Spectroscopic investigation on the efficient organic nonlinear crystals of pure and diethanolamine added DAST

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 667–674

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Spectroscopic investigation on the efficient organic nonlinear crystals of pure and diethanolamine added DAST C. Karthikeyan a, A.S. Haja Hameed a,⇑, J. Sagaya Agnes Nisha a,b, G. Ravi c a

PG and Research Department of Physics, Jamal Mohamed College (Autonomous), Tiruchirappalli 620 020, Tamil Nadu, India Department of Physics, Bishop Heber College (Autonomous), Tiruchirappalli 620 017, Tamil Nadu, India c School of Physics, Alagappa University, Karaikudi 630 003, Tamil Nadu, India b

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

 The strongest vibrational modes

500

DAST B1 B2 B3 B4 B5 B6 peak sum

400 B3

PL Intensity (a.u.)

contributing to the electro optic effect were identified.  The energy of HOMO and LUMO explained the eventual charge transfer interaction within the molecule.  The density of states of the system showed the energy levels available to be occupied by electrons.  The strong peak at 609 nm indicated the yellow–orange emission.

300

200

100

B6

B4

B1 B2

B5

0

550

600

650

700

750

800

Wavelength (nm)

a r t i c l e

i n f o

Article history: Received 5 March 2013 Received in revised form 17 May 2013 Accepted 13 June 2013 Available online 27 June 2013 Keywords: DAST DFT FT-IR FT-Raman UV–Vis–NIR Spectra HOMO–LUMO

a b s t r a c t 4-N,N0 -dimethylamino-N-methyl-4-stilbazolium toyslate (DAST) and diethanolamine (DEA) added DAST crystals are grown by slow cooling method. The corresponding powder samples are examined by characterization studies such as XRD, FT-IR, FT-Raman, UV–Vis–NIR and photoluminescence studies. From the powder X-ray diffraction, their lattice parameter values are found out. Since the vibrational spectra of the molecules are considerably contributed to their linear and nonlinear optical effects, Infrared and Raman spectroscopic studies are carried out for the samples. The UV–Vis–NIR absorption spectra of the samples are used to find the nature of transitions occurred in the samples. Using the density functional theory, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) analyses are done in order to explain the transition and density of states (DOS). The first order hyperpolarizability is calculated by HF and B3LYP/6-311 G(d,p) basis sets for the DAST molecule. From the photoluminescence (PL) spectral studies, the strong excitation emissions are observed. Ó 2013 Elsevier B.V. All rights reserved.

Introduction Organic materials have large second-order nonlinear optical (NLO) susceptibilities with very good frequency conversion [1,2]. The optical molecular crystalline materials are composed of stable chromophoric molecules with large molecular hyperpolarizabilities with an optimized orientation for large macroscopic NLO ⇑ Corresponding author. Tel.: +91 431 2331135/2332235; fax: +91 431 2331435. E-mail address: [email protected] (A.S. Haja Hameed). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.06.060

effects. Good quality NLO crystals are required for their applications in fabricating second harmonic generating (SHG) elements and electro-optical (EO) devices [3]. Among the promising organic NLO materials, 4-N,N0 -dimethylamino-N-methyl-4-stilbazolium toyslate (DAST) is one of the best materials for very pronounced bulk quadratic NLO activity and the powder second harmonic generation (SHG) efficiency of DAST is 1000 times that of urea as a reference at 1907 nm laser emission. The largest nonlinear coefficients are obtained when all the molecules are aligned in the same direction [4]. The static first hyperpolarizability, bo is

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Fig. 1. Synthesizing scheme of DAST material.

evaluated to be 364  1030 e.s.u. from the hyper-Rayleigh scattering investigation and the electro-optic coefficient r11 is measured to be 160 ± 50 pm/V. The largest second-order NLO coefficient, d11 = 1010 ± 110 pm/V at 1318 nm, and the high band width of 140 GHz have rendered DAST crystal to be an efficient candidate for the ultra-fast electro optical modulation [5,6]. In the present study, diethanolamine (DEA: [CH2 (OH) CH2]2 NH) is selected for adding with DAST. The reason is that the amino group of DEA is having more energy to give the charge transfer (CT) for electronic absorption corresponding to the transition from the ground to the first excited state. Pure and DEA added DAST crystals are grown by slow cooling method. The powder samples are characterized by XRD, FT-IR, FT-Raman, UV–Vis–NIR and photoluminescence studies. Using density functional theory, HOMO–LUMO analysis is carried out. The first order hyperpolarizability is also calculated by HF and B3LYP/6-311 G(d,p) basis sets for the DAST molecule.

