Supramolecular chirogenesis in zinc porphyrins: Investigation of zinc-freebase bis-porphyrin, new mechanistic insights, extension of sensing abilities, and solvent effect

The mechanism of supramolecular chirality induction in achiral ethane-bridged syn bis-porphyrins on interaction with chiral amines (L*) was investigated by comparison of bis(zinc porphyrin), ZnZn (possessing two binding sites), with zinc-freebase bis-porphyrin, Zn2H (possessing one binding site) in CH2Cl2 and CCl4 solvents. UV-vis and CD analyses showed that the major contribution to the supramolecular chirality induction mechanism is the total number of supramolecular chiral steric interactions occurring in solution between the host's 3- and 7-ethyl groups and the guest bound only to the inside face of the host. This can be seen in that the full width at half-maximum (fwhm) values of the Soret band of ZnZn.L*(2) are significantly smaller than the Zn2H.L* species due to the presence of a higher percentage of species with chiral steric interactions which reduce the flexibility of the resulting complex. In the CD spectra the Cotton effects of Zn2H.L* are dramatically reduced by an average of 63% in comparison to ZnZn.L*2, which is closer to the 75% expected on consideration of the number of supramolecular steric interactions than the 33% expected from the number of chirally active species. The same chirality-inducing steric mechanism can also be observed in the lower degree of supramolecular chirality induction observed for the Zn2H.L* system in CCl4 than in CH2Cl2, with an average reduction of 45% in the CD intensity. The lower ability Of CCl4 to electrostatically interact with the ligand and porphyrin reduces the bulkiness of the corresponding substituents thus reducing the degree of chiral steric repulsion. This system possesses a high degree of chiroptical activity allowing the use of Zn2H for the determination of the absolute configuration of chiral ligands, by virtue of its one-point coordination property. Thus, it is possible to determine the absolute configuration of bis-amines, something not previously possible with ZnZn.