pH-Responsive Switchable Aggregation Phenomena of Xanthene Dyes Adsorbed on Tungsten(VI) Oxide Colloid Surface

The surface interactions between xanthene (XN) dyes, including rhodamine B, rhodamine 3B, rhodamine 19, rhodamine 6G, rhodamine 110, and rhodamine 123, and tungsten(VI) oxide (WO 3 ) colloid particles were investigated. These XN dyes were strongly adsorbed as a monolayer onto the WO 3 surface via the electrostatic interaction between their peripheral cationic amino substituents and negatively surface-charged WO 3 colloid particles, and most of the ones adsorbed eventually formed stable π-stacked dimers. The geometry of dimers formed on the WO 3 colloid surface is discussed on the basis of the molecular exciton theory framework. Cationic XN dyes formed the approximately ideal face-to-face H -dimers on the WO 3 colloid surface, whereas zwitterionic ones had a higher tendency to form the oblique J -dimers. Additionally, we have experimentally demonstrated the possibility of pH-induced switching between H - and J -aggregation modes of the XN dye’s dimers formed on the WO 3 colloid surface. The findings lead to a better understanding of organic dye’s adsorption/aggregation behaviors on the metal oxide surface.