Hydrogen-bonded organic framework of rigidly branched fluorophore: guest-adaptive cavity and phase-dependent light emission

We report a rational design of luminescent molecular materials that display dynamic behavior in the solid state. Stimuli-responsive porous materials constructed at the molecular level are finding useful applications in sensing, storage, and separation. However, it is challenging to create such materials since the molecules tend to pack closely, preventing the creation of void spaces. To address this challenge, we have invented a letter X-shaped luminescent molecule that exploits quadruple C–H⋯N hydrogen bonds (H-bonds) to construct highly branched 1D assemblies. In polar solvents that can compete with weak H-bonds, the molecule adopts interdigitation rather than 1D assemblies. Regardless of the primary mode of intermolecular interactions, the molecules cannot find shape complementarity with each other, resulting in porous networks held together by weak van der Waals forces. A combination of spectroscopic, single-crystal, and powder X-ray crystallographic studies revealed that one polymorph exhibits reversible crystal-to-crystal transformations that can be triggered by external stimuli and optically read through visually discernible changes in the fluorescence. We further elucidate the underlying molecular mechanism of this optical response, which is driven by changes in the molecular conformation and aggregation modes, tilting the alignment angle of the H-bonded substructure.