Electrochemical oxidation and ex-situ STM observations of bis(4-dimethylamino-2-dihydroxyphenyl)squaraine dye layers on HOPG electrodes

Electrochemical oxidation of insoluble highly ordered bis(4-dimethylamino-2 -dihydroxyphenyl)squaraine (1-1OHSQ) dye layers adsorbed on highly oriented pyrolytic graphite (HOPG) electrodes was studied in aqueous electrolytes. Staircase cyclic voltammetry obtained in chloride electrolytes revealed hys teresis characterized by large peak separations (100-200 mV) and sharp redo x peaks (full width at half maximum (fwhm) 10-60 mV) the shape and potentia ls of which depended on electrolyte concentrations. Small stochastic reduct ion peaks were observed at more negative potentials that are associated wit h the reduction of small domains of the oxidized 1-1OHSQ layers. Peak poten tials and peak shapes were also dependent on the identity of the electrolyt e anion species. The results support a reaction scheme where electrolyte an ions are incorporated into surface confined one-electron oxidized dye molec ular layers. From the results of peak potential shifts: the preference for ion-pairing is estimated to be in the order of: I- > Br- > Cl- > SO42- appr oximate to ClO4- approximate to F-. Ex-situ STM observations were performed to reveal structural changes upon electrochemical oxidation of 1-1OHSQ lay ers in LiCl electrolytes. Two polymorphs with different angles of the long- axis of molecules to the directions of the molecular row were observed for the oxidized samples. Both polymorphs had larger packing densities compared to the reduced form of the dye. (C) 2001 Elsevier Science B.V. All rights reserved.