Experimental procedure DAST was synthesized by the condensation of 4-methyl-Nmethyl pyridinium toyslate and 4-N-N-dimethylamino benzaldehyde in the presence of piperdine [7–9]. The 4-methyl-N-methyl pyridinium tosylate used in the condensation reaction was prepared from equimolar quantities of 4-picoline and methyl-paratoluene sulfonate. The entire synthesis process was conducted in dry nitrogen atmosphere to avoid the formation of the orange hydrated form of DAST. The scheme route to synthesis DAST material is shown in Fig. 1. The resulting DAST material was further purified by recrystallization from methanol. The recrystallized DAST crystals were used for the preparation of two equal amounts of saturated DAST solutions. 0.1 mol of DEA was added into one of the DAST solutions. DEA added DAST solution was stirred till DEA was dissolved completely in it. Then, both solutions were kept at the saturated temperature for 6 h to attain homogenization. Thereafter, the solution temperature was reduced at a cooling rate of

Fig. 2. Grown crystals of (a) pure DAST and (b) DEA added DAST.

0.5 °C/day. After a week, spontaneously grown crystals were obtained. The grown crystals of pure and DEA added DAST are shown in Fig. 2. The crushed powder of the grown pure and DEA added DAST crystals were used for the present studies.

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669

Stilbazolium H3C

+

N

CH3 N CH3 Toyslate O O

-

S

CH3

O

(a)

(b)

(c)

(d)

(e)

Fig. 3. The chemical and molecular structure of DAST (a and b). The projections of two unit cells of the DAST crystal along principal crystallographic axes (c) a-axis, (d) b-axis and (e) c-axis.

The powder X-ray diffraction data were collected for finely crushed powder of both pure and DEA added DAST crystals subjected to intense X-rays of Cu Ka radiation using X-pert PRO PANalytical powder diffractometer. The scan angle was varied from 10° to 80°. The Infra-Red spectra of pure and DEA added DAST samples were recorded in the range 400–4000 cm1 using Perkin–Elmer spectrometer using KBr pellet technique. FT-Raman spectra were

recorded using Burker RFS 25 spectrometer. The absorption spectra of pure and DEA added DAST samples were studied in the range 200–1100 nm by Lamda 35 spectrometer. The photoluminescence spectra were recorded in the wavelength from 540 to 800 nm using JASCO luminescence spectrometer. The density functional theory (DFT) was used for HOMO–LUMO analysis, by means of the hybrid functional DFT/B3LYP with the 6-311 G(d,p) basis set available in

Intensity (a.u.)

(b)

10

20

25

025 006

13-3

310 22-3 115

13-1 131

2θ (deg)

130

114 221

023

202

021

020

113

15

200

11-2

11-1

111

11-3

Intensity (a.u.)

004

(a)

30

Fig. 4. X-ray diffraction pattern of (a) DAST and (b) DEA added DAST crystals.

35

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Gaussian 09 Program [10–12]. For the calculation of first order hyperpolarizability of DAST molecule, HF and B3LYP/6-311 G(d,p) basis sets are used. Results and discussion X-ray diffraction analysis DAST crystal belongs to the non-centrosymmetric monoclinic space group Cc, with Laue symmetry of 2/m. The cell dimensions are a = 10.3650 Å, b = 11.3220 Å, c = 17.8930 Å; a = c = 90° and b = 92.24 with V = 2098.18 Å3 [4]. The molecular structure of the cation in DAST consists of two aryl rings adopting the expected trans arrangement about the ethylenic linkage. The chemical and optimized molecular structures of DAST are shown in Fig. 3a and b. The projections of two unit cells of the DAST crystal along principal crystallographic axes are shown in Fig. 3c–e. The crystals have highly oriented stilbazolium chromophores aligned at an angle of less than 20° with the respect to polar axis a. These fully conjugated nonlinear active chromophores are forced to align parallel to one another by tosylate counter ions, leading to extremely large electro-optical anisotropic properties [13,14]. From the X-ray powder diffraction pattern of pure and DEA added DAST crystals shown in Fig. 4a and b, the lattice parameter values are calculated using 2h values of peaks corresponding to the (hkl) planes using the monoclinic crystallographic equation given below.

