Transient Absorption Studies of the Pacman Effect in Spring-Loaded Diiron(III) μ-Oxo Bisporphyrins

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Abstract
Picosecond transient absorption spectroscopy of diiron(III) μ-oxo bisporphyrins appended to xanthene, (DPX)Fe2O and (DPXM)Fe2O, and dibenzofuran (DPD)Fe2O have been investigated in order to decipher the effect of a spring-loaded cleft on their photophysics and attendant oxidation photocatalysis. The tension of the cofacial pocket is systematically tuned with the bridge span and meso-substitution opposite to the bridge; the distances of the relaxed cofacial pockets and clamped Fe−O−Fe pockets are known from X-ray crystallography (ΔdM-M(relaxed − clamped) = 4.271 Å (DPD), 2.424 Å (DPXM), 0.208 Å (DPX)). The photophysical and chemical properties of these cofacial platforms are compared to the unbridged diiron(III) μ-oxo analogue, (Etio)2Fe2O. Photon absorption by the diiron(III) μ-oxo chromophore prompts Fe−O−Fe photocleavage to release the spring and present a PFeIVO/PFeII pair (P = porphyrin subunit); net photooxidation is observed when oxygen atom transfer to substrate occurs before the spring can reclamp to form the μ-oxo species. The inherent lifetimes of the PFeIVO/PFeII pairs for the four compounds are surprisingly similar (τ[(DPD)Fe2O] = 1.36(3) ns, τ[(DPX)Fe2O] = 1.26(5) ns, τ[(DPXM)Fe2O] = 1.27(9) ns, and τ[(Etio)2Fe2O] = 0.97(3) ns), considering the structural differences arising from tensely clamped (DPD and DPXM), relaxed (DPX), and unbridged (Etio) cofacial architectures. However, the rates of net oxygen atom transfer for (DPD)Fe2O and (Etio)2Fe2O are found to be 4 orders of magnitude greater than that of (DPX)Fe2O and 2 orders of magnitude greater than that of (DPXM)Fe2O. These results show that the spring action of the cleft, known as the Pacman effect, does little to impede reclamping to form the μ-oxo species but rather is manifest to opening the cofacial cleft to allow substrate access to the photogenerated oxidant. Consistent with this finding, photooxidation efficiencies decrease as the steric demand of substrates increase.

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