Azido bridged binuclear copper(ii) Schiff base compound: Synthesis, structure and electrical properties

Article abstract of DOI:10.1039/c9nj02672j

An azido bridged dinuclear complex [Cu(L^-)(μ_1,1N₃)]₂ (1) was synthesized by a 1?:?1 condensation of N-cyclohexyl-1,3-propanediamine and 5-bromosalicylaldehyde (HL). The complex was subsequently characterized based on elemental analyses, IR, single-crystal X-ray diffraction, a Hirshfeld study, FESEM, ESI-MS, powder XRD and also DFT studies successfully. The X-ray crystal structure of complex 1 revealed that the Cu(ii) ion exhibited a definite five-coordinate square pyramidal coordination arrangement with a centro-symmetric μ 1,1-azido bridging in the end-on mode and formed a dimeric structure. The optical direct band gap of complex 1 was estimated to be 2.84 eV. The current-voltage characteristics of the Al/1/ITO structure were analyzed considering the thermionic emission theory of the Schottky barrier diode. The transient behaviour of the test structure was found to be interesting as it showed explicit frequency-selective phenomena. In view of an application-specific perspective for the material, compound 1 can be exploited as an electronic material for the development of electronic filters.

Full text of DOI:10.1039/c9nj02672j

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Azido bridged binuclear copper (II) Schiff base compound:
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synthesis, structure and electrical properties
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Mrinmoy Ghosha, Sandip Sahaa*, Abhijit Banerjee b*, Dieter Schollmeyerc, Ananda
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Sarkard, and Saikat Banerjeee
aDepartment of Chemistry, Acharya Prafulla Chandra College, New Barrackpore, Kolkata-700131, India
bDepartment of Electronic Science, Acharya Prafulla Chandra College, New Barrackpore, Kolkata-
700131, India
cInstitut fur Organische Chemie, Universit at Mainz, Duesbergweg 10-14 55099 Mainz, Germany
dDepartment of Physics, Acharya Prafulla Chandra College, New Barrackpore, Kolkata-700131, India,
eAshuti Netaji High School (H.S.), Village+P.O.- Ashuti, P.S.- Maheshtala, South 24 Parganas, PIN-
700141,India
Abstract
Azido bridged dinuclear Complex [Cu(L- )(1-1N3)]2 (1) has been synthesized by 1:1
condensation of N-cyclohexyl-1, 3 propanediamine and 5-bromosalicylaldehyde(HL) and
subsequently characterized based on elemental analyses, IR, single-crystal X-ray diffraction,
Hirshfeld study, FESEM, ESI-MS, powder XRD and also DFT studies successfully. The X-ray
crystal structure of complex 1 reveals that, the Cu (II) ion exhibits a definite five-coordinate
square pyramidal coordination arrangement with a centro-symmetric µ 1, 1-azido bridging in the
end-on mode and forms a dimeric structure. The optical direct band gap of the complex 1 has
been estimated 2.84 eV. The current–voltage characteristics of Al/1/ITO structure have been
analyzed considering the thermionic emission theory of Schottky barrier diode. The transient

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behaviour of the test structure is found to be interesting by showing explicit frequency selective
phenomena. In view of an application specific material’s perspective, compound 1 can be
exploited as an electronic material for the development of electronic filters.
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Keywords: Coordination Polymer; Schottky diode; Electronic filter.
____________________________________________________________________________
*Corresponding authors. Fax.+91-33-2537-8797; e-mail: sandipsaha2000@yahoo.com and
abhijit@apccollege.ac.in
1. Introduction
In the recent years, synthetic inorganic materials have achieved considerable interest in the
production of a variety of metal-organic frameworks (MOFs)1-4 and coordination polymers (CPs)
5-9, particularly when the materials have attractive electronic properties. In CPs metal ions and
ligands are the representatives of the nodes and spacers respectively. Instead of free metal ions,
occasionally the metal-ligand complexes are considered as nodes. A variety of ligands are
commonly obtainable having the N and O as donor atoms, which can be used for the synthesis of
metal organic frameworks (MOFs)10-11 and coordination polymers (CPs)12-15. Nevertheless,
binuclear complexes containing two metal ions have attracted all interest in electrical
conductivity, catalytic applications and magnetism16-18.The conductivity of these materials is
mainly correlated with their corresponding band gap, which remains well between the limiting
band gap values generally considered for semiconductor. As a consequence, the electrical
properties of such binuclear complexes have drawn significant interest compared to the other
material properties. Numerous investigations have already revealed that Zn2+ and Cd2+ (d10

