(Invited) Application of Ethane-Bridged Bis-Porphyrin Structural Motif for Effective Supramolecular Chirogenesis, Sensor Development, and Light Harvesting Systems
Authors: Borovkov Victor , Gathergood Nicholas 
Published: February 2020

Porphyrin and multi-porphyrin systems play a vital role in various natural processes and in artificial assemblies resulting in various practical applications in the numerous research and technoligical fields such as catalysis and nonlinear optics, polymer and materials science, light harvesting and different sensors, molecular and chiral recognition, various supramolecular and chiroptical devices, chirogenesis and absolute configuration determination.
Recently, it was found that a simple ethane-bridged bis-porphyrin motif, 1 (M = Zn, Mg, Cu, 2H), can be effectively used for various application purposes owing to the specific molecular and supramolecular functionality. In particular, the unique functional property of this bis-porphyrin host is based upon the structural semi-flexibility of ethane linkage resulting in the environmentally assisted syn-to-anti conformational switching (for monodentate guests) and tweezer structure formation (for bidentate guests), which can be effectively employed in different areas.1-10

For example, in the case of monodentate chiral guests there is the unidirectional screw formation in the anti form of 1 (M = Zn, Mg, 2H) upon the corresponding host-guest interaction. The direction of screw governs by the size difference of ligand’s substituents at the chirogenic center resulting in chirality transfer to the bis-porphyrin host and induction of the noticeable (moderate-to-strong) exciton couplet CD signals in the absorption region of porphyrin.1-6 This chirogenic phenomenon was efficiently utilized for determining the absolute configuration of various chiral guests, thus making it possible to apply 1 as effective and universal chirality sensors for different types of organic molecules as in solution and in solid state.
To expand further the applicability of these supramolecular systems Langmuir–Schaefer thin-films on the basis of 1 have been prepared and employed as highly selective and sensitive sensors for aromatic amines.7-9 For example, the monometallated copper complex of 1 is able to detect the presence of aniline in aqueous solution at a concentration as low as 1 nM.9 The sensory mechanism includes the corresponding host-guest interaction resulting in the syn-to-anti conformational switching, which can be easily detected by various spectroscopic methods.
Another type of functional materials on the basis of the syn-anti flexibility of 1 (M = Zn) was nanoparticles.10 It was found that upon encapsulation into the semiconducting poly (9-vinylcarbazole) (PVK) polymer the conformation is switched to the anti form and the subsequent efficient excited energy transfer (up to 96%) from the PVK host to the energy acceptor, 1, was observed, thus opening further prospects in designing a new class of porphyrin-based functional nanoparticles for the application in effective light harvesting system.
Consideration of the 12 Principles of Green Chemistry for supramolecular and chiroptical devices will also be highlighted.11

The authors would like to acknowledge funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement No. 621364 (TUTIC-Green).

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