Structural, thermal and optical characterization of a Schiff base as a new organic material for nonlinear optical crystals and films with reversible noncentrosymmetry

Spectrochimica Acta Part A 79 (2011) 1757–1761

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Structural, thermal and optical characterization of a Schiff base as a new organic material for nonlinear optical crystals and films with reversible noncentrosymmetry Mario Rodríguez a , Gabriel Ramos-Ortíz a,∗ , José Luis Maldonado a , Víctor M. Herrera-Ambriz a , Oscar Domínguez b , Rosa Santillan b , Norberto Farfán c , Keitaro Nakatani d a

Centro de Investigaciones en Óptica A.P. 1-948, 37000 León, Gto., Mexico Departamento de Química, Centro de Investigación y de Estudios Avanzados del IPN, 07000, Apdo. Postal. 14-740, México D.F., Mexico c Facultad de Química, Departamento de Química Orgánica, Universidad Nacional Autónoma de México, 04510, México, D.F., Mexico d Laboratoire de Photophysique et Photochimie Supramoléculaires et Macromoléculaires (UMR 8531 du CNRS), Ecole Normale Supérieure de Cachan, Avenue du Président Wilson, 94235, Cachan, France b

a r t i c l e

i n f o

Article history: Received 15 February 2011 Received in revised form 12 May 2011 Accepted 16 May 2011 Keywords: Single crystal growth Organic compound Nonlinear optical materials

a b s t r a c t Macroscopic single crystals of (E)-5-(diethylamino)-2-((3,5-dinitrophenylimino)methyl)phenol (DNP) were obtained from slow cooling of chloroform or dichlorometane saturated solutions at controlled temperature. X-ray diffraction analysis showed that this compound crystallizes in a noncentrosymmetric space group (P21 21 21 ). Thermal analysis was performed and indicated that the crystals are stable until 260 ◦ C. Second-order nonlinear optical properties of DNP were experimentally investigated in solution through EFISH technique and in solid state through the Kurtz-Perry powder technique. Crystals of compound DNP exhibited a second-harmonic signals 39 times larger than of the technologically useful potassium dihydrogenphosphate (KDP) under excitation at infrared wavelengths. In addition, the second-order nonlinear optical properties of DNP were also studied at visible wavelengths through the photorefractive effect and applied to demonstrate dynamic holographic reconstruction. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Organic molecules that possess nonlinear optical (NLO) properties have emerged as interesting construction blocks for a new generation of photonic applications due to their large second- and third-order hyperpolarizabilities, which in many cases are comparable or higher than those for inorganic materials [1]. In the field of molecular engineering is well established that “push–pull” molecules formed by a -conjugated system substituted by an electron donor group in one end of the -backbone, and by an electron acceptor group in the other end, comprise significant second- and third-order nonlinear optical responses [2,3]. This is because the -conjugated system provides an effective pathway for the redistribution of electronic charge across the entire length of conjugation when the perturbation from an external electric field is present. The intense research in the topic of nonlinear materials during the last two decades has produced many -conjugated organic molecules with high NLO responses [2–5], but despite these advances more

∗ Corresponding author. Tel.: +52 477 441 42 00; fax: +52 477 441 42 09. E-mail addresses: [email protected] (G. Ramos-Ortíz), [email protected] (J.L. Maldonado). 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.05.051

development is needed before they can be used in commercial applications such as optical signal processing, high-speed electrooptic modulators, data storage and optical frequency conversion [6]. In this context, research about organic NLO molecules in the solid state, especially crystals, is the bridge to bind fundamental research and real applications. The search of efficient second-order NLO crystals is, in fact, the exploration of “polar crystals” in which the macroscopic properties reflect the internal asymmetric molecular arrangement [7]. Thus, the improvement of the second-order NLO response in organic crystals is commonly based on molecular units possessing high molecular first hyperpolarizability (ˇ) (“push–pull” architecture) that can be packed in a chiral arrangement. Schiff bases are important organic compounds obtained from reversible condensation between amino and carbonyl groups, which is one of the most fundamental reactions in chemistry [8]. Azomethine or imine also known as Schiff bases, have been applied successfully in several areas, such as biological chemistry [9], materials science [10], and organic synthesis [11]. On the other hand, polar crystals grown from Schiff bases have been obtained through the vanishing of their dipole moment [12], insertion of chiral fragments [13] or groups that favor the hydrogen bond interactions [14]. In this work we report the synthesis of a