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systems) CPs19-25 have attractive photochemical and photophysical properties, particularly when
the alkoxy-phenol type ligand are employed along with and the matter is corroborated by their
exceptional stability and unique electronic properties. These complexes have suitable band gaps
and good electrical conductivity, commonly crucial for electronic applications. However in
recent years, synthesis and characterization of heterometallic coordination polymer has also
gained much attention by confirming excellent electrical and optoelectronic properties. For
instance, Roy et.al. have reported a Ni(II)/Pb(II) heterometallic coordination polymer which has
been successfully applied in thin-film photosensitive Schottky device26. Moreover, several recent
works have already confirmed that a choice of Cu(II) CPs also have interesting electrical or
optoelectronic properties27-29. The matter has been substantiated mainly by investigating the
optical band gap and also by analyzing the dark and illuminated current-voltage characteristics of
the Schottky barrier diodes fabricated using these materials. Looking to this matter it might be
interesting to investigate on the electrical properties of Copper(II) salt based anion mediated
diverse arrangements. In view of that, considering the dimeric structure of compound 1, a
purposeful experimental procedure has been adopted to analyse the electrical properties of the
complex 1 in the present work. In the present context it may be mentioned here that, the
synthesis and characterization of such type of complexes having Schiff base ligands with
divalent metal ions have been the point of interest in recent years among different workers.
Among a wide variety of ligands, the salicylaldehyde derived Schiff bases have attracted special
attentions.30, 31 In this study, we have reported the syntheses, single crystal structure and
characterizations of a dimeric Cu(II) complex [Cu(L-)(1-1N3)]2 (where HL = 3-
(cyclohexylamino) propyliminomethyl-4-bromophenol), which exhibits electrical conductivity
like a semiconductor. The current-voltage characteristics of this material in Al/1/ITO structure is
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similar to that of a Schottky barrier diode and the material can be applied in the development of
electronic filters.
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2. Experimental
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2.1. Materials and instrumentation
All reagents and solvents for the synthesis and analysis were commercially available and
used as received without further purification. N-cyclohexyl -1,3- propanediamine and 5-
bromosalicylaldehyde were purchased from Sigma–Aldrich, Copper perchlorate monohydrate,
sodium azide, methanol were of AR grade and used as received. All other chemicals used were
of analytical grade.
IR spectra were recorded in KBr pellets within the range 5000-450 cm-1on a Perkin-
Elmer Spectrum Version 10.4.1. The Solid state UV-visible spectrum was obtained using JAZ-
spectrometer (Ocean Optics). Elemental analyses were carried outusing a Heraeus CHN-O-Rapid
elemental analyzer.
2.2. Synthesis of the complex 1
Caution! Since the metal azide and perchlorate are potentially explosive, only small amount of
the material should be prepared and it should be handled with care.
5-Bromosalicylaldehyde (0.201 g, 1 mmol) in 10 mL of methanolic solution was added to a 10
mL methanolic solution of N-Cyclohexyl 1, 3-propanediamine (0.156 g, 1 mmol) and the
resulting mixture was refluxed for 3 h (Scheme 1). Then a methanolic solution (10 mL) of
Cu(ClO4)2 .6H2O (0.370 g, 1 mmol) was added to it and again refluxed for 2 h.Then 2 mL
aqueous solution ofNaN3 (0.13 g, 2 mmol) was then added to it and stirred at about 2 h. The

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resulting solution was then filtered and kept for slow evaporization at room temperature. A blue
block shaped crystals of the complex 1 with 67 % yield was formed about 2 weeks late (Scheme
2).Analytical Calculation for complex 1.C32 H44 Br2 Cu2 N10O2: C, 43.3; H, 5.0; N, 15.8. and
Found: C, 43.5 ; H, 5.1 ; N, 16.0.%. FT-IR (KBr cm-1) νmax: 1621 (-CH=N), 2039 (νN3) and 772
(νBr) (Fig. S1).
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Synthetic route:
N
H
O
H
MeOH
Reflux 3 h
OH
N
NH2
N
HC
+
OH
Br
Br
HL
Scheme 1: Synthesis route of HL
N
H
Cu(ClO4)2 .6H2O
Reflux 2 h
N
[Cu (L-)(1-1N3)]2
HC
OH
NaN3 , Stirring 2 h
Br
HL
Scheme 2: Synthesis of Complex 1.

