Strain-Gated Photochemical Fe²⁺ Uncaging in Ferrocene-[n]Cycloparaphenylenes: A Photomechanophore Paradigm

Ferrocene is a benchmark metallocene known for its robust chemical stability. Yet, mechanical activation of ferrocene or its irradiation when heavily distorted by strain leads to Fe–cyclopentadienyl bond scission. However, a direct link between simultaneous mechanical force transduction and photoactivation has never been established. Here, we show that light can harness stored mechanical energy to induce the metal–ligand bond dissociation. A family of ferrocene–cycloparaphenylene nanohoops (Fc[n]CPP, n = 6–8) releases Fe²⁺ efficiently under visible light, even when the ferrocene unit itself is unstrained. X-ray crystal structure analysis, quantum yield measurements, and excited-state calculations reveal that reactivity scales with strain localized in the aromatic hoop-like ligand, not with distortion at the metal center. This correlation identifies the hoop as an internal stress reservoir that lowers the triplet-state barrier for Fe–cyclopentadienyl cleavage. By coupling mechanical bias to photochemical reactivity, this work establishes a concept of photomechanophores, molecular units in which light and mechanical activation operate in concert. Thereby, we lay foundation of a general framework for designing tunable photocages and new force-responsive materials that can be modulated by light.