Evaluation of the Tunneling Constant for Long Range Electron Transfer in Azobenzene Self-Assembled Monolayers on Gold

Abstract

The electron transfer at interfaces is of primary importance in chemical, biological, and other natural processes, which has led to scientists making continuous efforts to pursue idealmodels to study the basic issues for the electron transfer processes.1 After Nuzzo and Allara reported the formation of highly organized monolayers based on the Au-sulfur interaction,2 the creation of welldefined self-assembled monolayers (SAMs) has provided an idealmodel in the study of interfacial electrochemistry. It presents us with a unique opportunity to investigate the fundamental issues such as the distance dependence and the effect of interfacial structure on the long range electron transfer.3-17 Miller and Becka5 have systematically explored theuseof an inert self-assembledmonolayer on gold as a blocking layer toward dissolved redox molecules. From the dependence of the electron transfer rate of a series of redox couples on the thickness of the monolayer film, the tunneling constant in the hydroxyl thiolwasmeasured independentof theredoxcouple,5while Li6 reported the value in the long-chain alkanethiol monolayers. In themeantime,Chidsey7-9 andFinklea10-14 have constructed reversible redox center (ferrocene or pentaammine (pyridine) ruthenium) tethered selfassembled monolayers on gold and investigated their electron-transfer kinetics in detail. Recently, the investigation on the distance dependence of electron transfer in electroactive self-assembled monolayers has been extended to viologen,15 quinone,16 and a ferrocene LBSAMcompositebilayer.17 Theelectron tunneling constant of respective system was addressed experimentally. However, up to date, no experimental data have yet been provided concerning the tunneling constant for long range electron transfer in SAMs with attached complex redox centers such as azobenzene, which possesses protonation reaction and structural change. In this note, we will describe our results about the evaluation of the electron tunneling constant for electron transfer of azobenzene self-assembled monolayers on gold. The novelty of thiswork is on the functional groupazobenzene. It has a multistep reduction/oxidation behavior during which themolecular structuredefinitely changes (Scheme 1). It might be of great interest to compare the results of azobenzene SAMswith those simple redox center derived SAMs. Further, together with the previous studies on the azobenzene SAMs (as examples, see refs 18-21), the present investigation does help us to gain deep insights into the electrochemical reaction nature of azobenzene organized molecular assemblies.