Schiff Base Pillar-layered Metal-organic Frameworks: From Synthesis to Applications

ABSTRACT Pillar-layered metal-organic frameworks (PL-MOFs) are advanced crystalline porous substances built using metal ions, oxygen- and nitrogen-donor linkers. The pillar-layered strategy emerged as a very effective way to incorporate the Schiff base functionalities into pore cages of MOFs by using N-donor Schiff base pillar ligands. The Schiff base functionalities provide active sites for enhanced performance in sensing, catalysis, adsorption, extraction, and gas separation areas. Although the PL-MOFs have gained remarkable advances in the last few decades, Schiff base PL-MOFs have been reviewed to a lesser extent up to this point. In this way, a review summing up their performance is profoundly expected. This review covered the recent developments connected with Schiff base PL-MOFs, including synthesis and applicability in various potential fields. It also included the challenges and forthcoming pathways for fulfilling the research and development needs of Schiff base PL-MOFs. ABSTRACT


Introduction
Metal-organic frameworks (MOFs) are an attractive porous materials owing to their high surface area, high surface-to-volume ratio, and tunability. [1][2][3][4] Pillar-layered MOFs (PL-MOFs) recently appeared as a subclass of MOFs, composed of metal centers connected by O-donor ligands and pillared by N-donor ligands. [5,6] In opposition, MOFs are mainly comprised of a single sort of ligands (either O-donor or N-donor). The pillared strategy brings noteworthy adaptability to MOFs concerning pore properties (i.e., diameter, size, topology, and surface area, etc.) and pore environment (i.e., functionalization). [7,8] In contrast, single linker MOF systems are unable to provide such kind of regulation over the pore environment as two different kinds of functional groups cannot be introduced within the same pore cage by using a single sort of organic linkers decorated with the one particular type of functional moieties. Contrarily, pillar MOFs provide a better design of the pore environment as they are amenable to the introduction of variety of functional groups within the same pore cage of MOFs to allow the high degree of controllability of host-guest interactions. And this can be easily achieved by using O-donor linkers and N-donor pillar linkers decorated with the different functional moieties, which makes the pillar MOFs a better applicant for a desired application. [7] An extensive range of comparable PL-MOFs having similar functionalities, but tunable pore properties may be generated by varying the length of pillar linker. [9] Pillar linker length in particular differs from 2.7 Å to 15.6 Å for creating the PL-MOFs. [10][11][12][13] It is worth mentioning that the use of longer pillars usually produced unstable MOFs because their structure collapsed upon guest removal due to the instability of large open MOF pores. However, there are certain previous reports available in the literature on stable MOFs prepared using longer pillar ligands. [14,15] For instance, two stable Schiff base PL-MOFs, i.e., TMU-25 and TMU-26, were prepared from longer Schiff base pillar ligand, namely, N 4 ,N 4' -bis(pyridin-4-ylmethylene)-biphenyl-4,4'diamine. Interestingly, by using such a long Schiff base pillar ligand the two metal ions are separated by 24.282(5) Å for TMU-25 and 24.286(5) Å for TMU-26. [10] Notably, the stability of such long pillars ligand based MOF frameworks can be enhanced by the intermolecular interactions (e.g. hydrogen bonding and π-π stacking) between adjacent pillar linkers and by the framework interpenetration, which make the whole frameworks more stable. [14,15] Such sort of variation in pore properties through variation of pillar linker length is helpful to generate the library of high performance of PL-MOFs for desired applications. In addition, the pore environment of comparable PL-MOFs might also be varied for their upgraded performances. This was achieved by varying the specific functional moieties on pillar linkers through their proper planning and designing. [16][17][18] These specific functional moieties act as an active sites which regulate the hydrophilicity/hydrophobicity, polarity, and acidity/basicity etc. of the resultant PL-MOFs. [19][20][21] Interestingly, further specificity of PL-MOFs for a desired application can also be enhanced by introducing same or different functional moieties on both O-and N-donor linkers. [22] Due to presence of these well-designed active functional moieties and improved pore properties, PL-MOFs showed enhanced performance compared to single linker MOF materials for numerous prospective areas like catalysis, gas separation, supercapacitors, adsorption, and molecular sensing. [7,[23][24][25][26][27][28] To date, different functional moieties including imine, heterocyclic rings, amide etc. have been introduced into pore cages of PL-MOFs. [22,[29][30][31] In recent years, Schiff base PL-MOFs were established as the most significantly explored research area and many scientific experts attempted to plan and integrate such sort of MOFs using variety of N-donor Schiff base pillar ligands to achieve the specific functions. The reason being that Schiff bases are inexpensive and facilely synthesized via one-step reaction process. [32] In addition, Schiff base functionalities provide active sites for upgrade performance in sensing, catalysis, adsorption, and gas separation areas. To the best of the author's knowledge, there is no review specifically focusing on Schiff base PL-MOFs. The goal of this review is to provide the most updated critical overview of Schiff base PL-MOFs utilized for the diverse applications. Finally, this review also provided the proposed future perspectives for further enhancement of Schiff base PL-MOFs.

Synthetic strategies for Schiff base PL-MOFs
Schiff base PL-MOFs can be prepared by varying synthetic methodologies, such as slow evaporation, diffusion, refluxing, solvothermal, mechanochemical, ultrasonic, solvent-assisted linker exchange (SALE), and layer-by-layer (LBL) (Figure 1). Table 1 summarizes the different Schiff base PL-MOFs including their synthetic strategy, ligands (for structure refer to Figures 2  and 3), and applications. In the slow evaporation method, solvent in which precursors (O-and N-donor linkers and metal salt) were dissolved was evaporated at room temperature. [83] However, this method requires long time despite poor crystallinity of the resultant materials. To improve the crystallinity, single crystals of Schiff base PL-MOFs were synthesized by the diffusion method. In this method, the solution of organic linkers is slowly diffused into the solution of metal salt. [73] Again this method is a timeconsuming process and gives a poor yield. To overcome these shortcomings, bulk quantities of Schiff base PL-MOFs were prepared via a reflux method in which a combination of metal salt and organic linkers was heated at 70°C in a round-bottom flask. [73] This method is simple, convenient, and fast compared to the aforementioned methods for synthesis of bulk amounts of Schiff base PL-MOFs.
In case of solvothermal method, a combination of metal source and organic linkers were heated in a sealed reactor under autogenous pressure. The major advantage of this method is the comparatively higher yield and better crystallinity. However, to achieve the desired temperature and pressure conditions special instruments like autoclaves are required. [84][85][86] In addition, the mechanochemical and ultrasonic methods, which are economical (costeffective) and eco-friendly synthetic approaches, results in the synthesis of Schiff base PL-MOFs in a short duration of time (10-60 min). [87,88] The mechanochemical method avoid the use of toxic organic solvents. [89,90] The ultrasonic method resulted the generation of nanostructures of Schiff base PL-MOFs with reduced particle size and high surface area. [69,91] Further, for upgrading the properties of Schiff base PL-MOFs, a postsynthetic modification method known as SALE was also chosen. [92] In this method, a new Schiff base PL-MOF can be assembled by replacement of Schiff pillar ligand of parent MOF with another Schiff base pillar ligand. Although this method produces MOFs with better functionalities and improved surface area, it is timeconsuming and can create ligand missing "defects" in the resulting Schiff base PL-MOF. LBL method has been used for development of Schiff base PL-MOF thin films. In this method, MOF thin films can be created by sequential dipping of functionalized organic-surface e.g., natural silk fiber into metal ion solution and organic linker solution. [74,93,94] The silk fiber surface having extremely lot of reactive carboxylic (-COOH) moieties has been preferred for deposition of MOF thin film.   [33] 2. TMU-16 L 3 L 4 Solvothermal Fluorescent sensor for Fe 3+ and Cd 2+ ions [34] Adsorbent for removal of PHP azo dye [35] Adsorption of cationic dyes [36] Adsorption of MO dye [37] Reversible adsorption of iodine [38] Catalyst for preparation of pyrano[2,3-d]pyrimidines [39] 3. TMU-5S H 2 L 5 L 4 Solvothermal Ratiometric fluorescent sensor for Ca 2+ [40] Ratiometric fluorescent sensor for TNP [41] 4. TMU-17-NH 2 H 2 L 6 L 2 Solvothermal Fluorescent sensor for Fe 3+ ions [42] Catalyst for the preparation of tetrahydro-chromenes [43] 5.  [46] Reflux Catalyst for the carbon dioxide fixation of olefins [47] Mechanochemical 8.

