Synthesis, crystal structure, and vibrational and DFT simulation studies of benzylammonium dihydrogen phosphite

Journal of Physics and Chemistry of Solids 123 (2018) 150–156

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Synthesis, crystal structure, and vibrational and DFT simulation studies of benzylammonium dihydrogen phosphite

T

Asma Ben Racheda, Wassim Maaleja,∗, Philippe Guionneaub, Nathalie Darob, Tahar Mhiria, Habib Fekic, Zakaria Elaouda a

Laboratory of Physical Chemistry of the Solid State, Faculty of Sciences, University of Sfax, BP 1171, 3000, Sfax, Tunisia Institut de Chimie de la Matière Condensée de Bordeaux, Centre National de la Recherche Scientifique, University of Bordeaux, 87 avenue du Dr A. Schweitzer, 33608, Pessac, Bordeaux, France c Laboratory of Applied Physics, Faculty of Sciences, University of Sfax, BP 1171, 3000, Sfax, Tunisia b

A R T I C LE I N FO

A B S T R A C T

Keywords: Organophosphite X-ray diffraction Vibrational study Density functional theory UV-Visible spectroscopy Photoluminescence

Single crystals of a new organophosphite compound, C7H12NPO3, were grown by the slow evaporation technique and characterized by X-ray diffraction, IR absorption spectroscopy, Raman scattering spectroscopy, UV-visible spectroscopy, photoluminescence measurements, and thermal analysis. This salt crystallizes at 120 K in orthorhombic symmetry with space group Pbca and cell parameters a = 14.0377 (12) Å, b = 8.0915 (7) Å, and c = 16.2980 (2) Å. The same symmetry, Pbca, is shown by X-ray diffraction at 298 and 350 K. To minimize the thermal effects, the crystal structure determined at 120 K was used as the reference. In the crystal structure, the phosphite anions, H2PO3−, are connected to the organic cations, C6H5CH2NH3+, through N–H...O hydrogen bonds that form infinite parallel two-dimensional planes. Theoretical calculations were performed with density functional theory at the B3LYP/6-31G (d,p) level of theory to study the molecular structure and vibrational spectra of C7H12NPO3. The optimized geometry and the calculated frequencies are in good agreement with the experimental data. The UV-visible spectrum shows characteristic absorption bands in the region from 300 to 800 nm, which suggests that C7H12NPO3 is potentially suitable for optical applications. The absence of absorption in the visible region might enable achievement of microscopic nonlinear optical response with nonzero values. In addition, photoluminescence measurements showed that this compound exhibits green emission at 502 nm at room temperature.

1. Introduction Hybrid compounds are increasingly studied because of their capability in offering important opportunities to combine the remarkable features of organic compounds with those of inorganic materials. These compounds are potentially good materials for exhibiting electric [1–3], magnetic [4–6], catalytic [7–9], optical [10–13], and nonlinear optical [14–16] properties. Particularly, the domain of hybrid optics has been very productive not only scientifically but also in terms of applications such as optical data storage, optical signal processing, laser technology, and optical communication [17–19]. In this context, few studies have attempted to explain the behavior of benzylamine in the solid state. However, the combination of hydrogen-bonded organic molecules such as benzylammonium cations, obtained by slow evaporation or hydrothermal synthesis, is a very general method for realization of a large variety of novel organic-

∗

inorganic materials such as phosphites and phosphates [20,21]. Following this line of work, we are searching for other organic-inorganic phosphates consisting of C6H5CH2NH3+ and H2PO3−, which is derived from phosphorous acid (H3PO3). Only a few organic-inorganic phosphites have been reported [22–29], which may be due to difficulties in preparing high-quality single crystals for structural analysis. In the present work, we report the synthesis and characterization of a new organophosphite compound with general chemical formula C7H10NH2PO3, namely benzylammonium dihydrogen phosphite (BDP). We present the structural characterization by single-crystal X-ray diffraction, Fourier transform (FT) IR spectroscopy, FT Raman spectroscopy, photoluminescence measurement, and thermal analysis. A reliable assignment of the vibrational bands in the IR and Raman spectra is needed. For this purpose, density functional theory (DFT) calculations were performed. To supplement our theoretical calculations, correlation between vibrational spectra and computed results is used to

Corresponding author. E-mail address: [email protected] (W. Maalej).

https://doi.org/10.1016/j.jpcs.2018.07.017 Received 27 January 2018; Received in revised form 22 July 2018; Accepted 23 July 2018 Available online 26 July 2018 0022-3697/ © 2018 Elsevier Ltd. All rights reserved.

