Dipole orientation dependent symmetry reduction of chloroaluminum phthalocyanine on Cu(111)

We demonstrate a dipole orientation dependent symmetry reduction of 4-fold symmetric chloroaluminum phthalocyanine (ClAlPc) molecules on a Cu(111) surface by combined low temperature scanning tunneling microscopy (LT-STM) and density functional theory (DFT) calculations. Unexpected symmetry reduction from 4-fold (C4) to 2-fold (C2) was observed for Cl-down (dipole up) adsorbed ClAlPc, while molecules adopted Cl-up (dipole down) configuration reserved the C4 symmetry. DFT calculations indicated strong charge accumulation at the interface region between Cu surface and the Cl atom in Cl-down adsorbed ClAlPc due to the electron transfer from the bonded Cu atoms. This can result in charge redistribution within the phthalocyanine (Pc) macrocycle, and the formation of anionic Pc with an uptake of 1.3 e, which can be subjected to Jahn-Teller distortion. The inequivalent charge distribution onto the four lobes would be further enlarged due to the conformational distortion. The two down-bended lobes with more electrons interact stronger with the substrate and are much closer to the surface, leading to the C2 symmetry with one pair of up-bended lobes brighter and longer than their perpendicular counterparts for Cl-down adsorbed ClAlPc.