A new organic-inorganic hybrid compound (C10H28N4)[CuCl4][BF4]2: Structural, optical, thermal studies and DFT-TDDFT calculations

Article abstract of DOI:10.1016/j.molstruc.2021.131441

The new organic-inorganic hybrid compound 1,4-bis (3-aminiopropyl) piperazine-1,4-dium bis(tetrafluoroborate) tetrachlorocuprate (II) was synthesized and determined by X-ray diffraction analysis. The structure consists of two isolated [BF₄]^? tetrahedrons, [CuCl₄]^2- square plane and tetra organic cations, held together by hydrogen bonds, forming a three-dimensional network. Different types of contacts have been quantified through the calculations of Hirshfeld surfaces percentages and described by fingerprint plots. The vibrational, optical and thermal properties have been also investigated by Infrared spectroscopy, UV–Vis absorption, Photoluminescence and TG-DTA measurements, respectively. The titled compound exhibits absorption features attributed to the Ligand-to-Metal Charge Transfer (LMCT) phenomena and a blue light emission appealing to some optoelectronic applications. Moreover, Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TDDFT) calculations have been performed in order to more accurately infer the experimental finding, and great agreement has been provided.

Full text of DOI:10.1016/j.molstruc.2021.131441

Journal of Molecular Structure 1248 (2021) 131441
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
A new organic-inorganic hybrid compound (C10H28N4)[CuCl4][BF4]2:
Structural, optical, thermal studies and DFT-TDDFT calculations
Souhir Hadaouia, Zeineb Ouerghia, Slim Elleuchb, Riadh Kefia,∗
a Université de Carthage, Faculté des Sciences de Bizerte, Laboratoire de Chimie des Matériaux, 7021, Zarzouna, Tunisia
b Laboratoire de Physique Appliquée, Faculté des Sciences de Sfax, Université de Sfax, B.P. 1171, 3000 Sfax, Tunisia
a r t i c l e i n f o
a b s t r a c t
Article history:
The new organic-inorganic hybrid compound 1,4-bis (3-aminiopropyl) piperazine-1,4-dium
bis(tetrafluoroborate) tetrachlorocuprate (II) was synthesized and determined by X-ray diffraction
analysis. The structure consists of two isolated [BF4]− tetrahedrons, [CuCl4]2- square plane and tetra
organic cations, held together by hydrogen bonds, forming a three-dimensional network. Different types
of contacts have been quantified through the calculations of Hirshfeld surfaces percentages and described
by fingerprint plots. The vibrational, optical and thermal properties have been also investigated by In-
frared spectroscopy, UV–Vis absorption, Photoluminescence and TG-DTA measurements, respectively. The
titled compound exhibits absorption features attributed to the Ligand-to-Metal Charge Transfer (LMCT)
phenomena and a blue light emission appealing to some optoelectronic applications. Moreover, Density
Functional Theory (DFT) and Time-Dependent Density Functional Theory (TDDFT) calculations have been
performed in order to more accurately infer the experimental finding, and great agreement has been
provided.
Received 29 March 2021
Revised 29 August 2021
Accepted 2 September 2021
Available online 8 September 2021
Keywords:
Hybrid compound
CuCl4square plan
LMCT
Photoluminescence
Thermal analysis
DFT and TDDFT calculations
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
inert, and weakly coordinated molecule. It is often used in ionic
liquids and exhibits high thermal stability, for instance, in applica-
Research on hybrid-based metal materials becomes one of the
greatest and popular research domains due to their magnificent
features that combine both the properties of organic and inor-
ganic molecules. They possess wide applications in different ar-
eas, such as optoelectronics, photoluminescence, magnetism, catal-
ysis, and the medical or pharmaceutical field [1-4]. Among these
compounds, copper-based compounds attract considerable atten-
tion due to the d9 configuration. The Cu(II) is Jahn-Teller active,
indeed, one of the d orbitals occupies a single electron which re-
sults in a stereochemistry with four [5,6], five [7,8], and six coor-
dination [9] compounds predominating. Consequently, copper (II)
complexes have different molecular geometries, such as tetrahe-
dral, square planar, square pyramidal, and octahedral [10]. Con-
siderable attention has been also paid to tetrafluoroborates, due
to their moderate sizes and highly symmetrical shapes. The well-
known ball-shaped anions, such as BF4−, are stable and subject to
positional changes with varying temperature and weak interactions
in the crystals. The interaction of cations with monovalent tetrahe-
dral counter anions is expecte−d to generate phase transition mate-
rials [11,12]. Notably, the BF4 entity is a symmetrical, lipophilic,
tions such as electric double-layer capacitors [13], and in improv-
ing the performance of perovskite solar cells [14].
All these advantages make it particularly valuable to synthesize
and study physicochemical properties of the new hybrid Cu (II)
complex containing the BF4−entity and 1,4-bis (3-aminiopropyl)
piperazin 1–4 dium as the organic cation. This later was chosen
as a piperazine derivative, given the importance of this family and
owing to their great applications in pharmaceuticals as antituber-
culosis [15], antimalarial [16], antitumor [17], antiviral [18], and an-
ticancer [19] agents. Herein, we report the synthesis, crystal struc-
ture, vibrational, optical and thermal investigation of the new hy-
brid material: (C10H28N4)[CuCl4][BF4]2. Hirshfeld surface analysis
was also performed to gain a deep insight into the intermolecu-
lar interactions inside the compound and to understand its crystal
architecture. Furthermore, DFT and TDDFT calculations were used
to get more details and to aid in the interpretation of the experi-
mental vibrational and optical findings, respectively.
