Photoluminescent Coordination Polymers Based on Group 12 Metals and 1 H -Indazole-6-Carboxylic Acid

: Two new coordination polymers (CPs) based on Zn(II) and Cd(II) and 1 H- indazole-6-carboxylic acid (H 2 L) of general formulae [Zn(L)(H 2 O)] n ( 1 ) and [Cd 2 (HL) 4 ] n ( 2 ) have been synthe-sized and fully characterized by elemental analyses, Fourier transformed infrared spectroscopy and single crystal X-ray diffraction. The results indicate that compound 1 possesses double chains in its structure whereas 2 exhibits a 3D network. The intermolecular interactions, including hydrogen bonds, C–H ··· π and π ··· π stacking interactions, stabilize both crystal structures. Photoluminescence (PL) properties have shown that compounds 1 and 2 present similar emission spectra compared to the free-ligand. The emission spectra are also studied from the theoretical point of view by means of time-dependent density-functional theory (TD-DFT) calculations to conﬁrm that ligand-centred π - π * electronic transitions govern emission of compound 1 and 2 . Finally, the PL properties are also studied in aqueous solution to explore the stability and emission capacity of the compounds. the structural point of view, indazole is an aromatic heterocyclic molecule a benzene ring fused a pyrazole ring [9]. tautomeric forms tautomer B aromaticity mode to establish a dimeric paddle-wheel shaped entity, whereas the non-protonated nitrogen atom of the pyrazole ring links to a third Cu(II) atom in a monodentate way (see the highlighted modes in Scheme 2). Aside from this work mainly focused on the description of a new compound, it should be pointed out that some Co(II)-based complexes with indazole derivatives have shown a capacity to bind to DNA [12].


