Contribution of Electrostatic Cohesive Energy in Two-Dimensional J-Aggregation of Cyanine Dye

Well-ordered two-dimensional (2D) J-aggregates of cyanine dyes can be easily self-assembled on an atomically flat Ag(111) film covered with a halide monolayer, thus serving as a superior model system for studying the 2D J-aggregation of cyanine dyes. The 2D J-aggregate structure of a thiacarbocyanine dye has been examined in detail by optical absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), and kinetic analysis of molecular desorption in dye solvent. An angle-resolved XPS allowed easy determination of the vertical molecular orientation in the 2D J-aggregate. More importantly, an intramolecular chemical shift near 1 eV at maximum between the cationic dye chromophore and the counteranion, together with the kinetic information concerning the solvent-induced molecular desorption, illuminated the dominant contribution of an electrostatic Madelung energy to the overall cohesive energy for 2D J-aggregation. This collective Coulomb interaction also gives strong constraint in respect to the type of 2D molecular network so that only J-aggregation becomes energetically feasible in the case of the selected thiacarbocyanine dye.