2. Experimental
2.1. Synthesis of (C10H28N4)[CuCl4][BF4]2
∗
The new compound (C10H28N4)[CuCl4][BF4]2 was prepared at
room temperature by mixing 67.22 mg (0.5 mmol) of CuCl2 and
Corresponding author.
E-mail address: Riadhkef@yahoo.fr (R. Kefi).
https://doi.org/10.1016/j.molstruc.2021.131441
0022-2860/© 2021 Elsevier B.V. All rights reserved.

S. Hadaoui, Z. Ouerghi, S. Elleuch et al.
Journal of Molecular Structure 1248 (2021) 131441
Table 1
Crystallographic data of (C10 H28N4)[CuCl4][BF4]2.
Crystal data
Chemical formula
Formula weight (g/mol)
Crystal system
(C10H28N4)[CuCl4][BF4]2
583.32
Orthorhombic
Pnma
Space group
˚
a, b, c (A)
21.1762(13), 12.0411(8), 8.2842(5)
3
˚
˚
V (A )
2112.3(2) A
Z
4
Crystal size (mm3)
0.18 × 0.13 × 0.12
˚
Radiation (Wavelength (A))
Cu Kα, λ= 1.54178
F(000)
1180
Density (calculated), (mg/m3)
Diffractometer
1.834
Bruker Smart APEX-II CCD
4.176 to70.062
Theta range for data collection (°)
Index ranges
Absorption coefficient / [mm−1
-25<=h<=25, -14<=k<=14, -10<=l<=10
6.84
]
No. of measured, independent & observed
[I>2σ(I)] reflections
37704 / 9825 / 2103
Number of parameters
161
Abs. correction
multi-scan SADABS 2016
0.468 and 0.357
1.08
Max. and min. transmission
Goodness-of-fit on F2
Final R indices [I>2σ(I)]
R1 = 0.027, wR2 = 0.075
0.37 and - 0.40
−3
˚
Largest diff. peak and hole [e A
]
100 mg (0.5 mmol) of 1,4-bis (3-aminopropyl) piperazine in 30 mL
of ethanol. A mixture of a solution formed by 87.81 mg (1 mmol)
of Tetrafluoroboric acid and 36.45 mg (1 mmol) of HCl (1 M) in
20 mL of water was added drop by drop to this mixture under stir-
ring for three hours. The resulting solution was slowly evaporated
at room temperature until the formation of crystals of light green
color (yield 77%). Anal. Calc.: C, 20.57; H, 4.80; N, 9.60%. Found: C,
20.83; H, 4.94; N, 9.48%. The reaction equation can be schematized
as follows:
sphere with a finely ground sample of 11.8 mg. The C, H and N
microanalyses were reported with a Vario Elemental analyzer.
2.2.3. Hirshfeld surface analysis
Hirshfeld surface analysis was computed around the ortep of
the crystal using Crystal Explorer 3.1 [23]. It allows the visual-
ization of different types of intermolecular contacts in the crys-
tal by focusing on the close ones between atoms in neighbouring
molecules. The Hirshfeld surface was calculated using the normal-
ized contact distance surface dnorm which is generated, thereafter,
into 2D fingerprint plots, which is a two-dimensional summary of
the intermolecular interactions that we take into account in the
2HBF4 + C10 H24N4 + 2HCl + CuCl2 → (C10 H28N4)[CuCl4][BF4]2
crystal. The dnorm is given by the following equation: dnorm= (di-
2.2. Characterization
vdW
rivdW)/rivdW+ (de-revdW)/revdW, where rivdW and re
are the van
der Waals radii of the appropriate atoms internal or external to the
surface, di and de are the distances to the nearest atoms inside and
outside the surface [24]. The normalized contact distance is dis-
played using the red color for highlighting shorter contacts, white
for contacts around the vdW separation and blue for longer con-
tacts. The enrichment ratios of contacts are derived from the Hir-
shfeld contact surface analysis and are obtained for all the sum of
individual moieties present in the ortep to determine which type
of contacts is over or under-represented in the crystal packing. It
is defined as the ratio between the proportion of actual contacts in
2.2.1. X-ray crystallography
A single crystal for (C10H28N4)[CuCl4][BF4]2 was carefully se-
lected under a microscope in order to perform its structural anal-
ysis by X-ray diffraction. Data were collected at 100 K on a Bruker
Smart diffractometer equipped with an APEX II CCD Detector, us-
˚
ing a mirror monochromated Cu Kα radiation, λ = 1.5417 A. The
structure was solved by dual-space algorithm using the SHELXT
program [20], and then refined with full-matrix least-square meth-
ods based on F2 (SHELXL) [21]. All non-hydrogen atoms were re-
fined with anisotropic atomic displacement parameters. All hydro-
gen atoms were found in the difference Fourier map and refined
isotropically A final refinement on F2 with 9825 unique intensities
and 161 parameters converged at ωR(F2) = 0.075 (R(F) = 0.027) for
2103 observed reflections with I > 2σ(I). Crystal data are reported
in (Table 1). Drawings were made with Diamonds [22].
the crystal and the theoretical proportion of random contacts Exy
=
Cxy/Rxy, which are calculated from the surface contacts according
to the following formula: Rxx= Sx.Sx and Rxy= 2.Sx.Sy [25].
