# A novel 1-D square-pyramidal coordinated palladium (II) hybrid compounds [C9H16N2]PdX4 (X=Cl, Br) showing broadband emission, electrical properties and narrow optical band gap

H. Bouznif, F. Hajlaoui, K. Karoui, Nathalie Audebrand, Marie Cordier, Thierry Roisnel, N. Zouari

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H. Bouznif, F. Hajlaoui, K. Karoui, Nathalie Audebrand, Marie Cordier, et al.. A novel 1-D square-pyramidal coordinated palladium (II) hybrid compounds [C9H16N2]PdX4 (X=Cl, Br) showing broadband emission, electrical properties and narrow optical band gap. Journal of Solid State Chemistry, 2022, 311, pp.123149. 10.1016/j.jssc.2022.123149. hal-03691373

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A novel 1-D square-pyramidal coordinated palladium (II) hybrid compounds [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdX<sub>4</sub> (X=Cl, Br) showing broadband emission, electrical properties and narrow optical band gap

Hajer Bouznif a*, Fadhel Hajlaoui a, Karim Karoui b, Nathalie Audebrand c, Marie Cordier c,
Thierry Roisnel c, Nabil Zouari a

<sup>a</sup> Laboratoire Physico-chimie de l'Etat Solide, Département de Chimie, Faculté des Sciences de Sfax, B.P. 1171, 3000 Sfax, Université de Sfax, Tunisia

<sup>b</sup> Laboratoire des caractérisations spectroscopiques et optique des matériaux, Faculté des Sciences de Sfax, B.P. 1171, 3000 Sfax, Université de Sfax, Tunisia

<sup>c</sup>Univ Rennes, INSA Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) - UMR 6226, F-35000 Rennes, France

*Corresponding Authors' E-mail: fadhelh83@yahoo.fr

**Abstract**

Hybrid halide perovskite-like semiconductors are attractive materials for various optoelectronic applications, from photovoltaics to smart devices. Here, two new one-dimensional palladium halides, [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdCl<sub>4</sub> (1) and [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdBr<sub>4</sub> (2) showing electrical and optical properties were prepared by reaction of 2,4,6-trimethyl-m-phenylenediamine with PdX<sub>2</sub> (X = Cl, Br) in the corresponding concentrated halogen acids HX. Single-crystal X-ray diffraction analysis reveals that compounds [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdX<sub>4</sub> (X=Cl, Br) belong to isostructural phases and crystallize in the orthorhombic system with non-centrosymmetric space group *Pmc*2<sub>1</sub>. The basic unit of the crystal structure in each compound is composed of an independent [PdX<sub>4</sub>]<sup>2-</sup> anion charge balanced by a [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]<sup>2+</sup> cation. Remarkably, the central Pd(II) atom is coordinated by five halogen atoms PdX<sub>5</sub> forming a distorted square-pyramidal coordination geometry. These hybrid compounds present good thermal stability up to 500 K. The DSC and electric measurements show that no phase transition occurs in the compounds over the temperature range 300–400 K. Optical absorption measurements suggest that the two hybrids Pd(II)-based metal halides have a narrow optical band gap (Eg) of ~3.26 eV (X=Cl) and ~2.98 eV (X=Br) which makes them promising
materials in optoelectronic devices. Besides, compounds exhibit broadband green emission excited by UV light at room temperature. The electrical study shows the contribution of the grain and grain boundary in the conduction of these materials and confirms the optical result where the conductivity of the compound (1) is higher than the compound (2). The variation of the conductivity of grain confirms the activation energies calculated for the two regions.

