NVP-2

Supramolecular Assembly of a Zn(II)-Based 1D Coordination Polymer through Hydrogen Bonding and π···π Interactions: Crystal Structure and Device Applications

ABSTRACT: A new mixed-ligand divalent one-dimensional coordination polymer (1D CP) [Zn(adc)(4-nvp)2(H2O)2]n, (1) [H2adc = acetylenedi- carboxylic acid and 4-nvp = 4-(1-naphthylvinyl)pyridine] has been synthesized and well characterized by elemental analysis, infrared spectrum, single-crystal X-ray crystallography, powder X-ray diffraction pattern, and thermogravimetric analysis. The compound 1 constructs a 3D supramolecular network by the combination of hydrogen bonding, C−H···π, and π···π interactions. Interestingly, the material shows Schottky behavior which is exclusively analyzed with the help of thermionic emission and space charge- limited current theory. In addition, the Schottky barrier diode parameters for compound 1 demonstrate better device performance after light soaking. Hence, the compound has applicability in the fabrication of optoelectronic devices.

INTRODUCTION
In modern trends of materials science, coordination chemistryis one of the eye catching areas for its versatile applications.1−3 The advancement of laboratory to land application of this chemistry hides within the structural features. During the past few decades, polymeric coordination entities have been used in the field of materials chemistry for their architectural beauty aswell as wide spectrum of applications.4−10 These types of inorganic−organic hybrid polymeric coordination entities are known as coordination polymers (CPs). CPs are generally composed of metal ions or metal-ion clusters linked by organicligands in an array. Depending upon the applications, one can choose a particular type of organic ligands. However, researchers are interested in mixed organic ligands instead of single one, whereby the structural dimensionality can easily be tuned.11−13 Because of their structural diversity, CPs are potentially applicable in sorption and separation of gases, catalysis, fluorosensors, drug delivery, sensing studies, energy storage, magnetism, and ion exchanging.14−25 Recently, the electrical conductivity of these materials has been largely studied and interest continues unabated.26,27 The electrical properties of CPs are not very attractive because of the presence of dicarboxylic acids containing lot of insulating carbon atoms or chains present in molecular assembly. However, the judicious selection of auxiliary ligands may influence the charge transport properties and semiconducting behavior of the compound.28−31 It has been observed that the rich aromatic moieties happen to enhance charge mobility in CPs.32 In addition, thermal, chemical, and structural stability of these compounds tempted the researchers to use them in the field of semiconductors. Inspired by these ideas, our group designed and synthesized a number of CPs which are shown to possess electrical conductivity and are useful for the fabrication of electronic devices.33−36 Most of the cases, the structure property relationship is very much important for the application in this field.Herein, we present a Zn(II)-based one-dimensional CP (1D CP) [Zn(adc)(4-nvp)2(H2O)2]n (1), synthesized using actylenedicarboxylic acid (H2adc) as a linear organic linker along with co-ligand 4-(1-naphthylvinyl)pyridine (4-nvp). The structure of the compound 1 is supported by single-crystal X- ray diffraction measurement and various spectroscopic techniques.

Simultaneously, the presence of hydrogen bonding, C−H···π, and π···π stacking interactions leads to the construction of a three-dimensional (3D) supramolecular network in the solid-state crystal structure. The presence of π···π stacking interactions among the pyridine and naphthalene rings of the adjacent chains facilitates the charge transportproperties of the compound, resulting in the high electrical conductivity. In addition, stacked 4-nvp ligands may attain maximum planarity under photoirradiation, ensuring bettercharge mobility among the aromatic rings along with the significant enhancement of conductivity. Therefore, this material has the potential to serve as a photosensitive device.EXPERIMENTAL SECTIONMaterials and Physical Measurements. All the chem- icals were purchased from Sigma-Aldrich Chemical Co. Inc. and were used as received. Microanalytical data (C, H and N) were collected on a PerkinElmer 240C elemental analyzer. Infrared (IR) spectroscopy was performed in KBr (4500−500cm−1) using a PerkinElmer FT-IR RX1 spectrometer. Thethermogravimetric data were collected on PerkinElmer Pyris Diamond TG/DTA in the temperature range 30−550 °C under N2 atmosphere. The powder X-ray diffraction (PXRD) of the powdered sample was performed on a Bruker D8 ADVANCE X-ray diffractometer with Cu Kα radiation (λ = 1.548 Å) in the 2θ range of 5°−50°. The electronic spectrum of the compound was recorded with the help of a PerkinElmer UV−vis spectrophotometer (LAMBDA 35).

