Intramolecular chirality induction and intermolecular chirality modulation in BINOL bridged bisporphyrin hosts
Wenxin Lu, Likun Li, Huifang Yang, Luyang Zhao, Dongdong Qi, Yongzhong Bian, Jianzhuang Jiang
Dyes and Pigments
We have studied the intra- and inter-molecular chirogenesis events on a family of chiral bisporphyrin hosts H1-H3. The three pairs of enantiomers for H1-H3 were constructed by connecting two zinc(II) porphyrinates to a homochiral (R)-(+)- or (S)-(−)-1,1′-bi-2-naphthol (BINOL) with ester or ether linkages. Split Cotton effects were observed for all of the bisporphyrin hosts. For each enantiomer, the sign of the bisignate CD couplets directly correlates to the stereostructure of the BINOL linker, namely a (R)-BINOL linker leads to a negative CD couplet and vise versa. The CD intensities of the hosts are decreasing from H1 to H3 along with the increasing of the length and flexibility of the chemical linkages between BINOL and porphyrin subunits. The predictable CD signs and regularly changed CD intensities demonstrate the defined intramolecular asymmetric information transfer, by which the chirality of a BINOL linker is translated to a preferred chiral twist in the inter-porphyrin arrangement with tunable efficiency. Furthermore, the intermolecular binding of 4,4′-BiPyridyl (BiPy), as a typical achiral ditopic guest, to the series of hosts were detected by UV–Vis, 1H NMR and CD spectroscopic titrations. The results indicate a convergent and complementary 1:1 binding mode, in which a BiPy ligand binds to a host molecule by ditopic Znsingle bondN coordination forming a stable cyclic supramolecular complex. In particular, the intensity of CD signals decreases significantly for all of the supramolecular complexes in comparison with the chiral bisporphyrin hosts alone, though the CD sign does not change. The observation is rationalized by DFT molecular modeling, which suggests that upon the formation of 1:1 cyclic supramolecular complexes the chiral twist direction of the two porphyrin moieties does not change, while the Znsingle bondZn distance increases remarkably. The present results highlight the effects of intramolecular chirality induction and intermolecular chirality modulation in BINOL bridged porphyrin tweezer systems.
Circular dichroism, Stereochemistry, Materials