Applications and performance of Schiff base PL-MOFs
Schiff base PL-MOFs had been implemented in numerous areas like sensor, catalysts, photocatalysts, extraction sorbents, adsorbents, and gas separators owing to their active functional Schiff base sites and porous nature ( Figure 4). The accompanying subsections are devoted to the outline of various significant achievements accomplished in the diverse application areas of Schiff base PL-MOFs.

Sensors
Schiff base PL-MOFs were utilized as an excellent sensors. Schiff base group as target-specific identification sites, such as hydrazone (-CH = N-NH-CO-), azine-methyl (-CR = N-N = CR-; R = CH 3 ), azine (-CH = N-N = CH-), and imine (-CH = N-), moieties etc. selectively identify the goal analytes including heavy metal ions, anions, and organic molecules etc. Predominantly, nitrogen atoms in these identification sites interact with a goal analytes through hydrogen bonding and complexation interactions to boost the detection potential of PL-MOF sensors. Then again, incredible porosity and the high surface area of Schiff base PL-MOFs assist these identification moieties to engage promptly with goal analytes to upgrade the sensing ability. Table 2 summarizes the sensing performance of the different Schiff base PL-MOFs sensors. The subsection has been dedicated to outline the role of Schiff base sites in Schiff base PL-MOF sensors for detection of heavy metal ions, anions, and organic molecules etc. Further, this subsection has also been devoted to provide the quick overview of diverse detection mechanisms through which Schiff base PL-MOFs respond toward target analytes to get better insights of mode of action of Schiff base PL-MOF sensors. These particulars would be helpful for designing the new Schiff base PL-MOF sensors for diverse target analytes in future.

Metal ion sensors
Elevated level of Al 3+ resulting in Alzheimer's and Parkinson's diseases, bone softening, and memory damage. A Schiff base PL-MOF, [Cd(L 1 )(L 2 ) 1.5 ]·DMF (L 1 = 4,4′-methylenebis(3-hydroxy-2-naphthalene-carboxylic acid; L 2 = 1,4-bis (4-pyridyl)-2,3-diaza-2,3-butadiene) having 0.56 µM detection limit (DL) has been developed to selectively detect Al 3+ in water. [33] Such detection was based on the collapse of MOF framework upon interaction with Al 3+ that was confirmed through presence of ligand-oriented emission band at 483 nm in the fluorescent region of aluminum incorporated Schiff base PL-MOF. Exposure of Cd 2+ , being toxic and carcinogenic can damage liver and kidney etc. therefore its identification is significantly important. Zn 2 (L 3 ) 2 (L 4 )]⋅3DMF (coded as TMU-16; L 3 = 1,4-benzene-dicarboxylic acid; L 4 = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene) showed turn-on sensing behavior toward Cd 2+. [34] The Cd 2+ ions had been selectively coordinated by the nitrogen atoms of azine-methyl moieties of L 4 , resulting in turn-on sensing of TMU-16. Calcium participates in the contraction of muscle cells, construction of bones, as well as a cofactor for coagulation. Consequently, identification of the Ca 2+ level could be vital. A rhodamine B dye-sensitized Schiff base PL-MOF, TMU-5S (TMU-5 formulated as [Zn(L 5 )(L 4 ) 0.5 ] n ·(DMF) 1.5 ; H 2 L 5 = 4,4'-oxybisbenzoic acid) showed DL of 0.017 µM used to locates Ca 2+ in the aqueous phase. [40] The fluorescence enhancement, accompanied by bluemovement of emissions of rhodamine B dye as well as of MOF structure from 583 to 575 nm and 485 to 465 nm, respectively was observed upon association of Ca 2+ with azine-methyl moiety of L 4. [40] Iron participates in oxygen transport, regular working of central nervous system, DNA and RNA synthesis. Iron deficiency causes anemia while its excess causes conjunctivitis, retinitis, and choroiditis. Hence, detection of the iron is very important. Up to this point, Fe 3 + examination using various Schiff base PL-MOF sensors has been done successfully. Among these, TMU-16 appeared as great sensor with very low DL of 0.2 µM for Fe 3+. [34] Such selectivity of TMU-16 for ferric was laid out based on the coordination connection of azine-methyl sites of the Schiff base with Fe 3+ , which triggers turn-off emission of TMU-16. [34] [Cd(L 1 )(L 2 ) 1.5 ]·DMF became the succeeding sensor for identification of ferric in water possessing DL of 0.3 µM. [33] The binding of Fe 3+ ions with the -OH moieties of L 1 linker can bring about the quenching of emission of [Cd(L 1 )(L 2 ) 1.5 ]·DMF. In addition, competition absorption (CA) for excitation energy plays an additive role in detection. Zn(L 6 ) (L 2 )]·2DMF (coded as TMU-17-NH 2 ; H 2 L 6 = 2-aminoterephthalic acid) Schiff base PL-MOF ranks third as a good sensor which identify ferric in dimethyl formamide with a DL of 0.7 µM. [42] The Fe 3+ ions weakly associates with nitrogen atoms of azine and amine moieties of L 2 and H 2 L 6 ligands, respectively of TMU-17-NH 2 . [42] Such weak association declined the effectiveness of energy transfer from L 2 and H 2 L 6 ligands to Zn 2+ nodes, which in turn lowered the intensity of emission peak of TMU-17-NH 2 . Likewise, {[Cd(L 6 )(L 7 )]·2H 2 O} n (DL = 1.77 μM) isonicotinohydrazide), have also been fit correctly for particular recognition of Fe 3 + in aqueous medium. [44] The binding of hydrazone moieties of Schiff base ligand L 7 and amine moieties of H 2 L 6 with Fe 3+ ions reduced the energy shift efficiency from these ligands the framework nodes (Zn 2+ or Cd 2+ ), accordingly reduced the emission intensity of these PL-MOFs. [44] Lead, in abnormal concentrations can cause headache, loss of memory, mental retardation in children, and poor attention. Schiff base PL-MOFs, i.e., {[Zn(L 6 )(L 7 )]·H 2 O} n (DLs of 0.2 μM) and {[Cd(L 6 ) (L 7 )]·2H 2 O} n (DL of 0.1 μM) were additionally observed to be an exceptional particular sensors for determination of Pb 2+ in water. [44] In these Schiff base PL-MOFs, the amino group of H 2 L 6 and hydrazone functionality of L 7 work as a coordinating site for Pb 2+ . Hence, these PL-MOFs work as turn-off detectors for lead ions.

