Enhancement of Electrical Conductivity due to Structural Distortion

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Enhancement of Electrical Conductivity due to Structural Distortion from Linear to Nonlinear Dicarboxylato Bridged Zn(II) 1D-Coordination Polymers Kaushik Naskar, Sayantan Sil, Nilima Sahu, Basudeb Dutta, A.M.Z. Slawin, Partha Pratim Ray, and Chittaranjan Sinha Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01753 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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Crystal Growth & Design

Enhancement of Electrical Conductivity due to Structural Distortion from Linear to Nonlinear Dicarboxylato

Bridged

Zn(II)

1D-Coordination

Polymers Kaushik Naskara‡, Sayantan Silb‡, Nilima Sahua‡, Basudeb Duttac‡, A. M. Z. Slawind‡, Partha Pratim Rayb*‡and Chittaranjan Sinhaa*‡ aDepartment

of Chemistry, Jadavpur University, Jadavpur, Kolkata-700032,India.

E-mail: [email protected] bDepartment

of Physics, Jadavpur University, Jadavpur, Kolkata-700032, India.

E-mail: [email protected] cDepartment

dSchool

of Chemistry, Aliah University, New Town, Kolkata-700156, India.

of Chemistry, University of St. Andrews, Purdie Building, St. Andrews, Fife, KY16 9ST

(UK)

ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Coordination polymers are useful materials in different field of applications including the development of supramolecular electrical devices for the use of renewable energy sources. In this work, we have designed two new classes of mixed-ligand one-dimensional coordination polymers (1D CPs) [Zn(ADC)(PBT)2(H2O)2]n, (1) and [Zn(Succ)(PBT)2(H2O)2]n, (2) (ADC2-, Acetylene dicarboxylato; Succ2-, Succinato; PBT, 2-Pyridin-4-yl-benzothiazole) and were characterized by elemental analysis, infrared spectra (IR), single crystal X-ray diffraction data, powder X-ray diffraction (PXRD) and thermogravimetric analyses (TGA) data. In the structural motif Zn(II) is bridged by aliphatic dicarboxylato ligand (ADC/ Succ) and two axial positions are occupied by pyridyl-N of PBT along with water coordination. ADC acts as monodentate carboxylato-O ligand in the Compound 1, whereas in 2 Succ serves as carboxylato-O, O chelator. Both compounds 1 and 2 are isostructural and construct 3D supramolecular networks by the hydrogen bonds (bonding), π···π and along with weak C-H···π interactions. Fascinatingly, the Compound 1 exhibits ~700 times higher Schottky barrier diode (SBD) electrical conductivity (1.31×10-2 Sm-1) than Compound 2 (1.80×10-5 Sm-1). The impedence electrical conductivity of 1 (1.22×10-4 Sm-1) and 2 (3.24×10-6 Sm-1) differ significantly; besides, the direct current conductivities are 1.08×10-4 Sm-1 (1) and 5.55×10-6 Sm-1 (2). To shed light on the charge transport mechanism of the compounds, the mobility, transit time, and density of states at a quasi-Fermi level have been evaluated. Linear dicarboxylato bridging with sp-hybrid acetylene motif may be the reason for faster charge flow in 1 than sp3 hybrid nonlinear Succinato bridging Compound 2.


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Introduction Coordination polymers (CPs) have gained great attention in recent times in the field of materials science1-4 due to their attractive inherent properties that lead to the variety of applications, such as magnetism,5-8 catalysis,9-11 nonlinear optics,12,13 molecular wires,14−17 and molecular sensing.18,19 In the field of material chemistry, the first principle is to transfer technology from the laboratory to land (for potential applications). One of the focused research areas in the CPs is the design of light inspired higher conducting materials to meet the challenges of the energy crisis. So, research in material chemistry is directed to fabricate electronic devices using highly conducting compounds. To improve the quality of conducting CPs judicious choice of metal nodes, structure-directed organic linker and uninterrupted -conjugation are required. The development of electro-conductive materials is widely distributed in one-, two-, or threedimensional architectures.20-22 In fact, out of hundreds of CPs, only few exhibits recognizable electrical conductivity. Highly ordered crystal structure assists defect free charge transportation and hence improvement of electrical conductivity is observed.23 There are three significant charge transport mechanisms are noted, these are (i) through-space usually directed by πstacking, (ii) through the covalent bonds, as in molecular wires; and (iii) the charge hopping, which is operated in molecular conductors such as CPs or MOFs. The first two mechanisms are ideally based on band transport, whereas, the hopping mechanism is governed by Marcus theory. 24

