Synthesis and Characterization of Octacyano-Fe-Phthalocyanine

Octacyano-metal-phthalocyanine MPc(CN)₈ is a promising n-type stable organic semiconductor material with eight cyano groups, including a strong electron-withdrawing group at its molecular terminals. However, a thorough investigation of MPc(CN)₈ has not yet been conducted. Therefore, we synthesized FePc(CN)₈ and investigated its crystal structure, chemical and electronic states, electrical properties, photocatalytic activity, and magnetic properties. In this paper, we discuss the various properties of MPc(CN)₈ in comparison with those of FePc. X-ray diffraction measurements indicated that the crystal structure of FePc(CN)₈ was strongly influenced by the cyano groups and differed from the α- and β-forms of FePc. The space group P4/mcc structure of FePc(CN)₈ was similar to that of the x-form of LiPc. The ultraviolet-visible (UV-vis) absorption spectrum of FePc(CN)₈ was observed at wavelengths longer than that of FePc. Density functional theory-based molecular orbital calculations indicated that the energy gap of FePc(CN)₈ is smaller than that of FePc, which can lead to the observation of the Q-band in the UV-vis absorption spectrum of FePc(CN)₈ at longer wavelengths than that of FePc. Because FePc(CN)₈ has a wider optical absorption band in the visible region than FePc, its photocatalytic activity is approximately four times higher than that of FePc. The conductivity of FePc(CN)₈ was also higher than that of FePc, which is due to the larger overlap of π-electron clouds of the molecules in the crystal structure of FePc(CN)₈. Magnetic measurements revealed that FePc(CN)₈ exists in an antiferromagnetic ground state. The magnetic properties of FePc(CN)₈ are specific to its crystal structure, with direct exchange interactions between Fe²⁺ ions and π-electron-mediated interactions. In particular, the Pauli paramagnetic behavior at high temperatures and the antiferromagnetic behavior at low temperatures (Weiss temperature θ = -4.3 ± 0.1 K) are characteristic of the π-d system.