1 Introduction

In recent years, hybrid materials have whetted an extensive interest and drew the widest attention among scientists and researchers. In this respect, they have been the subject of several previous studies due to the outstanding combination of their inorganic and organic properties [1, 2]. This type of material exhibits promising properties such as magnetic, ferroelectric, electrical, conductivity, non-linear optics (NLO), catalysis and luminescence [3, 3, 4, 6], which implies that it can be used in a wide range of scientific fields.

Multiple hybrid materials were prepared based on the specific properties of the organic molecule, often with a prorogated amine [5,6,7]. Organic and inorganic solids were kept attached through interactions via various bonds such as hydrogen contacts, π–π functionalization and Van der Waals interactions [8, 9].

One of the most prominent properties of hybrid materials including organics based on pyridine and its derivatives, which has spilled much ink, is the photochemical property [10,11,12,13]. The inorganic part of the hybrids rely on transition metals and poor metals such as mercury, copper, tin, cadmium, lead and others. These materials display interesting physical properties [14,15,16,17]. As for the hybrid materials based on mercury halide and organic cations, they present a semiconductor behavior [18, 21], in addition to a wide structural diversity [19].

Complexes of halomercurates (II) with alkali metal ions and aliphatic or aromatic ammonium ions yielded significant results in several fields [23]. In this regard, our team is interested in examing such physical properties as optics, conductivity and luminescence of the new hybrid materials, which have been the subject of several articles [14,15,16,17,18,19,20,21, 23].

In order to explore new properties of organic–inorganic materials, this study is devoted to exploring the properties of this new compound bis(4-amino-2-chloropyridinium) tetrachloromercurate (II) monohydrate, (C5H6N2Cl)2[HgCl4]•H2O, (abbreviated (CAP)2[HgCl4]•H2O), using various significant techniques such as X-ray diffraction, FT-IR, Raman, solid-state NMR studies and UV–Visible spectroscopy via DFT calculations (B3LYP/LanL2DZ basis set). To quantify the intermolecular interactions involved in stabilising the crystal structure, analyses of the three-dimensional Hirshfeld surface and the molecular electrostatic potential (MEP) surface were carried out. Eventually, photoluminescence (PL), thermal analysis (DSC-TGA) and dielectric studies were carried out to grain a deeper insight into the conduction phenomenon in this new material.

2 Experimental

2.1 Preparation of (CAP)2[HgCl4]•H2O Compound

To obtain our new hybrid compound, (C5H6N2Cl)2[HgCl4]•H2O, we used the following starting materials, from an equimolar specified proportions (2:1): 4-amino-2-chloropyridine (CAP) (0.398 g; 3.09 mmol); mercury chloride (HgCl2) (0.199 g; 0.733 mmol) and hydrochloric acid (HCl) (38%; 5 mL). Using the slow evaporation method, this compound was prepared at room temperature. The above products were dissolved separately in hydrochloric acid, then mixed together in an approximate volume of 10 ml of water with magnetic stirring until complete dissolution of the solutes. Colourless crystals appeared with a prismatic-shaped salt having parallelogram bases (Fig. 1a, b, c). These crystals were washed with ether, carefully filtered and left for a day to dry. Subsequently, they were collected and used in various physical and chemical analyzes.

Fig. 1
figure 1

a Typical EDX spectrum, b photograph (magnification *10) c and SEM image of the crystals of the compound(CAP)2[HgCl4]•H2O having the same shape as prismatic

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2.2 Scanning Electron Microscopic Study of the Sample

Quantitative analysis under a scanning electron microscope performed on this material reveals the presence of all atoms other than hydrogen atoms: Hg, C, N, O and Cl without setting their percentage. The result of this analysis is indicated on the following spectrum Fig. 1a. The morphology of bis(4-amino-2-chloropyridinium) tetrachloromercurate (II) monohydrate crystals obtained with the scanning electron microscopy (SEM) is shown in Fig. 1b, c show an assemblage of crystal fragments having uniform distribution, a flat surface, regular edges that indicate good crystalline quality are clearly visible in the images.