The conducting properties of CPs have been influenced by the π-conjugation of the linkers,

metal ion type in the nodes and flexibility as well as the chemical functionality of the ligands. Dicarboxylates have been equipped as “linkers” in the construction of major classes of CPs for the various dimensionalities with different coordination modes and sizes.25 Aliphatic dicarboxylates are more flexible than aromatic dicarboxylates and have been used to design CPs.



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The electronic properties are influenced by the steric and electronic nature of organic

linkers.27, 28 Thus, the bulk electronic properties of the polymers may be harmonized by changing the linkers. The CPs synthesized from d10 metal ions (Zn2+, Cd2+) with the conjugated carboxylate organic linkers are found to exhibit semiconductor properties25, 29-32 and may be the active components in the photonic and electronic devices.33,34 In this work, we have synthesized two CPs with subtle differences in structure; aliphatic dicarboxylato ligand (ADC2-, Acetylene dicarboxylato; Succ2-, Succinato;) bridges Zn(II) and two axial positions are occupied by pyridyl-N of PBT (2-Pyridin-4-yl-benzothiazole) along with water coordination. Acetylene dicarboxylato (ADC2-) ligand acts as monodentate carboxylato-O ligand in the Compound 1 and succinate serves as carboxylato-O, O chelator in 2. The 3D supramolecular networks in these two compounds are constructed by the hydrogen bonds and CH···π interactions. These two compounds show electrical conductivity but among them, 1 shows remarkably enhanced (~700 times) efficiency than 2). In conjunction with the experiments, the quantum mechanical calculations using the DFT and TD DFT have been performed to get insight into the band gap, band positions, etc.

Experimental Section Synthesis 2-Pyridin-4-yl-benzothiazole (PBT) had been synthesized and characterized by literature method.35 [Zn(ADC)(PBT)2(H2O)2]n,1: A solution of PBT (M. Wt. 212.27, 42 mg, 0.2 mmol) in MeOH (2 mL) was slowly and carefully layered to a solution of Zn(NO3)2.6H2O (60 mg, 0.2 mmol) in H2O (2 mL) using 2 mL 1:1 (= v/v) buffer solution of MeOH and H2O followed by layering of


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H2ADC (23 mg, 0.2 mmol) neutralized with Et3N (0.027 mL, 0.2 mmol) in EtOH (2ml). The colorless prismatic crystals of [Zn(ADC)(PBT)2(H2O)2]n, 1 were obtained after few days (49 mg, yield 76%). Elemental analysis (%) calculated for C28H20N4O6S2Zn: C 52.71, H 3.16, N 8.78; found: C52.57, H 3.12, N 8.56. IR (KBr pellet, cm-1): 1609 (-C=N), 1556 νas(COO-),1319 νsys(COO-), 3252 ν(H2O) (SI†, Figure S1). [Zn(Succ)(PBT)2(H2O)]n, 2: It was synthesized following the identical procedure as adopted for 1, layering the solution of PBT in MeOH over Zn(NO3)2.6H2O (60 mg, 0.2 mmol) in water followed by H2Succ.Yellowish rectangular shaped crystals of [Zn(Succ)(PBT)2(H2O)]n, 2 were obtained after few days (43 mg, yield 68%). Elemental analysis (%) calculated for C28H22N4O5S2 Zn: C 53.89, H 3.55, N 8.98; found: C 53.99, H 3.58, N 9.01. IR (KBr pellet, cm-1): 1622 (C=N), 1551 νas(COO), 1397 νsys(COO), 3176 ν(H2O) (SI†, Figure S1). X-ray crystallography Single Crystal data and experimental information for data collection and structure refinement are reported in Table 1. Two good-shaped single crystal of 1 (0.18× 0.18× 0.15 mm) was used for data collection via Rigaku FRX RA generator (confocal optic) with P200 detector and the crystal of 2 (0.4× 0.09× 0.04 mm) was used to collect data from Bruker SMART APEX II diffractometer, having graphite-monochromated Mo-Kα radiation (λ= 0.71073 Å). Least squares refinements of all reflections within hkl range -20 ≤ h ≤21, −37 ≤ k ≤ 37, −22 ≤ l ≤ 20 (1) and −32 h 32, −6k  7, −17l  19 (2) were used to evaluate the crystal-orientation matrices and unit cell parameters. The intensity data were corrected for Lorentz and polarization effects.36 The collected data (I >2σ (I)) were integrated using SAINT program and the absorption correction was made with SADABS. Full matrix least-squares refinements on F2 were carried out using SHELXL-2014/737 with anisotropic displacement parameters for all non-hydrogen atoms and