**Keywords:** Pd(II)-based metal halides, Crystal structure, Semiconductor properties, Optoelectronic devices.

**Introduction**

Research on organic-inorganic hybrid materials with perovskite or perovskite-related crystal structures has become one of the most essential and popular study subjects due to its outstanding capabilities that integrate both the properties of organic and inorganic molecules within a single molecule [1-6]. The organic moiety supplies useful properties such as plastic mechanical properties, a lot of polarizabilities, structural variation, processability and fluorescence, whereas the inorganic moiety offers potential advantages such as good electrical mobility, band gap tunability, mechanical, thermal stability and interesting magnetic or dielectric transitions [7-11]. In the reported organic-inorganic hybrid materials, most are constructed with organic cations and inorganic semiconducting metal-organic salts of Ge(II) [12], Sb(III) [13, 14], Cu(II) [15], Pb(II) [16], Bi(III) [17-19] and Sn(II) [20, 21] halides anions. For instance, the material based lead iodide methylamine hybrid perovskite compound (CH3NH3)PbI3 is particularly attractive due to its unique structure and excellent optoelectronic properties [22-24]. The toxicity issue of lead precludes large-scale photovoltaic field application of lead-iodide perovskites. To address this toxicity issue, intensive research has been devoted to developing lead-free hybrid metal halide materials suitable by substituting lead by low-toxic metals [4, 25-30]. Meanwhile, transition metal halides have been studied for less hazardous perovskite-like structures in recent years [31, 32]. In fact, the layered hybrid materials are being studied in depth due to their intriguing structural phase transitions and semiconducting properties [33]. In that case, Pd(II)-based metal halides can be used to generate non-toxic hybrid materials. It will be fascinating to see if Pd(II)-based hybrid compounds with additional X-site anions, such as Br, Cl or I, can be synthesized [34]. In the same way, Pd(II) complexes and their derivatives received considerable attention due to their interesting optical property and potential application in the electronic devices [35, 36]. Moreover, compared with Pb(II), Pd(II)-based halides are more friendly to the environment
and then are more suitable for the practical application [37]. In fact, the Pd(II) hybrid
materials constitute a potential class of materials with atypical structural archetypes, which
are well-known to range from three dimensions 3-D to zero dimension 0-D [35-39]. Thus, the
introduction of organic moieties with a flexible structure has been conceived as an effective
strategy to construct new molecular phase transition systems with semi-conducting properties
[39-41]. Given the diversity of structural networks and properties attainable with
palladium(II) halides, we cite here the layered halide hybrid compounds (CH3NH3)2PdX4 (X
=Cl, Br, I) [34, 37], which demonstrates promising semiconductor character.

Although the structural diversity has been recognized through the regulation of organic
templates, rare corner-shared octahedra 1-D perovskite chain was structurally and optically
characterized [42]. In contrast, the corner-shared octahedral chain is apt to feature more
flexibility, softer crystal lattice and higher distortion level, which will promote the formation
of semiconducting chains of metal-halogen [43]. In addition, the majority of perovskite-like
1-D palladium halides are composed of PdX6 octahedra and limited hybrid materials are built
from the edge or face sharing of five-coordinated PdX5 units with square-pyramidal
configuration. These research insufficiencies bring about new structural constructing
opportunities to pursue new-type of 1-D semiconductor materials.

In order to investigate this type of materials with optical, photoluminescence (PL) and
electrical properties, we propose a strategy to combine 2,4,6-trimethyl-m-phenyl-enediamine
with palladium halides.

In this study, we report the synthesis, crystal structures, optical, photoluminescent and
electrical properties of two new 1-D hybrid perovskite-like namely, [C9H16N2]PdCl4 (1) and
[C9H16N2]PdBr4 (2). X-ray crystallography studies suggest that the presence of discrete
molecular building blocks, organic [C9H16N2]2+ cations, and inorganic square [PdX4]2- leads to
one-dimensional (1-D) crystal structures of [C9H16N2]PdX4 results in light emission. Under
the excitation of UV light (266 nm), the bulk crystals of compounds exhibit broadband green
emission, which covers the range from 400-800 nm peaked at 505 nm and 526 nm for (1) and
(2), respectively. Notably, compounds (1) and (2) showed semiconductor characteristics with
bandgaps (Eg) of 3.26 eV for (1) and 2.98 eV for (2). Such finding highlights an avenue to
design novel multifunctional perovskite-like materials.
# Experimental section

## Raw Materials.

Pd(II) chloride ($PdCl_2$), Pd(II) bromide ($PdBr_2$), hydrochloric acid HCl (37%), hydrobromic acid HBr (48%), 2,4,6-trimethyl-m-phenyl-enediamine were purchased from Sigma-Aldrich and used without further purification.

