Three-Dimensional-Coordination Polymer of Zn(II)-Carboxylate: Structural Elucidation, Photoelectrical Conductivity, and Biological Activity.

A newly designed mixed-ligand coordination polymer [Zn4(bdc)4(ppmh)2(H2O)] n (1) (H2 bdc = 1,4-benzene dicarboxylic acid, ppmh = N-pyridin-2-yl-N'-pyridin-4-ylmethylene-hydrazine) has been characterized using different physicochemical techniques. The structure has been confirmed by single crystal X-ray diffraction measurements. There are two pyridyl-N and one hydrazino-imine-N donor centers in ppmh, where two pyridyl-Ns bind simultaneously to two Zn(II) to serve as a bridging agent to form a coordination polymer. The 1,4-benzene dicarboxylato (bdc) is ligated via the aromatic dicarboxylato-O to form a one-dimensional (1D) chain. These two 1D chains about Zn(II) constitute a two-dimensional structure, which undergoes noncovalent interactions (C-H···π and π···π) to generate a three-dimensional supramolecular assembly. Electrical conductivity of 1 is higher by 1 order (1.37 × 10-6 S/cm) than that of the free ligand, ppmh (6.2 × 10-7 S/cm). Especially, the responsivity of the compound 1 was 56.21 mA/W, which is 11 times higher than that of the ligand ppmh (5.12 mA/W). The specific detectivity of the compound was 2.17 × 1010 Jones, which is also almost 10 times higher with respect to the specific detectivity of the ligand-based device (4.53 × 109 Jones). The results show that the compound can be valuable for optoelectronic fields. The biological studies suggest that compound 1 is antibacterial as well as a promising anticancer agent (LD50, 42.2 μg/mL against HepG2 cells), while ligands remain silent. Investigation of the mechanism of the cell killing activity of compound 1 accounts the generation of intracellular reactive oxygen species.

Introduction

Zinc is known for many centuries as one of the versatile metals and plays vital roles in the growth of civilization and industrial applications.[1−3] It is useful in corrosion resistance, painting, in battery, energy storage, etc. A Zn(II) ion (d10) is redox-innocent, has closed cell electronic configuration, and is diamagnetic, and its abundance is next to iron, in living beings.[4] Many researchers are now engaged in designing coordination polymers (CPs)/metal organic framework (MOF), where Zn(II) is used as a metal knot with the coordination of organic linkers like carboxylates, N-heterocycles, imines, azophenols, etc.[5−13] Thus, a linker may generate coordination polymers (CPs). The wide range of interest in these materials is due to their esthetic structural diversity, high thermal and chemical stability than that of simple coordination complexes, photochemical and electrochemical applications, high conductivity, etc.[12−26] Because of the electrical and photoelectrical properties, some of them are useful materials for electronic devices.[8]

Recently, our research group has reported a number of coordination polymers (CPs) using dicarboxylato ligands as linkers for Schottky diode applications.[27−31] Ambitious applications of dicarboxylato Zn(II)-MOF in different prospective fields have stimulated us to design some newer molecules for exploration of new activities. Herein, we report a Zn(II)-based CP, [Zn4(bdc)4(ppmh)2(H2O)] (1); the compound 1 shows Schottky diode barrier (SDB) activity (conductivity: 6.2 × 10–7 S/cm for N-pyridin-2-yl-N′-pyridin-4-ylmethylene-hydrazine (ppmh) and 1.37 × 10–6 S/cm for the compound 1); and the electrical conductivity is enhanced on irradiation of light. The density functional theory (DFT) calculation of the optimized geometry of the compound has been attempted for the explanation of the electronic and conducting features. Interestingly, the compound, 1, shows considerable antibacterial activity [IC50, 190 ± 2.4 μg/mL (Escherichia coli) and IC50, 185.96 ± 3.04 μg/mL (Staphylococcus aureus)] along with anticancer activity.

Results and Discussion

Structural Descriptions

The ligand, ppmh, crystallized by slow evaporation of methanol solution belongs to a monoclinic system and space group P21/n with Z = 8. The condensation of −CHO and −NH2 has been supported by the presence of C=N, N(5)–C(5) 1.273(2) Å (Figure and Table S1). The presence of noncovalent interactions (C–H···π, π···π) makes the structure of the molecule robust. There are π···π interactions (3.806 Å), CH···π interactions (2.826 Å) (CH···π 147.5°), and H-bonding interactions (2.227 Å, 175.28°) (2.57 Å, 160°) shown in Figure S1.

Figure 1

Molecular structure of ppmh.