Introduction
The study of coordination polymers (CPs) and metal-organic frameworks (MOFs) is at the forefront of modern inorganic chemistry due to their broad range of potential applications, spanning from magnetism and luminescence, through catalysis and sensing, to gas separation and storage, and biomedicine [1,2]. Through an adequate selection of their building blocks (metal ions and organic ligands), CPs and MOFs can be designed to enhance a particular property [3][4][5]. It is well known that nitrogen-containing heterocycles are molecules commonly employed as ligands owing to not only their good coordination ability, but also pharmacological relevance, given that they are important scaffolds widely present in numerous commercially available drugs [6]. The most famous are diazepam, isoniazid, chlorpromazine, metronidazole, barbituric acid, captopril, chloroquinine, azidothymidine and anti-pyrine. As a result of their diverse biological activities, nitrogen heterocyclic compounds have always been attractive targets to develop new active compounds. This is the case for 1H-indazole-6-carboxylic acid (H 2 L), a common moiety in the pharmaceutical industry [7]. Polysubstituted indazole-containing compounds furnished with different functional groups usually present significant pharmacological activities and serve as structural motifs in drug molecules (i.e., niraparib-anticancer drug, pazopanibapproved by the FDA for renal cell carcinoma, bendazac and benzydamine-antiinflamatory Inorganics 2021, 9,20 2 of 12 drugs) [8]. From the structural point of view, indazole is an aromatic heterocyclic molecule with a benzene ring fused to a pyrazole ring [9]. It shows three tautomeric forms (Scheme 1) being tautomer A favoured over B and C due to its higher degree of aromaticity [10].
x FOR PEER REVIEW 2 of 12 pazopanib-approved by the FDA for renal cell carcinoma, bendazac and benzydamineantiinflamatory drugs) [8]. From the structural point of view, indazole is an aromatic heterocyclic molecule with a benzene ring fused to a pyrazole ring [9]. It shows three tautomeric forms (Scheme 1) being tautomer A favoured over B and C due to its higher degree of aromaticity [10]. H2L is presented here as an ideal candidate to form CPs or MOFs as it possesses multiple coordination possibilities, not only derived from its carboxylate group, but also from its pyrazole function. Thus, a great variety of coordination modes are possible, according to similar ligands containing carboxylate and pyrazole chemical functions in crystallized complexes (Scheme 2). Until now, only one complex based on this ligand has been reported so far [11]. In that work, Kruger et al. described in detail four substituted indazole derivatives containing pyridine or carboxylic functionalities upon coordination with Cu(II) ions in solution and solid state. In the complex, 1H-indazole-6-carboxylate acts as a bridging ligand showing a tridentate coordination mode: the carboxylate group coordinates to two Cu(II) atoms in a syn,syn mode to establish a dimeric paddle-wheel shaped entity, whereas the non-protonated nitrogen atom of the pyrazole ring links to a third Cu(II) atom in a monodentate way (see the highlighted modes in Scheme 2). Aside from this work mainly focused on the description of a new compound, it should be pointed out that some Co(II)-based complexes with indazole derivatives have shown a capacity to bind to DNA [12].
Possible coordination modes of 1H-indazole-6-caboxylate ligand. Note that only those two modes highlighted ve been described in bibliography whereas the rest correspond to potential binding modes.
On the other hand, H2L may also present interesting photoluminescence (PL) properties due to its aromatic nature and the presence of carboxylic groups, with potentially strong light absorption [13]. When these indazole-carboxylate ligands are coordinated to metal centres in the crystal structure of a CP, PL tends to be enhanced by means of the well-known crystal-induced luminescence effect [14]. Among others, metal ions from group 12 are particularly appropriate for their use in PL as they present a closed-shell electronic configuration in which d-d transitions cannot occur [15,16]. In fact, many CPs and MOFs formed by these metal ions have been reported during the last decade [17,18], some of which present not only strong and bright fluorescent emissions, but also long- H 2 L is presented here as an ideal candidate to form CPs or MOFs as it possesses multiple coordination possibilities, not only derived from its carboxylate group, but also from its pyrazole function. Thus, a great variety of coordination modes are possible, according to similar ligands containing carboxylate and pyrazole chemical functions in crystallized complexes (Scheme 2). Until now, only one complex based on this ligand has been reported so far [11]. In that work, Kruger et al. described in detail four substituted indazole derivatives containing pyridine or carboxylic functionalities upon coordination with Cu(II) ions in solution and solid state. In the complex, 1H-indazole-6-carboxylate acts as a bridging ligand showing a tridentate coordination mode: the carboxylate group coordinates to two Cu(II) atoms in a syn,syn mode to establish a dimeric paddle-wheel shaped entity, whereas the non-protonated nitrogen atom of the pyrazole ring links to a third Cu(II) atom in a monodentate way (see the highlighted modes in Scheme 2). Aside from this work mainly focused on the description of a new compound, it should be pointed out that some Co(II)-based complexes with indazole derivatives have shown a capacity to bind to DNA [12]. pazopanib-approved by the FDA for renal cell carcinoma, bendazac and benzyda antiinflamatory drugs) [8]. From the structural point of view, indazole is an aromat erocyclic molecule with a benzene ring fused to a pyrazole ring [9]. It shows three meric forms (Scheme 1) being tautomer A favoured over B and C due to its higher d of aromaticity [10]. H2L is presented here as an ideal candidate to form CPs or MOFs as it possesse tiple coordination possibilities, not only derived from its carboxylate group, but also its pyrazole function. Thus, a great variety of coordination modes are possible, acco to similar ligands containing carboxylate and pyrazole chemical functions in crysta complexes (Scheme 2). Until now, only one complex based on this ligand has be ported so far [11]. In that work, Kruger et al. described in detail four substituted ind derivatives containing pyridine or carboxylic functionalities upon coordination Cu(II) ions in solution and solid state. In the complex, 1H-indazole-6-carboxylate ac bridging ligand showing a tridentate coordination mode: the carboxylate group c nates to two Cu(II) atoms in a syn,syn mode to establish a dimeric paddle-wheel s entity, whereas the non-protonated nitrogen atom of the pyrazole ring links to a Cu(II) atom in a monodentate way (see the highlighted modes in Scheme 2). Aside this work mainly focused on the description of a new compound, it should be point that some Co(II)-based complexes with indazole derivatives have shown a capa bind to DNA [12]. Scheme 2. Possible coordination modes of 1H-indazole-6-caboxylate ligand. Note that only those two modes highlighte in black have been described in bibliography whereas the rest correspond to potential binding modes.
On the other hand, H2L may also present interesting photoluminescence (PL) erties due to its aromatic nature and the presence of carboxylic groups, with pote strong light absorption [13]. When these indazole-carboxylate ligands are coordina metal centres in the crystal structure of a CP, PL tends to be enhanced by means well-known crystal-induced luminescence effect [14]. Among others, metal ions group 12 are particularly appropriate for their use in PL as they present a closed electronic configuration in which d-d transitions cannot occur [15,16]. In fact, man and MOFs formed by these metal ions have been reported during the last decade [ some of which present not only strong and bright fluorescent emissions, but also Scheme 2. Possible coordination modes of 1H-indazole-6-caboxylate ligand. Note that only those two modes highlighted in black have been described in bibliography whereas the rest correspond to potential binding modes.
On the other hand, H 2 L may also present interesting photoluminescence (PL) properties due to its aromatic nature and the presence of carboxylic groups, with potentially strong light absorption [13]. When these indazole-carboxylate ligands are coordinated to metal centres in the crystal structure of a CP, PL tends to be enhanced by means of the well-known crystal-induced luminescence effect [14]. Among others, metal ions from group 12 are particularly appropriate for their use in PL as they present a closed-shell electronic configuration in which d-d transitions cannot occur [15,16]. In fact, many CPs and MOFs formed by these metal ions have been reported during the last decade [17,18], some of which present not only strong and bright fluorescent emissions, but also long-lived phosphorescence that may be traced by the naked eye [19][20][21]. Moreover, the presence of these ions may also promote ligand-to-metal charge transfer (LMCT) as metal ions possess empty orbitals that can be populated in the excited state, and therefore the PL emission may be modulated with regard to the ligand-centred (LC) emissions [22,23]. Irrespective of the luminescence mechanism occurring in these systems, the interest for group 12-based compounds has increased given their potential application as not only lighting devices, but also as luminescence-based molecular detectors [24], thermometers [25] and anti-counterfeiting inks, among others [26]. Particularly for indazole derivatives playing as ligands, many Zn-/Cd-indazole complexes have already proved efficient luminescent CPs under UV irradiation [27].
Considering all the above, in this work we present the synthesis, structural characterisation and PL properties of two new coordination polymers based on group 12 metals and 1H-indazole-6-carboxylic acid of general formula [Zn(L)(H 2 O)] n (1) and [Cd 2 (HL) 4 ] n (2). Their emission characteristics have been studied both from the theoretical and experimental points of view, involving the measurements in the solid state as well as in aqueous medium.