## Synthesis.

$[C_9H_{16}N_2]PdCl_4$ (1): to a solution of $PdCl_2$ (1 mmol) in 10mL $H_2O$ was added a solution of 2,4,6-trimethyl-m-phenyl-enediamine (2 mmol) in 10 mL of aqueous solution HCl (1M). The mixed solutions were stirred to give a clear red solution, which was allowed to evaporate slowly on a hot platform at about 50 °C. Stick red crystals were formed in one day, which were isolated by filtration and dried in vacuo.

The purity of compound (1) was confirmed by an elemental microanalysis: Anal. (Calcd): C, 26.92% (27.1%); H, 3.92% (3.51%); N, 6.81% (7.02%). Yield: 81.7% based on Pd.

$[C_9H_{16}N_2]PdX_4$ (2): according to the similar procedure to synthesis of compound (1) based on materials $PdBr_2$ (1 mmol) and 2,4,6-trimethyl-m-phenyl-enediamine (2 mmol) in 10 mL concentrated HBr (1M) led to red prism crystals.

The purity of compound (2) was confirmed by an elemental microanalysis: Anal. (Calcd): C, 19.09% (18.74%); H, 2.95% (2.42%); N, 5.12% (4.86%). Yield: 83.2% based on Pd.

## Infrared Spectroscopy.
Infrared measurements were obtained using a PerkinElmer FT-IR Spectrum 1000 spectrophotometer. Samples were diluted with spectroscopic grade KBr and pressed in to a pellet. Scans were run over the range of 400-4000 cm⁻¹. As shown in Figures S1-S2, the IR spectra of two compounds are similar, in which the medium peaks at 3400/3425 cm⁻¹ and 3200/3195 cm⁻¹ are ascribed to the N-H and methyl C-H stretching vibrations, and the strong absorptions at 1561/1547 cm⁻¹ are attributed to v(-C=C) in aromatic ring.

## Single Crystal X-ray Diffraction.
Single-crystal X-ray diffraction data of $[C_9H_{16}N_2]PdCl_4$ (1) were collected at 150 K on an APEX-II Bruker AXS diffractometer and processed with the APEX 2 program suite [44]. For the compound $[C_9H_{16}N_2]PdBr_4$ (2), the data were performed at 150 K on an D8 VENTURE Bruker AXS diffractometer and processed with the APEX 3 program suite [45]. The used X-ray wavelength was the Mo-Kα radiation ($\lambda$=0.71073 Å). Frames integration and data reduction were carried out with the program SAINT [46]. The program SADABS [47] was employed for multiscan-type absorption corrections. Structure was determined by dual space methods using the SHELXT program [48] and refined with full-matrix least-square methods based on F² (SHELXL-2014) [49].
through the WinGX program suite [50]. All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. The software Diamond [51] was used to create the graphic representations of the crystal structures. Detailed information of the crystallographic data and structures refinements are presented in Table 1. The relevant selected bond lengths and angles are listed in Tables S1-S4.

### Thermal measurements.

Thermogravimetric Analysis (TGA) of [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdCl<sub>4</sub> (1) and [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdBr<sub>4</sub> (2) were performed on raw powders with a TGA ‘SETSYS Evolution’ under nitrogen atmosphere. The thermograms were collected on of mass m = 5mg and 17mg sample for (1) and (2), respectively, in the temperature range between 298 and 773 K (heating rate of 5°C.min<sup>-1</sup>).

Differential Scanning Calorimetry (DSC) measurements were recorded on raw powders with NETZCSCH DSC 200 F3 instrument (Pt crucibles, Al<sub>2</sub>O<sub>3</sub> as a reference) under nitrogen atmosphere. The DSC measurements of compounds:(1) and(2) were carried out in the temperature range between 273K and 623K with heating rate of 5°C.min<sup>-1</sup> (m = 5,2 mg).

### Optical Absorption Spectroscopy.

The UV-visible spectra for compounds [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdCl<sub>4</sub> (1) and [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdBr<sub>4</sub> (2) were measured on a conventional UV visible absorption spectrometer Shimadzu UV-3101PC at room temperature in the range of 200–800 nm. The band gap energy was calculated according to the linear fit equation and by Tauc plot [52].

$$ (\alpha h\nu)^2 = B (h\nu - E_g) \quad (1) $$

where B is a constant, E<sub>g</sub> is the optical band gap and α is the optical absorption coefficient deduced from the absorbance using the relation:

$$ \alpha = \frac{2.303 A}{d} \quad (2) $$

where A is the absorption and d the thickness of the samples.

