Two Zn(II) Based Metal Complexes of New Pyrimidine Derived Ligand

Article Cite This: J. Phys. Chem. C 2018, 122, 8724−8734

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Two Zinc(II)-Based Metal Complexes of New Pyrimidine Derived Ligand: Anion-Dependent Structural Variations and Charge Transport Property Analysis Saugata Konar,*,§ Arka Dey,¶ Somnath Ray Choudhury,# Kinsuk Das,∥ Sudipta Chatterjee,† Partha Pratim Ray,*,¶ Joaquín Ortega-Castro,⊥ Antonio Frontera,*,⊥ and Subrata Mukhopadhyay‡ §

Department of Chemistry, The Bhawanipur Education Society College, 5, Lala Lajpat Rai Sarani, Kolkata, Kolkata 700020, India Department of Physics, Jadavpur University, Jadavpur, Kolkata 700032, India # Central Chemical Laboratory, Geological Survey of India, 15A & B Kyd Street, Kolkata, Kolkata 700016, India ∥ Department of Chemistry, Chandernagore College, Hooghly, West Bengal 712136, India † Department of Chemistry, Serampore College, Serampore City, West Bengal 712201, India ⊥ Departament de Química, Universitat de les Illes Balears, Crta. de Valldemossa km 7.5, 07122 Palma de Mallorca (Baleares), Spain ‡ Department of Chemistry, Jadavpur University, Jadavpur, Kolkata 700032, India ¶

S Supporting Information *

ABSTRACT: Two zinc(II) based metal complexes, [Zn(Pymox)Cl2] (1) and [Zn6(Pymox)6(μ2-O)3] (2), where Pymox = 3-[(4,6-dimethyl-pyrimidine-2-yl)-hydrazono]-butan2-one oxime, have been synthesized and characterized by elemental analysis, IR, and single crystal X-ray diffraction studies. Single crystal X-ray analysis confirms that one of the synthesized products (1) is a mononuclear complex and another (2) is a hexanuclear Zn complex. The optical band gap energy in both the complexes (3.43 eV in 1 and 2.36 eV in 2) from solid state UV measurement explores semiconductor behavior of the synthesized materials. The dielectric parameters such as charge transfer resistance, room temperature dc conductivity and electron lifetime measurement shows the superiority of complex 2 over complex 1. Therefore, the Schottky barrier diode (SBD) electronic devices were fabricated by using these two complexes with aluminum (Al) and indium tin oxide (ITO) in sandwich configurationITO/1 or 2/Al. Both the devices exhibit sound rectification behavior with better photosensing property for complex 2 based SBD under irradiation of light, in comparison to dark conditions. The electric current measurement for complex 2 based SBD also exhibits enhanced photoconduction properties under irradiation of light when current is measured several times under a constant bias voltage by putting light on and off with successive repetitions. The detailed discussion of photo response properties of both the complexes in the solid state device have been reported along with their applicability in photosensitive devices. Finally, DFT calculations have been carried out to rationalize the experimental differences in band gap and conductivity observed for compounds 1 and 2.

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INTRODUCTION Over the past decade, oxime and their metal-coordinated complexes have engrossed attentions of scientific community for their rich physicochemical properties, reactivity patterns, and potential applications in many important chemical processes.1 Interestingly in these oxime derived ligands, the free −OH group usually engaged in self-assembled polymetallic grid type metallo-organic framework which may play some crucial role in controlling their optical, electronic, and magnetorelated properties.2−5 Considerable interest has also been paid in the syntheses and study of molecular complexes which are able to exhibit semiconducting behavior.6 Semiconducting organic solid materials are frequently grouped into the © 2018 American Chemical Society

categories of molecular crystals, charge transfer complexes and polymers. Inorganic semiconductors stand on the threshold of a bright and extraordinarily exciting prospect. An organic semiconductor can be synthesized with properties comparable to those exhibited by inorganic semiconductor materials such as development for transistors and the wide range of now-existing derivative devices and components of the electronics industry. It is fundamentally important to choose a metal ion which should percolate its importance in the metallo-organic Received: November 24, 2017 Revised: February 20, 2018 Published: March 26, 2018 8724

