Crystal structure, vibrational spectra and non-linear optical properties of diethylenetriammonium hexabromobismuthate: C4H16N3BiBr6

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1235–1243

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Crystal structure, vibrational spectra and non-linear optical properties of diethylenetriammonium hexabromobismuthate: C4H16N3BiBr6 Hajer Dammak a,⇑, Habib Feki a, Habib Boughzala b, Younes Abid a a b

Laboratoire de Physique Appliquée (LPA), Université de Sfax, Faculté des Sciences, BP.1171, 3000 Sfax, Tunisia Laboratoire des Matériaux et de Cristallochimie, Institut préparatoire des études ingénieurs de Nabeul, 800, Mrezga, Nabeul, Tunisia

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 A detailed interpretation of infrared

arb. units

and Raman spectra of C4H16N3BiBr6 were reported.  Nonlinear optical properties of C4H16N3BiBr6 were studied using DFT calculations.  HOMO–LUMO energy gap explains the charge transfer interactions in the molecule.  UV spectrum was interpreted and assigned based en TD-DFT calculation.

2,0

2,5

3,0

3,5

4,0

4,5

5,0

Energy (eV)

a r t i c l e

i n f o

Article history: Received 12 May 2014 Received in revised form 26 July 2014 Accepted 31 August 2014

Keywords: Organic–inorganic Zero-dimensional quantum dots Absorption DFT

a b s t r a c t A new organic–inorganic material, diethylenetriammonium hexabromobismuthate (C4H16N3)BiBr6, was synthesized and characterized by X-ray diffraction, infrared absorption, Raman spectroscopy scattering and optical absorption. The crystal lattice is composed of discrete [BiBr6] anions surrounded by diethylenetriammonium cations. The title compound crystallizes in the non-centro-symmetric space group P212121 of orthorhombic system. Theoretical calculations were performed using density functional theory (DFT) at B3LYP/LanL2DZ level of theory for studying the molecular structure, vibrational spectra and non-linear optical (NLO) properties of the investigated molecule in the ground state. Good consistency is found between the calculated results and the experimental structure, IR, and Raman spectra. The results also show that the title compound might have important NLO behavior and can be a potential new nonlinear optical (NLO) material of interest. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Recently, much attention has been devoted to the large family of organic–inorganic metal halides based perovskites due to their special structural features and physical properties. This important class of low dimensional materials exhibit distinctive optical properties such as strong and sharp photoluminescence [1], ⇑ Corresponding author. Tel.: +216 20 640 743. E-mail address: [email protected] (H. Dammak). http://dx.doi.org/10.1016/j.saa.2014.08.150 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

electroluminescence and highly efficient non linear optical effects [2] which make these hybrid systems attractive candidates for electronic and optoelectronic devices [3,4]. As an important class of low dimensional hybrid materials, organic–inorganic perovskite like family of the type RxMyXz (where R is protonated amine, M is a metal and X is a halide) has received considerable interest in the past decades. In the case of bismuth-halide systems, some interest has been directed towards halobismuthates (III) compounds in combination with organic cation, due to the potential semiconducting behavior, as well as the rich structural diversity displayed