2.3 Characterization

2.3.1 Crystal Data and Structure Determination

A Bruker Kappa Apex II CCD diffractometer equipped with a monochromatic MoKα graphite beam (0.71073 Å) was used for the crystalline study of a well-chosen single crystal of dimensions (0.39 × 0.32 × 0.26) mm3. Intensity data were used at room temperature. The final unit cell parameters and crystal orientation matrix were obtained by a least-square fit of 4021 reflections and the structure was solved and refined using the SHELXS program which is included in the WINGX software [21]. Firstly, direct methods located the heavy atoms Hg and Cl. N, C and O atoms were next located using successive Fourier difference maps. Hydrogen atoms were fixed geometrically. The final refinement led to the following reliability factors: R = 0.048 and Rw = 0.216. Table 1 illustrates the experimental details of the structure determination.

Table 1 Summary of crystal data, intensity measurements and refined parameters of (CAP)2[HgCl4]·H2O compound
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The graphical illustrations were generated using the following programs: ORTEP [22], DIAMAND 2.1 [23] and MERCURY3.1 [24].

2.3.2 Spectroscopy Equipment

FT-IR spectrum of the sample was recorded within the range 400–4000 cm−1 using a Perkin Elmer L120-00 spectrometer. The Raman spectrum of sample was recorded in the range 50–4000 cm−1 HORIBA JOBIN–YVON (T64000) spectrometer triple monochromator instrument using the 514.5 nm line spectra-physics argon ion laser.

13C NMR experiment was conducted on a Bruker MSL 300 spectrometer operating at 75.48 MHz for 13C. The powdered sample was packed in a 4 mm diameter rotor and allowed to rotate at speeds up to 10 kHz in a Doty MAS probehead. The spectra were acquired by the use of cross-polarization for proton with 5 ms contact time.

Optical absorption spectrum by spin-coating of the films (compound in powder form (result of grinding the crystals)) was deduced from direct transmission measurements carried out using a conventional UV–Vis spectrophotometer (Hitachi, U3300) that has a dual-beam monochromator, covering a spectrum from 200 to 2400 nm and using two sources: (i) Xenon lamp for the UV–Visible domain, (ii) Halogen lamp for the infrared range.

2.3.3 Hirshfeld Surface Analysis

The 3–D Hirshfeld surfaces [29] in the crystal structure were constructed resting on the electron distribution calculated as the sum of spherical atom electron densities. For each point on the 3–D Hirshfeld surface, we determined both de (which is the distance from the 3–D Hirshfeld surface to the nearest nucleus outside the surface) and di (which is the corresponding distance to the nearest nucleus inside the surface). For further information about the intermolecular interactions in the crystal structure, they were estimated utilizing the 3–D Hirshfeld surface analysis [25, 26] whereas the associated 2–D fingerprint plots were specified via the Crystal Explorer 3.1 [27] which is based on the structure of the CIF file. The mapped Hirshfeld surfaces were made transparent to allow the visualization of the molecular moiety, in a similar orientation for the crystal. In particular, the Hirshfeld surface mapped on a dnorm clearly highlighting, with the spots, in addition to the circular depressions (dark red) the donor (de > di) and the acceptor (de < di), which are indicative of intermolecular interactions of hydrogen bonding contacts. We can therefore assert that, the π–π stacking interactions are verified by the existing evidence of the adjacent red and blue triangles on the shape index surface.

The normalized contact distance (dnorm) resting on both de and di, and the van der Waals (VdW) radii of the atom, indicated by Eq. (1) allowed the identification of the regions of particular importance to intermolecular interactions [28].

$${\mathrm{d}}_{\mathrm{norm}}=\frac{{\mathrm{d}}_{\mathrm{i}}-{\mathrm{r}}_{\mathrm{i}}^{\mathrm{vdw}}}{{\mathrm{r}}_{\mathrm{i}}^{\mathrm{vdw}}}+\frac{{\mathrm{d}}_{\mathrm{e}}-{\mathrm{r}}_{\mathrm{e}}^{\mathrm{vdw}}}{{\mathrm{r}}_{\mathrm{ei}}^{\mathrm{vdw}}}\boldsymbol{ }$$
(1)

With rivdw and revdw are the van der Waals radii of the atoms.