The crystal structure of [Zn4(bdc)4(ppmh)2(H2O)] (1) is monoclinic crystal system having the space group P21 with Z = 2. The square pyramidal unit of the coordination entity is shown in Figure , and the selective bond parameters are listed in Table S2. The asymmetric unit contains four Zn(II) centers that are assigned as Zn01, Zn02, Zn03, and Zn04. The pairs of Zn(II) centers Zn01, Zn03 and Zn02, Zn04 are equivalent. Zn01 is bonded to one ppmh and three bdc ligands; ppmh is just appended from the metal center, and bdc acts as a bridging motif via chelation and in a rectilinear fashion. The Zn02 center is chelated by the two N-donor centers (pyridyl-N, imine-N) from a ppmh ligand and coordinated from another ppmh, and two bdc ligands bind in a monodonating fashion. Thus, Zn01 forms ZnO4N and Zn02 forms ZnN3O2 coordination arrangements. The bond lengths Zn01–O(bdc) lie in the accepted range (Zn01–O1, 1.997(7) Å; Zn01–O8, 2.533(9) Å; Zn01–O9, 1.975(8) Å, and Zn01–O15, 1.984(8) Å; and Zn01–N2, 2.035(8) Å).

Figure 2

Asymmetric unit of metal organic polymeric compound 1.

These bond parameters support the distorted geometry around the central metal ion. The above-described coordination unit undergoes repetition to form a polymeric structure (Figure ). The polymerization has taken place via coordination of bdc and propagates in two directions, and from the ppmh ligand, it has extended to form a polymer. The higher dimension of the molecule has also been prompted by different interactions (Figure ).

Figure 3

(a) One-dimensional polymeric chain. (b) Two-dimensional network.

Figure 4

(a) H-bonding interactions in compound 1. (b) Supramolecular assembly of compound 1.

However, the phase purity of the powdered sample of crystalline ppmh and compound 1 has been verified by powder X-ray diffraction (PXRD). The simulated (at room temperature) and as-synthesized PXRD patterns are well matched, which supports the purity and consistency of the bulk material used for application purposes (Figure S3).

Optical Analysis

UV–vis absorption analysis of the thin film of the materials was done to analyze the extent of light absorption as well as to determine the band gap of the material. The UV–vis absorption spectrum (250–900 nm) is shown in Figure . It can be seen from the figure that the compound 1 shows higher absorbance than ppmh.

Figure 5

UV–vis absorption spectra of compound 1 and ppmh.

The band gap of the materials has been calculated from Tauc’s plot following αhν = A(hν – Eg), where h denotes the Planck constant, α signifies the absorption coefficient, ν symbolizes the photon frequency, A represents a constant, Eg indicates the optical band gap, and n = 1/2 (for direct transition). Tauc’s plot for the materials is shown in Figure .

Figure 6

Tauc’s plots for (a) ppmh and (b) compound 1.

The band gap for ppmh is found to be 3.28 eV, and the band gap of compound 1 is 3.13 eV. The lower band gap and higher absorbance capability of compound 1 indicate its better potential for photosensitive device application.

There are two types of ligands, symmetrical dicarboxylato bdc2– and antisymmetric N-pyridin-2-yl-N′-pyridin-4-ylmethylene-hydrazine (ppmh) coordinated to Zn(II). The structural distortion mainly arises from the coordination of the N-donor (ppmh) ligand to Zn(II). The bond parameters are as follows: Zn–N(pyridyl): Zn02–N5, 2.045/Zn04–N8, 2.038; Zn–N(hydrazino): Zn02–N3, 2.441; Zn04–N6, 2.452 Å. Thus, hydrazino-N is weakly bonded with Zn(II) and is sensitive to light irradiation. There are some examples of photoisomerization of Zn(II)-arylazoheterocycles, which undergo cleavage of the Zn–N(azo) bond upon light irradiation.[32−34]

The same idea may be extended to the example where the increased structural distortion by enhancing the Zn–N (hydrazino) bond length upon light irradiation causes increment of conductivity. This distortion may reduce the band gap and increases photoconductivity. The energy gap is the major parameter determining conductivity or semiconductivity of a material; if it is decreased to 4 eV, semiconductivity is observed. In addition, the conjugation in compound 1 decreases the energy gap between the conduction and valence bands. Thus, charge transfer possibility increases. The geometric pattern and proper orientation in the compound 1 can lead to intermolecular interactions (e.g., π–π stacking, etc.) assisting efficient charge transfer within compounds.[35]

I–V Measurement and Analysis

The I–V curves of the devices made from ppmh (Figure a) and the compound 1 (Figure b) under dark and light conditions have shown rectification, which implies the formation of a Schottky barrier diode (SBD). The conductivity of ppmh is found to be 6.2 × 10–7 S/cm and is increased to 1.37 × 10–6 S/cm for the compound 1. The increased conductivity of the compound 1 can be supported by the band gap values of the materials, which are less than those of the free ligand, ppmh. Also, the compound-based SBD has shown better rectification behavior (Table ) compared to the ligand-based device. The on/off ratio of the SBD fabricated with compound 1 is 93, whereas it is 32 for ppmh.

