CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 4 445-447
Communications Second Harmonic Generation in para-Nitroaniline through Recrystallization in a Strong Continuous Electric Field Etelvina de Matos Gomes* and Michael Belsley Departamento de Fı´sica, Universidade do Minho, 4710-057, Braga, Portugal Received January 29, 2003
ABSTRACT: We show that the normally centro-symmetric crystalline structure of para-nitroaniline is altered by recrystallization under the influence of an intense continuous electric field. This electric field induced symmetry breaking is confirmed by the generation of an optical second harmonic signal. The possibility of influencing the process of crystal nucleation from solution using an externally applied electric field could lead to powerful new methods of crystal engineering with important applications in pharmaceuticals and optoelectronics. Several years ago, Garetz et al.1 showed that nucleation of urea in an aqueous solution can be dramatically accelerated (by up to 13 orders of magnitude) through the application of an intense pulse of infrared laser radiation. The physical mechanism behind the effect remains unclear, although this is certainly not a single molecule effect. Instead, Garetz et al.1,2 suggest that the intense electric field associated with the laser pulse acts to align small clusters of molecules that have formed in pre-aged solutions, somehow facilitating their organization into a stable proto-crystal. Except for the accelerated nucleation and a tendency to grow in needlelike structures along the polarization direction of the laser radiation, the crystals grown seem to possess physical properties equivalent to that of urea grown under conventional methods. Here we report that by recrystallizing the organic compound para-nitroaniline under an intense continuous electric field of 2 × 105 V/m, we can obtain an entirely new crystalline form that is capable of generating an optical second harmonic signal. The applied electric field has induced a symmetry breaking of the normally centrosymmetric monoclinic crystal structure. This induced symmetry breaking can be especially important in nonlinear optical applications. For bulk crystalline materials to generate second order nonlinear optical signals, such as optical second harmonic generation, a non-centro-symmetric crystalline structural arrangement is required. However, many strong molecular candidates for second order nonlinear optical materials crystallize naturally in centro-symmetric structures. We were motivated to experiment with recrystallization under an intense electric field by recent work that showed it is possible to improve the crystalline quality and rate of nucleation of proteins using an external electric field.3 * To whom correspondence should be addressed. E-mail: emg@fisica. uminho.pt.
Application of an electrical potential to ice crystals formed from the vapor phase has also been shown to induce an enhanced diffusion of polar molecules near the dendrite tip,4 while structural modifications have been found in crystalline polymers formed in the presence of a static electric field.5 Previously, we have shown that the application of electric fields during recrystallization can lead to an increase in the second harmonic nonlinear response of the organic nonlinear crystal 2-methyl-4-nitroaniline by as much as a factor of 3.6 para-Nitroaniline (pNA) represents a model system for nonlinear optical studies, being a substituted benzene molecular crystal with a donor amine group, NH2, and an acceptor nitro group, NO2, in the para position, that crystallizes in a centro-symmetrical structure, space group P21/n.7,8 Although the molecule exhibits a large secondorder hyperpolarizability9 due to its high dipole moment,10 no bulk second-order nonlinear optical effects are observed in crystals grown under ambient conditions due to the existence of a centro of symmetry. Crystals were grown at room temperature using methanol as solvent. A solution of para-nitroaniline was prepared far from saturation, one part of which was recrystallized under a dc electric field and the other part was recrystallized outside the electric field (hereafter referred to as pNA-E and pNA, respectively). The details of the experimental setup used to create the electric field have been described elsewhere.6 Small crystals of sub-millimeter size precipitated after roughly 50 h. Evidence that the center of symmetry has been broken in the crystals grown under the electric field was obtained by performing second harmonic generation experiments using the Kurtz powder test.11 Polycrystalline pNA-E samples of 3 mm diameter, 0.5 mm thickness with a nominal grain size ranging from 50 to 160 µm were compared with polycrystalline urea with the same grain size and similar sample preparation. The fundamental beam of a Q-switched Nd:YAG laser with a 1064 nm wavelength, pulse duration of roughly 7 ns and a 10 Hzrepetition rate was weakly focused on the samples. The
10.1021/cg0340143 CCC: $25.00 © 2003 American Chemical Society Published on Web 06/07/2003
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Figure 1. Second harmonic generation signal in pNA-E and urea, measured on polycrystalline samples as a function of grain size. Crystals of pNA grown under ambient conditions do not give rise to a detectable second harmonic signal.