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complex 1 was further refined with Olex2 1.2-alpha crystallographic package.38 All calculations were carried out using, SHELXL 2014/737, SHELXS 2014/739, PLATON 9940 and ORTEP-341 program.

Table 1. Crystallographic Data of 1 and 2 1 2 CCDC No. 1867287 1867290 Empirical formula C28 H20 N4 O6 S2 Zn C28 H22 N4 O5 S2 Zn Formula weight 637.97 623.01 Crystal system Monoclinic Monoclinic Space group P21/n C 2/c a (Å) 18.166(3) 28.0265(16) b (Å) 31.212(5) 5.9046(3) c (Å) 18.900(3) 16.2193(9) α (o) 90.00 90.00 o β( ) 95.080(4) 98.493(2) γ (o) 90.00 90.00 3 V (Å ) 10674(3) 2654.6(3) T (K) 93 293(2) Z 16 4 Dcalcd(Mg/m3) 1.568 1.561 1.129 1.130  (mm-1) 0.71073 0.71073  (Å) 1.6 −25.0  range () 3.125.0 Total reflections 94620 16757 Unique reflections 18642 2293 Refine parameters 1998 182 c a 0.0836 0.0675 R1 [ I > 2 (I) ] b wR2 0.2776 0.1788 Goodness-of-fit 0.999 1.074 Difference between peak and hole (e Å-3) 1.35, -0.83 1.23, -1.16 aR = Σ||Fo| − |Fc||/Σ|Fo|, bwR = [Σw(Fo2− Fc2)2/Σw(Fo2)2]/2, 1 2 for 1,w = 1/ [2(Fo2) + (0.1590P)2+ 0.0000P], where P= (Fo2) + 2Fc2)/3 and for 2, w=1/[ 2(Fo2)+( 0.0986P)2+17.2742P], where P= (Fo2+2Fc2)/3 c The reason of A-level Alerts and the data analyses are described in SI†


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Theoretical Calculation The optimized structures of 1 and 2 were performed by using Density Functional Theory (DFT) at the B3LYP level.42 The theoretical calculations were done by the Gaussian 09 program package with the aid of the GaussView visualization program.43 For C, H, N, O the 6-31G (d) basis set were assigned and LanL2DZ basis set with effective core potential without any symmetry constrain was employed for Zn. The vibrational frequency calculations were evaluated for these two compounds to ensure that the optimized geometries represent the local minima and there were only positive eigenvalues. The single crystals of X-ray coordinates of the 1 and 2 had been used in the calculations. Vertical electronic excitations based on B3LYP optimized geometries were computed using the Time-Dependent Density Functional Theory (TD-DFT) formalism in DMSO using conductor-like polarizable continuum model (CPCM) to consign the low lying electronic transitions in the experimental spectra. Gauss Sum44 was used to calculate the contributions of various functional groups in each molecular orbital. The electronic structure and related properties of the compounds were to be known from DFT computation studies.