Figure 7

I–V characteristic curves for (a) ppmh and (b) compound 1 under dark and light irradiation.

Table 1

Schottky Diode Parameters of ppmh and the Compound 1a

		S	R (mA/W)	D* (Jones)	on/off	n	Rs (dV/dln(I))	Rs (H)	B.H. (eV)
ppmh	dark	9.98	5.12	4.53 × 109	32	4.26	7.71 × 107	6.70 × 107	0.65
	light				65	3.87	7.11 × 106	7.50 × 106	0.61
compound 1	dark	20.95	56.21	2.17 × 1010	93	3.41	1.37 × 106	1.58 × 106	0.60
	light				129	2.91	6.56 × 105	7.03 × 105	0.58

Note: S = photosensitivity, R = responsivity, D* = specific detectivity, n = ideality factor, Rs = series resistance, ϕb = Schottky barrier height.

The photoresponse of the devices is important to assess their application potential in optoelectronics. Thus, transient photocurrent measurement was also done for the SBDs under 1.5 G light illumination (100 mW/cm2), and the corresponding result is shown in Figure . The transient light-induced current response is measured by switching the light on and off at an interval of 60 s for 600 s. As can be seen in Figure , the devices show good photoresponse upon irradiation of light. The photocurrents are steady and reproducible for several switch on–off cycles. The response is rectangular, which indicates the efficient movement of excited electrons to the external circuit. However, the response speed is better for compound 1-based SBD compared to ppmh-based SBD. Most importantly, the device fabricated with compound 1 has shown much higher photoresponse behavior compared to the free ligand, ppmh. The higher photoconductivity of the compound 1 can be explained using UV–vis absorption spectra and band gap analysis. The enhanced light absorption and smaller band gap of compound 1 resulted in better photoconductivity of the compound 1-based Schottky diode.

Figure 8

Transient photocurrent response of the ligand ppmh and compound 1.

The photosensitivity (S) has been calculated from eq where IPh = IL – ID (IL is the current generated on light irradiation and ID is the current obtained in the dark) is the photocurrent.[37,38] Photosensitivity of the ppmh-based SBD diode is 9.98, whereas it is 20.95 for the compound 1. The figure of merit of the SBD as a photodetector can be assessed by its responsivity (R) (eq )[36−39] and specific detectivity (D*). The responsivity was measured fromwhere PIn is incident optical power per unit area and A is the area of the diode. The responsivity of the compound 1 is 56.21 mA/W, which is 11 times higher than that of the free ligand ppmh (5.12 mA/W). We also measured the specific detectivity of the devices using eq (36,37)where q denotes the charge of electron and ID denotes the dark current. The specific detectivity of the compound 1-based SBD is about 5 times higher in magnitude than that of the ppmh-based SBD. The results indicate that the synthesized compound 1 has better potential for photodetector and other optoelectronic device applications.

The diodes are further analyzed by thermionic emission theory of Schottky diode (eq )[36−40]where I indicates the forward current, I0 means the reverse saturation current, V denotes the applied bias, q denotes the electronic charge, k denotes the Boltzmann constant, and T denotes the absolute temperature; n signifies the ideality factor, a constant considered for nonideal behavior of the diode. Representation of I0 can be done by eq (40)where A, A*, and ϕb signify the effective diode area, Richardson constant, and Schottky barrier height, respectively. The effective diode area was 7.065 × 10–6 m2.

Determination of the ideality factor, series resistance, and barrier height of a Schottky diode is necessary to assess the device performance. These parameters have been derived from Cheung’s eqs –8[40−42]andwhere Rs signifies the series resistance of the diode. The dV/dln(I) versus I and H(I) versus I plots are portrayed in Figure . The series resistance and ideality factor are calculated from the slope and y axis intercept of the dV/dln(I) versus I graph, respectively. The y axis intercept of the H(I) versus I graph is used to determine the barrier height of diodes. The series resistance is also determined from the slope of the H(I) versus I graph.

The ideality factor (Table ) for both devices is deviated from unity. This refers to the mixed interface of the metal–semiconductor (MS) junction and trapping states in the interfacing layer, which acts as a localized generation–recombination center.[37−40] Other causes may be the bias dependency on the barrier height, the presence of a thin native oxide layer at the interface, and electron tunneling within the barrier.[37−42] Upon illumination, the ideality factor approaches unity, which means improvement in junction quality. For the complex-based SBD, the ideality factor is closer to 1, implying better metal–semiconductor junction formation. Series resistance (Rs) yielded from two methods (i.e., Rs derived from the slope of dV/dln(I) vs I and H(I) vs I graphs) (Figure and Table ) was pretty much consistent, and Rs of compound 1-based SBD is much lower with respect to the series resistance of the ppmh-based device.

