The Effect of Molecular Structure on Voltage Induced Shifts of Charge Transfer Excitation in Surface Enhanced Raman Scattering

The Raman intensity versus electrode voltage behavior of a series of substituted pyridines and saturated nitrogen heterocycUc compounds was studied at two laser excitation energies. The voltage (resonance) maximum shifts to more positive potential with increasing excitation energy for the substituted pyridines (case I) and to more negative potential with increasing excitation energy for the saturated nitrogen heterocyclic compounds (case II). The voltage maxima can be correlated with the Hammett sigma function for substituted pyridines and with pK a for the saturated nitrogen heterocycles. The results are consistent with charge transfer from the metal to molecule (adcluster-molecule complex) in the case I system and from the molecule (adcluster-molecule complex) to the metal in the case II system.