Results and discussion The Compound 1 crystallizes in the monoclinic system and space group P 21/n with Z= 16 and the Compound 2 crystallizes with same system and space group C 2/c with Z 4. Asymmetric unit in 1 contains distorted octahedral geometry with ZnO4N2 coordination sphere. The structure (Scheme 1, 1) shows that two ADC2- act as monodentate carboxylato-O donor to Zn(II) (Zn(1)– O(161), 2.127(3) Å; Zn(1)–O(198), 2.158(3) Å) and two H2O molecules occupy on the equatorial sites (Zn(1)–O(16), 2.104(3) Å; Zn(1)–O(17), 2.092(4) Å) and two pyridyl-N from two PBT ligands (Zn(1)–N(1), 2.159(4) Å; Zn(1)–N(21), 2.158(4) Å) in the axial position



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(Scheme 1). In Compound 2 the Zn(II) ion is bridged by chelated Succinato-O, O ligand (Succ2-) and two axial positions are occupied by pyridyl-N of PBT along with water coordination (Scheme 1, 2).

Scheme 1. Synthesis of CPs, [Zn(ADC)(PBT)2(H2O)2]n, 1 and [Zn(Succ)(PBT)2(H2O)]n, 2. Overall topology of the Compound 1 is isotypical with 2. However, among the supramolecular interaction hydrogen bonding and π…π entities play an important role to construct 2D structure in Compound 1. The connectivity of the adjacent carboxylate-O atoms with Zn(II) centers results in a 1-D coordination polymer (Figure 1).


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Figure 1. A segment of 1D coordination polymers of 1 and 2.

The H-bonded O---O separation is 2.709 Å in 1, which is longer than that of 2 (1.960 Å) (Figure 2) and along with some selected π…π interaction are summarized in Figure 3(a) and centroid to centroid distances were listed in Table S1.



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Figure 2. View of supramolecular aggregated 2D network formed by hydrogen bonding in 1 and 2. Compound 1 has a small number of weak C–H…π interaction distances (summarized in SI†, Figure S2. (a)) those are symmetrical in both sides of the ring and span in the range of 3.459(19)– 3.685(8) Å (Table S1) which helps to assemble the 3D aggregate structure. Asymmetric unit in 2 contains distorted pentagonal bipyramidal Zn(II) centre (Figure S3) which is chelated by four O atoms from Succ anions (Scheme 1) (Zn(1)–O(1), 2.345(5) Å; Zn(1)–O(2), 2.460(5) Å), another O atom from one aqua ligand is placed in equatorial sites (Zn(1)–O(3), 2.221(12) Å) in Scheme 1 and two N atoms from two PBT ligands (Zn(1)–N(1), 2.387(6) Å) in the axial position. The aqua ligand creates a strong intermolecular hydrogen bond with coordinated O atom of bridging Succ ligands with the O---O separation of 1.960 Å (Figure 2). Moreover, these 1D chains are aggregated through π…π interactions and weak C–H…π along the PBT ligands with edge-to-face distances are in the range of 3.131–3.310 Å (Figure S2.(b)) in ac plane.


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Figure 3. π···π interactions were viewed for (a) Compound 1 and (b) for compound 2. Therefore, 1D zigzag chains aggregate through hydrogen-bonding and π···π interactions (Figure 3(b), Table S2) to assemble the 2D architecture of Compound 2. These cooperative hydrogen bonding (Figure 2) and π···π interactions (Figure 3 (a, b)) fabricate a 3D supramolecular arrangement for both 1 and 2. To verify the thermal stability of the compounds, thermogravimetric analyses (TGA) were executed with the powdered sample within the temperature range of 23-600 °C under N2 atmosphere. The TGA analysis reveals that compound 1 is stable upto ~112 °C whereas Compound 2 is thermally stable upto ~133 °C (Figure S4). Powder X-ray diffraction (PXRD) has been performed at room temperature and all of the main peaks of PXRD patterns of as-synthesized 1 and 2 matched well with those simulated from


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single crystal data signifying the phase purity of the bulk (Figure S5). To authenticate morphology and elemental analysis of our synthesized compounds 1 and 2 we performed SEM image and energy-dispersive X-ray spectroscopy (EDS) analyses via field emission scanning electron microscopy (FESEM; JEOL, JSM-6700F). (Figure S6 and Table S3)

