Stereochemical Stability and Absolute Configuration of Atropisomeric Alkylthioporphyrazines by Dynamic NMR and HPLC Studies and Computational Analysis of HPLC-ECD Recorded Spectra

The conformational and stereodynamic properties of a pyrene-substituted ethylthioporphyrazine (PzPy), promising as a dye for organic optoelectronic applications and light-harvesting systems, have been investigated by DFT computations and variable-temperature NMR and HPLC. In PzPy, rotation around the pyrene–macrocycle bond is hindered and the molecule displays atropisomerism, existing as a pair of enantiomers, thus representing the first chiral alkyl-porphyrazine to be reported in the literature. The rotational barrier (ΔG‡≈ 22.0 kcal mol–1), measured by variable-temperature HPLC experiments, allows chromatographic separation of the enantiomers on a chiral stationary phase below 0 °C, whereas racemization spontaneously occurs at room temperature. The absolute configurations of the eluted enantiomers were also determined by computational analysis of their electronic circular dichroism spectra. The influence of alkyl chain length and metal complexation on the molecular stereodynamic properties have also been investigated by studying its novel octylthio derivative (PzPy-octyl) and PdIIcomplex (PdPzPy), respectively. The latter also is the first example of a PdII–alkylthioporphyrazine complex. Given that PzPy has been used in the preparation of hybrid materials with nanocarbons for optoelectronic applications, the presence of an axial chirality element may play a key role in its interaction with semiconducting chiral nanotubes.