Novel 4,4'-Fluoresceinoxy Bisphthalonitrile Showing Aggregation-Induced Enhanced Emission and Fluorescence Turn off...

A novel 4,4'-fluoresceinoxy bisphthalonitrile FPN is synthesized from fluorescein and 4-nitrophthalonitrile by aromatic nucleophilic ipso nitro substitution reaction. The structure of FPN constitutes phthalonitrile-fluorescein-phthalonitrile, acceptor-donor-acceptor, A-D-A form and the solvatochromic study of newly synthesized compound FPN was done in hexane, cyclohexane, CHCl₃, DCM, DMF, acetonitrile, ethanol and in methanol. The aggregation behavior of FPN was investigated in good-poor solvent mixture DMF-water in various proportions and the molecule was found to be exhibiting Aggregation Induced Emission Enhancement AIEE for volume percentage of water beyond 50% with a significant hypsochromic shift of 70 nm in the emission maxima from 458 to 388 nm. This phenomenon is termed as Aggregation Induced Blue Shifted Emission Enhancement AIBSEE and was reported in substituted phthalonitrile for the first time. The chemo sensing activity of FPN with various transition metal ions also has been checked by fluorescence spectroscopy where the new molecule FPN exhibited fluorescence turn OFF behaviour towards Fe³⁺ ion in acetonitrile-methanol ACN-MeOH solution. The binding stoichiometry of FPN with Fe³⁺ was verified by Job's plot analysis and Density Functional Theory DFT-B3LYP computational methodology by using Gaussian 09 software. Graphical Abstract A novel 4,4'-fluoresceinoxy bisphthalonitrile FPN was synthesized by base catalyzed coupling reaction of fluorescein with 4 nitrophthalonitrile. The newly synthesized compound has been characterized using UV-vis, FTIR, 1HNMR, fluorescence spectral data and single crystal X-ray diffraction studies. The compound FPN was found to be exhibiting positive solvatochromism in various organic solvents and Aggregation Induced Blue Shifted Emission Enhancement AIBSEE in DMF-aqueous mixture when water volume exceed above 50%. As a metal ion chemo sensor FPN exhibited a selective fluorescence turn OFF behavior towards Fe3+ ion in the stoichiometric ratio 1:2 in acetonitrile-methanol mixture and limit of detection LOD of Fe3+ by FPN was calculated to be 3.665 μM.