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by these systems. As already pointed out in several published works, the structural chemistry of bismuth halides in the solid sates form various structures mainly with an anionic sublattice built up of the MX6 which can be connected in one of three way: face, edge or corner sharing forming naturally isolated molecules (0D), infinites chains (1D) two (2D) or three (3D) dimensional networks [5–16]. However, in conjunction with some recent works on organic–inorganic metal halides compounds, the crystal structure and vibrational studies of some halobismuthate (III) have been investigated in our laboratory [17–20]. In this work, we synthesized a novel organic–inorganic compound, C4H16N3BiBr6 which crystallizes in non centrosymmetric space group showing enhanced NLO activity. We report in this paper the XRD, Fourier transformation infrared (FT-IR), Raman spectroscopy, and UV absorption experiments of the title compound. A reliable assignment of vibrational bands in the IR and Raman spectra is needed. For this purpose density functional theory (DFT) calculations were performed. In the light of our theoretical calculations, correlation between vibrational spectra and computed results help unambiguous identification of vibrational modes and provide deeper insight into the bonding and structural features of complexes inorganic– organic molecular systems. The energy difference between HOMO and LUMO orbital which is called energy gap is a critical parameter in determining molecular electrical transport properties. Then, the HOMO–LUMO gap of a molecule will play an important role in determining its NLO properties [21,22]. The non linear optical properties will be discussed in relation to a computational approach, to rationalize the origin of the effect. Experimental Synthesis The (C4H16N3) BiBr6 crystals were grown by slow evaporation at room temperature. (C4H16 N3Br3) precipitates are first formed by adding an aqueous solution of HBr to diethylenetriamine (C4H13N3). Under ambient condition, [NH3(CH2)2NH2(CH2)2NH3]3+ 3Br and BiBr3 were dissolved in a concentrated HBr solution in the presence of water in a stoichiometric ratio. Four weeks later, transparent crystals are formed. The purity of synthesized compound was improved by successive recrystallization process. X-ray data collection A single crystal was selected for the diffraction experiments. The X-ray data collection was carried out on an Enraf-Nonus C4D4 four circle diffractometer using Mo Ka radiation (k = 0.71073 Å) at 293 K. The crystal structure was solved by the direct method and refined by the full matrix least-square technique using the SHELX-97 crystallographic software package [23]. The basic crystallographic data and the details of the measurements and refinement are summarized in Table 1. Supplementary crystallographic data for this article in CIF format are available as Electronic Supplementary Publication from Cambridge Crystallographic Data Centre (CCDC 989072). This data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/ retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Rood, Cambridge CB2 1EZ, UK (fax: (international): +44 1223/336 033; e-mail: [email protected]). Spectroscopic measurements Raman scattering was performed at room temperature using a LABRAM-Jobin Yvon set up. The excitation line was the 630 nm from a Neon laser in the range 50–4000 cm1. The incident laser

Table 1 Crystal data and structural refinement for (C4N3H16) BiBr6. Crystal data

Details

Empirical formula Formula weight Temperature (K) Wavelength (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) Volume (Å3) Z Densitycalc Crystal size (mm3) Crystal color h range Limiting indices h, k, l Goodness-of-fit Rint

(C4N3H16)BiBr6 794.64 293 0.71073 Orthorhombic P212121 7.134 (3) 13.995 (3) 16.225 (6) 1619.91 (10) 4 3.2851 mg m3 0.4  0.2  0.02 Yellow 2.51–26.97 9 6 h 6 3, 1 6 k 6 17, 20 6 l 6 20 1.178 0.0744

power was limited to 5 mW to avoid sample heating degradation. The laser beam was focused on to the sample through a 50  microscope objective. The IR spectrum was recorded in the 400– 3500 cm1 region on a BRUKKER spectrometer using KBr pellet technique with a spectral resolution of 2 cm1. Computational details Density functional theory (DFT) computations were performed by using the Lee–Yang–Parr correlation functional B3LYP with the LanL2DZ basis set [24,25] implemented within Gaussian 03 program [26] to derive the complete geometry optimizations, first hyperpolarizability and normal modes analysis. In order to take into account the effect of intermolecular interactions on geometrical parameters and vibrational spectroscopy, we have considered an appropriate cluster model built up from one (BiBr6)3 and two (C4H16N3)3+ cations linked by N–H  Br hydrogen bonds. All the parameters were allowed to relax and all the calculations converged to an optimized geometry which corresponds to an energy minimum as revealed by the lack of imaginary values in the calculated wavenumbers. Prior to compare the calculated vibrational wavenumbers with the experimental counterparts, the former have been scaled by 0.963 scaling factor [27] to correct the evaluated wavenumbers for vibrational anharmonicity and deficiencies inherent to the computational level used. The vibrational modes were assigned on the basis of PED analysis with the aid of VEDA 4 program [28]. Results and discussions Structure description The title compound crystallizes in the non centrosymmetric orthorhombic space group P212121 with four formula units in unit cell (Z = 4). The cell dimensions are: a = 7.134 (3) Å, b = 13.995 (3) Å, c = 16.225 (6) Å and V = 1619.91 (10) Å3. The asymmetric unit cell contains one bromobismuthate [BiBr6]3 anion and one crystallographically independent tri-protonated diethylentriamine cation as shown in (Fig. 1a). The spaces between the inorganic entities are filled by organic cations assuring their connection by means of the N–H  Br hydrogen bonds, forming a zerodimensional network. The [BiBr6]3 unit possesses a configuration of distorted octahedral, so that the central bismuth (III) ion is surrounded by six bromine atoms as shown in packing diagram