The value of dnorm was highlighted using a red–white–blue color scheme corresponding to the negative value (red regions), zero (white regions) and positive (blue regions) for shorter contacts and for contacts around the VDW separation and for longer contacts, respectively.

The 2–D fingerprint plots appeared as expanded two-dimensional with the (de) and (di) distance scales installed on the graph axes. They can be decomposed to highlight particular atom pairs in close contacts [34].

The void surface was located where the sum of the promolecule density in the unit cell is less than a user definable threshold, where the void surface met the boundary of the unit cell, and capping faces were generated to create an enclosed volume. In addition to visualizing voids, Crystal Explorer equally reported the volume of the void and its surface area.

2.3.4 Computational Methods

The fully optimized ground state geometry and harmonic vibrational wavenumbers were computed at the DFT level using the Lee–Yang–Parr correlation fuctional (B3LYP) in conjunction with the LanL2DZ basis set [29, 36] by means of Gaussien 09 software package [30]. On this basis, all the parameters were allowed to relax and all the calculations converged to an optimized geometry, which corresponds to a minimum of energy as revealed by the lack of imaginary values in the calculated frequency. An empirical scaling factor of 0.961 was used to offset the systematic error [38] by basis set incompleteness, neglect of electron correlation and vibrational anharmonicity. The calculated vibrational wave numbers were assigned deploying the GaussView 05 program [31] in order to generate visual presentations and verify the normal mode assignments. The molecular electrostatic potentials (MEPs) were calculated to investigate the reactive sites of synthesized crystal. To account for the chemical selectivity or reactivity sites, the electron density-based local reactivity descriptors such as Fukui functions were computed [32]. The electronic properties such as HOMO and LUMO energies and absorption wavelengths were determined by time-dependent DFT (TD-DFT) approach [33] at the same level of theory. It is necessary to clarify that the sample was also optimized in ethanol solvent.

2.3.5 Thermal Analysis

The thermal behavior was examined under an air flow in Mettler Toledo DSC 822e calorimeter and Perkin Elmer Pyris 6 TGA equipment, at a heating rate of 5 K.min−1.

2.3.6 Electrical Measurements

Electrical measurements of the (CAP)2[HgCl4]·H2O sample, which was pressed into pellets 8 mm in diameter and 1.3 mm thick, were carried out with the TEGAM 3550 ALF automatic bridge controlled by a microcomputer between 318 and 408 K in a frequency range from 209 Hz to 46 MHz.

3 Results and Discussion

3.1 X-ray Diffraction Analysis

The sample (CAP)2[HgCl4]·H2O was characterized by single-crystal X-ray diffraction at room temperature. It revealed that this new hybrid belongs to the triclinic system with the space group Pī, whose elementary point displays two unitary forms.

The asymmetric unit contains two monoprotonated cationic entities (4-amino-2-chloropyridinium), an anionic entity [HgCl4]2− in the form of an isolated tetrad and a water molecule H2O, drawn with a 50% probability of thermal ellipsoids, as depicted in Fig. 2a. This unit contains both an anionic and a cationic charge, ensuring the electrical neutrality of the crystal structure. At this point, the best possible geometry was undertaken using DFT/B3LYP/LanL2DZ basis set, which accepts a structure input file in its crystallographic data. It is indicated in Fig. 2b in accordance with the experimental structure as well as the relaxation of all parameters; that the calculations converge to optimize geometries, which correspond to true energy minima.

