Arene-Fused <i>o</i> -Carboranes via C–B(3) Bond Formation: Synthesis and Properties

Arene-fused <i>o</i>-carboranes represent a class of solid-state luminescent materials. Here we report a transition-metal-free method for constructing naphthyl- and biphenyl-fused <i>o</i>-carboranes via intramolecular nucleophilic substitution of the B(3)-H bond by <i>in situ</i> generated aryllithium intermediates under mild conditions. Single-crystal X-ray diffraction confirmed B(3)-C(aryl) bond formation, affording novel five- and six-membered fused motifs. The resulting derivatives exhibit weak or negligible emission in solutions but display strongly red-shifted and markedly enhanced fluorescence in the solid state. Naphthyl-fused species show particularly high photoluminescence efficiencies, with quantum yields up to 56%, attributable to stronger intermolecular interactions that restrict vibrational relaxation. DFT calculations reveal that C-B fusion stabilizes LUMO energies and narrows HOMO-LUMO gaps relative to C-C fused analogues, consistent with their absorption profiles. NICS analyses further establish higher aromaticity in the naphthyl-fused series. This work introduces an accessible route to π-extended carborane-arene hybrids and delineates their structure-property relationships, underscoring their potential as solid-state luminescent materials.