Spectroscopic evidence of strong π-π interorbital interaction in a lead-phthalocyanine bilayer film attributed to the dimer nanostructure

We demonstrate spectroscopic evidence of 2t level splitting of the highest occupied molecular orbital (HOMO) state, which is characterized by twice the intermolecular transfer integral t, with ultraviolet photoelectron spectroscopy in a well-ordered bilayer of lead-phthalocyanine deposited on graphite. The HOMO state splits clearly by 2t=0.35 eV upon bilayer formation at 295 K and the splitting increases to 0.41 eV at 47 K. The splitting is ascribed to dimer nanostructures with antiparallel molecular electric dipoles.