Boron Monofluoride Complexes Bearing π-Expanded Coumarin Subunits: Facile Synthesis, Crystal Structure, and Solid-State Enhanced Emission.

Boron-containing compounds have received special interest over the last few years owing to their favorable photophysical characteristics, stimulating their use as fluorescent sensors, in bioimaging and mechanofluorochromic materials. However, their synthesis is often time-consuming and requires specialized protocols. Here, we report the design and synthesis of a novel class of monofluoroborate compounds featuring π-expanded coumarin subunits, with simple access in relatively high yields, and investigate, by computational and experimental approaches, their photophysical properties in solution and in the solid state. These compounds exhibit intense absorption in the 350-450 nm range and show fluorescence in the solid state, while are not luminescent in solution. The absence of emission in fluid solution is ascribed to ultrafast radiationless decay via conical intersection, as suggested by computational data. Ultrafast pump-probe transient absorption spectroscopy in fluid solution confirms the excited-state computed data, indicating that the excited state decays within few picoseconds to the ground state. The structural rearrangement needed for reaching the conical intersection is inhibited in the solid state thanks to the presence of dimers with restricted geometrical flexibility, as evidenced by crystallographic studies and calculations, giving rise to solid-state emission.