Orientation and Order of Xanthene Dyes in the One-Dimensional Channels of Zeolite L: Bridging the Gap between Experimental Data and Molecular Behavior

Supramolecular organization in nanochannels is governed by both the nature of the channels and the size, shape, and charge of the guests, and may also depend on the cosolvent in the host. Oxonine (Ox⁺) and pyronine (Py⁺) cationic dyes in zeolite L (ZL) could provide important insight on this issue, but their orientation in the composite materials is not understood yet. Theoretical modeling of Ox⁺/ZL and Py⁺/ZL composites indicates that the 70–80° orientation, deduced from optical microscopy data but incompatible with host–guest geometry contraints, is not stable and reveals two possible orientations for these dyes in ZL nanochannels: perpendicular or parallel to the channel axis. In the hydrated composite, corresponding to optical microscopy experiments conditions, perpendicular Ox⁺/Py⁺ is the favored orientation, suggesting a larger portion of perpendicularly oriented dye molecules and a smaller fraction aligned. Optical microscopy data might therefore be reinterpreted as the vector sum of components arising from a population of molecules with nonuniform orientation. The stability order is reversed at dry conditions, indicating that the organization of xanthene dyes in ZL channels is governed by water. Our study explains this cosolvent-driven supramolecular organization regime, here unraveled for the first time, and highlights the role of molecular flexibility in switching the dye orientation.