Solid-state emission spectra were obtained by using a HORIBA JobinYvon (Fluoromax-4) fluorescence spectrophotometer.Synthesis of Compound 1. A solution of 4-nvp (0.046 g, 0.2 mmol) in MeOH (2 mL) was cautiously layered to a solution of Zn(NO)3·6H2O (0.06 g, 0.2 mmol), in H2O (2 mL) using 2 mL 1:1 (=v/v) buffer solution of CH3CN and H2O. Then, a solution of H2adc (0.023 g, 0.2 mmol) neutralized with Et3N (0.021 g, 0.2 mmol) in 2 mL EtOH was layered upon it. After three days, light yellow color rectangular crystals of [Zn(adc)(4-nvp)2(H2O)2]n, (1) were obtained in 65% yield (0.088 g). Elemental analysis (%) calcd for C38H26N2O6Zn: C, 67.92; H, 3.90; N, 4.17. Found: C, 68.01; H, 3.98; N, 4.10. IR (KBr pellet, cm−1): 1621νas(COO−), 1379 νsys(COO−).X-ray Crystallography. A suitable single crystal of the compound 1 was used for data collection by a Bruker SMART APEX II diffractometer, having graphite-monochromated MoFor electrical characterization of the devices, the current− voltage (I−V) characteristics were measured under both dark and illumination conditions and recorded with the help of a Keithley 2635B source meter by a two-probe technique. All themeasurements were performed at room temperature and under ambient conditions.Computational Method. The GAUSSIAN-09 program package38 was used to generate optimized geometry using density functional theory (DFT). A hybrid DFT-B3LYP39 functional was used throughout the calculations. The LanL2DZ basis set was utilized for all the elements including metal ion Zn(II). To assign the different low-laying electronic transitions in the experimental spectra, the time-dependent DFT40−42 calculations of the compound was done. Gauss sum43 was used to calculate the fractional contribution of various groups to each molecular orbital.

RESULTS AND DISCUSSION
Single-crystal X-ray crystallography revealed that compound 1 crystallizes in the monoclinic space group Pc with two Zn(II) centers (Zn01 and Zn02). In this compound, two asymmetric units are present because of the imposing of chirality in the structure. The flack parameter value of the structure is 0.002(12). The coordination atmospheres around the metal ions are identical. Zn01 has a distorted octahedral geometry ligated by two O atoms from two adc anions in monodentate fashion (Zn−O, 2.075(11)−2.156(10) Å) and two N atomsfrom two 4-nvp ligands (Zn−N, 2.200(13)−2.172(13) Å) inthe equatorial position, and by O atoms from two aqua ligands at the axial sites (Zn−O, 2.082(8)−2.148(8) Å) (Figure 1a). The Zn02 center also adopts distorted octahedral geometry. Two O atoms from two different adc anions and two N atoms from two 4-nvp ligands define the equatorial plane [Zn−O, 2.075(11)−2.156(10) Å; Zn−N, 2.200(13)−2.172(13) Å],while O atoms from two aqua ligands are at the axial sitesKα radiation (λ = 0.71073 Å). The structure was solved by using SHELX-9737 with anisotropic displacement parameters for all nonhydrogen atoms. Hydrogen atoms were constrained to ride on the respective parent atoms. The crystal data and experimental details for data collection of 1 are reported in Table S1, and selected bond lengths and bond angles are listed in Table S2.Conductivity Measurements. In order to perform the electrical study, the multiple metal−semiconductor (MS) junction device has been fabricated in indium tin oxide (ITO)/compound 1/Al sandwich configuration. For this purpose, the thin film of well dispersion of as-synthesized compound 1 has been deposited on the precleaned ITO coated glass substrate with the help of SCU 2700 spin coating unit and dried subsequently.

