Direct voltammetric observation of reversible and irreversible oxidations of two-dimensional J-aggregates of cyanine dye

The redox properties of two-dimensional (2D) J-aggregates of a typical thiacarbocyanine dye have been studied directly by the standard electrochemical method in conjunction with the recently developed technique for controlled layering of 2D J-aggregates on a self-assembled cysteamine monolayer on an atomically flat Au(111). The cyclic voltammograms measured at widely ranging scan rates from 10 mV/s to 100 V/s illuminated unique mechanistic features of the J-aggregate oxidation, which proved to be a convolution of reversible and irreversible oxidations involving the charge transfer of up to 4 electrons/molecule. The oxidation-induced irreversible bleaching of the J-band was accompanied by negligibly small changes in its peak position and sharpness, suggesting that the irreversible reaction occurs preferentially along the domain edges or boundaries. The two different phases of oxidation could be more or less clearly separated under the fast scan mode (scan rate larger than 10 V/s), giving us the unprecedented opportunity to directly observe the partial reversible oxidation of the 2D J-aggregate involving a limited amount of charge-transfer much less (15%) than that of the hypothetical one-electron oxidation of all dye molecules. This limitation probably represents energetic constraint against the accumulation of too many repulsive positive holes. For a series of J-aggregates partitioned into progressively smaller segments with a coadsorbed diluent dye, the corresponding reversible potentials were 0.18-0.10 V more negative than that expected for the monomer. The results indicate that the J-aggregation caused concurrent upward and downward shifts in the dye HOMO and LUMO levels, respectively. The way the J-aggregation changes the redox levels depends critically on the nature of positive holes in the given aggregate framework.