Synthesis and Characterization of Octacyano-Cu-Phthalocyanine

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 P1̅ 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.