DOI: 10.1021/acs.jpcc.7b11579 J. Phys. Chem. C 2018, 122, 8724−8734

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The Journal of Physical Chemistry C

a solution of the Schiff base ligand, PymoxH, in the same solvent (0.331 g, 1.5 mmol) with constant stirring. The stirring was continued for another 2 h and filtered. The yellow solution was kept at room temperature which produced yellow crystals suitable for X-ray diffraction after a few days. The crystals were isolated by filtration and air-dried. Yield: 75.0%. Anal. Calcd for C11H19Cl2N5O2Zn (1): C, 33.88; H, 4.88; N, 17.97. Found: C, 33.84; H, 4.85; N, 17.99. IR (KBr pellet/cm−1): 3320 (m), 1582 (s), 1511 (s), 1423 (s), 1330 (s), 1013 (m), 903 (m). Synthesis of [Zn6(Pymox)6(μ2-O)3] (2). Complex 2 was prepared similarly using Zn(ClO4)2·6H2O instead of ZnCl2· 6H2O. Yield: 67%. Anal. Calcd for C60H86N30O10Zn6(2): C, 40.45; H, 4.83; N, 23.60%; Found: C, 40.41; H, 4.81; N, 23.63%. IR (KBr pellet/cm−1): 3317 (m), 1576 (s), 1512 (s), 1421 (s), 1335 (s), 1012 (m), 907 (m). (Caution! Perchlorate salts of metal complexes with organic ligands are potentially explosive. Only small amounts of materials should be prepared and to be handled with extreme care. The complexes described in this report have, so far, been found to be safe when used in small quantities). X-ray Crystallographic Analysis. Single crystal X-ray diffraction intensity data of 1 and 2 were collected at 150(2) K using a Bruker SMART APEX-II CCD diffractometer equipped with graphite monochromated MoKα radiation (λ = 0.71073 Å) source in the φ and ω scan mode. Cell parameters refinement and data reduction were carried out using the Bruker SMART and Bruker SAINT softwares9 for both the complexes. An empirical absorption correction SADABS10 was applied. The structures of the complexes were solved by conventional direct methods and refined by the full-matrix least-squares methods using F2 data. SHELXS-97 and SHELXL97 programs11 were used for the structure solution and refinement, respectively. The non-hydrogen atoms were refined anisotropically until convergence was attained. In 1, a methanol molecule was present as the solvent molecule which was removed using SQUEEZE.12 Thus, one carbon atom, four hydrogen atoms, and one oxygen atom (of the methanol molecule) were added to the chemical formula to adjust the density, molecular mass, and F000 value. In 2, a water molecule was present as the solvent molecule which was removed using SQUEEZE. Thus, two hydrogen atoms and one oxygen atom (of water molecule) were added to the chemical formula similarly. A summary of crystal data and relevant refinement parameters are shown in Table S1. Theoretical Methods. In this work, the experimental structures of the complex 1 and 2 were optimized with the density functional based tight binding theory using the DFTB+ program13 code of Accelrys, Inc. The internal coordinates and the unit cell of the crystals were optimized, finding small variations with respect to the experimental data. We have been used the 3ob Slater-Kosterbasis set library,14−17 that was specifically designed for third order DFTB and provides good quality geometries. The self-consistent charge (SCC) parameters were set to 1.0 × 10−8 using the Broyden method for charge mixing.18 The k-mesh points over the Brillouin zone were generated with fine quality using the Monkhorst−Packscheme.19 The long-range dispersion correction has been included in the calculations with the UFF-based Lennard-Jones correction. The energy positions of all the conduction bands were shifted upward by 0.47 for 1 and 0.87 eV for 2 through the scissors operator. Band structures were calculated along the k-vector of the first Brillouin zone of the crystal and total and

complexes (MOC’s) to exhibit interesting semiconducting properties. The ability of oxime containing ligands to stabilize reduced and oxidized forms of metal ions has importance in technological applications. In this consequences, initially salicyland pyridyl-based oximes are used in exploring manganese cluster chemistry which led to the synthesis of an unprecedented number of polynuclear manganese clusters with nuclearities ranging from 3 to 12, but in recent days it has been extended to other 3d, 4f, and 3d−4f clusters as well.7 Incidentally, we have seen that Cd(II) (having d10 electronic systems) clusters shows interesting photochemical and photophysical properties using ethoxy-phenol type ligand.5 Literature survey reveal that the unexceptional stability and unique electronic properties of such complexes can be attributed to their crystal geometry.8 Inspired by these works we tailored our strategy and tried to synthesize pyrimidine derived oxime type ligand and got success in achieving anion mediated diverse structural architecture with Zn(II) salts. We have concentrated ourselves to study the semiconducting behavior of these Zn(II) complexes and to the best of our knowledge this is the first report that shows Zn(II) complexes, at least these two that we synthesized, show interesting photophysical properties. We report here the syntheses; X-ray structures of two newly synthesized Zn(II) complexes (among which one is mononuclear and the other is a hexanuclear cluster) derived from a new pyrimidine-based oxime ligand, PymoxH (Scheme S1) along with its electrical and photophysical properties.

