Design and synthesis of phosphonium ionic liquids exhibiting strong fluorescence in various solvents and liquid or glassy state

Abstract

Liquid fluorescent ILs (LFILs) are rare materials that exhibit strong photoluminescent properties in the pure liquid state without the need for a solvent. While ILs are known for their optical properties in solution, fluorescence in the pure liquid state is typically weak or absent due to quenching phenomena. LFILs are extremely scarce in the literature, and only a few have been reported with absolute quantum yield (AQY) values to assess their fluorescence efficiency in the pure liquid state. A series of phosphonium ILs (PILs), containing fluorescent 5-(dimethylamino)-1-naphthalenesulfonate (DNS^-) and 9,10-anthraquinone-2-sulfonate (AQS^-), were synthesized as potential soft photoluminescent materials in solution and neat liquid state in very good to excellent yields. Additionally, mono and diphosphonium chloride ILs were prepared via an improved workup process to afford ILs with high purity (>99 %) compared to traditional methods. All fluorescent PILs possessed high thermal stabilities (T_d = 332–383 °C) as determined by thermogravimetric analysis (TGA). The differential scanning calorimetry (DSC) thermograms for the PILs-DNS and PILs-AQS revealed that all of them were ILs, with most being RTILs and the triphenylphosphonium containing PILs existing as glassy ILs (T_g = 21.13 and 30.13 °C) at room temperature. Their photoluminescent properties in the solution state were studied in various organic solvents, with the PILs-DNS possessing AQY values upwards of 0.95 in solution and [P_666,10]DNS exhibited an impressively high AQY of 0.35 in the liquid state. The PILs-AQS did not possess strong photoluminescent properties in solution and no fluorescence in the liquid or glassy state. The PILs-DNS and PILs-AQS demonstrated excellent photostability, exhibiting no significant photobleaching. Study of these novel PILs containing anionic fluorophores is necessary to contribute towards the scarcity of LFILs and offer prospects in a wide range of fields, including chemical/physical sensing, optoelectronics, and biological imaging.