Results and Discussion
The reaction of 1H-indazole-6-carboxylic acid gave rise to two compounds based on group 12 metals which exhibit a different structural dimensionality. In particular, the solvothermal reaction of the 1H-indazole-6-carboxylic acid ligand with zinc acetate salt (Zn(CH 3 COO) 2 ) using a 1:2 molar ratio in a N,N-dimethylformamide/water (DMF/H 2 O) mixture afforded a 1D CP, namely 1 (see Experimental Section for further details). Similarly, the use of cadmium acetate salt (Cd(CH 3 COO) 2 ) salt in the synthesis, successfully led to a 3D MOF, namely 2. This fact can be explained by the larger ion size of Cd(II), which may admit higher coordination numbers, involving the participation of additional ligands and increasing the metal-to-ligand connectivity.

Description of the Structures
Compound 1 crystallizes in the P2 1 /n space group and consists of a double chain structure in which Zn(II) ions are bridged by nitrogen atoms of L 2− in a bidentate way, giving rise to a stable and in plane Zn 2 N 4 dimeric core as a six membered ring ( Figure 1).
lived phosphorescence that may be traced by the naked eye [19][20][21]. Moreover, the pr ence of these ions may also promote ligand-to-metal charge transfer (LMCT) as metal io possess empty orbitals that can be populated in the excited state, and therefore the emission may be modulated with regard to the ligand-centred (LC) emissions [22,23]. respective of the luminescence mechanism occurring in these systems, the interest group 12-based compounds has increased given their potential application as not o lighting devices, but also as luminescence-based molecular detectors [24], thermomet [25] and anti-counterfeiting inks, among others [26]. Particularly for indazole derivativ playing as ligands, many Zn-/Cd-indazole complexes have already proved efficient lum nescent CPs under UV irradiation [27]. Considering all the above, in this work we present the synthesis, structural char terisation and PL properties of two new coordination polymers based on group 12 met and 1H-indazole-6-carboxylic acid of general formula [Zn(L)(H2O)]n (1) and [Cd2(HL (2). Their emission characteristics have been studied both from the theoretical and exp imental points of view, involving the measurements in the solid state as well as in aqueo medium.

