Doping-induced structural changes of conducting polyalkoxythiophene on the chemically modified gold surface:: An in situ surface enhanced resonance Raman spectroscopic study

In situ surface enhanced resonance Raman spectroscopy (SERRS) with excitation at 1064 nm has been used to monitor the dynamic changes in charge transfer, structure, and orientation during the redox doping of a self-assembled monolayer of conducting poly-3-(3'-thienyloxy)propanesulfonate (P3TOPS) on a gold surface modified by 2-mercaptoethylamine (MEA). The SERRS spectra were compared with Fourier transform Raman spectra of solid 3TOPS monomer and with reduced and oxidized forms of P3TOPS in solution. The oxidative doping of P3TOPS has been carried out in the potential range between -0.3 and +0.7 V, vs Ag/AgCl. Our SERRS data indicate that the oxidation process goes through initial preferential generation of polaron charge carriers with followed generation of bipolarons at higher oxidation levels. At low doping levels, thiophene rings are more coplanar than at higher ones. The observed potential dependence of the normalized integrated SERRS intensities of P3TOPS in the range of the inter-ring stretch vibrations exhibits a bell-shaped curve with a maximum at 0 V. In the potential window from -0.1 to +0.1 V, the highest number of doped coplanar neighboring molecular units is achieved. Reorientation of alkoxy side chain closer to the surface normal starts at -0.1 V. At higher potentials applied, the trans --> gauche conformational transformation of the alkoxy side chain was observed along with the massive growing number of bipolarons and simultaneous deplanarization of the polymer chain.