2.2.4. Computational details
The theoretical study consists of performing DFT and TDDFT
calculations type using the B3LYP functional with the LANL2DZ
as a basis set within the Gaussian 09 software. The cluster cho-
sen from X-Ray diffraction data is composed of one [CuCl4]2-
square plane, two [BF4]2− tetrahedrons and one piperazinium
cation [C10H28N4]4+ (Fig. 1). The DFT theory allows to a complete
geometry optimization and IR normal modes calculations when the
absence of imaginary frequencies was checked. The TDDFT calcula-
tion type provides the vertical excitations transitions and then the
theoretical UV-Visible absorption spectrum. The GaussView 5.0.8
software [26] is used to visualize the calculated IR normal modes
scaled by 0.961 [27] and to generate the IR and the UV–Vis ab-
2.2.2. Physical measurements
The IR spectrum was obtained, using the NICOLET IR 200
FT-IR infrared spectrometer in the range of 4000–400 cm−1
.
Solid-state absorption spectrum was registered at room tempera-
ture with a Perkin Elmer Lambda 35 UV–Vis spectrophotometer
equipped with an integrating sphere in the range of 200–1000 nm.
The emission spectrum was recorded for the solid sample with
Perkinelmer LS55 Spectrofluorometer at room temperature. Simul-
taneous Thermogravimetry-Differential thermal analysis (TG-ATD)
was performed using “Labsys” operating from 27 up to 400 °C tem-
perature at an average heating rate of 5 °C/min in Argon atmo-
2

S. Hadaoui, Z. Ouerghi, S. Elleuch et al.
Journal of Molecular Structure 1248 (2021) 131441
Fig. 1. (a) Ortep of the structure of (C10 H28N4)[CuCl4][BF4]2, the labelled part indicates the asymmetric unit of the compound. (b) Optimized geometry of the cluster model
used in the DFT and TDDFT calculations.
sorption spectra from Lorentzienne functions with a bandwidth of
15 cm−1 and 15 nm, respectively.
the title compound are τ4(B1) = 0.99 for the B(1)F4 tetrahedron
(β = 110.6° and α = 109.74°) and τ4(B2) = 0.99 for the B(2)F4
tetrahedron (β =110.2° and α = 110.0°), these values show that
the two bore atoms adopt a perfect tetrahedral geometry. More-
over, for the CuCl4 entity, τ4 = 0.21, indicating a similar config-
uration for the square planar geometry. The value of τ4 can take
an intermediate value which corresponds between tetrahedral and
square planar, this is the case of the compound (C12H20N2)[CuCl4],
where this value is 0.51 [31]. In the crystal lattice, the isolated
[BF4]2− tetrahedra and [CuCl4]2- entity form layers parallel to the
(a, c) plane at y = ¼(2n + 1). The crystal structure is stabilized by
intermolecular hydrogen bonding interactions. The organic cations
are inserted between the inorganic layers and interact with them
through hydrogen bonds with the chloride and fluoride anions,
generated by the cyclic azote and the amine groups as donors (see
Fig. 2). Hydrogen band parameters are given in Table 3. The net-
work of hydrogen bonds N–H···Cl, N–H···F, C–H···Cl and C–H···F is
characterized by intermolecular distances HꢀꢀꢀA varying between
3. Results and discussion
3.1. Structure description
The title compound 1,4-bis(3-aminiopropyl) piperazine-1,4-
dium bis (tetrafluoroboric) tetrachloridocuprate(II) crystallizes in
the orthorhombic system with Pnma space group and the follow-
˚
˚
ing unit cell parameters a = 21.1762(13) A, b = 12.0411(8) A and
˚
c = 8.2842(5) A (see Table 1). The experimental and theoretical
structural parameters for all the components of the structure are
gathered in Table 2. The mean values of the relative errors between
the calculated and observed distances and angles are respectively,
3.3% and 8.4% for the [CuCl4]2− square plane, 3.9% and 2.7% for
the [BF4]2− tetrahedron, 1.7% and 1.7% for the organic cation. These
relatively acceptable agreements lead us to confirm the accuracy of
the calculations and the suitability of the chosen cluster for further
calculations.
˚
2.11 and 2.78(2) A. The angles vary between 162(3) and 164.7° This
network stabilizes the structure and plays a significant role in the
formation of the three-dimensional framework.
