A Family of Cd(II) Coordination Polymers Constructed from 6-aminopicolinate and Bipyridil co-Linkers: Study of their Growth in Paper and Photoluminescent Sensing of Fe3+ and Zn2+ ions

In this work, we report on five novel coordination polymers (CPs) based on the linkage of the [Cd(6apic)2] building block [where 6apic = 6-aminopicolinate] by different bipyridine-type organic spacers to form different coordination compounds with the following formulae: [Cd(µ-6apic)2]n (1), {[Cd(6apic)2(µ-bipy)]·H2O}n (2), {[Cd(6apic)2(µ-bpe)]·2H2O}n (3), [Cd(6apic)(µ-6apic)(µ-bpa)0.5]n (4) and {[Cd2(6apic)4(µ-tmbp)]·7H2O}n (5) [where bipy = 4,4´-bipyridine, bpe = 1,2-di(4-pyridyl)ethylene, bpa = 1,2-di(4-pyridyl)ethane (bpa) and tmbp = 1,3-di(4-pyridyl)propane]. Most of the synthetized compounds form infinite metal-organic rods through the linkage of the building block by the bipyridine-type linker, except in the case of compound 4 whose assembly forms a densely packed 3D architecture. All compounds were fully characterized and photoluminescent properties were studied experimentally and computationally through density functional theory (DFT) calculations. All compounds display, upon UV excitation, a similar blue emission of variable intensity depending on the linker employed for the connection of the building unit, among which compound 2 deserves to be highlighted for its room temperature phosphorescence (RTP) with an emission lifetime of 32 ms that is enlarged to 79 ms at low temperature. These good photoluminescence properties, in addition to its stability in water in a large pH range (between 2-10), motivated us to study compound 2 as a sensor for the detection of metal ions in water, showing high sensitivity for Fe3+ by means of fluorescence turn-off mechanism and unspecific turn-on response for Zn2+, respectively. Furthermore, the compound is processed as a paper-based analytical device (PAD) in which the phosphorescent emission is preserved, improving the sensing capacity toward the mentioned metal ions.