Coordination Polymers Based on Highly Emissive Ligands: Synthesis and Functional Properties

Coordination polymers are constructed from metal ions and bridging ligands, linking them into solid-state structures extending in one (1D), two (2D) or three dimensions (3D). Two- and three-dimensional coordination polymers with potential voids are often referred to as metal-organic frameworks (MOFs) or porous coordination polymers. Luminescence is an important property of coordination polymers, often playing a key role in their applications. Photophysical properties of the coordination polymers can be associated with intraligand, metal-centered, guest-centered, metal-to-ligand and ligand-to-metal electron transitions. In recent years, a rapid growth of publications devoted to luminescent or fluorescent coordination polymers can be observed. In this review the use of fluorescent ligands, namely, 4,4′-stilbenedicarboxylic acid, 1,3,4-oxadiazole, thiazole, 2,1,3-benzothiadiazole, terpyridine and carbazole derivatives, naphthalene diimides, 4,4′,4′′-nitrilotribenzoic acid, ruthenium(II) and iridium(III) complexes, boron-dipyrromethene (BODIPY) derivatives, porphyrins, for the construction of coordination polymers are surveyed. Applications of such coordination polymers based on their photophysical properties will be discussed. The review covers the literature published before April 2020.

Luminescence is an important property of coordination polymers, often playing a key role in their applications. Luminescence is a non-coherent radiation that occurs upon the excitation of atoms, ions or molecules. Luminescence arises when certain transitions (called spontaneous radiative transitions) of these species from the states with higher energy to the states with lower energy, including the ground state, take place. Depending on the excitation method, different types of luminescence are differentiated. Thus, photoluminescence occurs upon excitation by an optical radiation (usually in UV range), electroluminescence-when excited by an electrical field. The processes that accompany the luminescence are often visualized in Jablonski diagrams (Figure 1). Absorption of light occurs in a very short femtosecond timeframe and correspond to the excitation of the particle from the ground state (S 0 ) to an excited state (S 1 , S 2 , . . . ). It should be noted that each state has its own set of vibrational levels, which are populated upon excitation with different probabilities and when combined, form an absorption spectrum. After the absorption of a photon, the most probable process is called the internal conversion or vibrational relaxation. This process is longer that the excitation (picosecond timeframe) and is accompanied by a structural relaxation of the excited molecule. The excess energy is converted into heat and the relaxation is thus a non-radiative process. The molecule can exist in this excited state for nanosecond and longer and then returns to the ground state, emitting a photon in a process called fluorescence. Other events that can occur after the excitation include non-radiative relaxation upon collision of the excited molecule with other particles or intersystem crossing to the lowest excited triplet state (T 1 ). Relaxation from the triplet state to the ground state with photon emission is called phosphorescence. Transition back to the S 1 state is also possible, followed by a delayed fluorescence.
Materials 2020, 13,2699 2 of 67 atoms, ions or molecules. Luminescence arises when certain transitions (called spontaneous radiative transitions) of these species from the states with higher energy to the states with lower energy, including the ground state, take place. Depending on the excitation method, different types of luminescence are differentiated. Thus, photoluminescence occurs upon excitation by an optical radiation (usually in UV range), electroluminescence-when excited by an electrical field. The processes that accompany the luminescence are often visualized in Jablonski diagrams (Figure 1). Absorption of light occurs in a very short femtosecond timeframe and correspond to the excitation of the particle from the ground state (S0) to an excited state (S1, S2, …). It should be noted that each state has its own set of vibrational levels, which are populated upon excitation with different probabilities and when combined, form an absorption spectrum. After the absorption of a photon, the most probable process is called the internal conversion or vibrational relaxation. This process is longer that the excitation (picosecond timeframe) and is accompanied by a structural relaxation of the excited molecule. The excess energy is converted into heat and the relaxation is thus a nonradiative process. The molecule can exist in this excited state for nanosecond and longer and then returns to the ground state, emitting a photon in a process called fluorescence. Other events that can occur after the excitation include non-radiative relaxation upon collision of the excited molecule with other particles or intersystem crossing to the lowest excited triplet state (T1). Relaxation from the triplet state to the ground state with photon emission is called phosphorescence. Transition back to the S1 state is also possible, followed by a delayed fluorescence. Coordination polymers are complex systems consisting of metal ions, one or more ligand types, inclusion of solvent molecules or other guests in voids is also possible. Emission of light by the coordination polymers can arise from various types of electron transitions-intraligand (ligandcentered), metal-centered, metal-to-ligand and ligand-to-metal charge transfer (MLCT and LMCT), Figure 1. Electron transitions in guest molecules encapsulated in the pores of the coordination polymers can also influence their photophysical properties.
The photophysical properties of the coordination polymers are used to create electroluminescent materials for LEDs [39][40][41], as contrast agents in biomedical imaging, theranostics and photodynamic therapy [42,43]. In recent years, more attention is given to nonlinear optical properties of the coordination polymers, including the second harmonic generation, multi-photon absorption, upconversion luminescence and lasing [44][45][46][47][48][49]. The most extensive area of the use of the Coordination polymers are complex systems consisting of metal ions, one or more ligand types, inclusion of solvent molecules or other guests in voids is also possible. Emission of light by the coordination polymers can arise from various types of electron transitions-intraligand (ligand-centered), metal-centered, metal-to-ligand and ligand-to-metal charge transfer (MLCT and LMCT), Figure 1. Electron transitions in guest molecules encapsulated in the pores of the coordination polymers can also influence their photophysical properties.
The photophysical properties of the coordination polymers are used to create electroluminescent materials for LEDs [39][40][41], as contrast agents in biomedical imaging, theranostics and photodynamic therapy [42,43]. In recent years, more attention is given to nonlinear optical properties of the coordination polymers, including the second harmonic generation, multi-photon absorption, upconversion luminescence and lasing [44][45][46][47][48][49]. The most extensive area of the use of the luminescent properties of the coordination polymers is the development of sensors for various analytes -cations and anions in aqueous and non-aqueous solutions [50][51][52], gases (oxygen, nitric(II) oxide, carbon monoxide, ammonia, water vapor, etc.) [53][54][55][56], volatile organic compounds (aromatic hydrocarbons, aromatic nitro compounds, amines, etc.) [57][58][59][60], biologically important compounds (vitamins, pharmaceutical substances, toxins, DNA and RNA) [61][62][63]. The analytical signal in sensors of this type, as a rule, is associated either with a decrease in the luminescence intensity in the presence of an analyte (the "quenching" effect), or with its increase (the "turn-on" effect). The wide range of applications and the variety of building blocks of luminescent coordination polymers causes a rapid increase in the number of publications on this topic in the last 10-15 years. Thus, the first works devoted to the study of the photophysical properties of the coordination polymers appeared in 1997 [64], in recent years 400-500 publications devoted to this area were published annually, and to date, more than 4300 works have already been published, according to Scopus search results using the query "(luminescent OR fluorecscent) AND ((MOF OR metal-organic framework) OR coordination polymer)" (Figure 2). Materials 2020, 13,2699 3 of 67 luminescent properties of the coordination polymers is the development of sensors for various analytes -cations and anions in aqueous and non-aqueous solutions [50][51][52], gases (oxygen, nitric(II) oxide, carbon monoxide, ammonia, water vapor, etc.) [53][54][55][56], volatile organic compounds (aromatic hydrocarbons, aromatic nitro compounds, amines, etc.) [57][58][59][60], biologically important compounds (vitamins, pharmaceutical substances, toxins, DNA and RNA) [61][62][63]. The analytical signal in sensors of this type, as a rule, is associated either with a decrease in the luminescence intensity in the presence of an analyte (the "quenching" effect), or with its increase (the "turn-on" effect). The wide range of applications and the variety of building blocks of luminescent coordination polymers causes a rapid increase in the number of publications on this topic in the last 10-15 years. Thus, the first works devoted to the study of the photophysical properties of the coordination polymers appeared in 1997 [64], in recent years 400-500 publications devoted to this area were published annually, and to date, more than 4300 works have already been published, according to Scopus search results using the query "(luminescent OR fluorecscent) AND ((MOF OR metal-organic framework) OR coordination polymer)" (Figure 2). Recently, several reviews on luminescent coordination polymers were published, but almost all of them were devoted to their sensory properties [65][66][67][68]. In addition, in most reviews, the emphasis was placed on coordination polymers based on lanthanides with metal-centered luminescence [69][70][71], and only one work of 2019 was devoted to a review of luminescent MOFs based on transition metals, but its area was also limited by the sensory properties of MOFs with respect to biologically relevant metal ions [51]. Within this review, data on the coordination polymers with ligand-centered luminescence and their functional properties of will be surveyed. The classification of the coordination polymers will be based on the types of ligands responsible for the appearance of luminescent properties.