Anions sensors
Dichromate ( [46] Such selectivity might be justified by good overlap among emission spectra of these Schiff base PL-MOFs and absorbance spectra of Cr 2 O 7 2− /CrO 4 2anions. Also, electron transfer happen from hydrazone moiety of Schiff base ligand (L 9 ) to the these anions recommended   [49] (Continued) photo-induced electron transfer (PET) as significant quenching pathway. [46] In some other record Parmar

Response mechanisms of Schiff base PL-MOFs
Owing to the high crystallinity of Schiff base PL-MOF sensors, the detection or response mechanisms for these sensors can be deeply studied and profoundly understood thanks to the advanced X-ray characterization techniques. In this subsection, we present the three frequently involved mechanisms, including photoelectron transfer (PET), fluorescence resonance energy transfer (FRET), and competition absorption (CA) which are mainly responsible for the fluorescence signal modulation of Schiff base PL-MOFs upon interaction with target analytes. Here, we provided the detailed overview of conditions that should be fulfilled for these mechanisms to operative. This section informed the readers how the properties of Schiff base PL-MOFs (like excitation and emission energy range and the energy level of the molecular orbitals of the ligands of the Schiff base PL-MOFs) played a key role in the selective detection of target analyte. This knowledge would be useful for designing the Schiff base PL-MOF sensors in future by controlling their properties.

Photoelectron transfer (PET).
It is well known that the Lewis basic and/ or electron rich nitrogen atoms of Schiff base recognition sites and conjugated rings of O-donor or Schiff base ligands in Schiff base PL-MOFs are able to interact with target analytes including explosives, metal ions, anions, and aromatic compounds, through diverse interactions such as coordination, hydrogen bonding, and π-π staking interactions. Such a strong host-guest interactions may facilitate the PET process upon photo excitation. In general, PET is an excited-state charge-transfer process in which a photoelectron is transferred from the excited donor to the ground-state acceptor. [96] It becomes possible for the photoelectron to be transferred from the donor to the ground-state acceptor instead of relaxing back to the ground state, only when the energy level of the lowest unoccupied molecular orbital (LUMO) of the donor is higher than that of the acceptor, which, if occurs, will result in fluorescence quenching of the donor. Density functional theory (DFT) calculations are usually employed to analyze the LUMO energy levels of donors and acceptors. In most cases, molecular orbital calculations are consistent with the observed fluorescence quenching. As illustrated in Figure 5, when the CB (conduction band, or LUMO) of the Schiff base PL-MOF lies above the LUMO of the analyte, photoelectrons can transfer from the Schiff base PL-MOF to the analyte efficiently. Through this process, the luminescence of the Schiff base PL-MOF may be quenched and that can be deemed as a response signal for the analyte. Generally, the lower the LUMO energy level of the analyte, the easier the photoelectron transfer, and thus, the higher the quenching efficiency. For instance, higher the quenching efficiency of the analyte higher the selectivity of Schiff base PL-MOF toward that particular analyte. This response mechanism has been identified in the detection of explosives and organic contaminants using Schiff base PL-MOF sensors.

Fluorescence resonance energy transfer (FRET).
Another frequently used fluorescence sensing technique is FRET, in which an acceptor is promoted to an excited state while a simultaneously excited donor returns to the ground state through a nonradiative energy transfer to acceptor. [98] This nonradiative energy transfer results in the donors emission being quenched and the acceptors emission being enhanced. Its effectiveness is typically influenced by two factors: (i) the degree of spectral overlap, or the emission and absorption spectra of the host and guest and (ii) the dipole-dipole interaction, or the distance and relative orientation of the host and guest. [99,100] If the UVvis absorption spectrum of the guest analyte molecules and the Schiff base PL-MOFs emission spectrum overlap, (spectral overlap can be experimentally determined), FRET may happen and cause the MOFs fluorescence to be quenched ( Figure 6). Besides the extent of spectral overlap, the efficiency of energy transfer can also be modulated through the host-guest dipole-dipole interaction. This mechanism has been used for sensing of inorganic ions (e.g., Cr 2 O 7 2and CrO 4

Competition absorption (CA).
One more often involved mechanism through which Schiff base PL-MOF sensors selectively respond toward target analyte is the competition absorption. This mechanism is operative only when the analytes absorption spectra overlaps with the Schiff base PL-MOF excitation spectra (Figure 7). [101] In this mechanism, the Schiff base PL-MOF and the analyte are likely to compete with one another for the excitation light. It makes sense that the analyte absorbs excitation light of the Schiff base PL-MOF would reduce the total amount of energy available to the MOF for excitation, leading to less populated excited states, and subsequently the Schiff base PL-MOF fluorescence being quenched. This mechanism has been used for sensing of inorganic ions (e.g., Cr 2 O 7 2and Fe 3+ ) by Schiff base PL-MOF sensors as discussed earlier.

Catalysts
Schiff base PL-MOFs were emerged as an excellent heterogeneous catalysts. Most of these MOFs are water stable and give better performance in ecofriendly water solvent. These can avoid the use of toxic organic solvents. Particularly, nitrogen atoms of active catalytic sites (act as Lewis bases), including imine, azine, azine-methyl, and hydrazone groups etc. in these MOFs can efficiently catalyze the varied organic reactions. To compare the  [97] with permission from the Royal Society of Chemistry.
catalytic efficiency of these catalysts, turnover frequency (TOF) is used. TOF value is defined as the number of moles of substrate converted per mole of catalyst per hour. [102][103][104] TOF value can be calculated by dividing the percentage yield divided by the mole percent of the catalyst divided by the time of reaction in hour. The subsection has been dedicated to outline the role of Schiff base sites in Schiff base PL-MOF catalysts to catalyze the Knoevenagel condensation, carbon dioxide fixation, heterocyclic synthesis etc.

