Remarkable Electronic Effect on the meso-Tetra(thienyl)porphyrins

Abstract

Complexes derived from meso-tetra(thienyl)porphyrins (TThP) and meso-tetra(pyridyl)porphyrin (TPyP) containing peripheral ruthenium complexes with general formulas {TPyP[RuCl(dppb)(5,5'-Mebipy)]₄}(PF₆)₄, {TThP[RuCl(dppb)(5,5'-Mebipy)]₄}(PF₆)₄, and {TThP-me-[RuCl(dppb)(5,5'-Mebipy)]₄}(PF₆)₄ [5,5'-Mebipy = 5,5'-dimethyl-2,2'-bipyridine and dppb = 1,4-bis(diphenylphosphino)butane] were synthesized and characterized by spectroscopy techniques (¹H- and ³¹P{¹H}-NMR, IR, UV/vis, fluorescence, and electron paramagnetic resonance (EPR)), cyclic voltammetry, coulometry, molar conductivity, and elemental analysis. Voltammetry and UV/vis studies demonstrated differentiated electronic properties for ruthenium appended with TThP and TThP-me when compared to ruthenium appended with TPyP. The UV/vis analysis for the ruthenium complex derived from TThP and TThP-me, as well as the Soret and Q bands, characteristics of porphyrins, showed a band at 700 nm referring to the Ru → S electronic transition, and porphyrin TThP-me showed another band at 475 nm from the Ru-N transition. The attribution of these bands was confirmed by spectroelectrochemical analysis. Cyclic voltammetry analysis for the ruthenium complex derived from TPyP exhibited only an electrochemical process with E_1/2 = 0.47 V assigned to the Ru(II)/Ru(III) redox pair (Fc/Fc⁺). On the other hand, two processes were observed for the ruthenium complexes derived from TThP and TThP-me, with E_1/2 around 0.17 and 0.47 V, which were attributed to the formation of a mixed valence tetranuclear species containing Ru(II) and Ru(III) ions, showing that the peripheral groups are not oxidized at the same potential. Fluorescence spectroscopic experiments show the existence of a mixed state of emission in the supramolecular porphyrin moieties. The results suggest the formation of Ru(II)-Ru(III) mixed valence complexes when oxidation potential was applied around 0.17 V in the {TThP[RuCl(dppb)(5,5'-Mebipy)]₄}(PF₆)₄ and {TThP-me-[RuCl(dppb)(5,5'-Mebipy)]₄}(PF₆)₄ species.