Electronic Structure of Cu-Phthalocyanine Metal–Organic Framework

Abstract

Metal phthalocyanine metal–organic frameworks (MPc-MOFs) possess a crystalline structure consisting of stacked molecular layers that form a square lattice composed of metal phthalocyanine (MPc) units. The unique structure of these MPc-MOF molecular layers is theoretically predicted to belong to the Leib or fes (square octagon) lattice. Although these lattice models possess fascinating electronic structures, such as Dirac bands and flat bands around the energy gap, they remain the least-studied Dirac materials to date. In this study, the π-bands of CuPc-MOF were investigated owing to their similarity to the Dirac bands of the fes lattice, as the Cu 3d band avoids the Fermi level. We synthesized CuPc-MOF using the molten-salt method, with ZnCl2 as the flux. X-ray diffraction and transmission electron microscopy studies revealed that synthesized CuPc-MOF consisted of flake-like microcrystals formed by overlapping molecular layers and that the crystals grew poorly in the stacking direction. Density functional theory (DFT) energy band calculations for the CuPc-MOF bulk crystals suggested that CuPc-MOF is metallic, as energy bands with large dispersion widths develop in the stacking direction of the molecular layers. However, the experimental results differ from the theoretical predictions. The visible and near-NIR absorption spectra indicate that CuPc-MOF has an optical energy gap of 0.31 eV. Furthermore, the density of states (DOS) of CuPc-MOF, directly observed using ultraviolet photoelectron spectroscopy (UPS), indicates that synthesized CuPc-MOF is nonmetallic. This discrepancy is attributed to the fact that the synthesized CuPc-MOF consists of flake-like microcrystals. In fact, the DFT-calculated energy band structure, assuming a single-molecular layer structure for the CuPc-MOF, strongly resembled that of the fes lattice and explained the observed UPS results. As expected, the DOS immediately below the Fermi level of the CuPc-MOF showed barely any contribution from the Cu 3d orbitals, and the π orbitals gave rise to a linear DOS. These results are in good agreement with the UPS results. Finally, electrical measurements revealed that CuPc-MOF was more electrically conductive than typical organic semiconductors. In the future, a wider range of physical properties of CuPc-MOF and its detailed electronic structure should be determined to understand its similarities to the fes lattice.