Charge transfer and SEIRAS studies of 1,4-benzoquinone functionalized mixed monothiol/dithiol self-assembled monolayers

Although substantial information can be obtained from electrochemical measurements, much greater detail concerning molecular structure can be obtained by coupling such measurements with molecular spectroscopy. To this end, electrochemical and in situ surface enhanced infrared spectroscopy (SEIRAS) was performed to analyze 1,4-benzoquinone (BQ) terminated self-assembled monolayers. Monolayers were derived via the Michael addition of BQ to a pre-formed mixed monolayer composed of methyl and thiol terminated functionalities. This approach resulted in relatively robust but non-ideal redox-active monolayers. Spectroscopic and electrochemical measurements have allowed us to determine the pH dependence of both the apparent formal potential and the heterogeneous standard rate constant for proton coupled electron transfer (PCET) for this 2e−/2H+ redox system. While the former is in excellent agreement with predictions of step-wise PCET, the latter deviates from the expected kinetic response.