Photoredox

How Intramolecular π-Stacking Controls Photoinduced Ligand Release in Ru(II) Photocages

Context

Ruthenium(II) polypyridyl complexes are widely used as light-activated photocages, where coordination to the metal temporarily suppresses ligand activity until visible light triggers release. A central challenge in this field is achieving efficient ligand dissociation under mild irradiation conditions, as many Ru(II) complexes relax through non-dissociative excited-state pathways. Improving ligand release efficiency without fundamentally changing the metal center or ligand framework remains a key design goal.

What's New

This work shows that introducing intramolecular aromatic stacking within a Ru(II) complex can dramatically enhance light-induced ligand release. By adding extended aromatic groups to one of the polypyridyl ligands, the complex becomes subtly distorted from ideal octahedral geometry. This structural distortion makes dissociative excited states more accessible after light absorption, leading to a much higher probability of ligand release. Importantly, this improvement is achieved through molecular design rather than changes in oxidation state, charge, or ligand identity.

Why It Matters

The study establishes intramolecular pi stacking as a general design principle for controlling photochemical reactivity in Ru(II) complexes. A small, internal structural modification leads to a large functional outcome, enabling efficient ligand release under visible light. This insight provides a practical strategy for designing more effective photocages for applications in photochemistry, chemical biology, and light-controlled drug delivery, where low light doses and predictable reactivity are essential.

Limitations & Open Questions

While intramolecular pi stacking significantly enhances light-induced ligand release, several limitations remain. The demonstrated effects are based on a limited set of Ru(II) complexes and aromatic substituents, leaving open the question of how broadly this design principle applies across different ligand frameworks and metal centers. In addition, ligand release efficiencies were primarily evaluated under controlled solution conditions, and performance in fully aqueous or biologically complex environments may differ. The increased accessibility of dissociative excited states is also accompanied by shorter excited-state lifetimes, which may not be desirable for applications requiring long-lived excited states or competing photochemical pathways. Finally, the study focuses on ligand dissociation as the primary outcome, and does not explore potential side reactions or long-term photostability under prolonged irradiation.
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References

Journal of the American Chemical Society (2026)

DOI: https://doi.org/10.1021/jacs.5c16353

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