Helical Single Crystals of Porous Hydrogen-Bonded Organic Frameworks as a Candidate for Morphologically Functional Organic Materials.
Abstract
Modulation and control of the crystalline morphology of porous materials are important for their functionality yet remain underdeveloped. In this study, we report the first example of a porous hydrogen-bonded organic framework (HOF) with helical crystalline morphology. The helical HOF was prepared using pyrene-based tetracarboxylic acid. The formation of helical-shaped and needle-shaped crystals of the HOFs can be controlled by the solvent evaporation rate and temperature during heating. X-ray diffraction analysis indicates that both crystals have the same layered porous structure. Based on these data and theoretical calculations, we propose the hypothesis that certain structural defects are locally generated, which induces nonuniform crystal growth and consequently results in the formation of the helical crystals. The obtained HOFs exhibit thermal stability and permanent porosity with Brunauer-Emmett-Teller (BET) surface area up to 1602 m2 g-1. Interestingly, the helical HOFs exhibited dynamic expansion and contraction through the desorption and adsorption of a guest solvent. Moreover, doping with a second component in the helical HOF enables the modulation of fluorescence properties and crystalline morphology into a double helix. These results highlight the potential of a new category of organic materials: helical porous crystals and morphologically functional organic materials.