Modification of Gold’s Work Function upon Adsorption of Mercaptobiphenylcarbonitrile: Experimental Evidence for a Theoretical Prediction

Tuning the energy of the frontier orbitals of an adsorbed molecule to match the Fermi level of the electrode is a fundamental requirement for efficient charge injection in molecular electronic devices. In this paper, we present electrochemical, impedimetric, spectroscopic, and scanning electrochemical microscopy (SECM) data that were used to study the effects of the adsorption of 4′-mercaptobiphenylcarbonitrile (HS2PCN) on the work function of gold. The adsorption process was studied and indicated the formation of a loosely packed self-assembled monolayer (SAM, Δ<i>G</i>_ads = −43.3 kJ mol^–1) following the immersion of the gold substrate in an ethanolic solution of HS2PCN. An increase in the immersion time resulted in the accumulation of negative charge density on the gold surface ascribed to the bonding dipoles resulting from the charge rearrangement at the metal–SAM interface that generates interfacial dipoles as a result of a charge-transfer process. As a consequence, a modification of about 1.2 eV is estimated in the work function of the gold surface modified with HS2PCN. Electron-transfer rate constants (<i>k</i>⁰), as measured via SECM, showed a strong dependence on the net charge of the redox probes and increased on going from negatively (ca. 1.14 × 10^–3 cm s^–1) to positively charged species (>1.0 cm s^–1). Such behavior is ascribed to the polarity of the interface of the HS2PCN SAM on gold, which is negatively charged because of the electron-withdrawing property of the nitrile fragment