Unraveling the spectroscopic/electrochemical behavior of a tetraruthenated zinctetrapyridylporphyrazine complex

A comparison of the chemistry of the tetraruthenated zinctetrapyridilporphyrin and zinctetrapyridylporphyrazine species has been performed, aiming to improve the understanding of the electronic coupling (or back-bonding) between the macrocyclic center and the peripheral [Ru(bipy)₂Cl]${}^{+}$ complexes. In the case of the supramolecular tetrapyridylporphyrins, the bridging pyridyl groups are located at the meso-carbon position so that their rotational properties decrease the electronic coupling between the peripheral complexes and the porphyrin center. However, in the tetrapyridylporphyrazine system, the interaction with $\pi$-backbonding Ru(II) complexes is greatly enhanced because the pyridyl groups are fused into the macrocyclic ring with stronger electron acceptor properties. Consequently, the fluorescence emission from the porphyrazine center can be directly pumped from the peripheral complexes by exciting in the Ru${}^{\text{I}\text{I}}\to$ZnTPyPz(π^*) MLCT band, in contrast with the analogous porphyrin species. The electrochemical behavior of the ZnTRuPyPz complex exhibits the characteristic redox waves of [Ru${}^{\text{I}\text{I}\text{I}/\text{I}\text{I}}$(bipy)₂Cl] at 1.00 V, showing a 0.080 V increase in comparison with the ZnTRuPyP complex, consistent with stronger π-back bonding interactions with the porphyrazine center. This observation was also corroborated by the pronounced spectroelectrochemical changes in the porphyrazine Q bands induced by the oxidation of the peripheral ruthenium complexes to Ru(III). The spectroscopic and electrochemical results indicate that the porphyrazines can be better than the porphyrins for developing electronically conjugated supramolecular systems.