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EXPERIMENTAL SECTION Materials and Measurements. All reactions were carried out in aerobic condition. All chemicals used were of reagent grade and used as received. Diacetylmonoxim (97%) was purchased from Aldrich Chemical Company, USA, and was used without further purification. IR spectra were recorded on a PerkinElmer RXI FT-IR spectrophotometer with the sample prepared as a KBr pellet, in the range 4000−400 cm−1. Elemental analyses (C, H and N) were performed on a PerkinElmer 240C elemental analyzer. The field emission scanning electron microscopy (FESEM) images were taken by a FEI make Inspect F-50 scanning electron microscope. The optical characterization was performed with the help of Shimadzu 2401 PC UV−vis spectrophotometer. The frequency dependent capacitance was recorded by the computer controlled Agilent make precision 4294A LCR meter. The electrical characterization was performed with the help of Keithley 2400 Source meter, interfaced with PC. Synthesis. Synthesis of PymoxH (3-[(4,6-Dimethylpyrimidine-2-yl)-hydrazono]butan-2-one Oxime). The PymoxH ligand was synthesized by refluxing a methanolic solution (30 mL) of diacetylmonoxim (1.01 g, 10 mmol) and 2-hydrazino4,6-dimethyl pyrimidine (1.38 g, 10 mmol) for 1 h in the presence of two drops of acetic acid. A yellow microcrystalline solid compound separated out. The solid was filtered off, washed several times with cold methanol and dried in vacuum over fused CaCl2. Yield: 54.0%. MP (°C): 202. Anal. Calcd for C10H15N5O: C, 54.28; H, 6.83; N, 31.65. Found: C, 54.24; H, 6.81; N, 31.72. IR (KBr pellet/cm−1): 3336 (m), 1597 (s), 1529 (s), 1432 (s), 1344 (s), 1021 (m), 916 (m). MS (m/z): 221.26 (M+, 100%). 1H NMR (300 MHz, DMSO-d6, δ): 11.36 (s, 1H, −OH), 9.71 (s, 1H, Pym ring), 6.61 (s, 1H, -NH), 2.26 (s, 6H, −CH3), 2.18 (s, 3H, −CH3), 1.87 (s, 3H, −CH3). Synthesis of [Zn(Pymox)Cl2] (1). To a methanol solution (30 mL) of ZnCl2·6H2O (0.366 g, 1.5 mmol) was slowly added 8725


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The Journal of Physical Chemistry C partial density of states (TDOS and PDOS, respectively) were plotted with respect to the Fermi level.

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RESULTS AND DISCUSSION Crystal Structure Description of [Zn(Pymox)Cl2] (1). The perspective view of molecular structure of complex 1 with atom numbering scheme is displayed in Figure 1. Crystallo-

Figure 2. Formation of Supramolecular dimer by neutral monomeric [Zn(Pymox)Cl2] units. This assembly is viewed along a axis. Weak C− H···π interactions involving H4C atom is also shown. Color code: Zn, magenta; O, red; N, blue; C, light purple; H, aquamarine; Cl, yellow.

sum of van der Waals radii of H and C is 2.90 Å). These supramolecular pairs are also interconnected by C−H···π interactions and hydrogen bonds. The same pyrimidine ring is also engaged here, giving rise a C−H···π···H−C type of association (Figure S1). The distance between H10C and the centroid of the pyrimidine ring is 3.107(2) Å [C10···Cg(3) = 3.404(3) where Cg(3) is the centroid of the ring defined by the atoms N(4)/C(5)/N(5)/C(9)/C(8)/C(7)]. The shortest separation distance reflecting this interaction is H10C···C9 = 2.76 Å, which is below the sum of the corresponding van der Waals radii (the sum of van der Waals radii of H and C is 2.90 Å). This pairing of supramolecular dimers leads to the formation of a 2D arrangement along bc plane. Crystal Structure Description of Complex of [Zn 6(Pymox) 6 (μ2 -O) 3 ].H 2O (2). The molecule 2 is a hexametallic neutral monomer. Crystallographic and bond metrical parameters are listed in Tables S1 and S2 respectively. It crystallizes in the orthorhombic space group Pbcn with half of one [Zn6(Pymox)6(μ2-O)3] cluster in the asymmetric unit. The molecular structure of 2 is depicted in Figure 3 and its core structure in Figure 4. The asymmetric unit of 2 comprises three Zn2+ ions, three tetradentate ligand Pymox and one μ2-oxygen. The Zn1 atom is linked by pyrimidine N atom N5, imine N atom N2, oxime N atom N1, another ligand oxime O atom O1 and μ2-O atom O2. It has distorted square pyramidal geometry, as is evident from the τ value of Zn1, which is found to be 0.43. The Zn1 atom is displaced out the best plane formed by N5, N2, N1, and O1 by 0.128 Å. On the other hand, the Zn2 atom is coordinated to pyrimidine N atom N10, imine N atom N7, ligand N atom N6, and another ligand O atom O4. It has distorted square pyramidal geometry, as is evident from the τ value of Zn1 which is found to be 0.45. Among this hexametallic Zn(II) complex, there are three μ2-oxobridging and the other three are bridged by N−O in an alternate manner. In 2, the coordination number of all six Zn(II) atoms are five and to the best of our knowledge and based on CSD search this hexanuclear Zn(II) polynuclear metal cluster appears to be the first report of complexing with pyrimidine derived oxime based ligand. Morphology Analysis. The morphological study was executed by the images of thin film of the synthesized complexes 1 and 2 taken from a FEI make Inspect F50 field emission scanning electron microscope (FESEM). The

