Universality of Ethane-Bridged Bis-Porphyrin Structural Motiff for Chirality and Molecular Sensing
Authors: Borovkov Victor 
Published: February 2020

Covalently linked porphyrins are of particular interest for the scientific community as efficient artificial systems widely used in various practical applications in the numerous research and technoligical fields such as polymer and nano science, light harvesting and nonlinear optics, different sensors and catalysis, molecular and chiral recognition, various supramolecular and chiroptical devices, chirogenesis and absolute configuration determination.
Recently, we have found that a simple ethane-bridged bis-porphyrin structural motif, 1 (M = Zn, Mg, Cu, Co, 2H), can be effectively served for various application purposes owing to the specific molecular and supramolecular functionalities. In particular, the unique functional property of this bis-porphyrin host is based upon the structural semi-flexibility of covalent linkage between two porphyrin units resulting in the environmentally assisted syn-anti conformational switching and tweezer formation depending upon the guest structure, which can be effectively employed in different sensing areas.1-11

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 exciton couplet CD signals in the porphyrin absorption region.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 (LS) 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 Cu 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 sensing application of the LS film of monometallated Co complex of 1was found in the field of detection of volatile organic compounds like alcohol vapors and oxidizing gas as NO2 by using magneto-optical surface plazmon resonance spectroscopy.10 The detection mechanism is based upon the molecular recognition correlating with the alcohol molecular structure.
More simple sensing system was developed on the basis of spin-coated thin films of the monometallated Zn complex of 1.11 The obtained thin films were successfully employed to detect ammonia and acetone as the relevant analytes in aqueous solutions. This was a result of the supramolecular interaction between each of these two compounds and the bis-porphyrin active layer, inducing the corresponding tweezer conformation easily detectable by UV–vis absorption spectroscopy in the range of concentrations between 1 ppm and 20 ppm for both the analytes.
Further developments and prospects toward new sensing systems on the basis of heterometallic bis- and multi-porphyrin structures will be discussed.

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