Probing charge states in molecular junctions using Raman spectroscopy

We examine the Raman spectra of a series of molecules in different charge states and suggest ways of controlling these charge states in charge transport experiments. Our study uses density functional theory (DFT)-based quantum chemical methods. The molecules we study are benzene derivatives, which accommodate transport either through the HOMO or the LUMO (where the HOMO and the LUMO denote the highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively). We have investigated whether observable differences in the Raman spectra will occur upon electron addition or removal. We find substantial frequency shifts upon electron removal or addition. These shifts tend to be uniformly downward in the case of LUMO transport, whereas, for HOMO transport, this unidirectionality is lacking and it is due to the fact that anions are usually less strongly bound than neutrals, whereas removing an electron from a bonding-type orbital usually gives localized modification in bond length and strength.