Figure 2. Differential scanning calorimetry results showing the fusion transformation in pNA and pNA-E crystals.
second harmonic signal was separated from the incident fundamental beam using a combination of dichroic mirrors and a 0.2 m monochromator. The signal was detected using a photomultiplier tube (type 1P28 at 900V bias) and read using a fast digital oscilloscope (Tektronix TDS 520 with a bandwidth of 500 MHz). Averages over 100 laser shots were acquired to reduce the influence of the shot-to-shot variations in laser power. It was observed that pNA-E generates a small but clear optical second harmonic signal approximately 5 times smaller than urea, Figure 1. In contrast, pNA samples grown outside the electric field did not produce a second harmonic signal that could be distinguished from the detection noise. While in urea, as is characteristic of any phase matchable crystalline material, the second harmonic intensity falls off with a decrease in grain size, in pNA-E the signal intensity was found to be independent of the grain size. Further evidence that the samples grown under an electric field differ from those grown under ambient conditions was obtained through a thermal analysis with differential scanning calorimetry as shown in Figure 2. Both pNa and pNA-E samples start to undergo fusion at roughly the same temperature with the fusion temperature calculated to be 420.41(2) and 420.53(2) K for pNA and pNA-E, respectively. However, the latent heat involved in the process differs significantly with values calculated to be 253.1 J/g for pNA and 216.6 J/g for pNA-E, implying that the variation in the internal energy required for fusion is
Communications smaller in samples grown under an electric field as the transformation occurs at a constant volume. This result confirms that some subtle structural lattice changes have occurred during crystal growth under the electric field, with the result that the breaking of crystal bonds during the fusion transformation is easier in electric field grown samples. Thermal analysis was performed on a NETZSCH 204 differential scanning calorimeter instrument using a temperature scan rate of 2.0 K/min from 300 to 470 K. The samples were sealed in a container, and there was no mass exchange with the exterior. The associate baseline linearity was ( 0.2 mW, while instrument resolution is estimated to be ( 0.1%. The full physical mechanism responsible for the observed differences between the physical properties of pNA-E and pNA remains unclear at present. When recrystallized under dc electric fields of the order of 105 V/m, the ratio the molecular dipolar energy to the thermal energy, ∆E/ kbT is of order of 5 × 10-5 (using the molecular dipole moment of 7,2D quoted for pNA in methanol10). This suggests that there is a very small orientational alignment induced in individual pNA molecules by the applied field, although fields of the same order of magnitude have been used to successfully pole pNA molecules intercalated in a thin tetramethylammonium saponite film.12 We hypothesize that the main effect of the applied field is to slightly alter the effective charges on the donor and acceptor groups in the molecules. When these molecules are incorporated into the crystalline structure they could lead to slightly altered in their bond lengths, angles, and relative orientation of the molecules in the unit cell, provoking a small symmetry breaking of the basic pNA center-symmetric crystalline structure. The broken symmetry is most likely connected to the hydrogen bond system. Etter et al.13 have shown that in nitroaniline crystal structures, polar hydrogen bonded nitroaniline chains are the prenucleation sites for crystal growth, these chains being necessarily acentric.14 It has also been found that in the presence of an electric field the dipole moment of donor-acceptor molecules increases as the field increases and a variation in the hydrogen bonding length also occurs in hydrogen bonds chained crystalline structures, in particular in urea where hydrogen bonds can