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2.3. Xray Crystal structure determination of complex 1.
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X ray crystal study was done and the crystal data for the complex is given in Table 1. Total
independent datas-19431 for complex 1 was collected on a STOE IPDS2T diffractometer
equipped with a graphite monochromator Mo-Kα radiation (k = 0.71073 Å). The structure of the
complex 1 was solved by SHELXT32.The structure refinement has been performed by full-matrix
least squares based on F2 with SHELXL-2014 (Sheldrick, 2014)33. All non-hydrogen atoms were
refined anisotropically. The C-bound H atoms were constrained to ideal geometry and were
included in the refinement in the riding model approximation. Data for molecular geometry,
intermolecular interactions and pictures were produced using, PLATON-200934and
ORTEP3.235programs.Crystallographic data for complex 1 has been deposited at the Cambridge
Crystallographic Data Center (CCDC). CCDC No. of complex 1 is 1879738.Copy of the data
can be obtained free of charge on application to the CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (Fax: +44 1223 336 033; E-mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.
cam.ac.uk/conts/retrieving.html).

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Table 1: Crystallographic data for [Cu(L- )(1-1N3)]2
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Crystal Data
Complex
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Empirical formula
C32 H44 Br2 Cu2 N10 O2
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Formula weight
887.67
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Crystal dimension (mm3)
0.040 x 0.060 x 0.130
Crystal system
Space group
a (Å)
Triclinic
P -1
8.8242(7)
b (Å)
10.4560(8)
c (Å)
11.0688(8)
α (º)
66.252(6)
β (º)
70.368(6)
γ (º)
84.614(6)
V (Å3)
879.44(13)
Z
1
h,k,l
-11 ≤ h ≤ 11 -13 ≤ k ≤ 13 -14 ≤ l ≤ 14
Temperature (K)
Dcalc (g cm-3)
µ (mm-1)
F(000)
193K
1.676
3.527
450
θ ( º)
2.340-28.279
Total data
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11105
0.0727
0.2271
0.982
Unique data
R
Rw
Goodness-of-fit on F2, S
Rint
0.0860

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2.4. Hirshfeld Surface analysis
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Hirshfeld surface analysis have been done using Crystal Explorer version 17.5.36 The normalized
contact distance (dnorm) based on di and de has been determined by the given equation where rvdW
is the van der Waals (vdW) radius of the appropriate atom internal or external to the surface.
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dnorm
=
dnorm becomes negative for shorter contacts than vdW separations and becomes positive for
contacts greater than vdW separations, and is displayed using a red–white–blue color scheme,
where red highlights shorter contacts, white is used for contacts around the vdW separation, and
blue is for longer contacts.37
2.5. DFT study of Complex 1:
The electronic geometry of the Cu complex of its singlet and triplet states were fully optimized
38-41
in the gas phase without imposing any symmetry restriction using the DFT method
using
B3LYP, Becke's three parameter hybrid density functional incorporated into the GAUSSIAN 09
software package.42 All elements, except Cu and Br, were assigned the 6-31G+(d) basis set.
LanL2DZ with an effective core potential for the Cu and Br was used. The calculated electronic
density plots for frontier molecular orbitals were prepared by using the GaussView 5.0
software.43 The cif file of 1 has been taken as the initial input of the calculation.
2.6. FESEM study
The FESEM micrograph of the powder sample has been procured by FEI INSPECT F 50.

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2.7. ESI MS study
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Mass spectrum of the complex 1 has been recorded in 1:1 acetonitrile and methanol solution at
room temperature using Agilent 6250 Accurate-Mass Quadrupole Time-OF-Flight (Q-TOF)
Instrument.
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2.8. Powder X-ray diffraction study
The X-ray diffraction of the sample 1 in powder form has been measured by a Bruker D8
Advance X-ray diffractometer using Cu-Kα (λ=1.5418Å) and the experimental procedure is
similar to that of the adopted process discussed in ref. 31b.
2.9. Fabrication and characterization of the device using complex 1.
A plain glass slide, an indium tin oxide (ITO) coated glass slide (25mm×15mm×2mm; surface
resistivity~10Ω/sq, transmittance (%)>85%) and an aluminium coated glass slide are
systematically cleaned by distilled water, trichloroethylene, acetone and methanol in an
ultrasonic cleaner. Subsequently, a known amount of complex1 is dissolved in 1:1 mixture of
methanol (CH3OH) and acetonitrile (CH3CN) solvent and the solution is stirred at room
temperature for 30 minutes to make it homogeneous.The solution is directly deposited on the
plain glass slide and the ITO coated surface by drop casting method. The aluminium coated slide
is then pressed over the solution coated ITO substrate in such a way that the layer-by-layer
formations turned into glass-ITO-1-aluminium-glass structure. Both the samples are kept
untouched at room temperature for 48 hours to let the moisture and solvent evaporate from the
coated surface.When a completely desiccated solid coat of 1 is realized, the sample coated glass