Knoevenagel condensation catalysts
The Knoevenagel condensation is a very useful C = C bond-forming methodology to form an α,β-unsaturated carbonyl compounds by reaction of an activated hydrogen compound and a carbonyl compound. [105,106] Numerous homo and heterogeneous catalysts have been reported, however frequently need a high reaction temperature and suffer from low stability. A few research groups have employed Schiff base PL-MOFs to catalyze such type of reactions due to their inherent properties (e.g., thermally and chemically stable features, excellent surface area, large surface area to volume ratio, uniformly distributed catalytically active and readily accessible Schiff base active sites, and structural diversity). It is worth mentioning here that Schiff base groups (e.g., hydrazone (-CH = N-NH-CO-), azine-methyl (-CR = N-N = CR-; R = CH 3 ), azine (-  (Table 3). [6,53,[59][60][61] To compare catalytic efficiency of the aforementioned catalysts, the Knoevenagel condensation of malononitrile with benzaldehyde has been studied as model reaction. Among these, TMU-55 and HTMU-55 catalysts were best and second best performer with their corresponding TOF values of 2,376 and 2,088 h −1 , respectively. [59] The existence of more basic azine-methyl group in TMU-55 compared with HTMU-55 responsible for its better performance. [59] The relative ordering of TOF between these Schiff base PL-MOF systems is organized for synthesis of benzylidenemalononitrile as TMU-5 ( (Table 3). [6,53,[59][60][61] The TOF-based . Reproduced from Ref. [97] with permission from the Royal Society of Chemistry. evaluation indicates that type of Schiff base sites in these PL-MOFs plays a central role for their catalytic efficiency. Catalytic comparison between TMU-4, TMU-5, and TMU-6 can be explained on the basis of electron lone pair-lone pair repulsion on adjacent nitrogen atoms of azine group (in TMU-4 and TMU-5) that increased the reactivity of azine group (as Lewis basic sites) in the catalytic reaction. [53] Though these repulsions are absent in TMU-6 in which two imine nitrogen atoms of Schiff base pillar ligand are separated by the phenyl ring. In addition to electronic repulsion, TMU-5 has 3D, narrow interconnected pores (pore size of about 4.4 × 6.2 Å) because of presence of methyl groups near basic center significantly improve the interaction between substrate and basic sites and responsible for their high catalytic activity compared to TMU-4 (pore size of about 5.3 × 9 Å) and TMU-6 (pore size of about 9.1 × 8.9 Å) having 1D, large pores. [53] In contrast, {[Zn(L 16 )(L 7 )]·2H 2 O} n and {[Cd(L 16 )(L 7 )]·2H 2 O} n showed the comparable catalytic activity as TMU-5. The reason behind that both these MOFs act as bifunctional catalysts as it involves two catalytically active sites; metal of framework as Lewis acid sites and amide group of Schiff base ligand as basic sites. [60] A possible mechanism for the Knoevenagel condensation reaction catalyzed by Schiff base PL-MOFs is shown in Figure 8. The nitrogen atoms of Schiff base groups (e.g., azine) present freely in the pore cages of Schiff base PL-MOFs, activated the active hydrogen compounds (e.g., malononitrile, ethyl acetoacetate etc.) via abstraction of highly acidic proton to generate the nucleophile. [61] The generated nucleophile is the active species which could attack the carbonyl group of aldehyde derivative to produce the Knoevenagel condensation products. It should be mentioned here that Lewis acidic metal sites bearing Schiff base PL-MOFs have been also widely reported to promote the Knoevenagel condensation through simultaneous activation of carbonyl group of an aldehyde derivative by Lewis acidic sites and malononitrile by the Lewis basic Schiff base sites from a mechanistic standpoint. Such type of Schiff base PL-MOF catalysts having both acidic and basic sites are known as synergistic or cooperative catalysts because in these catalysts both acidic and basic sites cooperate together to promote the Knoevenagel condensation.

Carbon dioxide fixation catalysts
Six Schiff base PL-MOF catalysts including ZnMOF-NH 2 , ZnMOF-1,  (Table 4). [45,47,65,66] To compare catalytic efficiency of these aforementioned catalysts, the carbon dioxide fixation of styrene to styrene oxide was studied as model reaction. Among these, ZnMOF-NH 2 catalyst was best performer with corresponding TOF value of 11 h −1 . The presence of hydrazone and amine functionalities on the linkers act as catalytic sites in ZnMOF-NH 2 which were responsible for its better performance. The relative ordering of TOF between these Schiff base PL-MOF systems is organized for synthesis of styrene oxide as ZnMOF-NH 2 ( (Table 4). [45,47,65,66] The TOF-based evaluation indicates that functional groups present on organic linkers of Schiff base PL-MOF catalysts plays an integral role in enhancement of catalytic role in carbon dioxide fixation.
From the mechanistic point of view, the CO 2 fixation reaction involved three main basic steps: epoxide ring opening, CO 2 insertion, and ring closure to form the five-membered cyclic carbonate. Based on above reports, the mechanism for the CO 2 fixation catalyzed by Schiff base PL-MOFs (e.g., CoMOF-1' containing both Lewis acidic metal sites and Lewis basic Schiff base sites) was proposed and presented in Figure 9. [65] As clear from the Figure 9, the hydrogen bonding of an oxygen of an epoxide to the Lewis basic -NH sites of the hydrazone group of the Schiff base ligand increases the electrophilicity at the carbon atom of ring which is thus activated toward nucleophilic attack. In addition, the Lewis acidic metal sites of Schiff base PL-MOFs may also activate the epoxide ring via coordination interactions. After activation, the nucleophile Br − anion generated from tetra-butyl ammonium bromide attacked the less-hindered carbon atoms of both the hydrogenbonded and coordinated epoxides leading to the ring opening of epoxides. This ring opening generated the oxygen anion of the opened epoxy ring which quickly interacts with the nearby polar CO 2 molecules to form an alkyl carbonate anion, and subsequent ring closure step takes place to produce the corresponding cyclic carbonate and catalyst get regenerated for the next cycle.  [61] It is worth mentioning here that the existence of basic functional sites such as -NH in the hydrazone moiety of Schiff base ligand of the Schiff base PL-MOFs can increase the CO 2 capture, which in turn can favor chemical fixation of CO 2 . Overall, it can be inferred that the Lewis acidic metal sites and Lewis basic Schiff base sites within the Schiff base PL-MOFs efficiently promote the CO 2 fixation reactions to achieve the satisfactory product yields.

Degradation catalysts
Ye et al. offered the Schiff base PL-MOF for the selective oxidative catalytic degradation of methyl orange (MO) in 2015-a cobalt, 5-hydroxyisophathalic acid (L 19 ), and L 4 based single crystal, microcrystals, and nanocrystals MOF [Co(L 4 )(L 19 )] ∞ . [67] A comparison between single crystals and micro-/nanocrystals, revealed the latter have better catalytic degradation efficiency. The micro-/nanocrystals more effectively promote the selective catalytic degradation of MO with 90% degradation achieved only in 40 min. [67] The selectivity of this MOF toward the MO dye was due to presence of N = N group which is easily oxdisable. Wang  Oxidative degradation efficiency of Nibased MOF (88% selectively degrade the MO) was reported to be higher than Co-based MOF (70% selectively degrade the MO). [68] The difference of degradation efficiency clearly revealed that metal of isostructural framework of both catalysts was responsible for their catalytic activity.

Photocatalysts
Schiff base PL-MOFs were also emerged as an excellent photocatalysts ( Table 5). The subsection has been dedicated to outline the role of Schiff  [65] base sites in Schiff base PL-MOF photocatalyst to degrade the organic and inorganic targets.

