Facile Construction of a New Hybrid Conjugate via Boron Cage Extension

Abstract

Aromatic polycyclic systems have been extensively utilized as structural subunits for the preparation of various functional molecules. Currently, aromatics-based polycyclic systems are predominantly generated from the extension of two-dimensional (2D) aromatic rings. By contrast, polycyclic compounds based on the extension of three-dimensional (3D) aromatics such as boron clusters are less studied. Here we report three types of boron cluster-cored tricyclic molecular systems which are constructed from a 2D aromatic ring, a 3D aromatic nido-carborane and an alkyne. These new tricyclic compounds could be facilely accessed by Pd-catalyzed BH activation and the subsequent cascade heteroannulation of carborane and pyridine with an alkyne in an isolated yield of up to 85% under mild conditions without any additives. Computational results indicate that the newly generated ring from the fusion of the 3D carborane, the 2D pyridyl ring, and an alkyne is non-aromatic. However, such fusion not only leads to 1H chemical shift considerably downfield shifted owing to the strong diatropic ring current of the embedded carborane, but also devotes to new/improved physicochemical properties including increased thermal stability, the emergence of a new absorption band, as well as a largely red-shifted emission band and enhanced emission efficiency. Besides, a number of bright, color-tunable solid emitters spanning over all visible light are obtained with absolute luminescence efficiency of up to 61%, in contrast to aggregation-caused emission quenching of Rhodamine B containing a 2D-aromatics fused structure. This work demonstrates that the new hybrid conjugated tricyclic systems might be promising structural scaffolds for the construction of functional molecules