Boron-Doped Polycyclic π-Electron Systems with an Antiaromatic Borole Substructure That Forms Photoresponsive B-P Lewis Adducts

Heteroatom doping is a powerful strategy to alter the electronic structure of polycyclic aromatic hydrocarbons (PAHs). Especially boron doping endows PAH scaffolds with electron-accepting character and Lewis acidic centers. Herein, we report that embedding a five-membered borole ring into a polycyclic skeleton imparts the pi-system with antiaromatic character and thereby induces unique properties and behavior. A series of borole-embedded pi-conjugated compounds were synthesized from teraryl precursors via a borylation/intramolecular electrophilic C-H borylation sequence. The obtained compounds exhibit planar structures with distorted geometries around the boron center and form columnar slipped face-to-face pi-stacked structures. Among these compounds, a pyrene-fused derivative shows an intense emission with a high quantum yield in solution. This compound also exhibits high Lewis acidity, which reflects the antiaromatic character and strained structure of the borole substructure. This compound forms a Lewis acid-base adduct even with weakly Lewis basic phosphorus-containing polycyclic pi-systems. Analyzing the crystal structure of the thus-obtained adduct revealed a complex between the boron- and phosphorus-embedded pi-systems with a direct B-P dative bond. This complex undergoes photodissociation in the excited state and exhibits an emission exclusively from the base-free borole-embedded pi-system.

Automated summary

Incorporating a five-membered borole ring into a polycyclic aromatic hydrocarbon imparts the pi-system with antiaromatic character, inducing unique properties and behavior. Synthesized borole-embedded pi-conjugated compounds exhibit planar structures and columnar face-to-face pi-stacked structures. One derivative shows intense emission with high quantum yield and high Lewis acidity.