Distorted octahedral supramolecular cages based on spirobiindane with enhanced emission

Abstract

Although the structural framework for constructing regular polyhedral supramolecular cages from symmetric ligands is relatively well established, assembling distorted polyhedra using ligands of lower symmetry remains a considerable challenge. Herein, we report two distorted octahedral supramolecular cages, S1 and S2, based on a spirobiindane-derived rigid core with varying π-conjugation lengths and heteroatoms. The nearly perpendicular spirobiindane unit provides a rigid structural platform and promotes the formation of distorted cage-like structures. Octahedron S1 exhibited a high fluorescence quantum yield (Φ_F) of up to 72.87% in pure DMF, and the Φ_F increased to 96.24% in a DMF/H₂O mixed solvent with 20% water content, surpassing that of most reported metal–organic cages. In contrast, the π-extended and heteroatom system S2 shows progressively red-shifted and aggregation-induced emission enhancement; however, the strong electron-withdrawing effect of the phenazine unit and the enlarged cavity size likely result in a significantly lower Φ_F for S2 compared to S1. By reporting the synthesis of luminescent supramolecular cages with novel structures, this work further reveals how π-conjugation extension and heteroatoms modulate the emission behavior of supramolecular assemblies, thereby offering a new perspective for the rational design and construction of tunable luminescent supramolecular systems.