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Fig. 1. (a) Projection of (C4H16N3) BiBr6 structure along [1 0 0] direction and (b) view of the asymmetric unit of diethylenetriammonium hexabromobismuthate showing the atomic numbering scheme.

Table 2 Comparison between observed and calculated inter-atomic parameters. Parameters

Observed

Bond length (Å) Bi–Br1 Bi–Br2 Bi–Br3 Bi–Br4 Bi–Br5 Bi–Br6 N1–C1 N2–C3 N2–C2 N3–C4 C1–C2 C3–C4

2.868 (3) 2.940 (3) 2.852 (4) 2.791 (3) 2.902 (3) 2.781 (4) 1.46 (4) 1.49 (5) 1.53 (4) 1.44 (4) 1.41 (5) 1.50 (5)

Bond angles (°) Br1–Bi–Br2 Br1–Bi–Br3 Br1–Bi–Br4 Br1–Bi–Br5 Br1–Bi–Br6 Br2–Bi–Br3 Br2–Bi–Br4 Br2–Bi–Br5 Br2–Bi–Br6 Br3–Bi–Br4 Br3–Bi–Br5 Br3–Bi–Br6 Br4–Bi–Br5 Br4–Bi–Br6 Br5–Bi–Br6 C3–N2–C2 C2–C1–N1 C1–C2–N2 N2–C3–C4 N3–C4–C3

89.72 (9) 178.02 (10) 91.30 (10) 86.15 (10) 87.28 (13) 88.96 (10) 178.13 (10) 96.76 (10) 90.02 (11) 89.99 (10) 95.47 (11) 91.25 (14) 84.87 (10) 88.46 (11) 170.52 (13) 116 (3) 110 (3) 111 (3) 107 (3) 113 (3)

Calculated 2.9865 3.2529 2.9392 2.8994 3.1022 2.8314 1.5297 1.4740 1.4750 1.5267 1.5522 1.5475 86.59 174.30 95.46 83.98 84.66 86.59 177.76 100.36 91.17 90.27 98.12 94.57 79.97 85.85 165.87 116.31 111.74 109.84 111.20 110.70

(Fig. 1b). Selected bonds and angles are listed in Table 2. The Bi–Br bond lengths vary from 2.781 (4) Å to 2.940 (3) Å and the Br–Bi–Br bond angles fall in the range 84.87 (10)–96.76 (10) which suggests a slight distortion of the [BiBr6]3 octahedra. Such results are similar to the known data for hexabromobisuthates anion in homologous compound [29,30]. In C4H16N3BiBr6, the organic species interact with the inorganic entities via N–H  Br hydrogen bond. The various hydrogen bond parameters are summarized in Table 3. The N–C bond length range from 1.44(4) to 1.53 (4) Å. As seen in Table 2 most of the computed bonds are slightly longer than experimental one. This discrepancies can be explained by the fact that the calculations relates to the isolated molecule where the intermolecular Coulombic interaction with the neighboring molecules are absent, whereas the experimental result corresponds to

Table 3 Hydrogen bond geometry for (C4N3H16) BiBr6. D–H  A

D–H (Å)

D  A (Å)

H  A (Å)

D–H  A (°)