UV-Vis and Impedance Spectroscopy Study: Optical Characterization: The optical property of the synthesized materials was measured by UV-vis absorption spectra. The inset of Figure 4 represents the normalized absorption spectra of compounds 1 and 2 deposited in thin films by preparing a well dispersion in N, N-dimethylformamide (DMF) medium, which were recorded in the wavelength region of 200-800 nm. The energy absorption is observed in the UV region such as 337 and 315 nm for compound 1 and 2, respectively. The optical band energy gap of, 1 and 2 have been determined by Tauc’s equation (Eq. 1)45, 46: (αhν) = A(hν - E g )

n

(1)

Where, α is the absorption coefficient, Eg is the band gap energy, h is Planck’s constant,  is the frequency of light and the exponent n is the electron transition processes dependent constant, for direct transition n=½47,48, A is a constant which is considered as 1 for the general case.


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Figure 4. Tauc’s plot of compounds 1, 2 and normalized UV-vis absorption spectra (inset). The (αhν)2 vs. hν plot of the synthesized compounds are portrayed in Figure 4. By extrapolating the linear region of both the plot (αhν)2 vs. hν, the values of direct optical band gap energy for 1 and 2 were evaluated as 3.67 eV and 3.93 eV respectively. To further verify the optical band gap energy for compound 1 and 2 we used the Kubelka-Munk function (Figure S7, see supporting information). The measured band gap energy values from Tauc’s equation and Kubelka-Munk equation are nearly closed for compound 1 (3.61 eV) and 2 (3.92 eV). Impedance Spectroscopy Impedance Spectroscopy (IS) study was performed in the frequency range 40 Hz-10 MHz at room temperature. Figure 5(a) shows the Nyquist plot for two compounds under the dark condition. The higher frequency semicircular arc depicts the bulk contribution and an intermediate or low-frequency semicircular arc represents the grain-boundary or electrodespecimen effect.49 The intercept of the semicircular arc on real axis Z demonstrates the bulk resistance Rb (dc resistance) of our synthesized compounds. From Figure 5(a) it is clearly seen that Compound 1 has lower resistance than Compound 2. The lower resistance signifies the better possibility of charge transfer. Figure 5(b) shows the ac conductivity plot for 1 and 2. At


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lower region of frequency, the extrapolation of Y-axis gives the value of dc conductivity (σDC), which is attributed to the long-range translational motion of the charge carriers.49

Figure 5. Nyquist and frequency dependent AC conductivity plot for compound 1 and 2. In the high-frequency region, the term Aωs is responsible for the frequency dependent behavior and the dispersion nature.50 The frequency dependence of AC conductivity (σAC) may be described for free and bound carriers.51 The dependence of the conductivity with frequency can be expressed by the following power law equation (Eq. 2)52, 53: (2)

𝝈(𝝎) = 𝝈𝒅𝒄 + 𝝈𝒂𝒄

Where, σ(ω) is the total conductivity, σDC is the DC conductivity and σAC is the AC conductivity. At high-frequency region, the increase of conductivity described through Jonscher’s power law49 and it is defined in Eq. 3: 𝝎 𝒔

𝝈𝒂𝒄 = 𝝈𝒅𝒄[𝟏 + (𝝎𝑯) ]

(3)

Where, s is the dimensionless frequency component, ω(=2πf) is the angular frequency and ωH is the hopping frequency of the charge carrier. At room temperature, the dc conductivity (σDC) of 1 (1.22×10-4 Sm-1) is about 375 times higher than 2 (3.24×10-6 Sm-1).


