A brief discussion of MOFs was introduced in the introduction section of the thesis. The last part of the thesis demonstrates the strategic approach to the synthesis of sulfamate-, sulfate- and hydroxy-exchanged MOF-808.
Sensing
The other two hydrogen atoms of the water molecules are not included in the least-squares refinement. The hydrogen atoms of the water molecules are not included in the least-squares refinement. The hydroxyl hydrogen atoms of the H2DOBDC2- ligands were not included in the least-squares refinement.
The emerging peak at around 260 nm in the UV-Vis spectrum of the MOF with TNP in chloroform indicates the formation of a stable GSC (Figure 2.7). The fluorescence spectra showed red shift (by 15 nm) of the emission peak in the case of TNP (Figure 2.16a). In the case of water, calculation of the overlap integral of the MOF emission and analyte absorption (Table 2.7) indicates the highest overlap integral value for TNP (Figure 2.11c).
The interactions within the pores were validated based on the PXRD pattern change in the presence of the analytes. At low analyte concentrations, no noticeable changes were observed in the powder X-ray diffraction (PXRD) patterns of the MOF (Figure 2.21). However, as the amount of analyte was increased, moderate changes in the PXRD patterns were noted (Figure 2.22).
The combination of these changes leads to an overall decrease in the mean fluorescence lifetime of the MOF. The fluorescence lifetime of the MOF in water remained almost the same even in the presence of the analytes, indicating that the behavior of the quenching process is static (Figure 2.23b and Table 2.8). Longer operating range of the RET process in water led to the more effective detection of TNP in water.
The acidity of the grafted SA depends on the way the SA binds to the metal centers.
Proton Conductive MOFs
Review and Analysis of Molecular Simulations of Methane, Hydrogen and Acetylene Storage in Metal_Organic Frameworks. Metal–organic frameworks (MOFs) have been widely used as fluorescence-based sensors for cations, anions, small molecules, explosives, and solvents due to their variability in pore size and easy modulation of the functional moiety to regulate host–guest interactions 1 - 4 .
Experimental section
Least squares refinement of the structural model was performed under geometric constraints such as DFIX and DANG. Least squares refinement of the structural model was performed under geometry constraints and displacement parameter constraints such as DFIX, DANG, FLAT and SIMU.
Results and discussion
Excellent stability in water and good quantum yield (0.14) of the MOF are promising for sensing NACs in water. The fluorescence spectra of the MOF in water were also recorded with excitation at 352 nm and monitored in response to the addition of analytes (Figure 2.5c and 2.6a). In this case, negligible change in the UV-Vis spectrum of the MOF with TNP was recognized compared to the UV-Vis spectrum of the MOF, only the convolution of the spectra of the MOF and TNP was noted (Figure 2.15).
As depicted in Figure 2.17, there is no spectral overlap between the emission of 1c and the absorption of the analyte in chloroform, which rules out the RET process in chloroform. Additionally, to check the role of the RET process in quenching, I/I0 was plotted against the overlap integral for TNP, DNP, and NP (Figures 2.18-2.20). Surprisingly, the fluorescence lifetime study of the MOF in chloroform in the presence of TNP showed decrease in the 'average' lifetime, which is a typical feature of dynamic quen ching (Figure 2.23a and Table 2.8).
Decrease in the τ3 component can be related to the formation of a TNP@MOF complex component, as deduced from the crystal structure. On the other hand, the lifetime reduction of the τ2 component may be due to change in the environment or in some non-interacting fluorophores. On the other hand, the r0 value of MOF was 0.09 with a rotation time constant of approx. 150 ps.
The results of time-dependent fluorescence anisotropy measurements of the MOF in water indicate that similar EM occurs among ordered fluorophores across the network of the MOF, as in the case of the MOF in chloroform. The sensitivity of the MOF to TNP in water is slightly higher than that in chloroform even though its quenching efficiency is slightly lower in water.
Conclusion
Light harvesting in microscale metal-organic frameworks by energy migration and interfacial electron transfer quenching. Selective and sensitive aqueous phase detection of 2,4,6-trinitrophenol (TNP) using an amine-functionalized metal-organic framework. Exploitation of guest accessible aliphatic amine functionality of an organometallic framework for selective detection of 2,4,6-trinitrophenol (TNP) in water.
Novel microporous metal-organic framework demonstrating unique selectivity for the detection of high explosives and aromatic compounds. The energy differences between the LUMO of MOF and the LUMO of the analytes are calculated using the Gaussian 09 package with B3LYP/6-31G* basis set under the polar continuum of the chloroform medium. Continuous increase in the population of the world has led to the development of renewable energy sources. 1 Proton exchange membrane fuel cells (PEMFC) are considered as one of the best prospects for green energy sources due to their higher energy density and less pollution. 2 Current state-of-the-art materials used in PEMFC if the proton exchange membrane is used, sulfonated fluoropolymer is called Nafion.3,4 However, due to its low operating temperature, poor cyclability and higher cost, there is an extreme urge to develop new proton exchange membrane (PEM) material.
