Designing of Pb(II)-Based Novel Coordination Polymers (CPs): Structural Elucidation and Optoelectronic Application.

[Pb2(bdc)1.5(aiz)] n (1) and [Pb2(bdc)1.5(aiz)(MeOH)2] n (2) (H2bdc = 1,4-benzene dicarboxylic acid, aiz = (E)-N'-(thiophen-2-ylmethylene)isonicotinohydrazide) have been synthesized, and structural characterization has been established by X-ray analysis and thermogravimetric analysis (TGA). Here, bdc2- links two Pb(II) centers and the aiz ligand binds the metal centers in two different manners: chelating and monodonating. Thus, polymerizations have taken place from the combination of mixed ligand system. Optical band gaps have been studied via UV measurements. Again, the experimental and calculated (from density functional theory (DFT)) band gaps agree well and the semiconducting properties of synthesized polymeric materials have been approved. Thus, optoelectronic and photonic devices can be made by this type of coordination polymers (CPs). The I-V representative curves of 1 (device-A) and 2 (device-B) in both dark and illuminated conditions show that device-A has a higher magnitude of current than device-B. Dark- and photo-conductivity values of device-A are calculated as 2.94 × 10-6 and 6.12 × 10-6 S m-1, respectively, whereas for device-B, the values of dark- and photo-conductivity are 2.92 × 10-7 and 3.66 × 10-7 S m-1, respectively, at room temperature.

Introduction:

Lead(II), a heavy toxic metal, is affecting almost every organ in the body.[1] Still, lead is used worldwide because of its application in energy storage devices for their reliability and cost-efficiency such as lead-acid batteries and lead-carbon batteries[2] along with lithium-ion battery.[3,4] The coordination chemistry of lead is not so popular unlike transition metals as Pb(II), d10, neither is magnetically active nor synthesizes colorful complexes. However, the group 14 Pb(II) has a large radius and has also flexible stereochemical activity with distinctive coordination preferences, which provide unique opportunities to construct an interesting network.[5] Besides, Pb-halide perovskites APbBr3 are potential photoactive materials and have shown promise for low-cost photoconducting and solar energy conversion materials.[6] This has prompted us to design Pb(II)-based metal–organic coordination polymers (MOCPs).[7] Emerging trends of chemistry have been continuously flagging due to the synthetic compounds and their applications. The metal–organic framework (MOF)[8−18] is one of the potential materials in this direction. The metal–organic coordination polymeric compounds are also useful to construct photonic and optoelectronic devices.[19−22] Semiconducting materials made by polymeric organic compounds have been used over the last few years, but the insufficient thermal stability lagged them behind. Overall electronic properties[23−28] of these types of materials may be tuned by the thoughtful selection of organic ligand entities. High thermochemical stability of the coordination polymer arises due to the presence of a strong covalent bond, which is generated by the self-assembly (via secondary interactions)[29−32] process. Nonmetallic materials can also be used as semiconductor materials,[24] but sometimes these compounds are lagging behind due to the complicated synthetic procedure, nonreproducibility, and thermal and chemical instability. In the case of metal-introduced hybrid materials, these problems can be overcome.[19−22] The highly ordered molecules in the coordination polymers produce unitary cell-like arrangements.[33] Different secondary interactions assemble the motifs to form a large surface area to collect photons in illumination conditions, which causes transfer of excited electrons from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO).[34,35] The photoconductivity over the polymeric molecule–electrode interface occurs via excitation of charge carriers and migration over the lattice followed by charge separation. In this aspect, we are able to design two Pb(II)-based coordination polymers[36−41] and are useful to fabricate optoelectronic devices.

In the present work, two Pb(II)-based coordination polymeric compounds [Pb2(bdc)1.5(aiz)] (1) and [Pb2(bdc)1.5(aiz)(MeOH)2] (2) have been synthesized (Scheme ), and electrical conductivity has been studied. A higher magnitude of current is observed in the I–V characteristic curves for 1 than 2, in both dark and irradiated conditions. Dark- and photo-conductivity values of 1 are calculated as 2.94 × 10–6 and 6.12 × 10–6 S m–1, respectively, whereas for 2, the values of dark- and photo-conductivity are 2.92 × 10–7 and 3.66 × 10–7 S m–1, respectively. Thus, these compounds may be possible options for optoelectronic device fabrication.

