Single Crystalline, Non-Stoichiometric Hydrogen-Bonded Organic Frameworks Showing Versatile Fluorescence Depending on Composition Ratios and Distributions.

Hydrogen-bonded organic frameworks (HOFs) composed of multicomponent molecules in a non-stoichiometric composition have drawn great interest due to their tunable properties. However, the photobehavior of the single crystals of such mixed HOFs has not been explored. Here, we report on the synthesis, characterization and photobehavior of single crystalline non-stoichiometric HOFs (NS-HOFs). NS-HOFs (BN-1) with various composition ratios were successfully obtained as single crystals from two analogue tetratopic carboxylic acids, possessing naphthalene and benzothiadiazole cores (NTTA and BTTA, respectively). The heterogeneous distribution of the components was thoroughly confirmed by time-resolved fluorescence microscopy and local crystallographic analysis using focused synchrotron X-ray radiation. The versatile fluorescence of BN-1 behavior depends on the composition ratio and distribution of the component in the single crystals. We observed not only fluorescence bands with various colors such as purple, blue, green and white, depending on the composition ratios, but also different emission bands from a single crystal. We provide details on their emission lifetimes following the composition, emission color and targeted region on the crystal. This work is the first example of single crystal studies applied to organic porous co-crystals and demonstrates unique and versatile optical properties of carboxylic acid-based NS-HOFs. The results provide a concept of creating functional mixed porous materials capable of different and tunable optical properties.