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The direct current measurement of compound 1 and 2 are performed with the help of Keithley 2635B source meter in the voltage range -2V to +2V. The measured conductivity values are nearly the same to the values extracted from impedance data (Figure S8, see supporting information). Electrical Characterization The band gap of the compounds (1, 2) lies well within the semiconducting limit which has prompted us to further check the electrical conductivity and its possible application in metalsemiconductor hetero-structure such as Schottky barrier diode23,

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(SBD). Hence, for a better

understanding of the charge transport phenomenon, the electrical characterization was carried out for the thin film devices of the well-dispersed solution of compounds deposited on top of ITOcoated glass substrates. To analyze the electrical properties, Current-voltage (I-V) measurement of 1 and 2 based thin film devices were performed in the range ±2.0 V with a Keithley 2635B source meter under the dark condition. The current-voltage characteristics curve in Figure 6 exhibits non-linear rectifying behavior which is the signature of the Schottky diode. The rectification ratio (i.e., on/ off ratio) of the devices under the dark condition were measured as 59.41 and 38.68 for 1 and 2, respectively. Logarithmic presentation of I as a function of V is shown in the inset of Figure 6.



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(a)

(b) Figure 6. Current-voltage (I-V) characteristics curve of Compound 1 and Compound 2 based Schottky barrier diode.


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The underlying mechanism of charge transport has been explored by analyzing the I-V characteristics curve with the help of thermionic emission (TE) theory.54 The diode analysis was performed with the help of following equations (Eqs. 4-6)55: 𝒒𝑽 ―𝒒𝑽 (𝜼𝑲𝑻 )[𝟏 ― 𝒆𝒙𝒑( 𝜼𝑲𝑻 )]

(4)

𝑰 = 𝑰𝟎𝒆𝒙𝒑

(

𝑰𝟎 = 𝑨𝑨 ∗ 𝑻𝟐𝒆𝒙𝒑 b =

kT q

*

ln

AA T

―𝒒∅𝒃 𝑲𝑻

)

(5)

2

(6)

I0

Where, I is current, I0 is the saturation current deduced from the straight line intercept of ln (I) at V=0, V is the forward-bias voltage, q is the electronic charge, k is the Boltzmann constant, T is the temperature in Kelvin, A is the effective diode area (7.065×10−6 m2), η is the ideality factor and A* is the effective Richardson constant (1.20×106 AK-2m-2), respectively. The barrier heights of 1 and 2 have been obtained as 0.55 eV and 0.63 eV, respectively. It is very important to note that the rectification ratio of 1 based device increased whereas the barrier height decreased compared to 2 based devices. So, Compound 1 is expected to be the better candidate for the fast switching device.56 For a better insight into the charge transport phenomena, we investigated the I-V curves in details. The current conduction mechanism is governed by the power law (I ∞ Vm)57, where m is the slope of the I vs. V curve. The characteristic I-V curves (Figure 7) under dark conditions in the logarithmic scale revealed that it can be differentiated in two slopes for compounds 1 and 2. The two regions have slopes of nearly 1 and 2. In region-I, when the slope is ~1, current follows the relation I ∞ V, which refers to the Ohmic regime.



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Figure 7. log I vs. log V plot (a) Compound 1 and (b) Compound 2 under dark condition. The I-V characteristics in this region can be attributed due to the thermionic emission of current. In region-II, the slope is about 2, where current is proportional to V2. In this region, the current originated from space charge, designated as space charge limit current (SCLC) mechanism by discrete trapping level.58 If the injected carriers are more than the background carriers, the injected carriers spread and create a space charge field. The currents are controlled by this field and are known as SCLC.59 The device performance greatly depends on the mobility and transit time of the carriers. So, the mobility of 1 (8.16×10-5 m2V-1s-1) improved by 380 times of 2 (2.15×10-7 m2V-1s-1) under the dark condition from I vs. V2 plot (Figure 8) using by MottGurney space-charge-limited-current (SCLC) equation (Eq. 7)60, 61: I=

9μeff ε 0εr A V 2 ( 3) 8 d

(7)


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Figure 8. Current (A) vs V2 (voltage2) plot of SCLC region under dark condition for (a) 1 and (b) 2. Where, I is the current (A), µeff is the effective mobility of the charge carrier, 𝜀0 is the permittivity of free space, and 𝜀𝑟 is the dielectric constant. The value of the dielectric constant of the compounds was measured from capacitance (C) vs. frequency (f) plot (Figure 9) by using the equation (Eq. 8)62:

