Application of Octaethylporphyrin Structural Motif for Supramolecular Chirogenesis
Authors: Borovkov Victor 
Published: February 2020

Supramolecular chirogenesis is a smart combination of supramolecular chemistry and chirality associated with the processes of asymmetry induction, amplification, modulation in non-covalently linked multicomponent assemblies.1Porphyrin stuctures are of particular interest to study this phenomen owing to their unique spectral, physico-chemical, and synthetic properties. Octaethylporphyrin (OEP) is one of the simplest porphyrins, yet being easily accessible via the standard synthetic procedure or commercially available from many reagent suppliers.
Therefore, monomeric ZnOEP was firstly tested in the process of supramolecular chirality induction. Hence, upon interation with chiral amines (L) the induced optical activity in inherently circular dichroism (CD) silent ZnOEP was almost neglidgible that indicated inefficiency of the chirality transfer process in these supramolecular systems.2 However, in the case of solid state (a glassy KBr matrix) there was the time-dependent formation of (ZnOEP.L)n chiral aggregates posessing a high degree of optical activity with the anisotropy factor of 0.015.3 The chirality transfer mechanism was based upon the unidirectional arrnagement of ZnOEP molecules.
In contrast to monomeric ZnOEP, dimeric (ZnOEP)2 exhibited considerable optical activity upon interaction with chiral guests.2,3 Thus, monodentate ligands resulted in formation of the stable 1:2 supramolecular chiral complexes and induction of the noticeable (moderate-to-strong) exciton couplet CD signals in the absorption region of porphyrin. It was established that the mechanism of asymmetry transfer from chiral guests to achiral (ZnOEP)2 consists of two major steps. At the first step there is formation of the host-guest complex owing to abilities of chiral guests to interact with (ZnOEP)2 resulting in the initial syn-to-anti conformational switching. The second step is another type of structural deformation in (ZnOEP)2 induced by the size difference of ligand’s substituents at the chirogenic center resulting in asymmetric screw formation in the resulting complexes. For example, when the bulkiness order of the guest substituents around the chiral center coincides with the substituents’ priority rule, the (R)-enantiomer yields a negative first Cotton effect and a positive second Cotton effect, while the guest with the (S) absolute configuration produces CD signal of the opposite signs, thus making this sensing protocol to be effectively used for determining the absolute configuration of different classes of organic molecules. In the solid state the situation is different; (R)-ligands upon interaction with (ZnOEP)2 induce a positive first Cotton effect, whilst the corresponding (S)-ligands produce a mirror image CD spectra.3 Remarkably, the induced chirality in the solid state exhibits enhanced intensity, with an inverted sign in comparison to that in the solution phase owing to formation of chiral aggregates of regular structure, in which the intermolecular screw sense is opposite to the intramolecular screw sense of the single molecule. The generated CD sign unambiguously correlates with the induced helicity allowing straightforward determination of the absolute configuration of various chiral guests and thus making it possible to apply (ZnOEP)2 as effective and universal chirality sensors for different types of optically active compounds as in solution and in the solid state.
For investigation of the weak host-guest interaction mode a special tetrameric (NiOEP)4 host has been designed.4 This host satisfies the major following requirements: (1) total inertness towards most of the conventional host-guest binding modes, (2) possession of an open cavity between the two bis-porphyrin subunits owing to the syn-orientation in (NiOEP)4, which allows chiral guest molecules to be incorporated into it and thus to interact with the host in a stereospecific manner, and (3) presence of a spectrally separated B|| electronic transitions aligned along the diacetylene bridges and located at the low energy region of the porphyrin B band due to the enhanced interporphyrin interactions through the unsaturated linkage. The chirogenic process in (NiOEP)4 was monitored upon interaction with various chiral solvents and resulted in induction of optical activity in the region of B|| electronic transitions due to the corresponding host-guest interactions. In particular, (R)-solvents yield a positive first and negative second Cotton effects, while the (S)-enantiomers gives the opposite signs. The apparent mechanism of chirality induction in (NiOEP)4 includes penetration of the solvent molecules inside the interporphyrin cavity to enhance the solvent-solute interactions and thus transfer the chiral information to an achiral host by inducing a unidirectional screw in (NiOEP)4
and following the guest’s absolute configuration.

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