Accepted Manuscript

Synthesis, crystal structure, vibrational studies, optical properties and DFT calculation
of a new luminescent material based Cu (II)

Wafa Ben Hmida, Aycha Jellali, Haithem Abid, Besma Hamdi, Houcine Naili, Ridha
Zouari

PII: S0022-2860(19)30192-9
DOI: https://doi.org/10.1016/j.molstruc.2019.02.062
Reference: MOLSTR 26219

To appear in: Journal of Molecular Structure

Received Date: 25 November 2018

Revised Date: 14 February 2019
Accepted Date: 15 February 2019

Please cite this article as: W. Ben Hmida, A. Jellali, H. Abid, B. Hamdi, H. Naili, R. Zouari,
Synthesis, crystal structure, vibrational studies, optical properties and DFT calculation of a new
luminescent material based Cu (II), Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/
j.molstruc.2019.02.062.

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 ACCEPTED MANUSCRIPT
Synthesis, crystal structure, vibrational studies, optical properties and DFT calculation
of a new luminescent material based Cu (II)

Wafa BEN HMIDAa, Aycha JELLALIa*, Haithem ABIDb, Besma HAMDIa, Houcine NAILIc,
Ridha ZOUARIa
a
Laboratory of Materials Science and Environment, Faculty of Sciences, University of

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SFAX, PB 1171, 3000 SFAX, Tunisia.

b
Laboratory of Applied Physic, Faculty of Sciences, University of SFAX, PB 1171, 3000

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SFAX, Tunisia.

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c
Laboratory of Physical Chemistry of the Solid State, Faculty of Sciences, University of
SFAX, PB 1171, 3000 SFAX, Tunisia.

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* E-mail corresponding: jellali.aycha@gmail.com
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Abstract:
In this work, we report the synthesis, molecular structure, the vibrational spectral (FT-IR, FT-
Raman) analysis and optical properties of the investigated novel non linear optical bis(2, 2, 6, 6–
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Tetramethylpiperidinium–1–yl) oxidanyl tetrachlorocuprate (II), having a general chemical formula

(C9H19NO)2[CuCl4] abbreviated as (TEMPO-H)2[CuCl4]. The molecular structure crystallises in the
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orthorhombic system with the P212121 space group. The crystalline stability is installed by a zero-
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dimensional network using hydrogen bonds between O–H…Cl, N–H…Cl and C–H…O. In addition, a
3-D Hirshfeld surface analysis was performed to study molecular interactions and connect them with
2-D fingerprint schemes to detect the relative contribution of these interactions to the crystalline
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structure. Molecular structure, vibrating wave number, non-linear optical characterization (NLO), and
Mulliken were calculated with B3LYP / LanL2DZ (DFT the Density Functional Theory) using
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GAUSSIAN09. The optimized geometry and the vibrational spectra calculated results compared to
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experimental data, showed a good overall agreement. The UV–Visible absorption and the
photoluminescence (PL) spectroscopy of (TEMPO–H)2[CuCl4] were also presented. The
unaided-eye-detectable blue luminescence emission comes from the excitonic transition in the
CuCl4 anions. Finally, the electronic property was determined by time-dependent DFT (TD-DFT)
approach.

Keywords: Crystal structure, DFT, Absorption, Photoluminescence, First hyperpolarizability,

TD–DFT.
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1. Introduction

In recent years, the design and synthesis of inorganic organic solids that rely on the
concept of crystal engineering have attracted considerable attention regarding the application
of these compounds. Recently, varions applications have been recorded owing to their
significant magnetic, luminescence, nonlinear optical (NLO), catalytic, electrical and
ferroelectric properties [1–5]. More specifically, the development of photonic and

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optoelectronic technologies depends largely on the growth of NLO materials with high non-
linear optical responses and the emergence of novel and more efficient materials [6].

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Organometallic compounds such as NLO are particularly important in terms of their ability to
provide significant opportunities to combine the beneficial properties of inorganic materials,

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which in turn provide good conductivity, mechanical and thermal stability, amplitude,
magnetic displacements or insulation, structural diversity, ease of processing, high NLO

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coefficients and efficient fluorescence.
For this reason, considerable efforts have been performed to synthesize a new organic-
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inorganic material (TEMPO-H)2[CuCl4], which crystallizes in non centrosymmetric space
group showing enhanced NLO activity. On the other side, copper (II) possesses an association
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of different dimensional orders: zero (0D), one (1D), two (2D) or three (3D) dimensional
networks [7–11]. In other words, the Cu2+ ion can acquire a variety of coordination numbers
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and geometries: tetrahedral [12], trigonal bipyramidal [13] square pyramidal, and octahedral
[14] in view of the relative flatness of the potential surfaces and the presence of a d9 electronic
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system proving the presence of Jahn–Teller effect in this complex [15]. The crystallographic
study is an important source that can be used to derive information about the intermolecular
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interactions involved in the crystal packing [16, 17]. In addition, the application of Hirshfeld
surface analysis is increasing in the field of crystallography. This approach has also become
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an intrinsic tool for the investigation of intermolecular interactions in the crystal packing.
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Research on copper coordination compounds, structural properties as well as vibrational and