The ortep view of the title compound shows that the crys-
tal structure contains one tetracation (1,4-bis(3-aminiopropyl)
piperazine-1,4-dium), one [CuCl4]2− and two [BF4]− The asymmet-
ric unit of the title compound consists of half of CuCl4, two halves
of BF4 and half C10H28N4 organic cation (Fig. 1(a)). The atoms
B(1), B(2), Cu(1), Cl(1), Cl(3), F(1), F(2), F(5) and F(6) are located
in a special position, which is the m symmetry plane perpendic-
ular to the b axis. These fragments verify the electric neutrality
of the crystal with the empirical formula (C10H28N4)[CuCl4][BF4]2.
The atomic arrangement of the compound shows that the copper
(II) atom is surrounded by four chlorine anions, while the boron
by four fluorine atoms. The bond distances around the copper (II)
and boron atom reported in Table 2 are consistent with the lit-
erature [28,29]. The geometry of four-coordinated metal complex
may conveniently be measured by the Yang τ4- parameter [30],
τ4 = [360 - (β + α)]/141 with β and α being the two largest an-
gles, that is zero for a perfect square planar geometry and one
for a perfect tetrahedral geometry. The calculated τ4 values of
In the case of the [C10H28N4]4+ cation, the selected bond
lengths and angles fall within the normal ranges. Indeed, N–C, C–C
distances and C–C–C, C–N–C, N–C–C, angles are in the same or-
der of magnitude with those observed in similar hybrid chlori-
docuprate (II) with the same organic cation [32,33]. The conforma-
tion of the piperazinium cycle can be described in terms of Puck-
ering coordinates, namely the evaluation of the parameters q2, q3,
Q, θ and ϕ [34]. The values for the cycle formed by C2–C1–N1–C2–
˚
˚
˚
C1–N1 are equal to Q = 0.7449 A, q2 = 0.7449 A, q3 = 0.000 A,
θ = 90.00 °, ϕ = 122.77 ° and correspond well to the most stable
chair conformation.
3.2. Hirshfeld surface analysis
Hirshfeld
surfaces
to
the
title
compound
(C10H28N4)[CuCl4][BF4]2 and the associated two-dimensional
fingerprint plots, were drawn to quantify the percentage of con-
3

S. Hadaoui, Z. Ouerghi, S. Elleuch et al.
Journal of Molecular Structure 1248 (2021) 131441
Table 2
˚
Experimental and theoretical bond length (A) and angles (°) in (C10 H28N4)[CuCl4][BF4]2. The theoretical value in brackets is the relative
error [((dexp-dthe)/dexp)x100].
Bond
Experimental
Theoretical
Bond
Experimental
Theoretical
Cu1—Cl1
Cu1—Cl2
2.2568(6)
2.2900(4)
2.2936(1.6)
2.3730(3.6)
Cu1—Cl2i
Cu1—Cl3
2.2900(4)
2.2783(6)
2.4188(5.6)
2.3283(2.2)
Bond
Experimental
Theoretical
Bond
Experimental
Theoretical
F1—B1
F2—B1
F3—B1
F3i—B1
1.409(3)
1.493(6.0)
1.477(5.5)
1.3860(0.2)
1.4437(4.3)
F4—B2
F5—B2
F6—B2
F4iii—B2
1.3892(19)
1.403(3)
1.4519(4.5)
1.482(5.7)
1.373(0.5)
1.4492(4.3)
1.401(3)
1.3836(18)
1.3836(18)
1.379(3)
1.3892(19)
Bond
Experimental
Theoretical
Bond
Experimental
Theoretical
N1—C1ii
N1—C2
N1—C3
N2—C5
C1—C2
C1—N1
C3—C4
1.505(2)
1.5005(19)
1.515(2)
1.490(2)
1.515(2)
1.505(2)
1.519(2)
1.521(1.1)
1.5253(1.7)
1.539(1.6)
1.526(2.4)
1.529(0.9)
1.526(1.4)
1.554(2.3)
C4—C5
N1—C2
N1—C3
N2—C5
C1—C2
C3—C4
C4—C5
1.517(2)
1.5005(19)
1.515(2)
1.490(2)
1.515(2)
1.519(2)
1.517(2)
1.544(1.8)
1.5276(1.8)
1.527(0.8)
1.533(2.9)
1.534(1.3)
1.554(2.4)
1.548(2.1)
Angle
Experimental
Theoretical
Angle
Experimental
Theoretical
Cl1—Cu1—Cl2
Cl1—Cu1—Cl2i
Cl1—Cu1—Cl3
90.887(13)
90.886(13)
167.25(3)
95.451(5.0)
70.723(22.2)
163.89(2.0)
Cl2i—Cu1—Cl2
Cl3—Cu1—Cl2i
Cl3—Cu1—Cl2
162.99(3)
90.996(13)
90.995(13)
153.48(5.8)
100.514(10.5)
95.676(5.1)