Organic molecules photocatalysts
A group of three azine-and imine-functionalized MOFs (TMU-4, −5 and −6) were introduced by Masoomi et al. for photodegradation of Congo red (CR) from water under UV irradiation without H 2 O 2 oxidant. [54] HOMO (also known as valence band coded as VB) and LUMO (also known as conduction band, coded as CB) band gap and BET surface area significantly influence the photocatalytic performance of these MOFs. In typical photocatalytic procedure, when Schiff base PL-MOFs irradiated with the photons of suitable energy, the electrons on VB jump to CB, creating holes (h+) in VB. [54,107] Positive holes could either directly oxidize the adsorbed organic contaminants, or produce very reactive hydroxyl radicals (OH˙) by capturing an electron (e − ) from a water molecule, which is oxidized to the OH˙ active species (Figure 10). [107] In the meantime, an electron in the CB reduces the adsorbed dioxygen (O 2 ) on photocatalyst to the superoxide radical (O 2-) which also degraded the adsorbed contaminants or organic dye molecules effectively to Figure 9. Carbon dioxide fixation catalyzed by the Lewis acidic metal sites and Lewis basic hydrazone sites of Schiff base PL-MOF i.e., CoMOF-1. Reproduced from Ref. [65] with permission from the Royal Society of Chemistry.
promote the photocatalytic process. So, the correct band gap value of Schiff base PL-MOFs is essential feature for excellent photocatalytic degradation of pollutants. The use of different N-donor Schiff base ligands led to the formation of frameworks with different topologies, with different band gaps, and thus different photocatalytic activities. The longer the conjugation length of the N-donor Schiff base ligand, the closer the HOMO and LUMO, and smaller the band gap. Thus, the smaller the band gap then the easier the electron transfer, and higher the photocatalytic efficiency. Furthermore, the larger the surface area of Schiff base PL-MOFs, the higher the organic contaminant, i.e., dye adsorption and the better the photocatalytic performance. Accordingly, TMU-6 having smaller band gap (2.1) and high BET surface area (650 m 2 g −1 ) showed excellent photocatalytic performance and remove 98.3% CR in shorter time (90 min) under UV irradiation. This is due to the presence highly conjugated Schiff base pillar, L 13 in TMU-6 that induces the reduction of band gap for easier transfer of electron from ligand to the metal center. [54] However   [64] visible or UV source (Table 5). [64] The BET surface area of these MOFs were 400.8, 447.3, 547.9, and 243.6 m 2 g −1 , respectively. [64] Further, the maximum phenol removal efficiency of these MOFs were 80.4%, 82.5%, 87.6%, and 33.3%, respectively. [64] The high surface area and easy availability of azine groups of Schiff base pillar (L 4 ) of Zn-based MOFs were the main reasons for their better performance compared with Cd-based MOFs for phenol degradation.

Inorganic molecules photocatalysts
TMU-4′ and TMU-6′ were developed through ligand exchange technique and applied for the photocatalytic reduction of Cr(VI) ( Table 5). [ [92] Compared to parent MOFs, SALE MOFs showed enhanced Figure 10. A schematic illustration of organic dye degradation over Schiff base PL-photocatalyst under UV irradiation. Reproduced from Ref. [107] with permission from Elsevier. photodegradation efficiency of Cr(VI) reduction about 22.1 and 23.5%, respectively. [92] In addition, the Cr(VI) photoreduction was significantly inhibited in the presence of electron scavenger AgNO 3 revealed that electrons play a function in Cr(VI) reduction using TMU-6′. The mechanism for photocatalytic degradation of Cr(VI) by TMU-6' is illustrated in Figure 11. [92]

Adsorbents
Schiff base PL-MOFs were good adsorbent for removal of dyes, antibiotics, iodine, and phenols. The subsection has been dedicated to outline the role of Schiff base sites for the selective adsorption of organic and inorganic targets.