Coordination Polymers Based on 4,4′-Stilbenedicarboxylic Acid
As of this day, 4,4′-stilbenedicarboxylic acid (H2sdc, Scheme 1), is an organic linker in the array of dicarboxylates widely used for the construction of coordination polymers. It is often encountered as a part of reticular syntheses due to its predictable geometry and availability. Its relatively large conjugated electron system, as well as a certain degree of flexibility make it interesting for the synthesis of luminescent MOFs [72,73]. "Rigidifying" of the ligand conformation in the resultant MOF often leads to the enhancement of stilbene-based luminescence, which allows the preparation of highly emissive and stable materials [74]. Recently, several reviews on luminescent coordination polymers were published, but almost all of them were devoted to their sensory properties [65][66][67][68]. In addition, in most reviews, the emphasis was placed on coordination polymers based on lanthanides with metal-centered luminescence [69][70][71], and only one work of 2019 was devoted to a review of luminescent MOFs based on transition metals, but its area was also limited by the sensory properties of MOFs with respect to biologically relevant metal ions [51]. Within this review, data on the coordination polymers with ligand-centered luminescence and their functional properties of will be surveyed. The classification of the coordination polymers will be based on the types of ligands responsible for the appearance of luminescent properties.

Coordination Polymers Based on 4,4 -Stilbenedicarboxylic Acid
As of this day, 4,4 -stilbenedicarboxylic acid (H 2 sdc, Scheme 1), is an organic linker in the array of dicarboxylates widely used for the construction of coordination polymers. It is often encountered as a part of reticular syntheses due to its predictable geometry and availability. Its relatively large conjugated electron system, as well as a certain degree of flexibility make it interesting for the synthesis of luminescent MOFs [72,73]. "Rigidifying" of the ligand conformation in the resultant MOF often leads to the enhancement of stilbene-based luminescence, which allows the preparation of highly emissive and stable materials [74]. Scheme 1. 4,4′-Stilbenedicarboxylic acid, its derivatives and co-ligands used for the preparation of luminescent coordination polymers.
Bauer et al. [75] prepared two H2sdc-based MOFs [Zn3(sdc)3(dmf)2]n and {[Zn4O(sdc)3(dmf)]·CHCl3}n by varying synthetic conditions. It was discovered that crystal structure packing density influenced - interligand interactions, more dense structure demonstrating a redshift and broadening of the emission band compared to a less densely packed coordination polymer (441 nm and 390 nm correspondingly). The emission of both MOFs was ascribed to intraligand excitations. The characteristic lifetimes were longer compared to the free ligand, indicative of an increased rigidity of sdc 2-linkers in the coordination network. MOF formulated as [Zn4O(sdc)3(dmf)]·CHCl3}n demonstrated luminescence sensitivity to inclusion of guest solvent molecules [75].
Bauer et al. [75] prepared two H 2 sdc-based MOFs [Zn 3 (sdc) 3 (dmf) 2 ] n and {[Zn 4 O(sdc) 3 (dmf)]·CHCl 3 } n by varying synthetic conditions. It was discovered that crystal structure packing density influenced π-π interligand interactions, more dense structure demonstrating a red-shift and broadening of the emission band compared to a less densely packed coordination polymer (441 nm and 390 nm correspondingly). The emission of both MOFs was ascribed to intraligand excitations. The characteristic lifetimes were longer compared to the free ligand, indicative of an increased rigidity of sdc 2linkers in the coordination network. MOF formulated as [Zn 4 O(sdc) 3 (dmf)]·CHCl 3 } n demonstrated luminescence sensitivity to inclusion of guest solvent molecules [75].
Later the same authors have extended this study by synthesizing an additional series of MOFs [79]. Among them, the new structure with H2sdc as a ligand, {[Zn2(sdc)2(AnEPy)]·2DMA·1.5H2O}n (AnEPy-trans,trans-9,10-bis(4-pyridylethynyl)anthracene) demonstrated an emission maximum at 560 nm and a quantum yield of 21%. The authors explored the effect of structural variation of MOFs on two-photon excited emission. However, it was found that both the quantum yields and twophoton absorption cross-sections did not exhibit comprehensible structure−property relationship.
Self-catenated rob-type net {[Zn2(dmtrz)2(sdc)]·6H2O}n (MAC-11, Hdmtrz-3,5-dimethyl-1H-1,2,4-triazole), consisting of Zn-triazolate 2-D layers linked together by H2sdc was described in [81]. It was found that the framework undergoes thermo-induced phase transformation, accompanied by a photoluminescence response. The authors assume that the observed red-shift of 35 nm (418 nm to 453 nm) is due to the flattening of Zn(dmtrz) layers, as well as the changes in the coordination mode of H2sdc, leading to the enhanced interaction between the carboxylate ligand and Zn 2+ centers.
Later the same authors have extended this study by synthesizing an additional series of MOFs [79]. Among them, the new structure with H 2 sdc as a ligand, {[Zn 2 (sdc) 2 (AnEPy)]·2DMA·1.5H 2 O} n (AnEPy-trans,trans-9,10-bis(4-pyridylethynyl)anthracene) demonstrated an emission maximum at 560 nm and a quantum yield of 21%. The authors explored the effect of structural variation of MOFs on two-photon excited emission. However, it was found that both the quantum yields and two-photon absorption cross-sections did not exhibit comprehensible structure−property relationship.
Self-catenated rob-type net {[Zn 2 (dmtrz) 2 (sdc)]·6H 2 O} n (MAC-11, Hdmtrz-3,5-dimethyl-1H-1,2,4-triazole), consisting of Zn-triazolate 2-D layers linked together by H 2 sdc was described in [81]. It was found that the framework undergoes thermo-induced phase transformation, accompanied by a photoluminescence response. The authors assume that the observed red-shift of 35 nm (418 nm to 453 nm) is due to the flattening of Zn(dmtrz) layers, as well as the changes in the coordination mode of H 2 sdc, leading to the enhanced interaction between the carboxylate ligand and Zn 2+ centers.
One of the complexes prepared in [84] utilizes H 2 sdc as co-ligand for building of the Zn 2+ based framework.
One of the coordination polymers, {[Zn(sdc)(bim)]·DMF} n , demonstrated one of the best values for MOF luminescence quantum yields-82% before activation (λ em = 455 nm, λ ex = 390 nm). Upon further investigation, the increase of the quantum yield of the sdc-based luminescence was attributed to the rigidifying of the framework with increased interpenetration.
Several Zn-MOFs are described in [87]. Upon excitation at 350 nm, structures formulated as [Zn 3 (sdc) 3 (py) 2 ] n , [Zn 3 (sdc) 3 (4,4 -bpy)] n , {[Zn 3 (sdc) 3 (bpea)]·3H 2 O} n (py-pyridine, 4,4 -bipy-4,4 -bipyridine, bpea-1,2-bis(4-pyridyl)ethane, Scheme 1) exhibited emission bands at 460, 487 and 469 nm (Table 1), showing different degrees of blue shift compared to the free ligand. Authors state that the shift is due to the interchromophore coupling and metal coordination as well as intraligand π-π* transitions. - [82] The authors of [88] studied the luminescent properties of two coordination polymers [Zn(sdc)(H 2 O)] n , [Cd(sdc)(H 2 O)] n , however, only data for the Zn structure is present. It demonstrated strong emission bands at 435 and 459 nm in the solid state at room temperature upon excitation at 387 nm. Increased emission intensity of the coordination polymers compared to the free ligand was attributed to a change of π*-π transitions of the free ligand to π*-n transitions upon coordination. Interestingly enough, the authors mention that the emission of Cd based coordination polymer is weak compared to the Zn one.
As one of goals of the work [90], luminescence properties of two polymers, [Zn 2 (sdc) 2

Coordination Polymers Based on Sulfur Heterocyclic Derivatives
Luminescent materials are often based on sulfur heterocycles, among which 4-hydroxythiazole is worth noting as responsible for the remarkable bioluminescence phenomenon [109].   [108]. Excitation of the Cd II coordination polymers at 350 nm leads to a fluorescent emission with peaks at 425 nm (for 4-bpo) and 378 nm (for 3-bpo) nm. It is interesting to note that no emission was observed for {[Zn(4-bpo)(bdc)]·CH 3 OH} n . The authors attribute the quenching effect to high-energy C-H and/or O-H oscillators in MeOH lattice molecules.

Coordination Polymers Based on Sulfur Heterocyclic Derivatives
Luminescent materials are often based on sulfur heterocycles, among which 4-hydroxythiazole is worth noting as responsible for the remarkable bioluminescence phenomenon [109].

Thiazole Derivatives
Zhai et al. prepared a series of Zn II /Cd II MOFs based on 2,5-bis(4-pyridyl)thiazolo [5,4- [110]. All of these MOFs displayed good emission in the solid-state at room temperature from ≈460 nm to ≈560 nm ( Table 2). The free ligand Py 2 TTz showed two emission peaks at 439 nm and 452 nm upon excitation at 409 nm. The observed red-shift of MOF emission was attributed to the LMCT.    3+ and Al 3+ ions in 5 × 10 −3 M solutions in DMF. It is interesting to note that Fe 3+ ions had a dramatic luminescence quenching effect, while Al 3+ caused luminescence intensity enhancement [111]. XPS measurements showed weak interaction between Fe 3+ and nitrogen atoms of free pyridyl groups, which was proposed as responsible for luminescence quenching.