The photoluminescence (PL) spectra were performed at room temperature on a Perkin Elmer MPF-44B spectrometer using a Xenon light source with an excitation wavelength of 266 nm. The spot size and the excitation power of the xenon lamp were 5 mm in diameter and 0.1 mW, respectively. During measurements, samples were placed in a vacuum cell and the PL measurements were performed at low pressure (10<sup>-3</sup> torr) in order to avoid any modification of the emission band by the photo-oxidation of the surface resulting from the Xenon lamp heating.
## Electric Properties and Impedance Spectroscopy.

The electrical measurements of the real and imaginary components of the impedance parameters (Z' and Z") were measured on pellet disks of about 8 mm in diameter and a thickness of approximately 1.3 mm in the frequency range of 1–106 Hz with a SOLARTRON SI 1260 impedance equipment coupled to a dielectric interface in the temperature range of 300–400 K. A circular copper electrode with a diameter of 8 mm was deposited on the surfaces of the pellet.

## Results and discussion

### Crystal structure.

In order to investigate the details of the structures of [C₉H₁₆N₂]PdCl₄ (1) and [C₉H₁₆N₂]PdBr₄ (2) materials, single-crystal X-ray diffraction data were collected at 150 K. The analyses reveal that compounds (1) and (2) are isostructural and crystallize in the orthorhombic space group Pmc2₁ (No.26) (Table 1). Each asymmetric unit consists of discrete [PdX₃]⁻ (X=Cl or Br) anion and half of 2,4,6-trimethyl-m-phenyl-enediammonium [C₉H₁₆N₂]²⁺ cation (Figure 1). The structures of (1) and (2) contain a 1-D zigzag [PdX₄]²⁻ chain along the c axis charge balanced by a [C₉H₁₆N₂]²⁺ cations. As shown in Figures 2 and 3, one crystallographically independent Pd atom, located in a special position, is coordinated by five halogen (X= Cl or Br) atoms to form a [PdX₅] square pyramid geometry, which can be considered a corner-sharing pentahedron. The [PdX₅] square pyramid shares two neighboring planar halogen atoms according to reverse arrangement, which results in the zigzag [PdX₄]²⁻ chain. Meanwhile, four of the halide ligands constitute the basal plane [Cl1, Cl2, Cl3, Cl2⁽²⁾] for (1) and [Br1, Br2, Br3, Br⁽²⁾] for (2), and one Cl3⁽vi⁾ ion for (1) and one Br1⁽vi⁾ ion for (2) are in the apical position (Figure 3). The four short basal Pd–X bonds lengths range between 2.2956 (18) and 2.3132 (9) Å for (1) and between 2.4321 (16) and 2.4463 (7) Å for (2). When the chlorine atoms are replaced by bromine atoms, the position of the axial ion is slightly changed from 3.248(2) to 3.374(1) Å. Thus, the distances between Pd1 and Cl3⁽vi⁾, Pd1 and Br1⁽vi⁾ are too long to form the coordination bond, which belongs to intermolecular interactions.

To evaluate the distortion level of 1-D zigzag [PdX₄]²⁻ chains, we calculate the distortion of [PdX₅] square pyramid arising from the variation of Pd–X bond length by the following formula:

$$ \Delta d = \frac{1}{5} \sum \left[ \frac{(dn-d)}{d} \right]^2 $$
in which *dn* is the individual Pd-X bond length and *d* represents the average bond length.

The calculated *Δd* parameters for PdCl₅ and PdBr₅ are 0.022 and 0.020, respectively. Therefore, the coordination geometry around the Pd(II) ion can be described as a distorted polyhedron (Figure 3 (c) and (d)). The distortion level from bond length is higher than that of 1-D (2,2'-(ethylenedioxy)bis(ethylammonium))PbBr₄ (*Δd* = 2.32×10⁻³) indicating the nature of its potential broadband light emission [53]. The organic [C₉H₁₆N₂]²⁺ cations fill the space to separate the parallel 1-D [PdX₄]²⁻ chains with N-H···X hydrogen bonding interactions between them. Within the protonated [C₉H₁₆N₂]²⁺ cation, the bond lengths of C–C and C–N ring are in the range from 1.379(6) to 1.518(10)Å and between 1.385(10)Å and 1.514(10)Å in (1) and (2). Meanwhile, the C–C–C and N–C–C angles are in the range of 116.3(7)–123.1(5)° and 116.0(9)–123.1(7)°, respectively. All bond lengths and bond angles fall within the normal ranges. The 2,4,6-trimethyl-m-phenyl-enediammonium ion size and shape of the cation skeleton structure play an important role in rendering 1-D zigzag chains of compounds [C₉H₁₆N₂]PdX₄ (X=Cl, Br) with interesting structural and optoelectronic properties.