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slide is taken for optical measurements and the second sample of the material placed between the
ITO and aluminium layers is used for electrical measurements. The formation and homogeneity
of both the samples are monitored using a metallurgical microscope (Banbros: BMU 101) and
Fig. 1 shows the surface micrographs of such deposition. The thickness (~ 1.28 mm) of the
deposited material has been measured by a digital electronic micrometer (Toolsden DM0251).
The optical measurements are carried out on the complex 1 coated glass substrate within 200-900
nm wavelength using UV-VIS spectrometer (JAZ; Ocean Optics) at room temperature. The
second sample having the glass-ITO-1-aluminium-glass structure is subjected for electrical
measurements. Two separate aluminium contact leads are bonded through silver epoxy
[<0.025Ω/sq (0.001inch thick)], one over the exposed ITO layerand other on the exposed Al
coating. The current-voltage measurements are performed using Keithley 2401 source
measurement unit within -1.5 V to +1.5 V dc bias and the capacitance-voltage measurements are
carried out using a high precision LCR meter (SM6020). Additionally, the frequency response of
the test structure is investigated considering a set of ac signals generated using a function
generator (Tektronix AFG 3022C) and the output waveforms are investigated using a digital
storage oscilloscope (SMO40CA). All the measurements are performed at room temperature
(~300K).
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Fig. 1 Surface micrographs of ITO-1-aluminium structure.
3. Results and discussion
3.1. Synthesis and IR Spectroscopy
HL ligand was synthesized by the condensation of N-cyclohexyl -1, 3 propanediamine with
5-bromo salicylaldehyde 1:1 ratio in methanol. The produced ligand was not further purified,
instead it was used for preparation of complex 1.Then the ligand (HL) was refluxed with
methanolic solution of Copper perchlorate hexahydrate followed by stirred with sodium azide in
water to prepare the dinuclear compound. This tridentate N, N, O-donor Schiff base (Scheme 1)

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was coordinated with Copper(II) which has a general preference for trigonal bipyramidal
geometry. IR spectra of compounds the distinct bands observed at 1621 cm-1 are due to
azomethine(-CH=N) group of the Schiff bases present in the complex 1. The peak appears at 772
cm-1correspond to bromo group of 5-bromosalicylaldehyde. The asymmetric stretching vibration
mode of the azide (as) group appear at 2039 cm-1 and is attributed to the presence of an end-on
azide bridge in complex 1 (Fig. S1).
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3.2. Description of the structures Complex 1
The complex [Cu (L-)(1-1N3)]2 is an azido-bridged Cu(II) dimer complex as shown in
Fig. 2. The bond distances and bond angles for the complex 1 is given in Table 2 and Table
S1.The copper(II) ion has a distorted penta coordinated geometry in which the basal plane is
composed of the one oxygen atom and three nitrogen donor atoms. Among these, one nitrogen
atom is from the bridging azide anion, and the other two nitrogen atoms are from the Schiff base.
The remaining coordinate position is occupied by the phenoxo oxygen of tridentate Schiff base.
The fifth coordination site of the copper(II) center is occupied by the nitrogen atom of the
symmetry related azide group (symmetry code: -x, -y, -z) forming the Cu1–Cu1A bridge
(internuclear distance between the two copper atoms 3.156 Å). The Cu1–N20–Cu1A angle is
found to be ca. 93.9(3) in this complex. This Cu1–N (azide)–Cu1A angle is very much similar
with the reported end-on double azido bridged binuclear complexes.44-47 In the equatorial
positions, the bond lengths of Cu1–N20 and Cu1A–N20 are 2.037(7), 2.275(8) Å, respectively.
This inequality in bond lengths gives the azido complex a rare variety of asymmetric end-on
double-bridged structure. The Cu1A–N20 bond is showing little longer distance but is not

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unusual as quite a few end-on azido bridged copper(II) complexes possess Cu–N(azide)
bondlength of the order of 2.5 Å.44-47
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Fig. 2 Ortep view of complex 1. Atoms are shown as 30% thermal ellipsoids. H atoms are
omitted for clarity.