Scheme 1

Synthesis of Compound 1 and 2 upon the Reaction of Pb(NO3)2 with Aiz and H2bdc Ligands

Result and Discussion

Crystal Structures

The confirmatory molecular arrangements of 1 and 2 are obtained by single-crystal X-ray diffraction (SCXRD). The asymmetric unit in 1 contains two metal centers (Pb01 and Pb02), and they are in the distorted pentagonal geometry with a PbO4N coordination sphere. In Pb01, the center metal ion binds through bdc2– in two different manners, chelating (by two O-atoms from a carboxylic acid group) and bridging (by two O-atoms, one is μ1-O and another is μ2-O, from two different carboxylic acid groups), and resulted in polymerization. Again, there is a monodonation of pyridyl-N atom from the “aiz” ligand. Thus, every Pb(II) center is coordinated with three bdc2– and one aiz ligands. The Pb–O and Pb–N bond lengths (Pb01–O004, 2.435(10); Pb01–O2, 2.943(10); Pb01–O005_d, 2.763(9); Pb01–O006, 2.285(10); and Pb01–N00A_c, 2.533(12) Å) also confirm the distorted nature of the geometry (Figure a).

Figure 1

(a) View of the repeating dimeric unit of 1, (b) one-dimensional (1D) ribbon-like polymeric arrangement of the molecule along the “b” axis, and (c) two-dimensional (2D) square-grid net of 1 viewed along the “a” axis.

The similar PbO4N geometrical atmosphere also appears around the Pb02 center. In this case, the central metal ion is chelated from both the ligands: dicarboxylic acid and aiz along with bridging-O of the second bdc2– unit. The Pb02–O and Pb02–N bond lengths are Pb02–O1, 2.322(10); Pb02–O2_a, 2.651(10); Pb02–O3, 2.392(12); Pb02–O005, 2.890(9); and Pb02–N1, 2.447(15) Å (Figure a). Due to presence of two types of Pb(II) centers and two kinds of bridging units (bdc2– and aiz), 1 adopts a 1D ribbon-like polymeric structure (Figure b), and the 2D square-grid arrangement is achieved by the chelating and monodonating nature of aiz with Pb(II) (Figure c). During the formation of structural architecture, there are a few prominent supramolecular hydrogen-bonding (intramolecular: N2–H2A···S1 = 2.07 Å (∠N2–H2A···S1 = 143°), C3–H3···O2 = 2.59 Å (∠C3–H3···O2 = 131°), and C00S–H00S···O004 = 2.42 Å (∠C00S–H00S···O004 = 127°) along with intermolecular: C4–H4···O1 = 2.56 Å (∠C4–H4···O1 = 159°) and C00X–H00X···O005 = 2.51 Å (∠C00X–H00X···O005 = 134°)) interactions, which play a crucial role in maintaining the molecular structure. As a result, 1 produces a three-dimensional (3D) structure assisted from π···π interactions (Figure ). The asymmetric unit of 2 also holds two metal centers (Pb1 and Pb2) of hexa-coordinated geometry with a PbO5N coordination sphere. In the Pb1 center, the metal ion binds in a chelating manner by two different bdc2– units along with bridging-O (μ2-O) with another bdc2– unit and monodonation of the pyridyl-N atom from the aiz ligand and satisfies the hexa-coordinating (Pb1–O3d, 2.638(9); Pb1–O4c, 2.500(7); Pb1–O5c, 2.526(7); Pb1–O6, 2.532(8); Pb1–O7, 2.711(8); and Pb1–N4, 2.449(9) Å) geometrical arrangement (Figure a).

Figure 2

(a) Supramolecular π···π interactions in 1 along the b axis. (b) Supramolecular 3D aggregated netlike structure along the a-axis.

Figure 3

(a) View of the repeating dimeric unit of 2. (b) Supramolecular π···π interactions in 2 (along the a-axis). (c) 1D polymeric arrangement of the molecule (along the a-axis).

But in the Pb2 environment, two solvent molecules (MeOH) are coordinated with the metal center and it is also chelated by aiz and bdc2– ligands (Pb2–O1, 2.301(8); Pb2–O2, 2.414(7); Pb2–O3, 2.795(8); Pb2–O9, 2.788(12); Pb2–O10b, 2.808(9); Pb2–N2, 2.425(9) Å). Again, the monomeric unit undergoes polymerization from dicarboxylic acid along with aiz and is supported from the π···π supramolecular interaction (Figure b) to form 2D arrangements (Figure c).

The hydrogen bonding, intermolecular (O(9)–H(9)···O(4) = 1.97 Å (∠O(9)–H(9)···O(4) = 168°) and C(26)–H(26B)···O(6) = 2.50 Å (∠C(26)–H(26B)···O(6) = 164°)) and intramolecular (C(3)–H(3)···O(3) = 2.50 Å, ∠C(3)–H(3)···O(3)133), is responsible for the formation of assembly of the molecules and constructs a 3D supramolecular structure (Figure ).