The crystal structures of (1) and (2) are stabilized by N-H···X hydrogen-bonding interactions. Within the intermolecular bonds, the distances of N-H···Cl interactions fall in the range of 3.182(4)–3.351(4)Å, and the N-H···Cl bond angles range from 116 to 161° in (1). Similarly, the N-H···Br interactions are in the range of 3.349(7)–3.575(8)Å, and the N-H···Br bond angles range between 113 to 164° in (2). The N-H···Cl distances and the bond angles in (1) were slightly changed comparing with those observed in (2), by replacing the chlorine by bromine atoms. The hydrogen bond parameters are given in Tables S3-S4.

## Thermal analysis and stabilities.

TGA and DSC measurements were carried out to highlight the phase transitions and the thermal stability of the [C₉H₁₆N₂]PdX₄ (X = Cl (1), Br (2)) compounds.

[C₉H₁₆N₂]PdCl₄(1). TGA-thermogram shows that the compound seems to be stable up to 500 K. Above this temperature, a gradual weight loss appears at 520 K and that is considered like a limit of thermal stability of the compound (Figure 4(a)). The weight loss can be attributed to the decomposition of the organic part together with the chloride is removed to support the charge neutrality and corresponds to the formation of PdCl₂ (observed weight loss, 56%, theoretical, 55.5%). This loss of mass is manifested on the DSC curve by the appearance of broad endothermic peaks around 500 K. A slight mass gain of 1.6%, between 700 and 780K, is attributed to the formation of PdO (ICDD 41–1107) [54]. A similar decomposition has been observed in other hybrid-halide compounds such as (CH₃NH₃)₂PdX₄ (X=Cl, Br) [35-37]. The
DSC measurements show an endothermic peak located at 480K. For this compound, no phase
transition can be identified (Figure S3).

[C9H16N2]PdBr4(2). The TG thermogram shows that the compound (2) seems to be stable up to 520 K 5 (Figure 4(b)). Above this temperature, a gradual weight loss appears that is considered like a limit of thermal stability of the compound (observed weight loss, 53%, theoretical, 53.8%) which can be assigned to the degradation of the organic group and the departure of two bromide atoms, leading most likely to the formation of PdBr2. This decomposition process is related on the DSC curve (Figure S3) by an intense endothermic peak around 500K. A progressive mass loss of 1.5% took place over the range 670–780K, affording PdO (ICDD 41–1107) as residue [54].

**Optical and Photoluminescence Properties.**

Figures 5 a and b shows the absorbance spectra of (1) and (2) compounds at room
temperature. The deconvolution of these spectra based in the Lorentzian function showed five
peaks at 251, 318, 366, 492 and 665nm for (1) and at 255, 323, 357, 487 and 643nm for the
(2) materials. The last peak at higher wavelength associated to the red color of the two
compounds (1) and (2). Based in the literature [55], in the compounds which contains the
transition metals, such as the palladium, d orbitals have an important role in the spectroscopic
properties which is depend of the several coordination geometries (Figure S4). In the linear
molecule, there are three levels for molecular orbital where two degenerated pairs. For an
octahedron, there are two sets of different energy molecular orbitals: the three t2g orbitals (dxу,
dyz, dxуy) and the two orbital eg (dx2-y2, dz2) (Fig S4????)