Figure 9

dV/dln(I) vs I and H(I) vs I curves for ppmh-based SBD under (a) dark and (b) light conditions; compound 1-based SBD under (c) dark and (d) light conditions.

The Schottky barrier height is another pivotal factor for evaluation of the performance of a Schottky diode. The barrier heights for ppmh- and compound 1-based SBDs under dark are estimated as 0.65 and 0.60 eV, respectively (Table ). Under light irradiance, these values were 0.61 and 0.58 eV, respectively (Table ). Thus, the barrier height of compound 1-based SBD is a bit smaller than that of the SBD based on ligand ppmh, which indicates its fast switching action.

To analyze the charge transport characteristics of the devices, we performed impedance spectroscopy. The Nyquist plots for both the devices are shown in Figure . The figure displays that the diameter of the semicircle for the compound 1-based SBD is much smaller, implying reduced charge recombination and faster charge transfer in compound 1-based SBD compared to those in the ppmh-based device, resulting in higher current and better performance of the device. (Figure S4).

Figure 10

Nyquist plots for both the devices: ppmh and compound 1.

DFT Computation and the Band Gap

In the present study, lattice matching and deformation potentials have been utilized to acquire the Schottky electrical contact, where energy difference within the conduction and valence bands is referred to as the deformation. In a single molecule, ΔE (=ELUMO – EHOMO, eV) is very useful. The optimization of structural geometry of the ligands (H2bdc and ppmh) has been performed. In the case of network compounds, the absolute deformation potentials are required to determine the band gap, which influences the electronic features of metal and ligands. In the coordination compounds of the d10 metal ion system, the band edges are usually explained via the electronic state of the ligands and the geometric pattern of the framework. The optimized structure of the coordination unit of CP is needed to calculate ΔE (Figure ), which correlates with the band gap found from Tauc’s plot.

Figure 11

DFT-computed energy of molecular orbitals and the energy difference between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of H2bdc, ppmh, and compound 1.

The energy values of HOMO and LUMO of the compound 1 are −6.44 and −3.23 eV, respectively. The energy difference is 3.21 eV. Again, the calculated band gaps of 3.97 eV (HOMO = −6.00 eV and LUMO = −2.03 eV) for ppmh and 4.88 eV (HOMO = −7.58 eV and LUMO = −2.70 eV) for H2bdc are higher than those for the compound 1 (Figure ). Thus, it is quite clear that the compound 1 is relatively higher conductive than the component ligands. In the irradiation condition, this theoretical view has a good agreement with the experimental one. The time-dependent (TD)DFT table is shown in the Supporting Information (Table S3). In ppmh, an intense band can be seen at 378 nm, which may be due to the HOMO-to-LUMO transition. In compound 1, the transition is shifted to a longer wavelength, 396 nm, which can be assigned to the HOMO-to-LUMO + 1 transition

Biological Study

Antimicrobial Activity and Reactive Oxygen Species (ROS) Measurement

One of the recent and promising applications of coordination polymers or metal organic frameworks (MOFs) is the exploration of biomedical activity.[43] Hydrazones display diversified biology-related activities, viz., anticancer, antimicrobial, antidepressant, anticonvulsant, anti-inflammatory, antiplatelet, analgesic, antimalarial, antifungal, antitubercular, antiviral, cardio-protective, etc., and establish an important class of compounds for development of latest drugs.[44]  The ligand N-pyridin-2-yl-N′-pyridin-4-ylmethylene-hydrazine carries an imine, −CH=N–, function and has potential pyridyl-binding sites to capture and penetrate the cell wall, has affinity to H+-binding, shows coordination with metal ions, and can disrupt the protein structure through noncovalent interactions. Hence, ppmh and its complexes have endogenous bioactive motifs that may be useful in examining their antimicrobial activity. The minimum inhibitory concentration (MIC) has been evaluated by the microdilution method in Luria broth according to the reported protocol.[43−46]

The in vitro cytotoxicity of H2bdc, ppmh, and the compound 1 has been estimated for checking the cytotoxic effects on the HepG2 cell line. The treatment of cells was performed with five different concentrations (20, 40, 60, 80, 100, and 120 μg/mL) for 24 h, followed by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The cell viability assay (Figure ) reports that the % survivability of cells is decreased in parabolic dependence with increasing concentration of the coordination polymer 1 with an LD50 value of 42.2 ± 2.3 μg/mL, while H2bdc and ppmh do not show any impact at any concentration (up to 120 μg/mL) on the growth states of cells. We have taken cisplatin as the positive control whose LD50 value is 12.6 ± 2.8 μg/mL. This implies the compound 1 is active against the HepG2 cell line and comparatively less toxic and cheaper than cisplatin. To check the leaching of Zn2+ from the compound 1 in the solution phase during biological studies, the 1H NMR spectra of 1 in the solvent dimethyl sulfoxide (DMSO)-d6 are recorded even after 24 h; the spectra do not display any change. This implies that the compound 1 is stable in solution (Figure S7).