Results and Discussion
The reaction of 1H-indazole-6-carboxylic acid gave rise to two compounds based group 12 metals which exhibit a different structural dimensionality. In particular, the s vothermal reaction of the 1H-indazole-6-carboxylic acid ligand with zinc acetate s (Zn(CH3COO)2) using a 1:2 molar ratio in a N,N-dimethylformamide/water (DMF/H mixture afforded a 1D CP, namely 1 (see Experimental Section for further details). Sim larly, the use of cadmium acetate salt (Cd(CH3COO)2) salt in the synthesis, successfu led to a 3D MOF, namely 2. This fact can be explained by the larger ion size of Cd( which may admit higher coordination numbers, involving the participation of additio ligands and increasing the metal-to-ligand connectivity.

Structural Description of [Zn(L)(H2O)]n (1)
Compound 1 crystallizes in the P21/n space group and consists of a double ch structure in which Zn(II) ions are bridged by nitrogen atoms of L 2− in a bidentate w giving rise to a stable and in plane Zn2N4 dimeric core as a six membered ring ( Figure   Figure 1. Representation of the 1D polymeric chain in which Zn2N4 planar six membered ring is observed (zinc, nitrogen, oxygen, and carbon are represented in green, blue, red, and grey, respectively; hydrogen atoms are omitted for clarity).
The Zn(II) ion is also coordinated to a carboxylate moiety of the indazole derivat ligand in a monodentate way, which extends the dimeric entity into infinite 1D cha running along the crystallographic [100] direction. The coordination sphere of Zn is co pleted by the coordination of a water molecule (see Table S1 in the ESI for further inf mation about bond lengths and angles). The ZnN2O2 coordination sphere can be describ as a tetrahedron, although Zn ions show a geometry close to an axially vacant trigo bipyramid according to continuous-shape-measures (CShMs) using SHAPE software (T bles S2 and S3, in the ESI) [28].  The Zn(II) ion is also coordinated to a carboxylate moiety of the indazole derivative ligand in a monodentate way, which extends the dimeric entity into infinite 1D chains running along the crystallographic [100] direction. The coordination sphere of Zn is completed by the coordination of a water molecule (see Table S1 in the ESI for further information about bond lengths and angles). The ZnN 2 O 2 coordination sphere can be described as a tetrahedron, although Zn ions show a geometry close to an axially vacant trigonal bipyramid according to continuous-shape-measures (CShMs) using SHAPE software (Tables S2  and S3, in the ESI) [28].
The packing of the double chains is ruled by intermolecular interactions, among which hydrogen bonding interactions established between coordination water molecules and carboxylate oxygen atoms are to be highlighted ( Figure 2). In particular, the coordinated water molecule is involved in hydrogen bonding interactions in which non-coordinated carboxylate oxygen atoms belonging to adjacent chains act as receptors. Additionally, the angle formed among neighbouring chains allows for the formation of C-H···π interactions between aromatic rings, reinforcing the stability of the supramolecular crystal building (see Figure S5 in the ESI). and carboxylate oxygen atoms are to be highlighted ( Figure 2). In particular, the coordinated water molecule is involved in hydrogen bonding interactions in which non-coordinated carboxylate oxygen atoms belonging to adjacent chains act as receptors. Additionally, the angle formed among neighbouring chains allows for the formation of C-H•••π interactions between aromatic rings, reinforcing the stability of the supramolecular crystal building (see Figure S5 in the ESI).

Structural Description of [Cd2(HL)4]n (2)
Compound 2 crystallizes in the triclinic P-1 space group. The asymmetric unit contains two non-equivalent Cd(II) atoms and four ligand molecules. Each Cd(II) ion is connected to two monodentate indazole nitrogen atoms and four oxygen atoms of the carboxylate group of the ligand. Cd1 and Cd2 ions are doubly linked by ancillary syn-anti carboxylate moieties of 1-H-indazole-6-carboxylate ligands (namely A, C and D). However, the carboxylate group of B ligand presents a different coordination pattern, in which O1B connects in a monodentate way to both Cd1 and Cd2 atoms giving rise to alternating five and six membered rings (Figure 3, see also the view along b axis in Figure S6 in the ESI), whereas O2B atom remains unconnected to any metal centre.