Dispersion of compound 1 has been made in N,N-dimethylformamide (DMF) by mixing and sonicating the right proportion (40 mg/mL) of compound 1 in a vial. Then, the dispersion of compound 1 has been depositedon the top of the ITO-coated glass by spinning first at 800 rpm for 4 min and thereafter, at 1200 rpm for 6 min. Afterward, the as-deposited thin film has been dried in a vacuum oven at 100°C for several minutes to evaporate the solvent fully. Thethickness of the developed thin film has been measured by a surface profiler as ∼1 μm. The aluminum electrodes have been deposited under pressure 10−6 Torr by maintaining the effective area as 7.065 × 10−2 cm2 with a shadow mask in the vacuum coating unit 12A4D of HINDHIVAC.(Zn−O, 2.082(8)−2.148(8) Å) in the Zn02 center. The connectivity of neighboring dicarboxylate oxygen atoms with Zn(II) centers results in a 1D chain structure (Figure 1b). The aqua ligands in 1D chains make strong intermolecular hydrogen bonds with coordinated O atoms of adc ligands (O···O separation of 2.799−2.85 Å), resulting in the formation of a 2D hydrogen bonding network in the bc plane (Figure 1c). In addition, pyridine and naphthalene rings of 4-nvp ligands of adjacent chains undergo face-to-face π···π interactions in the ac plane with centroid−centroid distances of 3.789−3.813 Å(Figure 1d). Furthermore, there exist C−H···π interactionsbetween the parallel chains with edge-to-face distances in the range of 2.779−3.001 Å (Figure S1).44,45 These hydrogen bonding, π···π, and C−H···π interactions in 1 fabricate a 3D supramolecular architecture by stacking the 1D chains together (Figure S2).Thermogravimetric Analysis and PXRD Analysis.

Thermogravimetric analysis (TGA) was carried out with the freshly prepared sample in the temperature range 30−550 °C under N2 atmosphere. The TGA result shows that the compound 1 is stable up to 120 °C (Figure S3) and can be used in device fabrication. The final residue is assumed to be ZnO (ca. 12.04%), which is corresponding to the experimentalis considered as 1 for the ideal case. From the Tauc’s plot, the direct optical band gap (Eg) of the compound 1 has been estimated as 3.33 eV by extrapolating the linear region of the plot (αhν)2 versus hν (Figure 2) to α = 0 absorption.DFT Computation and Band Gap. For Schottky electrical contact, lattice matching and deformation potentials have been used. The deformation corresponding to the energy gap between the conduction and valence band refers to the energy difference between the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) (ΔE = ELUMO − EHOMO, eV). In the case of polymeric coordination compounds, the band gap is determined by using absolute deformation potentials.47 As the polymeric coordination compounds, that is, CPs are mainly composed of inorganic and organic hybrid materials, the band gap is supposed to be influenced by the electronic nature ofboth the components. For the CPs made of d10 metal ions, the band edges are obtained by the electronic states of the ligand system as well as the geometry factor of the framework.48 Therefore, the optimized structure of the coordination unit of 1 has been used to calculate ΔE (3.52 eV, Figure 3), which is in good agreement with the band gap obtained from Tauc’s plot.weight of residue (11.26%). To ensure the purity of the bulk compound, PXRD has been performed at room temperature with the powdered sample. Good correspondence exists of the entire peak positions of the simulated pattern with the as- synthesized 1, suggesting the phase purity of the bulk (Figure S4).UV−Vis Spectroscopy and Band Gap. In this study, the optical characterization has been carried out using the UV−vis spectrum of the compound 1.