Figure 12

Cell viability assay of H2bdc, ppmh, and compound 1 on human liver cancer cells, HepG2.

The profile of inhibition of H2bdc, ppmh, and compound 1 has been presented in Figure . From the result, it is evident that the ligands have insignificant impact on the bacterial growth, while the compound 1 shows a concentration-dependent decrease in the growth of both types of bacteria. In the case of E. coli, the IC50 is 190 ± 2.4 μg/mL, and S. aureus shows IC50 of 185.96 ± 3.04 μg/mL. We have taken ampicillin as positive control whose IC50 values are 56 ± 1.9 μg/mL for E. coli and 42 ± 2.8 μg/mL for S. aureus. Thus, Zn(II) coordination polymer, i.e., compound 1, is more efficient than its ligands H2bdc and ppmh.

Figure 13

Analysis of the activity of H2bdc, ppmh, and compound 1 by the MIC method in (a) E. coli and (b) S. aureus. The data is the average of three experiments ± standard deviation (SD). * Represents P value < 0.05, ** represents P value < 0.01, and *** represents P value < 0.001.

Induced intracellular oxidation, superoxide formation, and the oxidative stress in both Gram-negative (E. coli) and Gram-positive (S. aureus) bacteria have been detected using ROS measurements. The intracellular ROS level is proportional to the intensity of fluorescence activity.

Normally, the 2,7′-dichlorofluorescein diacetate (DCF-DA) goes into the cell as well as reacts with the reactive oxygen to display a green fluorescent compound, dichlorofluorescein (DCF). In summary, a stock solution of DCF-DA (10 mM) was prepared in methanol and was further diluted with phosphate-buffered saline (PBS) to a working concentration of 100 μM. Treatment of HepG2 cells was performed with the compound at LD50 for 12 h at 37 °C, followed by rinsing with ice-cold 1× PBS and incubation with 100 μM DCF-DA for 30 min in the dark at 37 °C. The fluorescence intensity was estimated both spectroscopically (Hitachi, Japan) and under a fluorescence microscope in HepG2 cells (Leica, Japan) at excitation and emission wavelengths of 485 and 520 nm, respectively.

To understand the mechanism of the inhibition of the growth of bacterial cells, the intracellular generation of ROS inside the bacterial cells was tested. The ROS production was measured by utilization of DCF-DA [intracellular ROS indicator for the compound 1-treated bacterial cells (Figure )]. The results suggest that the compound 1 causes significant increase in the generation of the intracellular ROS in the case of both E. coli and S. aureus cells, which ultimately results in the bacterial cell death. The production of ROS along with DCF fluorescence intensity was similar in both types of bacteria. Thus, this study depicts the antibacterial ability of compound 1 involving the production of intracellular ROS. More ROS generation implies maximum cell death. This intracellular ROS production disturbs the electron transport chain within the cell, fragments genetic materials, and damages the bacterial cell membrane.[46−48] This happens probably due to the effective penetration of compound 1 into the Enterococcus faecalis cells, which inhibits the bacterial cell growth and serves as an antibacterial agent.

Figure 14

Intracellular ROS generation of compound 1 in (a) E. coli and (b) S. aureus by spectrophotometry. The data is the average of three experiments ± SD. * Represents P value < 0.05, ** represents P value < 0.01, and *** represents P value < 0.001.

Estimation of reactive oxygen species (ROS) is performed using both fluorescence microscopy and spectrofluorometry where 2,7′-dichlorofluorescein diacetate (DCF-DA) served as a specific probe. The determination of reactive oxygen production is done in HepG2 tumor cell lines after the treatment of cells with compound 1, at their respective LD50 dose for 12 h. The fluorescence microscopic images reveal that the green color fluorescent intensity is increased for both treated cells compared to control cells in HepG2 cell lines after 12 h (Figure ).

Figure 15

Intracellular ROS generation of compound 1 in HepG2 cells by fluorescence microscopy and spectrophotometry. (The data is the average of three experiments ± SD. * Represents P value < 0.05, ** represents P value < 0.01, and *** represents P value < 0.001.)

Anticancer Activity

Despite a notable improvement in the ground of cancer diagnosis and treatment, cancer is a growing threat to the world. Furthermore, due to high side effects caused by most promising chemotherapy, metastatic cancer needs more effective chemotherapeutics to minimize these problems. Again, the global dissemination of resistant bacterial strains is one of the most serious present-day challenges in hospital-acquired infections, which needs to be taken care of in a more economical and healthy way.