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identify vibrational modes and to provide deeper insight into the bonding and structural features of inorganic-organic molecular systems.

Table 1 Crystal data and structural refinement for C7H12NPO3 crystal. Crystal data

2. Experimental

Color Shape Formula weight(g mol−1) Volume(Ǻ3) Density(Mg m−3) Crystal system Space group Temperature(K) a(Ǻ) b(Ǻ) c(Ǻ) Z F(000) Crystal dimensions (mm3) Diffractometer θ range for data collection(°) Radiation, λ(Å), monochromator Reflections collected/unique Observed reflections Fo > 4σ(Fo) Rint Range of h, k, l Refined parameters Goodness of fit Final R1 and wR2 w

2.1. Chemical synthesis H3PO (purity 99%) was added dropwise with magnetic stirring at room temperature to a concentrated solution of benzylamine of high purity (purity 99%) in a stoichiometric ratio of 1:1. The interaction between acid and base gives a precipitate that disappears on the addition of distilled water. The reaction scheme is as follows:

H2 O C6H5CH2NH2 + H3PO3 ⎯⎯⎯⎯⎯⎯→ C6H5CH2NH3·H2PO3 By slow evaporation at room temperature, single crystals with parallelepiped shapes of suitable dimensions were obtained. Chemical analysis of phosphorus and acidic protons was performed [30]. The composition was as follows: calculated 16.37% P, 7.04% N, and 44.44% C; found 16.41% P, 7.24% N, and 44.35% C.

2.2. Thermal analysis and spectroscopic characterization Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) measurements were performed on our heating the sample in a platinum crucible from 298 to 535 K in a SETARAM thermal analyzer (TG-ATD92) under air at a heating rate of 5 K min−1. The IR spectrum was recorded in the range from 500 to 4000 cm−1 with a PerkinElmer Spectrum 100 F T-IR spectrometer. The resolution of the spectrum was ± 2 cm−1. Back-scattering Raman spectra in the region from 50 to 4000 cm−1 were obtained with a Horiba Jobin Yvon LabRAM HR 800 dual spectrophotometer with incident laser excitation at 632 nm. The resolution was 2 cm−1. Thin films of BDP were grown on a quartz substrate by spin coating at 2000 rpm for 20 s; the crystals were first dissolved in aqueous solution. Optical absorption of the spin-coated films was deduced from direct transmission measurements performed with a conventional UV-visible spectrometer (Hitachi, U-3300). The room temperature photoluminescence spectrum was recorded with a PerkinElmer LS 55 spectrometer with excitation at 350 nm.

Colorless Parallelipipedic 189.15 1851.2(3) 1.357 Orthorhombic Pbca 120(2) 14.0377(12) 8.0915(7) 16.2980(2) 8 800 0.14 × 0.08 × 0.08 APEX II 3.2 < θ < 27.5 Mo Kα, 0.71073, graphite 15,285/2119 1617 0.045 −17/18, −10/10, −19/21 158 1.06 0.035/0.087 1/[σ2(F02) + (0.043P)2+ 0.6274P], where P = (F02+2Fc2)/3

Table 2 Summary of crystal data at different temperatures for C7H12NPO3. Temperature (K)

Final R1 and wR2

Volume (Å3)

Unit cell dimensions (Å)

120(2)

0.035/0.087

1851.2(3)

298(2)

0.064/0.201

1883.8(4)

350(2)

0.065/0.168

1887.8(17)

a = 14.0377(12) b = 8.0915(7) c = 16.2980(2) a = 14.0938(15) b = 8.0853(10) c = 16.532(2) a = 14.111(7) b = 8.087(3) c = 16.543(10)

