▎ 摘　　要

He et al. demonstrated dehydrogenative coupling of single porphines to graphene edges by scanning probe technique with submolecular resolution, revealing specific bonding motifs and electronic features directly, which created a new hybrid material with tunable functionalities [Nat. Chem. 9 (1) (2017) 33-38]. Motivated by this work, we study the electron transport properties of four types molecular junctions with a single porphine molecule coupled between two graphene nanoribbons as have been observed in the above experiment by density functional theory calculations. It is found that both the electron transmission near the Fermi level and the current at low bias can be enhanced by manipulating the covalent coupling between the porphine molecule and graphene nanoribbons. Based on this behavior, we design three asymmetric junctions to realize rectification effect, and as anticipated, a perfect rectifier is obtained by one of them. The mechanism is well understood by the distributions of local density of states. The design of molecular devices with covalent coupling provides an efficient method to tune the electron transmission states, which is of great importance in developing integrated circuits in future.