Three new turn-on fluorescent sensors for the selective detection of Zn²⁺: Synthesis, properties and DFT studies

The design and synthesis of three new 1,8-naphthalimide-based fluorescent sensors (1–3) for the detection of Zn²⁺ in aqueous solution is described. The structural architect of these sensors contains 1,8-naphthalimide scaffold as a fluorophore attached to 2,2′-dipicolylamine (DPA) and bis(2-quinolinylmethyl)amine (DQA) receptors through an amide linkage. The addition of Zn²⁺ to the solutions of sensors (1–3) led to enhanced fluorescence intensity, ranging between 2.5 and 14 folds. At physiological pH (pH = 7.4), these sensors exhibited high selectivities for Zn²⁺ over a wide range of competing metal cations, displaying high sensitivities with a limit of detections of 120, 81.7 and 79.2 nM, respectively. This suggests that these sensors can detect chronic Zn²⁺ concentration for freshwater (>1.84 μM), designated by the U.S. Environmental Protection Agency. DFT simulations performed on the more stable stacked conformations of unbound and Zn²⁺ bounded states suggested that the latter display higher density of excited states than the unbound sensors. Moreover, the stacked conformer of sensor 3 was significantly more stable as compared to sensors 1 and 2, which was attributed to a stronger Van Der Waals (VDW) interaction between DQA and 1,8-naphthalimide. The Zn²⁺ binding leads to enhanced electronic coupling between the HOMOs and LUMOs, making excited states more populated which then undergoes geometric relaxation before emitting light and relaxing back to the ground states. The lower energy separation (5.0 eV) between the HOMO and the first Zn²⁺ d-orbital in sensor 3 as compared to sensors 1 and 2 results in enhanced density of the generated states and subsequently higher intensity upon binding with Zn²⁺.