N1–H1A  Br1 (1) N1–H1A  Br4 (1) N1–H1B  Br5 (2) N1–H1C  Br4 (2) N2–H2A  Br1 (3) N1–H1C  Br2 (3) N3–H3A  Br1 (4) N2–H2B  Br2 (4) N3–H3C  Br3 (5) N2–H2B  Br4 (5) N3–H3A  Br6 (6) N3–H3B  Br2 (7) N3–H3B  Br3 (7)

0.89 0.89 0.89 0.89 0.90 0.89 0.89 0.90 0.89 0.90 0.89 0.89 0.89

3.412 3.373 3.391 3.438 3.352 3.474 3.567 3.426 3.265 3.476 3.361 3.450 3.310

2.681 2.870 2.532 2.851 2.696 2.691 2.905 2.602 2.429 2.943 2.801 2.872 2.594

140.01 118.22 163.40 124.82 130.57 147.31 132.53 152.51 156.44 119.47 122.24 124.00 138.06

(0.028) (0.028) (0.027) (0.028) (0.026) (0.027) (0.031) (0.026) (0.031) (0.025) (0.030) (0.030) (0.030)

Equivalent positions: (1) x + 1, +y, +z; (2) x + 1/2, y + 1/2, z + 1; (3) x + 1, +y1/2, z + 1/2; (4) x + 2, +y1/2, z + 1/2; (5) x + 1/2 + 1, y + 1/2, z + 1; (6) x + 2, +y1, +z; (7) x + 1, +y1, +z.

H. Dammak et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1235–1243

149

1238

(a) N

C

65

C

Br

N

C

Br

Bi

N N

168

C Br

Raman intensity

Br

(b)

75

Br C

Br C

N C N 300

250

200

1646 2723

2500

2000

1464

946

1500

760

1488

987

1402 1323 1262 1166 1127 1062

1642 1597

1550

2858

3142 3057 3000

3396

(b)

1000

-1

Wavenumbers (cm ) Fig. 3. (a) Simulated IR spectrum and (b) experimental IR spectrum of the (C4H16N3)BiBr6.

595

774

959

1096

2785

2951

3059

(a)

1535

1474

comparison with the previously reported vibrational studies of similar compounds. As mentioned above, the crystal structure of the title compound consists of isolated BiBr6 octahedra surrounded by organic cations. Then, in order to take into account the effect of intermolecular N–H  Br hydrogen bonds on geometrical parameters and vibrational spectroscopy, the cluster composed of one BiBr6 octahedra surrounded by two protonated diethylenetriaminium cations is perfect to model the system. The DFT optimized geometry of the title compound is presented in Fig. 2. The IR

2849

3142

3378

To gain more information on the crystal structure, we have undertaken a vibrational study using Raman scattering and infrared absorption at room temperature. In this work, we tried to give most precise assignment of the observed bands on the basis on our calculations as a preliminary source in the discussion and also by

3000

50

Fig. 4. (a) Simulated Raman spectrum computed at B3LYP/LanL2DZ basis and (b) experimental Raman spectrum recorded in the low frequency range.

1296

Vibrational studies

3500

100

wavenumbers (cm )

interacting molecules in the crystal lattice. The maximum difference does not exceed 0.32 Å for the bond lengths and 4.65° for the bond angles. This result shows that the cluster approach is sufficient to the analysis of the spectra in the solid state.

Transmittance

150 -1

Fig. 2. Optimized molecular structure of [(C4H16N3)2 BiBr6]3.

823

C

H. Dammak et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1235–1243

1461

1293

487

766

373 330

829

961 913

1122

1063 1024

1178

1450

(b)

1800

1600

1400

1000

314

504 436

822 779

944 916

752

975

1200

1053 1038

1258 1209 1171 1122

1335

1415

1487

1583

1559 1527

Raman intensity

1398

1537

1624

(a)

1239

800

600

400

Wavenubers (cm-1) Fig. 5. (a) Simulated Raman spectra computed at B3LYP/LanL2DZ basis set and (b) experimental Raman spectrum recorded at room temperature in the range 300–1800 cm1.