Organic molecules adsorbents
Masoomi et al. detailed the adsorption of dye, first and foremost, utilizing Schiff base PL-MOF, TMU-8 in 2015. [71] A dual ligand Cd-based MOF, TMU-8 (formulated as [Cd 2 (L 5 ) 2 (L 2 ) 2 ] n ·3.5DMF) exhibited much higher removal efficiency of 97.3% (97.3 mg g −1 ) of CR from aqueous solution. [71] TMU-8 was also applied as a successful adsorbent for the elimination of reactive black 5 from water. [108] The maximum uptake capacity of this MOF for reactive black 5 was viewed as 79.36 mg/g at optimal pH 2. The removal mechanism was suggested to be the hydrophobic, π-π, and H-bonding interactions. [108] Further, the improvement of CR uptake of TMU-8 MOF was achieved by the replacement of 4,4'-oxybisbenzoic acid ligand (H 2 L 5 ) with benzene-1,3,5-tricarboxylic acid (H 2 L 20 ) ligand resulted the formation of [Cd 3 (L 20 ) 2 (L 2 ) 2 ] having more H-bonding sites for removal of CR. [72] This MOF effectively adsorb CR and Neutral Red (NR) with maximum uptake capacities of 192.3 and 243.9 mg/g, respectively. In addition to H-bonding, π-π associations among aromatic rings of L 8 and L 1 and the aromatic rings of CR and NR play a key role for the adsorption of the dye molecules on the outer surface of the Cd-MOF particles. [72] Further, for selective uptake of dyes the free specific functional groups may be added within MOFs. The selectivity of Schiff base decorated MOFs toward cationic/anionic dyes can be enhanced by the incorporation of uncoordinated sites such as -COOH, -NH 2 groups which can interact with dyes. Considering this criteria, {[Zn 2 (L 20 ) 2 (L 2 )(H 2 O) 2 ](C 2 H 5 OH) 3 } n was synthesized using benzene-1,3,5-tricarboxylic acid ligand and L 2. [73] This coordination polymer has a two-dimensional network structure made out of [Zn 2 (L 20 ) 2 (H 2 O) 2 ] n motif connected through the terminal N atoms of Schiff base pillar L 2 . This coordination polymer can selectively adsorb cationic dyes such as Rh B (rhodamine B), MB (methylene blue), and MV (methyl violet) (24.36, 21.55 and 17.15 mg/ g) from aqueous solution due to presence of free carboxyl groups in the layered gap that efficiently interact with the cationic dyes via H-bonding interactions ( Figure 12). [73] Moreover, due to its selectivity toward cationic dyes this coordination polymer was used as filler in a chromatographic column for separation of cationic dyes from a combination of anionic dyes and cationic dyes in water. Furthermore, amine groups could also upgrade the adsorption capacities of Schiff base functionalized MOFs toward dyes. Based on this strategy, the adsorption capacity of TMU-16 (composed of L 4 and terephthalic acid) was enhanced by replacing the terephthalic acid (L 3 ) with 2-aminoterephthalic acid ligand (H 2 L 6 ) during synthesis resulted the formation of isostructural amine functionalized TMU-16-NH 2 . [37] MO was efficiently adsorb by the TMU-16-NH 2 with adsorption capacity of 393.7 mg/g which is higher compared to TMU-16 (350 mg/g). [37] High adsorption capability of TMU-16-NH 2 was reinforced by the strong electrostatic interactions as well as H-bonding interactions between MO and TMU-16-NH 2 having an additional amine group compared to TMU-16. The electrostatic interactions are highly pH dependent which in turn, resulted in the variation of adsorption capacity of TMU-16/TMU-16-NH 2 with pH. At acidic pH, the adsorption of anionic dye MO increases due to increased electrostatic attraction between the MO molecules and accumulated positive charge on the surface TMU-16/ TMU-16-NH 2 adsorbent resulted from the protonation of amine as well as azine-methyl groups. [37] However, at basic pH the MO adsorption decreases due to increased repulsion between the MO molecules and accumulated Figure 11. TMU-6' photocatalyst based reduction of Cr(VI) reduction under visible light. Reproduced from Ref. [92] with permission from the Royal Society of Chemistry.
negative charge on the surface TMU-16-NH 2 adsorbent resulted from the deprotonation of amine group. Due to the pH dependent adsorption behavior of TMU-16/TMU-16-NH 2 , these adsorbents were also utilized for adsorption of cationic dyes at basic pH (pH = 12) because at this pH MOF surface covered by the negative charge for interaction with cationic dyes via H-bonding and electrostatic interactions. The adsorption capacity of TMU-16-NH 2 for tested cationic dyes such as Janus Green B, Brilliant Cresyl Blue, Toluidine Blue, Safranin O (700.72, 639.72, 610, and 645 mg g −1 ) was higher compared to TMU-16 (632.6, 512.2, 534.5, and 575.19 mg g −1 ). [36] In addition, the adsorption behavior of both previously mentioned MOFs was also studied for the removal of azo dye i.e. phenazopyridine hydrochloride (denoted PHP). Of these two, TMU-16-NH 2 adsorbed PHP at a faster rate 95% after 13 h. [35] However, TMU-16 adsorbed only 69% PHP from aqueous solution.
Besides the functional sites, the synthesis method also influences the adsorption efficiency of Schiff base functionalized MOFs. TMU-7 was synthesized by varying synthesis methods viz. solvothermal, mechanochemical, and ultrasonic methods and the respective MOFs named as TMU-7S, TMU-7 M, and TMU-7 U, respectively and utilized for the adsorption of CR dye from water. [69] It was observed that the removal efficiency of ultrasonically synthesized TMU-7 U (97% in 45 min) was much higher compared to solvothermally synthesized TMU-7S (64.3% in 2 h) and mechanochemical synthesized TMU-7 M (80% in 1.5 h). [69] This happens due to the enhanced surface area or porosity of MOF by the ultrasonic conditions. The BET surface area of TMU-7 U was found to be the 393 m 2 g −1 which was higher compared to TMU-7C (nonporous) and TMU-7 M (BET surface area = 243 m 2 g −1 ). The adsorption Figure 12. The feasible electrostatic and H-bonding associations among MOF and MV dye. Reproduced from Ref. [73] with permission from the Royal Society of Chemistry. capacity of CR by TMU-7 U is 97 mg/g and this follows the first order reaction kinetics. [69] Considering the above-discussed advantage of ultrasonic method, this method was also used for the preparation of TMU-5 and TMU-6 adsorbents having different Schiff base groups to understand their influence on adsorption. Both TMU-5 and TMU-6 were applied for the removal of RhB from aqueous solution. The adsorption process of these MOFs was governed by the acid-base interactions and are well fitted with pseudo-first order model. It was found that the adsorption capacity of TMU-5 (96.2%) was higher compared to TMU-6 (92.8%). [57] This was due to the higher basicity of azinemethyl group in TMU-5 compared to imine group in TMU-6. In addition, TMU-5 was highly stable and showed recyclability up to five recycle sessions with no loss of adsorption capacity.
To further enhance the adsorption capacity of Schiff base PL-MOFs, the composites of these MOFs were prepared by coating the surface with silk fiber via layer-by-layer deposition technique. These composites effectively adsorb pollutants via cooperative function of Schiff base sites of MOFs and free carboxylic acid groups of silk fiber. Based on the above discussed criteria, a fabricated TMU-5@silk fiber was synthesized by the stepwise deposition of TMU-5 on the surface natural silk fiber. [74] This composite was the best performer for the selective adsorption of MO from water with adsorption capacity of 4.892 mg/g. The selective adsorption of MO by this composite was governed by the acid-base as well as H-bonding associations among MO and active groups of the TMU-5@silk fiber (i.e. free carboxylic acid of silk fiber and azine-methyl groups of TMU-5). [74] To further enhance the adsorption capacity of Schiff base functionalized MOF@silk fiber composites, these composites were prepared by the using the MOFs having amine sites in addition to Schiff base sites which can also provide the sites for efficient adsorption of pollutants. Using this approach a Schiff base functionalized MOF@silk fiber, TMU-16-NH 2 @silk fiber composite was prepared by the stepwise deposition of TMU-16-NH 2 (having both Schiff base as well as free amine group) on the surface natural silk fiber. [76] This composite adsorb 1.271 g MO via H-bonding interactions between MO and azine-methyl/amine groups of TMU-16-NH 2 as well as free -COOH group of silk fiber. [76] Abazari et al. offered Schiff base PL-MOF for 2,4-dichlorophenol removal in 2018-single crystal [Zn(L 6 )(L 2 )] ∞ which was obtained via slow diffusion or ultrasonic methods to give nanostructures. [70] A comparison between single crystal MOF and nanostructured version, exposed the latter to have a better adsorption capacities. The nanostructured version remove the 91% of 1,2-dichlorophenol from aqueous solution in 90 min, however a single crystal removes only 68%. [70] Interestingly, nanostructured version was featured with excellent recyclability up to five cycles with no loss of adsorption capability.
Abbasi et al. presented the two nanostructured Schiff base PL-MOF composites e.g., TMU-5@silk fiber and TMU-4@silk fiber for the adsorptive removal of morphine from water. [75] The adsorption capacity of TMU-5@silk fiber (100% morphine removal achieved in 24 h) was reported to be higher than TMU-4@silk fiber (100% morphine removal achieved in 48 h). [75] This can be attributed to the fact that TMU-4 has three-dimensional honeycomb framework with one-dimensional open channels (aperture size of 5.3 × 9 Å) and TMU-5 has narrow interpenetrated framework with three-dimensional interconnected pores (aperture size of 4.4 × 6.2 Å). Irrespective of its narrower pores than TMU-4, TMU-5 is porous having BET surface area = 582.4 m 2 /g while TMU-4 is nonporous to nitrogen at 77 K. [75] Furthermore, the azine groups located on N-donor ligands are more basic in TMU-5 also improved the morphine adsorption. The driving force for adsorption of morphine is increased entropy which results from the increased hydrophobic interactions when morphine replaced the water within the MOF cavity. Adsorption of morphine on TMU-5@silk fiber is spontaneous and endothermic. [75] In another report, Abbasi et al. demonstrated the use of TMU-16-NH 2 @silk fiber to remove morphine from aqueous solution. The TMU-16-NH 2 @silk fiber (1.0 g) could effectively adsorb morphine to the extents of approximately 2.00 g. [76] The relatively higher adsorption of morphine was attributed to the strong hydrogen bonding between the TMU-16-NH 2 @silk fiber and morphine. In 2015, the methyldopa (MD) drug was completely removed from the aqueous solution by using Schiff base functionalized MOF composites, TMU-17-NH 2 @silk fiber in a short time 30 min. [77]