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Table 2: Bond distance (Å) and bond angles (◦) for complex 1
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Bond distances (Å)
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Cu1- Cu1A
Cu1-O7
3.156(1)
1.926(6)
1.961(7)
Cu1-N13
Cu1-N20
Cu1-N20A
2.065(6)
2.037(7)
2.275(8)
Cu1-N9
Bond angles (◦)
N9-Cu1-N13
94.6(3)
Cu1-N20-Cu1A
O7-Cu1-N9
93.9(3)
91.7(3)
167.0(3)
N9-Cu1-N20
O7-Cu1-N20
173.1(3)
87.5(3)
O7-Cu1-N13
3.3. Hirshfeld Surface Analysis
Supramolecular interactions are further studied using Hirshfeld Surface analysis. Complex 1 was
mapped over dnorm (range of -0.1 to 1.5Å), shape index (range of -1.0 – 1.0Å) and curvedness
(range of -4.0 – 0.4) respectively and presented in fig. S2 (A). Surfaces are kept transparent for
visualization of different supramolecular interactions. No characteristic H-bonding interaction
was found during mapping. This observation was further supported from decomposed fingerprint

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plot. Total extent of X…H/H…X (X=N, O, Br) contacts were found to be only around 31.5% or
so of the total surface. Br···H/H···Br, N···H/H···N and O···H/H···O interactions comprise of
11.60%, 15.80% and 4.10% of the total Hirshfeld surface, respectively. The bright red area in the
Hirshfeld surfaces is mainly due to intermolecular π-π interaction. Fig. S2 (B).
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3.4. DFT study of Complex 1
DFT optimization of the structure of coordination motif has been used to calculate the
difference in energy of HOMO and LUMO, ∆E (Fig. S3), which matched well with the band gap
using Tauc plot (Fig. 4). DFT calculation shows that the total energies of optimized copper dimer
of its singlet and triplet states are -64287.2919184eV and -64288.6443241eV, respectively. The
dimer is dinuclear with two Copper(II) metal centres, the ground state is a triplet with two
unpaired electrons, one in each Cu(II) atom.27 Therefore, ground state with triplet configuration
has the possibility to affect the band gap as two SOMO orbitals are involved.27 In this regard,
theoretical calculation in ground state tripletconfiguration gives band gap 3.62 eV for α-spin and
2.32 eV for β-spin. The experimentally measured band gap (2.84 eV) well agrees with
computationally calculated the average band gap (2.97 eV) due to SOMO orbitals.
3.5. FESEM study
The surface morphology and also the homogeneity of the sample have been tested by the
FESEM micrograph (Fig. 3). It has been observed that the surface of the sample is blobby and
continuous in nature.

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Fig.3 The FESEM surface micrograph of the sample in two different scales.
3.6. ESI MS study
To get an idea of the purity and stability of the complex, we have performed the electrospray
ionization mass spectroscopy (ESI-MS positive) before and after the deposition of the material.
The ESI-MS positive peaks of the molecular weight and fragments are recorded using 1:1
CH3OH and CH3CN solution and the related results are shown in Fig. S4 (a,b,c,d) respectively.
The mass spectrum of the complex 1 (Fig.S4(a)) shows peaks at m/z 402.01, 849.03 and 911.96
respectively correspond to [CuLH]+, [Cu2L2(N3)H]+ and [Cu2L2(μ1,1N3)2Na]+ respectively. The
study of fragmentation pattern (Fig.S4 (b) and S 4(c)) of possible complex adduct before and
after deposition clearly confirms the purity and stability of 1, which has further been verified by
comparing the experimental and theoretical isotopic patterns (Fig. S4 (d)).

