Supramolecular Chirogenesis in Zinc Porphyrins: Interaction with Bidentate Ligands, Formation of Tweezer Structures, and the Origin of Enhanced Optical Activity

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Abstract
The complexation behavior, binding properties, and spectral parameters of supramolecular chirality induction in the achiral host molecule, syn (face-to-face conformation) ethane-bridged bis(zinc porphyrin), upon interaction with chiral bidentate guests (diamines and amino alcohols) have been studied by means of UV−vis, CD, fluorescence, 1H NMR, and ESI MS techniques. It was found that the guest structure plays a decisive role in the chirogenesis pathway. The majority of bidentate ligands (except those geometrically unsuitable) exhibit two major equilibria steps: the first guest ligation leading to formation of the 1:1 host−guest tweezer structure (K1) and the second guest molecule ligation (K2) forming the anti bis-ligated species (1:2). The second ligation is much weaker (K1 ≫ K2) due to the optimal geometry and stability of the 1:1 tweezer complex. The enhanced conformational stability of the tweezer complex ensures an efficient chirality transfer from the chiral guest to the achiral host, consequently inducing a remarkably high optical activity in the bis-porphyrin.

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