Derivatives of 2,1,3-Benzothiadiazole
A series of luminescent MOFs based on 2,1,3-benzothiadiazole derivatives is reported in [112]. In this work, Cd II , Zn II , Co II , Ni II coordination polymers were synthesized under solvothermal conditions with 4,7-bis(4-pyridyl)-2,1,3-benzothiadiazole (dpbt, Scheme 4) as a ligand. Terephthalic (H 2 bdc) and isophthalic acids (H 2 ipa) were used as co-ligands. Solid-state PL spectrum of the free dpbt shows emission at 465 nm upon excitation at 371 nm. {[Cd(dpbt)(bdc)]·2H 2 O} n shows emission at 464 nm upon excitation at 356 nm and {[Zn 2 (dpbt) 2 (ipa) 2 ]·2DMA} n displays emission at 513 nm upon excitation at 375 nm. The emission of Cd-MOF was assigned to n*-π and/or π*-π transitions, while the emission red-shift of Zn-MOF allowed to assume LMCT contribution. Zn II -MOF was demonstrated to be perspective for detection of nitro compounds. The fluorimetric titration of MOF suspension by the addition of nitro compounds results in quenching of the MOF photoluminescence. The highest PL quenching degree of as high as 99.7% was observed for the picric acid at the concentration of 0.1 mM. Strong quenching was explained a lower LUMO energy level of PA in comparison to that of dpbt, while other nitro compounds demonstrated higher LUMO levels. Similar results were obtained for Cd II -MOF [112]. A series of luminescent MOFs based on 2,1,3-benzothiadiazole derivatives is reported in [112]. In this work, Cd II , Zn II , Co II , Ni II coordination polymers were synthesized under solvothermal conditions with 4,7-bis(4-pyridyl)-2,1,3-benzothiadiazole (dpbt, Scheme 4) as a ligand. Terephthalic (H2bdc) and isophthalic acids (H2ipa) were used as co-ligands. Solid-state PL spectrum of the free dpbt shows emission at 465 nm upon excitation at 371 nm. {[Cd(dpbt)(bdc)]·2H2O}n shows emission at 464 nm upon excitation at 356 nm and {[Zn2(dpbt)2(ipa)2]·2DMA}n displays emission at 513 nm upon excitation at 375 nm. The emission of Cd-MOF was assigned to n*-π and/or π*-π transitions, while the emission red-shift of Zn-MOF allowed to assume LMCT contribution. Zn II -MOF was demonstrated to be perspective for detection of nitro compounds. The fluorimetric titration of MOF suspension by the addition of nitro compounds results in quenching of the MOF photoluminescence. The highest PL quenching degree of as high as 99.7% was observed for the picric acid at the concentration of 0.1 mM. Strong quenching was explained a lower LUMO energy level of PA in comparison to that of dpbt, while other nitro compounds demonstrated higher LUMO levels. Similar results were obtained for Cd II -MOF [112].  ions [113]. The emission enhancement in the presence of low concentrations of Al 3+ and Cr 3+ ions was tentatively ascribed to their coordination to dpbt ligands. Song et al. prepared four zinc coordination polymers based on 4,7-bis((E)-2-(pyridine-4yl)vinyl)-2,1,3-benzothiadiazole (bptda, Scheme 4) with diverse topologies [114]. Polycarboxylate possessing different bend angles (4,4 -dicarboxydiphenylamine (H 2 dpa), 4,4 -oxybisbenzoic acid (H 2 oba) and 4,4 -sulfonyldibenzoic acid (H 2 sdba)) were used as co-ligands, coordination polymers [Zn(bptda)(dpa) 2  A benzothiadiazole-decorated UiO-68 was synthesized by Mallick et al. [116]. In this work luminescent analogue of p-terphenyl-4,4"-dicarboxylic acid, 4,4 -(2,1,3-benzothiadiazole-4,7-diyl)dibenzoic acid (H 2 btdb, Scheme 4) was used as a ligand ( Figure 6). The free ligand shows emission at 480 nm upon excitation at 365 nm in water, while a water suspension of MOF Zr-btdb-fcu demonstrates a 21 nm red-shift and emission at 501 nm. Additionally, studies of the luminescent sensing of volatile organic amines (methylamine, ethylamine, triethylamine, aniline, p-phenylenediamine) in aqueous solution were performed. Increasing the concentrations of aniline and p-phenylenediamine led to a drastic decrease of MOF fluorescence intensity. In contrast, fluorescence turn-on effect was observed in the presence of aliphatic amines such as methylamine [116]. The turn-on effect was explained by the bonding between the protonated methylamine molecules and nitrogen atoms of btdb 2− linkers, leading to suppressed twisting motion of the ligand, which reduces the probability of nonradiative relaxation pathways. Interaction between the protonated methylamine and the framework was confirmed by DFT calculations.  [116]. In this work luminescent analogue of p-terphenyl-4,4″-dicarboxylic acid, 4,4′-(2,1,3-benzothiadiazole-4,7diyl)dibenzoic acid (H2btdb, Scheme 4) was used as a ligand ( Figure 6). The free ligand shows emission at 480 nm upon excitation at 365 nm in water, while a water suspension of MOF Zr-btdbfcu demonstrates a 21 nm red-shift and emission at 501 nm. Additionally, studies of the luminescent sensing of volatile organic amines (methylamine, ethylamine, triethylamine, aniline, pphenylenediamine) in aqueous solution were performed. Increasing the concentrations of aniline and p-phenylenediamine led to a drastic decrease of MOF fluorescence intensity. In contrast, fluorescence turn-on effect was observed in the presence of aliphatic amines such as methylamine [116]. The turn-on effect was explained by the bonding between the protonated methylamine molecules and nitrogen atoms of btdb 2− linkers, leading to suppressed twisting motion of the ligand, which reduces the probability of nonradiative relaxation pathways. Interaction between the protonated methylamine and the framework was confirmed by DFT calculations. The same ligand H2btdb in combination with 4,4'-(1H-benzo[d]imidazole-4,7-diyl)dibenzoic acid was used for the preparation of mixed-ligand Zr-fcu-MOF in which energy transfer between the linkers was observed [117]. A strong overlap between the emission band of benzimidazole ligand and the absorption band of H2btdb ensured a superior efficiency of energy transfer of 90%, making it a promising light-harvesting platform.
Zn-MOF with H2btdb ligand was prepared by Wei et al. [118]. This MOF exhibited strong luminescent properties both in the solid state and in MeOH suspension. In the solid-state {[Zn(btdb)(DMA)]•H2O}n exhibited an intense emission band at ~494 nm upon excitation at 370 nm, in methanol suspension it demonstrated emission at 491 nm upon excitation at 350 nm. As it was Reproduced with permission from [116]. Copyright 2019 American Chemical Society.
The same ligand H 2 btdb in combination with 4,4'-(1H-benzo[d]imidazole-4,7-diyl)dibenzoic acid was used for the preparation of mixed-ligand Zr-fcu-MOF in which energy transfer between the linkers was observed [117]. A strong overlap between the emission band of benzimidazole ligand and the absorption band of H 2 btdb ensured a superior efficiency of energy transfer of 90%, making it a promising light-harvesting platform.
Zn-MOF with H 2 btdb ligand was prepared by Wei et al. [118]. This MOF exhibited strong luminescent properties both in the solid state and in MeOH suspension. In the solid-state {[Zn(btdb)(DMA)]·H 2 O} n exhibited an intense emission band at~494 nm upon excitation at 370 nm, in methanol suspension it demonstrated emission at 491 nm upon excitation at 350 nm. As it was shown in the article, Zn-MOF had luminescent response for Cd 2+ ions in methanol. The emission intensity enhanced distinctly in the presence of Cd 2+ ions, thus, upon addition of 3 equivalents of Cd 2+ the emission intensity increased by 3.5 times [118]. Tian et al. prepared Co II MOF based on 4,7-bis(1H-imidazol-1-yl)-2,1,3-benzothiadiazole (bibt, Scheme 4) and 1,3,5-benzenetricarboxylic acid (H 3 btc) as co-ligands [119]. MOF {[Co 3 (bibt) 3 (btc) 2 (H 2 O) 2 ]·solvents} n , JXUST-2 exhibited emission at 396 nm in solid state. After being immersed in DMA it showed emission peak at 530 nm upon excitation at 394 nm. The free bibt ligand demonstrated emission band around 540 nm upon excitation at 394 nm. JXUST-2 was tested for selective detection of metal ions and exhibited a turn-on response to Fe 3+ , Cr 3+ and Al 3+ .
MOFs were dependent on the type of dicarboxylate used (Figure 7).  Vasylevsyi et al. prepared Zn II and Cd II coordination polymers based on 9,10-di(1H-imidazol-1-yl)-anthracene ligand (dia) [131]. The solid-state PL spectrum of the free ligand showed an emission maximum at 433 nm upon excitation at 375 nm with QY = 28%. The excitation and emission maxima of the coordination polymers (Table 3) corresponded well to those of the free ligand and were thus assigned to intra-ligand n-π* and π-π* transitions.   Table 3) demonstrated emission at 419 nm (λ ex = 343 nm) and 428 nm (λ ex = 350 nm), respectively [133]. The photoluminescence of MOFs was attributed to ligand-centered excitations. Both MOFs could detect Fe 3+ and Al 3+ ions in aqueous solutions. The presence of Fe 3+ ions lead to the complete quenching of the luminescence, while Al 3+ caused a significant luminescence enhancing effect. In addition, nitrobenzene and 2,4,6-trinitrophenol could be detected through luminescence quenching effect.  Table 3) [134]. The MOF adopted a 3D unimodal 4-c CdSO 4 topology. A free bis(triazolyl) ligand upon excitation at 280 nm showed an emission peak at 380 nm, while the emission band of Zn-MOF was observed at 432 nm under the same excitation, a notable red-shift was attributed to LMCT. This MOF had a luminescent turn-off response on Fe 3+ ions, luminescence lifetime was also reduced from 362.17 ns to 44.63 ns with the increase of Fe 3+ concentration, these changes were attributed to weak binding of bmbip-1,3-bis(2-methylbenzimidazol-1-yl)propane, bdmbip-1,5-bis(5,6dimethylbenzimidazol-1-yl)pentane) were prepared by Zhang [139]. The photophysical characteristics are given in Table 3 [139].