Structural Description of [Cd 2 (HL) 4 ] n (2)
Compound 2 crystallizes in the triclinic P-1 space group. The asymmetric unit contains two non-equivalent Cd(II) atoms and four ligand molecules. Each Cd(II) ion is connected to two monodentate indazole nitrogen atoms and four oxygen atoms of the carboxylate group of the ligand. Cd1 and Cd2 ions are doubly linked by ancillary syn-anti carboxylate moieties of 1-H-indazole-6-carboxylate ligands (namely A, C and D). However, the carboxylate group of B ligand presents a different coordination pattern, in which O1B connects in a monodentate way to both Cd1 and Cd2 atoms giving rise to alternating five and six membered rings (Figure 3, see also the view along b axis in Figure S6 in the ESI), whereas O2B atom remains unconnected to any metal centre.  CShMs indicate that different ligand coordination modes affect the connectivity of the metal centres, which leads to the formation of a distinct crystal structure. When comparing 1 and 2 compounds, the coordination spheres of Cd1 and Cd2 are described as octahedra according to SHAPE measurements (Tables S2 and S3 in the SI). M•••N2 distances are slightly shorter than in compound 1 (in the 1.969 and 2.293-2.316 Å ranges, respectively), similarly to the M•••O1carboxylate bond distances (between 1.935 and 2.320 Å, see Table S1 in the ESI). As a result, a 3D framework is obtained in the case of 2 by the further linkage of the carboxylate groups to Cd(II) atoms along a metal-carboxylate rod ( Figure 4). Considering the connectivity of the metal ions and HL ligands, this framework may be described as a 5,6T24 topological network with the (3 2 .4 2 .5 2 .6 3 .7)2(3 2 .4 4 .5 4 .6 2 .7 3 ) point symbol, as previously observed in the MOF of [Al2(OH)2(H2O)2(C10O8H2)] or MIL-118A [29].
To end up with the structural description, it is worth mentioning that compound 2 presents some remarkable supramolecular interactions that reinforce its packing. Unlike with hydrogen bonding and C-H••• π interactions governing the crystal structure of 1, 2 contains π•••π stacking interactions. In particular, the aromatic rings of HL promote strong face-to-face contacts among the whole structure (see Figure S5 in the ESI). CShMs indicate that different ligand coordination modes affect the connectivity of the metal centres, which leads to the formation of a distinct crystal structure. When comparing 1 and 2 compounds, the coordination spheres of Cd1 and Cd2 are described as octahedra according to SHAPE measurements (Tables S2 and S3 in the SI). M···N2 distances are slightly shorter than in compound 1 (in the 1.969 and 2.293-2.316 Å ranges, respectively), similarly to the M···O1 carboxylate bond distances (between 1.935 and 2.320 Å, see Table S1 in the ESI). As a result, a 3D framework is obtained in the case of 2 by the further linkage of the carboxylate groups to Cd(II) atoms along a metal-carboxylate rod (Figure 4). Considering the connectivity of the metal ions and HL ligands, this framework may be described as a 5,6T24 topological network with the (3 2 .4 2 .5 2 .6 3 .7) 2 (3 2 .

Fourier Transformed Infrared (FTIR) SpectroscopyP
The analysis of the FTIR spectra of 1 and 2 confirms the coordination of zinc(II) and cadmium(II) ions to the N-containing carboxylate ligand ( Figure S1, see the ESI). FTIR To end up with the structural description, it is worth mentioning that compound 2 presents some remarkable supramolecular interactions that reinforce its packing. Unlike with hydrogen bonding and C-H··· π interactions governing the crystal structure of 1, 2 contains π···π stacking interactions. In particular, the aromatic rings of HL promote strong face-to-face contacts among the whole structure (see Figure S5 in the ESI).

Fourier Transformed Infrared (FTIR) Spectroscopy
The analysis of the FTIR spectra of 1 and 2 confirms the coordination of zinc(II) and cadmium(II) ions to the N-containing carboxylate ligand ( Figure S1, see the ESI). FTIR spectra of both compounds confirmed a shift in the wavelengths in comparison to the pure linker, suggesting the formation of interactions between the linker and the metals. The main vibrations of 1H-indazole-6-caboxylic acid associated with the

