Dye-Loading Mechanismin Electrochemical Self-Assemblyof CuSCN/Stilbazolium Dye Hybrid Thin Films

Abstract

The mechanism of dye loading has been studied for the electrochemical self-assembly of nanostructured CuSCN thin films hybridized with cationic stilbazolium dyes with dimethylamino- (DAS⁺), methoxy- (MTS⁺), and cyano- (CNS⁺) substituents with increasing electron-withdrawing capabilities. After numerous film syntheses and analyses, it turned out that they all obey the same physicochemical model, namely, switching from diffusion-limited to surface reaction-limited dye loading. A universal mathematical model has been proposed and nicely applied to the three systems to describe the rate of dye precipitation in the two regimes and the border of the mechanistic switching as a function of dye-specific parameters, namely, diffusion coefficient and stability of coordination to the surface sites, under a given rate of CuSCN precipitation. This rule also explains the change of the hybrid structure from dye occluded in CuSCN grains under diffusion-limited dye precipitation to nanoscale segregated CuSCN and dye solid under that limited by the stability of the surface complex. The stoichiometry of the surface complex has empirically been identified as [CuSCN site][dye]_0.5, for which the stability constants are quantified as 71.9, 81.2, and 114.1 mol^–0.5 cm^2.5 for DAS⁺, MTS⁺, and CNS⁺, respectively. The increasing dipole moment and Mulliken charge at the methylpyridinium moiety, as supported by density functional theory calculations, result in the increase of the stability constant as well as the upward shift for the upper limit of the diffusion-limited dye loading, thereby the switching border.