Figure 1. Molecular structure of 1 with partially labeled atoms. Color code: Zn, magenta; O, red; N, blue; C, light purple; H, aquamarine; Cl, yellow.

graphic and bond metrical parameters are listed in Tables S1 and S2 respectively. Complex 1 crystallizes in space group P21/ c and the unit cell of 1 comprises of four molecules. The geometry at the metal center is distorted square pyramidal as is evident from the τ value20 (Adison parameter, for ideal trigonal bipyramidal geometry it is 1 and for ideal square pyramidal it is equal to 0) of Zn1 which is found to be 0.10. Here the neutral ligand acts as a tridentate one and chelates to one Zn(II) through its pyrimidine nitrogen N5, imine nitrogen N4, and oximate nitrogen atom N1, whereas the residual coordination numbers are satisfied by two chlorine atoms (Cl1 and Cl2). In 1, among the four equatorial bonds Zn1−Cl1 (2.455(4) Å) bond is the longest whereas Zn1−N2 (2.315(8) Å) bond is shortest. The more or less equivalence of Zn−Cl bond lengths in equatorial and axial positions is attributed to the large size of the chlorine. The mean square plane was constituted by N1N2N5Cl1 and the metal Zn(II) is shifted 0.795 Å toward the axially coordinated chlorine (Cl2) atom. Both cis and trans angles are shifted from the ideality due to steric crowding. The complex is interesting because to the best of our knowledge it is the first report of pentacoordinated Zn(II) complex derived from pyrimidine based oxime type ligand. Being d10electronic configuration Zn(II) generally prefers tetrahedral geometry to release steric strain as this d10 system does not possesses any crystal field stabilization. Centrosymmetrically related [Zn(Pymox)Cl2] units are pair wise connected through self-complementary N3−H3···Cl1 hydrogen bonds to form a supramolecular dimer (Figure 2). Weak C−H···π interactions involving H4C atom of the diacetylmonoxim moiety of the Pymox ligand and pyrimidine ring also contributed to the formation of the supramolecular dimer. The distance between H4C and the centroid of the pyrimidine ring is 3.131(2) Å [C4···Cg(3) = 3.764(3) where Cg(3) is the centroid of the ring defined by the atoms N(4)/ C(5)/N(5)/C(9)/C(8)/C(7)]. The shortest separation distance reflecting this interaction is H4C···C9 = 2.69 Å, which is below the sum of the corresponding van der Waals radii (the 8726


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Figure 6 (inset). The absorption spectrum of complex 1 depicts strong energy absorption in the UV region at ∼360 nm whereas complex 2 exhibits energy absorption in the visible region near at ∼525 nm. The optical band gap of the as synthesized complexes was determined by Tauc’s equation:23 αhν = A(hν − Eg )n

(1)

where “α” is the absorption coefficient, “Eg” is the band gap, “h” is Planck’s constant, “ν” is the frequency of light, and the exponent “n” is the electron transition processes dependent constant. “A” is a constant which is considered as 1 for ideal case. To estimate the direct optical band gap of the synthesized material, the value of the exponent “n” in the above equation was taken as 1/2.22 The plot of (αhν)2 vs. hν of the synthesized complexes are demonstrated in Figure 6. By extrapolation of the linear region of the plot (αhν)2vs. hν to α = 0 absorption, the values of direct optical band gap (Eg) of 1 and 2 were evaluated as 3.43 and 2.36 eV respectively. The optical band gaps in 1 and 2 thus obtained are well within the limit of band gaps of materials to be taken as semiconductors. Moreover, the presence of strong absorbance in the UV and visible region and the hierarchical structure prophesies some conducting nature of the synthesized complexes. These results motivated us to further check the electrical transport properties of the synthesized complexes in detail and we carried out the dielectric study of our complexes. Dielectric Measurement. Impedance spectroscopy has been widely used to study the charge transport behavior of crystalline materials. This analysis provides a correlation between the electrical and structural properties of the material. For a detailed investigation of the transport properties of as prepared complexes 1 and 2, the impedance measurement was carried out. For electrical measurements, the polished pellets of both the synthesized complexes (1 and 2) were taken and high purity ultrafine silver paste was used as electrode on the opposite surfaces of the pellets. Capacitance (C), impedance (Z) and phase angle (θ) of the samples were measured as a function of frequency (40 Hz−11 MHz) using a computercontrolled LCR-meter Agilent 4294A. The complex plane impedance plots i.e. the Nyquist plots for both 1 and 2 are shown in Figure 7. The impedance measurement of the complexes reveal a prominent and a tiny arc of semicircles contributed by semiconducting grains in the high frequency region and insulating grain boundaries in the low frequency region. This semicircle at the high frequency region is related to the electrode resistance and also reflects the charge transfer resistance at the electrode/composite interface. From Figure 7, it is clear that the radii of the complex 2 is decreased when compared with complex 1, indicating that complex 2 facilitates better interfacial charge transfer. Figure 8 shows the Bode plots of the two complexes 1 and 2. Comparing Bode phase plots, we see that the characteristic peak position of 2 shifts to a lower frequency rather than the characteristic peak position of 1. This characteristic frequency is related to the inverse of the recombination lifetime or electron lifetime. From the peak of the characteristic graph the electron lifetime can be determined using eqn 2.24

