? Is directional motion preserved under stronger confinement or higher occupancy? Can energy or force generated by rotation be transferred to the host structure?

Relevant For

Molecular machine and nanomechanics research.Supramolecular host–guest chemistry.Photochemistry and photophysics.Bio-inspired confined motion systems.
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References

Journal of the American Chemical Society (JACS) (2026)

DOI: https://doi.org/10.1021/jacs.5c16349