2.3. Crystal structure

3. Theoretical method

The experimental conditions used for collection of single-crystal Xray diffraction data are reported in Table 1. A parallelepipedic crystal with size of 0.14 × 0.08 × 0.06 mm3 was mounted on a Bruker APEX II diffractometer that used Mo Kα radiation (λ = 0.71073 Å) at 120 K. The BDP crystal structure was solved by direct methods with use of SHELXS in the orthorhombic Pbca system with unit cell dimensions a = 14.0377 (12) Å, b = 8.0915 (7) Å, and c = 16.2980 (2) Å, and was then refined by means of SHELXL [31]. After successive refinements based on F2, we obtained a reliability factor R of 0.035. Anisotropy thermal displacement parameter refinement was used for all non-hydrogen atoms. The hydrogen atoms attached to carbon and nitrogen were placed at geometrically calculated positions and refined with an appropriate riding model. A variable-temperature single-crystal X-ray diffraction experiment was performed and led to the determination of the crystal structures at 298 and 350 K. The final coordinates of BDP at different temperatures are given in Tables S1, S2, and S3. Data collection revealed no significant differences between these crystal structures apart from thermal motion. A selection of the structural data at different temperatures is presented in Table 2. In the rest of this article, the 120 K crystal structure is used for the structural description. The crystal structures have been deposited with the Cambridge Crystallographic Date Centre under numbers 1827223, 1827224, and 1827225.

DFT computations were performed with the Lee-Yang-Parr B3LYP correlation functional with the 6-31G (d,p) basis set [32,33] implemented within Gaussian 03 [34] to derive the complete geometry optimizations and normal mode analysis. To take into account the effect of intermolecular interactions revealed by the crystal structure determination on geometrical parameters and vibrational spectra, we considered an appropriate cluster model built from one [H2PO3]- anion and one [C6H5CH2NH3]+ cation linked by N–H…O hydrogen bonds. All the parameters were allowed to relax, and all the calculations converged to an optimized geometry that corresponds to an energy minimum as revealed by the lack of imaginary values in the calculated wavenumbers. Before comparison of the calculated vibrational wavenumbers with the experimental counterparts, the former were scaled by 0.963 [35] to correct them for vibrational anharmonicity and deficiencies inherent to the computational level used. 4. Results and discussion 4.1. Description of the structure The asymmetric unit of BDP contains one [H2PO3]- anion and one [C6H5CH2NH3]+ cation. Selected experimental bond lengths and angles 151

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117.51 (7)°. Moreover, the O–P–H1P bond angles are between 104.6 (6)° and 109.1 (8)°. These values indicate distorted tetrahedral ions [36]. The planes of [H2PO3]- layers are located at z = 1/4 and z = 3/4 within the unit cell. These anions are connected through strong O (1)–H (O1)…O (3) hydrogen bonds (d (O…O) < 2.73 Å) [37,38], so infinite two-dimensional [H2PO3−]n chains parallel to the b-axis are formed. The [C6H5CH2NH3]+ cations are located at z = 0 and z = 1/2. The C–C, C=C, and C–N bond lengths are 1.504 (2) Å, 1.382(3)–1.392(2) Å, and 1.492 (2)Å, respectively. The C–C–N and C–C–C bond angles are between 112.65 (14)° and 121.19 (17)° (see Table S4 for the geometrical parameters of BDP at 298 and 350 K). The benzylammonium cations are connected to each other through van der Waals bonds, but are also connected to the anionic chains through N–H…O hydrogen bonds. The various hydrogen bond parameters are summarized in Table 4. All the hydrogen atoms attached to nitrogen atoms are connected by hydrogen bonds to O2 and O3 oxygen atoms of the phosphite anions. There are two types of hydrogen bonds: the relatively strong O–H… O bonds with a short D…A distance of 2.5832 (17) Å, and the weak N–H…O bonds with D…A distances ranging from 2.7555 (19) to 2.8121 (19) Å. Van der Waals interactions and hydrogen bonds give rise to a three-dimensional network in the structure and add stability to this salt. The optimized structural parameter bond lengths and bond angles determined from X-ray data and theoretically calculated for the appropriate cluster model at the B3LYP level with the 6-31G (d,p) basis set are listed in Table 3. For the dihydrogen phosphite anion, the average P–O bond length determined with the 6-31G (d,p) basis set is 1.542 Å, while the P–O–P bond angle is 111.91°; these values are in good agreement with the average P–O bond length and P–O–P bond angle determined from the crystal data: 1.523 Å and 111.81°, respectively. In the dihydrogen phosphite anion the P–O1 bond length is greater (1.572 Å) that the P–O2 (1.498 Å) and P–O3 (1.499 Å) bond lengths. This is due to the presence of an acidic hydrogen atom. The geometrical parameters (bond lengths and angles) of the dihydrogen phosphite anion are in good agreement with those reported in L-argininium phosphite [39], melaminum dihydrogen phosphite monohydrate [29], and L-prolunium phosphite [40]. For the benzylammonium cation the calculated average C–C bond length is 1.409 Å, while the C–C–C bond angle is 120.24°; these values are in good agreement with the average C–C bond length and C–C–C bond angle determined from the crystal data: 1.404 (3) Ǻ and 120.18 (17)°, respectively. These connections are similar to those observed in related compounds [1,41–55]. As seen in Table 3, the optimized parameters (bond lengths and angles) are in good agreement with the experimental data. The largest discrepancies do not exceed 0.044 Å for bond lengths and 1.468° for