Figure 3. Molecular structure of 2. Color code: Zn, magenta; O, red; N, blue; C, light purple; H, aquamarine (the hydrogen atoms are omitted for clarity).

Figure 4. Molecular core structure of 2.

characteristic FESEM micrographs of thin film of synthesized complexes 1 and 2 are demonstrated in Figure 5 that clearly reveals both the synthesized complexes have their own but different characteristic morphology. Complex 2 has definite rod shape morphology whereas complex 1 has arbitrary shape with no definite structure. The conductivity of the material depends on their morphology.21 In other words the aspect ratio of a material has an influence on its electrical conductivity.22 As the synthesized complexes have different aspect ratio, it can be speculated that they also possess different electrical conductivities. From this point of view, we initiated the study of their (both complexes) electrical properties. UV−Vis Spectroscopy. The optical characterizations of both the complexes were performed with the help of Shimadzu 2401 PC UV−vis spectrophotometer in the range 200−900 nm. Here, the absorption spectrum was recorded for the deposited thin films of both the complexes 1 and 2 by preparing a well dispersed solution in DMF and presented in

τn = 8727

1 2πfmin

(2) DOI: 10.1021/acs.jpcc.7b11579 J. Phys. Chem. C 2018, 122, 8724−8734

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Figure 5. FESEM images of complexes (a) 1 and (b) 2 respectively.

Figure 6. Tuac’s plot and UV−vis absorption spectra (inset) for complexes 1 and 2.

Figure 8. Bode plot for complexes 1 and 2.

Figure 7. Nyquest plot for complexes 1 and 2.

measurements provide information about the interior of the semiconductor which is a region of relatively low conductivity even when the conduction process is electrode-limited.25 Frequency-dependent ac conductivity (σ(ω)) having a form of σ(ω) ∝ωs, where ω is the angular frequency and the parameter s ≤ 1.This frequency dependence of ac conductivity may be attributed to free and bound carriers. The conductivity

The electron lifetime of the complexes 1 and 2 thus determined are listed in Table 1. From these studies it can be concluded that the characteristic frequency is related to the electron lifetime. The longer electron lifetime corresponds to a smaller frequency. Figure 9 shows the frequency ( f) dependence of the AC conductivity of the complexes 1 and 2. The ac conductivity 8728


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The Journal of Physical Chemistry C Table 1. Dielectric Parameters of Synthesized Complexes samples complex 1 complex 2

charge transfer resistance (Ω) 1588 1136

dc conductivity (S·m−1)

electron lifetime (s)

−5

−8

9.08 × 10 1.25 × 10−4

5.51 × 10 4.38 × 10−8

relative permittivity (F·m−1) 7.726 × 102 1.007 × 103

saturation level the relative permittivity of the complexes was calculated employing following equation:23

εr =

decreases with the increase in frequency when it depends on free carriers.26 The frequency dependence of the conductivity obeyed the empirical law of frequency dependence given by the power law of the form in (3): (3)

Here σ(ω) is the total conductivity, σdc is the dc conductivity and σac is the ac conductivity. The frequency-dependent part of conductivity σAC has been observed to obey the relation σac = Aωs

(5)