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3.7. PXRD study
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8
The experimental XRD pattern of the bulk product (before and after deposition) is in excellent
conformity with the simulated XRD pattern generated from single crystal X-ray diffraction data
(CIF) using the CCDC Mercury software (Fig.S5). This indicates that the bulk material is true
representative of the single crystal.
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3.8. Solid state UV and I-V characteristic of the Complex 1:
The UV–Vis spectra of the solid film of complex 1 are studied based on the experimental
absorbance and transmittance data. Fig. 4 shows the absorbance plot inside the photon
wavelength range of 200–900 nm. Interestingly, a distinct band-to-band fundamental absorption
peak is observed in the absorbance plot near 430 nm and thereafter the absorbance of such
material shows a decreasing trend with the increase of wavelength. In this connection it could be
mentioned here that a similar copper(II) binuclear structure has been reported recently, having
the absorption peak near 400 nm.27 The insignificant dispersion and the small peaks observed in
the absorbance plot can be explained in terms of the sub-band transitions and the resonance of
the lattice. The optical direct bandgap (Eg) of the sample is estimated using the well known
Tauc’s method by the following relation.48, 49
h  B(h  Eg )n
(1)

Where is the absorption coefficient, B is a constant, h is the Plank’s constant and ν is the
frequency. For allowed direct transition, the value of n is ½ and is applied in eqn.1.

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Fig. 4. Measured absorbance versus wavelength plot of the complex. Inset shows the
transmission spectra of the sample.
Fig. 5 shows the(h )2 vs. h plot of the sample estimated using the absorption coefficient. To
assess the optical direct band gap of the sample graphically, the linear portion of this plot is
extrapolated up to h axis, where the straight line intersects the h axis. In the present work,
the optical direct band gap of the sample is found to be 2.84 eV using this standard method. The
band gap value of this material is well between the limit of band gaps of other copper (II)
materials which are considered as semiconductors.27-29

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Fig.5 (h )2 Vs. photon energy plot of the complex estimated using the absorption coefficient.
Such a band gap of this material has prompted to investigate how this material behaves under
steady bias conditions. In view of that a systematic investigation on such sample is carried out
applying ±1.5 V dc bias considering a special layered structure of glass-ITO-1-aluminium-glass
sandwich configuration. Fig. 6 shows the layer-by-layer structure of the device and the current-
voltage characteristics generated thereof. Interestingly, the experimental I-V plot of the test
structure is found to be analogous to the current-voltage characteristics of a Schottky barrier
diode and henceforth all the electrical analyses of the ITO-1-aluminium structure is performed in
light of the Schottky barrier diode theory.

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Fig. 6 The current-voltage characteristics of the ITO-1-aluminium structure. Inset A shows the
glass-ITO-1-aluminium-glass layered test structure.
The current conduction mechanism through the Al/1/ITO structure is mainly attributed to the
thermionic emission across the junctions. Considering the thermionic emission current to be
dominant in the device, the measured current is analysed using the well-known current-voltage
relation
q(V  IR )




s
I  I exp
(2)
0
nkT



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Where
R
s is the series resistance, n is the ideality factor and V is the applied voltage. The reverse
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saturation current I0 can be expressed as
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q
kT




I  AA*T 2 exp 

b 
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(3)
0
Where A is the area of deposition, A*is the Richardson constant and

b is the barrier height.
When the applied voltageV  3kT q , the barrier height (

b ) and the ideality factor ( ) can be
n
estimated by the relations49

2 
kT
q
AA*T




b 
ln
(4)
I0


and
q
dV
n 
(5)
kT d(ln I)
In the present case, the reverse saturation current (I0) of the diode structure is estimated 2×10-6A
graphically from the measured I V characteristics. Barrier height( b ) and the ideality factor (
) of the diode are calculated using eqn. 4 and 5 considering the linear part of the forward bias

n
I V plot and found to be 0.76 eV and 5.54 respectively. As it is seen from fig.4, the linear
part of the plot is restricted within 0.1 V and the device current has experienced a large series
resistance inside the end-to-end diode structure. In fact, for a range of applied voltages the
forward to reverse current ratio varies within one order to two orders of magnitudes. The series
resistance of the test structure is estimated from the measured I V data and found to be within
the range of 104Ω. However, an insignificant voltage dependence of the series resistance is
observed when estimated using eqn.2. The mild voltage variation of the series resistance is
attributed to the presence of small potential barriers present at the metal-semiconductor ohmic

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contact regions.50 The origin of such high series resistance might be explained in terms of the
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cumulative contributions of a large amount of grain boundary present in the test material along
with the discrete energy levels of non-uniformly separated microcrystals and the non-ohmicity at
the contact ends of the test structure.51
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Fig.7. Voltage variation of the dynamic resistance and the conductance of the diode.
Moreover, to examine how the conductance ( dI dV ) of the device varies with applied bias, the
dynamic resistance ( dV dI ) is estimated using the measured I V data. Fig. 7 shows the
voltage variation of the dynamic resistance and the conductance of the diode structure. As the
reduction of dynamic resistance is field dependent, a linear drop of the dynamic resistance up to
1.2 V indicates the effect of voltage drop across the series resistance, which influences the
conductance of the device significantly.