Table 3 Comparison between observed and calculated geometrical parameters of C7H12NPO3.

Bond length (Å) P–O1 P–O2 P–O3 P–H1P C1–C2 C2–C3 C3–C4 C4–C5 C5–C6 C6–C1 C6–C7 N1–C7 Bond angle (°) O1–P–O2 O1–P–O3 O2–P–O3 O1–P–H1P O2–P–H1P O3–P–H1P C2–C1–C6 C3–C2–C1 C2–C3–C4 C3–C4–C5 C4–C5–C6 C1–C6–C7 C5–C6–C1 C5–C6–C7 N1–C7–C6

Experimental

Calculated

RMSD

1.572(13) 1.498(12) 1.499(12) 1.328(17) 1.386(3) 1.382(3) 1.384(3) 1.388(3) 1.390(2) 1.392(2) 1.504(2) 1.492(2)

1.616 1.515 1.496 1.390 1.392 1.396 1.384 1.389 1.401 1.395 1.508 1.494

0.044 0.017 −0.003 0.062 0.006 0.014 0 0.001 0.011 0.003 0.004 0.002

106.58(7) 111.34(7) 117.51(7) 104.6(6) 109.1(8) 106.9(8) 120.83(17) 120.00(18) 119.87(16) 120.03(18) 120.74(17) 120.25(17) 118.53(16) 121.19(17) 112.65(14)

106.91 110.44 118.39 97.88 113.31 107.98 120.233 120.446 119.317 120.629 120.353 121.106 119.998 119.816 112.473

0.33 −0.9 0.88 −6.72 4.21 1.08 −0.597 0.446 −0.553 0.599 −0.387 0.856 1.468 −1.374 −0.177

RMSD, root-mean-square deviation.

together with the calculated ones are presented in Table 3 in accordance with the atom numbering scheme given in Fig. 1a. The optimized geometry of the BDP model is presented in Fig. 1b. Examination of the crystal structure in the (b,c) projection shows an alternation of organic and inorganic layers along the c-axis (Fig. 2). The crystal structure is therefore based on infinite parallel two-dimensional planes built on [H2PO3]- tetrahedral ions and [C6H5CH2NH3]+ cations, confirming the hybrid nature of this compound. According to Table 3, the P–H1P distance in the [H2PO3]- anions is 1.328 (17) Å and the P–O distances range from 1.498 (12) to 1.499 (12) Å. The longest P–O distance is 1.572 (13) Å, which is due to the presence of the acidic hydrogen atom in the tetrahedron; the distance ranges from 1.557 (3) to 1.572 (13) Å for all structures at different temperatures. The O–P–O bond angles are between 106.58 (7)° and

Fig. 1. Atom numbering scheme for C7H12NPO3: (a) the experimental results and (b) the optimized geometry. 152

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Fig. 2. Projection along the a-axis of the atomic arrangement of C7H12NPO3. Table 4 Hydrogen bonds of C7H12NPO3. D–H···A

d (D–H) (Å)

d (H···A) (Å)

d (D···A) (Å)

∠(DHA) (°)

N1–H2N···O3a N1–H1N···O2b N1–H3N···O2c O1–H1O···O3a

0.87(2) 0.94(2) 0.89(2) 0.88(3)

1.94 1.82 1.92 1.74

2.8121(19) 2.7555(19) 2.7841(19) 2.5832(17)