For a complex of plane-square structure we find four levels molecular orbital. Except the dxz
and dyz orbital are degenerate. For this last geometry we can obtain four transitions: a
transition of dxуto dx2-y2 which is the lower energy absorption transition as observed in the
experimental absorption spectrum (Figure 5) at 492 and 487 nm for (1) and (2) respectively.
The higher energy HOMO-LUMO transition is a d-d metal-centered transition type involving
the promotion of an electron in the dyz orbital to the dx2-y2 orbital. This transition appears at
251 and 255nm for (1) and (2), respectively. The two other transitions between the orbitals
dxz---dx2-y2, dz2---dx2-y2 are detected in the experimental spectrum at 318 and 366nm for (1),
323 and 357nm for (2). The peaks at higher energy are used to determine the direct band gap
energy using Tauc's low [52].
The extrapolation of the line that intersects the abscissa axis gives a value of gap energy Eg equal to 3.26 eV for compound (1) and to 2.98eV for the compound (2) (Figures 6 a and b). These values indicate the character semi-conductor of these compounds.

Many scientific results indicate that structural deformation has an important effect on the band gaps of hybrid organic-inorganic perovskites and the band gap is directly related to the optical properties of them [55, 56]. Thus, the photoluminescence (PL) spectra of [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdX<sub>4</sub> (X=Cl, Br) are measured and shown in Figure 7. The emission wavelength is 505 nm and 526 nm for compounds (1) and (2), respectively, suggesting the broadband green light emissions [57]. Meanwhile, the full width at half-maximum (FWHM) of PL spectra is decreased by simple halogen substitution from Cl to Br, which is attributed to the electronic band gap fluctuations derived from the structural distortion in Cl are larger than that in the Br. Therefore, the spread in square-pyramidal distortions is predicted to strongly broadening emission spectra of Cl [58]. The light emission properties of [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdX<sub>4</sub> (X=Cl, Br) are easily tuned with halogen-substituted from Cl to Br. The Commission International de l'Eclairage (CIE) color coordinates are (0.32; 0.5) and (0.35; 0.46) for Cl and Br, respectively, which indicates that simple halogen substitution can provide an effective way to prepare pure green-light-Emitting semiconductor devices (Figure S5).

## Electrical properties and equivalent circuit

Figures 8 a and b show the Nyquist representations (-Z'' vs. Z') for (1) and (2) samples used to separate between all contributions in the conduction mechanism such as the grains and the grain boundaries. These spectra of the two samples are characterized by non-Debye behavior, which is manifested by depressed semi-circle in Nyquist representations. Two arcs are observed in these spectra corresponding to the grain at higher frequencies and to the grain boundaries at lower frequencies. The arc of the grain decreases with temperature rise, which confirms that the conduction mechanism is thermally activated for all measurement temperatures, and indicates the semi-conducting behavior of these compounds [59].

The intercept of the impedance arc by Z' axis show that the resistivity of compound (2) is lower (≈10<sup>6</sup>Ω) than that of compound (1) (≈10<sup>7</sup>Ω). This result indicates the conductivity of the Br based compound is higher and confirms the low gap energy obtained in this compound which proved the semi-conductor character.

The models of the equivalent circuits for the two samples, made by two cells each formed by a parallel combination of a resistance (R) with a constant phase element (CPE), confirm the presence of the grain and grain boundaries effect. The good agreement between the theoretical
and experimental curves indicates that these circuits described well the behavior of these samples. This simulation allows determining the resistance of the grain in order to calculate their conductivities for all temperature based on the equation following:

$$ \sigma_g = e/R_g S $$

where 'e' is the sample thickness, 'S', the area of the electrode and R_g, the bulk resistance. The thermal evolution of the conductivity (σ_g) of (1) and (2) compounds as a function of inverse temperature are reported in Figures 9 a and b. Two regions are observed in these variations described by the Arrhenius law and the activation energies are 0.2 eV and 0.47eV for compound (1), 0.18eV and 0.38eV for compound (2). The change of the slope at some temperature ~350K in the two compounds corresponds to the evaporation of the H₂O. Therefore, we considered the role of humidity, which affects on conductivity in hybrid metal halides [60-61, 62].

## Conclusion

In summary, two hybrid halide compounds: [C₉H₁₆N₂]PdX₄ (X= Cl(1), Br(2)) have been synthesized and structurally characterized. Their structures are made of corner-sharing 1-D chains based on square pyramid building units of [PbX₅]. The polyhedra unit distortions of 1-D [PdX₄]²⁻ chain lead to the formation of self-trapped excitonic photoluminescence at room temperature, which gives a green emission. The absorption spectra show that compound (1) has a medium band gap (Eg~3.26 eV), while compound (2) has a narrow band gap (Eg~2.98 eV), and both have potential application in semiconductor materials fields.