Benzimidazole Derivatives
Zong et al. prepared a Cd-MOF [Cd(tmb)(bbibp)] n based on 4,4 -bis(benzimidazo-1-yl)biphenyl (bbibp) and (1H-1,2,4-triazol-1-yl)methylenebis(benzoic acid) (H 2 tzmb), which represented a two-fold interpenetrated unimodal 4-connected 3D frameworks with a (6 5 ·8) topology [140]. The free bbibp ligand at the solid-state showed emission peak at 400 nm upon excitation at 275 nm, H 2 tmb demonstrated emission band at 420 nm upon excitation at 330 nm. However, when the ligands were coordinated by Cd 2+ the main emission peak of the MOF appeared at 360 nm upon excitation at 285 nm and was assigned to ligand-centered electron transitions. Ion detection by this Cd-MOF was also studied.  Table 3) with a strong emission peak at 377 nm upon excitation at 320 nm demonstrated a high quenching degree (up to 97.8%) in the presence of Fe 3+ ions [141]. Blue-shift of the emission band relative to the free ligand allowed to tentatively assign it to LMCT. Luminescence quenching by Fe 3+ ions was attributed to adsorption of these ions by the coordination polymer, confirmed by ICP analysis.
Zhao et al. prepared Tb-MOF based on 5-(1H-pyrazol-3-yl)isophthalic acid (H 2 pia) [146]. Free Zn-MOF {[Zn(dptmia)]·2H 2 O·0.5DMF} n (dptmia-(5-(3,5-di-pyridin-4-yl-[1,2,4]triazol-1ylmethyl)-isophthalic acid) with the emission peak near 430 nm (λ ex = 350 nm) was tested for the sensing of metal ions and organic compounds [148]. It was found that Fe 3+ ions in the aqueous MOF suspension lead to a considerable luminescence quenching. Among the organic analytes tested, 4-nitrophenol, 2,4-dinitrohenol and 3-nitrophenol showed the most notable quenching effects. Structural changes of MOF observed by PXRD method after its exposure to Fe 3+ ions, suggested a cation-exchange of Zn 2+ as a mechanism of luminescence quenching. Liu [149]. The free pyridine-based ligand exhibited an emission at 375 nm upon excitation at 312 nm, while the coordination polymer showed a strong emission peak at 428 nm under excitation at 320 nm. Luminescence quenching effect was observed in the presence of nitrobenzene in DMF solution, the emission intensity decreased by 50% at 75 ppm and complete quenching was achieved at 175 ppm of nitrobenzene, the quenching effect was attributed to energy transfer from the ligand to nitrobenzene molecules. Duan [152]. It demonstrated interesting thermochromic and solvatochromic effects (Figure 8). Compound {[Zn 2 (Htpim)(3,4-pydc) 2 ]·4DMF·4H 2 O} n showed emission at 423 nm under excitation at 380 nm. When this MOF was heated to 100 • C to remove water it displayed an emission peak at 466 nm and after heating at 160 • C (DMF removal) the emission shifted to 515 nm upon excitation at 380 nm.
Both linear and nonlinear photophysical properties of zinc(II) metal-organic frameworks based with trans-2-(4-pyridyl)-4-vinylbenzoate (pvb) ligands were reported in [154]. Two MOFs of dia topology {[Zn(pvb) 2 ]·DMF} n and [Zn(pvb) 2 ] n with 7-fold and 9-fold interpenetration were synthesized and structurally characterized. Single crystal-to-single crystal structural transformation of the first MOF to the second upon removal of DMF was observed. The intraligand emission wavelength was dependent on the presence of solvent molecules and was red-shifted for [Zn(pvb) 2 ] n owing to increased π-π interactions (λ em = 478 nm vs. λ em = 460 nm for {[Zn(pvb) 2 ]·DMF} n , Table 3). Nonlinear optical properties were characterized by measuring the second harmonic generation (SGH) and two-photon photoluminescence (2PPL). Desolvated MOF demonstrated a much stronger SGH response compared to the solvated product, 2PPL intensity was also approximately 14 times stronger for the desolvated MOF.  [153].
Both linear and nonlinear photophysical properties of zinc(II) metal-organic frameworks based with trans-2-(4-pyridyl)-4-vinylbenzoate (pvb) ligands were reported in [154]. Two MOFs of dia topology {[Zn(pvb)2]⋅DMF}n and [Zn(pvb)2]n with 7-fold and 9-fold interpenetration were synthesized and structurally characterized. Single crystal-to-single crystal structural transformation of the first MOF to the second upon removal of DMF was observed. The intraligand emission wavelength was dependent on the presence of solvent molecules and was red-shifted for [Zn(pvb)2]n owing to increased π-π interactions (λem = 478 nm vs. λem = 460 nm for {[Zn(pvb)2]⋅DMF}n, Table 3). Nonlinear optical properties were characterized by measuring the second harmonic generation (SGH) and two-photon photoluminescence (2PPL). Desolvated MOF demonstrated a much stronger SGH response compared to the solvated product, 2PPL intensity was also approximately 14 times stronger for the desolvated MOF.

Ligands with BODIPY-Type Fluorophores
Boron-dipyrromethene or BODIPY derivatives are notable for their uniquely small Stokes shift, high, environment-independent fluorescence quantum yields, often approaching 100% even in water, sharp excitation and emission peaks contributing to overall brightness [155][156][157][158]. The combination of these qualities makes BODIPY fluorophores promising for the construction of highly luminescent coordination polymers, some of which were reviewed in 2016 [159]. Zhou