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Fig. 1. Molecular structure of compound DNP.

new “push–pull” Schiff base with double meta-substitution (3,5nitro) that promotes the chiral packing in the solid state; this property was exploited to grow organic nonlinear crystals showing a second-harmonic generation (SHG) efficiency 6 times larger than that for urea and 39 times larger than that for potassium dihydrogenphosphate (KDP) at the IR wavelength of 1907 nm. Furthermore, the second-order nonlinearities of this Schiff base were also demonstrated at a visible wavelength (633 nm) through dynamic holography experiments. 2. Experiment For the preparation of DNP the reagents 3,5-dinitroaniline and 4-diethylaminobenzaldehyde were obtained from Sigma–Aldrich (USA) and used without any further treatment. 1 H and 13 C NMR spectra for the title compound were recorded on a JEOL Eclipse GX 270. Chemical shifts (ppm) were relative to (CH3 )4 Si for 1 H and 13 C. Infrared spectra were measured on a Perkin-Elmer Spectrum RX1 spectrophotometer using KBr pellets while UV–Vis spectra were obtained with a Perkin-Elmer Lambda 900 spectrophotometer. X-ray diffraction studies of single crystals were performed on an Enraf-Nonius diffractometer with a CCD detector ˚ monochromator: graphite). Frames were col(MoK˛ = 0.71073 A, lected at T = 293 K via ω/ϕ rotation. Direct methods SHELXS-86 [15] were used for structure solution and SHELXL-97 [16] program package for refinement and data output. Thermal analysis was carried out using a Thermobalance Mettler Toledo TGASDTA851e thermal analyzer. 3. Results and discussion 3.1. Synthesis

Fig. 2. Photograph of single crystals grown from compound DNP. Each tiny square is of 1 mm2 .

vents at 35 ◦ C. By slow solvent evaporation at room temperature only the formation of microcrystals was promoted; to carry out the X-ray diffraction analysis these microcrystals were used. In contrast, by preventing the solvent evaporation and from a slow decrease of the temperature (at a rate of 5 ◦ C per day, controlled by using a hotplate), bulk red crystals were obtained after 3 days with typical dimensions of about 2 mm × 2 mm × 0.5 mm and with good optical quality (Fig. 2). 3.3. UV–vis absorption To determine the transparency range of the DNP single crystals their UV–vis absorption spectra were acquired in the wavelength range from 200 to 1100 nm. Fig. 3 displays a typical absorption spectrum obtained with these crystals. This figure shows that the samples were optically transparent in the wavelength range from 650 nm to 1200, a feature that promotes possible optical applications in devices operating at the near infrared wavelength range, i.e., ultrafast modulators and switches [6,7]. Inset of Fig. 3 shows the absorption spectrum of compound DNP in chloroform solution, where the maximum absorption band at 413 nm is assigned to the n → * electronic transition. 3.4. FT-IR studies The functional groups present in the structure of compound DNP were confirmed through Fourier transform infrared (FTIR) spectrum analysis. In this case a sample was prepared by processing organic microcrystals with KBr into pellet form. The IR spectrum of DNP shows the O–H stretching peak at 3415 cm−1 and for C–O at