Compound 1 has shown to be a potential antibacterial agent against Gram-negative (E. coli) and Gram-positive (S. aureus) bacterial species. In the present experiments with compound 1, destruction of bacterial cell wall might have caused due to one or multiple steps of signaling cascades, resulting in defective cell wall synthesis or impaired cross-linking of polymer units, which occur due to the ROS generation in both Gram-negative and Gram-positive bacteria. Furthermore, the compound 1 enhanced cellular ROS in human liver cancer cells, which in turn culminates in the death of cancer cells. In conclusion, our study shows that the Zn(II) coordination polymer 1 is not only an effective antibacterial drug but also appears as a promising anticancer drug, which can be a subject of further detailed studies.

Conclusions

Keeping in mind the energy crisis and human health, multifunctional three-dimensional Zn(II)-MOF presented in this work may be a potential candidate to solve the problems. Pyridylhydrazone and 1,4-benzene dicarboxylic acid (H2bdc) both serve as bridging agents to bind Zn(II) and have been characterized by single crystal X-ray diffraction (SCXRD). This molecule undergoes higher dimension from the combination of arrangement of organic ligands and different supramolecular interactions mainly H-bonding and π···π interaction. Fascinatingly, the molecule shows enhancement of electrical conductivity under light conditions. Thus, the compound may be a useful candidate for optoelectronic device fabrication. The biological application of the coordination polymer shows effective antibacterial and anticancer activity. Thus, our compound may be considered as an active member in the class of multifunctional materials.

Experimental Section

Materials and General Methods

All of the chemicals used during this experiment purchased from Sigma-Aldrich were of reagent grade and were utilized without any purification in laboratory. Elemental analysis (C, H, N) was performed by a PerkinElmer 240C analyzer. A PerkinElmer spectrometer (Spectrum Two) was used during the Fourier transform infrared spectral measurement with the samples following the attenuated total reflectance technique. Thermal analysis was carried out on a PerkinElmer Pyris Diamond thermogravimetry/differential thermal analysis in the temperature range between 30 and 600 °C under a nitrogen atmosphere. The PXRD data of the grinded sample was collected on a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (λ = 1.548 Å) produced at 40 kV and 40 mA and was recorded in a 2θ range of 5–50.

The impedance measurement of the given samples as a function of frequency (40 Hz to 11 MHz) utilizing a computer-controlled Agilent make precision 4294A LCR meter. The electrical characterization was carried out via a Keithley 2400 sourcemeter, interfaced with a personal computer. Solid-state UV–vis spectroscopy was performed via a PerkinElmer UV–vis Lambda 365 instrument. For solid-state UV–vis spectroscopy, ppmh and compound 1 were dissolved in N,N-dimethylformamide (DMF) at 0.2 mM concentration and sonicated for 15 min. Then, a thin film of each was deposited over a glass plate and dried in vacuum for 1 day. After 1 day, UV–vis spectra have been recorded (Figure S7).

Thiazolyl blue formazan (MTT) and 2,7-dichlorofluorescein diacetate (DCF-DA) were purchased from Sigma-Aldrich. Phosphate-buffered saline (PBS) and Luria–Bertani (LB) broth were obtained from HiMedia Pvt. Ltd., India, and ethanol and glutaraldehyde from Merck, India. E. coli DH5 (MTCC-1652) and S. aureus (MTCC 25923) were obtained from the Institute of Microbial Technology, Chandigarh, India. Throughout the experiment, Millipore water was used. The in vitro cytotoxicity of ppmh and the compound 1 was estimated for checking the cytotoxic effects on the HepG2 cell line. The cells were treated with five different concentrations (20, 40, 60, 80, 100, and 120 μg/mL) for 24 h followed by the MTT assay. H2bdc, ppmh, and the compound 1 were dispersed in 1% DMSO to form a dilute suspension of 2 mg/mL using a bath sonicator for 30 min. When these were entirely dispersed in water, the average particle diameter and size distribution of microspheres were determined by dynamic light scattering (DLS) utilizing Zetasizer (NANO ZS90, Malvern Instruments Ltd., U.K.).

Synthesis of Compound

N-Pyridin-2-yl-N′-pyridin-4-ylmethylene-hydrazine (ppmh)

4-Pyridine carboxaldehyde (214 mg, 2 mmol) was dissolved in 5 mL of MeOH with a few drops of AcOH and stirred for 30 min. Then, 2-hydrazinopyridine (218 mg, 2 mmol) was slowly added in stirring conditions, and stirring was carried out for 1 h. The mixture was refluxed for 24 h followed by cooling; yellow rod-shaped crystals appeared in the solution. Crystals were filtered and washed with Et2O. They were then dissolved in hot minimum volume of MeOH to recrystallize by slow evaporation. Rod-shaped yellow crystals were then separated; yield, 90% (356 mg). IR: 1610–1720 cm–1 aromatic region, 1298 cm–1 C=N, 2856 cm–1 C–H, 3130–3300 cm–1 N–H (Figure S5). Elemental analysis (%) calcd for C11H9N4: C: 66.65, H: 5.08, N: 28.26. found: C: 66.60, H: 5.00, N: 28.34. 1H NMR spectrum in CDCl3 is given in Figure S6.

