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

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(NO 3 ) 2 ·6H 2 O 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 ( 1 H & 13 C), 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 : Ni 2+ ) 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 Zn 2+ , Cu 2+ and Hg 2+ . The emission intensity of MG is quenched with Cu 2+ /Hg 2+ but enhances with Zn 2+ in 1 : 1 ( MG : M 2+ ) stoichiometry. Therefore, MG mimics a sequence dependent molecular keypad lock for Cu 2+ ( C ), Hg 2+ ( H ) and Zn 2+ ( 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. Schiff base HL forms stable Ni( ii )-gel MG (differ from Ni( ii )-complex 1 ) with triethylamine that serves as reversible ON-OFF-ON fluorescent probe for Zn 2+ /Cu 2+ /Hg 2+ and as sequence dependent molecular keypad lock to protect secret information.