Compound DNP (Fig. 1) was prepared using the typical synthetic method for imine derivatives [8]. A ethanol solution of 3,5-dinitroaniline and 4-diethylaminobenzaldehyde was refluxed by 2 h, and after complete reaction the solution was cooled down to room temperature and then the product was precipitated and separated by filtration. The compound was purified by recrystallization processes in chloroform previous to the complete chemical characterization. 13 C NMR spectra for compound DNP showed a set of thirteen signals corresponding to the same number of carbon atoms in its structure. In particular, the signal at 163.7 ppm confirmed the iminic moiety and the signal at 164.1 ppm showed the presence of the phenoxy carbon, which is involved in the N···H–O intramolecular hydrogen bond. 3.2. Crystal growth Recrystallized powder of DNP was used to prepare saturated solutions with the use of chloroform or dichloromethane as sol-

Fig. 3. UV–vis absorption spectrum from a DNP single crystal (thickness: ∼1 mm). Inset: Absorption spectrum of compound DNP in chloroform solution.

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bond N···H–O in DNP promotes the planar conformation on the salicilydene fragment, with A···D distance of 2.633(3) Å and A···H–D angle of 146.2(5)◦ . However, for the complete -backbone structure a non planar conformation is observed, this is because the phenyl ring of the aniline moiety is deviated 47.4◦ from the plane of the salicilydene part. Crystal packing of compound DNP is controlled by intermolecular non classical hydrogen interactions promoted by NO2 groups. Crystallographic data for compound DNP have been deposited in the Cambridge Crystallographic Data Centre as supplementary publications No. CCDC 808275. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax: +44 1223 336 033; [email protected]; http://www.ccdc.cam.ac.uk). 3.6. Thermal analysis Fig. 4. IR spectrum for compound DNP.

1236 cm−1 . C C and C N stretching and annular stretching appear at 1637 cm−1 and 1585 cm−1 , respectively, and a band at 1338 cm−1 due to C–N bond. The nonsymmetric and symmetric stretching bands of N O bonds in NO2 groups appear at 1532, 1512 cm−1 and 1319, 1303 cm−1 , respectively (Fig. 4). 3.5. X-ray diffraction analysis X-ray diffraction analysis was carried out on monocrystals; this technique confirmed the structure of the compound DNP. The C−H hydrogen atoms were placed in geometrically calculated positions using a riding model. O−H hydrogen atoms were localized by difference Fourier maps and their bond distances and isotropic temperature factors were refined freely. The crystal packing of the new imine derivate is presented in Fig. 5. This compound crystallizes in the P21 21 21 space group and in an orthorhombic crystal system, with one isolated molecule in the asymmetric unit cell. The ˚ crystallographic data for this crystalline system are a = 6.6367(2) A, ˚ and c = 30.9362(9) A. ˚ The intramolecular hydrogen b = 8.1942(2) A,

In order to know the thermal behavior of the macroscopic crystals, thermogravimetric analysis (TG and DTA) of samples was carried out for the temperature range of 25–500 ◦ C in a nitrogen atmosphere at the heating rate of 10 ◦ C/min. The resulting thermogram and its differential thermogravimetric curve are shown in Fig. 6. TG graph indicates that the crystals of compound DNP are thermo stable until 260 ◦ C, after this temperature and until 320 ◦ C, a strong loss of weight is shown. The DTA thermogram shows an endothermic peak at 297 ◦ C, which is attributed to the melting point of the crystal and to a complete material decomposition. It is important to mention that the solvent used for the crystallization process was not present in the crystal structure according to the invariant behavior of the TG plot between 25 and 260 ◦ C. 3.7. Second-order nonlinear optical characterization The first hyperpolarizability (ˇ) was experimentally determined by using the electric field induced second-harmonic (EFISH) technique [17]. Briefly, the EFISH experimental apparatus is described as follows: a Nd:YAG laser emitting picosecond pulses (1.064 ␮m

Fig. 5. Crystal packing of compound DNP.

Fig. 6. TG and DTA analysis of crystals obtained from compound DNP.