Fig. 2
figure 2

Atom numbering scheme for the structure a experimental results and b theoretical optimized geometry

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The projection of this structure along the a-axis (Fig. 3) revealed that the organic cations are positioned between the infinite chains of the inorganic part and the water molecules, which have a different orientation. Projection along the b axis demonstrated that the organic molecules are parallel along the a-axis and equidistant. This assembly was then built by a succession of cationic dimers and isolated tetrahedra in addition to water molecules.

Fig. 3
figure 3

Projections of the (CAP)2[HgCl4]•H2O compound: a along the a-axis and b along the b axis

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However, in this structure, the organic part consists of two cations presenting a regular spatial configuration with experimental distances (Table 2) the C–C and C–N bond lengths ranging [1.324(14); 1.429(14)] Å and [1.308(14); 1.357(15)] Å, respectively. The main values of the C–Cl bond lengths vary from 1.697(9) to 1.712(11) Å. Additionally, the experimental values of C–C–C and N–C–C and N–C–Cl and C–C–Cl angles vary from 116.3(9)° to 120.8(10)°and 121.4(9)° to 123.3(8)° and 115.3(8)° to 116.3(7)° and 122.4(8)° to 123.2(8)°, respectively. It is worth noting that C–N–C angle is of the order 123.3(8)°. These values go in good agreement with those reported in previous literature [7, 10, 34, 35].

Table 2 Selected geometric parameters (Å, °) of (CAP)2[HgCl4]·H2O compound
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The mercuric ion Hg (+ II) adopts the geometry of a regular tetrahedral (Table 2). The Hg–Cl bond distances range from 2.447(3) to 2.476(3) Å whereas the Cl–Hg–Cl angles fall in the range 104.43(8)° to 115.95(10)° [9, 36].

Interestingly enough, a study of the calculated geometric characteristics revealed a good convergence with the experimental parameters. According to Table 2, the majority of the calculated link values are slightly longer than those of the experimental links, which can be explained by the fact that the theoretical calculation considers that the molecule is isolated in the gaseous phase at 0 K while the experimental result examines the molecule in the solid state, presenting therefore intermolecular interactions. Thus, the accuracy of the calculation is satisfactory and the B3LYP/LanL2DZ level is suitable for the compound investigated in this manuscript.

Figure 4 and Table 3 foreground the interactions of hydrogen bonding values present in this structure. The cohesion of the structure is ensured by hydrogen bonds: on the one hand between the organic part and the inorganic part and on the other hand with the water molecule and both organic–inorganic entities. These bonds are interatomic and are of the type O–H…Cl, C–H…Cl and N–H…Cl (chlorine of the anion).

Fig. 4
figure 4

Packing diagram showing the capped sticks of the (CAP)2[HgCl4]•H2O compound with dotted red lines represent hydrogen bonds

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Table 3 Main inter-atomic distances (Å) and bond angles (°) involved in the hydrogen bonds of (CAP)2[HgCl4]·H2O compound
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In addition, the surface mapped on the shape index (Fig. 5a) discloses the rationalized information from hydrogen bond interactions, such as the deep red area which highlights the hydrogen bond acceptor (concave) and the blue area which corresponds to the hydrogen bond donor (convex); thus indicating the stabilization of the crystal studied here [37]. Again, the presence of red and blue red triangles (Fig. 4b) adjacent with summit in the shape index goes in tandem with the presence of π– π interactions interacting between aromatic rings [8]. within this framework, these intermolecular interactions involved within the structure go in good agreement with the 2-D fingerprint plots (Fig. 6). The fingerprint plots for the new hybrid indicated the H…Cl/Cl…H interactions, with the most important one being 55.9% of the total Hirshfeld surface region. These values proved to be de = 1.6 Å, di = 1.00 Å / de = 1.00 Å, di = 1.6 Å, this yielded the de > di (acting as a donor) and the other de < di (acting as an acceptor). As a matter of fact, Fig. 7 emphasizes the domination of these interactions; allowing to conclude that hydrogen bonds play a very crucial role in the stability of the building. The rates of intermolecular interactions H…H and C…H/H…C amount to 14.1% and 9.1%, respectively of the Hirshfeld surface area. Thus; the Hirshfeld surface area revealed Cl…Cl and C…C interactions in the order of 4.8% and 3.2%. C…C interactions suggested that the value for the de = di = 1.8 Å is indicative of the presence of π–π interaction between organic molecules, which is portrayed in Fig. 8. Basically, the results found by the analyses of intermolecular interactions using the Hirshfeld surface go in good conformity with the previous structural description. The proportion of H…N/N…H interactions comprises 3.1%, whereas 2.2% of contribution comes from the Cl…C/C…Cl intermolecular interactions. We also outline the following interactions with their percentages: C…N/N…C, Cl…O/O…Cl, Cl…N /N…Cl, H…O/O…H, H…Hg/Hg…H and Cl…Hg/Hg…Cl and N…N, 1.6%, 1.5%, 1.3%, 1.1%, 1%, 0.8% and 0.1%, respectively. We equally record the following interactions with their percentages: C…N/N…C, Cl…O/O…Cl, Cl…N /N…Cl, H...O/ O…H, H…Hg/Hg…H and Cl…Hg/Hg…Cl and N…N, 1.6%, 1.5%, 1.3%, 1.1%, 1%, 0.8% and 0.1%, respectively.