Fourier Transformed Infrared (FTIR) SpectroscopyP
The analysis of the FTIR spectra of 1 and 2 confirms the coordination of zinc(II) and cadmium(II) ions to the N-containing carboxylate ligand ( Figure S1, see the ESI). FTIR spectra of both compounds confirmed a shift in the wavelengths in comparison to the pure linker, suggesting the formation of interactions between the linker and the metals. The main vibrations of 1H-indazole-6-caboxylic acid associated with the ƲC=N stretching vibration at 1633 cm −1 , and the asymmetric and symmetric vibrations of the carboxylate groups at 1683 and 1423 cm −1 are shifted when compared with the spectra of 1 and 2. The bands found at 1537 and 1589 cm −1 for complex 1 and 2, respectively, are attributed to the ƲC=N stretching vibration of the indazole ring [30]. Moreover, the strong absorption peak observed at 1558 and 1402 cm −1 for 1, and 1541 and 1411 cm −1 for 2, respectively, revealed the asymmetric and symmetric vibrations of the carboxylic groups [31]. Finally, the strong broad band in the range of 3317-3086 cm −1 was assigned to the O-H stretching vibration of the coordinated water molecule in complex 1 [32].

Luminescence Properties
As previously mentioned, complexes consisting of metal ions with d 10 electronic configuration are known to yield strong PL emissions. The completely filled d-orbitals disable ligand field d-d transitions, eliminating fluorescence quenching and allowing the occurrence of PL [14]. Thus, the development of d 10 -based compounds is interesting for photochemical, electroluminescence and sensing applications [17,33]. The extended aromaticity of the 1H-indazole-6-carboxylate ligand coordinated to Zn(II) and Cd(II) atoms suggests the existence of emissive properties of 1 and 2. The emission of these compounds are found to be similar to ligand emission, which may stem from the ligand-centred π-π* electronic transitions, as shown in Figure 5. Consequently, it can be suggested that the highly conjugated 1H-indazole-6-caboxylate ligand is the main part contributing to the emission [34]. An intense broad band at 350-450 nm dominates the emission spectra of all compounds upon 325 nm excitation (in view of the maxima found in the excitation spectra), among which the maxima at 362 and 388 nm, 363 and 381 nm, and 363 and 391 nm  Figure S1, see the ESI). FTIR ed a shift in the wavelengths in comparison to the n of interactions between the linker and the metals. 6-caboxylic acid associated with the ƲC=N stretching metric and symmetric vibrations of the carboxylate fted when compared with the spectra of 1 and 2. The r complex 1 and 2, respectively, are attributed to the zole ring [30]. Moreover, the strong absorption peak , and 1541 and 1411 cm −1 for 2, respectively, revealed tions of the carboxylic groups [31]. Finally, the strong 6 cm −1 was assigned to the O-H stretching vibration complex 1 [32].
lexes consisting of metal ions with d 10 electronic con-PL emissions. The completely filled d-orbitals disable ing fluorescence quenching and allowing the occurent of d 10 -based compounds is interesting for photonsing applications [17,33]. The extended aromaticity nd coordinated to Zn(II) and Cd(II) atoms suggests of 1 and 2. The emission of these compounds are ion, which may stem from the ligand-centred π-π* igure 5. Consequently, it can be suggested that the oxylate ligand is the main part contributing to the at 350-450 nm dominates the emission spectra of all (in view of the maxima found in the excitation spec-2 and 388 nm, 363 and 381 nm, and 363 and 391 nm stretching vibration of the indazole ring [30]. Moreover, the strong absorption peak observed at 1558 and 1402 cm −1 for 1, and 1541 and 1411 cm −1 for 2, respectively, revealed the asymmetric and symmetric vibrations of the carboxylic groups [31]. Finally, the strong broad band in the range of 3317-3086 cm −1 was assigned to the O-H stretching vibration of the coordinated water molecule in complex 1 [32].