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Fig. 7. (a) Diffraction efficiency and (b) refractive index modulation as a function of E for thick solid PR film with composition DNP:PVK:ECZ:C60 at 25:49:25:1 wt.%. Inset of (a): (i) photograph of the object, (ii) holographic reconstruction of the object obtained through the diffracted signal at an external applied electric field E = 45 V ␮m−1 .

at 10 Hz repetition rate) was used to pump a hydrogen cell (1 m long, 50 bar). The outcoming Stokes-shifted radiation at 1.907 ␮m generated by the Raman effect was used as the fundamental beam to carry out the EFISH experiments. This fundamental beam was focused into dichloromethane solutions of DNP (at concentrations of 5 mM and 10 mM) which were contained in a cell with quartz windows in a wedge configurations separated by 1 mm. The centrosymmetry of the solution was broken by dipolar orientation of the chromophores by applying a high voltage (5 kV) synchronized with the laser pulses. The second-harmonic signal (at 953.5 nm) was selected through a suitable interference filter, and detected by a photomultiplier tube. Therefore the ONL response was proportional to the product of ˇ␮ being ␮ the dipole moment of the chromophore. The EFISH measurements gave a ˇ␮ value of 3 × 10−46 esu D for compound DNP, which is 2.5 times smaller than that shown by a Schiff base reported by Lacroix [18]. The reduction of ˇ␮ is a consequence of the sustitution of the NO2 groups in 3,5-positions, situation that decreases in some extent the dipole moment of the molecule. Thus, althouhg there is a reduction in the dipolar moment, such substitution was employed to promote intermolecular links that lead to the formation of chiral crystals [19]. A more efficient NLO compound would, certainly, contain a NO2 group in the para-position to complete the push-pull character, but it is well know that such condition favors the crystallization in centrosymmetric space group [18]. Additionally, the SHG efficiency of DNP in solid state was evaluated by the Kurtz–Perry powder test [20,21]. Samples were prepared by crushing crystalline powder of compound DNP between two transparent glass plates. Samples were exposed to picosecond laser radiation at 1.907 ␮m, and the SHG signal was detected by a photomultiplier tube after eliminating the pump light with a color filter. The efficiencies were quantified versus a reference sample of powdered urea. The SHG efficiency in average was six times larger than that of urea. By comparing the SHG capacity of urea and that of the technologically useful KDP [22], it results that single crystals grown from DNP exhibited a SHG efficiency about 39 times higher than KDP. This study illustrates that chromophores with rather modest ˇ␮ value can exhibit sizeable SHG efficiencies, once an optimized molecular orientation is achieved in the solid state. To further assess the nonlinear behavior of DNP we implemented photorefractive (PR) characterization of solid state thick films containing such compound. The PR phenomenon consists in the reversible modulation of the refractive index of a material through the Pockels effect [23] and has many technological applications such as dynamic volume holography and reversible data storage [2,3]. PR films were prepared using the guest:host approach by mixing the push–pull Schiff base DNP with the photo-conducting polymeric matrix polyvinylcarbazole (PVK), the plasticizer ethylcarbazole (ECZ) and the sensitizer fullerene C60 at

