Abstract
The complexation of (3aR,7aR)-N-(3,5-bis(trifluoromethyl)phenyl)octahydro-2H-benzo[d]imidazol-2-imine (BTI), as a guest, to ethane-bridged bis(zinc octaethylporphyrin), bis(ZnOEP), as a host, has been studied by means of ultraviolet-visible (UV-Vis) and circular dichroism (CD) absorption spectroscopies, single crystal X-ray diffraction, and computational simulation. The formation of 1:2 host-guest complex was established by X-ray diffraction and UV-Vis titration studies. Two guest BTI molecules are located at the opposite sides of two porphyrin subunits of bis(ZnOEP) host, which is resting in the anti-conformation. The complexation of BTI molecules proceed via coordination of the imine nitrogens to the zinc ions of each porphyrin subunit of the host. Such supramolecular organization of the complex results in a screw arrangement of the two porphyrin subunits, inducing a strong CD signal in the Soret (B) band region. The corresponding DFT computational studies are in a good agreement with the experimental results and prove the presence of 1:2 host-guest complex as the major component in the solution (97.7%), but its optimized geometry differs from that observed in the solid-state. The UV-Vis and CD spectra simulated by using the solution-state geometry and the TD-DFT/ωB97X-D/cc-pVDZ + SMD (CH2Cl2) level of theory reproduced the experimentally obtained UV-Vis and CD spectra and confirmed the difference between the solid-state and solution structures. Moreover, it was shown that CD spectrum is very sensitive to the spatial arrangement of porphyrin subunits.
Highlights
IntroductionPorphyrins play an important role in different fields of science and technology, including catalysis [1,2,3], light harvesting [4,5,6], medicine [7,8,9,10], supramolecular systems [11,12,13,14,15,16,17,18,19,20,21], electronic devices [16,17,18], etc
Much attention has been paid to the phenomena of supramolecular chirogenesis, where a chiral guest determines the supramolecular chirality of the entire host-guest system upon binding to an achiral host molecule [12,13,14,15,20,23,24,25,26,27,28,29,30,31,32,33,34,35]
In the case of ethane-bridged bis(zinc porphyrin)s (bis(ZnOEP)s) (Figure 1), steric hindrance induced by coordination of a chiral guest forces the supramolecular system to adopt a screw conformation, with the chirality of a conformation, with the chirality of a guest determining either a clockwise or anticlockwise guest determining either a clockwise or anticlockwise arrangement of porphyrin units in arrangement of porphyrin units in bis-porphyrin hosthelicity
Summary
Porphyrins play an important role in different fields of science and technology, including catalysis [1,2,3], light harvesting [4,5,6], medicine [7,8,9,10], supramolecular systems [11,12,13,14,15,16,17,18,19,20,21], electronic devices [16,17,18], etc. Porphyrin-based systems have found broad application as chemical and chirality sensors [19,20,21,22] because of their notable property to form supramolecular assemblies with different guest molecules. These assemblies produce characteristic absorption bands in the low-energy regions of the corresponding UV-Vis and circular dichroism (CD) spectra, which are essentially shifted from absorption of the majority of analytes [23,24,25,26]. In the case of ethane-bridged bis(zinc porphyrin)s (bis(ZnOEP)s) (Figure 1), steric hindrance induced by coordination of a chiral guest forces the supramolecular system to adopt a screw conformation, with the chirality of a conformation, with the chirality of a guest determining either a clockwise or anticlockwise guest determining either a clockwise or anticlockwise arrangement of porphyrin units in arrangement of porphyrin units in bis-porphyrin hosthelicity [30,31,32].results
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