Electrochemical control of the second harmonic generation property of self-assembled monolayers containing a trans-ferrocenyl-nitrophenyl ethylene group on gold

The second harmonic (SH) intensity at gold electrodes modified with self-assembled monolayers (SAMs) of two novel molecules, which have trans-ferrocenyl-nitrophenyl ethylene and thiol groups as the second harmonic generation and surface-attached groups, respectively, was found to be strongly dependent on the oxidation state of the ferrocene group. The SH intensity increased when the ferrocene moiety of the SAMs was oxidized electrochemically to the ferricenium cation, and it decreased and returned to the original value when the ferricenium cation of the ferrocene moiety was reduced to neutral ferrocene. These changes were reversible and were repeated many times. The molecular orientation change in each SAM upon reduction/oxidation of the ferrocene moiety was investigated by in situ Fourier transform infrared reflection-absorption spectroscopy, and the hyperpolarizabilities of these molecules were calculated using a computational electronic structure model. The origin of the SH intensity change is discussed on the basis of these results.