[Zn4(bdc)4(ppmh)2(H2O)] (1)

Methanolic (2 mL) solution of ppmh (39.6 mg, 0.2 mmol) was slowly layered to a solution of Zn(NO3)2·6H2O (60 mg, 0.2 mmol) in water (2 mL) utilizing buffer solution of MeOH and H2O, 1:1 (v/v, 2 mL) followed by layering of H2bdc (33.2 mg, 0.2 mmol) neutralized with Et3N (0.021 g, 0.2 mmol) in EtOH (2 mL). Brown crystals of [Zn4(bdc)4(ppmh)2(H2O)] were obtained after 3 weeks (163.4 mg, yield 60%). Elemental analysis (%) calcd for C54H36N8O19Zn4: C: 47.60; H: 2.66; N: 8.22; found: C: 47.52; H: 2.73; N: 8.19. IR: 1559 cm–1 C=N, 1620 cm–1 C=O, 3564 cm–1 N–H, 2826 cm–1 C–H (Figure S5). And in case of free H2bdc the major peaks are obtained at 1782 cm–1 for acidic C=O group, 3250 cm–1 O–H, 1550–1650 cm–1 for aromatic region, which confirms the binding mode of H2bdc in compound. NMR spectra of compound 1 in DMSO-d6/D2O mixture (1:10 v/v) is shown in Figure S6.

Thermogravimetric analysis (TGA) (30–600 °C, under an inert N2 atmosphere): It shows first breaking of compound at 105 °C, indicating loss of water from the crystalline compound followed by characteristic weight loss at 200 °C. Thus, this long range of thermal stability supports the framework stability and applicability of the material in the field of electrical conductivity for compound 1. The TGA plots of compound 1 and ppmh are shown in Figure S8a,b, respectively, which shows the extreme level of thermal stability.

X-ray Crystallography

Yellow single crystal of ppmh and brown single crystal of the compound 1 were used for data collection using a Bruker SMART APEX II diffractometer equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Data collection parameters and crystallographic data are listed in Table . The molecular structure has been solved using the SHELX-97 package.[46] Non-hydrogen atoms of the compound were refined with anisotropic thermal parameters. The unit cell parameters and crystal-orientation matrices of the compound were determined by least-squares refinement of all reflections within the hkl range −18 < h < 18, −18 < k < 15, −16 < l < 16. The intensity data were amended for Lorentz and polarization effects. Crystal data were collected applying the condition I > 2σ(I). Hydrogen atoms of this molecular system were placed in their geometrically idealized positions and constrained to ride on their parent atoms. Selected bond lengths of ppmh are given in Table S1. Selected bond lengths and bond angles of compound 1 are listed in Table S2. The crystallographic data for ppmh and compound 1 are summarized in Table .

Table 2

Crystal Data and Refinement Parameters for ppmh and the Compound 1

formula	C11H10N4 (ppmh)	C54H36N10O17Zn4 (1)
fw	198.23	1362.47
cryst syst	monoclinic	monoclinic
space group	P21/n	P21
a (Å)	13.2104(7)	10.589(5)
b (Å)	8.5066(5)	17.347(7)
c (Å)	18.3849(10)	16.501(5)
β (deg)	101.936(2)	91.35(3)
V (Å3)	2021.34(19)	3030(2)
Z	8	2
Dcalcd (g/cm3)	1.303	1.493
μ (mm–1)	0.084	1.641
λ (Å)	0.71073	0.71073
data [I > 2σ(I)]/params	3529/271	9644/766
GOF on F2	1.051	1.067
final R indices (I > 2σ(I))a,b	R1 = 0.0590	R1 = 0.0501
	wR2 = 0.1588	wR2 = 0.1817

R1 = ∑||Fo| – |Fc||/∑|Fo|.

w2 = [∑w(Fo2 – Fc2)2/∑w(Fo2)2]1/2, w = 1/[σ2(F02) + (0.0693P)2 + 0.9697P] for ppmh and w = 1/[σ2(F02) + (0.1289P)2 + 2.7519P] for compound 1, where P = (F02 + 2Fc2)/3.

Computational Method

The structural optimization of full geometry of the compound 1 and ligand (H2bdc and ppmh) was performed utilizing density functional theory (DFT) with the GAUSSIAN-09 program package. The SCXRD coordinates of ppmh and compound 1 were used in the calculation. During the theoretical calculation, a hybrid level DFT-B3LYP was used. The basis set LanL2DZ was exploited for all of the elements including Zn(II) in the metal organic network compound and ligand systems.[48−53] To denote the different important electronic transitions based on B3LYP/B3LYP optimized geometry were theoretically computed for the time-dependent density functional theory (TDDFT) formalism. The vibrational frequency calculation was also implemented for these three compounds to ensure that the DFT-optimized geometries represent the local minima and there were only positive eigenvalues. The fractional contribution of different molecular orbital components present in the molecule was calculated via Gauss sum operation[47−57] (Tables S3–S5).