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3.6. Capacitor behaviour of the complex 1:
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Fig.8. Capacitance vs. frequency plot of the Al/1/ITO diode structure
Fig.8 shows the experimental capacitance plot of the Al/1/ITO test structure as a function
of frequency. It is well-known that in an idealized case, the capacitance of a metal-
semiconductor rectifying junction is usually frequency independent. However, in the present
case, the frequency dependence of the test structure, particularly at low frequency is attributed to
the diffusion capacitance contributed from the minority carriers present in the device. The other
two variety of device capacitance, namely the depletion capacitance and geometrical capacitance
are purely frequency independent. Nevertheless, one can explain the low frequency capacitance
observed in the present case in terms of the carrier capture and emission processes involved with

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the interface states, which becomes insignificant at relatively higher frequencies. In a structural
point of view, considering an electron rich material, the total capacitance of the test structure
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may be analysed by a general expression 52
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qV
kT




q2 D p exp
1
2
p
n0


qs0 Nd
2(Vd V )
0
L


CTotal



(6)




2f kT
The first term of the expression of the total capacitance is contributed from the diffusion of
minority carriers, where Dp and pn0 are the hole diffusion coefficient and hole density,
respectively in an n-type semiconductor and
second term of eqn. 6 corresponds to the depletion capacitance of the Al/1/ITO structure, where
s , Nd and Vd are the permittivity of the semiconductor, donor density and diffusion potential
f
is the frequency of the applied ac signal. The

respectively. The third term of the expression is contributed from the geometrical capacitance
and is dependent on the thickness of the deposited material. To investigate how the device
performs under ac condition, a set of square wave signals ranging between 10 kHz to 10 MHz
are applied to the test structure. Fig. 9 shows the input-output waveforms of such ac signals at 10
kHz, 50 kHz, 100 kHz and 10 MHz respectively. Analyses on these output waveforms
corresponding to the input signals clearly confirm that the test structure bypasses the signals at
50 kHz and 100 kHz in a more convenient manner compare to that of the other frequency
signals. In view of an application oriented material’s perspective, this issue has been found
interesting as the phenomena can be exploited in terms of the development of electronic filter in
the frequency selective circuits.

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Fig.9. Output waveforms of the Al/1/ITO diode structure for the applied input signals at 10 kHz,
50 kHz, 100 kHz and 10 MHz respectively.
Conclusion
In summary, we have reported a novel copper (II) coordination dimer, which has been further
studied under dc and ac conditions. Optical studies revealed that the direct optical band gap of
the said material is 2.84 eV, which suggests that the complex 1 belongs to semiconductor family.
The current-voltage characteristics of a typical Al/1/ITO structure shows a similar characteristics
generally observed in Schottky barrier diodes and the relative electrical analyses are carried out
based on the thermionic emission theory of Schottky barrier diode. Additionally, the frequency
behaviour of the Al/1/ITO structure shows that this material has a frequency selective property,
which can be utilized in electronic filters.

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Conflict of Interest
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There are no conflicts of interest to declare.
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Acknowledgements
Financial supports from UGC [Sanction no.F.38-5/2009(SR)] and DST [Sanction no. SR/FT/CS-
060/2009], New Delhi to S. S. is gratefully acknowledged. This work is supported by the
infrastructure provided in the Department of Chemistry and Electronic Science, Acharya Prafulla
Chandra College under DST-FIST scheme (Sanction order: SR/FST/College-008 dated
15/02/2012). The authors are thankful to Dr. Sanjay Kumar and Miss Rituparna Mondal, Dept. of
Physics, Jadavpur University for SEM data and Dr. Saktibrata Bhowmik (Principal, Acharya
Prafulla Chandra College), for his support to carry out this work. We are also thankful to Dr. T.
K. Paine and Mr. Abhijit Bera, Dept. of Chemical Science, IACS for ESI MS study.
Supplementary Data
CCDC 1879738 contains the supplementary crystallographic data for 1. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.

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