New Fluorescent Conjugates Displaying Solvatochromic Properties

Four new fluorescent dyes based on bis(phenyl‐ethynyl‐)‐2‐naphthyl ( BPEN ) were designed and synthesized. To improve the solvatochromic property and enhance the brightness of the fluorescent BPEN , an electron‐donating unit of azetidine and/or an electron deficient group of ‐ NO 2 was introduced. The corresponding fluorophores are denoted as T1 , T2 , T3 and T4 , respectively. Moreover, to facilitate derivatization of the probes, two ethoxy carbonyl residues were grafted onto the side positions of BPEN . Spectroscopic studies demonstrated that introduction of azetidine leads to superior solvatochromic properties and largely enhanced fluorescence quantum yields as evidenced by the fact that T3 shows more than 150 nm shift in its maximum emission when dissolved in solvents of very different polarities and displays high fluorescence quantum yields in the solvents studied. However, T2 , which is the one bearing a ‐ NO 2 group, is non‐fluorescent. Theoretical analysis and Lippert‐Mataga modeling revealed the intra‐molecular charge transfer ( ICT ) nature of the solvatochromic behavior of the compounds. Further test reveals that the fluorophores, in particular T3 , are sensitive to the presence of trace water in less polar solvents, such as THF and 1,4‐dioxane. Moreover, it is believed that the new fluorophores may serve as building blocks for creating environment‐sensitive fluorescent sensors.