Figure 4

(a) View of a 2D molecular sheet having alternative tetragonal grid. (b) 3D supramolecular interdigitated structure of molecular arrangement.

Thermal Stability of 1 and 2

Thermogravimetric analyses (TGA, 30–700 °C) revealed that 1 is more stable (150 °C) than 2 (120 °C) (Figure S1) and the loss in weight may be due to the release of coordinated solvent molecules. Finally, the network breaks after 225 °C for 1 and 270 °C for 2. To verify the phase purity of 1 and 2, the powder X-ray diffraction (PXRD) test was also undertaken (Figures S2 and S3).

DFT Computation

The experimental UV–vis spectra of 1 and 2 in dimethyl sulfoxide (DMSO) have sharp absorption at ∼323 and 360 nm, respectively. The time-dependent density functional theory (TD-DFT) calculation in the triplet spin state (using the DMSO solvent and conductor-like polarizable continuum model (CPCM) model) was performed to investigate the cause of UV–vis transitions in 1 and 2. The TD-DFT-calculated energies of the specific bands along with the nature of transitions, theoretical lambdas, and oscillation strengths (f) are listed in Table S1. Some molecular orbital diagrams with energy are also listed (Tables S2–S5). The λ (calcd) values for 1 and 2 are 327.72 and 360.09 nm, respectively, and these may be interligand charge transfer transitions (ILCTs) and the experimental wavelengths are 323 and 362 nm correspondingly. For compound 1, the transition from HOMO → LUMO + 1 (β) has contributions mainly due to the absorption bands at 323 nm. However, 2 shows the absorption peaks at 362 nm, and this may be due to the HOMO – 1 → LUMO + 1 (α) transition. The energy difference between HOMO and LUMO, ΔE = ELUMO – EHOMO (eV), is also obtained from DFT computation in the triplet state (Figure ), and it is correlated with the obtained band gap (normally considered as the difference between conduction and valence bands) from Tauc’s plot by the UV spectrum. Minor discrepancy between experimental and calculated band gaps is obtained in the triplet than in the singlet spin state (Figure S4). The band gaps (Tauc’s plot) of the synthesized 1 and 2 were estimated as 3.46 and 3.01 eV, respectively, while band gaps from DFT computation are 2.84 and 2.52 eV (β spin), respectively.

Figure 5

Energy difference between HOMO and LUMO of 1 and 2 in the triplet state (DFT).

Optical Characterization

The optical characterization was studied (λ, 300–600 nm). Figure (inset) exhibits normalized optical absorbance spectra of 1 and 2. The optical band gap of the compounds resembles the excitation of an electron to the conduction band from the valance band (obtained using Tauc’s equation).[42,43]where α is the absorption coefficient; Eg is the band gap energy, h is Planck’s constant, ν is the frequency of light, n is the electron transition process-dependent constant (in direct transition, n = 1/2), and A is the constant, which is considered as 1 for the common case.[44,45] The (αhν)2 vs hν plot of compounds under investigation is shown in Figure . In the (αhν)2 vs hν plot, the linear region of both the plots is extrapolated and the direct optical band gap energies of 1 and 2 are calculated as 3.46 and 3.01 eV, respectively.

Figure 6

Tauc’s plots of 1 and 2; inset: UV–vis absorption.

Electrical Characterization

Figure shows I–V characteristics of device-A (Al/1/ITO) and device-B (Al/2/ITO) under dark and illuminated conditions, and it shows that device-A has a higher magnitude of current than device-B. At room temperature (RT), dark- and photo-conductivity values of device-A are calculated as 2.94 × 10–6 and 6.12 × 10–6 S m–1, respectively, whereas for device-B, the values of dark- and photo-conductivity are 2.92 × 10–7 and 3.66 × 10–7 S m–1, respectively. The inset of Figure shows the logarithmic presentation of current as a function of voltage.

Figure 7

I–V plots (a) device-A and (b) device-B under dark and illumination conditions.

