Extreme Confinement Effects on the Incorporated Dyes in Metal–Organic Frameworks

Confinement of fluorescent dyes is known to enhance fluorescence properties by reducing aggregation and restricting molecular motion, but few studies have attempted to modulate the extent of confinement. In this work, we explored extreme confinement by exploiting the rigid structure of metal–organic frameworks (MOFs). Other than the commonly known restriction of peripheral substituents in fluorescent molecules for aggregation-induced emission (AIE)-like effects, the more powerful confinement surprisingly led to buckling of the chromophore core, leading to reduced fluorescence lifetime. We name these effects buckling-induced quenching (BIQ). By studying 14 dyes in zeolitic-imidazolate framework 8 (ZIF8), we systematically analyzed their confined behaviors, establishing strong correlations: The reduction of chromophore planarity always leads to a decrease of fluorescence lifetimes, whereas reduction in the longest dimension of the confined molecule, while maintaining chromophore planarity, always leads to an increased lifetime. Confinement in the larger cavities of ZIF71 leads to signs of alleviation, in good agreement with our hypotheses. The BIQ effects provide an important complement for the well-known confinement effects, and the extreme confinement serves also as an important reference for more subtle effects in various applications.