εr =

Cd ε0 A

(8)

Where, C is the capacitance (at saturation), d is the thickness and rest are the same as defined previously. Transit time (τ) is defined as the time, which is required by a carrier to travel from anode to cathode. It can be expressed as the summation of the average total time spent by each electron as a free carrier plus total time spent in the tarp63. The transit time (τ) of the charge carrier is estimated with the help of the following equation (Eq. 9)64: τ=

9ε 0εr A V ( ) 8d I

(9)



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Figure 9. Capacitance versus frequency plot for (a) compounds 1 and (b) compounds 2. The density of states (DOS) near the Fermi level was determined from the SCLC region of I-V characteristics using den Boer method (Eqs. 10, 11)65, 66: N' (EF ) =

2ε 0 ε r (V2 - V1 ) qd 2 ΔEF

ΔEF = kT ln(

I 2 V1 I1 V2

(10)

)

(11)

Where, 𝑁′(𝐸𝐹) is the DOS, d is the thickness of the film, EF the quasi-Fermi level, T is the roomtemperature in Kelvin scale, V1 and V2, I1 and I2 are different voltages and currents applied to the diode in the space charge region. All the obtained values of charge transport parameters for 1 and 2 are listed in Table 2 with other parameters. Longer transit time generally indicates the occurrence of more number of trapping states67,

68

but for Compound 1 the transit time is

decreased due to the higher carrier mobility. The performance indicating parameters like mobility, transit time and DOS are improved for Compound 1. Therefore, from all the measured values listed in Table 2, it is very distinct that the device fabricated by Compound 1 is better than 2.


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21


Table 2. Comparison of I-V Characteristic Parameters of 1 and 2 sample

barrier height (ϕb) (eV)

carrier mobility (μeff) m2v-1s-1

transit time (τ) (Sec)

DOS N (EF) (V-1m-3)

0.55

SCLC conductivity (σ) (Sm-1) 1.31×10-2

1

8.16×10-5

5.26×10-9

9.30×1021

2

0.63

1.80×10-5

2.15×10-7

2.24×10-6

10.79×1021

DFT Computation and Band Gap To generate Schottky electrical contact the lattice matching and deformation potentials have been used and the deformation commonly refers difference between the highest occupied and lowest unoccupied molecular orbitals energy i.e. band gap (ΔE=ELUMO – E HOMO, eV)69. In 1 and 2 the absolute deformation potentials (ADPs) are used to determine the band gap70. These polymers have an organic and inorganic moiety, so the band gap may be influenced by the electronic contribution of both. In compounds 1 and 2, the Zn has d10 electronic configuration, the band edges are often defined by electronic states on the organic moiety along with geometry strain of the network. The structural agreement has also been verified by comparing the bond distances and angles between the DFT optimized and X-ray determined structures. DFT optimization of the structure of the coordination motif of 1 and 2 have been used to calculate ΔE (the difference in energy of HOMO and LUMO) (Compound 1 and 2) (Figure 10); which may match with the band gap obtained from Tauc’s plot (Figure 4). Energy of MOs has been increased in 2 compared to Compound 1 which may be due to electron drifting in the metal coordination sphere which follows the experimental results (Figure S9 and Table S4). The calculated (TD DFT) transitions HOMO-1 LUMO+1 (, 335 nm; f, 0.0494); HOMO-4 → LUMO+2 (, 256.04 nm; f, 0.2110) etc have been assigned as ILCT charge transferences for



22

1, and in 2 the composition differs significantly i.e. HOMO-1 → LUMO (, 323.41 nm; f, 0.5675); the HOMO-9  LUMO+1 (, 270.44 nm; f, 0.0307) is also an ILCT charge transfer transition (Figure S9, S10, Table S5 and Table S7). A small increment in the calculated band gap may be assigned to the geometry factor which has not been considered in the use of calculation of a single motif. The contribution of an acid part in Compound 1 exists in HOMO-4 → LUMO+2 and HOMO-5 → LUMO+1 having lower energy than that of in Compound 2 the acid part exists in HOMO-9 → LUMO+1 and HOMO-15 → LUMO which refers the band gap in 1 is lower than Compound 2 (Figure S10 and Table S6). This direction is also seen in HOMO → LUMO band gap in both the compounds. As a result, Compound 1 shows more conductivity than that of 2.