179.1(19) 177.2(19) 161.2(18) 160(2)

a b c

(2) (2) (2) (3)

Symmetry code −x+3/2, y−1/2, z. Symmetry code x, y−1, z. Symmetry code −x+1, y−1/2, −z+3/2.

bond angles. These discrepancies can be explained by the fact that the calculations relate to the isolated molecule, where intermolecular Coulombic interactions with the neighboring molecules are absent, whereas the experimental results correspond to interacting molecules in the crystal lattice. The largest difference between the experimental and calculated values is associated with the P–H bond (0.062 Å) and the O–P–H angle (6.72°). Since strong deviation from experimental P–H bonds may arise from the low scattering factors of hydrogen atoms in the X-ray diffraction experiment, we can consider these values to be close to the experimental ones. As seen, the calculated geometric parameters represent a good approximation and can be used as a foundation to calculate the vibrational spectra of BDP.

Fig. 3. Differential thermal analysis (TDA) and thermogravimetric analysis (TGA) of C7H12NPO3.

4.3. Vibrational analysis To gain more information on the crystal structure, we undertook a vibrational study using IR absorption and Raman scattering. The experimental FT-IR spectrum between 400 and 4000 cm−1 and the simulated one are shown in Fig. 4. Experimental Raman spectra as well as the theoretical spectra computed with the 6-31G (d,p) basis set are shown and Figs. 5 and 6. The detailed interpretation of the vibrational modes was performed on the basis on our DFT calculations as the primary source of assignment and by comparison with the findings of spectroscopic studies of similar compounds. Selected observed and computed bands as well as the proposed assignment are given in Table 5.

4.2. Thermal studies TGA and DTA experiments were performed by our heating BDP from 298 to 535 K. The thermal analysis results are presented in Fig. 3. The endothermic peak observed at 433 K in the DTA thermogram accompanied by an important weight loss which tends towards 40% in the TGA thermogram corresponds to the melting point of BDP followed by its degradation. In addition, the sample is stable between 298 and 398 K as revealed by the TGA thermogram.

4.3.1. Vibration of benzylammonium cations 4.3.1.1. NH3+ group vibration. The NH3+ asymmetric and symmetric stretching bands usually appear at 3330 and 3080 cm−1, respectively 153

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Table 5 Calculated vibrational frequencies (scaled) and measured IR and Raman band positions with the proposed assignments.

Fig. 4. (a) Simulated IR spectrum and (b) experimental IR spectrum of C7H12NPO3.

νIR (cm−1)

νRaman (cm−1)

νcal (cm−1)

Assignments

– – – – – – 2976 w 2886 w 2750 w 2688 w 2410 w 2388 w 2176 w 1663 w – 1509 w 1486 w 1389 w 1367 w 1238 w 1195 w 1076 w 1052 w 1005 s 942 w 859 w 773 w 750 m 711 vw 602 w 555 – – – – – – – –

3187 vw 3079 vs 3064 vw 3056 m 3032 w 3018 m 2982 s 2911 vw 2776 vw – – – – 1626 w 1607 vw 1494 vw – 1407 vw 1377 w 1244 w 1204 vw 1070 m 1043 vs 1009 w 964 w 844 m 777 m – – 640 w 512 w 487 w 390 vw 350 vw 174 sh 147 m 121 m 100 vs 76 w

3326 3088 3074 3062 3040 2974 2971 2933 – – 2466 – – 1687 1609 1503 1492 1434 1385 1283 1203 1154 1116 1018 965 863 784 777 732 654 543 492 407 366 185 154 132 106 88

υasym (NH3+) υ(C–H) υ(C–H) υ(C–H) υ(C–H) υsym (NH3+) υasym (CH2) υsym (CH2) Combination, overtone Combination, overtone υ(PH) υ(PH)+ υsym (CH2) – δasym (NH3) + υ(C–C)ring δasym (NH3) + υ(C–C)ring δsym (NH3) + υ(C–C)ring δsym (NH3) + υ(C–C)ring δasym (CH2) δasym (CH2) t (CH2) + β(C–H) υasym (PO3) + β(C–H) υsym (PO3) + β(C–H) δ(PH) + β(C–H) δ(PH) + ρ (NH3) υ(P–OH) + γ (C − H ) γ (C–H) ρ (CH2) + γ (C − H ) υ(C–C) + β(C–C–C) γ (C–C–C) β(C–C–C) β(C–C–C) δasym (PO3) + γ (C−C−C) δsym (PO3) δsym (PO3) + τ (NH3) τ (NH3) + γ (C−C−C) τ (NH3) + γ (C−C−C) τ (HCCC ) + γ (C−C−C) τ (CNHO) + γ (C−C−C) τ (CNHO) + γ (C−C−C)

