Hysteresis-like Behavior in MBANP Crystals Adenilson O. dos Santos,† Luis H. Avanci,† Lisandro P. Cardoso,*,† Ronaldo Giro,† Se´rgio B. Legoas,‡ Douglas S. Galva˜o,† and John N. Sherwood# Instituto de Fı´sica Gleb Wataghin, UNICAMP, C. P. 6165, CEP 13083-970, Campinas SP, Brazil, Departamento de Fı´sica, Universidade Federal do Amazonas, CEP 69077-000, Manaus-AM, Brazil, and Pure and Applied Chemistry Department, University of Strathclyde, Glasgow, G1 1XL, U.K. Received October 7, 2003;
CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 5 1079-1081
Revised Manuscript Received March 23, 2004
ABSTRACT: In a previous study of (-)-2-(R-methylbenzylamino)-5-nitropyridine (MBANP) [Avanci, L. H. et. al., Phys. Rev. B 2000, 61, 6507], X-ray multiple diffraction assessments of the small changes in the unit cell parameters due to a strain produced by E B , together with the resulting variations in the (hkl) peak position and the observed strain, were used to calculate the piezoelectric coefficients of this material. In the present work, we report the extension of these measurements to the determination of the variation of the unit cell parameters of this material under the influence of an externally applied dc electric field. Rocking curves were measured using a SIEMENS P4 singlecrystal diffractometer equipped with encoders to ensure omega axis step sizes smaller than 1 mdeg. At the beginning of the experiments, the electric field was increased from zero to 3.2 × 105 V/m and then decreased to zero. It was then reversed in polarity from zero to -3.2 × 105 V/m and returned to zero again to complete the E-cycle. The measured strain versus E-cycle showed an interesting butterfly wing shape hysteresis behavior. Quantum mechanical calculations on isolated MBANP molecules show that the main features of the hysteresis shape can be explained in terms of field-induced changes in the charge profiles and geometry of isolated MBANP molecules. Introduction In recent years, large high-quality noncentrosymmetric organic single crystals, grown for nonlinear optical examination, have shown other interesting and potentially useful physical properties.1 In addition to exceptional optical nonlinearities, the space groups of these materials are all of the type that should produce piezoelectricity.2 An examination of the material metanitroaniline (mNA) yielded a surprising ferroelectric behavior that was qualitatively explained in terms of the geometrical distortions of isolated mNA molecules.3 The the main features of this behavior could be explained in terms of changes in the acceptor-donor properties of isolated mNA molecules. Another such material, (-)-2-(R-methylbenzylamino)-5-nitropyridine (MBANP), has been the subject of thorough ultrasonic4 and multiple diffraction studies, which have led to the evaluation of most of its elastic and piezoelectric tensors.5 MBANP, molecular formula C13H13N3O2, crystallizes in the monoclinic space group P21, point group 2 with unit cell (Figure 1b) dimensions of a ) 5.392 Å, b ) 6.354 Å, c ) 17.924 Å, and β ) 94.60°. The MBANP molecule (Figure 1a) is butterfly shaped containing two aromatic rings angled at 84.6°. However, the possibility of small angle deformations in the solid state can lead to an alteration in this angle. Charge transfer occurs between the amino nitrogen (donor) and nitro nitrogen (acceptor) so that the molecular dipole moment b p has its largest component in the [001] direction at an angle of 33.25° to the unique axis B b. * To whom correspondence should be addressed. Lisandro P. Cardoso, Instituto de Fı´sica Gleb Wataghin, CP 6165, CEP 13083-970, Campinas - SP, Brazil. E-mail:
[email protected]. † Instituto de Fsica Gleb Wataghin, UNICAMP. ‡ Universidade Federal do Amazonas. # University of Strathclyde.
Figure 1. (a) Schematic representation of the MBANP molecule and (b) unit cell of the MBANP crystal.
Intermolecular hydrogen bonding occurs in the [011] direction between molecules related by a [1,1,0] translation. Although hydrogen bonding does not have a great effect on the molecular shape, it does play an important role in imposing a strong net alignment of the molecular dipoles within the material. The alignment of the
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molecular dipoles contributes to the optical nonlinearity of the material. In MBANP, the chiral methylbenzylamino group causes noncentrosymmetry and the nitro (acceptor) and amino (donor) groups provide the polarizable functionality, both of which are needed for second harmonic generation. Hyperpolarizability in MBANP is due chiefly to the dipole moment of the pyridine fragment. In this work, the same experimental procedure as was used to study the effects of an external electric field (E) on the mNA crystal lattice was employed in the examination of MBANP crystals. Experimental Section Samples for examination of dimensions a × b × c ) 10 × 20 × 5 mm3 were cut from large optically perfect crystals grown from saturated methanol solutions.1 In the experiments, the X-ray beam was incident on the narrower platelike sample face (c-axis). The peak shifts for the primary diffraction (0 0 10) rocking curves were obtained by measuring the change in the corresponding Bragg angle (θ):
∆c + cot β ∆β ) -cot θ00l∆θ00l c
Figure 2. Lattice distortion through (0 0 10) rocking curves as a function of applied electric field.