1 2

d

¼

! 2 2 2 2 h k sin b l 2hl cos b þ þ  2 2 ac c2 sin b a2 b 1

ð1Þ

V ¼ abc sin b

ð2Þ

The calculated lattice parameter values of pure and DEA added DAST crystals are shown in Table 1. The lattice parameter values of DAST crystals are found to be slightly varied in DEA added DAST crystals due to the influence of DEA molecules in the DAST crystal lattice. Infrared and Raman spectroscopic studies The direction of the charge-transfer axis of the stilbazolium is defined by the two nitrogen atoms. The two molecules are displayed as they are packed in the crystal structure as shown in Fig. 3c–e where the projections of two unit cells of the DAST crystal are along the principal crystallographic axes. It is quite necessary to interpret the Infrared and Raman spectra of DAST samples while considering the optical effects in the crystals. In particular, for contribution to the linear electro-optic effect, a certain vibrational mode has to be Raman and Infrared active. It is found that the Infrared and Raman spectra show the strongest modes in DAST and DEA added DAST samples as shown in Fig. 5a and b. The description of the modes is given in Table 2. There are four strongest modes which are infrared as well as Raman active. The four strong vibrational modes A, B, C and D are contributing to linear electro-optic effect of DAST crystals and from the vibrational spectra, the values corresponding to the modes are at 1161 cm1, 1181 cm1, 1346 cm1 and 1577 cm1

Table 1 Lattice parameter values of pure and DEA added DAST crystals. Sample

a (Å)

b (Å)

c (Å)

V (Å)3

DAST [4] DAST + DEA

10.3660 10.4023

11.3223 11.3242

17.8910 17.8971

2085.81 2088.02

respectively. The largest contribution of electro-optic effect is caused from the aromatic ring deformations and CH3. The aromatic ring deformations at A and B modes are active in both IR and Raman spectra because of the symmetry lowering of the molecule. Such deformations change the bond length between the carbon atoms, which build the charge transfer axis. The vibration mode C is due to CH3 umbrella vibration. This symmetric bending modes yield a large positive charge localized on the hydrogen, which further supports the presence of hyperconjugation. Furthermore, the symmetrical umbrella mode of the methyl groups can contribute in the electro-optic effect of DAST. The CH3 umberlla vibration can be understood since the in-phase displacement of the three H atoms changes the electronic environment for the donor and acceptor of the stilbazolium chromophore and therefore the polarizability is modified [15]. The strongest vibration mode D takes part in collective stretching and shrinking of the skeletal C@C and CAC intra-ring and inter-ring bonds. This is mainly involved in the delocalization of the p electrons. The strongest vibrational modes at 1577 cm1 are resulted from in-phase symmetric stretching vibrations of single CC bonds and shrinking of CC double bonds, t (C@C/CAC). This vibration spreads over the whole p-conjugated parts of the molecule. This vibration involves the intermolecular charger transfer from the donor and acceptor and gives rise to a large variation in the dipole moment [16]. This vibration reproduces the evolution from an aromatic to a quinonoid structure in the ethylenic bridge, which carries out the phenomenon of the electron/phonon coupling in the conjugated material [17]. The pelectron movement from donor to acceptor can make the molecule highly polarized through the single-double path when it changes from the benzenoid form (ground state) into the quinonoid form (first excited state). These vibrational modes are contributing to electro-optic effect of DAST crystals. There is small shifts in the position of spectrum obtained for the DEA added DAST as compared to that of pure DAST. Fig. 5c–f shows all the vibration modes, the higher number region spectra due to symmetric and asymmetric stretching modes of NH for pure and DEA added DAST samples. The NH stretching is observed at 3201 cm1 from the IR spectra of DAST sample. The ring CAH stretching vibrations appear to be very weak, which is due to the steric interaction that induces effective conjugation and charge carrier localization resulting in phenyl ring twisting [18]. This vibration is observed at 3168 cm1 from the IR spectra of DAST sample. The aromatic CAH stretching vibrations are found at 3016 and 3070 cm1 from the IR spectra of pure and DEA added DAST samples. The symmetric NACH3 stretching appears at 2887 cm1 in the IR spectra of DAST. The combined overtones are observed at 2792 and 2475 cm1 from the spectra of DAST and DEA added DAST samples. The CAN stretching and C@C stretching are found at 2358 cm1 and 1619 cm1 respectively in the case of DAST sample. The trans stilbene appears at 1580 cm1 from the IR spectra of DAST sample. From the Raman spectra of DAST and DEA added DAST samples, the trans stilbene C@C stretching is observed at 1577 and 1586 cm1. The CAH in bending vibrations is observed at 1215 and 1212 cm1 respectively in DAST and DEA added DAST samples. The sulfonate functional group is characterized by IR absorptions at 1161 and 1172 cm1. The corresponding wave numbers are found at 1175–1181 cm1 and 1161–1167 cm1 from the Raman spectra of pure and DEA added DAST samples. From the IR and Raman spectra, SO3 symmetric stretching vibration appears at 1000–1027 cm1 and 1040–1070 cm1 respectively in the case of pure and DEA added samples. The 1,4-distribution aromatic ring vibration appears at 820 and 848 cm1 from the IR spectra of both samples. The Cis orientation of the substitute at the olefinic double bond is found at 677 and 714 from the respective IR spectra of the 1 . samples. SO 3 deformation vibration mode is observed at 501 cm

671

-1 1586 cm

(b)

IR Intensity (a.u.)