asym, asymmetric; m, medium; s, strong; sh, shoulder; sym, symmetric; t, twisting; vs, very strong; vw, very weak; w, weak; β, in-plane bending; γ, out-ofplane bending; δ, deformation; υ, stretching; ρ, rocking; τ, torsion.

Fig. 5. (a) Simulated Raman spectrum and (b) experimental Raman spectrum of C7H12NPO3 in the range from 50 to 1800 cm−1.

[56]. As the N–H group is involved in the hydrogen bonds, the frequency of this mode is shifted toward lower values. In BDP, the NH3+ group interacts with the phosphite anion through N–H...O hydrogen bonds. Thus, the asymmetric stretching mode of the NH3+ group appears in the Raman spectrum at 3187 cm−1 as a band with a wide shoulder and is predicted by DFT calculation to occur at 3326 cm−1. The symmetric stretching mode is observed in the Raman spectrum at 3018 cm−1 as a medium-intensity band. In the IR spectrum, these modes are probably masked by the broad band centered at 2900 cm−1. The NH3+ asymmetric deformation vibrations usually appear in the region from 1610 to 1660 cm−1 and the symmetric deformation vibrations usually occur in the region from 1485 to 1550 cm−1 [57]. For BDP, the NH3+ asymmetric deformation vibration modes are observed as weak bands at 1663 cm−1 in the IR spectrum and at 1626 and 1607 cm−1 in the Raman spectrum. The medium-intensity IR bands at 1486 and 1509 cm−1 are assigned to the symmetric deformation vibrations of the NH3+ cation. As seen from Table 5, the δasym (NH3) and δsym (NH3) modes are coupled with the C–C ring stretching mode. Bands related to rocking and torsional modes were observed and assigned (Table 5).

Fig. 6. (a) Simulated Raman spectrum and (b) experimental Raman spectrum of C7H12NPO3 in the range from 2600 to 3400 cm−1.

4.3.1.2. CH2 group vibrations. The asymmetric and symmetric stretching modes of the CH2 group usually occur in the region from 154

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bending modes were consistent with the experimental spectral values. 4.3.2. Phosphite anion vibration The detailed assignment of the phosphite anion H2PO32− was given by Baran et al. [63]. In the present work, the stretching mode ν (P − H ) is observed at 2410 and 2388 cm−1 in the IR spectrum, with no counterpart in the Raman spectrum. The bending mode δ (P − H ) is observed at 1052 and 1005 cm−1 in the IR spectrum and at 1043 and 1009 cm−1 in the Raman spectrum. The stretching mode ν (P − OH ) appears at 942 cm−1 in the IR spectrum and at 964 cm−1 in the Raman spectrum; these values are in good agreement with the computed value of 958 cm−1. The asymmetric and symmetric stretching modes related to –PO3 groups are identified at 1195 and 1076 cm−1, respectively, in the IR spectrum. In the Raman spectrum these modes appear at 1204 and 1070 cm−1, respectively. The asymmetric and symmetric bending modes of –PO3 groups were identified and assigned (Table 5). Our assignment agrees well with the previous work on organophosphates [22–29,63]. 4.4. Optical studies

Fig. 7. (a) Room temperature absorption and (b) photoluminescence of C7H12NPO3.

Fig. 7 shows the experimental UV-visible absorption and photoluminescence spectra of the spin-coated film of BDP at room temperature. As seen there is no absorption in the visible region, and the compound is transparent in the visible region. On the other hand, the photoluminescence spectrum shows a broad and intense green band at 502 nm. This broadening is probably caused by population of different vibrational levels of the excited states.

