Effect of the substituent on metal-molecule hybridization

Local electronic structures of benzoate (C6H5COO−) and aminobenzoate (NH2C6H4COO−) isomers (meta and para) adsorbed on Cu(1 1 0) have been investigated with scanning tunneling microscopy (STM) and spectroscopy (STS) at 4.7 K. STS and molecular orbital calculation study indicate that the energy level of the lowest unoccupied molecular orbital (LUMO) of free meta-aminobenzoate molecule is largely shifted to an lower energy level by adsorption on Cu(1 1 0), whereas an energy shift is relatively small for benzoate and para-aminobenzoate molecules. The large energy shift of LUMO level for meta-aminobenzoate is ascribed to the strong metal-molecule hybridization. It turns out that the electron donation from amino substituent to the phenyl ring plays a key role in this hybridization, which is also influenced by the substituted position.