Synthesis and properties of non-planar octaphenyl-substituted phthalocyaninato manganese(III)

A trivalent manganese complex of a non-planar 1,4,8,11,15,18,22,25- octaphenylphthalocyanine derivative, 3, has been synthesized by treating metal-free 1,4,8,11,15,18,22,25-octaphenylphthalocyanine, 2, with manganese(II) chloride in refluxing DMF. X-ray crystallography has revealed that the complex adopts a highly-deformed skeletal conformation due to steric congestions between the substituted bulky phenyl groups. Although mass spectrometry results provide no indication of the presence of axial ligands, the crystal data clearly show that the manganese ion is coordinated axially by a chloride ion in addition to the four equatorial isoindole nitrogen atoms. The oxidation state of the manganese center of 3, therefore, has been assigned to 3+. Electronic absorption spectra revealed that the Q-band of 3 shows a significant bathochromic shift compared to that of the planar Mn(III) phthalocyanine 4 and non-planar metal-free phthalocyanine 2. The analysis of MCD and UV-vis spectra suggests that the absorption band observed at 555 nm for 3 is LMCT in origin. Electrochemical investigations indicate that the first oxidation couple is ligand-centered. Since the HOMO of 2 is destabilized due to the synergetic effects of the electron-donating phenyl groups at the non-peripheral positions and the deformed π-electronic skeleton, the resulting manganese complex 3, is anticipated to also have a ligand-centered HOMO. The results of spectroelectrochemistry confirm that the first oxidation process of 3 is indeed ligand-centered. DFT calculations indicate that the MO amplitudes of the HOMO and LUMO localize mainly on the Pc ligand and the manganese ion, respectively, also supporting the assignments of the cyclic voltammograms. By combining the manganese ion and non-planar Pc ligand, a novel, stable near-infrared (NIR)-absorbing material has been achieved in the present study.