Impact of molecule-dipole orientation on energy level alignment at the submolecular scale

The molecular orientation--dependent electronic properties of monolayer dipolar molecule chloroaluminum phthalocyanine (ClAlPc) on Au(111) are investigated by ultraviolet photoemission spectroscopy and scanning tunneling microscopy. The relation between geometrical and electronic structures has been revealed in the binding energies of the highest occupied molecular orbital states and vacuum level (VL) shifts. Two molecular orientations, Cl-up- and Cl-down-oriented molecules, coexist in the as-grown monolayer ClAlPc films on Au(111) without the formation of staggered molecular pairs to cancel the dipoles and phase separation, as is the case on graphite. After annealing, only the Cl-up-oriented molecules remain on Au(111), as on graphite. Interestingly, an extraordinarily large VL shift of {-}0.89 eV is observed in the annealed monolayer ClAlPc film on Au(111), which is opposite to that of +0.46 eV on graphite even though the molecular dipoles are oriented similarly.