(1)
Rocking curves were measured using a SIEMENS P4 singlecrystal diffractometer equipped with encoders to ensure step sizes in the omega axis smaller than 1 mdeg. The wavelength used throughout the experiments was 1.3922 Å (Cu Kβ conventional radiation). The accuracy of the experimental equipment was checked by measuring several rocking curves for MBANP in the absence of the electric field. During all measurements, the full width at half-maximum (fwhm) of the (0 0 10) diffraction peaks were determined as a function of the applied electrical field Ey.. The field strength was increased from zero to 3.4 × 105 V/m and decreased back to zero. Then, it was reversed in polarity from zero to -3.4 × 105 V/m and finally, returned to zero again to complete the whole E-cycle. Theoretical Calculations. The calculations were carried out using the well-known semiempirical method, Austin method one (AM1)6 with a modified code that allows the introduction of an external electric field term into the Hamiltonian. The details of the methodology have been discussed elsewhere.7 Despite their simplicity, semiempirical and even simpler topological methods have been proven to be efficient tools for the investigation of the electronic structure of organic materials.8 The geometrical optimizations of the MBANP molecule were performed as a function of the electric field strength applied in the dipole moment direction (Figure 1a). In this direction, the applied electric field should maximize the charge transfer in the molecular system. N1 of the N-H group was chosen as the origin of the coordinate system for molecular calculations.
Results and Discussions The peaks shifts in the (0 0 10) rocking curves (eq 1), as a function of applied electric field on MBANP crystals are shown in Figure 2. The external applied electrical field induced a hysteresis-like variation in the unit cell parameters. The measured strain versus E-cycle showed an interesting and unusual “butterfly wing" shape. Exploring the symmetry properties of MBANP crystals, we can qualitatively express changes in lattice dimensions in terms of geometric changes of isolated MBANP molecules. Our results show that the major influence of an applied electric field is to cause a rotational movement of the benzene ring (in the benzyl group) and neighboring CH3 group around the N1-C9
Figure 3. Charge population in the NO2 group, N1 nitrogen site, and pyridyl ring as a function of the strength of electric field E applied in the dipole moment b p direction. See text for discussion.
bond (Figure 1a), and a distortion of N-C bond lengths. These changes can be linked to the charge population since both have the same qualitative behavior.7,9 The decision to use charge population analysis (instead of bond-length deformations) was based on the fact that charge population is an intrinsic property of the molecular structure and is very easy to visualize. The use of bond length deformations would require geometric corrections (projections over the plane) for each value of the electric field, a very tedious process that does not add any helpful information to our qualitative analysis. In Figure 3 we present the results of the calculations for the charge populations over the NH group, NO2 group) and the intermediate pyridyl ring. When the electric field is positive along the b p direction, the NO2 and NH groups transfer charge to the pyridyl ring. For negative values of E, the NH group transfers charge to both the NO2 group and the pyridyl ring. In both cases (E positive and negative), the pyridyl ring is always receiving charge. These distinct changes and nonlinear behavior can be related to different types of geometrical deformations: simple elongation of bond lengths in the direction of electron transfer and elongation followed by torsional change in the direction of electron loss. The torsional change of the benzyl group around the NH group is characterized by a positive rotation around the N1-C9 axis until the maximum E strength (positive value) is reached. After that, as E is decreased, we observe a negative rotation as we pass through the zero
Hysteresis-like Behavior in MBANP Crystals
value of E to the lowest negative E value. Finally, as E decreases toward zero through the negative axis, we observe a positive rotation again. This behavior is repeated successively as the electric field changes through positive to negative values. These changes result in a macroscopic memory effect since the relaxation times will have different time scales. Since the facility for charge transfer is related to the hardness of the molecular structures (strain/ stress),9 we would expect to see an inverse steeper dependence of charge population and ∆c/c with the electric field. This effect was observed experimentally in MBANP; however, it has already been observed in case of mNA crystals.3 Molecular interactions will also be very important in defining the macroscopic behavior of MBANP crystals. As the molecules are arranged in different ways in the crystal, the rotation of the benzene ring as a function of electric field will present different orientations. This effect could result in a memory effect. Further calculations are being performed to investigate this possibility. In summary, the above results suggest strongly that the observed hysteresis can be directly associated with changes in the donor-acceptor properties of isolated molecules.
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Acknowledgment. This work was supported by Brazilian agencies FAPESP, CNPq, CAPES, and FAEPUNICAMP. References (1) Halfpenny, P. J.; Morrison, H.; Ristic, R. I.; Shepherd, E. E. A.; Sherwood, J. N.; Simpson, G. S.; Yoon, C. S. Proc. Royal Soc. (London) 1993, A440, 683. (2) Avanci, L. H.; Cardoso, L. P.; Girdwood, S. E.; Pugh, D.; Sherwood, J. N.; Roberts, K. J. Phys. Rev. Lett. 1998, 81, 5426. (3) Avanci, L. H.; Braga, R. S.; Cardoso, L. P.; Galva˜o, D. S.; Sherwood, J. N. Phys. Rev. Lett. 1999, 83, 5146. (4) Gilmour, S.; Pethrick, R. A.; Pugh, D.; Sherwood, J. N. Philos. Mag. 1993, B67, 855. (5) Avanci, L. H.; Cardoso, L. P.; Sasaki, J. M.; Girdwood, S. E.; Roberts, K. J.; Pugh, D.; Sherwood, J. N. Phys. Rev. B 2000, 61, 6507. (6) Dewar, M. S.; Zoebish, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902. (7) Dantas, S. O.; dos Santos, M. C.; Galva˜o, D. S. Galva˜o, Chem. Phys. Lett. 1996, 256, 207. (8) Galva˜o, D. S.; dos Santos, D. A.; Laks, B.; de Melo, C. P.; Caldas, M. J. Phys. Rev. Lett. 1989, 63, 786; 1990, 65, 527. (9) Toledano, J. C.; Toledano, P. The Landau Theory of Phase Transitions; World Scientific: Singapore, 1987; and references therein.
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