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 (<b>Cu-P</b>) and non-peripheral (<b>Cu-NP</b>) positions. <b>Cu-NP</b> showed a marked near-IR maximum absorption compared to <b>Cu-P</b> and, accordingly, a smaller HOMO–LUMO energy gap, calculated via the electrochemical results and simulations in the gas phase, as well as for <b>Cu-NP</b> from its crystallographic data. An electron-spin resonance (ESR) technique is used to extract the <i>g</i> values from the powder spectra that are taken at room temperature. The <i>g</i> values were determined to be <i>g</i><sub>∥</sub> = 2.160 and <i>g</i><sub>⊥</sub> = 2.045 for <b>Cu-P</b> and <i>g</i><sub>∥</sub> = 2.150 and <i>g</i><sub>⊥</sub> = 2.050 for <b>Cu-NP</b>. These values indicate that the paramagnetic copper center in both phthalocyanines has axial symmetry with a planar anisotropy (<i>g</i><sub>∥</sub> > <i>g</i><sub>⊥</sub>). The ESR spectra in solution could be obtained only for <b>Cu-P</b>. Curie law is used to fit the experimental data of the magnetic susceptibility versus temperature graphs, and the Curie constant (<i>C</i>) and diamagnetic/temperature-independent paramagnetic (α) contributions are deduced as 0.37598 (0.39576) cm<sup>3</sup>·K/mol and −23 × 10<sup>–5</sup> (25 × 10<sup>–5</sup>) cm<sup>3</sup>/mol respectively for <b>Cu-P</b> and <b>Cu-NP</b>. The room temperature magnetic moment value (1.70 μ<sub>B</sub>) is close to the spin-only value (1.73 μ<sub>B</sub>) for the peripheral complex, showing that there is no orbital contribution to μ<sub>eff</sub>. In contrast, at room temperature, the value of the magnetic moment (1.77 μ<sub>B</sub>) is above the spin-only value, showing an orbital contribution to the magnetic moment. <b>Cu-NP</b>’s room temperature magnetic moment value is larger than the value for <b>Cu-P</b>, 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 <b>Cu-P</b> and <b>Cu-NP</b> complexes have square-planar coordination. This result is consistent with the determined <i>g</i> values. The spin densities were determined experimentally, and the results suggest that the positions of the substituents affect these values (0.469 for <b>Cu-P</b> and 0.490 for <b>Cu-NP</b>).