2883 2848

2919 2897 2857 2820

2983 2955 3104 3066

(b)

4000

2785

3057

2964

3141

3373

3439

Raman intensity

(a)

3800

3600

3400

3200

3000

2800

2600

Wavenumbers (cm-1) Fig. 6. (a) Simulated Raman spectra computed at B3LYP/LanL2DZ basis set and (b) experimental Raman spectrum recorded at room temperature in higher frequency range 2500–4000 cm1.

spectrum measured between 400 and 3500 cm1 and the simulated one are shown in Fig. 3a and b respectively. Experimental Raman are presented in three different regions namely a low wavenumber region (50–300 cm1) Fig. 4, medium wavenumber region (300–1800 cm1) Fig. 5 and a high wavenumber region (2500– 4000 cm1) Fig. 6. For visual comparison we have superposed experimental and simulated Raman spectra in different regions. The frequencies of the calculated and observed Raman and infrared peaks are quoted in Table 4. The vibrations of (BiBr6)3 The bands which appear in the frequency range (50–300 cm1) are associated to translational and librational vibrational modes of the organic and the inorganic groups in the unit cell. As there are many vibration modes in a relatively narrow frequency range,

the overlapping between the Raman peaks leads to a reduced number observed vibrations modes. The strong band located at 75 cm1 is attributed to Br–Bi–Br binding mode. This mode is predicted at 65 cm1 according to B3LYP/LanL2DZ calculations. The very strong Raman band at 168 cm1 is due to the symmetric Bi–Br stretching mode coupled with cation rotation. As seen in Table 4, this mode is well predicted by theoretical method at 149 cm1. It can be seen that agreement between observed and calculated frequencies is reasonable showing that DFT/B3LYP/LanL2DZ performed for the cluster presented in Fig. 2 has accurately modeled the system. The vibration of diethylenetriammonium cation The bands observed between 400 and 4000 cm1 are assigned to internal modes of organic cation. Numerous functional groups þ and skeletal groups such as NHþ 3 ; NH2 ; CH2 , C–C, C–N, C–C–N

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Table 4 Calculated vibrational wavenumbers (scaled), measured infrared and Raman position (cm1) with the proposed assignment. Experimental

Calculated

Infrared

Raman

B3LYP/LanL2DZ

3396 b,m 3142 w 3057 b 3000 w – 2953 vw – – – 2858 w 2805 w

 3104 b,m 3066 b,m – 2983 s 2955 vs 2919 w 2897 w 2879 w 2857 m 2820 m 2727 b,w – 1583 m 1559 s 1527 s 1487 w 1450 vs 1438 m 1415 m 1366 w 1335 m 1317 w 1258 w 1209 m 1171 m 1122 m 1053 sh 1038 m 975 s 944 m 916 w 822 w 779 vw 752 s 504 m 436 m 314 m 168 vs 75 s

3378 3142 3059 3010 2964 2951 2914 2883 2877 2849

1642 sh 1597 s 1550 m – 1488 vs 1464 m 1445 w 1402 vw 1383 vw 1323 m 1262 vw – 1209 vw 1166 m 1127 m 1062 w 1026 sh 987 s 946 w 923 sh 823 w 794 vw 760 s – – – –

2723 1646 1624 1537 1535 1474 1461 1435 1398 1380 1347 1293 1263 1244 1178 1122 1063 1024 961 959 913 829 – 766 487 452 – 149 65