For better understanding of the charge transport mechanism in devices, the thermionic emission (TE) theory is used,[46] and according to this theory, the current of a diode may be stated as follows, eqs –4[47]wherewhere I0 is the saturation current, q is the electronic charge, k is the Boltzmann constant, T is the temperature in kelvin, V is the forward bias voltage, η is the ideality factor, ϕB is the effective barrier height at zero bias, A is the diode area (7.065 × 10–6 m2), and A* is the effective Richardson constant (1.20 × 106 A m–2 K–2). According to Cheung, the forward bias I–V characteristics (in terms of series resistance) may be stated as[48]eq and the series resistance is calculated[49] from eqs –8where IRS indicates the voltage drop across the series resistance of the device.and H(J) can be stated asFrom the dV/dln(J) vs J plot, the series resistance, RS, and ideality factor, η, for all of the devices in dark- and photo-conditions are determined by the slope and intercept, respectively (Figure ). Using the y-axis intercept of the H(J) vs J curve, the potential barrier height (ϕb) for the devices is calculated, and the slope of this plot provides a second determination of the series resistance. Obtained RS, η, and ϕb are listed in Table .

Figure 8

dV/d ln(J) vs J and H(J) vs J curves for (a) 1- and (b) 2-based Schottky barrier diodes in dark- and photo-condition.

Table 1

Schottky Diode Parameters

			d(V)/d(ln J) vs J graph		H(J) vs J graph
compound	measured condition	conductivity (δ) (S m–1)	ideality factor (η)	series resistance (RS) (Ω)	barrier height (ϕb) (eV)	series resistance (RS) (Ω)
1	dark	2.94 × 10–6	1.32	5.35 × 104	0.76	5.34 × 104
	light	6.12 × 10–6	1.11	2.25 × 104	0.73	2.15 × 104
2	dark	2.92 × 10–7	0.93	4.48 × 105	0.80	4.32 × 105
	light	3.66 × 10–7	0.97	3.64 × 105	0.78	3.44 × 105

The value of η in dark conditions deviates from ideal behavior, and the difference may arise for the presence of inhomogeneities of the Schottky barrier height, hole recombination in the depletion region, series resistance, and existence of interface states.[50,51] The ideality factor, η, value moves toward unity upon illumination of light. After absorbing light, the series resistance, RS, of the compounds in both cases decreases and signifies its applicability in the field of optoelectronic devices.

By using the power law (I ∞ V), the current conduction mechanism is described,[52] where m refers to the slope of the I vs V curve. The current–voltage plot (Figure ) exhibits two different regions under forward bias. At low bias voltage, region-I, the sample exhibits an Ohmic nature, i.e., the current is directly proportional to the applied bias voltage (I ∞ V). The region II corroborates the variation of current with square of forward bias voltage (I ∞ V2), and in this region the conduction mechanism is explained by the space-charge-limited current (SCLC) mechanism dominated by the discrete trapping level.[53,54]

Figure 9

Logarithmic plots of the I–V characteristic curves in dark and light conditions: (a) device-A and (b) device-B.

The injected carriers spread and make a space charge field when it is more than that of background carriers, and the currents are controlled by the field and known as SCLC.[55] The performance of the device greatly depends on the mobility and transit time of carriers. Thus, from the I vs V2 plot, the mobility has been evaluated (Figure ) taking the Mott–Gurney space-charge-limited current (SCLC)[56,57] (eq )where I is the current, ε0 is the permittivity of free space, μeff is the effective mobility of electron, d is the thickness, and εr is the dielectric constant. The dielectric constant (εr) of the material is derived by the capacitance vs frequency plot (C–F) (Figure ) taking[58] (eq )The values of mobility for 1 and 2 are 8.31 × 10–7 and 2.25 × 10–7 m2 V–1 s–1 under dark conditions and 1.43 × 10–5 and 2.57 × 10–6 m2 V–1 s–1 under photo conditions, respectively. This has been admirable compared to some other materials of this type. Some examples of semiconducting materials and their corresponding photosensitivities are listed in Table S6.

Figure 10

Capacitance versus frequency plot (a) for 1 and (b) for 2.

The transit time, τ, is the time required by a carrier to travel to the cathode from anode, and it may be stated as a summation of the average time spent by each electron (as a free carrier) plus the total time spent in the trap.[59] The transit time of the charge carrier is calculated using eq (60)In dark conditions, the effective mobility values of the carriers are estimated as 8.31 × 10–7 and 2.25 × 10–7 m2 V–1 s–1 for 1 and 2, respectively. Effective carrier mobility (μeff) values, after irradiation of light, improve to 1.43 × 10–5 (1) and 2.57 × 10–6 m2 V–1 s–1 (2), respectively. In dark conditions, the transit time (τ) is longer, which leads to higher trapping probabilities, but the situation is reverse after illumination of light, which may be due to the higher carrier mobility.[59] The values of μeff and τ of 1 and 2 are presented in Table .