It is to be mentioned that the compound produces stable dispersion in DMF; therefore, thinfilm on normal glass substrate has been utilized for solid-state UV−Vis spectroscopy. Here, the optical spectrum of compound 1 (inset of Figure 2) has been recorded in therange 250−600 nm, and the direct optical band gap of the film has been estimated from the UV−vis spectrum using Tauc’s equation.46A minor variance in the computed band gap could be designated to the geometry factor that has not been taken into the consideration during theoretical calculations using a single unit of the polymer. All the transitions in the compound 1 are intraligand charge transfer (ILCT) in nature. The relationships of theoretical electronic transitions with experimental data of 1 are listed in Table S3.Photophysical Properties. In this work, room-temper- ature luminescence emission properties of the compound 1 and free 4-nvp ligand were studied. The results show that 4- nvp ligand displays a strong emission with maxima at 418 nm (Figure S5) upon excitation at absorption maxima ∼380 nm, corresponding to the ILCT. This charge transfer can be ascribed as n → π and π → π transitions. It is to be mentioned that the emission intensity of 1 has been found to be similarat 418 nm. Notably, the average lifetime of compound 1improves relating to the 4-nvp ligand (Figure S6).Dielectric Characterization. As the obtained value of direct optical band gap fell well in the semiconductor region, it further prompted us to look into the inherent conductivity of the compound 1. Hence, we have performed the electrical characterization of 1 in terms of the dielectric study. Here, the study has been performed by estimating capacitance (C), impedance (Z), and phase angle (θ) of the compound 1 as a function of frequency (40 Hz to 11 MHz).

For this purpose, polished pellets of the compound 1 have been taken along with high purity ultrafine silver paste used as the electrode on the opposite surfaces of the pellets.For the compound 1, the complex plane impedance plot, that is, Nyquist plot is shown in Figure 4A, and the logarithmic angular frequency dependent phase plot (Bode plot) is represented in Figure 4B. The Nyquist plot of the compound 1 comprises a depressed semicircular arc which is assumed to be contributed by semiconducting grains in the high frequency region. The semicircle at the high frequency region is related to the electrode resistance. From the radius of the semicircle, the charge transfer resistance Rct (dc resistance) of the compound 1 at the electrode/composite interface has been estimated. The life time (τn) of the charge carrier has been calculated from the Bode plot (Figure 4B). The phase plot exhibits a peak at ωmax which is inversely related to the charge carrier that is electron life time (τn = 1/ωmax).49 The life time of the charge carriers obtained from the Bode plot has been listed in Table 1.where ε0, εr, C, d, and A are the permittivity of free space, dielectric constant of the compound 1, capacitance (at saturation), thickness, and effective area of the pellet, respectively. Using eq 2, the dielectric constant (εr) of the compound 1 has been estimated, and all the calculated parameters are shown in Table 1.Electrical Properties. The optical band gap falling in semiconducting regime and large charge carrier life-time prophesies compound 1 to behave as a light-sensitive semiconducting device. Hence, to get deep into the charge transport mechanism, electrical characterization has been realized by a metal (Al)−semiconductor (compound 1) (MS) junction thin-film device. To examine the electrical properties, we have recorded current−voltage (I−V) character- istics of compound 1 under dark and illumination conditions (Figure 6).

