Evaluating Host-Guest Interactions in a Metal-Organic Framework Using a Polarity-Sensitive Probe

We report in situ observations of molecular interactions, and photo-dynamics for the polarity-sensitive fluorescence dye 4-dimethylamino-4'-nitro-stilbene (DNS) when encapsulated inside of metal-organic frameworks (MOFs) by utilizing single-molecule, single-particle fluorescence spectroscopy. While the fluorescence intensity from DNS molecules adsorbed onto the surface of MOF-177 particles exhibited a strong solvent polarity dependence toward external solvent exchanges, the intensity from molecules encapsulated inside of well-defined MOF-177 particles slightly changed when the solvent was switched from toluene to chloroform. Furthermore, the photostability of the DNS molecules was significantly enhanced by incorporation into the MOF host. These observations strongly suggest that guest molecules encapsulated in MOFs should be protected from external solvent and gas molecules. The photoblinking behavior of encapsulated single DNS molecules was also observed and analyzed in terms of the probability density.