Spectroelectrochemical Investigation of Double-Walled Tubular J-Aggregates of Amphiphilic Cyanine Dyes

The amphiphilic cyanine dye 3,3‘-bis(2-sulfopropyl)-5,5‘,6,6‘-tetrachloro-1,1‘-dioctylbenzimidacarbocyanine (C8S3) self-assembles in aqueous solution to form double-walled, tubular J-aggregates with ∼13 nm diameters and lengths up to several hundred nanometers. The redox and light absorption properties of immobilized J-aggregates on transparent, conductive indium tin oxide (ITO) electrodes have been studied directly using cyclic voltammetry (CV) in conjunction with UV−vis spectroscopy to elucidate unique mechanistic features of J-aggregate oxidation. Morphological properties were examined using in situ atomic force microscopy (AFM). Irreversible J-aggregate oxidation appears to occur primarily along the outer wall of the tubular structure as evidenced by the potential-induced irreversible bleaching of J-band absorption. Voltammetric studies as a function of scan rate and pH indicate that J-aggregate oxidation involves both electrochemical and chemical steps in which dimerization and subsequent dehydrogenation of the J-aggregate leads to the formation of a new dehydrogenated dimer oxidation product. This dehydrogenated dimer exhibits an absorbance band near 560 nm along with a reversible reduction peak characteristic of a surface-confined, redox-active species. Excellent correlation of J-aggregate redox potentials with spectroelectrochemical data is obtained that allows us to understand energetic thresholds for electron transfer in C8S3 tubular J-aggregates.