A multi-cation responsive Ni(ii)-supramolecular metallogel mimics a molecular keypad lock via reversible fluorescence switching

Abstract

Synthesis of a bidentate N,O-donor Schiff base fluorescent ligand 5-(diethylamino-2-((4-(diethylamino-2-((4-(diethylamino)phenylimino)ethyl)phenol) (HL) adopting a new preparation procedure and its complexes with Ni(II) (1) and Zn(II) (2) has been illustrated. Structures of HL and 1 have been elucidated using X-ray single crystal analysis. Moreover, HL leads to the formation of a mechanically stable Ni(II)-gel (MG) upon treatment with Ni(NO₃)₂·6H₂O in the presence of triethylamine (TEA) using THF/MeOH (1 : 1) solvents at rt. The gelator HL, complexes 1–2 and MG have been characterized by different spectroscopic and microscopic techniques including NMR (¹H & ¹³C), FT-IR, ESI-MS, SEM, powder-XRD, rheology, UV/vis and fluorescence analysis. Rheological studies suggested good mechanical and thermal stability, whereas SEM analysis reveals a porous earth crust-like morphology of MG. Notably, 1 : 1 complexation between HL and Ni(II) forms a stable gel (MG), whereas 2 : 1 (HL : Ni²⁺) complexation leads to partial gelation. Formation of the Ni(II)-MG leads to slight “Turn-OFF” fluorescence relative to HL with a limit of detection (LOD) of 7.76 × 10^–9 M; however, MG is considered as the “ON” state due to moderate emission. Remarkably, Ni(II)-MG further displayed reversible “ON–OFF–ON” fluorescence switching behavior through detection of Zn²⁺, Cu²⁺ and Hg²⁺. The emission intensity of MG is quenched with Cu²⁺/Hg²⁺ but enhances with Zn²⁺ in 1 : 1 (MG : M²⁺) stoichiometry. Therefore, MG mimics a sequence dependent molecular keypad lock for Cu²⁺ (C), Hg²⁺ (H) and Zn²⁺ (Z) to give the maximum output. Association and quenching constants were calculated by the Benesi–Hildebrand method, and from the Stern–Volmer plot the LOD was determined to be 4.2 × 10^–6 M, 5.8 × 10^–6 M and 7.8 × 10^–6 M for MG with Zn(II), Cu(II) and Hg(II), respectively. To date, Ni(II) based MGs have been explored only toward electrochemical, thermal and conduction studies; however, the present work demonstrates the fluorescent reversible cation detection behavior of Ni(II)-MG to act as a molecular keypad lock for development of password protection devices.