non-linear optical (NLO) properties with DFT calculations using the theoretical level B3LYP
/ LanL2DZ for new hybrid materials, was carried out in order to determine physic-chemical
properties as well as energy gap of materials. The ultimate objective is the investment of these
properties for technological applications. Having the motivations stated above and encouraged
by this pioneering work, we attempt to present a novel hybrid compound (TEMPO-
H)2[CuCl4]. Our central focus is upon the crystal structure which is characterized by X-ray
diffraction as well as vibrational spectra study (Raman and infrared spectroscopy) and optical
properties studies (absorption and photoluminescence). In the light of our theoretical
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calculations, the relationship between vibrational spectra and calculated results makes it
possible to clearly identify vibration models and better understand the binding and structural
features of hybrid compounds, the theoretical study of others, the characteristics of linearity
and the excessive priority of the mixture. Functional Density Theory (DFT / B3LYP /
LanL2DZ) is the basis of all calculations. After all, TD-DFT calculations were also carried
out in order to analyze the electronic property which was investigated and interpreted.

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2. Materials and measurements

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2.1. Synthesis

(TEMPO-H)2[CuCl4] single crystals were synthesized through the reaction in

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stoichiometric 1:2 amounts of Cupper (II) chloride (CuCl2) anhydrous and (2,2,6,6-
Tetramethylpiperidin-1-yl) oxidanyl (C9H18NO): (TEMPO). First, (TEMPO) solution

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dissolved in an aqueous solution of HCl (37%) was added to the solution of anhydrous
Cupper (II) chloride in anhydrous ethanol. Several weeks after, yellow plate-shaped crystals
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of very high quality were obtained by evaporation at room temperature (Fig. 1). The dried
precipitates were then washed with diethyl ether. Only one crystal was selected for X-ray
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diffraction analysis. Copper and chlorine atoms were chemically analysed in order to confirm
the formula determined by structural refinement [18]. The density of this compound was
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measured at room temperature by pycnometer using CCl4.

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2.2. Crystal data and structure determination

A suitable single crystal of the title compound was chosen in order to perform its
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structural analysis by an X-ray diffraction. It was selected under a polarizing microscope and
was mounted on a cactus needle. The intensity data were collected on a Bruker Kappa Appex
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II CCD area detector system equipped with graphite monochromatic MoKα radiation (0.71073
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Å) at 293(2) K. Lattice parameters were refined from setting angles of 252 reflections in the
2.34 < θ < 26.589. The empirical absorption corrections were performed based on a multi-
scan [19]. A total of 4971 reflections were collected using the ω–2θ scan technique of which
2982 have I > 2σ(I) and were used for structure determination. Then, the Patterson with the
SHELXS program of the SHELXTL package and the organic moieties were obtained from
successive Fourier calculations using SHELXL-2016 [20], which readily established the
heavy atom position and facilitated the identification of the light atoms from different Fourier
maps. The pertinent experimental details of the structure determination for the new
compounds are presented in Table S 1 (supporting material). All hydrogen atoms are placed
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geometrically and refined isotropically. The structural graphics of the asymmetric unit are
created with ORTEP [21] and the other figures with the DIAMOND program [22].

2.3. Hirshfeld surface analysis

The intermolecular interactions ensuring the structure cohesion are visualized by

Hirshfeld surface analysis and the 2–D fingerprint plots which identify each type of

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intermolecular interactions. The Hirshfeld surfaces and 2–D fingerprint plots were prepared in
this paper by means of Crystal Explorer (Version 3.1) [23] based on the structure of the CIF

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file. The Hirschfield surface is set using (dnorm). The measured connection distance (dnorm) is
determined on the basis of de and di by

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 − 


 − 



 = +


 

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Where rivdw and revdw are the van der Waals radii of the atoms. di and de are the distance from
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Hirshfeld surface to nearest nucleus inside and to nearest nucleus outside, respectively. The
dnorm value can be negative or positive. The dnorm values are set using a red-white-blue color
scheme [24, 25]. The short contact is red, the white indicates the contact around vdW, and the
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blue is for longer contact. Fingerprint areas represent a combination of bi-directional de-di
and di-de measures. The graph shows that the areas visible in the fingerprint correspond to a
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single molecule, such as de> di and the other di>de. The 2-D fingerprint segment provides
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quantitative analysis of the molecular interconnections found in the molecule and presents this
information in a plot of color [26].
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2.4. Spectroscopic measurement

In order to quantify for the title compound, we have undertaken spectroscopic studies.
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First, the FT-IR spectrum of the sample was performed between 4000 and 400 cm−1 on the
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Perkin Elmer L120-00 spectrometer with a 2 cm−1 resolution using KBr pellet. Second, the
FT-IR spectrum of the sample was carried out between 4000 and 400 on the Perkin Elmer
L120-00 spectrometer with a 2 cm resolution using KBr Pellet. For example, the pellet was
prepared by mixing 15 mg of the powder sample with 300 mg of KBr (drying KBr at 383 K)
and compressing it into a disk. The FT-Raman spectrum was then recorded at room
temperature using a HORIBA JOBIN–YVON (T64000) spectrometer in an area of 4000 and
50 cm−1. The excitation line spectra-physics argon laser is of 514.5-nm.
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Then optical absorption spectrum of the films was deduced from direct transmission
measurements carried out using a conventional UV–Vis absorption spectrometer (Hitachi,
U3300).The room temperature photoluminescence spectrum was recorded using a Dilor XY
setup with an excitation wavelength of 385 nm from an Argon laser.

