PROOF COPY [B-11016] 023308JOB PROOF COPY [B-11016] 023308JOB Enhancement of the electro-optic properties in hybrid organic – inorganic crystals by molecular engineering Jean Paul Salvestrini and Mustapha Abarkan Laboratoire Mate ´ riaux Optiques, Photonique et Syste ` mes, Centre National de la Recherche Scientifique FRE 2304, Universite ´ de Metz et Supelec, 2 rue E. Belin, 57070 Metz, France Julien Zaccaro and Alain Ibanez Laboratoire de Cristallographie, Centre National de la Recherche Scientifique, UPR 5031, Universite ´ J. Fourier et Institut National Polytechnique de Grenoble, BP 166, 38042 Grenoble Cedex 09, France Received December 19, 2002; revised manuscript received April 18, 2003 The electro-optic properties and their frequency dispersion of the 2-amino-5-nitropyridium monohydrogen L-tartrate (2A5NPLT) are investigated at room temperature at a wavelength of 632.8 nm. The results show a large increase (more than three times higher) of the electro-optic coefficient values in the 2A5NPLT crystal compared with the 2A5NPDP crystal, which is one of the first crystals engineered with the 2-amino-5- nitropyridine highly polarizable chromophore. © 2003 Optical Society of America OCIS codes: 160.4330, 160.2100, 190.0190. 1. INTRODUCTION In the past two decades, extensive research has shown that organic crystals can exhibit nonlinear and thus electro-optic (EO) properties that are comparable with or even better than those of the best inorganic materials. 1,2 This is due to highly polarizable molecules that involve a conjugated system of  electrons such as polyenes or aro- matic compounds. Nevertheless, because of their poor chemical stability and low thermal and mechanical resis- tance, the use of organic molecular crystals is currently limited in their industrial application. To overcome these difficulties, a strategy has been de- veloped that aims to build cohesive acentric crystalline structures based on host – guest chemistry. 3 The attach- ment of highly polarizable organic cations onto various anionic matrices leads to short and three-dimensional (3-D) networks of short hydrogen bonds. Several polar- izable molecules were selected in this crystal engineering and particularly 2-amino-5-nitropyridine (2A5NP). A great number of hydrogen bond salts having a 3-D acen- tric structure based on polar herringbone motifs were ob- tained with this push– pull molecule such as dihydrogen phosphate 4 or dihydrogen arsenate. 5 These salts possess enhanced stability (chemical, thermal, mechanical) com- pared with corresponding molecular organic crystals. Moreover, they exhibit a wider transparency range and bulky crystal morphology. 6,7 Unfortunately, as we have shown recently, 8,9 the EO properties of the first two compounds did not reach our expectation. Indeed, the larger value of the effective EO coefficient that we measured in the 2-amino-5- nitropyridine dihydrogen phosphate (2A5NPDP) crystal was equal to 11 pm/V. This value is of the same order of magnitude as that of the inorganic KDP family type. 10 As has been shown in Refs. 8 and 9, the EO properties of the hybrid organic – inorganic crystals can be attributed mainly to organic cations (the contribution of the inor- ganic subnetwork to the EO coefficient can be neglected). Thus, we believe that the low EO coefficient value for these crystals arises from the misalignment between the charge transfer axis of the organic chromophores in the crystal and the light polarization direction. Indeed, the herringbone motif, which is favorable to second-harmonic generation, corresponds to a quite significant misalign- ment angle (35 deg in the case of 2A5NPDP crystal) and thus yields to low EO properties. From the crystal engineering mentioned above, a new type of hydrogen bond salt structure was discovered. 11,12 This new material, 2-amino-5-nitropyridinium monohy- drogen L-tartrate (2A5NPLT), was shown to have 2A5NP + cations nearly aligned with the crystallographic b axis (see Fig. 1). In the following we propose to char- acterize the EO properties of this compound to check if this quite good alignment of the dipole moments yields an enhancement of the EO properties. 2. PREPARATION OF 2A5NPLT CRYSTAL AND SAMPLE High-purity 2A5NPLT salt was synthesized from the dis- solution of purified 2A5NP (multiple sublimation) and L-Tartaric acid in hot acetic acid aqueous solution (14 M). Crystals were grown from solution both by the standard temperature-lowering method 13 and a rapid growth tech- nique derived from the one developed for KH 2 PO 4 . 14,15 With the latter technique, we obtained single crystals, Salestrini et al. Vol. 20, No. 8/ August 2003/ J. Opt. Soc. Am. B 1 0740-3224/2003/080001-05$15.00 © 2003 Optical Society of America PROOF COPY [B-11016] 023308JOB