Fig. 5
figure 5

The dnorm a and shape index b Hirshfeld surfaces and the full fingerprint plot of (CAP)2[HgCl4]•H2O compound

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Fig. 6
figure 6

2–D Fingerprint plots analysis of (CAP)2[HgCl4]•H2O compound

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Fig. 7
figure 7

The Hirshfeld surface mapped dnorm and the fingerprint plot of the H…Cl/Cl…H interactions of (CAP)2[HgCl4]•H2O compound

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Fig. 8
figure 8

The π–π interactions between the chains of aromatic ring and the Hirshfeld surface mapped shape index the π–π interactions of (CAP)2[HgCl4]•H2O compound

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The contributions of the different interactions to the Hirshfeld surfaces were calculated, suggesting the dominance of H…Cl/Cl…H interactions on the total surface of Hirshfeld (Fig. 9).

Fig. 9
figure 9

Relative contribution of various intermolecular interactions to the Hirshfeld surface area of (CAP)2[HgCl4]•H2O compound

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The Crystal Explorer allowed the calculation of the porosity of this material, whose visualization of the voids with the isosurfaces of electron density were exposed to a level of 0.002 a.u. As can be inferred from Fig. 10, the resulting volume of the void and its surface area surface, were determined as: Volume (93.91 Å3) (20.24%) and area (358.13 Å2) (78.15%) per unit cell. This result may be assigned to the response of molecular crystals to external pressure.

Fig. 10
figure 10

Crystal void of (CAP)2[HgCl4]•H2O compound

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3.2 MAS–NMR

The experimental study using the 13C NMR technique (Fig. 11) of the studied compound revealed the presence of five intense peaks (blue spectra) between 90 and 170 ppm. Owing to the Dmfit software [38], the deconvolution of the experimental spectrum entailed the appearance of ten peaks, which coincided with the total number of carbon atoms in unequal positions in this structure present in the asymmetric unit of this hybrid crystal. This result can confirm the structural results and purity of the crystalline phase of the synthetized product. Table 4 highlights an attempt of displacement of the pyridinic ring attribution. The latter was enacted in the same way as the molecule presented in previous works [5,6,7, 34, 35].

Fig. 11
figure 11

Plots of experimental and fitted curves of the isotropic band of the 13C spectrum of (CAP)2[HgCl4]•H2O compound

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Table 4 The chemical shifts of carbons in 13C NMR spectra of (CAP)2[HgCl4]·H2O compound
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3.3 Vibrational Studies

