RESUMO
Octacyano-metal-substituted 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, most MPc(CN)8 have not been thoroughly investigated. Therefore, CuPc(CN)8 was synthesized in this study and its crystal structure, chemical and electronic states, thermal stability, and electrical properties were investigated. This article discusses the various properties of CuPc(CN)8, as compared to those of CuPc and FePc(CN)8. The previously reported FePc(CN)8 is an organic semiconductor molecule with a molecular structure similar to that of CuPc(CN)8. X-ray diffraction (XRD) measurements revealed that CuPc(CN)8 has a crystalline structure in the P1Ì space group. The crystal structure forms an in-plane network parallel to the molecular plane through multiple hydrogen bonds by the cyano groups at the molecular terminals. Interestingly, the crystal structure, especially the molecular stacking, of CuPc(CN)8 differs from that of FePc(CN)8. The absorption edge observed in the ultraviolet-visible spectrum of CuPc(CN)8 shifted to a longer wavelength than that of CuPc, which was attributed to the energy gap of CuPc(CN)8 being smaller than that of CuPc owing to the influence of the cyano groups at the molecular terminals, according to the molecular orbital calculation results using density functional theory. Ultraviolet photoelectron spectroscopy measurements confirmed that CuPc(CN)8 had a stronger n-type character than CuPc because of the orbital energy stabilization by the cyano groups. Thermogravimetry/differential thermal analysis measurements revealed that the thermal stability of CuPc(CN)8 was significantly higher than that of FePc(CN)8. CuPc(CN)8 exhibited photoconduction upon visible-light irradiation, and its electrical conductivity was higher than that of CuPc, which was attributed to a reduction in the electron injection barrier at the electrode interfaces.
RESUMO
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 ß-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.
