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 CH 2 Cl 2 and CCl 4 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 CCl 4 than in CH 2 Cl 2, with an average reduction of 45% in the CD intensity. The lower ability of CCl 4 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 .