Table 2

Charge Transport Parameters

compound	measured condition	effective carrier mobility (μeff) (m2 V–1 s–1)	transit time (τ) (s)
1	dark	8.31 × 10–7	1.06 × 10–6
	light	1.38 × 10–6	6.34 × 10–7
2	dark	2.25 × 10–7	3.51 × 10–6
	light	3.67 × 10–7	2.15 × 10–6

Impedance Spectroscopic Measurement

The impedance spectroscopic study is carried out within the frequency range 40 Hz to 10 MHz taking the oscillating voltage of 200 mV by an Agilent 4294A LCR meter at room temperature. Nyquist plots for two compounds 1 and 2 are shown in Figure . The higher-frequency semicircular arc represents the bulk contribution, and the intermediate- or low-frequency semicircular arc represents the grain-boundary or electrode-specimen effect.[61] the bulk resistance Rb (dc resistance) of the sample is obtained from the intercept of the semicircle on real axis Z′, and from this figure, it is clearly specified that 1 possesses lower resistance than that of 2. This lower bulk resistance of 1 results in enhanced possibility of charge transfers and reduces the charge recombination chances.

Figure 11

Nyquist plots for 1 and 2.

Conclusions

Here, two coordination polymeric compounds with Pb(II) as node, bdc2− and aiz as linkers, were isolated. The Pb(II) coordination systems have been synthesized, structurally elucidated, and well characterized. Isostructural compounds are utilized for studying electrical conductivity. The I–V characteristics are determined for 1 and 2 in dark and illuminated conditions. From this study, at room temperature, dark- and photo-conductivity values of 1 are calculated as 2.94 × 10–6 and 6.12 × 10–6 S m–1, respectively, whereas for 2, the values of dark- and photo-conductivity are 2.92 × 10–7 and 3.66 × 10–7 S m–1, respectively. In the absorption of light, the compounds show increased electrical conductivity. Thus, the compounds may be the potential alternative for optoelectronic device fabrication. Higher distortion and greater polarization in the structure of 1 may be the reason for the faster charge mobility and higher conductivity than those of 2.

Experimental Section

The experimental details that include materials and physical methods used in this work, general X-ray crystallography, device fabrication, and characterization are added in the corresponding Supporting Information.

Synthesis of 1

(E)-N′-(Thiophen-2-ylmethylene)isonicotinohydrazide (aiz) was prepared by stirring of isoniazid and thiophene-2-carbaldehyde in dry MeOH solution for 6 h. A solution of aiz (0.046 g, 0.2 mmol) in dimethylformamide (DMF)/MeOH (1:10 v/v) (2 mL) was gently and sensibly layered in a solution of Pb(NO3)2 (0.066 g, 0.2 mmol), in 2 mL of H2O taking 2 mL of DMF buffer solution followed by layering with H2bdc (0.033 g, 0.2 mmol, neutralized with Et3N, 0.04 g, 0.4 mmol) in 2 mL of EtOH. Brown needle-shaped crystals of 1 were found after 2 weeks (0.123 g, yield 69%). Elemental analysis (calcd %) for C23H15N3O7Pb2S: C 30.97, H 1.70, N 4.71; found: C 30.91, H 1.63, N 4.78. IR (KBr disk, cm–1): 1586 νas(COO−), 1360 νs(COO−) and 1646 ν (−C=N−) (Figure S5).

Synthesis of 2

2 was prepared following the same technique as used for the synthesis of 1, except the use of buffer solution, H2O/MeOH (1:1, v/v) instead of DMF. The red prismatic crystals of [Pb2(bdc)1.5(aiz)(MeOH)2] (2) were acquired after seven days (0.142 g, yield 74%). Elemental analysis (calcd %) for C25H22N3O9Pb2S: C 31.44, H 2.32, N 4.40; found: C 31.41, H 2.38, N 4.45. IR (KBr disk, cm–1): 1578 νas(COO−), 1352 νs(COO−), 1654 ν (−C=N−) and 3209 ν (−OH, methanol) (Figure S6).

Theoretical Calculations

The Gaissian-09 program with the DFT-B3LYP hybrid function was used to carry out DFT computations for determining the correlation between theoretically and experimentally obtained band gaps.[62,63] For C, H, and N, the 6-31G (d) basis set was allocated, and for Pb, LanL2DZ was used. The SCXRD coordinates were taken for the compounds during the calculations. In the experimental spectra, low-lying electronic transitions of the compounds were acquired from TD-DFT calculations.[64,65] GaussSum was used to calculate the fractional contribution in the metal-ion-centered molecular orbital and ligand-based molecular orbital.[66]