Schottky Device and I–V Measurements

To fabricate the Schottky devices, first, the indium tin oxide (ITO)-coated glass substrate was cleaned by 2-propanol, acetone, and distilled water sequentially and repeatedly by ultrasonication bath for 20 min, and the Schottky device was fabricated by the as-synthesized compound 1 sandwiched between indium tin oxide (ITO) and metal (Al). The solution of compound 1 was prepared in N,N-dimethylformamide (DMF) by the ultrasonication technique. Then, it was spin-coated on a precleaned ITO-coated glass substrate (at a spinning speed of 600 rpm for 1 min) to produce a thin film via an SCU 2700 spin-coating unit. This spin-coating step was repeated four times followed by drying under vacuum conditions. For the characterization of the prepared thin film, the thickness was measured by the surface profiler at 1 μm. An aluminum (Al) electrode was deposited on the active film to construct the metal–semiconductor (MS) junction by the thermal evaporation technique via a vacuum coating unit (12A4D HINDHIVAC) under 10–6 Torr pressure, and the effective diode area was maintained as 7.065 × 10–6 m2 with a shadow mask. Current–voltage (I–V) measurements of the devices were performed in the voltage range −1 to +1 V with a Keithley 2635B sourcemeter under dark and light conditions.[37] Every measurement was carried out at normal temperature and under ambient conditions. To measure the conductivity of the materials, the Ohmic electrode was deposited on the material and the I–V measurement was done. The conductivity was measured from the slope of the resultant I–V graph.

Antimicrobial Activity

The antibacterial activities of H2bdc, ppmh, and the compound 1 were studied[43−45] against Gram-positive bacteria, S. aureus, and Gram-negative bacteria, E. coli, as standard. In brief, 10 μL of the bacterial strain containing 2.5 × 105 colony forming unit (CFU)/mL bacteria was separately added to 1 mL of Luria–Bertani (LB) broths with different concentrations of ppmh and compound 1. For preparation of a pure suspension of compound 1, the samples were sonicated in a bath-type ultrasonicator. Here, the pure suspension acts as a dissolved solution, accurately reflecting the amount of particles available in solution to attack microorganisms. Several concentrations of 1 were added to the bacterial media containing the bacterial strains followed by incubation for 24 h. After that, the MIC values were obtained by inspecting the turbidity of the bacterial growth. The values correspond to 99% inhibition of bacterial growth. All assays were performed in a biosafety cabinet. All of the experiments were triplicated to obtain the standard deviation. The antibacterial effect is calculated using M% = B – C/B × 100, where M denotes the mortality rate (%), B denotes the mean number of bacteria in the control samples (CFU/mL, where CFU signifies the colony forming unit), and C denotes the mean number of bacteria on the treated samples (CFU/mL).

Bacterial ROS Measurement

A fluorescent dye, 2,7-dichlorofluorescein diacetate (DCF-DA, Sigma), was utilized as an indicator of ROS generation. A fresh broth (4 mL) was inoculated with approximately 104 CFU/mL for overnight culture and treated with different concentrations of compound 1 (10, 20, 50, and 100 μg/mL, respectively), while the untreated sample was kept as control at 37 °C for 1 h. Following this, 10 μM dichlorofluorescein diacetate was added to the bacterial cell suspension and incubated for 30 min in the dark to record the fluorescence intensity in a microplate reader (BioTek, Germany) with excitation and emission at 504 and 529 nm, respectively.

Cell Line Culture and Cytotoxicity Assay

Human liver cancer cells (HepG2) were obtained from the National Center for Cell Science Pune, India. The growth of cells was performed in Dulbecco’s modified Eagle’s medium with 10% fetal bovine serum and penicillin/streptomycin (100 units/mL) at 37 °C and 5% CO2. All of the treatments were executed at 37 °C and at a cell density allowing exponential growth.

On exposure to various concentrations of ppmh and compound 1, the viability of HepG2 cells was determined by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay.[36] In summary, around 1 × 104 cells per well of 96-well plates were exposed to the compounds at different concentrations of untreated as control samples, 20, 40, 60, 80, and 100 μg/mL for 24 h of incubation at 37 °C and 5% CO2. Following this, the cells were incubated again with 10 μL of MTT solution (stock 1 mg/mL) for 4 h at 37 °C and 5% CO2 following a wash with 1× phosphate-buffered saline (PBS), and the formazan crystals were dissolved in MTT solubilization buffer to measure the absorbance at 570 nm using a microplate reader (BioRad). The data were formulated comparing with the control ones.