Luminescent Zn() coordination polymers as efficient fluorescent sensors for highly sensitive detection of explosive nitroaromatics

The hydro(solvo)thermal reactions of Zn(NO 3 ) 2 ·6H 2 O, N -(pyridin-3-yl)-4-(pyridin-4-yl)-1,8-naphthalimide (NI-bpy-34) and aromatic polycarboxylic acids afforded the corresponding zinc coordination polymers (CPs) [Zn 2 (1,4-bdc)(1,4-Hbdc) 2 (NI-bpy-34) 2 ] ( 1 , 1,4-H 2 bdc = benzene-1,4-dicarboxylic acid), [Zn 2 (2,6-ndc)(2,6-Hndc) 2 (NI-bpy-34) 2 ]·H 2 O ( 2 , 2,6-H 2 ndc = naphthalene-2,6-dicarboxylic acid), and [Zn(Hbtc)(NI-bpy-34)(H 2 O)]·H 2 O ( 3 , H 3 btc = benzene-1,3,5-tricarboxylic acid). CP 1 features a 1D ladder structure with lateral arms which expands into a two-fold interweaved supramolecular 2D + 2D → 2D network through H-bonds. CP 2 adopts a complicated 2D thick-layer structure which expands into a 3D H-bonded (4,6)-connected net with a (4 4 6 2 )(4 4 6 11 ) topology with three-fold interpenetration. CP 3 adopts a 2D grid-like (4,4)-layer structure. The three CPs all emit strong blue fluorescence in toluene suspension which is effectively turned off in the presence of a variety of electron-deficient aromatic nitro compounds. The quenching efficiencies and the Stern-Volmer quenching constants ( K sv ) both indicate that 1,4-dinitrobenzene (79% quenching, K sv = 2.11 × 10 3 M −1 ), 3-nitrophenol (95% quenching, K sv = 1.61 × 10 4 M −1 ), and nitrobenzene (87% quenching, K sv = 3.91 × 10 3 M −1 ) represent the highest quenching responses for CPs 1-3 , respectively, for which the limits of detection were calculated to be 8.45 (1.42), 2.47 (0.34), and 5.14 (0.63) μM (ppm), respectively. Thus, CPs 1-3 can be used as promising fluorescence sensors for the highly selective and sensitive detection of aromatic nitro compounds. The mechanism of the quenching sensing of aromatic nitro compounds can mainly be explained by a photoinduced electron transfer process. On the other hand, through metal-ion exchange, the framework Zn 2+ ions in CP 3 were partially substituted by Cu 2+ ions by about 45% via single-crystal to single-crystal (SCSC) transformations. The resulting Cu 2+ -exchanged material 3 -(Cu : Zn) displays decreased fluorescence emission due to paramagnetic fluorescence quenching caused by d 9 configuration Cu 2+ ions. Further, the exchange kinetics have been studied. Zn CPs 1-3 exhibit strong blue/cyan fluorescence emissions in toluene suspension for highly selective and sensitive detection of explosive nitroaromatics through remarkable fluorescence quenching responses. In addition, the framework Zn 2+ ions in 3 were partially substitute