Thermal and photoinduced electron transfer reactions of phthalocyanine complexes of Zn(II) and Cu(II) in acetonitrile

Phthalocyanine that has four peripheral 2-methoxyphenyl substituents at the α-position and its Zn(II) and Cu(II) complexes were synthesized. Chemical oxidation by the Cu(II) ion and electrochemical oxidation of these metal complexes were investigated spectrophotometrically in acetonitrile. The UV-visible absorption spectra of these metal complexes and their one-electron oxidized π-cation radicals showed no concentration dependence, indicating that these species exist as monomers in solution. Kinetics of the thermal electron transfer reaction from each phthalocyanine complex to Cu²⁺ and the photoinduced electron transfer reaction of the Zn(II) phthalocyanine complex with V(V) and V(IV) Schiff base complexes were studied using conventional spectrophotometric and transient absorption techniques, and the electron transfer rate constants were analysed using the Marcus cross relationship. The obtained rate constants of the electron self-exchange reaction between the parent phthalocyanine complexes and their π-cation radicals were in the order of 10⁹ to 10¹¹ M^-1 s^-1 at T = 298.2 K. These large electron self-exchange rate constants are consistent with the phthalocyanine-centred redox reactions where small reorganization energies are required with little structural change during the electron transfer process.

Effect of the Substitution Pattern (Peripheral vs Non-Peripheral) on the Spectroscopic, Electrochemical, and Magnetic Properties of Octahexylsulfanyl Copper Phthalocyanines

In order to investigate the substitution position effect on the spectroscopic, electrochemical, and magnetic properties of copper phthalocyanines, a detailed structure-property analysis has been performed by examining two copper phthalocyanines that are octasubstituted by hexylsulfanyl chains respectively in the peripheral (Cu-P) and non-peripheral (Cu-NP) positions. Cu-NP showed a marked near-IR maximum absorption compared to Cu-P and, accordingly, a smaller HOMO-LUMO energy gap, calculated via the electrochemical results and simulations in the gas phase, as well as for Cu-NP from its crystallographic data. An electron-spin resonance (ESR) technique is used to extract the g values from the powder spectra that are taken at room temperature. The g values were determined to be g_∥ = 2.160 and g_⊥ = 2.045 for Cu-P and g_∥ = 2.150 and g_⊥ = 2.050 for Cu-NP. These values indicate that the paramagnetic copper center in both phthalocyanines has axial symmetry with a planar anisotropy ( g_∥ > g_⊥). The ESR spectra in solution could be obtained only for Cu-P. Curie law is used to fit the experimental data of the magnetic susceptibility versus temperature graphs, and the Curie constant ( C) and diamagnetic/temperature-independent paramagnetic (α) contributions are deduced as 0.37598 (0.39576) cm³·K/mol and -23 × 10^-5 (25 × 10^-5) cm³/mol respectively for Cu-P and Cu-NP. The room temperature magnetic moment value (1.70 μ_B) is close to the spin-only value (1.73 μ_B) for the peripheral complex, showing that there is no orbital contribution to μ_eff. In contrast, at room temperature, the value of the magnetic moment (1.77 μ_B) is above the spin-only value, showing an orbital contribution to the magnetic moment. Cu-NP's room temperature magnetic moment value is larger than the value for Cu-P, demonstrating that the orbital contribution to the magnetic moment depends upon the substituent position. The magnitudes of the effective magnetic moment values also support that both Cu-P and Cu-NP complexes have square-planar coordination. This result is consistent with the determined g values. The spin densities were determined experimentally, and the results suggest that the positions of the substituents affect these values (0.469 for Cu-P and 0.490 for Cu-NP).