Ligands with BODIPY-Type Fluorophores
Boron-dipyrromethene or BODIPY derivatives are notable for their uniquely small Stokes shift, high, environment-independent fluorescence quantum yields, often approaching 100% even in water, sharp excitation and emission peaks contributing to overall brightness [155][156][157][158]. The combination of these qualities makes BODIPY fluorophores promising for the construction of highly luminescent coordination polymers, some of which were reviewed in 2016 [159].
In another work by the same group, composite material of coordination polymer {[Zn2(L 2 )2(bpdc)2]·H 2O}n and Pt nanoparticles was used for efficient hydrogen production with the rate of 4680 mol·g −1 ·h −1 one of the highest among MOF photocatalysts [162].
In another work by the same group, composite material of coordination polymer {[Zn2(L 2 )2(bpdc)2]·H 2O}n and Pt nanoparticles was used for efficient hydrogen production with the rate of 4680 mol·g −1 ·h −1 one of the highest among MOF photocatalysts [162].
In another work by the same group, composite material of coordination polymer {[Zn2(L 2 )2(bpdc)2]·H 2O}n and Pt nanoparticles was used for efficient hydrogen production with the rate of 4680 mol·g −1 ·h −1 one of the highest among MOF photocatalysts [162].
In another work by the same group, composite material of coordination polymer {[Zn2(L 2 )2(bpdc)2]·H 2O}n and Pt nanoparticles was used for efficient hydrogen production with the rate of 4680 mol·g −1 ·h −1 one of the highest among MOF photocatalysts [162].
Spectroscopic investigations showed that in the case of {[Cd 2 (L 3 ) 2 (bpp)]·2H 2 O·EtOH} n an uncommon J-dimer absorption band was observed at λ max = 705 nm with a long tail into the NIR region at room temperature. Low quantum yields (less than 1%) were explained by the formation of dimers by BODIPY moieties and concentration quenching. Interestingly enough, QY of the coordination polymer {[Cd 2 (L)(bpe) 3 (NO 3 ) 2 ]·2H 2 O·DMF·EtOH} n is twice as high as of other complexes (1.7%), since the structure of this framework did not allow the formation of dimers.
The same group reported three Cd-MOFs built from NDI-pyrazolate ligand (PzNDI,  [170]. Coordination polymers FJU-67 and FJU-69 demonstrated photochromic behavior due to the photochemical radical generation. While the free ligand showed an emission peak at 590 nm upon irradiation at 345 nm, the complexes exhibited slightly blue-shifted (by 20 nm and 13 nm) emissions bands, attributed to MLCT. As a result that the photoresponsive ability of FJU-67 was found to be slightly higher, the authors suggested that the interpenetrating mode plays a definite role in influencing the efficiency of the photoinduced electron transfer reactions, resulting in different degrees of photoresponse [170].
The same group reported three Cd-MOFs built from NDI-pyrazolate ligand (PzNDI,  [170]. Coordination polymers FJU-67 and FJU-69 demonstrated photochromic behavior due to the photochemical radical generation. While the free ligand showed an emission peak at 590 nm upon irradiation at 345 nm, the complexes exhibited slightly blue-shifted (by 20 nm and 13 nm) emissions bands, attributed to MLCT. As a result that the photoresponsive ability of FJU-67 was found to be slightly higher, the authors suggested that the interpenetrating mode plays a definite role in influencing the efficiency of the photoinduced electron transfer reactions, resulting in different degrees of photoresponse [170].
The first synthesis of a coordination polymer with amino-acid functionalized NDI ligand (GlyNDI, Table 5) was reported in [174]. Compound [Zn(GlyNDI)(dmf) 2 ] n had a 1D zig-zag chain structure and exhibited strong fluorescence centered at 458 nm upon excitation at 320 nm. The mechanism of the luminescence was described as metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT), supported by intermolecular π-π interactions.
Xu et al.  Table 5) [175]. All of the complexes were photochromic and their photosensitivity increased in the order Ba(II) < Ca(II) < Cd(II) < Zn(II), consistent with the order of electronegativity for these ions. Photoluminescent properties were also surveyed. The emission was centered at 585 nm for Cd and Zn compound when excited at 280 nm. For Ca compound the emission maximum was at 499 nm and for Ba it was at 467 nm. In addition, these compounds exhibited a color change upon exposure to nitrite ions and different solvents ( Figure 11).  Table 5) [175]. All of the complexes were photochromic and their photosensitivity increased in the order Ba(II) < Ca(II) < Cd(II) < Zn(II), consistent with the order of electronegativity for these ions. Photoluminescent properties were also surveyed. The emission was centered at 585 nm for Cd and Zn compound when excited at 280 nm. For Ca compound the emission maximum was at 499 nm and for Ba it was at 467 nm. In addition, these compounds exhibited a color change upon exposure to nitrite ions and different solvents ( Figure 11).    [176]. The first compound exhibited three emission peaks at 424, 452 and 475 nm upon excitation at 380 nm. Multiply emission peaks were attributed to ligand-centered excitation and MLCT. The second coordination polymer exhibited emission centered at 484 nm upon excitation at 350 nm. Due to their photochromism, upon irradiation with UV light, the intensities of their luminescence decreased to 52% and 61% of their original values.
Another material with a photocontrolled tunable luminescence was reported in [178]. Being a host-guest compound with polyoxometalates as guests, it had an overall formula [C 88 H 100 F 4 W 24 N 16 O 96 P 2 Zn 3 ] n and underwent color change upon UV-irradiation. It emitted light with a maximum near 450 nm when excited at 350 nm. It was found that this compound could serve as an effective photocatalyst for selective oxidation of benzylic alcohols [178].
Liu et al. reported three isostructural 1D coordination polymers with the formula [Zn(dpndi)X 2 ] n , where X = Cl − , Bror I − [179]. All compounds showed emission at around 595 nm upon excitation at 490 nm, which was completely quenched under UV light for [Zn(dpndi)Cl 2 ] n and [Zn(dpndi)Br 2 ] n , but no obvious changes were observed for [Zn(dpndi)I 2 ] n . This difference was explained by the strength of lone pair-π interactions being the highest for the structure with iodine ions, thus, the π-acidity of dpndi moiety being the lowest one.
In the work of Shang et al., MOF [Zn(AlaNDI)] n was synthesized with the use of chiral R-or S-alanine-functionalized NDI ligands [181]. The resulting structure displayed a number of interesting properties. Among them is an intense luminescence centered at 475 nm (360 nm excitation) which could be quenched by Naproxen-a non-steroidal anti-inflammatory drug, which is chiral. Interestingly, if non-racemic MOF is used, enantiodifferentiating fluorescence sensing is possible.
One of the five compounds studied in [182] was a 1D coordination polymer with the formula {[Cd 2 (3-imntd) 2 I 4 ]·2H 2 O} n (3-imntd-N,N -bis(3-imidazol-1-yl-propyl)naphthalene diimide, Table 5). It demonstrated two-centered ligand-based emission at 410 nm and 590 nm upon excitation at 309 nm. Since both of the emission bands matched the same bands of the free ligand 3-imntd, they were assigned to intra-ligand emissions. The longer-wavelength band was assigned to the transition between imidazole and NDI with perpendicular orientation, while the shorter-wavelength band was assigned to the same transition with the coplanar orientation of the groups [182].
Dhankhar et al. reported a pillar-layered framework [Zn 2 (NH 2 bdc) 2 (dpndi)] n [183]. Water suspension of this compound exhibited strong ligand based (π*-n and π*-π) emission at 430 nm under 330 nm light. This suspension was screened for the ability to detect the presence of nitroaromatic compounds (NAC) in solution ( Figure 12). It was found that all NAC cause fluorescence quenching, but 2,4,6-trinitrophenol (TNP) had the strongest effect, quenching nearly 92% of the initial intensity and shifting the maximum to 460 nm, which could be used for selective detection of TNP. DFT studies suggested that both photo-induced electron transfer and Förster resonance energy transfer processes are responsible for the luminescence quenching. under 330 nm light. This suspension was screened for the ability to detect the presence of nitroaromatic compounds (NAC) in solution ( Figure 12). It was found that all NAC cause fluorescence quenching, but 2,4,6-trinitrophenol (TNP) had the strongest effect, quenching nearly 92% of the initial intensity and shifting the maximum to 460 nm, which could be used for selective detection of TNP. DFT studies suggested that both photo-induced electron transfer and Förster resonance energy transfer processes are responsible for the luminescence quenching. Coordination polymer {[Zn2(hfipbb)2(dpndi)]⋅8DMF}n (H2hfipbb-4,4′-(hexafluoroisopropylidene)bis(benzoate)) was reported in [184]. It exhibited strong LMCT emission with the maximum at 415 nm upon excitation at 360 nm. Redox and photochromic properties have also been surveyed. Coordination polymer {[Zn 2 (hfipbb) 2 (dpndi)]·8DMF} n (H2hfipbb-4,4 -(hexafluoroisopropylidene) bis(benzoate)) was reported in [184]. It exhibited strong LMCT emission with the maximum at 415 nm upon excitation at 360 nm. Redox and photochromic properties have also been surveyed.
An interesting work from Kitagawa et al. [186], in which MOF [Zn 2 (bdc) 2 (dpndi)] n was prepared for the first time, shows how useful the NDI-based frameworks can be. This interpenetrated framework was able to incorporate molecules of various organic volatiles, which was accompanied by changes in crystal structure and shifts of luminescence maximum to a specific position for each compound ( Figure 13). Furthermore, quantum yield and luminescence lifetime were also affected. For a full range of compounds see Table 5.
Deng et al.
reported the synthesis of complex [Cd(3-pmntd) 2 (NO 3 ) 2 ] n (3-pmntd-N,N -bis(3-pyridylmethyl)naphthalene diimide), a luminescent material, which upon excitation at 447 nm exhibited broad emission band centered at 557 nm [187]. The emission band was structured compared to a wide band of the free ligand, which suggested that charge-transfer transitions contributed to the emission along with the ligand-centered excitation.
Yang et al. prepared NDI-based Ag-MOF, which was formulated as [Ag(2,6-ndc) 0.5 (dpndi) 0.5 (H 2 O)] n [188]. It is a stable supramolecular material with good electrical conductivity and interesting luminescent properties. Its MLCT emission was centered at 370 nm (excitation at 320 nm) and experienced a strong turn-on effect upon exposure to dichloromethane, while in other organic solvents the luminescence was quenched. This makes the aforementioned coordination polymer a good candidate for selective dichloromethane probes.
Magnesium is an ion much rarer encountered compared to Zn [189]. Dry samples showed strong emission band centered at 570 nm upon excitation at 515 nm, but upon exposure to solvents a gradual blue-shift consistent with the increase in polarity of the solvent occurred (Figure 14), bringing the maximum to 625 nm at the highest solvent polarity (EtOH). The luminescence was quenched in presence of both aliphatic and aromatic amines. Interestingly, sensing experiments were conducted with the as-synthesized MOF in the solid state, whereas in most works the studies are carried out with MOF suspensions in some solvent.
