Identification of an Overlooked Halogen-Bond Synthon and Its Application in Designing Fluorescent Materials

Research on new supramolecular synthons facilitates the progress of materials design. Herein, the ability of sp(2) carbonyl oxygen atoms to act as halogen-bond acceptors was established through cocrystallization. Four sets of carbonyl compounds, including aldehydes, ketones, esters, and amides, were selected as halogen-bond acceptors. In the absence of strong hydrogen bonds, 14 out of 16 combinations of halogen-bond donors and acceptors could form cocrystals, whereby the supramolecular synthon C=O center dot center dot center dot X acts as the main interaction. Further, the geometric parameters of the C= O center dot center dot center dot X interaction were statistically revealed on the basis of the crystallographic database. The bifurcated interaction mode that has been observed in other halogen-bond synthons rarely occurs in the case of C=O center dot center dot center dot X. The robustness of C=O center dot center dot center dot X makes its application in crystal engineering possible and opens up new opportunities in designing multicomponent fluorescent materials, as indicated by multicolor emission of cocrystals D through C=O center dot center dot center dot X interactions.