2.5. Computational methods

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Molecular geometry optimizations and vibration frequency calculations are performed
using the Gaussian 09 program package [27]. The starting structure is used from the

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crystallographic data of Table S 1 (supporting material). Geometrical improvement of the
complex condition is performed without any constraint on geometry [28]. In order to take into

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account the effect of isolated halogen atoms in the structure, as well as the interactions
between molecular parameters, engineering parameters and vibrational spectroscopy,
technical improvements were introduced to the appropriate cluster model at

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DFT/B3LYP/LanL2DZ [29–31] level of density functional theory. A standard test scale of
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0.961 was used to compensate for systematic errors resulting from an incomplete basis,
neglect of electron correlation and vibration anharmonicity [32]. GaussView [33] has been
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used to create visual displays and verify the normal mode assignments. In addition, the
electronic properties such as HOMO and LUMO were determined by DFT (TD-DFT).
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3. Results and discussion

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3.1. Structure description

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X-ray single crystal diffraction was used for the identification of the synthesized
compound. It revealed that the (TEMPO-H)2[CuCl4] compound crystallizes in the
orthorhombic system with a non-centrosymmetric space group P212121. The dimensions of the
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cell are as follows: a=11.911 (18), b=11.911(10), c= 17.832(4) Å, V = 2530.0 (7) Å3 with four
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formula units in unit cell (Z=4). The asymmetric unit of the structure drawn with 20%
probability thermal ellipsoids, together with the atomic numbering scheme is exhibited in Fig.
2 (a). It comprises two crystallographically independent protonated organic cations denoted
([TEMPO–H]+ (1) and [TEMPO–H]+ (2)) protonated cations ligand and one [CuCl4]2– anion.

The projection of the atomic arrangement of (TEMPO-H)2[CuCl4] in the (a, b) plane is

presented in Fig.3. This projection presents also zigzag layers formed by the tetrahedral
[CuCl4]2− which lead to chains along the crystallographic b-axis at approximately z = 0 and z
=0.5. In this structure, the Cu (II) has a tetrahedral geometry with four chloride atoms to form
 ACCEPTED MANUSCRIPT
a [CuCl4]2− anion. The tetrahedral is deformated with D2d symmetry. The expected distortions
are predicted by Jahn–Teller effect’s influence.

In order to find the most optimized geometry, theoretical calculations were carried out
using DFT/B3LYP/LanL2DZ basis sets. All parameters were allowed to relax and all
calculations converged to an optimized geometry corresponding to a minimum of energy as
revealed by the lack of imaginary values in the calculated frequency. The optimized geometry

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of the studied compound model is presented in Fig. 2(b). The geometrical parameters of our
compound are summarized in Table 1. The essential structural parameters were compared to

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similar systems leading to a good agreement with experimental data. Thus; Cu‒Cl distances
vary between 2.231(3) Å and 2.260(3) Å whereas, Cl‒Cu‒Cl angle values range from

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99.14(12)° to 130.94 (15)° [34‒36]. The average values of the Baur distortion indices for the
distances and angles are calculated by Eqs. (2) and (3) [37].

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  |  |
ID Cu − Cl = ∑ !

 
(1)
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 $ |" " |
ID Cl − Cu − Cl = ∑ !
#
 # "
(2)
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where, d is the (Cu–Cl) distance, a is the (Cl–Cu–Cl) angle, m is the average value, n1= 4, and
n2= 6. The values of these indices are 0.0006 and 0.00099, respectively. These values indicate
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the existence of hydrogen bond interactions between the inorganic and the organic entities
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forming a periodic zero-dimentional structure (0 D).

Fig.3 displays all hydrogen atoms of the oxygen atom participating in the formation of
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hydrogen bonds O–H…Cl and non-classical C–H…Cl hydrogen bonding interactions

between the [TEMPO–H]+ cations and the tetrahedral [CuCl4]2– anion. Besides, Table 2
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depicts the hydrogen bond parameters in this structure. The examination of the organic part
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clearly unveils that the ring of atoms of pyridine rings is bound to the outside of the ring by a
protonated oxygen. It is formed by two organic cations [TEMPO]+ (1) and [TEMPO]+ (2)
characterized by a torsion angle H(1A)‒O(1)‒N(1)‒H(1)=17° and H(2)‒O(2)‒N(2)‒H(2A)=
‒11°, respectively. In fact, The C–C , C–N and O–N distances in the (TEMPO)+ (1) cations
range from 1.319 Å to 1.396 Å and the C–C–C, C–C–N, C–N–C and C–N–O angles vary
from 105.6° to 132.2°. These values are in good agreement with those currently observed in
similar organic compounds [38, 39].