Inorganic molecules adsorbents
TMU-16 and TMU-16-NH 2 were applied for reversible adsorption of iodine. Fast decline of intensity of UV/vis peak (520 nm) of I 2 confirmed that later might adsorb I 2 1.4 times quicker compared to former ( Figure 13). The dark brown iodine solution quickly diminished to dull in approximately 30 min (using TMU-16-NH 2 as adsorbent) and about 2 h (using TMU-16 as adsorbent) ( Figure 13). [38] When iodine loaded MOFs were soaked in dry ethanol, the desorption of I 2 takes place which is 1.8 times faster in TMU-16 (due to presence of larger nonfunctionalized 1D channels of 13.4 × 11.8 Å) compared to TMU-16-NH 2 (smaller 1D amine functionalized channels of 7.1 × 4.6 Å that make weak N-H⋯I interactions result in faster sorption process and slower desorption process). [38]

Extraction sorbent
Schiff base PL-MOFs were utilized as an excellent extraction sorbents for removal of target analytes from environmental samples ( Table 6). The most appealing feature of these extraction sorbents is the presence of Schiff base active sites and conjugated aromatic rings which provides superior collaboration among MOF host and the target analytes (inorganic and organic) for their pre-concentration and separation.

Inorganic molecules extraction sorbents
Three notable Schiff base PL-MOFs including TMU-4, TMU-5, and TMU-6 have been tested for extraction as well as pre-concentration of heavy metals from ecological aqueous samples followed by flow injection inductively coupled plasma optical emission spectrometry (ICP-OES) investigation. [62] Associations among Lewis base sites of these Schiff base PL-MOFs (nitrogen and oxygen atoms) and Lewis acids (metal ions) were responsible for their selective adsorption. Among tried MOFs, TMU-5 having more basic azinemethyl Schiff base sites compared to TMU-4 having azine Schiff base sites and TMU-6 having imine sites. Likewise, as TMU-5 displayed the greatest separation efficiencies, it was chosen aimed at consequent solid phase extraction (SPE) investigations. [62] In the presence of ideal conditions including 7 mg sorbent, 5 min extraction time, desorption solvent 0.4 M EDTA, 5 min desorption time; the TMU-5 displayed noble linearity about 0.05-100 µg L −1 with limit of detection 0.01-0.5 µg L −1. [62] The introduced SPE method owns numerous benefits together with excessive extraction efficiency, straightforwardness, and slight natural waste. In addition, as the TMU-5 kept up with remarkable stability in aqueous medium over extensive pH range, it became Figure 13. (a, c) TMU-16-NH 2 and (b, d) TMU-16 displaying their color alteration and fall in UV-vis intensity after I 2 adsorption. Reproduced from Ref. [38] with permission from the Royal Society of Chemistry.
identified as the appropriate material for heavy metal ions removal from tap water, river water, and mineral water. Combination of Schiff base PL-MOFs with other materials to make a composite can lead to improved extraction efficiency toward target compounds compared to pristine Schiff base-PL-MOF. The Schiff base PL-MOF composite materials provide the synergistic benefits of both MOF as well as other material involved in composite synthesis. For example, TMU-4/polyethersulfone (TMU-4/PES) lined stainless steel wire has been applied for HS-SPME of organophosphorous pesticides (OPPs) followed by analysis by gas chromatography with a nitrogen-phosphorus detector (GC-NPD). [109] Under optimized conditions such as 75 ± 1°C extraction temperature, 40 min extraction time, 30% w/v addition of NaCl, 220°C desorption temperature, and 6 min desorption time, the good linearity with limit of detection 0.005-0.008 µg mL −1 with the range 0.015-50 µg mL −1 . The coated fiber also showed acceptable reproducibility (5.9-10.1%) and noble recovery (88-108%). [109] The high affinity of composite can be explained by the synergistic effects of mechanical features of PES polymer and high surface area of TMU-4. [109] The coated fiber displayed great stability and reusability up to 100 consecutive cycles with no detectable harm of extraction efficiency. Similarly, magnetic core sell composites of Schiff base PL-MOFs were synthesized which can be easily retrieved by a magnet. For example, Schiff base PL-MOF based core-shell magnetic composites Fe 3 O 4 @TMU-8 has been utilized for pre-concentration of heavy metal ions from water via magnetic solid-phase extraction (MSPE) accompanied by flow injection inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis. [78] Under the optimum condition such as 10 mg sorbent, pH 10, 11 min extraction time, 0.5 M HNO 3 desorption solvent; the LODs of 0.3-1 μgL −1 have been attained by means of linear dynamic range of 1-100 μgL −1 . PF and satisfactory relative recoveries (RR) are 66-232 and 17.83-62.70% respectively. The L 2 pillar ligand of TMU-8 has azine group (Lewis basic sites) that interact with metal ions via Lewis acid-base associations for selective extraction.

Organic molecules extraction sorbents
TMU-4, −5, and −6 lined stainless steel (SS) wires were utilized for headspace solid phase microextraction (HS-SPME) and pre-concentration of polycyclic aromatic hydrocarbons (PAHs) from river water, hookah water, and soil samples followed by GC-MS analysis. [56] The hydrophobic and π-π associations among target compounds and Schiff base pillar of sorbent framework have been liable for their specific adsorption onto the described sorbents. As per examination study, TMU-6 showed the most noteworthy performance among other fibers lined over MOFs for removal of PAHs. [56] This perception hence recommends that a more reasonable environment has been created for the extraction of PAHs by TMU-6 owing to presence of highly aromatic L 13 (Schiff base pillar) which provides better π-π associations with target  [109] compounds. [56] The fabricated fiber lined over TMU-6 showed good linearity (0.02-50 µg L −1 ), low LODs (0.005-0.008 µg L −1 ), good relative recovery (89-105%) under ideal tested conditions. Similarly, TMU-6 sorbent was also applied for the SPE and pre-concentration of three plasticizer compounds (di-n-butyl phthalate (DBP), Di-2-(ethylhexyl) phthalate (DEHP), and dioctyladipate (DOA)) from aqueous medium accompanied by evaluation via gas chromatography with flame ionization detector (GC-FID). [55] π−π stacking and hydrophobic (among studied plasticizers and Schiff base pillars) hydrogen bonding (among oxygen atoms of ether group of 4,4ʹoxybisbenzoic acid ligand and nitrogen atoms of imine groups of MOF and all three analytes) interactions were responsible for adsorption of target analytes. [55] Under optimized conditions including 7 mg adsorbent, 10 min extraction time, 2 min desorption time, ethyl acetate as desorption solvent, and 0.14 mL eluting solvent the good linearity was obtained in the range 0.5-100 μgL −1 with limit of detection 0.2-0.7 μgL −1 , suitable pre-concentration factor (PF) 92-295 and satisfactory recoveries 88-110%. [55] This sorbent has several advantages including convenience, good sensitivity, high efficiency, and implemented correctly to determine plasticizers in practical aquatic samples. A magnetic framework composites with core-shell structure (Fe 3 O 4 @TMU-21) was used for MSPE of trace pyrethroid residues from fruit juice samples followed by high-performance liquid chromatography-ultraviolet detection (HPLC-UV) analysis. [79] Hydrophobic, H-bonding, and π-π associations must brought about effective removal of target compounds over the exterior portion of Fe 3 O 4 @TMU-21. Under optimal conditions such as 4 mg Fe 3 O 4 @TMU-21, 80 μL volume of eluent, butanol as eluent, 6 min extraction time, and 2 min desorption time, the best linearity has been acquired of value of 0.5-250 μgL −1 . LOD, PF, and RR of the target compounds has been achieved of value of 0.1-0.05 μgL −1 , 463-498 and 93.0-104.5%, respectively. [79] The proposed MSPE method has numerous own benefits like simple, fast, reliable, sensitive, costeffective, low sorbent requirements, and satisfactory recovery. Moreover, as the prepared fabricated MFCs showed excellent water, chemical and thermal stability, good magnetic properties, free active sites, excellent extraction efficiency, and recyclability for six extraction cycles without loss of extraction efficiency, it turned into the proper material for removal of target compounds from fruit juices.  -nitroterephthalic acid) were reported for the selective CO 2 capture. [51,63,81,82,110] These were, in turn, the first, second, third, fourth, fifth, sixth,, and seventh top performers. The adsorption value of these MOFs for CO 2 capture were 84.26, 61.16, 59.15, 51.6, 50.6, 38.9, and 15 cc/g, respectively. [51,63,81,82,110] Existence of strong interactions between CO 2 molecules and Lewis basic Schiff base sites (-CH = N-) in the Schiff base pillar ligand and highly polar groups such as -NO 2 /-SO 2 /-COOH etc. in the O-donor ligand in the Schiff base PL-MOF framework were suggested as the main reasons for selective CO 2 adsorption.

