Orientational Behavior of Single Molecules in Molecular Sieves:  A Study of Oxazine Dyes in AlPO₄-5 Crystals

Abstract

Three differently sized oxazine dye molecules, oxazine-1, oxazine-170, and oxazine-750, with molecular widths of 0.85, 1.0, and 1.1 nm, respectively, have been built into the one-dimensional pores of AlPO 4 -5 with a pore diameter of 0.73 nm during a template assisted synthesis. There is evidence that a deformed oxazine-1 molecule can tightly fit into these pores, whereas the larger molecules produce defect sites. The orientations of the used dyes were measured with polarization dependent fluorescence confocal microscopy. Ensemble measurements on the samples containing the oxazine dyes in high concentration reveal a preferential orientation of the molecules along the main axis of the crystal. However, the ensemble measurements cannot reveal the orientational distribution function of tilt angles. Therefore, this distribution function was obtained by measuring the orientations of many individual molecules using single molecule spectroscopy. The thinnest molecule, oxazine-1, shows a Gaussian type distribution of tilt angles with a half width at half-maximum of ξ = 18°. The medium sized oxazine-170 shows a broader Gaussian tilt function with ξ = 48°, whereas the broadest molecule oxazine-750 does not show a preferential orientation. These results clearly show that there is a structure directing influence of the host on the guest molecules in dependence of their size, even if the molecule is larger than the pore size. Additionally, distinct orientational jumps of individual oxazine-1 molecules were observed. Dynamic events such as these cannot be revealed by conventional ensemble spectroscopy.