the concentration of 25:49:25:1 wt.%, respectively. Details of sample preparation and holographic experimental characterization can be found elsewhere [24,25]. Briefly, for sample preparation, a small piece of the ready PR polymer was melted at 130–135 ◦ C between two ITO-coated transparent glasses. The PR film thickness (110 ␮m) was controlled by using calibrated glass spacers. In the PR experiments an external electric field (E) is applied through the ITO electrodes of the composite film containing DNP, this breaks the centrosymmetry of the film and induces secondorder nonlinear optical effects. The PR polymer films were tested by means of the widely used holographic technique of Four Waves Mixing (FWM) [2] with a He–Ne laser (633 nm). In this technique the refractive index modulation of the PR sample is estimated from plots of light diffraction efficiency at different values of E. Fig. 7(a) shows the diffraction efficiency plot with a maximum value of 15.8% at E = 72 V/␮m while Fig. 7(b) shows the refractive index modulation (n) plot estimated from the diffracted signal. The maximum n value is 0.6 × 10−3 which is larger than that of some of the best existing PR inorganic crystals such as lithium niobate (LiNbO3 ) and lithium tantalate (LiTaO3 ) [3,6]. This diffracted signal is rather small in comparison with highly efficient PR films [25], however, in terms of photonic applications is possible to have holographic image reconstruction with just 3% of diffraction efficiency [26]. As a demonstration of dynamic holographic application, a 2dimensional image of an object (numeral “4 ) was recorded in a PR polymer film doped with compound DNP. Here the field E was 45 V/␮m and according to Fig. 7(a) a 2.8% of diffraction efficiency is achieved. Photographs of the object and the holographic reconstruction of its image are shown in the inset of Fig. 7(a). In this case the holographic process is reversible (dynamic) as the image can be erased or reconstructed by simply eliminating or applying the electric field E.

4. Conclusions A new Schiff base with a moderate “push–pull” character was synthesized. EFISH experiments confirmed the molecular secondorder nonlinear optical response in solution with a value for the ˇ␮ product of 3 × 10−46 esu·D. Although the double meta substitution of NO2 groups in the molecular structure of DNP presumably diminishes the molecular dipole moment, it generates a packing in a noncentrosymmetric arrangement capable of producing SHG signals 6 and 39 times more efficient than that for urea and KDP, respectively, under excitation at IR wavelengths. The second-order nonlinear optical features of the Schiff base molecule DNP were also studied in amorphous films in which reversible noncentrosymmetric arrangement was induced through the application of an external electric field to observe the PR effect; the latter was subsequently used for holographic reconstruction at visible wavelengths.

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Acknowledgements Author Mario Rodriguez thanks to CONACyT for postdoctorate fellowship. This work was supported by CONACyT projects 55250, 49512, and 58783, and UNAM (PAPIIT IN-214010). The authors thank Martin Olmos for technical assistance and Marco Leyva for X-ray analysis. References [1] S.R. Forrest, M.E. Thompson, Chem. Rev. 107 (2007) 923–925. [2] H.S. Nalwa, S. Miyata (Eds.), Nonlinear Optics of Organic Molecules and Polymers, CRC Press, Boca Raton, FL, 1997. [3] P. Günter (Ed.), Nonlinear Optical Effects and Materials, vol. 72, Springer-Verlag, New York, 2000. [4] M. Baldo, Nat. Photonics 3 (2009) 458. [5] S.R. Marder, J. Mater. Chem. 19 (2009), 7392- 3793. [6] R.W. Munn, C.N. Ironside, Principles and Applications of Nonlinear Optical Materials, CRC Press, Boca Raton, FL, 1993 (chapters 6 and 7). [7] D.S. Chemla, J. Zyss, Nonlinear Optical Properties of Organic Materials and Crystals, Academic Press, New York, 1987. [8] C.D. Meyer, C.S. Joiner, J.F. Stoddart, Chem. Soc. Rev. 36 (2007) 1705–1723. [9] S. Ren, R. Wang, K. Komatsu, P. Bonaz-Krause, Y.C.-E. Zyrianov, C. McKenna, Csipke, Z.A. Tokes, E.J. Lien, J. Med. Chem. 45 (2002) 410–419. [10] M. Sliwa, S. Letard, I. Malfant, M. Nierlich, P.G. Lacroix, T. Asahi, H. Masuhara, P. Yu, K. Nakatani, Chem. Mater. 17 (2005) 4727–4735. [11] M.-D. Zhou, J. Zhao, J. Li, S. Yue, C.-N. Bao, J. Mink, S.L. Zang, F.E. Kühn, Chem. Eur. J. 13 (2007) 158–166.

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