## Conflicts of interest

There are no conflicts to declare.

## Appendix

CCDC 2141657-2141658 contain the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.

## Acknowledgments

The authors thank the University of Sfax-Tunisia for financial support and CDIFX (UMR 6226, Univ. Rennes 1) for X-ray diffraction data collection.
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Figures and tables captions

**Figure 1:** The asymmetric units of [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdCl<sub>4</sub>(1) (a) and [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdBr<sub>4</sub> (2) (b). Hydrogen-bonding interactions are omitted for clarity. *Symmetry codes:* (') -x+1, y, z; (‘) -x, y, z.

**Figure 2:** The packing structure of the compounds(1) (a) and (2) (b) along the *a*-axis. The zigzag [PdX<sub>4</sub>]<sup>2-</sup> (X=Cl, Br) chains are connected by five coordinated PdX<sub>5</sub> square pyramids through corner-sharing. The 1-D zigzag [PdX<sub>4</sub>] chains compounds (1) (c) and (d). Hydrogen bonds have been omitted for clarity.

**Figure 3:** The structural assembly process of compounds (1) and (2): (a), (b) the packing structure of compounds (1)-(2) along the *c*-axis, (c), (d) the molecular structure of PdX<sub>5</sub> (X=Cl, Br). The structures of crystals (1) and (2) induce a change in the Pd–X bond lengths. *Symmetry codes:* (ii) -x, y, z; (vi) -x, -y, -1/2+z.

**Figure 4:** TGA curves for [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdCl<sub>4</sub>(1) (a) and [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdBr<sub>4</sub> (2) (b)

**Figure 5:** Absorption spectra for [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdCl<sub>4</sub>(1) (a) and [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdBr<sub>4</sub> (2) (b)

**Figure 6:** The direct band gaps extracted from the Tauc plot for [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdCl<sub>4</sub>(1) (a) and [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdBr<sub>4</sub> (2) (b)
**Figure 7:** Photoluminescence spectra of [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdCl<sub>4</sub>(1) (a) and [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdBr<sub>4</sub> (2) (b).

**Figure 8:** Nyquist diagram spectra for some temperatures of compounds (1) (a) and (2) (b).

**Figure 9:** Variation of the ln(σ<sub>g</sub>) versus 1000/T of compounds (1) (a) and (2)(b).

**Table 1.** Crystal data for [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdCl<sub>4</sub>(1) and [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdBr<sub>4</sub>(2)

<figure><img src="image_2.png" /></figure>

**Figure 1:** The asymmetric units of [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdCl<sub>4</sub>(1) (a) and [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdBr<sub>4</sub> (2) (b).