For example, the use of unsaturated metal sites, increasing the acidity of the pores, anchoring or encapsulating proton carriers in the pores and post-synthetic modification of functionalities in the pores. proton carriers in the pores has emerged as a successful strategy to achieve a proton conductivity value of 10-2 S/cm or higher. The encapsulation of acidic ammonium-based cations in a MOF containing weak bases such as carboxylate, sulfate, or phosphate as a grafted functional group exhibits a synergistic effect of conjugate acid-base pairing for enhanced proton conduction.47-49 The simultaneous encapsulation of the above-mentioned weak bases together with the ammonium-based acids as acid-base pair in MOF is reported only once, i.e. in situ encapsulation of the acid-base pair during the reaction50. The double bond nature of the S-N bond of the SA in bidentate bond mode results in the amino protons being more acidic and leads to the more efficient proton conduction pathway via a hydrogen bonding network involving the more acidic amino protons.
Experimental section
After the completion of 2 days, solid was filtered and washed with water 4 times in 1 day. Solid was then quickly exchanged 4 times with acetone (for 1 hour) followed by exchange with chloroform for 4 times in duration of 1 day. Measurements were made in a two-electrode assembly with stainless steel discs as electrode and samples were held between them in the form of solid pellets.
The entire cell assembly was kept in a humidity chamber to control temperature and humidity. First, the effect of water on the samples was analyzed by keeping the temperature constant at 30 °C and varying the relative humidity (RH) between 60 and 95%.
Results and discussion
SA-grafting was performed by dispersing the activated MOF-808-OH-150 in an aqueous sulfamic acid solution for 24 h. Since there are six possible formic acid molecules that can be replaced in MOF-808-FA, if we use a di-ionic molecule such as SU, we can only replace three molecules, while we can use a mono-ionic molecule, possibly six molecules . offset by an increase in proton carrier density in the pore. The similar PXRD pattern and morphology of SA-grafted MOF-808 (MOF-808-SA) to that of MOF-808 indicates that the framework is stable after molecular exchange (Figures 3.2 and 3.4).
The reduced pore volumes and elemental analyzes of MOF-808-SA show the presence of SA ions within the pore. It is well known that SU ion prefers the bridging bidentate binding mode on zirconium metal ions both in MOF-808-2.5SU-15055 and on zirconium or metal oxide surface.56-57. The heat treatment of MOF-808-4SA-60 at 150 °C removes the ligated water molecule near the grafted SA in monodentate binding mode, and converts the SA in monodentate binding mode to the SA in bridging ligand binding mode.
The proton conductivity of all samples at 95% RH, with the exception of MOF-808-OH-150, showed a direct relationship with increasing temperature. While the proton conductivity of MOF-808-OH-150 first increases up to 50 °C and reaches the highest value of S/cm, and then decreases slightly as the temperature further increases. It is clear that for monodentate MOF-808-4SA-60, the activation energy is much higher due to the possible rotational motion of SA at higher temperatures.
But in the case of MOF-808-4SA-150 and MOF-808-2.5SU-150, where SA and sulfate ions are bound in a bidentate mode with higher stiffness, the activation energies are much lower compared to MOF-808-4SA-60. which also indirectly supports the existence of monodentate SA in the case of MOF-808-4SA-60. To compare the proton conductivity of grafted SA with that of grafted SU, we prepared MOF-808-2SA-150 with two SA molecules in the bridging bidentate binding mode and compared its proton conductivity with the conductivity of MOF-808-2.5SU-150 with 2.5 SU molecules in to the same bridging bidentate binding mode.
Conclusion
Modulation of proton diffusion and conductivity in metal–organic frameworks by incorporation of available free carboxylic acid groups. Oxalate-bridged bimetallic complexes {NH(prol)3}[MCr(ox)3](M) MnII, FeII, CoII; NH(prol)3+ = Tri(3-hydroxypropyl)ammonium) exhibiting coexisting ferromagnetism and proton conduction. High proton conductivity and spectroscopic studies of metal-organic framework materials impregnated by strong acids.
A new type of double chain-based 3D lanthanide(III) metal–organic framework exhibiting proton conduction and tunable emission. Grafting of free carboxylic acid groups on the pore surface of 3D porous coordination polymers for high proton conductivity. Encapsulation of Mobile Proton Carriers into Structural Defects in Coordination Polymer Crystals: High Anhydrous Proton Conductivity and Fuel Cell Applications.
Controlling crystalline proton-conducting pathways by water-induced transformations of hydrogen-bonding networks in a metal-organic framework J. High proton conductivity at low relative humidity in an anionic Fe-based metal-organic framework. Two-in-one: inherent anhydrous and water-assisted high proton conductivity in a 3D metal-organic framework.