Angle
Experimental
Theoretical
Angle
Experimental
Theoretical
F2—B1—F1
F3—B1—F1
F3i—B1—F1
F3i—B1—F2
F3—B1—F2
F3—B1—F3i
108.7(2)
104.9(3.4)
110.29(0.5)
113.42(3.4)
104.93(3.7)
108.73(0.2)
113.7(2.8)
F4—B2—F4iii
F4iii—B2—F5
F4—B2—F5
F6—B2—F4
F6—B2—F5
F6—B2—F4iii
110.0(2)
112.3(2.1)
104.40(3.5)
105.59(2.4)
112.43(2.0)
105.8(3.7)
115.35(4.7)
109.74(14)
109.74(14)
108.98(14)
108.98(14)
110.6(2)
108.24(15)
108.24(15)
110.20(14)
109.9(2)
110.20(14)
Angle
Experimental
Theoretical
Angle
Experimental
Theoretical
C1ii—N1—C3
N1ii—C1—C2
N1—C2—C1
N1—C3—C4
C2—N1—C1ii
C2—N1—C3
C5—C4—C3
N2—C5—C4
108.25(12)
112.03(13)
110.18(13)
116.19(14)
108.81(12)
112.84(12)
112.61(14)
110.41(14)
113.49(4.8)
111.40(0.6)
110.04(0.1)
111.76(3.8)
108.30(0.5)
112.58(0.2)
111.59(0.9)
114.56(3.8)
C2—N1—C1
C1—N1—C3
C2—N1—C3
N1ii—C1—C2
N1—C2—C1
N1—C3—C4
C5—C4—C3
N2—C5—C4
108.81(12)
108.25(12)
112.84(12)
112.03(13)
110.18(13)
116.19(14)
112.61(14)
110.41(14)
110.58(1.6)
111.37(2.9)
111.36(1.3)
110.68(1.2)
110.30(0.1)
114.04(1.9)
116.12(3.1)
110.25(0.1)
Symmetry codes: (i) x. −y + 3/2. z; (ii) −x + 1. −y + 1. −z; (iii) x. −y + 1/2. z.
Fig. 2. Projection of the structure along the a axis (a). Organic molecules were omitted for clarity; Atomic arrangement of the organic and anionic part along the b axis (b).
4

S. Hadaoui, Z. Ouerghi, S. Elleuch et al.
Journal of Molecular Structure 1248 (2021) 131441
Fig. 3. Hirshfeld surface mapped with dnorm function for the title compound, showing hydrogen bonds with neighbouring molecules.
Table 3
Hydrogen bonding parameters (A.°) of (C10 H28N4)[CuCl4][BF4]2.
negligible contacts marking FꢁF (0.8%), and CuꢁH (0.2%) contacts.
Consequently, the contact analysis for this compound suggests
that the HꢁX (X = F, Cl) interactions are the driving forces in the
molecular arrangement and the crystal packing formation.
˚
D – H ··· A
D – H
H···A
D···A
D – H···A
N1 – H1 ··· Cl2
N1 – H1 ··· Cl3
N2 – H2B ··· F1
N2 – H2C ··· F4
C2 – H2D ··· F3
C4 – H4A ··· F3
C1 – H1A ··· F6
C3 – H3B ··· F2
C5 – H5B ··· Cl3
0.87(2)
0.87(2)
0.92(3)
0.90(3)
0.99
2.36(2)
2.78(2)
2.15(3)
2.11(3)
2.335
3.1131(14)
3.4667(14)
2.9330(18)
2.9733(19)
3.300
144.7(18)
136.9(17)
143(3)
162(3)
164
The enrichment ratios of contacts are regrouped in Table 4.
As expected, HꢁF and HꢁCl contacts are the driving forces in
the crystal, they appear with enrichment ratios larger than unity:
EHꢁCl= 1.64, EHꢁF = 1.53 establishing necessarily most of the sur-
face interactions as SH represents 54% in this set of molecules. The
FꢁCu contacts show a very high enrichment ratio EFꢁCu= 3.55,
thus, they are the stabilizing interactions in the crystal packing.
Furthermore, the structure is characterized by impoverished HꢁH
contacts with a value under the unity EHꢁH = 0.56, considering
that hydrogen is involved in other interactions such as HꢁF, HꢁCl,
and HꢁCu contacts. Moreover, the FꢁCl contacts reveal a signifi-
cant Under-representation enrichment ratio around 0.42 due to the
slightly favored SCu surface contacts indicating their low signifi-
cance in the crystal. The FꢁF and CuꢁH contacts are considered
negligible with a very low enrichment ratio and, therefore, are not
meaningful in the crystal formation.
0.99
2.345
3.313
165
0.99
2.543
3.447
151
0.99
2.442
3.348
151
0.99
2.61
3.583
164.7
Symmetry codes: (i) x. y. z + 1; (ii) −x + 3/2.−y + 1. z − 1/2.
tacts present in the title compound. The dnorm representation
shows a surface with a color scheme: red, blue and white (Fig. 3).
