Electrochemical and Spectroelectrochemical Studies of Porphyrazines and Related Precursors

Abstract

This thesis presents UV-visible, electrochemical, and spectroelectrochemical studies carried out in non-aqueous media of homo/heterobimetallic pyrazinoporphyrazine compounds, i.e.[(M′Cl2)LM] and [(PtCl2)(CH3)6LM](I)6, where L =tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazinato anion,M =ZnII, MgII(H2O) or PdII and M′ =PdII or PtII, 2,3-di(2-pyridyl)-6,7-dicyano-1,4-quinoxaline, [(CN)2Py2Quin], its complexes [(CN)2Py2QuinMCl2] (M = PdII, PtII) and related pyrazine derivatives. The [(M′Cl2)LM] compounds, as obtained from the mononuclear species [LM], undergo only subtle UV-visible spectral changes and exhibit practically unchanged half-wave potentials for reduction; thus, peripheral coordination to the [LM] macrocycles of a single PdCl2 or PtCl2 unit at one of the external dipyridinopyrazine fragments only minimally disturbs the s/pelectronic distribution within the entire porphyrazine unit. In contrast, quaternization by CH3I of the six unligated pyridine N atoms of the species [(PtCl2)LM] leading to formation of the hexacations [(PtCl2)(CH3)6LM]6+results in a significant bathochromic shift (5–15 nm) of the Q-band positions, thus suggesting an enhanced electron-withdrawing effect determined by an incremented displacement of the s/pelectronic system towards the periphery of the macrocycle. Accordingly, there is a facilitated thermodynamic uptake of electrons upon going from [(PtCl2)(CH3)6LM]6+ to [(PtCl2)(CH3)6LM]n(n =5+ →2+). Noteworthy, the UV-visible spectra of the salt-like species [(PtCl2)(CH3)6LM](I)6in water at c =5 ×10-5M) indicate the presence of a monomer-dimer equilibrium, persistent even at very low concentrations (ca. 5 ×10-7M). The newly examined triad of neutral quinoxaline compounds and their corresponding pyrazine counterparts, demonstrated that the one-electron reduction of these compounds in nonaqueous solvents generally resulted in a bathochromic shift of bands in the 250-400 nm region of the spectrum, accompanied by the formation of intense additional absorptions in the region of 500-900 nm. These spectral bands of the monoanions are theoretically examined in the present study by TDDFT calculations and interpreted in terms of single electron excitations between the Kohn-Sham orbitals of the gas-phase optimized structures.