[3]Radialene Fluorophores with pH‐Switchable Emission and Stable Absorption Maxima

The synthesis and characterization of the first push–pull quino [3]radialene fluorophore are reported, integrating a bis(dicyanomethylene)-[3]radialene acceptor with a benzimidazole donor. It exhibits minimal solvatochromism, and vibronic coupling of the [3]radialene motif dominates the emission properties. Upon protonation, the fluorescence quantum yield rises from 22% to 71%, with a significant redshift.

The first push–pull quino [3]radialene fluorescent dye is reported. Herein, the novel bis(dicyanomethylene)-[3]radialene electron acceptor is connected to a benzimidazole donor. With protonation, a substantial redshift of fluorescence wavelength is observed, while the absorption maximum remains stable. This process is accompanied with an increased fluorescence quantum yield to about 70%. Further, the findings are explained by a combined experimental and theoretical approach, and it is found that vibronic coupling plays a crucial role. This study highlights the yet unexplored potential of [3]radialene-based motifs for the design of environment-responsive fluorophores.

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