Stereochemical stability and absolute configuration of atropisomeric thioalkyl-porphyrazines by dynamic NMR and HPLC studies and computational analysis of HPLC-ECD recorded spectra

The conformational and stereodynamic properties of a pyrene-substituted thioethyl-porphyrazine (PzPy) promising as dye for organic optoelectronic applications and light harvesting systems, have been investigated by Density Functional Theory computations and variable temperature NMR and HPLC. In PzPy rotation around the pyrene-macrocycle bond is hindered and then the molecule displays atropisomerism existing as a couple of enantiomers, thus representing the first chiral alkyl-porphyrazine reported in the literature. The rotational barrier (G‡ ~ 22.0 kcal/mol), measured by variable temperature HPLC experiments, allows chromatographic enantiomers separation on a chiral stationary phase below 0°C, while racemization spontaneously occurs at room temperature. The absolute configuration of eluted enantiomers has been also determined by computational analysis of their Electronic Circular Dichroism spectra. The influence of the alkyl chain length and the metal complexation on the molecular stereodynamic properties have been also investigated by studying its novel thiooctyl derivative PzPy-octyl and Pd(II) complex PdPzPy, respectively. The latter also constitutes the first example of a Pd(II) thioalkyl-porphyrazine complex. Given that the use of PzPy in the preparation of hybrid materials with nanocarbons for optoelectronic applications has been reported, the presence of an axial chirality element can have a key role in its interaction with semiconducting chiral nanotubes.