Abstract Control over topological edges of molecular carbons (MCs) is of importance for achieving diverse molecular topologies and desirable physical properties. However, it remains very challenging for heteroatom‐doped MCs due to the synthetic difficulty. Herein, we report control over the edge structures of boron‐doped MCs (BMCs) via the sequential cyclization strategy. Three BMC molecules that feature the C56B2 or C84B2 polycyclic π‐skeletons with selective cove/fjord or cove/bay edges, respectively, were synthesized through the rational combination of Mallory photoreaction and Scholl reaction. We not only obtain the largest boron‐doped π‐system reported so far, but also disclose that fine control of their edges and length greatly affects electronic structures and thereby photonic properties of BMCs, such as tunable aromaticity, decreased band gaps, as well as redshifted absorptions and fluorescence. Remarkably, the C56B2 molecule exhibits stimulated emission behavior and amplified spontaneous emission property, both of which have never been reported for pristine boron‐doped π‐systems, thus demonstrating the potential of BMCs as optical gain materials for laser cavities.
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