Here, ε0 is the permittivity of free space, εr is the relative permittivity of the synthesized material, C is the capacitance (at saturation), and d and A are the thickness and effective area of the pellet. Using the above formula the relative dielectric constant (εr) of the materials were estimated and shown in Table 1. We can thus infer that the complex 2 is a better contender in the view of electrical conductivity than the complex 1. Inspired by these results, we attempted to design Schottky device of both the complexes to further check the applicability inactive electronic devices. We, therefore, calculated the Schottky parameters and studied the electrical behavior of devices. Fabrication of Schottky Device. The Schottky devices of the complexes 1 and 2 were fabricated in sandwich like ITO/1 or 2/Al configuration. Indium tin oxide (ITO) coated glass substrates were cleaned with soap solution, acetone, ethanol and distilled water sequentially in an ultrasonic bath. The material was dispersed in Dimethyl Sulfoxide (DMSO) medium and ultrasonicated well to form a stable dispersion. A thin film of the material was then prepared on the ITO coated glass by spin coating technique at 1200 rpm for 2 min and subsequently dried. The thickness of the developed thin film was measured as 1 μm with a help of surface profiler. Aluminum was chosen as the rectifier metal contact and deposited on the films by thermal evaporation technique to construct metal−semiconductor junction. The effective diode area was maintained at 7.065 × 10−6 m2 by shadow mask. For electrical characterization of the devices, the current− voltage (I−V) characteristic was measured both under dark and under illumination condition and recorded with the help of a Keithley 2400 source meter by two-probe technique. All the measurements were performed at room temperature and under ambient conditions.

Figure 9. Dependence of ac conductivity on frequency

σ(ω) = σdc + σac

1 C×d × ε0 A

(4)

where A is a constant and s is a number which depends upon frequencies at room temperature. The relative dielectric constant was measured on pellet. Our synthesized complexes (1 and 2) was pelletized into a disc of diameter 7.32 mm and thickness 1.8 mm. Figure 10 illustrates the curves showing the variation of the capacitance (C) as a function of the frequency ( f) at constant bias potential. The room temperature capacitance of both the complexes (1 and 2) is shown to be frequency dependent at relatively low frequencies. The capacitance decreases with increasing of frequency and becomes saturated at higher frequency. From the

Figure 10. Capacitance versus frequency graph for determination of dielectric constant 8729


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Figure 11. Current−voltage characteristics curve for ITO/complex 1/Al and ITO/complex 2/Al structured thin film devices.

Scheme 1. Schematic Illustrations of Analogous Routes for Charge and Energy Transport in Our Samples As Represented for 2a

The possible mechanism for charge transport: through-bond conduction via metal nodes and linkers. Photoexcitation followed by fluorescence resonance energy transfer or exciton hopping between chromophoric framework units. a

Electrical Properties. Metal ions with closed shell electron configurations such as zinc (Zn) are preferred to the hybrid organic−inorganic structure (MOFs, mettallo-organic compounds, intercatenated coordination polymers (ICPs)) for photovoltaic application because it (Zn) minimizes the opportunities for exciton quenching by fast decay to the ground state or to the LUMO, rather than undergoing charge injection. The required and essential traits of these kinds of metallo-organic complexes being relevant in the field of photovoltaic application is light absorption, which should be ranged in the visible and near IR region (∼400−900 nm). The presence of strong absorption in the visible and near visible wavelength region of our synthesized complexes prophesies some impact of incident radiation on the charge transport phenomena. Moreover, here in this report, the value of optical band gap for complex 2 demonstrates superiority than the complex 1 which may interpret some better possibility of formation of metal−semiconductor barrier with that complex 2. Hence, for better understanding of the charge transport phenomenon, electrical characterization was accomplished by the thin films devices of the well dispersed solution of

complexes deposited on top of ITO coated glass substrates. To analyze the electrical properties, we have measured the current at corresponding applied bias voltage sequentially within the limit ±2 V. Figure 11 represents the current−voltage characteristics curve for complexes 1 and 2 based thin film devices. The conductivity of the thin films was measured under dark and AM 1.5G radiation from the current−voltage characteristic curve. Evidently, the current−voltage characteristic curve in Figure 11 shows the devices based on our synthetic materials 1 and 2 exhibits rectifying behavior which signifies its Schottky behavior. Rectification ratio of the complex 1 was obtained as 10.58 at dark and 12.72 under photo condition whereas for complex 2 it was measured as 22.42 and 38.61 under dark and photo conditions respectively. The rectification ratio for complex 1 increases ∼20% after light soaking whereas for complex 2 it increases up to 72% in same experimental condition. This result depicts that complex 2 has better sensitivity upon light irradiance. The photosensitivity at +2 V of the complexes 1 and 2 was also computed as 1.64 and 2.22 respectively. The electrical conductivity of the fabricated films 8730