Assignment with PED (P10%)

mas(NH3)(99) mas(NH3)(99) ms(NH3)(97) ms(NH3)(98) mas(NH2)(84) ms(NH2)(85) mas(CH2)(92) mas(CH2)(86) ms(CH2)(92) ms(CH2)(89) ms(CH2) ms(NH2)(85) + mas(CH2) (14) das(NH3)(78) das(NH3)(79) + ms(CH2) (13) ds(NH3) + das(NH2) ds(NH3)(80) das(CH2)(74) das(CH2)(74) ds(CH2)(83) ds(CH2)(80) x(CH2)(78) x(CH2)(67) + x(NH2)(29) twist(NH2)(76) twist(CH2)(74) q(NH3)(78) + twist (CH2) (13) q(NH3)(67) m(C–N)(77) mas(C–C–N)(79) mas(C–N–C)(72) m(C–C)(76) mðC—CÞð64Þ þ qðNHþ3 Þð23Þ q(NH3)(67) ms(C–C–N)(73) + q(NH3)(13) ms(C–N–C) + q(NH3) q(CH2)(77) s(NH3)(64) + d(C–C–N)(22) s(NH3)(32) + d(C–C–N)(53) s(NH3) + d(C–N–C) + d(C–C–N) m(Bi–Br)(54) d(Br–Bi–Br)(59)

m, stretching; q, rocking; x, wagging; twist, twisting; d, scissoring; s, torsion. Subscripts: asym, asymmetric; sym, symmetric; w, weak; vw, very weak; s, strong; vs, very strong; m, medium; sh, shoulder; b, broad.

and C–N–C present in diethylenetriammonium cation. These groups are manifested in IR and Raman spectra in different range with different intensity. þ Vibration of NHþ 3 and NH2 groups. The N–H stretching vibration appears strongly and broadly in region 3450–3360 cm1 [20]. A careful inspection of the IR spectrum of the title compound, shows a broad band centered at 3396 cm1 and medium intensity broad band extending from 3200 to 2900 cm1 with peaks at 3142, 3057, 3000 and 2953 cm1. The spectral region 3200–2950 cm1 in Raman spectrum is characterized by two broad and medium intensity bands at 3104 and 3066 cm1, strong band at 2983 cm1 and a very strong band at 2955 cm1. We have assigned all these bands to N–H stretching mode. As the N–H group of the þ central (NHþ 2 ) and both (NH3 ) moiety of organic cation are involved in hydrogen bonds, the wavenumbers of this mode is shifted towards lower values. In fact, the XRD study of the title compound reveals that each of the three (two) hydrogen atoms in (NHþ 3 ) or (NHþ 2 ) moiety is involved in N–H  Br hydrogen bonds with the lengths of the order 3.288–3.540 Å. Thus one can also expect the various stretching N–H modes from 3400 to 2950 cm1 depending

on the strength of hydrogen bonds. It is interest to note that all these wavenumbers are pure stretching modes as expected from the PED calculation. In the IR spectrum the shoulder band shown at 1642 cm1 and the strong one at 1597 cm1 are assigned to the asymmetric das(NH3) bending mode. The corresponding predicted theoretically values are at 1646 and 1624 cm1. The two strong bands at 1559 and 1527 cm1 in Raman spectrum are assigned to the symmetric ds(NH3) bending mode. The wagging, þ rocking and the torsional modes related to (NHþ 2 ) and (NH3 ) are observed and assigned. As seen, the bending wavenumbers are not mush shifted indicating that the linear distortion is much greater than the angular distortion. Vibration of CH2 group. The 2950–2800 cm1 range correspond to the asymmetric and symmetric stretching vibrations of the CH2 group which are intense in Raman spectrum and probably overlapped by the hydrogen bonds stretching modes in IR spectrum. The asymmetric ethyl stretching vibrations are observed in Raman spectrum as weak bands at 2919 and 2897 cm1 whereas the symmetric corresponding vibrations appear at 2879 and 2857 cm1. The observed IR band located at 1323 cm1 and at 1335 cm1 in Raman spectrum is assigned to the wagging modes of CH2 and NHþ 2 groups. As seen in Table 4, all calculated B3LYP/LanL2DZ values related to CH2 group are in good agreement with the experimental data. The wagging, twisting and rocking modes of the CH2 group are observed and they lie within the expected range suggesting that the interaction of the ethyl group with environment is not strong. Vibrations of C–N, C–C, C–N–C and C–C–N groups. The Raman band observed at 1122 cm1 and the corresponding IR bands at 1127 cm1 can be assigned to C–N stretching mode. The strong band located at 975 cm1 in Raman spectrum and at 987 cm1 in IR spectrum is assigned to C–C stretching mode. The medium intensity band observed at 944 cm1 in Raman spectrum and at 946 cm1 in IR spectrum is assigned to C–C stretching mode coupled with the rocking mode of NHþ 3 . The C–C–N asymmetric stretching modes is observed at 1062 cm1 in IR spectrum and at 1053 cm1 in Raman spectrum and predicted at 1063 cm1. In the Raman spectrum the band observed at 822 cm1 arises from C–C–N symmetric stretching mode coupled with the rocking mode of NHþ 3 . The C–C–N binding mode appears as medium bands at 504 and 436 cm1 in Raman spectrum coupled with sðNHþ 3 Þ mode. In the case of tetrakis (dimethyammonium) bromide hexabromobismuthate, two bands observed at 1078 and 1072 cm1 were assigned to the asymmetric C–N–C mode [20]. In our case, we have assigned the medium band located at 1038 cm1 in Raman spectrum and at 1026 cm1 in IR spectrum to the asymmetric C–N–C stretching mode whereas the symmetric mode is observed at 794 cm1 in IR spectrum and at 779 cm1 in Raman spectrum. A detailed assignment of the skeletal vibration is given in Table 4.

