A Zn(II) coordination polymer and its photocycloaddition product: syntheses, structures, selective luminescence sensing of iron(III) ions and selective absorption of dyes.

One coordination polymer [Zn2(L)2(bpe)2(H2O)2] (1) (L = 4,4'-((1,2-phenylenebis(methylene))bis(oxy))dibenzoic acid; bpe = (E)-1,2-di(pyridin-4-yl)ethene) was prepared and structurally determined. Compound 1 has a chain structure in which its pair of bpe ligands is arranged in a head-to-tail manner with their C=C bonds being close enough for a [2 + 2] cycloaddition reaction. Upon exposure to UV light, compound 1 undergoes a single-crystal-to-single-crystal (SCSC) [2 + 2] photodimerization to generate one 2D coordination polymer [Zn(L)(rctt-tpcb)0.5(H2O)] (1a) (rctt (regio cis, trans, trans)-tpcb = tetrakis(4-pyridyl)cyclobutane). The tpcb ligands in the crystals of 1a show an intriguing in situ thermal isomerisation. The nanospheres of 1 can be obtained by recrystallization in DMSO/alcohol. The nanospheres of 1a can also be readily produced from the corresponding nanospheres of 1 by the photocyclodimerization method. Compared with those of 1a, the nanospheres of 1 display highly selective sensing of Fe(3+) ions over mixed metal ions through fluorescence quenching. Moreover, the nanospheres of 1a can rapidly adsorb CR (congo red), MB (methylene blue) or RhB (rhodamine B) over MO (methyl orange) from aqueous solutions. This work offers a new photoinduced post-synthetic method for the synthesis of multifunctional MOFs, which show luminescence sensing of Fe(3+) ions and dye adsorption properties.