Ion Exchange Processes and Environmental Effects in Chromophoric Self-Assembled Superlattices. Manipulation of Microstructure and Large Enhancements in Nonlinear Optical Response

This contribution reports on the anion exchange characteristics of self-assembled monolayers composed of high-β stilbazolium chloride moieties as assessed by X-ray photoelectron spectroscopy, optical spectroscopy, and polarized second harmonic generation (SHG) spectroscopy. At moderate chromophore surface coverages (~1 × 10¹⁴ molecules/cm²) in acetonitrile or methanol, replacement of ~60% of the Cl-counterions by I-, р-aminobenzenesulfonate, or 4-[4-[(diethylamino)phenyl]azo]benzenesulfonate (ethyl orange) is observed. However, negligible exchange is detected at higher chromophore surface coverages (≥2 × 10¹⁴ molecules/cm²) or when the monolayer is functionalized with a siloxane overlayer. Polarized SHG measurements at 1064 nm reveal ion exchange-induced increases in χ_zzz⁽²⁾ of 34%, 25%, and 44% for I-, р-aminobenzenesulfonate, and ethyl orange, respectively. However, changes in the average chromophore orientation angle with respect to the surface normal are ≤2°, implicating changes in anion-cation pairing as the source of the large enhancements in χ_zzz⁽²⁾.