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

Arene-fused o -carboranes represent a class of solid-state luminescent materials. Here we report a transition-metal-free method for constructing naphthyl- and biphenyl-fused o -carboranes via intramolecular nucleophilic substitution of the B(3)–H bond by in situ 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.