Luminescence Properties
As previously mentioned, complexes consisting of metal ions with d 10 electronic configuration are known to yield strong PL emissions. The completely filled d-orbitals disable ligand field d-d transitions, eliminating fluorescence quenching and allowing the occurrence of PL [14]. Thus, the development of d 10 -based compounds is interesting for photochemical, electroluminescence and sensing applications [17,33]. The extended aromaticity of the 1H-indazole-6-carboxylate ligand coordinated to Zn(II) and Cd(II) atoms suggests the existence of emissive properties of 1 and 2. The emission of these compounds are found to be similar to ligand emission, which may stem from the ligand-centred π-π* electronic transitions, as shown in Figure 5. Consequently, it can be suggested that the highly conjugated 1H-indazole-6-caboxylate ligand is the main part contributing to the emission [34]. An intense broad band at 350-450 nm dominates the emission spectra of all compounds upon 325 nm excitation (in view of the maxima found in the excitation spectra), among which the maxima at 362 and 388 nm, 363 and 381 nm, and 363 and 391 nm can be distinguished for the ligand, and compounds 1 and 2, respectively; which imbues all compounds with blue emission. The similar emission band of 2 and the free ligand must be attributed to the fact that 2 possesses the protonated form of the ligand (HL − ) whereas it is completely deprotonated (L 2− ) in 1. In a comparative scale, the ligand spectrum shows two well-defined maxima (not that easily identified for the compounds) and relatively higher intensity ( Figure S2, SI). It is worth noticing that the observed luminescence resembles to that shown by other previously reported CPs containing other isomers of indazolecarboxylates [35,36]. must be attributed to the fact that 2 possesses the protonated form of the ligand (HL whereas it is completely deprotonated (L 2− ) in 1. In a comparative scale, the ligand spec trum shows two well-defined maxima (not that easily identified for the compounds) an relatively higher intensity ( Figure S2, SI). It is worth noticing that the observed lumines cence resembles to that shown by other previously reported CPs containing other isomer of indazole-carboxylates [35,36]. In order to get a deeper insight into the emission mechanism, TD-DFT calculation were performed on suitable models of compounds 1 and 2. The calculated spectra repro duce fairly well the experimental ones, indicating that the process is driven by single transitions occurring between the molecular orbitals shown in Figure 5. In both cases, th electron density in HOMO orbitals, HOMO-2 and HOMO-4 for compound 1 and 2, re spectively, extend over the bonds over the whole ligand molecule (signifying a π orbita In order to get a deeper insight into the emission mechanism, TD-DFT calculations were performed on suitable models of compounds 1 and 2. The calculated spectra reproduce fairly well the experimental ones, indicating that the process is driven by singlet transitions occurring between the molecular orbitals shown in Figure 5. In both cases, the electron density in HOMO orbitals, HOMO-2 and HOMO-4 for compound 1 and 2, respectively, extend over the bonds over the whole ligand molecule (signifying a π orbital) whereas LUMO orbitals, LUMO+2 and LUMO+1 for compound 1 and 2, respectively, feature a π* character. Therefore, it can be stated that the transitions involved in the photoluminescence of compound 1 and 2 are mainly of π*←π nature induced by ligand centred emission.
Inspired by the potential biomedical properties of the ligand on the basis of its similar structure to other indazole derivatives [7,8], we studied the stability and fluorescence performance of these compounds in aqueous media in order to explore their performance as luminescent probes. First, the stability of both compounds was confirmed by recording UV-Vis absorption spectra on aqueous solutions of both compounds immediately after their solution and also after 24 h (see Figure S7 in the ESI). These spectra show the presence of three main absorption bands (sited at ca. 215, 265 and in the 340 nm for 1 and 220, 265 and 305 nm for 2), corresponding to intraligand and/or ligand-to-metal charge transfers occurring in the complexes. It is worth noticing that these bands are in good agreement with the experimental and TD-DFT computed excitation bands, finding only slight shifts that may be attributed to the different media in which the spectra are recorded (water for UV-Vis and solid state for excitation spectra). Moreover, these solutions were also employed to measure the PL emission spectra of both compounds. As observed in Figure  S4, the emission spectra acquired in this medium do not significantly differ from those measured in solid state but for a drop in the intensity of the signal, which is an expected behaviour given that the capacity of this solvent to quench the PL is a largely reported effect [37]. All these results suggest that these new compounds could show potential activity as luminescent probes in some particular biological media (i.e., as biosensor), a fact that a priori excludes most of biological tissues owing to their low transparency (high light absorption capacity) to the blue emission (λ em = 350-450 nm) shown by these compounds.

Materials and Physical Measurements
All the reagents were purchased commercially and used without any previous purification. Elemental analysis (C, H and N) were carried out at the Centro de Instrumentación Científica (University of Granada) on a Fisons-Carlo Erba analyzer model EA 1108 (Thermo Scientific, Waltham, MA, USA). FTIR spectra (400-4000 cm −1 ) were recorded on a Nicolet FT-IR 6700 spectrometer (Thermo Scientific, Madrid, Spain) in KBr pellets.

