Luminescent cadmium(II) coordination polymers of 1,2,4,5-tetrakis(4-pyridylvinyl)benzene used as efficient multi-responsive sensors for toxic metal ions in water

Reactions of Cd(NO₃)₂·4H₂O with 1,2,4,5-tetrakis(4-pyridylvinyl)benzene (4-tkpvb) and 5-tert-butylisophthalic acid (5-tert-H₂BIPA), 1,3,5-benzenetricarboxylic acid (1,3,5-H₃BTC) or 1,4-naphthalenedicarboxylic acid (1,4-H₂NDC) under solvothermal conditions afforded three two-dimensional (2D) Cd(ii) coordination polymers [Cd(4-tkpvb)(5-tert-BIPA)]_n (1), [{Cd(4-tkpvb)(1,3,5-HBTC)}·0.5DMF]_n (2) and [Cd(4-tkpvb)(1,4-NDC)]_n (3). Compounds 1-3 were structurally characterized by IR, elemental analysis, powder X-ray diffraction, and single crystal X-ray diffraction. Compounds 1-3 possess unique 2D networks in which 1D double chains [Cd₂L₂]_n (L = 5-tert-BIPA or 1,3,5-HBTC) (1-2) or 1D linear chains [Cd(1,4-NDC)]_n (3) are linked by 4-tkpvb ligands. Upon UV light excitation, a 4-tkpvb solution in DMF showed an emission band centered at 446 nm with a shoulder at 475 nm. The addition of Hg²⁺ ions into the 4-tkpvb solution in DMF remarkably changed its colour from colourless to yellow under natural light, or from blue to grey yellow under UV light, which were clearly visible to the naked eye. Compounds 1-3 suspended in water could emit yellow-green light under UV light irradiation. The representative compound 1 was confirmed to be an uncommon multi-responsive luminescent sensor for Hg²⁺, CrO₄^2- and Cr₂O₇^2- ions in water by the luminescence quenching method. The detection limits for these species were 0.15 μM (Hg²⁺), 0.08 μM (CrO₄^2-) and 0.12 μM (Cr₂O₇^2-), respectively. The luminescence quenching mechanism studies revealed that these quenching processes were involved in either the interaction of Hg²⁺ with free pyridyl groups in 1 or the overlap between the absorption band of CrO₄^2- or Cr₂O₇^2- and the excitation and/or emission bands of 1.