-1 1345 cm

-1 1167 cm -1 1181 cm

Raman intensity (a.u.)

-1

1577cm 1346 cm

-1

-1 1161 cm -1 1181 cm

(a)

IR Intensity (a.u.)

Raman intensity (a.u.)

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1000

1200

1400

1600

1800

1000

1200

-1

1400

1600

1800

-1

Wavenumber (cm )

Wavenumber (cm )

(d) Raman intensity

IR Intensity (a.u.)

(c)

(f) Raman intensity

IR Intensity (a.u.)

(e)

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm-1)

500

1000

1500

2000

-1

Wavenumber (cm )

Fig. 5. Expanded optically active region of interest in Infrared and Raman vibrational spectra for (a) DAST and (b) DEA added DAST. Full IR spectra of (c) DAST and DEA added DAST and Full Raman spectra of (e) DAST and (f) DEA added DAST.

Table 2 The four strongest vibrational modes of pure and DEA added DAST samples. DAST

DAST + DEA

Mode of vibrations

Assignments

1577

1586

D

1346 1181 and 1161

1345 1181 and 1167

C B and A

Typical CAC@CAC in plane stretching frequency (carbon backbone between the aromatic rings of the stilbazolium chromophore) CH3 umbrella deformation In plane aromatic ring deformations, as typical for para substituted benzenes

UV–Vis–NIR spectroscopic studies The absorption spectra (Fig. 6a and b) of pure and DEA added DAST samples dissolved in methanol show broad peaks with max-

imum absorption at 478 nm and 477 nm corresponding to the energy difference between the ground state and the first excited state of the chromophores. The absorption wavelength is observed in the visible region. The optical band-gap is estimated as 2.32 eV for the DAST sample and the respective transition is p–p. The absorption peak value (474 nm) is found to be a good agreement with the DAST sample reported by Marder et al. [19]. But there is a small shift in the absorption peak of DEA added DAST sample with an increase in the optical band gap, which is found as 2.33 eV due to the effect of DEA. The increase in the band gap energy is due to the fact that NH group of DEA is giving more energy for the charge transfer (CT) of electronic transition from the ground to the first excited state. The electronic absorption corresponding to the transition from the ground state to the first excited state is mainly described by one electron excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) [20,21]. HOMO represents the ability to donate an electron

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(a)

1.5

Absorbance

DAST+DEA

(b)

Eg = 2.32 eV

Eg = 2.33 eV 2

2.0

DAST

(αhυ)

2.5

( α h υ )2

672

2.20

2.25

2.30

2.35

2.40

2.45

2.50

2.55

2.60

2.25

2.30

Photon energy (hυ)

2.35

2.40

2.45

2.50

2.55

Photon energy (eV)

1.0

0.5

0.0 400

600

800

1000

Wavelength (nm)

400

600

800

1000

Wavelength (nm)

Fig. 6. UV–Vis–NIR absorption spectra of (a) DAST and (b) DEA added DAST samples.

Fig. 7. The atomic orbital compositions of the frontier molecular orbital of DAST.

whereas LUMO represents the ability to obtain an electron. The HOMO is delocalized over the whole C–C backbone of the p-conjucated phenyl ring, ethylenic bridge and over the dimethylamino group (N (CH3)2). The LUMO is located on the pyridine ring and the vinyl group. Consequently, the HOMO–LUMO transition implies an electron density transfer from, the more aromatic part of the p-conjucated system including the electron donor group to its more quinonoid side and mainly to the electron withdrawing end. Moreover, the HOMO and LUMO topologies show certain

overlap of two orbitals in the middle region of the p-conjucated systems, which is a prerequisite to allow an efficient CT transition [22]. The frontier molecular orbitals of DAST are shown in Fig. 7. The energy of HOMO, LUMO and HOMO–LUMO gap are 0.20137 a.u, 0.09947 a.u and 0.30084 a.u., respectively. The N(CH3)2 group closer to the donor group results from the charge transfer interaction of the phenyl ring and the amino group in the electron donor side of the NLO chromophores. The existence

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673

Fig. 8. Density of states (DOS) of DAST molecules.

