[3]Radialene fluorophores with pH-switchable emission and stable absorption maxima

We report on the first push-pull quino [3]radialene fluorescent dye. Here, the novel bis(dicyanomethylene)-[3]radialene electron acceptor was connected to a benzimidazole donor. With protonation a substantial red-shift 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, we explain the findings by a combined experimental and theoretical approach and find 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.