The red spots indicate the shortest intermolecular contacts N–
HꢁCl, and N–HꢁF, which represent the closest intermolecular
interactions in this compound. The 2D fingerprint plots provide
additional information on intermolecular interactions (Fig. 4). The
reciprocal FꢀꢀꢀH/HꢀꢀꢀF, ClꢀꢀꢀH/HꢀꢀꢀCl and HꢀꢀꢀH intermolecular
interactions are the most abundant contacts in the studied com-
pound with respective values: 44.4%, 30.6% and 16.4%, (Fig. 4(b),
(c), (d)). The XꢀꢀꢀH/HꢁX (X = F, Cl) intermolecular interactions
involved in the structure appear as distinct spikes in the 2D
fingerprint plot and occupy the most significant area of the total
Hirshfeld surfaces (75%), as shown in Fig. 4, indicating hydrogen
bonds in the structure, including HꢁX contacts (Table 3). The HꢁH
contacts are the third frequent interaction due to the abundance
of hydrogen atoms on the molecular surface. They appear as the
scattered points along with a peak appeared in the middle of the
region of the 2D fingerprint plot. The positions of the peak are
3.3. IR vibrational analysis
The IR spectroscopy was often used in order to present a
complementary characterization technique and to verify the func-
tional groups present in the crystal and investigate their vibra-
tional behavior in the solid state. Both experimental and theoret-
ical IR spectra of (C10H28N4)[CuCl4][BF4]2 are gathered in Fig. 5.
The wavenumbers and assignments of the observed and computed
IR vibrational modes are depicted in Table 5.
At high frequencies, the IR spectrum exhibits wide absorption
bands bringing to the NH3, CH2 and NH symmetric and asym-
metric stretch modes. The use of theoretical calculation helped
us to accurately identify the related wavenumbers of each vibra-
tional mode (Table 5). The corresponding deformations modes of
these groups are observed at lower frequency range, and they
are also overlapped and manifest relatively broad bands. Indeed,
we have assigned the δas(NH3), δ(CH2), ω(CH2) and ρ(CH2) de-
formation modes to the peaks located at 1595, 1475, 1380 and
˚
at de = di slightly higher than 1.1 A. The ClꢁF contacts appear
as two sharp symmetric spikes in the 2D fingerprint maps with
˚
prominent spikes at de + di = 3.5 A with a contribution of
3.9% of the total Hirshfeld surface (Fig. 4(e)). The CuꢁF contacts
˚
appear as two sharp symmetric spikes at de + di= 3.4 A with a
contribution of 3.7% of the total surface. Further, we can notice
5

S. Hadaoui, Z. Ouerghi, S. Elleuch et al.
Journal of Molecular Structure 1248 (2021) 131441
Table 4
Actual contact surfaces. derived random contacts and enrichment ratios of (C10 H28N4)[CuCl4][BF4]2.
Actual Contacts(C%)
Random Contacts(R%)
Enrichment (E)
F···H/H···F
Cl···H/H···Cl
H···H
44.4
30.6
16.4
3.9
28.94
18.63
29.16
9.24
1.04
7.18
2.1
1.53
1.64
0.56
0.42
3.55
0.11
0.09
Cl···F/F···Cl
F···Cu/Cu···F
F···F
3.7
0.8
Cu···H/H···Cu
0.2
Surface (S%)
SH = 54
SF = 26.8
SCl = 17.25
SCu = 1.95
Fig. 4. The 2D fingerprint plots and the contribution of real contacts to the Hirshfeld surface of (C10 H28N4)[CuCl4][BF4]2.
752 cm−1, respectively. Moreover, the characteristic vibrations of
BF4−are present in the IR spectrum often as shoulders due to
their lower intensities. Their assignments are done based mainly
on theoretical calculation results and on reported vibrational stud-
ies of tetrafluoroborate containing materials [35-38]. We therefore
attributed the shoulders detected at 1295, 903, 895, 822 and 521
−
cm−1 to stretching and deformation modes of the BF4 ion. It is
worthily noting that the IR absorption bands relating to [CuCl4]2−
vibrational modes are not observed experimentally since they oc-
cur at low frequency regions (below 300 cm−1). Finally, the bands
observed in the range of 1700–2500 cm−1 are attributed to the
harmonic combination bands.
3.4. Optical properties
The room temperature experimental UV-visible optical spec-
trum of (C10H28N4)[CuCl4][BF4]2 is superposed with the TDDFT
generated theoretical spectrum in Fig. 6. Here, since the used
organic cation do not exhibit any conjugation of double bonds
and the BF4 groups also does not presents any absorption char-
acteristics in this region [38-43], so we do not expect to ob-
Fig. 5. Experimental and theoretical IR spectra of (C10 H28N4)[CuCl4][BF4]2.
6

S. Hadaoui, Z. Ouerghi, S. Elleuch et al.
Journal of Molecular Structure 1248 (2021) 131441
Table 5
Assignments of the experimental and theoretical IR absorption bands.
Experimental
Theoretical
Assignment
Experimental
Theoretical
Assignment
t(CH2)
3595 w
3525 w
3386 vw
3362 vw
νa(NH3)
1329 vw
1318 vw
1279 vw
1246 vw
1234 vw
3435 m
3235 vs
3315 vw
νs (NH3)
νa(CH2)
3077 vs
3060 m
1295 vw.sh
1190 s
νa(BF4)
1156 m
3044 w
νa(NH3ꢁCl)
1137 vw. sh
1113 vw
1059 vw
1027 vw
998 vw
ρ (NH3)
ν(CC)
3021 s
ν(NH)
3000 vs
2991 vw
2967 vw
2861 s
νs(CH2)
1040 vs
2845 m
1595 w
νa(NHꢁF)
δa(NH3)
ρ(NH3)
ν(CN)
1672 vw
1656 vw. sh
1644 vw. sh
973 w
966 w
903 m.sh
895 w.sh
822 w.sh
752 w
912w
νa(BF4)
ρ(CH2)
1568 w.sh
1562 vw
1507 vw
δs(NH3)
δ(NH)
873 m
851 s
794 vw
750 vw
1475 s
1474 vw.sh
1460 vw
δ(CH2)
672 vw
580 vw
νs(BF4)
δ (CCN)
δ(BF4)
1407 vw
1368 vw
598 vw. sh
521 vw
1380 w
ω(CH2)
477 vw
460 vw
ν: stretching. δ: deformation. ω: wagging. t: twisting. ρ; rocking.