An interesting work from Kitagawa et al. [186], in which MOF [Zn2(bdc)2(dpndi)]n was prepared for the first time, shows how useful the NDI-based frameworks can be. This interpenetrated framework was able to incorporate molecules of various organic volatiles, which was accompanied by changes in crystal structure and shifts of luminescence maximum to a specific position for each compound ( Figure 13). Furthermore, quantum yield and luminescence lifetime were also affected. For a full range of compounds see Table 5.  [187]. The emission band was structured compared to a wide band of the free ligand, which suggested that charge-transfer transitions contributed to the emission along with the ligand-centered excitation.
Yang et al. prepared NDI-based Ag-MOF, which was formulated as [Ag(2,6ndc)0.5(dpndi)0.5(H2O)]n [188]. It is a stable supramolecular material with good electrical conductivity and interesting luminescent properties. Its MLCT emission was centered at 370 nm (excitation at 320 nm) and experienced a strong turn-on effect upon exposure to dichloromethane, while in other organic solvents the luminescence was quenched. This makes the aforementioned coordination polymer a good candidate for selective dichloromethane probes.
Magnesium is an ion much rarer encountered compared to Zn and Cd in luminescent MOFs. There are, however, some examples. Mallick et al. prepared Mg-based MOF, which was determined to be [Mg2(bindi)2(dmf)2(H2O)]n [189]. Dry samples showed strong emission band centered at 570 nm  Calcium-based MOF {[Ca2(bindi)(dmf)4]·2DMF}n with excellent thermal stability and photochromic properties was described in [190]. Pristine sample showed 575 nm centered emission under 505 nm excitation light, which was attributed to the LLCT and/or LMCT.
An unprecedented linear correlation between the full width at half maximum (FWHM) of PL bands and the π-π distances between aromatic ligands was demonstrated in a work utilizing γaminobutyric acid-functionalized NDI [191]. Sixteen MOFs were prepared and characterized. They were formulated as follows:  Calcium-based MOF {[Ca 2 (bindi)(dmf) 4 ]·2DMF} n with excellent thermal stability and photochromic properties was described in [190]. Pristine sample showed 575 nm centered emission under 505 nm excitation light, which was attributed to the LLCT and/or LMCT.
An unprecedented linear correlation between the full width at half maximum (FWHM) of PL bands and the π-π distances between aromatic ligands was demonstrated in a work utilizing γ-aminobutyric acid-functionalized NDI [191]. Sixteen MOFs were prepared and characterized. They were formulated as follows: [Li 2 Table 5.
Similar photophysical behavior was reported for another Cd-ntb MOF [Cd 5 (ntb) 4 (H 2 O) 2 ] n , for which strong luminescence quenching (λ em = 417 nm, λ ex = 352 nm) was observed in the presence of 2,4,6-trinitrophenol and 2,4-dinitrophenol [195]. It should be noted that the emission response was selective for two mentioned compounds and no quenching was observed in the presence of other nitroaromatic derivatives.
Among 17 tested metal ions, only Fe 3+ lead to complete quenching of emission at 549 nm ( 5 D 4 → 7 F 5 transition) with the detection limit of 8·10 −6 M in MeOH-H 2 O, 1:4 solution. At the same time, Al 3+ ion lead to a significant enhancement of emission band at 463 nm, detection limit 7·10 −7 M [196].
Wen et al. prepared MOF {[Cd 3 (ntb) 2 (bimb)]·6DMA} n (bimb-4,4 -bis(imidazol-1-yl)biphenyl), which demonstrated ligand-centered emission at 470 nm upon excitation at 360 nm [197]. A notable solvatochromism was observed for this MOF and the emission changed from purple (423 nm) in methanol to indigo (461 nm) in water to cyan (490 nm) in ethanol. Since no solvatochromism was detected for the free ligand, the shifts in emission maxima were attributed to the formation of exciplexes between the solvent and MOF excited state.
Solvent-dependent structural diversity of manganese(II) coordination polymers with H 3 ntb was explored in [198]. Four new coordination polymers, [Mn 3 (ntb)(HCOO) 3 (Table 6) with coordination-induced shifts relative to the free ligand (λ em = 448 nm, λ ex = 333 nm). Coordination polymer based on two ligands with extended π-systems, H 3 ntb and 1,1,2,2-tetrakis(4-(pyridin-4-yl)phenyl)ethane (tppe), {[Zn 3 (tppe) 0.5 (ntb) 2 ]·4DMA·7H 2 O} n was reported by Wu et al. [199]. Crystal structure of this MOF resembled a 3D framework of {Zn 3 (ntb) 2 } layers interconnected by tppe pillars (Figure 15). MOF demonstrated significant porosity with the opening window sizes of 18.4 × 14.3 Å 2 and void volume as high as 72.7%. The emission spectrum of this compound comprised a single band at 475 nm, which was assigned to the fluorescence of both ligands in the framework. High porosity of the MOF made it promising for fluorescent detection of nitroaromatic compounds in the vapor phase. Thin films of MOF were fabricated and after exposure to vapors of different aromatic nitro-derivatives showed luminescence quenching, the highest degree of which was achieved in case of mono-nitro compounds, such as 2-nitrotoluene, 2-nitrophenol and nitrobenzene [199].  [200]. It demonstrated a remarkable sensitivity to 4-hydroxy-4′-nitrobiphenyl (HNBP), which quenched the emission near 450 nm with the detection limit as low as 50 nM. It should be noted that other biphenyls, including structurally similar to HNBP 4-phenylphenol and 4-nitrobiphenyl caused almost no changes in emission intensity. Molecular force field calculations indicated bonding between HNBP and MOF via close O⋯H contacts and C-H⋯π interactions. In addition, MOF proved to be efficient and recyclable methylene blue photodegradation catalyst [200].
A water-stable indium-organic framework {(Me2NH2)[In(ntb)4/3]·2DMF·3H2O}n was reported in [201]. It demonstrated blue emission at 478 nm in the solid state, and after the exchange of dimethylammonium cations to 4-[p-(dimethylamino)styryl]-1-ethylpyridinium dye dual emission with maxima at 479 and 590 nm was observed. The obtained dye-encapsulated MOF exhibited sensing towards Hg 2+ ions, which caused almost complete luminescence quenching with the detection limit of 1.75 ppb. Quenching of luminescence also occurred in the presence of dichromate anions Cr2O7 2-and nitro-explosives in water.
A water-stable indium-organic framework {(Me 2 NH 2 )[In(ntb) 4/3 ]·2DMF·3H 2 O} n was reported in [201]. It demonstrated blue emission at 478 nm in the solid state, and after the exchange of dimethylammonium cations to 4-[p-(dimethylamino)styryl]-1-ethylpyridinium dye dual emission with maxima at 479 and 590 nm was observed. The obtained dye-encapsulated MOF exhibited sensing towards Hg 2+ ions, which caused almost complete luminescence quenching with the detection limit of 1. 75 [202]. Zn and Cd MOFs showed an intense emission bands at 513 nm and 515 nm attributed to π*→n or π*→π transitions. Luminescence quantum yield of Zn-MOF was higher compared to Cd-MOF, which was explained by heavy atom effect of cadmium.
A pillar-layered MOF {(Me 2 NH 2 ) 2 [Cd 3 (ntb) 2 (bdc)]}·4DMF} n was prepared by Wang et al. [204]. MOF exhibited emission at 440 nm in DMF suspension, a red-shift of emission compared to the free ligand DMF solution (424 nm) was attributed to ligand-to-metal charge transfer. The emission was solvent-dependent and was quenched in the presence of nitroaromatic compounds, for which the quenching constants (K SV ) from Stern-Volmer equation were determined. Picric acid had the highest K SV value of 96.52. Photoinduced electron transfer was proposed as a possible mechanism for fluorescence quenching [204].
An aldehyde-decorated MOF, [Cd 3 (ntb) 2 (apa)] n (apa-2-amino-3-pyridinecarboxaldehyde) was reported in [205]. Post-synthetic oxidation of this MOF by hydrogen peroxide lead to a coordination polymer Cd-TCOOH containing both amino and carboxyl groups simultaneously. Cd-TCOOH in HEPES buffer (pH 7.4) displayed emission at 450 nm. Influence of a wide range of mono-, di-and trivalent metal cations on the luminescence intensity of Cd-TCOOH was evaluated and only for Al 3+ an emission turn-on effect was detected. The detection limit for Al 3+ in water of 0.54 ppb is one of the best reported MOF sensors. The detection mechanism was associated with Al 3+ coordination to Brønsted acidic sites of Cd-TCOOH. Presence of both Brønsted acidic and basic sites in Cd-TCOOH it allowed to selectively detect lysin without interference from other common aminoacids [205].
Coumarin 343 (C343) and Coumarin 6 (C6) were loaded into MOF cavities and the photoluminescence spectra were measured. Upon excitation at 381 nm the emission of the framework near 450 nm was quenched, while the emission of the dyes (530 nm for C6 and 510 nm for C343) was enhanced, indicating energy transfer from the network to the dyes molecules [207]. Wu [208]. It exhibited an intraligand emission peak at 410 nm upon excitation at 300 nm. The luminescence was completely quenched in the presence of 300 ppm of nitrobenzene or 220 ppm of Fe 3+ ions in DMF suspensions of MOF.
The above-mentioned series was extended in [214] by the preparation of Eu and Gd MOFs of the same composition. Eu-MOF exhibited the typical 5 D 0 → 7 F 1 (591 nm) and 5 D 0 → 7 F 2 (614 nm) transitions, while for a mixed-lanthanide Eu 0.02 Dy 0.18 -MOF white emission with CIE coordinates (0.3336, 0.3168) was achieved. In addition, this mixed-metal MOF demonstrated sensing abilities for water in ethanol, on increasing the water concentration band at 614 nm gradually decreased, while the emission intensity of the ligand at 416 nm increased. The ratio of these two intensities linearly correlated with the water content in the range of 0 to 10%, making the MOF perspective for water determination in bio-ethanol fuels [214].
An unusual 3D lithium-organic framework, [Li 4 (bcbaip)(dmf) 2 ] n (HNU-31) was prepared and characterized by Feng et al. [216]. It demonstrated emission at 402 nm, which dramatically enhanced in the presence of Al 3+ ions. Other metal ions did not interfere with the detection and a low detection limit of 4.4 µM was achieved. Since the emission maximum gradually shifted from 402 to 420 nm with the aluminum concentration increase, but no MOF degradation occurred, the authors assumed that Al 3+ interacts with the framework [216].
Ligand with an even more extended conjugated system, tetraphenylphenylenediamine (H 4 tppd, Scheme 7) was used by Mayer et al. to prepare zinc(II) and cadmium(II) coordination polymers, that demonstrated two-photon-absorption-induced fluorescence (TPPF) [219]. Coordination polymer [Zn 2 (tpbd)(dma) 2 ] n resembled a two-dimensional network, while [Cd 2 (tpbd)(H 2 O) 4 ] n was a three-dimensional framework. Both coordination polymers demonstrated photoluminescence near 520 nm with relatively high quantum yields of 24% (Zn-CP) and 52% (Cd-CP). The two-photon-absorption cross-section was nine time higher of Cd-CP compared to Zn-CP, which was ascribed to a higher degree of interpenetration in Cd-MOF, leading to higher chromophore density and stronger excitonic interactions.