3.2. Molecular Hirshfeld Surfaces calculations

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Hirshfeld surfaces and their associated 2D fingerprint plots were used to determine
interactions between different molecules in (TEMPO-H)2[CuCl4]. The Hirshfeld surfaces of
(TEMPO-H)2[CuCl4] are generated using an expanded surface resolution with the dnorm
surfaces mapped over a fixed color scale of −0.092 to 1.456 Å (Fig. 5). In dnorm surfaces, the
large circular deep red colored depressions indicate hydrogen bonding contacts and other
spots which are due to O–H…Cl and C–H…Cl, interactions (Fig. 6).

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As expected, drawing 2D fingerprint plots from the surface of analysis presented in
Hershfield Fig.7, the 2D fingerprint plots from Hirshfeld surface analysis allows quantitative

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data to be obtained on percentage of element contribution interactions in the molecule. In this
case, the decomposition of fingerprint plots H...H reaction is widespread and constitutes

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68.8% of the total area of Hirshfeld surface. We can assert that hydrogen atoms have a source
of electricity owing to the partial positive charge, δ+ of H atoms. H...Cl/Cl...H corresponds to

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25.5% of the total surface structure provided by the important role of hydrogen bonds in
creating a strong cohesion between molecules. In addition, O...H/H...O as well as
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H...Cu/Cu...H interactions are (3.2%) and (0.5%), respectively.
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Fig. 8 shows the analysis of the Hirshfeld surface mapped with dnorm and fingerprint
plots of tetrachlorocuprate (II) in our compound. The decomposition of the fingerprint plots
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shows that the dominance of H...Cl contacts is by 95% of the total Hirshfeld surface area. The
Cu...H interaction contributes by 2.8%, while 2.2% of total contribution comes from Cl...N
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contacts.

The voids surface of the grown crystal is illustrated in Fig. 9. The voids in the
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crystalline material have been visualized by constructing (0.002 au) isosurface of procrystal
electron density. The resulting void surface volume is of 356.13 Å3 per unit cell, and its
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surface area is of 1070.51 Å2 while from single crystal XRD analysis, the volume of the unit
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cell is of 2530.0 (7) Å3. The percentage of void in the unit cell corresponds to 14.07%.

3.3 Vibrational Frequency Analysis

The crystal structure of the metal compounds can be easily verified by studying
Infrared and Raman spectra, which are compared to their theoretical spectra. Added to that,
using the DFT method at B3LYP/LanL2DZ level of theory tends to overestimate the
fundamental modes. Therefore, scaling factors have to be used for both Raman and Infrared
theoretical spectra to obtain a considerable better agreement with the experimental data. This
scaling factor depends on the calculation method, the atoms in the molecules, and sometimes
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on even the vibration type. Thus, the superposed experimental and calculated IR and Raman
spectra of (TEMPO-H)2[CuCl4] are shown in Figs. 10 and 11. All domain assignments are
presented in Table 3. It’s work noting that based on our calculations and comparison with
literary studies [38–42] similar results are reported.

The assignments occur in two different regions: high wavenumber (4000–400 cm–1)
and low wave number (4000–50 cm–1). [CuCl4]2– anions do not provide stretching and

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bending vibrations, but methods associated with deformation as well as organic vibration
cations. These groups are defined at different wavelengths with different spectral densities.

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3.3.1. Internal modes

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Vibration analysis of the hydroxyl group (O–H) is the most sensitive to the
environment. Thus, it displays significant changes in spectra of protons.