vw: very weak. w: weak. m: medium. s: strong. vs: very strong.
Fig. 7. In inset Tauc plot and energy gap determination of the compound.
Fig. 6. Experimental and theoretical UV–Vis spectrum of (C10 H28N4)[CuCl4][BF4]2.
serve organic or BF4 characteristic features, but we will assign
the observed bands to electronic transitions occurring within the
[CuCl4]−2 square planar groups. In effect, such kind of materials al-
ways exhibit absorption features in the UV region related to charge
transfer transitions and characteristic of Jahn–Teller distortion [44].
Moreover, they also display weak structured features below the
band gap energy associated to ligand-field transitions [44-49].
For (C10H28N4)[CuCl4][BF4]2, the exhibited UV-Visible features
are fortunately typical of copper chlorides complexes [CuCl4]−2 in
square planar coordination [44-49]. As it can be seen in Fig. 6,
there are three well resolved absorption bands in the UV region, at
274, 321 and 364 nm, assigned to the well-known ligand-to-metal
charge transfer “LMCT” transitions. The LMCT transitions arise from
promoting an electron from the predominantly π bonding orbitals
on the Cl ligand to one of the d molecular orbitals of the copper
metal and are much more intense [45]. Moreover, we detected a
very broad and notably intense absorption band above 400 nm re-
lated to the dd electronic transitions within the Cu atoms.
For more convenience, the TDDFT calculation provided a useful
tool to learn more about the observed UV-Visible optical electronic
transitions [44]. In this study, the theoretical absorption spectrum
provides four relatively intense bands in the UV region at 238,
7

S. Hadaoui, Z. Ouerghi, S. Elleuch et al.
Journal of Molecular Structure 1248 (2021) 131441
Fig. 8. Orbital frontier representation and energy of some molecular orbitals calculated using TDDFT theory.
Table 6
Experimental and TDDFT calculated electronic transitions. Oscillator strength and contribution with assignments according to orbital
frontier representation.
Experimental
Theoretical
Wavelength (nm)
Wavelength (nm)
Oscillator strength
Nature
Assignment
737
681
736.37
698.10
656.06
606.42
515.82
437.31
384.52
325.02
277.97
238.90
0.0193
0.0158
0.0146
0.0134
0.0467
0.0890
0.4226
0.9129
0.0775
0.3609
HOMO → LUMO (92%)
dd
dd
dd
dd
dd
dd
CT
CT
CT
CT
HOMO-1 → LUMO(89%).
HOMO-2 → LUMO(95%).
570
HOMO-1 → LUMO+1(87%)
HOMO-1 → LUMO+1(89%)
428
365
321
299
276
HOMO-2 → LUMO+1(87%)
HOMO-5 → LUMO (49%) HOMO-3 → L+1(40%)
HOMO-5 → LUMO+1(47%) HOMO-8 → LUMO(45%)
HOMO-8 → LUMO+1 (52%) HOMO-10 → LUMO(43%)
HOMO-10 → LUMO+1 (83%)
277, 325 and 384 nm. Besides, there are also six electronic tran-
sitions at 437, 515, 606, 656, 698 and 736 nm. Although the com-
puted intensities don’t much well with those detected experimen-
tally, the position and wavenumbers of the bands can be consid-
ered in agreement. In order to get more information about the
computed electronic transitions origin, we performed a more de-
tailed analysis based on the frontiers orbitals generations (Fig. 8)
and we awarded a bands assignment (Table 6). Here, we firstly
worthily authenticate the above-mentioned hypothesis of the non-
implication of both the tetrafluoroborate ion and the organic cation
in the optical absorption transitions. Besides, the six computed fea-
tures at low energetic level are found to be due to electronic tran-
sitions from the three highest HOMOs levels toward the two lowest
LUMOs levels (Table 6). From Fig. 8, these molecular orbitals ex-
hibit electronic densities principally located on the metal Cu atom
(Cu_3d). Consequently, these features included electron transitions
of the internal crystal field of d levels of Cu, as proposed in the ex-
perimental analysis. On the other hand, the calculated four features
in the UV region correspond to electronic transitions from deeper
HOMOs levels toward the LUMO and LUMO+1 molecular orbitals.
8

S. Hadaoui, Z. Ouerghi, S. Elleuch et al.
Journal of Molecular Structure 1248 (2021) 131441
Fig. 10. TG-DTA curve of (C10 H28N4)[CuCl4][BF4]2.
retical value (12.52%). Beyond 310 °C, a large endothermic peak is
observed on the DTA curve, the maximum at 335 °C, is accompa-
nied by a significant mass loss on the TG curve corresponding to
the decomposition of the organic part of this hybrid. A black de-
posit of carbon was obtained at the end of the experience.
