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

Abstract

The hydro(solvo)thermal reactions of Zn(NO₃)₂·6H₂O, 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) [Zn₂(1,4-bdc)(1,4-Hbdc)₂(NI-bpy-34)₂] (1, 1,4-H₂bdc = benzene-1,4-dicarboxylic acid), [Zn₂(2,6-ndc)(2,6-Hndc)₂(NI-bpy-34)₂]·H₂O (2, 2,6-H₂ndc = naphthalene-2,6-dicarboxylic acid), and [Zn(Hbtc)(NI-bpy-34)(H₂O)]·H₂O (3, H₃btc = 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 (4⁴6²)(4⁴6¹¹) 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 × 10³ M⁻¹), 3-nitrophenol (95% quenching, K_sv = 1.61 × 10⁴ M⁻¹), and nitrobenzene (87% quenching, K_sv = 3.91 × 10³ 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 Zn²⁺ ions in CP 3 were partially substituted by Cu²⁺ ions by about 45% via single-crystal to single-crystal (SCSC) transformations. The resulting Cu²⁺-exchanged material 3-(Cu : Zn) displays decreased fluorescence emission due to paramagnetic fluorescence quenching caused by d⁹ configuration Cu²⁺ ions. Further, the exchange kinetics have been studied.