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With reference to literary studies [43,44], O–H stretching vibrations were found
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around 3500 cm–1, whereas the presence of hydrogen bonds can reduce the stretching
frequency of O–H to 3500 and 3200. Thus, in our current work, similar symmetric vibrations
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of O–H are observed between 2888–2751 cm–1 and 2638 cm–1 in the FT–IR and FT–Raman
spectra, respectively. The calculated value is 2747 cm–1. Further analysis of this task reveals
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that the weak band at 1174 cm–1 in the FT–IR spectrum is for asymmetric and symmetric of
O–H and N–O, respectively. The DFT calculation estimates these patterns at 1157 cm–1. In
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addition, the average range is set at 743 and 724 cm–1 in the FT–IR and FT–Raman spectra
according to stretching vibration of O–H and (C–C) symmetric stretching while the expected
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value at 709 cm–1 calculated according to the B3LYP/LanL2DZ method. The stretching CH3
groups appear strongly and widely in the high frequency region 3416–2957 cm–1. First, the
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calculated and experimental frequencies need to be reasonably accepted. The low FT–IR band
is specified at 3416 cm–1, which may be due to in the CH3, an asymmetric stretching mode.
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The calculations (DFT) give this position at 3192 cm–1. In addition, observed intense bands at
3087 and 3000 cm–1 are assigned in infrared spectra and Raman to CH3 and CH2 groups.
However, the theoretically calculated value of this mode is 3149 cm–1. However, the very
strong FT-IR band is set at 2957 cm–1 and the Raman band is very strong at 2938 cm–1 with
long CH3 oscillations. This also indicates a good agreement with the expected DFT number at
2938 cm–1. The weak band in FT–IR spectrum set at 1640 cm−1 is assigned to the scissoring
mode of CH3, which corresponds to a good compatibility with the calculated value at 1610
cm−1. In addition, the CH3 twisting and CH2 scissoring modes are observed at 1509 cm−1 in
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the FT-IR spectrum and at 1480 cm−1 in the FT-Raman spectrum. The DFT calculations give
these modes at 1499 cm−1. Furthermore, CH3 deformation mode is observed empirically in
FT–IR at 1087–1019 cm−1 and 1057 cm−1 in the FT-Raman spectrum. The bands attributed to
CH3 and (C-C) asymmetric stretching vibration are observed at 1249 cm–1 in the FT–IR
spectrum and at 1221 cm–1 in the Raman spectrum. There is a strong band at 944 cm–1 in the
FT–IR spectrum and a medium intensity band at 955 cm–1 in the FT-Raman spectrum, that are

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attributed to CH2 deformation mode and (C–N) twisting mode. The calculated value is 921
cm–1.

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Therefore, the CH3 modes do not differ significantly from the expected value, which
indicates that the interactions of methyl groups with the environment are not strong. The

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numbers of vibration waves of the CH2 modes are based on the direct environment.

Symmetric stretching band of (C–C) has a vibration at 895 cm−1 in the IR spectrum.

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The (C–N) symmetric stretching and the (C–C) twisting vibrations are observed at 519 cm−1
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with a strong band in the FT–IR spectrum and with a theoretical band in the same position,
while the peak in the FT-Raman spectrum is observed at 560 cm−1. The theory is calculated at
649–580 cm−1. Finally, the strong band is found at 495 cm−1 in the FT–IR spectrum and at
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499 cm−1 in the Raman spectrum representing oscillation models of (C–C–C) symmetric and
(C-N-C) stretching that are calculated at 485 cm−1.
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3.3.2. External modes

Vibration analysis of isolated isolation [CuCl4]2− is less than 370 cm–1, which is
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allocated for discussion below.

At this level of analysis, symmetric and asymmetric stretching of Cu–Cl in Raman

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spectra is very strong at 356 and 274 cm−1. In theory these modes must be of 367 and 247
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cm−1. The band observed in the range 199 to 197 cm−1 refers to the stretching vibration of
[CuCl4]2− while DFT gives this model at 131 cm−1. The Cl–Cu–Cl symmetric stretching mode
is at 77 cm−1 and theoretically is at 69 cm−1. In general, the difference between observable and
calculated frequencies is due to the fact that solid state molecular interactions are not taken
into account in gas theory calculations.
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4. Optical study
4.1 Absorption and photoluminescence spectra

The electronic structure of the molecule in ground state can be determined by the wave
function of the electron moving within the molecule [45]. The absorption spectroscopy was
carried out according to the Frank–Condon principle, which allows studying electronic
transitions.

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Fig. 12 and 13, show the superposition of the experimental and theoretical UV-Visible
absorption spectra and the photoluminescence spectrum of the (TEMPO–H)2 [CuCl4] at room

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temperature, respectively.

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As a matter of fact, UV-Vis spectrum of the (TEMPO–H)2 [CuCl4] material (Fig. 12)
presents two bands in the wavelength region 350-700 nm. As can be seen from this spectrum,
satisfactory agreement between the experimental and calculated results is recorded. Thus, first

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a strong absorption band detected at 406 nm (3.05 eV) refers to photoinduced exciton formed
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by the transition from the top of the valence band consisting of Cl (3p) orbital to the bottom of
the Cu (3d) conduction band. In addition, the second band at 675 nm (1.83 eV) refers to a d–d
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transition in the orbits of metal ion Cu2+. Such conformity is reported in literature [35, 46].
Regarding the value of band gap at 2.385 eV, the title compound can be attributed to the
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highest energetic level in the conduction band. This energy is almost equal to 0.5 eV, which
clearly proves the high stability of the excitons and the intense luminescence that can be seen
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with the naked eyes even at room temperature. We can deduce therefore that the dielectric and
the zero dimensional quantum have a confinement effect.
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In the present case, a simple mode exemplifies the formation and recombination
process of the exciton in this sample exposed in Scheme1. Under the excitation of 385 nm
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irradiation, an electron (-) is excited from the valence band (VB) composed by Cu (3d) + Cl
(3p) orbitals to the conduction band (CB) formed by the Cu (4s) orbital, leaving a hole (+) in
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the VB. The electron (-) and the hole (+) move freely in the CB and VB, forming an exciton.
The recombination of the electron and hole in the exciton yields to a blue emission at 552 nm.
Moreover, the photoluminescence (PL) spectrum of the (TEMPO–H)2 [CuCl4] complex (Fig.
13) exhibits broad and strong band blue luminescence at 552 nm when excited at 385 nm,
which is also shown in the inset. This luminescence mainly comes from the electronic
transition in the inorganic [CuCl4]2– anions, rather than the organic layers, because the simple
organic molecule employed is transparent in the visible region [47].
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4.2 HOMO–LUMO energy gap