Table 5 The electric dipole moment l (D), the average polarizability a (1024 esu) and the first hyperpolarizability b (1031 esu) computed at B3LYP/LanL2DZ.

lx ly lz l axx ayy azz axy axz ayz atot

3.3352 17.4205 1.3501 17.7881 35.3807 188.9426 141.7672 10.3741 28.5269 5.3215 18.9740

bxxx bxxy bxyy byyy bxxz bxyz byyz bxzz byzz bzzz btot

12.0120 88.9986 41.5014 54.3071 64.2001 67.6417 58.7446 33.4827 38.8045 28.4678 7.0021

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H. Dammak et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1235–1243 Table 6 Comparison of static first hyperpolarizability, polarizability, dipole moment and HOMO–LUMO energy gap of the constituents of the title compound. Compounds 3

(BiBr6) [NH3(CH2)2NH2(CH2)2NH3]+3 [NH3(CH2)2NH2(CH2)2NH3](BiBr6)

l (D)

atot (1024 esu)

btot (1031 esu)

DEHL (eV)

0.3101 2.8302 17.7881

25.8743 7.8783 18.0848

0.2808 2.5004 7.0021

5.42 7.27 4.30

The crystallization of the title compound in non-centrosymmetric space group prompted us to explore its second-order NLO properties. The calculation of first static hyperpolarizability from the GAUSSIAN 03 output has been explained previously and well detailed in our previous publications [30,31]. Electronic structure calculation was performed for the title compound in order to understand the electronic structure and to assign the observed absorption bands (see next section). For this reasons, we have performed theoretical TD-DFT calculations. In Table 5 are listed the B3LYP/LanL2DZ results of the electronic dipole moment li (i = x, y, z), polarizability aij and the first hyperpolarizability bijk for the chosen cluster presented in Fig. 2. The calculated dipole moment is equal to 17.788 D (Debye). The highest value of dipole moment is observed for component ly. In this direction the value is equal to 17.420 D. The calculated polarizability atot is equal to 18.974 1024 esu. The first hyperpolarizability btot of the title compound is equal to 7.0021  1031 esu and small dominated by the bxxy component which is equal to 88.998 au. Domination of particular component indicates a substantial charge delocalization in this direction. As seen, btot has the same order of the reference crystal KDP (b (KDP) = 6.85  1031 esu). This results show that the title compound can be good material for NLO applications. In order to investigate the role of hydrogen bonds on the electronic structure and the non-linear behavior, we have calculated ltot, atot and btot of the constituents of the title compound. Table 6 shows that the static hyperpolarizability btot is larger for the cluster than btot for all constituents. As can be seen, the compound having the higher dipole moment results in the higher btot value and the corresponding highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO–LUMO) energy gap is quite low compared with inorganic anion and organic cation. This clearly indicates that the hydrogen bonding between the charged species reduced the energy gap considerably and plays an important role not only in stabilization of the crystal structure, but also in enhancement of the polarizability and the hyperpolarizability of the crystal. It is interest to note the calculated first hyperpolarizability of the title compound is predicted to be about two times that of our previous investigated compound ((CH3)2NH2)3BiBr6 (b = 3.5747  1031 esu) [20]. Hence, the large b value shows that the studied compound is a good material for NLO applications.

