No abstract

The rationalization of various aspects of the mechanism of chirality induction in supramolecular assemblies based upon the complexes formed between an ethane-linked bisoctaethylporphyrin and chiral ligands is described. The influence of various controlling factors such as bonding strength, host-guest steric interactions, equilibria and thermodynamic parameters, temperature and solvent effects, role of the center metal ion, stoichiometry, and phase transition on the chirality induction processes have been investigated, and the results and implications are discussed. As a result of such detailed understanding, it is possible to employ this bisporphyrin system as an effective chirality sensor for the determination of absolute configuration.

The achiral syn folded (face-to-face conformation) host molecule of the ethane-bridged bis(zinc porphyrin) transforms into the corresponding chiral extended anti bis-ligated species in the presence of enantiopure amine guests. The mechanism of the supramolecular chirogenesis is based upon the screw formation in bis(zinc porphyrin), arising from steric interactions between the largest substituent at the ligand's asymmetric carbon and peripheral alkyl groups of the neighboring porphyrin ring pointing toward the covalent bridge. The screw direction is determined by the guest's (amines) absolute configuration resulting in a positive chirality induced by (S)-enantiomers due to formation of the right-handed screw, and a negative chirality produced by the left-handed screw of (R)-enantiomers. The screw magnitude is strongly dependent upon the structure of the chiral guests. The amines with bulkier substituents result in stronger CD signals and larger (1)H NMR resonance splittings of enantiotopic protons. This system possesses a high degree of chiroptical activity, which allows the differentiation of one of the smallest homologous elements of organic chemistry, that is, the methyl and ethyl groups attached to the asymmetric carbon, and additionally, which senses a remote chiral center at a position beta to the amine binding group.

The achiral syn folded conformer (face-to-face) of the ethane-bridged bis(Zn porphyrin) gradually transforms into the chiral extended anti form in the presence of enantiopure guest molecules (alcohols or amines) upon lowering the temperature from 293 to 183 K. The mechanism of the supramolecular chirality induction is based upon the formation of right-or left-handed screw diastereomers of the anti form. The split absorption maxima which are caused by the exciton coupling of the corresponding B transitions match the bisigned Cotton effects. The amplitude of the CD bands is found to be dependent on the bulk and ligation strength of the chiral guest, while the sign of the couplets is observed to be determined by the absolute configuration of the external ligand. The formation of the screw structure in the anti conformation is also confirmed by 1 H NMR.

Unique temperature-dependent syn-anti conformational switching in bis-and monozinc ethane-bridged porphyrin dimers takes place in nonpolar solvents containing a small amount of alcohol (1-5%). These dimers adopt a syn conformation at any studied temperatures in the absence of alcohol added, while, in the presence of alcohol, a decrease in temperature from 310 to 183 K results in the gradual shift of the conformational equilibrium toward the anti conformer. Apparently the mechanism of this unprecedented phenomenon is based on the enhanced alcohol ligation to zinc porphyrins at low temperature which is capable of destroying the strong π-π interporphyrin interactions. This process is monitored by variable temperature (VT) UV-vis and VT 1 H NMR spectral methods and the corresponding thermodynamic parameters are evaluated using a van't Hoff type analysis. Strong exciton coupling between the B transitions of the anti dimers are observed at low temperatures.

The complexation behavior, binding properties, and spectral parameters of supramolecular chirality induction in the achiral host molecule, syn (face-to-face conformation) ethane-bridged bis(zinc porphyrin), upon interaction with chiral bidentate guests (diamines and amino alcohols) have been studied by means of UV-vis, CD, fluorescence, (1)H NMR, and ESI MS techniques. It was found that the guest structure plays a decisive role in the chirogenesis pathway. The majority of bidentate ligands (except those geometrically unsuitable) exhibit two major equilibria steps: the first guest ligation leading to formation of the 1:1 host-guest tweezer structure (K(1)) and the second guest molecule ligation (K(2)) forming the anti bis-ligated species (1:2). The second ligation is much weaker (K(1) >> K(2)) due to the optimal geometry and stability of the 1:1 tweezer complex. The enhanced conformational stability of the tweezer complex ensures an efficient chirality transfer from the chiral guest to the achiral host, consequently inducing a remarkably high optical activity in the bis-porphyrin.

Supramolecular chirality induction and inversion processes can be controlled by phase transition. The achiral syn (face‐to‐face) conformer of the ethane‐bridged bis(zinc octaethylporphyrin) (see figure) interacts with enantiopure monoamines in solution and solid‐state phases to yield the corresponding anti conformer. The chirality sign in the solid state is opposite to that in solution because of the formation of an aggregated species with inverted intermolecular helicity.

The role of the ligand's structure and absolute configuration in the supramolecular chirality induction in achiral bis(zinc porphyrin) has been studied. The amines with bulkier substituents resulted in stronger CD signals due to increased helical displacement in the anti conformer. All the amines with an R absolute configuration gave a negative first Cotton effect and positive second Cotton effect, while the ligands with an S absolute configuration produced CD signals with opposite signs due to formation of the left- and right-handed screw diastereomers, respectively.