The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO) are very important parameters for electrical and optical properties.
Furthermore, HOMO and LUMO are essential for understanding the chemical stability, as
well as the chemical reaction of the molecule [48–50]. The frontier molecular orbitals are
calculated as shown in Fig. 12 using TD-DFT/B3LYP/LanL2DZ. The orbital energy of

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HOMO is directly associated with the ionization potential and the orbital energy of LUMO
which is directly related to the orbital electron affinity. We found the HOMO and LUMO

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energy values at –6.343 and –3.958 eV, respectively. The value of the energy gap value
between HOMO and LUMO is 2.385 eV. Therefore, this value and the non-centrosymmetric

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character are indicated that this material behaves as a semiconductor, which suggests the same
range of highly efficient photovoltaic materials and may be therefore considered as a potential

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material for harvest home solar radiation in solar cell applications. In addition, it may have
interesting additional applications in the field of optoelectronics.
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4.3 First hyperpolarizability
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The study of nonlinear optics provides a large amount of information on the

interaction of electromagnetic fields applied in different materials in order to generate new
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electromagnetic fields or modify them in the wavenumber, phase or other physical properties
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[51].
In this context, the crystal (TEMPO-H)2[CuCl4] belongs to the orthorhombic system
with a non-centrosymmetric space group P212121 which invites us to study its own NLO
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properties. The values of electronic dipole moment, polarization and hyperpolarization links
are calculated by Gaussian 09 using the DFT method for base B3LYP / LanL2DZ from Table
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4. Polarization is a phenomenon based on the creation of a dipole moment induced by an

external electric field %&'( ) that can be presented as a Taylor series expansion of the total
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dipole moment, µtot induced by the field:

1 1
*%&'( ) = *0 − , -. &. − , 2.3 &. &3 − , 5.36 &. &3 &6 − ⋯
2! 3!
. .3 .36

where α is the linear polarizability, the subscripts i,j,k represent the different
components of x, y and z cartesian coordinates system, U(0) is the permanent dipole moment
and βijk are the first hyperpolarizability tensor components.
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The total dipole moment is calculated by the equation:

-898 = :-;< + -=< + -><

The isotropic linear polarizability and the first hyperpolarizability tensor can be
calculated by the following equations.
2;; + 2== + 2>>
2898 =
3

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5898 = :%5;< + 5=< + 5>< )

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where:
<
5;< = %5;;; + 5;== + 5;>> )

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<
5=< = %5=== + 5=>> + 5=;; )
<
5>< = %5>>> + 5>;; + 5>== )

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As shown in Table 4, the calculated dipole moment -898 is equal to 973.04197 D
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(Debay). The calculated polarization value αtot is equal to 0.470910–24 esu, while the value of
the first hyperpolarizability βtot is equal to the compound 10.29110–31 esu. Finally, the result
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obtained from the high value -898 calculated by the B3LYP method shows that the studied
container has appropriate electronic applications.
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4.4 Atomic Charge

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To better understand the distribution charge of (TEMPO-H)2[CuCl4], presenting the

graph in Fig. 14, it should be noted that the Mulliken population is directly related to the
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molecular vibrations that determine the structural changes in the light of atomic displacement
characteristics. In addition, it affects dipole moment, polarization, electronic structure and
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other properties of molecular systems. As a result, they are directly related to the chemical
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bonds present in the molecule [52]. In this case, the distribution of Mulliken charge studies of
these carbon atoms (C11, C15, C21 and C25) is associated with positive nitrogen atoms. In other
words, the oxygen and nitrogen atoms contain a negative charge while all hydrogen atoms
contain a positive charge. Copper atom is located in the center of the anion part which is a
positive charge due to the coupling with negatively charged halogen. In fact, a negative
oxygen (–0.364) was observed with a carrier coupling via hydrogen bonds, which guarantees
the stability of the crystalline structure.
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Conclusions
We have synthesized a new NLO material (TEMPO–H)2 [CuCl4]. It crystallizes in an
orthorhombic system with a non–centrosymmetric space group P212121. The crystalline
packing of the compound is stabilized by hydrogen bonding interactions O–H...Cl and C–
H...Cl. Thus, the analysis of the Hirschfield surface in the form of decomposed fingerprint
plots allowed a quantitative examination showing the intermolecular interactions in the