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Table 2. From this study it is clear that the ligand itself is a bad conductor of electronic charges or it hardly belongs to

was also determined in dark condition and under illumination conditions. In addition, the characteristics curve under light shows larger current which illustrates the photo response of the devices. The hybrid organic−inorganic structure provides numerous opportunities for light harvesting and energy transport. Light can be absorbed by the linkers, metal cations, or guest molecules and transferred by exciton hopping or fluorescence resonance energy transfer (FRET). The synthetic versatility of this kind of material would allow both light absorption and band alignment to be tuned. The advantageous properties of these kinds of materials for charge transportation are their inbuilt porosity, which can be used to contain absorbers and/or charge carriers. Hybrid organic−inorganic structured materials are well suited to serve as the light harvesting component by virtue of the ability to build structures with multiple light absorbers locked into a stable crystalline structure. Moreover, the porosity provides an additional design element by enabling another light absorber to be used. Until now, three types of charge carrier transport mechanisms have been identified in hybrid organic−inorganic structures: through-bond conduction, through-space conduction (charge delocalization due to the close approach of adjacent aromatic linkers), and guest molecules.27 In this study, the mechanism for light induced charge carrier transport would be through-bond conduction. In through-bond conduction, charge moves through continuous chains of covalent and coordination bonds in the material. Typically, this mechanism involves the so-called “hopping transport”. The possible conjecture of a light-induced charge carrier transportation pathway maneuver has been represented in Scheme 1. In the present study, the increase in conductivity of the materials after illumination of incident light reveals that the incorporation of metal ion within the organic framework makes it suitable for photovoltaic application. To ensure the influence of the metal ion conjugation on conductivity, we further carried out the same experiment with the ligand only (Figure 12)

Table 2. Electric parameters of synthesized complexes and only ligand based devices sample

condition

rectification ratio

complex 1

dark light dark light dark light

10.58 12.72 22.42 38.61 2.16 2.23

complex 2 ligand

photosensitivity 1.64 2.22 1.18

conductivity (S· m−1) 2.13 3.19 3.99 7.13 5.71 6.73

× × × × × ×

10−3 10−3 10−3 10−3 10−8 10−8

semiconductor family. Addition of Zn metal ion makes the ligand better conductor of electronic charge from its initial (free ligand) stages. Here we also compared experimentally obtained conductivity data of our synthesized complexes (1 and 2) with that of previously reported Zn(II) materials and displayed in Table S3 that shows better conductivity values of our synthesized complexes under dark condition in comparison to the previously reported Zn based complexes. Our synthesized materials 1 and 2 may thus be potentially promising candidates for the applications in optoelectronic devices. The transient photocurrent response is generated by the transfer of excited photoelectrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). This transient photocurrent response mainly reveals the conductance value and the number of free carrier produce at the time of light irradiation in the semiconductor.28 The transient photocurrent responses of the complexes 1 and 2 are investigated for several on−off cycles of irradiations. As shown in Figure 13, the photocurrent values

Figure 12. Current−voltage characteristics curve for ITO/ligand/Al structured thin film devices.

Figure 13. Transient photocurrent response for the ITO/complex 1/ Al and ITO/complex 2/Al devices.

whence it can be seen that the oxime ligand shows poor conductivity (at dark condition 5.71 × 10−8 S·m−1) as it has no inherent conductivity as well as it does not show any response under illumination condition (under illumination condition 6.73 × 10−8 S·m−1). The measured photosensitivity (∼1.18) of the ligand-based devices also depicts that the ligand shows no such response under illumination condition. All the measured device parameters for ligand-based device have been listed in

rapidly decreases as soon as the light turns off. It also comes back to a certain value when the lights turn on. This characteristic behavior depicts that most of the photogenerated electrons migrate to the ITO-coated substrates to yield photocurrent under visible light irradiation. In the present study, the transient photocurrent response of the complex 2 is found to be higher than that of complex 1. This result reflects a lower recombination rate and a more efficient separation of photogenerated electron−hole pairs for the complex 2 based 8731


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Figure 14. Electronic band structure of the ground state of compounds 1 (a) and 2 (b) crystals. (a) Points of high symmetry in the first Brillouin zone are labeled as follows: Z = (0, 0, 0.5); G = (0, 0, 0); Y = (0, 0.5, 0); A = (−0.5, 0.5, 0); B = (−0.5, 0, 0); D = (−0.5, 0, 0.5); E = (−0.5, 0.5, 0.5) and C = (0, 0.5, 0.5). The energy positions of all the conduction bands are shifted upward by 0.47 eV for 1 and 0.87 eV for 2 through a scissors operator. (b) Points of high symmetry in the first Brillouin zone are labeled as follows: G = (0, 0, 0); Z = (0, 0, 0.5); T = (−0.5, 0, 0.5); Y = (−0.5, 0, 0); S = (−0.5, 0.5, 0); X = (0, 0.5, 0); U = (0, 0.5, 0.5); R = (−0.5, 0.5, 0.5) .