The study of the vibration modes of the structural architecture of this compound, using infrared absorption and Raman diffusion, confirmed the presence of the envisaged chemical elements. This study was supported by the theoretical calculation of vibratory spectra using functional density theory with the B3LYP/LanL2DZ assembly base. The theoretical and experimental studies of each technique are plotted in Figs. 12 and 13. This part was followed by an attempt to assign the majority of the peaks to their corresponding vibratory mode while relying on previous works [5,6,7, 17, 34, 35]. These attributions are listed in Table 5 and discussed further in what follows: the stretching vibrations O–H, N–H and C–H were detected by the presence of peaks in the high frequency [3800–3000] cm−1. The IR spectrum indicated the presence of several bands in the 2800 cm−1–1700 cm−1 region, which are characteristic of the combined harmonics and vibrations. The latter are not present in the Raman spectrum from which they are excluded in the assignments. The NH2 deformation and C = C stretching vibrations were detected by the presence of peaks at 1655 cm−1 and 1631 cm−1, respectively. The region located between 800 and 400 cm−1 was assigned to NH scissoring and CH bending vibrations. The IR and Raman spectrum displayed strong peaks at 786 cm−1 and 717 cm−1, which were attributed probably to the carbonyl scissoring mode, and out of plane bending mode, respectively.

Fig. 12
figure 12

Superposition of the experimental (black) and the DFT computed (red) FT-IR spectra of (CAP)2[HgCl4]•H2O compound

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Fig. 13
figure 13

Superposition of the experimental (red) and the DFT computed (black) FT-Raman spectra of (CAP)2[HgCl4]•H2O compound

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Table 5 Observed and calculated frequencies (cm−1) of (CAP)2[HgCl4]·H2O compound
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Using Raman spectrum (Fig. 13), the Tetrahedron [HgCl4]2− vibration modes proved to be in the range of (40–400) cm−1. The attribution was difficult but relied on references [19, 36]. The bands observed at 258 cm−1 and 218 cm−1 can be attributed to asymmetric and symmetric stretchings, respectively. Asymmetric and symmetrical strain vibration peaks of the Hg–Cl bond in the [HgCl4]2− anion appeared at 186 cm−1 and 108 cm−1, respectively.

3.4 Optical Properties

3.4.1 UV–Vis Absorption

The experimental (black spectrum) and theoretical (red spectrum) UV–visible spectra at room temperature are displayed in Fig. 14. These spectra indicated a satisfactory agreement between the experimental and calculated results. It can be inferred that the (CAP)2 HgCl4·H2O exhibited a single intense band around 266 nm (4.66 eV) and 273 nm (4.53 eV) respectively, which refers to the optical energy band gap value equal to 4.66 eV, as illustrated in Fig. 15. The difference among the measured and calculated transition energies is 0.13 eV. This may account for the characteristic feature of mercury (II) complexes with a 5 d10 electronic configuration. According to literature studies [19, 39], this band was allocated to n → π* transitions, appearing in the corresponding free ligand, as a consequence of the presence of the aromatic ring. In addition, the energy gap value allowed the assumption that this hybrid material can be considered as an optically potential material with spectroscopic applications.

Fig. 14
figure 14

Experimental and theoretical optical absorption spectra at room temperature of (CAP)2[HgCl4]•H2O compound

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Fig. 15
figure 15

Molecular orbital surfaces for the HOMO and LUMO of (CAP)2[HgCl4]•H2O compound

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3.4.2 Molecular Electrostatic Potential Analysis

In order to explore the relative polarity of the molecule, we examined the molecular electrostatic potential (MEP), which is a visual method, unveiling an interesting property that can be derived from the density distribution of electrons. The mapped MEP surface was used for the qualitative interpretation of the electrophilic and nucleophilic sites [51]. The chemical polarity qualifier and the assessment of the charge distribution in the crystal the MEP map, were calculated by B3LYP, as presented in Fig. 16. The MEP figure portraying the visual representation of chemically active sites, presents different colors such as: electrophilic reactivity which is in red (negative regions) as well as nucleophilic reactivity which is in blue (positive regions). The potential increases through moving from red–orange–yellow–green–blue color, where the color code of these cards varies between − 0.053 a.u. and 0.053 a.u. deepest red and deepest blue, respectively.