Fig. 9. Solid state excitation and emission spectra of the compound.
These HOMOs orbitals are all Cl_3p in character, and then we con-
firm well the occurrence of the ligand to metal charge transfer
(LMCT) in the UV region.
4. Conclusion
Another important inspection is the energy of the gap of our
materials deduced using the expression of the Tauc’s method
[50] and by extrapolation of the linear part of the curve
with the energy axis (inset of Fig. 7(b)). It is obvious that
(C10H28N4)[CuCl4][BF4]2 exhibits a gap energy of 3.305 eV. This
value is however a little larger than those found for homologous
CuCl based materials (2.48 eV for (CH3NH3)2CuCl4 [44] and 2.45
for [NH3(CH2)4NH3]CuCl4 [46]), it is consistent with the theoret-
ically calculated (gap = 3.29 eV). It is worthily noted that B3LYP
underestimates HOMO-LUMO gaps and excited-state energies but
range-separated functionals are a way to correct for this deficiency
[51-54].
Herein, we have reported the synthesis and physico-chemical
characterization of a new compound (C10H28N4)[CuCl4][BF4]2. On
the structural level, it can be described by the isolated [BF4]2−
tetrahedra and [CuCl4]2- entity form layers parallel to the (a,c)
plane at y = ¼(2n + 1). Organic cations are inserted between
those layers and interact with them through hydrogen bonds with
the chloride and fluoride anions. The Hirshfeld surface analysis re-
vealed the percentage of intermolecular contacts of the title com-
pound. It showed the important interactions existing within the
crystal structure that led to the molecular packing. The infrared
spectrum showed the characteristic absorption band of different
functional groups present in the compound. The optical properties
were examined by optical absorption and photoluminescence mea-
surements. We detect dd and LMCT electronic transitions and light
emission typical of CuCl based materials. The gap energy deducted
by the Tauc method was found to be 3.3 eV. DFT and TDDFT calcu-
lations provided great aid in the interpretation of the spectroscopic
results and good agreement between experiment and theoretical
findings is worthily noticed. The thermal analysis revealed the sta-
bility up to 220 °C of the titled compound.
The emission and excitation spectra of (C10H28N4)[CuCl4][BF4]2
are shown in Fig. 9. It reveals a broad luminescence band in the
blue region generated from three well resolved peaks at 384, 411
and 437 nm. Since the organic molecule used is transparent in the
visible range, this emission is probably due to the electronic tran-
sition in the [CuCl4]2− square plane entities. Indeed, the recombi-
nation of an excited electron and a hole can lead to a blue emis-
sion at two very close wavelengths (411 and 437 nm). This agrees
well with the literature values for the hybrid compound based on
Cu (II) (C8NH6CH2CH2NH3)2[CuCl4] [55]. Moreover, the excitation
spectrum, which represents the variation of the intensity of the
emission band at 384 nm as a function of excitation wavelengths,
exhibits one band with a maximum at 265 nm. This proves well
the selected region of the excitation wavelength. All these optical
properties leave this material a good element for applications in
optoelectronics.
Author statement
Souhir Hadaoui and Zeineb Ouerghi - They wrote the whole ar-
ticle
Slim Elleuch - DFT- Gaussian 09 computation
Riadh Kefi - He is the Research Supervisor and he gave the plan
for this Paper.
3.5. Thermal analysis
Supplementary data
The simultaneous TG-DTA analysis of (C10H28N4)[CuCl4][BF4]2
(Fig. 10) was carried out in argon at a heating rate of 5 °C.min−1on
a sample of 11.8 mg. The DTA curve shows a first endothermic peak
at 220 °C relatives to the departure of two HF molecules, arising
from an intermolecular elimination reaction. This phenomenon is
confirmed by the TG curve which shows an experimental mass loss
of 6.45% compared to the percentage of two molecules of fluoric
acid in this material (6.8%). The little endothermic peak at 290 °C
accompanied by a second mass loss can be attributed to the depar-
ture of two HCl with a mass loss of 12.15% very close to the theo-
Crystallographic data of the structure have been deposited in
the Cambridge Crystallographic data center CCDC 2048573. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/data_
request/cif.
Declaration of Competing Interest
The aim of this study is the investigation of the physico-
chemical properties of the new synthesized compound
9

S. Hadaoui, Z. Ouerghi, S. Elleuch et al.
Journal of Molecular Structure 1248 (2021) 131441
(C10H28N4)[BF4]2[CuCl4]. We report in this manuscript the chem-
ical preparation, crystal structure discussion, thermal and optical
properties of the compound. We have also examined its inter-
molecular interactions using Hirshfeld surfaces analysis. Further-
more, we have undertaken a DFT calculation using the Gaussian
09 program to determine and confirm the structural parameters,
the vibrational infrared spectrum bands and provides the vertical
excitations transitions.
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Acknowledgements
This work was supported by the Tunisian National Ministry of
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