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 easil y self-assembled on an atomically flat Ag(lll) film covered with a halide m onolayer, thus serving as a superior model system for studying the 2D J-agg regation of cyanine dyes. The 2D J-aggregate structure of a thiacarbocyanin e dye has been examined in detail by optical absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), and kinetic analysis of molecular desorp tion in dye solvent. An angle-resolved XPS allowed easy determination of th e vertical molecular orientation in the 2D J-aggregate. More importantly, a n intramolecular chemical shift near 1 eV at maximum between the cationic d ye chromophore and the counteranion, together with the kinetic information concerning the solvent-induced molecular desorption, illuminated the domina nt contribution of an electrostatic Madelung energy to the overall cohesive energy for 2D J-aggregation. This collective Coulomb interaction also give s strong constraint in respect to the type of 2D molecular network so that only J-aggregation becomes energetically feasible in the case of the select ed thiacarbocyanine dye.