Fig. 16
figure 16

Molecular electrostatic potential map of (CAP)2[HgCl4]•H2O compound

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In this respect, the negative regions are mainly localized on the chlorine atoms, whereas the positive regions are mainly localized on the amino atoms N. This region may constitute a suitable site for the nucleophilic attack. These results provide useful information on the region from which the compound can have an intra- and an intermolecular interaction. This result converges towards the interpretation that this crystal presents a global electrostatic attraction between the anion [HgCl4]2− and the organic cation that is added to the favorable H…Cl hydrogen bond.

3.4.3 Mulliken Atomic Charges

It is well known that the Mulliken population indicates charge distributions on atoms and quantifies how the electronic structure changes under atomic displacement [44].

In addition, the Mulliken charge is directly related to the chemical bonds present in the studied crystalline structure. Figure 17 depicts the charge distribution of the different atoms of this complex (CAP)2[HgCl4]•H2O with all the carbon atoms displaying negative charges, except C atoms between both nitrogen ones. The latter are positively charged due to the negative charges of the nitrogen atoms. The metal Hg has a positive charge due to the fixation of halogens Cl which are negatively charged. Finally, all hydrogen atoms carry positive charges while both chlorine atoms carry negative charges. It is well confirmed that the load transfer by the hydrogen bonds (N–H…Cl) of the hybrid material has been analysed.

Fig. 17
figure 17

Atomic charge distribution of (CAP)2[HgCl4]•H2O compound

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3.4.4 Photoluminescence Behavior

We measured solid-state photoluminescence to determine its optical properties, which corresponds to the result of the absorption of photons by the material. We computed the photoluminescence (PL) of our compound at room temperature, with an excitation wavelength of 285 nm, as presented in Fig. 18. A wide fluorescence band from 346 to 381 nm emissions by our hybrid lies approximately in the luminescence range observed for other mercury-containing materials [HgCl4], corresponding described in literature [40, 41]. The corresponding CIE coordinates (0.149; 0.201); are located in the blue region of the CIE chromaticity diagram [42].

Fig. 18
figure 18

Emission spectrum of (CAP)2[HgCl4]•H2O compound

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In hybrid compounds based on haberdashery chloride, the luminescence of the excitons comes from the electronic transitions of the inorganic part. Excitation by a laser follows an electron (-) which will be excited from the valence band (VB). The latter is made up of a combination of Hg (6d) and Cl (4p), which is in the direction of the conduction band (CB) for the Hg(5 s) states, leaving a ( +) gap in (VB). The (-) electron and ( +) hole move freely in (CB) and (VB) forming an exciton [6]. Recombination of the electron with the hole in the exciton results in blue emissions. To illustrate the process of formation and recombination of excitons in our compound, we built up a simple model, as revealed in Fig. 19.

Fig. 19
figure 19

Simple model for the formation and recombination of the exciton in the (CAP)2[HgCl4]•H2O compound

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3.5 The Thermal Study

3.5.1 ATG and DSC Studies

The thermal behavior of this compound is the central focus of this part. Notably, the combination of gravimetric and calorimetric behavior pave the way for the following discussion.

The TGA curve (Fig. 20a) displays a weight loss of 3.06% between 360 and 423 K, which coincides with the unit formula of the studied compound. The calorimetric study (Fig. 20b) presents a small bump at 340 K, which can probably be ascribed to the loss of surface water molecule. The same curve has an intense peak at 370 K, indicating the loss of the structural water molecule that is detected in the thermogravimetric study. This endothermic peak at an enthalpy ΔH = 70.61 J/g implies that the compound exhibits thermal stability at higher temperatures.

Fig. 20
figure 20

a ATG spectrum and (b) DSC spectrum of (CAP)2[HgCl4]•H2O compound

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The calorimetric study is followed by a cooling cycle on the same sample. This demonstrates the total absence of an endothermic peak (Fig. 20b), after which, it undergoes another warming sweep cycle that shows the total absence of an endothermic peak. Thus, this confirms that the observed peaks are due to the loss of surface and structural water molecules.