Hydrogen-bonding interactions are omitted for clarity. Symmetry codes: (') -x+1, y, z; (") -x, y, z.
<figure><img src="image_3.png" /><figcaption>**Figure 2:** Packing structures of the compounds (1) (a) and (2) (b) along the *a*-axis. The zigzag [PdX₄]²⁻ (X=Cl, Br) chains are built from five coordinated PdX₅ square pyramids through corner-sharing. The 1-D zigzag [PdX₄] chains compounds (1) (c) and (2) (d). Hydrogen bonds have been omitted for clarity.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 3:** The structural assembly process of compounds (1) and (2): (a), (b) the packing structure of compounds (1) and (2) along the *c*-axis, (c), (d) the molecular structure of PdX<sub>5</sub> (*X*=Cl, Br) units. The structures of crystals (1) and (2) induce a change in the Pd–X bond lengths. *Symmetry codes*: (ii) $-x, y, z$; (vi) *x*, 1-*y*, 1/2+z.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 4:** TGA curves for [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdCl<sub>4</sub>(1) (a) and [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdBr<sub>4</sub> (2) (b)</figcaption></figure>
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 5:** Absorption spectra for [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdCl<sub>4</sub>(1) (a) and [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdBr<sub>4</sub> (2) (b)</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 6:** Direct band gaps extracted from the Tauc plot for [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdCl<sub>4</sub>(1) (a) and [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdBr<sub>4</sub> (2) (b)</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 7:** Photoluminescence spectra of [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdCl<sub>4</sub>(1) and [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdBr<sub>4</sub> (2).</figcaption></figure>
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 8:** Nyquist diagram spectra for some temperatures of compounds (1) (a) and (2) (b).</figcaption></figure>
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 9:** Variation of the ln(σg) versus 1000/T of compounds (1) (a) and (2)(b).</figcaption></figure>
<table><caption>**Table 1.** Crystal data for [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdCl<sub>4</sub>(1) and [C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdBr<sub>4</sub>(2)</caption><thead><tr><th></th><th>[C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdCl<sub>4</sub>(1)</th><th>[C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdBr<sub>4</sub>(2)</th></tr></thead><tbody><tr><td>Compound name</td><td>[C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdCl<sub>4</sub>(1)</td><td>[C<sub>9</sub>H<sub>16</sub>N<sub>2</sub>]PdBr<sub>4</sub>(2)</td></tr><tr><td>Empirical formula</td><td>C<sub>9</sub>H<sub>16</sub>Cl<sub>4</sub>N<sub>2</sub>Pd</td><td>C<sub>9</sub>H<sub>16</sub>Br<sub>4</sub>N<sub>2</sub>Pd</td></tr><tr><td>Diffractometer</td><td>APEX II</td><td>D8 VENTURE</td></tr><tr><td>Color</td><td>Red</td><td>Red</td></tr><tr><td>Formula weight (g.mol<sup>-1</sup>)</td><td>398.42</td><td>576.26</td></tr><tr><td>Temperature (K)</td><td>150</td><td>150</td></tr><tr><td>Crystal system</td><td>Orthorhombic</td><td>Orthorhombic</td></tr><tr><td>Space group</td><td>Pmc2<sub>1</sub></td><td>Pmc2<sub>1</sub></td></tr><tr><td>a (Å)</td><td>9.5712(8)</td><td>9.7840(14)</td></tr><tr><td>b (Å)</td><td>9.4095(10)</td><td>9.5580(16)</td></tr><tr><td>c (Å)</td><td>7.6844(7)</td><td>8.0431(13)</td></tr><tr><td>α, β, γ (°)</td><td>90</td><td>90</td></tr><tr><td>V (Å<sup>3</sup>)</td><td>692.06(11)</td><td>752.2(2)</td></tr><tr><td>Z</td><td>2</td><td>2</td></tr><tr><td>R<sub>int</sub></td><td>0.024</td><td>0.045</td></tr><tr><td>ρ<sub>calc.</sub>, (g.cm<sup>-3</sup>)</td><td>1.912</td><td>2.544</td></tr><tr><td>Absorption correction</td><td>Multi-scan</td><td>Multi-scan</td></tr><tr><td>Radiation (Å)</td><td>MoKα (0.71073)</td><td>MoKα (0.71073)</td></tr><tr><td>μ (mm<sup>-1</sup>)</td><td>2.088</td><td>11.835</td></tr><tr><td>Crystal Shape</td><td>Stick</td><td>Prism</td></tr><tr><td>Crystal size</td><td>0.450 x 0.090 x 0.040</td><td>0.26 x 0.17 x 0.06</td></tr><tr><td>F (000)</td><td>392</td><td>536</td></tr><tr><td>θ range [°]</td><td>3.036 to 27.488°</td><td>2.1 to 27.5°</td></tr><tr><td>Collected reflections</td><td>3193</td><td>3842</td></tr><tr><td>Data/restrains/parameters</td><td>1277/5/98</td><td>1677/1/83</td></tr><tr><td>R<sub>1</sub>, wR<sub>2</sub> [I > 2σ(I)]</td><td>0.022, 0.042</td><td>0.032, 0.076</td></tr><tr><td>R<sub>1</sub>, wR<sub>2</sub> [all data]</td><td>0.025, 0.0431</td><td>0.034, 0.077</td></tr><tr><td>Goodness-of-fit on F<sup>2</sup></td><td>1.040</td><td>1.129</td></tr><tr><td>Largest diff. peak/hole/e Å<sup>3</sup></td><td>0.39/-0.48</td><td>0.92/-1.42</td></tr><tr><td>Flack parameter</td><td>0.03(6)</td><td>0.04(2)</td></tr></tbody></table>