Abstract
Coumarins (2H-chromen-2-one), owing to their rigid, planar structure and outstanding photophysical properties, such as high fluorescence quantum yields, large Stokes shifts, and tunable emission in the visible range, represent a highly versatile and privileged scaffold for constructing functional π-conjugated small molecules that has found widespread applications in organic microe;ectronics. Incorporating electron-donor groups at the C7 position (typically amino or hydroxyl groups) and electron-withdrawing groups at the C3 or C4 positions creates a push-pull architecture that induces intense intramolecular charge transfer (ICT), significantly red-shifting emission and enhancing light absorption efficiency. Unlike planar organic acceptors, embedding a three-dimensional o-carborane core as an electron-withdrawing group can generate novel ICT pathways associated with aggregation-induced emission (AIE) or solid-state luminescence. We found that o-carboranyllithium, prepared from o-carborane and n-BuLi, smoothly reacted with the corresponding 3-bromocoumarins to give 4-carboranylcoumarin fluorophores in good to high yields via a one-pot sequence of Michael addition followed by elimination of hydrobromide (Scheme 1). The photophysical study demonstrated that the prepared carboranylcoumarins emitted in the visible range in solution. In addition, these fluorophores exhibited solid-state luminescence with absolute quantum yields of up to 10%.