3.5.2 Electrical Study

Electrical analysis is used to provide useful information about the behaviour of localised electrical charge carriers, leading to a better understanding of conduction processes.

The electrical properties study of a material is denoted by the expression of the complex impedance parameter term Z*with Z’and Z" represent respectively the real and imaginary parts of the impedance the complex impedance.

$${Z^{*}} = {Z^\prime } - {Z^{\prime \prime }} = Rs - 1/j\omega Cs$$

Rs: series resistance. ω = 2π f (f = resonance frequency). Cs: capacitance in series. j = (− 1)1/2 the imaginary factor. C0: vacuum capacitance of the circuit elements.

Figure 21 presents the Nyquists diagram (plot of Z’ versus Z”) at different temperatures (303–398 K). These spectra reveal that the increase in temperature leads to a decrease in Z". This decrease is well marked at temperatures above T = 363 K.

Fig. 21
figure 21

The impedance plots at T = [303–398] K of (CAP)2[HgCl4]•H2O compound

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These spectra demonstrate on the one hand, in the field of scanned frequency and in the lower temperature in 358 K, that the new organic–inorganic compound has a capacitive behavior from which it is impossible to determine the resistance. On the other hand, these spectra follow the cole–cole law for any temperature greater than 366 K. This corroborates that the electrode polarization phenomena have an outstanding contribution [44].

The Arrhenius modelling equation was utilised to understand the conduction mechanism.

$$\sigma T = \mathop \sigma \nolimits_0 \exp \left( { - \mathop E\nolimits_a /\mathop K\nolimits_b T} \right)$$

Ea is the activation energy, σ0 is the pre-exponential factor, Kb is the Boltzmann constant, and T is the absolute temperature.

The variation of activation energy as a function of 1000/T is depicted in Fig. 22. This variation is separated into three regions as follows, Region (I): ∆Ea1 = 0.004 eV and Region (II): ∆Ea2 = 0.27 eV and Region (III): ∆Ea3 = 0.8 eV. These values are suggestive that the H+ ion transport in the material is due probably to a jump mechanism [45].

Fig. 22
figure 22

Arrhenius plots Ln(σT) = f(1000/T) of (CAP)2[HgCl4]•H2O compound

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The thermal study undertaken TGA and DSC depicts a single endothermic peak assigned to the evaporation of water molecules, in the temperature range 298–420 K. From this perspective, it helps us draw the conclusion that the variation in activation energy with temperature cannot be due to a phase transition. The observed change is rather due to a change in the conduction mechanism. Departing from Fig. 22, the activation energy of the region (III) involved in this jump is greater than that calculated in the regions (II) and region (I). This refers to the fact the observed time in this frequency range is too short for all jumps (proton) to be successful, the neighborhood cannot be therefore comfortable with the new position of the proton after it jumps.

4 Conclusions

In the current research work, single crystals of (CAP)2[HgCl4]·H2O were synthetised from an aqueous solution by a slow evaporation at room temperature. The edifice was stabilized by not only hydrogen bonds (N–H…Cl, O–H…Cl and C–H…Cl) but also by the presence of cation–cation π– π interactions. The number of solid state 13C, MAS-NMR components goes in good agreement with that of the crystallographically independent site. The cohesion within this material was also examined by the Hirshfeld surface calculation method. The DFT calculations, optimized geometry and vibration spectrum coincided well with experimental data, which were interpreted by analogy with pervious works. The decrease in the HOMO and LUMO energy gap accounts for intraligand n→π* transitions.

The thermogravimetric and calorimetric measurements with differential scanning revealed the absence of phase transition. The obtained peaks are due to the evaporation of the molecules. The studies of luminescence activities confirmed that this compound exhibited a powerful fluorescence property at room temperature. Thus, the new crystal displays an outstanding electrical behavior, where the activation energy emitted by the impedance spectra can be assigned by the H+ proton jump mechanism.