of two methyl groups provides the additional negative charges to the amino nitrogen atom. It gives that HOMO–LUMO energy gap of the charge transfer is negative. The calculated Self Consistent Field (SCF) energy of DAST is 1625.77502121 a.u. at B3LYP/6311 G(d,p). The energy of HOMO and LUMO explains the fact that eventual charge transfer interaction is taking place within the molecule. The HOMO–LUMO energy value of DAST is found to be consistent with the value reported by Vijayakumar et al. [16] through normal mode analysis performed using the Becke’s three-parameter B3P86 exchange–correlation functional together with 6-31G (d) basis set. The density of states (DOS) of DAST is shown in Fig. 8. The DOS of the system describes the number of states per interval of energy at each energy level that are available to be occupied by electrons. The density distributions are not discrete like a spectral density but they are continuous. A high DOS at a specific energy level indicates many states that are available for occupation.

500

DAST B1 B2 B3 B4 B5 B6 peak sum

400

PL Intensity (a.u.)

B3

300

200

100

B6

B4

B1

Photoluminescence studies Fig. 9 shows the photoluminescence spectra of DAST sample. A good fit of six peaks B1, B2, B3, B4, B5 and B6 is made using Gaussian function. The solid lines represent the linear combination of six Gaussian peaks. B1 has the lowest wavelength and B6 has the highest wavelength. In the PL spectra of DAST, the six peaks are observed in the visible region. The peaks B1, B2, B3, B4, B5 and B6 are represented the green, yellow, yellow–orange, orange, orange and red emissions at 548 nm (2.2625 eV), 575 nm (2.156 eV), 609 nm (2.0359 eV), 641 nm (1.9342 eV), 674 nm (1.8395 eV) and 705 nm (1.7586 eV), respectively. The B1 and B2 bands (548 and 575 nm) are expected to occur from the dimethylamino group (N(CH3)2) present in DAST crystal. The B3 band at 609 nm is supposed to appear from another chromophore (sulfonate group) through molecule exhibiting twisted intermolecular charge transfer (TICT) process. The TICT molecule on electronic excitation forms a moderately nonpolar state, and this nonpolar excited state undergoes an intermolecular transfer of an electron from donor (dimethylamino group) to an acceptor (sulfonate group) connected through flexible single bond about which rotation is free. Here electron transfer occurs between two molecules through ionic interaction between dimethylamino („N+) and sulfonate (ASO 3 ) groups. The B4 and B5 bands at 641 nm and 675 nm indicate the olefinic double bond. The B6 band appears at 705 nm for the benzene ring. A slight variation in the peak positions and band gap energy values is observed in the case of DEA added DAST samples. The NLO response of the push–pull NLO chromophores consisting of the stilbazolium cations and the toluene sulfonate (tosylate) anions involves in the attainment of excellent optical properties of the samples.

B2 B5

Determination of hyperpolarizability

0

550

600

650

700

750

Wavelength (nm) Fig. 9. Photoluminescence spectra of DAST sample.

800

The mean polarizability (a), anisotropy of polarizability (Da) and average value of the first order hyperpolarizability (btot) can be calculated using the equations given in the literature [23]. The first order hyperpolarizability are calculated by HF and

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B3LYP/6-311 G(d,p) basis sets for the DAST molecule. The values of first hyperpolarizability btot of the DAST molecule are estimated as 130.130296  1031 e.s.u. and 155.658135  1031 e.s.u. from HF and B3LYP/6-311 G(d,p) calculations respectively. These values are found to be consistent with the reported values [24]. Conclusions The synthesis of 4-N,N0 -dimethylamino-N-methyl-4-stilbazolium toyslate (DAST) was carried out at dry nitrogen atmosphere. Pure and DEA added DAST crystals were grown by slow cooling method. The powder samples from the grown crystals were used for XRD and spectroscopic studies. From the X-ray powder diffraction studies, as compared to DAST crystals, a slight variation in the lattice parameter values of DEA added DAST crystals was observed due to the effect of DEA molecules. The strongest vibrational modes contributing to the electro optic effect were identified from the Infrared and Raman spectroscopic analyses. The optical band gaps were estimated as 2.32 eV and 2.33 eV for pure and DEA added DAST samples. The HOMO–LUMO transition implied that an electron density transfer was more from the aromatic part of the p-conjucated system. The density of states (DOS) of the system showed the energy levels available to be occupied by electrons. From the photoluminescence spectra, the strong peak at 609 nm (2.0359 eV) indicated the yellow–orange emission. Acknowledgements One of the authors (ASH) is grateful to University Grants Commission (UGC), New Delhi, India for sanctioning the financial assistance (UGC MRP F. No. 38-97/2009(SR)) to execute the major research project.

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