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

Doping of 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. While 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 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 THF. The demonstrated structural control and fine‐tuning of optoelectronic properties is of great significance to potential application as emissive materials and in photocatalysis.