The exciton chirality method (ECM) is commonly recognized as one of the best approaches to assign the absolute configuration of biaryls. This paper reports the first exception to this method for a simple biaryl system. ECD and VCD measurements in combination with DFT (B3LYP/6-311G*), TDDFT (CAM-B3LYP/6-311G*), and Coupled-Cluster (RI-SCS-CC2) calculations were used to determine the absolute configurations of axially chiral BODIPY DYEmers. The ECM fails to predict the sign of the intense CD couplet at 500 nm of the 1,1'-coupled dimer. The odd behavior was rationalized by considering the strong transition magnetic dipole associated with the 500 nm transition, which leads to an unexpected dominance of the μm coupling at the expense of the μμ one in these compounds. This is the first case in which a strong μm coupling hampers the use of the ECM, but this behavior should not be restricted to the BODIPY chromophore.

The 3+3-type synthesis of a pyrazole-based expanded porphyrin 22 H4, a hexaphyrin analogue named Siamese-twin porphyrin, and its homobimetallic diamagnetic nickel(II) and paramagnetic copper(II) complexes, 22 Ni2 and 22 Cu2, are described. The structure of the macrocycle composed of four pyrroles and two pyrazoles all linked by single carbon atoms, can be interpreted as two conjoined porphyrin-like subunits, with the two opposing pyrazoles acting as the fusion points. Variable-temperature 1D and 2D NMR spectroscopic analyses suggested a conformationally flexible structure for 22 H4. NMR and UV/Vis spectroscopic evidence as well as structural parameters proved the macrocycle to be non-aromatic, though each half of the molecule is fully conjugated. UV/Vis and NMR spectroscopic titrations of the free base macrocycle with acid showed it to be dibasic. In the complexes, each metal ion is coordinated in a square-planar fashion by a dianionic, porphyrin-like {N4} binding pocket. The solid-state structures of the dication and both metal complexes were elucidated by single-crystal diffractometry. The conformations of the three structures are all similar to each other and strongly twisted, rendering the molecules chiral. The persistent helical twist in the protonated form of the free base and in both metal complexes permitted resolution of these enantiomeric helimers by HPLC on a chiral phase. The absolute stereostructures of 22 H6(2+), 22 Ni2, and 22 Cu2 were assigned by a combination of experimental electronic circular dichroism (ECD) investigations and quantum-chemical ECD calculations. The synthesis of the first member of this long-sought class of expanded porphyrin-like macrocycles lays the foundation for the study of the interactions of the metal centers within their bimetallic complexes.

β,β'-Bisporphyrins are intrinsically chiral porphyrin dimers with fascinating properties. The configurational stability at their axes can be directed by variation of the central metal atoms. Herein, we present a regioselective functionalization of the monomeric 2-amino-tetraphenyl-porphyrin as a versatile substrate for dimerization by oxidative coupling. By simple variation of the reaction conditions (solvent and oxidant), the oxidation selectively gave either the axially chiral C,C-coupled diaminobisporphyrin in high yields or, under Ullmann conditions, the twofold N,C-linked achiral dimer, also in good yields. A generalized mechanism for the coupling reaction is proposed based on DFT calculations. The axially chiral β,β'-coupled porphyrin dimers were isolated as racemic mixtures, but can be resolved by HPLC on a chiral phase. TDDFT and coupled-cluster calculations were used to explain the spectroscopic properties of the aminoporphyrins and their dimers and to elucidate the absolute configurations of the C,C-coupled bisporphyrins.

The two chromophores in 2,2′-coupled BODIPY DYEmers are linearly arranged and possess a configurationally stable axis. Their successful enantiomeric resolution was shown by HPLC-UV and HPLC–ECD on-line measurements. In accordance with theoretical predictions, the main UV/Vis signal of the dimers is redshifted in comparison with that of the monomers, and no exciton couplets are observed in the electronic circular dichroism (ECD) spectra. Moreover, the experimental ECD spectra have a very low intensity over the wavelength region from 300 to 800 nm and the linear BODIPY DYEmers thus show cryptochirality. The reason for this phenomenon is the peculiar molecular arrangement together with the conformational behavior of the dimers. For each compound two different conformations are possible around the biaryl axis that exhibit nearly mirror-image ECD spectra. As a result, the overall ECD curves show only weak signals dominated by vibronic coupling effects. Thus, the method commonly used to determine absolute configurations by TDDFT calculations is not reliable for linear BODIPY dimers and only VCD investigations (experimental in combination with DFT calculations) could unambiguously determine the absolute configurations of the 2,2′-coupled BODIPY DYEmers

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