Investigation of the adsorption of 4-cyanopyridine on Au(111) by in situ visible–infrared sum frequency generation

We have investigated the configuration of 4-cyanopyridine on Au(111) electrodes in perchlorate solution by in situ visible–IR sum frequency generation (SFG). Thanks to the use of two IR tunable lasers (the free electron laser CLIO and a table laser based on an optical parametric oscillator) we have obtained SFG spectra of the system in the spectral range of both the aromatic cycle and the CN stretching modes. We present herein the first SFG spectra ever obtained under electrochemical conditions in the 9–12 μm range. Our results show a potential-dependent orientation of the adsorbed 4-cyanopyridine from a configuration where the molecule is adsorbed perpendicular to the electrode via the nitrogen of the pyridine ring at negative potentials, to a flat adsorption configuration at intermediate potentials, and finally a perpendicular orientation again where the molecule is bound through the nitrogen of the cyanide end.