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structure. Based on DFT calculations, the geometry and vibration spectrum proves to be
compatible with the experimental data, which is interpreted to be similar to the homologous

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compounds. The total dipole moment, first-order hyperpolarizabilities and the energy gap of
HOMO–LUMO have been calculated by DFT/B3LYP/LanL2DZ method. At present, the

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study of the polarization and hyperpolarizability shows the importance of the role of hydrogen
bonds, especially in the creation of asymmetric structures. The UV–Visible absorption and
photoluminescence spectrum recorded at room temperature, allow their use as optoelectronic

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devices. Finally, the value of the energy gap (Eg) indicates that this material is a
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semiconductor corresponding to a good candidate for possible applications in the fields of
catalysis, intelligent therapeutic vectors, batteries, photovoltaic cells, radiation sensors,
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detector and LED.

Supplementary material
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This research paper contains supplementary crystallographic data in CIF format which
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are available as Electronic Supplementary Publication from Cambridge Crystallographic Data

Centre (CCDC number 1857073). The data can be obtained free of charge at
http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic
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Data Center, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +441223 336 033; E-mail:
deposit@ccda.cam.ac.uk.
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Acknowledgements
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The authors thank the members of unit of common services, at the University of Sfax
for their assistance in the measurements for X–ray diffraction. The authors are also grateful to
Prof Hamadi KHEMAKHEM, for his cooperation in the Raman spectroscopy measurement.
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Table1: Geometric parameters (Å, º).
Bond length (Å) Observed Calculated Bond angle (°) Observed Calculated
[CuCl4]2-
Cu–Cl1 2.239 (3) 2.331 Cl1–Cu–Cl2 100.87 (14) 96.861
Cu–Cl2 2.251 (3) 2.329 Cl2–Cu–Cl3 100.79 (11) 99.820
Cu–Cl3 2.231 (3) 2.396 Cl1–Cu–Cl3 130.94 (15) 133.231
Cu–Cl4 2.260 (3) 2.388 Cl3–Cu–Cl4 100.25 (13) 97.954
Cl1–Cu–Cl4 99.14 (12) 99.311
Cl4–Cu–Cl2 129.17 (15) 136.343

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[C9H19NO]+
O1–N1 1.412 (10) 1.363 O1—N1—C11 108.6 (9) 112.806
N1–C15 1.540 (15) 1.532 O1—N1—C15 105.6 (8) 117.259

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N1–C11 1.521 (16) 1.519 C11—N1—C15 122.4 (8) 126.368
C15–C19 1.502 (19) 1.544 O2—N2—C25 108.9 (7) 112.714
C15–C18 1.534 (17) 1.560 O2—N2—C21 109.2 (8) 117.496

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C15–C14 1.555 (18) 1.550 C25—N2—C21 118.6 (8) 126.344
C14–C13 1.540 (2) 1.540 C19—C15—C18 109.8 (12) 109.156
C13–C12 1.480 (2) 1.540 C19—C15—N1 107.0 (10) 109.820
C12–C11 1.503 (18) 1.552 C18—C15—N1 112.6 (11) 107.326

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C11–C17 1.519 (18) 1.546 C19—C15—C14 114.3 (13) 110.208
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C11–C16 1.520 (18) 1.557 C26—C21—N2 110.4 (8) 107.140
O2–N2 1.420 (12) 1.361 C27—C21—N2 105.1 (9) 110.123
N2–C21 1.556 (12) 1.523 C29—C25—C28 109.2 (9) 109.459
N2–C25 1.538 (13) 1.519 C29—C25—N2 105.7 (9) 108.230
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C21–C27 1.530 (16) 1.543 C28—C25—N2 112.2 (9) 108.508

C21–C22 1.509 (16) 1.345 C29—C25—C24 110.7 (12) 110.068
C21–C26 1.512 (16) 1.559 C28—C25—C24 113.0 (10) 112.252
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C22–C23 1.518 (16) 1.539 N2—C25—C24 105.8 (8) 108.220

C23–C24 1.486 (17) 1.533 C23—C24—C25 113.8 (11) 113.981
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C24–C25 1.540 (17) 1.552 C21—C22—C23 114.8 (9) 113.215

C25–C28 1.517 (16) 1.557 C12—C11—C17 109.6 (12) 109.926
C25–C29 1.513 (15) 1.546 C12—C11—C16 112.3 (12) 112.303
C17—C11—C16 109.6 (12) 109.458
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C12—C11—N1 106.4 (11) 108.168

C17—C11—N1 107.1 (10) 108.516
C16—C11—N1 111.7 (10) 108.561
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C24—C23—C22 109.9 (10) 109.791

C13—C12—C11 112.4 (12) 114.080
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C13—C14—C15 115.8 (12) 113.802

C12—C13—C14 109.9 (11) 109.964
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Table 2: Hydrogen-bond geometry (Å, º)

D—H···A D—H H···A D···A D—H···A

O1—H1···Cl3i 0.8200 2.1800 2.9912 173.00

O2—H2···Cl4ii 0.8200 2.2300 3.0292 167.00
C12—H12A···O2 0.9700 2.5700 3.4476 150.00

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C18—H18A···O1 0.9600 2.4400 2.8337 104.00
C26—H26A···O2 0.9600 2.5300 2.8506 100.00

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C28—H28C···O2 0.9600 2.4300 2.8717 108.00

Symmetry codes: (i) x−1/2, −y+1/2, −z; (ii) −x+1, y+1/2, −z+1/2

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Table 3: Observed and calculated vibration frequencies (cm-1) of the (C9H19NO)2[CuCl4].