Conclusions and prospects
PL-MOFs have widest structural diversity in terms of pore size and pore environment. Since in these MOFs, notwithstanding metal centers and O-donor linkers there is a third building block means the pillar linker that introduced the diversity in structure of PL-MOFs. To date, wide range of N-donor pillars are investigated like pyrazole, pyrazine, tetrazole, imidazole etc. Recently, Schiff bases have gained importance as a pillar linker because of their one-step synthetic protocol and easy availability of cost-effective precursors. Schiff bases are the condensation products of amines with aldehydes/ ketones. Schiff base pillars introduced weakly basic imine functionality into the MOFs which improves the interaction between substrate and MOF for desired application.
This review affords perception into Schiff base PL-MOFs with the capability for the extensive variety of applications like sensing, catalysis, gas separation, adsorptive removal of chemicals, and extraction sorbents. Compared with single linker MOF systems, Schiff base PL-MOFs are in the beginning phase of advancement. Hence, more endeavors are expected to plan a scope of Schiff base PL-MOFs. The design of PL-MOFs must consider real-world applications to give stable substances that work in practical conditions. Nonetheless, a few issues still need to be investigated during synthesis of Schiff base PL-MOFs. Certain Schiff base pillar linkers cannot withstand the harsh synthesis conditions of Schiff base PL-MOFs so stable Schiff base pillars must be selected for MOF synthesis. For example, partial decomposition of Schiff base ligand H 2 sal-TPD (having Schiff base moiety hanging freely as side moiety on the main backbone of linker which is involved in framework formation) takes place during the preparation of Fe-and Co-functionalized MOFs (sal-M-MOF, M = Fe or Co). [111] So, it must be ensured that the Schiff base moiety should be the part of main backbone of the Schiff base linker which is involved in MOF framework formation rather than it hanging freely as a side chain. Because the Schiff base moiety which is the part of main backbone of the linker are in conjugation with the aromatic rings which imparts stability to the Schiff base sites during MOF synthesis conditions. Furthermore, bulky Schiff base pillar ligand is difficult to construct MOFs due to its steric hindrance. Thus, selection of appropriate Schiff base ligand is the prerequisite in the synthesis method. The presence of undesired functional groups must be avoided in Schiff base pillar linker otherwise they could can interfere with crystallization of MOF crystals, giving rise to the formation of undesired products. The length of the pillar linker must be controlled because longer pillars lead to the unstable framework structure. Longer pillar linker based MOFs collapse during their application.
In spite of these difficulties, Schiff base PL-LMOFs have been emerged as the fastest developing area in the materials chemistry. Besides the outstanding achievements of Schiff base PL-MOFs, there are a number of key directions which should be considered to promote the further development of the Schiff base PL-MOFs and related applications.
(1) The water stability of Schiff base PL-MOFs is essential for their practical or in-field applications. And this can be enhanced by the introduction of hydrophobic groups e.g. alkyl or fluorinated (F, CF 3 , etc.) groups into the pillar linkers which have recently emerged as a highly efficient way to improve the water stability of resulting PL-MOFs. (2) By varying the length of pillar linker the size of pore can be easily controlled therefore large number of Schiff base PL-MOFs can be synthesized for selective studies like selective gas separation, selective size dependent catalysis etc. The size selective catalysis are also good future perspective for these MOFs. OPPs, heavy metals etc. these MOFs can also be applied for the extraction and pre-concentration of drugs and biological molecules. (7) Confining functional species into Schiff base PL-MOFs may play vital roles for diverse applications. To date, only dye molecules are loaded into the pores of Schiff base PL-MOFs. Other materials like metal nanoparticles, polymers, metal sulfides, metal oxides, and biomolecules are also suggested to be incorporated within the pores of Schiff base PL-MOFs, which may bring unexpected properties and remarkable functions.
(8) Synergistic effect is a also highly beneficial technique to further boost the performances of Schiff base PL-MOF materials. Therefore, insertions of other functional groups into other organic linkers and/or inorganic clusters together with the Schiff base moiety will probably generate more interesting and promising novel synergistic Schiff base PL-MOF materials. (9) Recently, postsynthetic modifications on the organic linkers or metal ions and postsynthetic exchange of linkers has become a prosperous area to modify the structures and functionalities of Schiff base PL-MOFs to improve the performance of special properties of these MOFs to achieve the target functions. (10) The fields of adsorption using Schiff base PL-MOFs are in their infancy and much more research is needed. In particular, the size and shape of their pores can be altered, which will have an important effect on the loading capacity of the adsorbents. (11) Another promising area for PL-MOFs is reducing them to the nanometer size. Specifically, PL-MOFs turned out to be a promising applicants for the preparation of nanofilms by step-by-step assembly.
A nanoscale PL-MOF is expected to have a bright future in a broader fields because they combines the benefits of both nanoparticles and pillar-layer structure motifs. (12) Present studies of Schiff base PL-MOFs mainly focus on their sensing, adsorption, and catalytic properties. Given the high structural stability and diversity, we have reason to believe that the Schiff base PL-MOFs have a potential in super capacitor, drug delivery applications and many other areas. (13) Finally, computational calculations should be opted which are highly useful to support the experimental findings.