B ← N Lewis Pair Fusion of N,N-Diaryldihydrophenazines: Effect on Structural, Electronic, and Emissive Properties

Doping of polycyclic aromatic hydrocarbons (PAHs) with boron and/or nitrogen is emerging as a powerful tool to tailor the electronic structure and photophysical properties. As N-doped analogues of anthracene, N,N-dihydrophenazines play important roles as redox mediators, battery materials, luminophores, and photoredox catalysts. Although benzannulation has been used successfully as a structural constraint to control the excited state properties, fusion of the N-aryl groups to the phenazine backbone has rarely been explored. Herein, we report the first examples of dihydrophenazines, in which the N-aryl groups are fused to the phenazine backbone via B←N Lewis pair formation. This results in structural rigidification, locking the molecules in a bent conformation, while also modulating the electronic structure through molecular polarization. B─N fusion in BNPz1-BNPz3 induces a quinoid resonance structure with significant C─N(py) double bond character and reduces the antiaromatic character of the central pyrazine ring. Borylation also lowers the HOMO/LUMO (highest occupied/lowest unoccupied molecular orbital) energies and engenders bathochromic shifts in the emission. Further rigidification in the solid state gives rise to enhanced emission quantum yields, consistent with aggregation-induced emission enhancement (AIEE) observed upon water addition to solutions in tetrahydrofuran (THF). The demonstrated structural control and fine-tuning of optoelectronic properties are of great significance to potential applications as emissive materials and in photocatalysis.