Precise Fusion of Conjugated Organoboranes: A Tetraboron-Doped Molecular Carbon with Narrowband Fluorescence and Microscale Lasing

Manipulating heterocyclic topologies is a powerful method to alter the electronic structures and achieve desirable excited-state properties of molecular carbons. Here, we disclose precise fusion of conjugated organoboranes as an efficient strategy to construct sophisticated boron-doped molecular carbons. This enables the successful synthesis of two tetraboron-doped molecular carbons via one-pot multifold Scholl reaction. They possess the C68B4 and C76B4 polycyclic π-skeletons, respectively, featuring the upper-/lower-edged boron-doping mode and ribbon lengths exceeding 2.3 nm, thus representing the longest boron-doped molecular carbons reported so far. Comprehensive studies reveal their distinctive electronic structures, excited-state dynamics, and properties. Notably, both molecules exhibit narrowband red fluorescence (full width at half-maximum < 24 nm), unexpected stimulated emission, and amplified spontaneous emission characteristics, which have been rarely integrated into a single organic molecule. We fabricated a proof-of-concept whispering-gallery-mode resonator based on the C76B4 molecule, which displays remarkable microscale lasing performance with a lasing wavelength of 725/731 nm, a threshold of 23.2 μJ cm-2, and a quality factor of >1800. This represents the first realization of lasing in boron/carbon-composited π-systems, highlighting the potential of boron-doped molecular carbons as optical gain media for organic lasers.