The conductivity of 1 under the dark condition is found to be 9.29 × 10−4 S·m−1, indicating the semiconducting nature of the material. However, the conductivity has been recognizably improved under photoillumination and measured as 16.58 × 10−4 S·m−1. It can be noticed from the I−V characteristics of Al/compound 1 interfaces that there exists a nonlinear rectifying nature under both the conditions, representing the formation of a Schottky barrier diode (SBD). The rectification ratio (Ion/Ioff) of the SBD at ±2 V has been obtained as 41 and 77 under dark and illumination conditions, respectively. The larger current from the character- istic curve under light exhibits the photoresponse of the device, which has been found to be 3.Further, the I−V characteristic of the compound 1 has beeninvestigated by the thermionic emission theory, employing Cheung’s method, and the important diode parameters have been estimated.46 For this purpose, we have analyzed I−V curves quantitatively in view of the following standard equations.46,50The series resistances (RS) and ideality factor (η) for the device under dark and illumination conditions were calculated from the slope and intercept of dV/d ln I versus I plot (Figure 7A), and the obtained values have been listed in Table 2. The ideality factor (η) of compound 1 base SBD has been estimated to be 23.18 and 43.26 under dark and light, respectively. Here, the obtained M−S junction is not ideal which can be addressed by considering the inhomogeneities of Schottky barrier height, existence of interface states, and seriesresistance.53,54 However, after light soaking, the value of the ideality factor of compound 1 approaches a more ideal value (closer to 1), indicating less interfacial charge recombination and better homogeneity of Schottky junctions.52 Therefore, itmay be concluded that compound 1 possesses less carrier recombination at the junction that is better barrier homogeneity under photoirradiation conditions.

The value of barrier height (ØB) has been determined from eq 7 using the ideality factor (η). For compound 1, the potential barrier height has been reduced under exposure of light. The decrease in barrier potential height can be attributed to the effect of generation and accumulation of photoinduced charge carriers near the conduction band. The series resistance (RS) can also be calculated from the slope of H versus I plot (Figure 7B).The potential height (ØB), ideality factor (η), and seriesobtained from both the plots are in good agreement and found to decrease upon light irradiation (Table 2). Here, the hanging 4-nvp ligands may attain maximum planarity under photo-irradiation which results in better π···π stacking interactions among the aromatic rings along with the significant increase in conductivity.To shed light on the charge transport phenomena at the MS junction, it is necessary to analyze the I−V curves in detail. The characteristic I−V curves in the logarithmic scale under both the conditions exhibit two slopes (Figure 8A) marked as region I and region II. The value of the slope is ∼1 in region I and current follows the relation I ∝ V, indicating the Ohmic regime, whereas the value of the slope is ∼2 in region II and current is proportional to V2 (Figure 8A), corresponding to atrap-free space charge-limited current (SCLC) regime.46,55 Here, the SCLC theory has been adopted to estimate the mobility of the compound.46,55The effective carrier mobility has been determined from the higher voltage region of I versus V2 graph (Figure 8B) by using the Mott−Gurney equation46,52,559μeff ε0εrA fı V2 hıwhere D is the diffusion coefficient and has been estimated by employing Einstein−Smoluchowski equation (eq 10).46 During the formation of the MS junction, diffusion length (LD) of the charge carrier plays a significant role in the device performance. The diffusion length (LD) has been further obtained from eq 11. All the estimated values of the parameters in the SCLC region have been listed in Table 3, which reveals that the charge transport properties of compound 1 have been enhanced upon light irradiation. The number of charge carriers increases under light and thereby, they increase in transportrate and mobility. The enhancement of diffusion length under illumination demonstrates that the charge carriers have to travel more length before recombination, which leads to the ultimate increase in current. The diode parameters of the compound 1-based SBD represent much better charge transfer properties after light irradiation. Therefore, this kind of material has a very promising prospect in device applications.

CONCLUSIONS
In summary, a Zn(II)-based 1D CP has been synthesized from linear dicarboxylate and 4-nvp ligands. 1D chains of the polymeric compound are stacked together via cooperative supramolecular interactions, namely, H-bonding, C−H···π, and π···π interactions to form 3D assembly. A detailed investigation of optical, dielectric, and electrical characterization was done for the compound. The measured optical band gap and the electrical conductivity of the compound fall well in the semiconductor regime. Furthermore, the compound reveals the Schottky diode nature and exhibits photosensing behavior upon illumination of visible light. Here, the presence of π···π stacking interactions among the 4-nvp ligands of adjacent rings assists the high charge carrier mobility of the material. Therefore, it appears that the compound can be used in photosensitive electronic NVP-2 devices.