2800 to 3000 cm−1 [58,59]. In BDP, the CH2 asymmetric and symmetric stretching modes are observed in the IR spectrum at 2976 cm−1 as very broad band and at 2886 cm−1 as a very weak band, respectively. In the Raman spectrum, these modes are observed at 2982 and 2911 cm−1. The observed IR band located at 1367 cm−1 and the corresponding band at 1377 cm−1 in Raman spectrum are assigned to the wagging mode of the CH2 group. This mode is predicted to be at 1385 cm−1 by DFT calculation. The observed IR band at 1238 cm−1 and the corresponding Raman band at 1244 cm−1 are associated with the twisting mode of the CH2 group. Rocking modes of the CH2 group were observed at 773 and 777 cm−1 in the IR and Raman spectra, respectively. As seen in Table 5, all calculated values related to CH2 groups are in good agreement with the experimental data.

5. Conclusions In the present work, the crystal structure, vibrational studies, thermal analysis, and optical properties of a new organophosphite compound have been reported. Single-crystal X-ray diffraction at 120, 298, and 350 K revealed that BDP does not undergo any structural phase transition and crystallizes in Pbca orthorhombic symmetry. The crystal packing of this organophosphite compound is built from alternating organic and inorganic layers. The benzylammonium cations are linked to phosphite layers by N–H…O hydrogen bonds, whereas the phosphite anions are joined in infinite [H2PO3−]n chains parallel to the b-axis through strong O–H…O hydrogen bonds. The structural data and the vibrational spectra calculated with the DFT method at the 6-31G (d,p) level of theory reasonably agree with the experimental results. On the basis of the agreement between the experimental and theoretical results, an assignment of all observed bands was proposed. The UVvisible spectrum shows good transparency in the visible region, and luminescence measurements show a broad and intense green fluorescence.

4.3.1.3. C–H vibrations. The C–H ring stretching occurs in the region from 3000 to 3100 cm−1, which is the characteristic region for the ready identification of the C–H stretching vibrations in the monosubstituted benzene ring [60]. In BDP, the C–H stretching vibrations are observed at 3079 cm−1 as a very strong band and at 3064, 3056, and 3032 cm−1 as shoulder bands in the Raman spectrum and are probably masked in the IR spectrum by the broad band located at 2900 cm−1. Bands involving the in-plane aromatic C–H bending vibrations appear in the range from 1000 to 1300 cm−1 [61]. In BDP, the C–H in-plane bending vibrations are observed at 1238, 1195, 1076, and 1052 cm−1 in the FT-IR spectrum and at 1244, 1204, 1070, and 1043 cm−1 in the Raman spectrum. They show good agreement with those theoretically computed with the 6-31G (d,p) basis set at 1283, 1203, 1154, and 1116 cm−1. The bands observed at 859 and 773 cm−1 in the FT-IR spectrum and at 844 and 777 cm−1 in the Raman spectrum are assigned to C–H out-of-plane bending vibrations. This mode shows good agreement with the DFT prediction (bands at 849 and 808 cm−1).

Acknowledgements We greatly thank the Institut de Chimie de la Matière Condensée de Bordeaux, especially the X-ray diffraction service and all members of the Molecule and Switchable Materials Group. Appendix A. Supplementary data

4.3.1.4. C–C vibrations. The C–C ring stretching vibrations occur in the range from 1430 to 1625 cm−1, and in general the bands are of variable intensity [56,62]. In the present work, the bands observed in the FT-IR spectrum at 1663, 1509, and 1486 cm−1 were assigned to C–C stretching vibrations. The counterpart vibrations in the Raman spectrum are observed at 1626, 1607, and 1494 cm−1. This mode is coupled with NH3 bending modes as can be seen in Table 5. The weak band located at 942 cm−1 in the FT-IR spectrum and at 964 cm−1 in the Raman spectrum is assigned to the ring breathing mode. The C–C–C inplane and out-of-plane bending modes were identified and assigned (Table 5). As seen from Table 5, the theoretically computed C–C–C

Supplementary data related to this article can be found at https:// doi.org/10.1016/j.jpcs.2018.07.017. References [1] A. Ben Rached, P. Guionneau, E. Lebraud, T. Mhiri, Z. Elaoud, J. Phys. Chem. Solid. 100 (2017) 25–32. [2] W. Maalej, A. Ben Rached, T. Mhiri, A. Daoud, N. Zouari, Z. Elaoud, J. Phys. Chem. Solid. 96–97 (2016) 92–99.

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