Energy-level alignment in 4′-substituted stilbene-4-thiolate self-assembled monolayers on gold

Molecular energy-level alignment around the Fermi level in self-assembled monolayers of E-stilbene-4-thiolate (SAM1), E-4′-(ethoxy)stilbene-4- thiolate (SAM2), and E-4′-(dimethylamino)stilbene-4-thiolate (SAM3) on Au(111) was studied by ultraviolet photoelectron spectroscopy. A comparison of the measured hole-injection barriers into SAM1-3 with the electrochemically estimated molecular ionization energies reveals that the influence of the 4′-substituent in the stilbene backbone on the hole-injection barrier is greatly suppressed. While predicted theoretically for a variety of densely packed π-conjugated thiolate monolayers before, we here provide conclusive experimental evidence for this phenomenon. The obtained results are compared to periodic density-functional theory calculations and are discussed in the light of electrostatic properties of dipolar monolayers.