be attributed to electrostatic15 and polarization interactions.16 Hysteresis-like variation of the lattice parameters in the organic disubstituted benzene crystal meta-nitroaniline has been observed under the influence of an external dc electric field applied to already grown crystals with a magnitude ranging from 0 to 106 V/m.17 This behavior has been qualitatively explained on the basis of bond lengths distortions of the acceptor and donor groups in the molecules. For these reasons, we believe that the non centro-symmetric structure pNA crystals grown under an intense electric field probably originates in molecular dipole-dipole and hydrogen bond interactions established during the crystal growth process in the presence of a static electric field. This field will enhance the acentric nature of the hydrogen bonding system of the prenucleation chainlike clusters, promoting an acentric overall crystalline structure. The preliminary results reported show that it is possible to break the center of symmetry of pNA, and consequently modify the physical properties of an organic compound by recrystallization under a strong dc electric field. Future work will be dedicated to understand in detail the structural changes involved, concentrating in particular on the hydrogen bond chain system and which we believe is responsible for the non-centro-symmetric structure. Full structural determination by single-crystal diffraction is
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planned for the near future. It would be interesting to carry out experiments combining the nonphotochemical lightinduced nucleation technique of Garetz et al.1,2 during the recrystallization under an intense continuous electric field. This could allow one to obtain new insights concerning the role of the applied continuous field on the prenucleation clusters. A determination of the elements of the nonlinear susceptibility tensor, using the Maker Fringes technique,18 is also planned once we are able to grow crystals of several millimeters in size under the electric field. We consider that the process of recrystallization of crystals formed by organic molecules with a high dipole moment in strong dc electric fields could break the center of symmetry commonly found in these crystals, making them potentially useful materials for applications based on second-order nonlinear optical effects. Acknowledgment. This work was supported by the Portuguese Fundac¸ a˜o para a Cieˆncia e Tecnologia (FCT) under the projects POCTI/1999/FIS/33657 and POCTI /FIS/ 10118/98.
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(5) Jieping, L.; Binyang, D.; Fengchao, X.; Fajun, Z.; Tianbai, H. Polymer 2002, 43, 1903-1906. (6) Nogueira, E.; de Matos Gomes, E.; Belsley M.; LancerosMendez, S.; Cunha, J. M.; Criado A.; Mano, J. Sol. State Sci. 2001, 3, 733-740. (7) Trueblood, K. N.; Goldish, E.; Donahue, J. Acta Cryst. 1961, 14, 1009-1012. (8) Tonogaki, B. M.; Kawata, T.; Ohba, S.; Iwata, Y.; Shibuya, I. Acta Cryst. 1993, B49, 1031-1039. (9) Clays, K.; Persoons, A. Phys. Rev. Lett. 1991, 66, 2980-2983. (10) Oudar, J. L.; Chemla, D. S. J. Chem. Phys. 1977, 66 (6), 2664-2668. (11) Kurtz, S. K.; Perry, T. J. Appl. Phys. 1968, 39, 3798-3813. (12) Ogawa, M.; Takahashi, M.; Kuroda, K. Chem. Mater. 1994, 6, 715-717. (13) Etter, M. C.; Kin-Shan Huang, K. S. Chem. Mater. 1992, 4, 824-827. (14) Panunto, T. W.; Urbanczyk-Lipkowska, Z.; Johnson, R.; Etter, M C. J. Am. Chem. Soc. 1987, 109, 7786-7797. (15) Dannenberg, J. J.; Haskamp, L.; Masunov, A. J. Phys. Chem. A 1999, 103, 7083-7086. (16) Danemberg, J. J. J. Phys. Chem. A 2001, 105, 4737-4740. (17) Avanci, L. H.; Braga, R. S.; Cardoso, L. P.; Galva˜o, D. S.; Sherwood, D. S. Phys. Rev. Lett. 1999, 83 (24), 5146-5149. (18) Bosshard, Ch.; Sutter, K.; Preˆtre Ph.; Hulliger J.; Florsheimer, Kaatz, P.; Gunter, P. In Organic Nonlinear Optical Materials; Garito A. F., Kajzar, F. Eds.; Gordon & Breach Science Publishers; Amesterdam, 1995; Vol. 1, Chapter 7, pp 129-132.
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