arb. units

Hyperpolarizability calculations

(a) (b)

2,0

2,5

3,0

3,5

4,0

4,5

5,0

Energy (eV) Fig. 7. Superposition of (a) optical absorption spectra of spin coated C4H16N3BiBr6 film on a quartz substrate measured at room temperature and (b) HOMO–LUMO electronic transition predicted by (TD-DFT) calculation.

LUMO PLOT (Excited state)

ELUMO = -3.8 eV

∆E = 4.30 eV

Optical absorption Fig. 7 shows the experimental UV–Vis absorption spectrum of the spin-coating film of diethylenetriammonium hexabromobismuthate at room temperature and the simulated spectrum predicted by TD-DFT/B3LYP/LanL2DZ level of theory. As seen, there are two distinct absorption peaks at 3.22 eV and 4.08 eV whereas theoretical calculation predicted only an intense electronic transition at 4.17 eV. The calculated band corresponds to electronic transition from the highest occupied molecular orbital (HOMO) to the lowest un-occupied molecular orbital (LUMO) and agree well with the observed band at 4.08 eV. The shift between calculated and measured transition energies is around 0.09 eV. Such shift is commonly observed in TD-DFT calculations employing the B3LYP

E HOMO = -8.16 eV

HOMO PLOT (Ground state) Fig. 8. Frontier molecular orbital of (C4H16N3)BiBr6.

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HOMO

LUMO

Fig. 9. Density of state (DOS) spectrum of (C4H16N3)BiBr6.

functional and is due to overestimation of the HOMO–LUMO gap [32]. The atomic orbital compositions of the Frontier molecular orbitals are sketched in Fig. 8. The Frontier molecular orbital are mainly composed by orbits of Bi and Br atoms. In the HOMO, maximum contribution comes from the halogen. In the LUMO, the percentage of contribution from the bismuth orbital is dominant along with some contribution from two bromide atoms. As an assignment attempt, the band with maximum energy at 4.08 eV can be consequently attributed to the absorption of a highest energetic level in the conduction band. The band with minimum energy at 3.22 eV is assigned to an excitonic absorption. In fact an electron is excited from the valence band (VB) to a permit level in the gap leaving a hole in the (VB). The electron bound to the hole moves freely forming a hydrogen like particle called exciton. The appearance of the exciton absorption with a relatively large binding energy resulted from both the dielectric and the zero dimensional quantum-confinement effects. It is worthy to note that the same result was found in our previous work [18]. A more advanced DFT calculations on the title compound’s larger clusters need to be performed in order to check this suggestion. Gauss-Sum 2.2 Program [33] was used to calculate group contributions to the molecular orbitals (HOMO and LUMO) and prepare the density of the state (DOS) as shown in Fig. 9. Conclusion In the present work, we have reported molecular structure, vibrational study and optical properties of a new organic–inorganic diethylenetriammonium hexabromobismuthate. The crystal lattice is composed of discrete [BiBr6] anions surrounded by diethylenetriammonium cations. The structural and vibrational spectra calculated by DFT/B3LYP/LanL2DZ level of theory agree satisfactory with experimental results. On the basis of agreement between experimental and theoretical results, assignments of all observed bands were examined and proposed in this investigation. The presence of intermolecular hydrogen bonds N–H  Br is confirmed from the lowering of N–H stretching mode. Moreover, the important role played by the hydrogen bonds was evidenced by the lowering of the energy gap and the enhancement of the second order non

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