Ligands Based on Highly Emissive Ruthenium(II) and Iridium(III) Complexes
Xie and co-workers designed and synthesized highly porous and phosphorescent Ir-containing coordination polymers for oxygen sensing via efficient and reversible luminescence quenching [220].  3 ] derivatives phosphorescence quenching was irreversible [220].  [221]. Upon excitation at 420 nm, the emission spectra of MOFs showed emission maxima at 599, 593 and 614 nm, respectively, consistent with the phosphorescent emission from MLCT excited states of the doped ligands.
Representative examples of luminescent Ir-Ln heteronuclear coordination polymers {[Ln[Ir(ppy)2(5,5′-dcbpy)]2(OH)]·H2O}n (Ln = Gd, Yb, Er, Nd) based on a highly efficient lightharvesting Ir antenna were reported in [222]. The Ir unit showed strong visible light absorption via 3 MLCT and sensitized the Ln(III)-based NIR emission by efficient d→f energy transfer. The NIR emission spectra of the coordination polymers were recorded upon excitation at 500 nm in the solid state. The observed emission band at 1535 nm was attributed to 4I13/2→4I15/2 transition for the Ir-Er complex, for the Ir-Yb complex the luminescence with a maximum at 980 nm was assigned to 2F5/2→2F7/2 transition. Three strong emission bands at 880 nm, 1056 nm and 1326 nm, corresponding to the 4F3/2→4I9/2, 4I11/2 and 4I13/2 transitions were observed for Ir-Nd complex.
Representative examples of luminescent Ir-Ln heteronuclear coordination polymers {[Ln[Ir(ppy) 2 (5,5 -dcbpy)] 2 (OH)]·H 2 O} n (Ln = Gd, Yb, Er, Nd) based on a highly efficient light-harvesting Ir antenna were reported in [222]. The Ir unit showed strong visible light absorption via 3 MLCT and sensitized the Ln(III)-based NIR emission by efficient d→f energy transfer. The NIR emission spectra of the coordination polymers were recorded upon excitation at 500 nm in the solid state. The observed emission band at 1535 nm was attributed to 4I 13/2 →4I 15/2 transition for the Ir-Er complex, for the Ir-Yb complex the luminescence with a maximum at 980 nm was assigned to 2F 5/2 →2F 7/2 transition. Three strong emission bands at 880 nm, 1056 nm and 1326 nm, corresponding to the 4F 3/2 →4I 9/2 , 4I 11/2 and 4I 13/2 transitions were observed for Ir-Nd complex. Li [223]. The coordination polymer displayed a strong emission band at 620 nm when excited at 500 nm in the solid state, similar to that of the iridium unit with the maximum emission peak at 628 nm, indicating that the luminescence is generated by the Ir unit arising from 3 [224]. The photophysical characteristics of the coordination polymers are given in Table 7. Lead(II) ions were introduced to the frameworks to promote phosphorescence-based sensitivity to oxygen, all coordination polymers were able to detect oxygen in real gas mixtures. Three magnesium coordination polymers based on highly luminescent Ir(III) units [Ir(ppy) 2 (4,4 -dcbpy)] were reported in [225].
All  (Table 7) compared to the free Ir(III) unit, which displayed a strong orange luminescence near 575 nm upon excitation at 468 nm.
All coordination polymers were isostructural 3D frameworks.
The solid-state photoluminescence emission spectrum of {[Cd 3 [Ir(3-cppy) 3 ] 2 (dmf) 2 (H 2 O) 4 ]·6H 2 O·2DMF} n , (λ ex 380 nm) showed a single peak at 519 nm in contrast to two maxima at 488 and 510 nm for Ir unit, corresponding to the 3 MLCT transition. The phosphorescence lifetime of this MOF (2.95 µs) was longer than that of the parent linker (1.93 µs). MOF exhibited excellent water stability which made it a highly selective and sensitive multiresponsive luminescent sensor for Fe 3+ and Cr 2 O 7 2− . The detection limits were 67. 8 and 145.1 ppb, respectively.
This MOF could also be used as an optical sensor for selective sensing of adenosine triphosphate (ATP 2− ) over adenosine diphosphate (ADP 2− ) and adenosine monophosphate (AMP 2− ) in an aqueous solution.
Liu and coworkers synthesized phosphorescent nanoscale coordination polymers using the [Ru(2,2 -bpy) 2 (5,5 -dcbpy)] bridging ligand and Zn 2+ or Zr 4+ nodes [229]. The UV/vis spectrum of Zn coordination polymer in ethanol showed a broad MLCT absorption band between 400 and 550 nm. It exhibited an emission maximum at 635 nm with a luminescence quantum yield of 2.1% and an average lifetime of 215 ns. Zirconium coordination polymer showed emission at 630 nm with a luminescent quantum yield of 0.8% and an average luminescence lifetime of 107 ns. In vitro viability assays for Zr nanoscale coordination polymer on H460 human non small-cell lung cancer cells were conducted and a significant MLCT luminescent signal was observed in the confocal z section images for H460 cells. Zhang 3 ]]·6H 2 O} n , which manifested a broad visible light MLCT absorption band between 250 and 650 nm [230]. The free ligand displayed a strong red photoluminescence with the emission maximum at 633 nm upon excitation at 400 nm, and MOF emission was centered at 657 nm. The red shift was attributed to the coordination of ligands with In(III) centers. The fluorescence lifetime of the ligand was determined to be 512 ns, while it is found to be 301 ns for the coordination polymer. Photocatalytic activity of MOF in visible light-induced photodegradation of methyl orange was demonstrated. Moreover, sensing properties of CP were also evaluated, and the result shows that CP can selectively detect the nitro explosives molecules.
Porous  3 ]]·12H 2 O} n were prepared by Kobayashi and coworkers [231]. Ruthenium ligands showed absorption bands at 467 and 484 nm and emission bands at 633 nm (for 4,4 -dcbpy derivatives) and 668 nm (for 5,5 -dcbpy derivatives), due to the singlet and triplet MLCT. All of the obtained coordination polymers showed a very broad absorption band below 600 nm and an emission band at~680 nm (  [232]. Absorption spectra showed that MOF exhibited efficient visible light harvesting with the absorption edge extended to around 650 nm due to the MLCT. The luminescence lifetime of MOF was 5.49 µs, which is more than one order of magnitude greater than that of [Ru] unit (483 ns). MOF could promote the visible light induced photocatalytic reduction of CO 2 [234]. The coordination polymer exhibited a dark-red broad emission centered at 691 nm without any vibronic progressions, which is largely shifted to the longer wavelength than that of the 3 MLCT. A reversible structural transition triggered by water adsorption/desorption was observed-the emission band shifted to the longer wavelength (by about 14 nm) after drying at 110 • C, the wavelength of the emission maximum gradually shifted to the shorter wavelength with increasing humidity. At 100% humidity, the luminescence was restored.
The maximum emission of the ligand [Ru(4,4 -H 2 dcbpy) 2 (2,2 -bpy)] and MOF were observed at 689 nm and 671 nm. The authors demonstrated the efficiency of the MOF-GO system for the determination of cocaine in serum sample. Polapally [236]. The UV-vis diffuse reflectance spectra of MOF showed the characteristic broad band with two peaks at 440 nm and 480 nm due to the MLCT. These bands were red-shifted compared to the discrete [Ru(4,4 -H 2 dcbpy) 3 ] 2+ , which showed a broad band centered at 427 nm. The emission of MOF in the solid state was near 605 nm when excited at 440 nm, which is shifted to shorter wavelength compared to ligand (622 nm). Copper(II) centers showed ferromagnetic interactions with θ = 34 K and C = 0.43 emu· mol −1 · K for its Curie-Weiss equation [236].
Two new porous coordination polymers {[Ln 7 (OH) 5 [Ru(4,4 -dcbpy) 3 ] 4 ]·4H 2 O} n (Ln = Ce 3+ , Nd 3+ ) were reported in [237]. Cerium(III) coordination polymer exhibited a broad dark red emission with the maximum at 684 nm and vapochromic effect associated with water vapor adsorption/desorption, arising from the 3 MLCT emission of Ru ligand. Upon excitation at 420 nm, Nd(III) coordination polymer showed emission at 884, 1054 and 1336 nm assignable to the 4f−4f emission of the Nd 3+ center ( 4 F 3/2 → 4 I 9/2 , 4 I 11/2 and 4 I 13/2 , respectively). In the excitation spectra (λem = 1054 nm), the 1 MLCT absorption bands in the visible region were clearly observed in addition to the sharp 4f−4f transitions. Kobayashi [H 5.5 (Ru(dpbpy) 3 ] 2 ]·4.5H 2 O} n (H 4 dpbpy-2,2 -bipyridine-4,4 -bis(phosphonic acid) were reported in [238]. It was shown that the porous structures collapsed on the removal of water molecules from the channels, but the original porous structure was reconstructed upon water adsorption. Both MOFs exhibited emission bands near 640 nm similar to that of Ru ligand due to the 3 MLCT of the Ru ligand. The emission band underwent a blue-shift on increasing the relative humidity from 0 to 75% and from 0 to 54%, respectively. The emission spectra obtained at 100% humidity were almost identical to those of the as-synthesized samples.