Synthesis of [Zn(L)(H 2 O)] n (1)
0.010 g (0.006 mmol) of 1H-indazole-6-carboxylic acid (H 2 L) was dissolved in 0.5 mL of DMF. Then, 0.5 mL of water was added to the ligand solution. In a separate vial, 0.0134 g (0.03 mmol) of Zn(CH 3 COO) 2 was dissolved in 0.5 mL of water. Similarly, once metal salt was dissolved, 0.5 mL DMF were added to the solution. Metal solution was added dropwise to the ligand solution, and the resulting colourless mixture was placed in a closed glass vessel and heated in an oven at 100 • C for 24 h. X-ray quality crystals of 1 were obtained during heating process under autogenous pressure and washed with water. Yield: 64% based on Zn. Anal Calcd. for C 8 11.59. In addition to the elemental analyses, the purity of all the samples was checked by FT-IR spectra.

Synthesis of [Cd 2 (HL) 4 ] n (2)
The same synthetic procedure was carried out to obtain complex 2, by replacing Zn(CH 3 COO) 2 by 0.01651 g (0.03 mmol) of Cd(CH 3 COO) 2 . X-ray quality crystals were obtained and washed with water. Yield: 54% based on Cd. Anal Calcd. for C 32

Crystallographic Refinement and Structure Solution
Single crystals of suitable dimensions were used for data collection. For compound 1 and 2, diffraction intensities were recorded on a Bruker X8 APEX II and Bruker D8 Venture with a Photon detector (Bruker, Madrid, Spain) equipped with graphite monochromated MoKα radiation (λ = 0.71073 Å). The data reduction was performed with the APEX2 software [38] and corrected for absorption using SADABS [39]. In all cases, the structures were solved by direct methods and refined by full-matrix least-squares with SHELXL-2018 [40]. The main refinement parameters are listed in Table 1. Details of selected bond lengths and angles are given in Table S2 in the ESI. CCDC reference numbers for the  structures are 1,948,382 and 1,948,383 for Cd and Zn coordination polymers, respectively.

Photophysical Measurements
UV-Vis absorption spectra were recorded on UV-2600 UV/vis Shimadzu spectrophotometer using polycrystalline samples of compounds 1 and 2. PL measurements were carried out on crystalline samples at room temperature using a Varian Cary-Eclipse fluorescence spectrofluorometer equipped with a Xe discharge lamp (peak power equivalent to 75 kW), Czerny-Turner monochromators, and an R-928 photomultiplier tube. For the fluorescence measurements, the photomultiplier detector voltage was fixed at 600 V, and the excitation and emission slits were set at 5 and 2.5 nm, respectively. Phosphorescence spectra were recorded with a total decay time of 20 ms, delay time of 0.2 ms and gate time of 5.0 ms. The photomultiplier detector voltage was set at 800 V, and both excitation and emission slits were open to 10 nm.

Conclusions
The reaction between 1H-indazole-6-carboxylic acid ligand and Zn(II) or Cd(II) leads the formation of two new coordination polymers with different dimensionalities. Compound 1 possesses a double chain structure, whereas compound 2 exhibits a 3D structure. Emissive properties of both complexes have been studied demonstrating that their photoluminescent emission is driven by the ligand centred π*←π transition. The similar luminescent properties between compound 2 and the linker may be consequence of the partially protonated HL − ligand present in 2. This work is pioneer in studying and comparing the luminescent properties of 1H-indazole-6-carboxylic acid (H 2 L) and its complexes, which represent a common moiety in pharmaceutical industry. In this regard, novel materials based on this ligand are being developed in our laboratory using lanthanide ions to enhance their luminescent properties.

Supplementary Materials:
The following are available online at https://www.mdpi.com/2304-674 0/9/3/20/s1, Figure S1: Infrared spectra of the ligand and compounds 1 and 2, Figure S2: Emission spectra of the ligand and compounds 1 and 2 under λ ex = 325 nm, Figure S3: Excitation spectra of compounds monitored at the emission maxima: (a) λ em = 381 nm for 1 and (b) λ em = 391 nm for 2, Figure S4: Comparative view of the absorption spectra of compounds (a) 1 and (b) 2 in solid state and aqueous solution, Figure S5: The most representative intermolecular interactions and packing modes for complexes 1 (up) and 2 (down). H bonds, π···π and C-H···π interactions are shown with dashed blue, green and orange lines, respectively, Figure S6: View along a (left), b (middle) and c (right) axis of complex 1 (up) and 2 (down), Figure S7: UV-Vis spectra of compounds (a) 1 and (b) 2 in aqueous solutions acquired at times (0 h and after 24 h), Table S1: Selected bond lengths (Å) and angles ( • ) for complexes 1 and 2,