Phthalocyanine-Based Organometallic Nanocages: Properties and Gas Storage

Phthalocyanine (Pc) molecules are well-known flexible structural units for 1D nanotubes and 2D nanosheets. First-principles calculations combined with grand canonical Monte Carlo simulations are used to obtain the geometries, electronic structures, optical properties, and hydrogen-storage capacities of nanocages consisting of six Pc molecules with six Mg or Ca atoms. The primitive Pc cage has T-h symmetry with twofold degeneracy in the highest occupied molecular orbital (HOMO), and threefold degeneracy in the lowest unoccupied molecular orbital (LUMO); the corresponding HOMO-LUMO gap is found to be 0.97 eV. The MgPc and CaPc cages have O-h symmetry with a HOMO-LUMO gap of 1.24 and 1.13 eV, respectively. Optical absorption spectra suggest that the Pc-based cages can absorb infrared light, which is different from the visible-light absorption in Pc molecules. We further show that the excess uptake of hydrogen on MgPc and CaPc cages at 298 K and 100 bar (1 bar=0.1 MPa) is about 3.49 and 4.74 wt%, respectively. The present study provides new insight into Pc-based nanostructures with potential applications.