A clear and unambiguous rationalization of chirality induction, amplification, and subsequent inversion processes has been demonstrated using an achiral Mg(II)porphyrin dimer (host) and a series of chiral diols (guests) upon stepwise formation of a 1:1 host−guest polymer and 1:2 host−guest monomer via intermolecular assembling and disassembling processes. Crystallographic characterizations are reported here for both the polymer and the monomeric complexes, which enable us to completely scrutinize the structural and geometrical changes systematically in rationalizing their optical properties. The sign of the CD couplets for both the polymer and monomer are just opposite between R and S guests, which suggests that the chirality is dictated solely by the stereogenic projection of the chiral centers. Stronger intra-and intermolecular coupling in the polymeric complexes is responsible for the highly enhanced CD couplets as compared to the monomer and have only intramolecular coupling as also observed in their X-ray structures. DFT studies clearly support the experimental observations. ■ INTRODUCTIONControlling supramolecular and macromolecular chirality has become a fascinating research area in the past two decades, not only for understanding the asymmetry of various biological systems but also due to its potential applications to materials for chiral sensing, chiral separation, asymmetric catalysis, etc. 1−3 Of particular interest is the precise regulation of chirality, which has been so far achieved by employing host− guest stoichiometry, metal coordination, solvents, anions, temperature, etc. 4,5 Porphyrins are of special interest for detailed investigations of the supramolecular chirality induction process owing to their interesting photophysical properties, versatile modification, great biological importance, and wide applicability. 5−7 Stoichiometry mediated chirality induction and inversion have been known to occur via an intramolecular 1:1 and 1:2 host−guest complexation mechanism. 6,7 There are only a few reports available in which the chirality induction and inversion processes have been rationalized with X-ray structures using the same host−guest system. 6c,d In the present work, we report stepwise chirality induction, amplification, and inversion processes occurring via intermolecular assembling and disassembling between the Mg(II)porphyrin dimer host and chiral diol guest by varying their stoichiometry. At a lower guest concentration, the 1D polymer of the 1:1 host−guest complex was formed, which upon great addition of the guest converted into the monomeric 1:2 host−guest complex. A clear rationalization of chirality induction, amplification, and inversion phenomena has been reported here, which are fully supported by single crystal X-ray structure determination and DFT calculations. ■ RESULTS AND DISCUSSIONA urea-bridged Ni(II)porphyrin dimer has been synthesized using the reported procedure. 8 After demetalation using concentrated sulfuric acid, magnesium was inserted into the free base porphyrin di...

A rationalization for the chirality transfer mechanism in the supramolecular host–guest assemblies of an achiral Zn­(II) porphyrin dimer (host) and a series of chiral diamines and diamino esters (substrates) via cyclic dimer formation has been reported for the first time. Stepwise formations of 2:2 host–guest cyclic dimers and 1:2 host–guest monomeric complexes have been observed via intermolecular assembling and disassembling processes. A large bisignate CD couplet was observed for the cyclic dimer, whereas the monomeric complexes exhibited negligible CD intensity. Crystallographic characterizations demonstrate that the strong intermolecular H bonding in cyclic dimers is responsible for their stability over the linear chain, which thereby display high-intensity bisignate CD couplets. In order to minimize the steric crowding within the host–guest assembly, the cyclic dimer switches its helicity toward the conformer having less steric hindrance. The cyclic scaffold is oriented according to the pre-existing chirality of the substrate in both the solid and solution phases: the substrates having R chirality display a negative CD couplet, whereas the substrates with S chirality display a positive couplet. Opposite signs for the CD couplets between R and S substrates suggest that the stereographic projection at the chiral centers solely dictates the overall helicity of the cyclic dimer. DFT studies further support the experimental observations.

Highly-flexible pyrrole-bridged Zn(II)porphyrin dimer has been successfully utilized as an efficient host which enables us an accurate determination of the absolute configuration directly for a large number of chiral amino...

Herein, we report chiral guest’s triggered spring-like contraction and extension motions coupled with unidirectional twisting in a novel flexible and ‘nano-size’ achiral trizinc(II)porphyrin trimer host upon step-wise formation of 1:1,...

A flexible Zn(II)porphyrin dimer has been utilized as an efficient host for the selective binding of several diols and diamines as substrates. Stepwise formation of a 1:1 host-guest polymer and 1:2 host-guest monomer via intermolecular self-assembling and disassembling processes, has been demonstrated with a series of diamine/diols. Transfer of chirality from the molecular to the supramolecular level has been rationalized with structural elucidation. The formation of the 1:1 host-guest polymer displays a bisignate CD couplet. X-ray structure demonstrates the presence of both clockwise and anticlockwise conformers, however, the conformer formed as per pre-organization in the substrate chirality contributes more towards the overall CD couplet in solution. The addition of the substrate in excess leads to the inversion of the CD couplet due to the formation of the monomeric complex. Architectural selectivity has been illustrated for the 1:1 host-guest complexation by simply tuning the length of the substrates. Crystallographic characterizations revealed linear 1D polymeric structures for the 1:1 host-guest complexes with longer diamines while shorter substrates stabilize cyclic dimer via intermolecular H-bonding between the urea bridge (cis-NH-CO-) of the two porphyrin dimers. Interestingly, the molecular structure of 1:1 host-guest complexes with diols displays a linear polymer despite the shorter length of the substrates. Along with weaker coordination of the alcoholic oxygen towards the Zn(II) ion, the presence of bulky substituents at the coordinating sites of the diols renders the formation of intermolecular H-bonding interactions. Thus, the nature of metal ion and their binding affinity towards the substrate, host-guest size complementarity, and the bulk of the substituents contribute cumulatively towards the architectural selectivity. The cyclic dimers display larger upfield shifts of the substrate protons in the 1H NMR spectra as compared to the linear polymers. Moreover, the computational calculations further substantiate the experimental observations.