Direct Voltammetric Observation of Reversible and Irreversible Oxidations of Two-Dimensional J-Aggregates of Cyanine Dye

Coexistence of multiple conformations in cysteamine monolayers on Au(111)

The structural organization, catalytic function, and electronic properties of cysteamine monolayers on Au(111) have been addressed comprehensively by voltammetry, in situ scanning tunneling microscopy (STM) in anaerobic environment, and a priori molecular dynamics (MD) simulation and STM image simulation. Two sets of voltammetric signals are observed. One peak at -(0.65-0.70) V (SCE) is caused by reductive desorption of cysteamine. The other signal, at -(0.25-0.40) V consists of a peak doublet. The pH dependence of the latter suggests that the origin is catalytic dihydrogen evolution. The doublet feature is indicative of two distinct cysteamine configurations. Cysteamine monolayer formation from initial nucleation to a highly ordered phase has been successfully observed in real time using oxygen-free in situ STM. Random cellular patterns, disordered adlayer formation accompanied by high step edge mobility, and ultimately a highly ordered (square root 3 x 4) R30 degrees lattice are observed sequentially. Pits are formed due to enclosure of the mobile edges during the adsorption process. In the highly ordered cysteamine layer, each unit has two spots with apparent 0.6 A height difference in STM images. The coverage 5.7 +/- 0.1 x 10(-10) mol cm(-2) determined by voltammetry supports that the spots represent two individual cysteamine molecules. A priori MD and density functional simulations hold other clues to the image interpretation and indicate that the NH(3)(+) groups dominate the tunneling contrast. A wide range of interface structures, showing variations in the sulfur binding site and orientation, gauche and trans conformers, and especially hydrogen-bonding interactions, are examined, from which it is concluded that the adsorbate structure is controlled by interactions with the solvent rather than with the substrate.

Photoinduced charge separation reactions of J-aggregates coated on silver nanoparticles

The photochemistry of cyanine J-aggregates on the surface of colloidal Ag nanoparticles is reported. The photochemistry is initiated through ultrafast photoexcitation of the plasmon band in Ag nanoparticles, producing an enhanced near-field that interacts with the J-aggregate monolayer. Through transient absorption spectroscopy, we show that photoexcitation of the plasmon in Ag nanoparticles leads to exciton dynamics that differ strongly from J-aggregates alone or for J-aggregate monolayers on bulk metal surfaces. Specifically, charge-separated states with a lifetime of approximately 300 ps between the J-aggregate and Ag colloid are formed. The reduction of the Ag nanoparticles is shown to be a multielectron process.