In Situ Raman Spectroelectrochemistry of Electron Transfer between Glassy Carbon and a Chemisorbed Nitroazobenzene Monolayer

Abstract

In situ Raman spectroscopy was used to monitor 4-nitroazobenzene (NAB) in an electrochemical cell, both as a free molecule and as a chemisorbed monolayer on a glassy carbon (GC) electrode surface. Reduction of free NAB exhibited two well-defined voltammetric couples in acetonitrile, and the accompanying spectral changes supported a mechanism involving two successive 1-e(-) transfers. Raman spectra of NAB chemisorbed to GC via diazonium ion reduction were obtained in acetonitrile with a high-sensitivity, line-focused CCD spectrometer. The chemisorbed NAB spectra were quite different from the free NAB spectra, and were sufficiently strong to monitor as a function of applied potential. In the potential range of +400 to -800 mV vs Ag/Ag(+), the intensity of the Raman bands associated with the phenyl-NO(2) moiety varied, implying an electronic interaction between the pi system of the graphitic substrate and the chemisorbed NAB molecules. Negative of -800 mV, a 1-e(-) voltammetric reduction peak was observed, which was reversible on the positive voltage scan. This peak was accompanied by significant spectral changes, particularly the loss of the N=N and NO(2) stretches. The spectra are consistent with formation of a quinoid structure containing a C=C double bond between the NAB and the graphitic surface. The electron transfer and spectral changes occurred over a wider potential range than expected for a conventional Nernstian equilibrium, but did not appear to be broadened by slow electron-transfer kinetics. The results imply a significant perturbation of electron transfer between the GC and the monolayer, caused by strong electronic coupling between the graphitic pi system and the NAB orbitals. Rather than a discrete electron transfer to a free molecule, the electron transfer to chemisorbed NAB is more gradual, and is presumably driven by the electric field at the electrode/solution interface.

Analytical Challenges in Molecular Electronics

“…In addition, XPS of a nominal 10-Å copper film on NAB shows a copper-nitrogen bond and partial loss of the N 1s band characteristic of the NO 2 group (406 eV). Combined with other evidence, the results indicate a carbon/NAB/copper molecular junction with partial reduction of the NAB centers and formation of covalent copper-nitrogen bonds (57 ). Similar XPS analysis of PPF/fluorene/copper junctions revealed no copper-carbon bonds, nor any detectable oxygen within the junction (37 ).…”

Electron Transport and Redox Reactions in Molecular Electronic Junctions

“…The change in the 1401/ 1450 cm À1 intensity ratio for the N=N stretch, and the decrease in the 1340 cm À1 is a clear signature for electrochemical reduction of NAB, as established previously in solution. [29] Further support for reduction of NAB is provided by UV/Vis absorption spectroscopy through partially transparent carbon/NAB/Al 2 O 3 / Pt junctions, [30] shown in Figure 6. A À4 V, 100 msec bias pulse causes an increase in absorbance at 420 nm, corresponding to formation of the "quinoid" form of the NAB anion.…”

Oxygen electroreduction on chemically modified glassy carbon electrodes in alkaline solution

“…Glassy carbon (GC) [1,4,[6][7][8][9][10][11][12][13][14][15][16][17][18][19], highly oriented pyrolytic graphite (HOPG) [4,6,20,21], boron-doped diamond (BDD) [21][22][23], carbon nanotubes (CNT) [24,25] and other carbon materials have been successfully grafted using the diazonium reduction method. A large variety of surface modifiers can be attached to carbon surfaces.…”