Observed frequency (cm-1) Calculated frequency (cm-1)

Assignments

FT-IR FT-Raman

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3416 w – 3190 νas (CH3)

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3087 s 3000 s 3149 νsym (CH3)+ νsym (CH2)

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2957 vs 2938 vs 2938 νsym (CH3)

2888–2751 m 2638 w 2747 νsym (O–H)

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1640 w – 1610 sciss (CH3)

1509 w 1480 w 1499 sciss (CH2), t (CH3)

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1466 m 1419 m 1426 νsym(C–C)

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1392 s 1323 w 1378 δ (CH2)

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1305 sh – 1317 t (CH2)

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1249 m 1221 m – δ (CH3), νas (C–C)

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1174 w – 1157 νas (O–H), νs (N–O)

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1118 m – 1118 t (CH2), νsym(C–C)

1087–1019 m 1057 m 1017 νsym (C–N), δ (CH3)

944 s 955 m 921 t (C–N), δ (CH2)

895 m 881 m – νsym(C–C), νsym( CH2)

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734 m 724 s 709 ν (O–H)

– 560 s 649, 580 νsym (C–N)

519 s – 519 νsym (C–N), t(C–C)

497 s 499 w 485 νsym (C–C–C), νsym (C–N–C)

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– 356 s 367 νas (Cu–Cl )

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– 274 s 247 νsym (Cu–Cl )

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– 199–197 m 131 ν [CuCl4]

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– 77 s 69 νsym (Cl–Cu–Cl )
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Abbreviations: s: strong, w: weak, v: very, sh: shoulder, m: medium; ν: stretching, as:
asymmetric, sym: symmetric, δ: deformation, sciss: scissoring; t: twisting.
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Table 4: The electric dipolemoment µ (D) the average polarizability αtot (10–24 esu) and first

hyperpolarizability βtot (10–31 esu) for the (C9H19NO)2[CuCl4].

µx 973 βxxx 0.8963795

µy –8.874313 βyyy 0.7119218
µz 1.714703 βzzz 1.067056

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µ 973.04197 βxyy –0.2336244
αxx 0.6196876 βyxx 0.02514030
αyy 0.4269339 βxxz –3.291180

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αzz 0.3663286 βxzz 6.110356
αxy 57.8424 βyzz –8.127770
αxz 2.128868 βyyz –0.1194208

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αyz –14.79907 βxyz –0.5784588
αtot 0.470983 βtot 10.295134

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Fig. 1: Atom numbering scheme of the title compound: (a) the experimental result, (b) the

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optimized geometry.

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Fig. 2: Projection of the atomic arrangement of (C9H19NO)2[CuCl4] structure along b axis.
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Fig. 3: Intermolecular hydrogen bonds of the (C9H19NO)2[CuCl4] compound.
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Fig. 4: Hirshfeld surface mapped over dnorm (a) of the asymmetric unit and 2D fingerprint

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Fig. 5: Hirshfeld surface mapped dnorm. Dotted lines indicate hydrogen bonds.

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Fig. 6: 2D fingerprint plots of the (C9H19NO)2[CuCl4] compound.
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Fig. 7: Hirshfeld surfaces mapped over dnorm of anionic part of the (C9H19NO)2[CuCl4] and 2D
fingerprintplot.
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Fig. 8: Crystal void of the (C9H19NO)2[CuCl4] compound.
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Fig. 9: Superposition of the experimental (black) and the DFT computed (red) FT-IR
spectra of the (C9H19NO)2[CuCl4] compound.
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Fig. 10: Superposition of the experimental (black) and the DFT computed (red) FT-Raman
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Fig. 11: Experimental and theoretical optical absorption spectra at room temperature of
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Scheme 1: Simple model for the formation and recombination of the exciton in

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(C9H19NO)2[CuCl4] compound.
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Fig. 12: The experimental room temperature photoluminescence spectrum
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Fig. 13: Molecular orbital surfaces for the HOMO and LUMO of the (C9H19NO)2[CuCl4]

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Fig. 14: Atomic charge distribution of the (TEMPO)2[CuCl4].
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Highlights

The (C9H19NO)2[CuCl4] crystal was solved in a orthorhombic system with P212121

space group.

The assignments of the vibrational modes based on DFT were reported and discussed.

This compound exhibits a strong blue emission at 552 nm.

The energy gap revealed an semi conductor property of the (C9H19NO)2[CuCl4]

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