Tetra-2,3-pyrazinoporphyrazines with Externally Appended Pyridine Rings. 15. Effects of the Pyridyl Substituents and Fused Exocyclic Rings on the UV–Visible Spectroscopic Properties of Mg(II)–Porphyrazines: A Combined Experimental and DFT/TDDFT Study

Two new Mg^II porphyrazine macrocycles, the octakis­(2-pyridyl)­porphyrazinato-magnesium­(II), [Py₈PzMg­(H₂O)], and the tetrakis-[6,7-di­(2-pyridyl)­quinoxalino]­porphyrazinato-magnesium­(II), [Py₈QxPzMg­(H₂O)], were prepared by Mg-template macrocyclization processes, and their general physicochemical properties were examined. The previously reported porphyrazine analog, the tetrakis-2,3-[5,6-di­(2-pyridyl)-pyrazino]­porphyrazinato-magnesium­(II), [Py₈PyzPzMg­(H₂O)], has been also considered in the present work. The UV–visible solution spectra in nonaqueous solvents of this triad of externally octapyridinated Mg^II complexes exhibit the usual profile observed for phthalocyanine and porphyrazine macrocycles, with intense absorptions in the Soret (300–450 nm) and Q band (600–800 nm) regions. It is observed that the Q band maximum sensibly shifts toward the red with peak values at 635 → 658 → 759 nm along the series [Py₈PzMg­(H₂O)], [Py₈PyzPzMg­(H₂O)], and [Py₈QxPzMg­(H₂O)], as the extension of the macrocycle π-system increases. TDDFT calculations of the electronic absorption spectra were performed for the related water-free model compounds [Py₈PzMg], [Py₈PyzPzMg], and [Py₈QxPzMg] to provide an interpretation of the UV–visible spectral changes occurring upon introduction of the pyrazine and quinoxaline rings at the periphery of the Pz macrocycle. To discriminate the electronic effects of the fused exocyclic rings from those of the appended 2-pyridyl rings, the UV–visible spectra of [PzMg] and [PyzPzMg] were also theoretically investigated. The theoretical results prove to agree very well with the experimental data, providing an accurate description of the UV–visible spectra. The observed spectral changes are interpreted on the basis of the electronic structure changes occurring along the series