Porphyrin Derivatives
Porphyrins and their derivatives play an important role in chemistry and biology; they participate in light-harvesting, oxygen transport and catalytic transformations [239][240][241][242]. Ligand based on porphyrins benefit from their rigid scaffold, photophysical properties and allow to obtain a wide range of coordination polymers with diverse luminescent properties [243][244][245].
[Zr 6 (µ 3 -OH) 8 [238]. It was shown that the porous structures collapsed on the removal of water molecules from the channels, but the original porous structure was reconstructed upon water adsorption. Both MOFs exhibited emission bands near 640 nm similar to that of Ru ligand due to the 3 MLCT of the Ru ligand. The emission band underwent a blue-shift on increasing the relative humidity from 0 to 75% and from 0 to 54%, respectively. The emission spectra obtained at 100% humidity were almost identical to those of the as-synthesized samples.

Porphyrin Derivatives
Porphyrins and their derivatives play an important role in chemistry and biology; they participate in light-harvesting, oxygen transport and catalytic transformations [239][240][241][242]. Ligand based on porphyrins benefit from their rigid scaffold, photophysical properties and allow to obtain a wide range of coordination polymers with diverse luminescent properties [243][244][245].

Tetrakis(4-Carboxyphenyl)porphyrin
Tetrakis(4-carboxyphenyl)porphyrin (H6tcpp, Table 8) is a D4h-symmetric tetratopic linker, which is widely used to construct MOFs. Rigid square geometry and free rotation of the terminal phenyl ring make it possible to obtain MOFs with various topologies. Zr-based MOF porphyrins have several topologies since the connectivity of Zr6 clusters can be tuned to 6-, 8-and 12-connetcted topologies, e.g., PCN-224 (she), PCN-222 / MOF-545 (csq) and MOF-525 (ftw) [246].  [238]. It was shown that the porous structures collapsed on the removal of water molecules from the channels, but the original porous structure was reconstructed upon water adsorption. Both MOFs exhibited emission bands near 640 nm similar to that of Ru ligand due to the 3 MLCT of the Ru ligand. The emission band underwent a blue-shift on increasing the relative humidity from 0 to 75% and from 0 to 54%, respectively. The emission spectra obtained at 100% humidity were almost identical to those of the as-synthesized samples.

Porphyrin Derivatives
Porphyrins and their derivatives play an important role in chemistry and biology; they participate in light-harvesting, oxygen transport and catalytic transformations [239][240][241][242]. Ligand based on porphyrins benefit from their rigid scaffold, photophysical properties and allow to obtain a wide range of coordination polymers with diverse luminescent properties [243][244][245].

Tetrakis(4-Carboxyphenyl)porphyrin
Tetrakis(4-carboxyphenyl)porphyrin (H6tcpp, Table 8) is a D4h-symmetric tetratopic linker, which is widely used to construct MOFs. Rigid square geometry and free rotation of the terminal phenyl ring make it possible to obtain MOFs with various topologies. Zr-based MOF porphyrins have several topologies since the connectivity of Zr6 clusters can be tuned to 6-, 8-and 12-connetcted topologies, e.g., PCN-224 (she), PCN-222 / MOF-545 (csq) and MOF-525 (ftw) [246].  [238]. It was shown that the porous structures collapsed on the removal of water molecules from the channels, but the original porous structure was reconstructed upon water adsorption. Both MOFs exhibited emission bands near 640 nm similar to that of Ru ligand due to the 3 MLCT of the Ru ligand. The emission band underwent a blue-shift on increasing the relative humidity from 0 to 75% and from 0 to 54%, respectively. The emission spectra obtained at 100% humidity were almost identical to those of the as-synthesized samples.

Porphyrin Derivatives
Porphyrins and their derivatives play an important role in chemistry and biology; they participate in light-harvesting, oxygen transport and catalytic transformations [239][240][241][242]. Ligand based on porphyrins benefit from their rigid scaffold, photophysical properties and allow to obtain a wide range of coordination polymers with diverse luminescent properties [243][244][245].

Tetrakis(4-Carboxyphenyl)porphyrin
Tetrakis(4-carboxyphenyl)porphyrin (H6tcpp, Table 8) is a D4h-symmetric tetratopic linker, which is widely used to construct MOFs. Rigid square geometry and free rotation of the terminal phenyl ring make it possible to obtain MOFs with various topologies. Zr-based MOF porphyrins have several topologies since the connectivity of Zr6 clusters can be tuned to 6-, 8-and 12-connetcted topologies, e.g., PCN-224 (she), PCN-222 / MOF-545 (csq) and MOF-525 (ftw) [246].  [238]. It was shown that the porous structures collapsed on the removal of water molecules from the channels, but the original porous structure was reconstructed upon water adsorption. Both MOFs exhibited emission bands near 640 nm similar to that of Ru ligand due to the 3 MLCT of the Ru ligand. The emission band underwent a blue-shift on increasing the relative humidity from 0 to 75% and from 0 to 54%, respectively. The emission spectra obtained at 100% humidity were almost identical to those of the as-synthesized samples.

Porphyrin Derivatives
Porphyrins and their derivatives play an important role in chemistry and biology; they participate in light-harvesting, oxygen transport and catalytic transformations [239][240][241][242]. Ligand based on porphyrins benefit from their rigid scaffold, photophysical properties and allow to obtain a wide range of coordination polymers with diverse luminescent properties [243][244][245].

Conclusions
To summarize, currently, a wide variety of highly emissive ligands is available to researchers, enabling them to tune the dimensionality and topology of the coordination polymers, as well as their functional properties, including sensing ability, thermochromism and photochromism, electroluminescence.
Despite the large number of luminescence data published for the coordination polymers, most works limit the results to the excitation and emission maxima, only some works report quantum yields, even in fewer works time-resolved photoluminescence experiments were carried out. The emission mechanisms in majority of the works are proposed tentatively, so the luminescence mechanisms of the coordination polymers are yet to be studied in detail using modern experimental techniques and DFT calculations.
It should be noted that derivatives of BODIPY dyes, ruthenium(II) and iridium(III) complexes, which demonstrate strong luminescence in the free state, but surprisingly, the coordination polymers based on these linkers did not exhibit high quantum yields (10% and less).
Many works that describe the synthesis of luminescent coordination polymers consider their sensory properties. However, it should be noted that the range of analytes studied is not very wide -in most cases these are simple aromatic compounds (benzene, toluene) and their nitro derivatives (nitrobenzene, di-and trinitrotoluene, nitrophenols), or a limited set of inorganic ions (Fe 3+ , Al 3+ , Cr 2 O 7 2− ). Sensing mechanisms are studied only in minor part of the works, so detailed evaluation of reasons for luminescent response, as well as inclusion of new analytes (biologically relevant molecules, environmental pollutants) seem to be promising areas for further research.