Negative Differential Resistance of Extended Viologen. Oscillations with Odd Harmonic Frequencies

The structure of extended viologens contains redox active pyridinium functions separated by a p-phenylene spacer. Our recent studies of six oligomers reported the single molecule conductivity [1], the transfer of periodicity to the deterministic chaos [2] and the verification of the Feigenbaum constant [3]. The present communication will show that the oligomer having three repetitive viologen units yields at potentials in the vicinity of the formal redox potential the negative differential resistance (NDR). Fig. 1. The complex impedance plot for reduction of extended viologen at four potentials near the standard redox potential. Panels C and D show NDR. Solvent was acetonitrile. The electrode was the mercury drop Resulting current oscillations are triggered by a small signal perturbation. Curiously, the Fourier spectrum shows the presence only of the odd harmonics of the perturbation frequency. Higher harmonics are of large amplitude and they are observed up to the 7th harmonic frequency. The observed effect will be interpreted in line with similar rare observations in enzymatic reactions, in acoustics and few other fields of physics. The selective generation of odd harmonics is related to the theory of the stochastic resonance [4]. Fig. 2. The Fourier transform of the current-time series observed during the reduction of extended viologen at the half-wave potential. Only the odd harmonics are observed References [1] V. Kolivo?ka, M. Valá?ek, M. Gál, R. Sokolová, J. Bulí?ková, L. Pospí?il, G. Mészáros, M. Hromadová, Single-molecule conductance in a series of extended viologen molecules. J. Phys. Chem. Lett. 4 (2013) 589. [2] L. Pospí?il, M. Hromadová, N. Fanelli, M. Valá?ek, V. Kolivo?ka, M. Gál, Extended viologen as a source of electric oscillations Phys. Chem. Chem. Phys. 13 (2011) 4365. [3] L. Pospí?il, M. Hromadová, M. Gál, M. Valá?ek, N. Fanelli, V. Kolivo?ka, Irregular polarographic currents obey Feigenbaum universality route from order to chaos. Collect. Czech. Chem. Commun. 74 (2009) 1559. [4] P. Jung , P. Hänggi, Amplification of small signals via stochastic resonance. Phys. Rev. A 44 (1991) 8032.