Devising Mixed-Ligand Based Robust Cd(II)-Framework From Bi-Functional Ligand for Fast Responsive Luminescent Detection of Fe3+ and Cr(VI) Oxo-Anions in Water With High Selectivity and Recyclability

Environmental issue related applications have globally surfaced as hottest areas of research, wherein luminescent metal-organic frameworks (LMOFs) with functionalized pores put unique signature in real-time monitoring of multiple classes of toxic compounds, and overcome many of the challenges of conventional materials. We report a two-fold interpenetrated, mixed-ligand Cd(II)-organic framework (CSMCRI-11) [Cd1.5(L)2(bpy)(NO3)]·DMF·2H2O (CSMCRI = Central Salt and Marine Chemical Research Institute, HL = 4- (1H-imidazol-1-yl)benzoic acid, bpy = 4,4′-bipyridine) that exemplifies bipillar-layer structure with two different Cd(II) nodes, and displays notable robustness in diverse organic solvents and water. Intense luminescence signature of the activated MOF (11a) is harnessed in extremely selective and fast responsive sensing of Fe3+ ions in aqueous phase with notable quenching constant (1.91 × 104 M−1) and impressive 166 ppb limit of detection (LOD). The framework further serves as a highly discriminative and quick responsive scaffold for turn-off detection of two noxious oxo-anions (Cr2O7 2− and CrO4 2−) in water, where individual quenching constants (CrO4 2−: 1.46 × 104 M−1; Cr2O7 2−: 2.18 × 104 M−1) and LOD values (CrO4 2−: 179 ppb; Cr2O7 2−: 114 ppb) rank among best sensory MOFs for aqueous phase detection of Cr(VI) species. It is imperative to stress the outstanding reusability of the MOF towards detection of all these aqueous pollutants, besides their vivid monitoring by colorimetric changes under UV-light. Mechanism of selective quenching is comprehensively investigated in light of absorption of the excitation/emission energy of the host framework by individual studied analyte.


23.
ICP analysis results and analysis S14 24. Table S2. ICP analysis results for samples S14 25. Table S3. Calculation of standard deviation of fluorescence intensity and limit of Detection for 11a towards Fe 3+ S14 26. Table S4. Calculation of standard deviation of fluorescence intensity and limit of detection for 11a towards Cr2O7 2-S15 27. Table S5. Calculation of standard deviation of fluorescence intensity and limit of detection for 11a towards CrO4 2-S15 28. Table S6. A comparison of quenching constants and corresponding LODs for various luminescent MOFs used for detection of Fe 3+ S16-S17 29. Table S7. A comparison of quenching constants and corresponding LODs for various luminescent MOFs used for detection of Cr2O7 2-/CrO4 2-S17-S18

Physical measurements
The infrared spectra (IR) of the samples were recorded using the KBr pellet method on a Perkin-Elmer GX FTIR spectrometer in the region of 400-4000 cm-1. Powder X-ray diffraction (PXRD) data were collected using a PANalytical Empyrean (PIXcel 3D detector) System equipped with Cu Kα (λ=1.54 Å) radiation. Microanalyses of the compounds were Conducted using elementary vario MICRO CUBE analyser. Thermogravimetric analyses (TGA) (heating rate of 5 °C/min under N2 atmosphere) were performed with a Mettler Toledo Star SW 8.10 system. The solvent-exchanged (methanol-exchanged) frameworks were then degassed overnight under vacuum at 120 °C to generate 11a. UV-Vis spectra recorded using Shimadzu UV-3101 PC spectrometer and the luminescence experiments were performed at room temperature using a Fluorolog Horiba Jobin Yvon spectrophotometer.

Chemicals
Analytical grade cadmium nitrate hexahydrate Cd(NO)3·4H2O (AR) and 4,4ˊ-bipyridine were purchased from Tokyo Chemical Industries private limited. All the solvents such as N, Nʹdimethylformamide (DMF) (Fisher Scientific), methanol (S. D. Fine Chemicals, India), were purchased and used without any further purification. Ligand (1H-imidazol-1-yl)benzoic acid was synthesised as mentioned in SI. All the metals salts used for the sensing experiments were procured commercially and used with any further analysis.

Single Crystal X-ray Crystallography
Single crystals with suitable dimensions were chosen under an optical microscope and mounted on a glass fibre for data collection. Intensity data for as synthesized colorless crystals of CSMCRI-11 were collected using graphite-monochromated MoKα (λ=0.71073 Å) radiation on a Bruker SMART APEX diffractometer equipped with CCD area detector at 173 K, The linear absorption coefficients, scattering factors for the atoms, and the anomalous dispersion corrections were taken from International Tables for X-ray Crystallography. The data integration and reduction were performed with SAINT 1 software. Absorption corrections to the collected reflections were accounted with SADABS 2 using XPREP. 3 The structure was solved by direct method using SIR-97 4 and was refined on F 2 by the full-matrix least-squares technique using the SHELXL-2014 5 program package. All H atoms were placed in calculated positions using idealized geometries (riding model) and assigned fixed isotropic displacement parameters using the SHELXL default. To give an account of disordered electron densities associated with solvent molecules, the "SQUEEZE" protocol in PLATON 6 was applied that produced a set of solvent free diffraction intensities. Final cycles of least-squares refinements improved both the R values and Goodness of Fit with the modified data set after subtracting the contribution from the disordered solvent molecules, using SQUEEZE program. The crystal and refinement data for solvent free CSMCRI-11 is listed in Table S1. Figure S1. (a) Asymmetric unit of CSMCRI-11 and (b) demonstration of π-π stacking interactions between the benzene rings of ligand in the structure.

Synthesis of ligand and CSMCRI-11
The ligand 4-(1H-imidazol-1-yl) benzoic acid was prepared by slight modification of the following known procedure. 1 Scheme S1. Synthetic scheme of ligand.
In a round bottom flask 8g of K2CO3(activated), 2.1g of imidazole and 3.63g of 4fluorobenzonitrile were dissolved in anhydrous DMF. Solution was stirred under N2 atmosphere and heated at 130 °C for 24 h. Afterwards, mixture was poured in ice cold water and allowed to stand for 24 h. The white ppt were filtered and was dissolved in ethanol (50ml) followed by dropwise addition of 6 N KOH solution (50 ml) and it was refluxed at 80 °C for 12 hours.
(a) (b) Figure S15. Spectral overlap between absorbance spectra of (a) cations (b) anions and absorption spectra/emission spectra of 11a in water.