Figure 10. DFT computed the energy of MOs and the energy difference between HOMO and LUMO of the compounds 1 and 2.


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23

Conclusions In summary, we have successfully constructed two 1D coordination polymers (CPs) of Zn(II), PBT, 2-pyridin-4-yl-benzothiazoleand varying aliphatic dicarboxylato ligand, such as ADC2-, Acetylene dicarboxylato; Succ2-, Succinato. The 1D polymeric chains herein stack together by the combination of H-bonding, ··· and weak C–H···π interactions to form 3D supramolecular aggregated structures. From impedance spectral analysis it can be seen that Compound 1 has lower resistance than 2 which implies that the 1 exhibits higher electrical conductivity compared to 2. Space charge limited current (SCLC) theory has been incorporated for the better understanding of charge transport phenomena through the metal-semiconductor junction for 1 and 2 based Schottky devices. However, 1 based metal-semiconductor hetero structure shows improved charge transport properties compared to 2 based devices due to the higher carrier mobility and reduced transit time. As the linear dicarboxylato bridging with sp-hybrid acetylene motif may be the reason for faster charge flow in 1 than sp3 hybrid nonlinear Succinato bridging 2. So, these kinds of material can pave the way for a very promising future in device application. ASSOCIATED CONTENT Supporting Information All the Supporting Information is available free of charge on the ACS Publications website. IR-Spectra of Compound 1 and 2, Crystallographic Data Analyses of Compound 1, Pentagonal bipyramidal motif of Compound 2, TGA plot of Compound 1 and 2, PXRD plot of Compound 1 and 2, SEM mapping and EDS spectrum of Compound 1 and Compound 2, Experimental Section, MO composition for Compound 1 and 2, TD-DFT results and assignment of charge transitions, energy, wavelength and oscillator strength (f) for compounds 1 and 2.



24

Accession Codes CCDC 1867287 and 1867290 (1 and 2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. AUTHOR INFORMATION Corresponding Author *Department

of Chemistry, Jadavpur University, Jadavpur, Kolkata 700032,India.

E-mail: [email protected] Present Address Department of Chemistry, Jadavpur University, Kolkata -700032, India.

Author Contributions ‡All the authors contributed equally. Funding Sources The authors acknowledge for the financial support from the Council of Scientific and Industrial Research (CSIR, Sanction No. 01(2894)/17/EMR-II) New Delhi, India. ACKNOWLEDGMENT The authors acknowledge for the financial support from the Council of Scientific and Industrial Research (CSIR, Sanction No. 01(2894)/17/EMR-II) New Delhi, India and SERB (New Delhi) PDF/ 2016/ 001813. ABBREVIATIONS CPs, coordination polymers; PBT, 2-Pyridin-4-yl-benzothiazole; H2ADC, acetylene dicarboxylic acid; H2Succ, succinic acid; IS, impedance spectroscopy.


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For Table of Contents Use Only

Enhancement of Electrical Conductivity due to Structural Distortion from Linear to Nonlinear Dicarboxylato Bridged Zn(II) 1D-Coordination Polymers Kaushik Naskar, Sayantan Sil, Nilima Sahu, Basudeb Dutta, A. M. Z. Slawin, Partha Pratim Ray and Chittaranjan Sinha

SYNOPSIS: Two new mixed-ligand one-dimensional coordination polymers (1D CPs) [Zn(ADC)(PBT)2(H2O)2]n,

(1)

and

[Zn(Succ)(PBT)2(H2O)2]n,

(2)

are

designed

for

supramolecular electrical devices application and interestingly the conductivity value of linear (ADC, 1) carboxylate linker based SBD has improved ~700 times in comparison with the nonlinear (Succ, 2) based SBD.


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Enhancement of Electrical Conductivity due to Structural Distortion

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