Figure 15. (a) Calculated partial density of states of Zn atom (Zn), Cl atoms (Cl), functional groups atoms of the ligand (FG) and aromatic part (Ar) of 1. (b) Calculated partial density of states of Zn atom (Zn), μ-O atoms (O), functional groups atoms (FG) and aromatic part (Ar) of 2.

device. All the measured electrical parameters for both the synthesized complexes are summarized below in Table 2. Theoretical Study. In the present theoretical study, we delve into the intrinsic properties of each crystalline semiconductor in order to explain differences between the two systems by analyzing the partial density of the states and bands of each compound (Figures S2 and S3). Moreover, a study of the differences in the electronic density of these materials is included in the ESI (Figure S4). Crystal structure analysis has been done by standard band theory and partial density of states calculation which indicates that complexes 1 and 2 are direct semiconductor (Figure 14). The theoretical band gaps from DFT calculations are in good accordance with the experimental values. Complex 2 has the lowest band gap of the two synthesized complexes, thus explaining its better conductance. The experimental bandgaps indicate that the materials belong to the semiconductor family which is also confirmed from the density of states (DOS) calculation as shown in Figure 15. From the examination of the partial DOS calculations some interesting issues arise. First, for complex 1 the valence bands are mainly dominated by Cl 3p states followed in smaller proportion by the 2p states of the imine functional group and the aromatic part of the organic ligand (Figure 15a). On the other hand the p-component of the

N1, C3 and N2 functional group atoms are the main component of the conduction bands in complex 1 (Figure 15a and S2). Second, the comparison of these results with the partial DOS of complex 2, reveals that the fundamental difference (apart from band gap reduction) resides in the atoms that participate in the valence bands. In this case, it is the pcomponent of the μ2-oxygen (O) bridges that presents the highest population in the valence zone (Figure 15). Moreover, the p-components of the N1, C3 and N2 atoms belonging to the organic ligand (FG) are the main contributors to the conduction bands in 2 similar to that observed in compound 1 (Figure 15b and S3).

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CONCLUSIONS In conclusion, two new Zn(II) based metal complexes (one is mononuclear Zn(II) complex 1 and the other (2) is a hexanuclear Zn(II)) have been prepared from newly synthesized pyrimidine based ligand. This structural variation depends on anion (chloride or perchlorate) diversity. Being d10 electronic configuration, Zn(II) generally prefers tetrahedral geometry to release steric strain as this metal ion does not possesses any crystal field stabilization. To the best of our knowledge our Zn(II) complexes (five-coordinated distorted square pyramidal geometry around metal center in 1 and 2) are 8732



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the first examples of a pyrimidine based ligand which show photophysical properties. In the present study 2 shows more ordered morphology with better optical band gap than 1. Complex 2 also exhibits better charge transport behavior that may be inferred from impedance spectroscopy analysis. Photophysical properties of as synthesized complexes were measured in ITO/1 or 2/Al configured Schottky devices. We obtained a detailed comparison of rectification ratio and photosensitivity, between complexes 1 and 2 under dark and light irradiance condition, which revealed that upon illumination the change of the above-mentioned characteristic parameters are noticeable. These result exhibits the photocurrent can be switched many times repeatedly without deterioration of the on/off ratio and might find applications in photoswitching devices. This unique property is exceptional and represents an important step toward the use of synthesized samples in optoelectronics and photovoltaics.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b11579. Scheme S1, representing the general structural formula of ligand PymoxH, Tables S1 and S2, showing crystallographic data and selected bond lengths and bond-angles parameters for complexes 1 and 2 respectively, Table S3, comparing experimentally found electrical conductivity data of our synthesized materials with reported Zn(II) complexes, Figure S1 illustrating supramolecular dimers of 1., Figures S2 and S3 showing calculated partial density of states of all atoms of 1 and 2, and Figure S4 representing electronic density differences of 1 and 2 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(S.K.) E-mail: [email protected]. *(P.P.R.) E-mail: [email protected]. *(A.F.) E-mail: [email protected]. ORCID

Saugata Konar: 0000-0001-5404-9107 Partha Pratim Ray: 0000-0003-4616-2577 Joaquín Ortega-Castro: 0000-0001-8131-0315 Antonio Frontera: 0000-0001-7840-2139 Notes

The authors declare no competing financial interest. Crystallographic data (excluding structure factors) for the two structures reported in this article have been deposited with the Cambridge Crystallographic Data Centre as CCDC 913125 and 913124 for complexes 1 and 2 respectively. Copies of the data can be obtainedo free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. Fax: + 44−1223− 336033. E-mail: [email protected].

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REFERENCES

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S Supporting Information *

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ACKNOWLEDGMENTS

Financial support from the UGC-UPE (II) program of Jadavpur University is thankfully acknowledged. A.F. thanks the MINECO of Spain (Projects CTQ2014-57393-C2-1-P and CTQ2017-85821-R FEDER funds). 8733


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Two Zn(II) Based Metal Complexes of New Pyrimidine Derived Ligand

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