X-RAY DIFFRACTION ANALYSIS OF ORGANIC NON-LINEAR OPTICAL CRYSTALS: NEW OPPORTUNITIES AND APPLICATIONS

Vladimir N. Nesterov¹, Mikhail Yu. Antipin^1,2, Ronald D. Clark,² and Mohan Sanghadasa³

¹Institute of Organoelement Compounds (INEOS), Russain Academy of Sciences, Vavilov St. 28, B-334, 117813, Moscow, Russia.
²Department of Physical Sciences, New Mexico Highlands University, Las Vegas, NM, 87701, USA.
³University of Alabama in Huntsville, Huntsville, Alabama, 35899, USA.

Keywords: non-linear optical materials, organic chromophores, X-ray diffraction, charge density analysis

Organic non-linear optical (NLO) compounds are a subject of intense studies because of the wide number of their possible applications. These materials may be used for second harmonic generation (SHG), frequency mixing, electrooptic modulation, and many other processes. In many cases the efficiency of organic NLO compounds is many times bigger than that of the best inorganic ones like KDP or LiNbO₃ due to very large molecular second-order polarizabilities, ultrafast response times and low dielectric constants. Molecular engineering methods are widely used nowadays for a design of new NLO materials, and X-ray diffraction analysis plays a key role in such a design.

In the present communication we report about X-ray diffraction study of a large series of different substituted derivatives of dicyanovinylbenzene C₆H₅-CH=C(CN)₂ (I) and some parent compounds. More than 20 new crystal structures were studied including different OMe-, (Alk)₂N- and Hal (F,Cl) derivatives of (I). Most of the compounds were found to demonstrate rather big molecular second-order susceptibilities () that was proven by the quantum-chemical calculations as well as SHG measurements in solutions (EFISH technique) and solid state (powder Kurtz test). It was shown for a series of studied dicianovinylbenzenes (using molecular mechanics and crystal packing analysis) what factors are responsible for the centrosymmetric or acentric crystal structure of a given compound. In particular, stacking interactions between planar molecules and weak C-H…N hydrogen bonds (C-H bond belongs to the group -CH=C(CN)₂) prevent a formation of acentric crystal structure. Several new (acentric) compounds of this series were found to exhibit strong SHG signal in solid state, in particular the o-fluoro- and p-dimethylamino-dicyanovinylbenzenes (II,III) and 4-(4-methoxyphenyl)-1,1-dicyano-1,3-butadiene (IV), 4-MeO-C₆H₄-CH=CH-CH=C(CN)₂. Molecular and crystal structures of these compounds have been analyzed in details and some conclusions were made about relations between their molecular packing arrays (in the space groups Pc, P2₁, and Pc, respectively) and NLO characteristics. In particular, powder samples of II and IV give strong SHG signals (Nd:YAG laser, l=1064 nm), exceeding 10-15 times that of urea, that is in agreement with the crystal packing features. On the contrary, the powder of compound III (having an "optimal" crystal packing and high value of b) is not active in SHG at this wavelength because of the strong absorption of the second harmonic light, but exhibits strong SHG signal at another wavelength of 1907 nm (using Raman shifter).

Another new application of the modern X-ray diffraction method in the study of NLO materials is related to analysis of the electron density distributions in crystals and direct estimation from the diffraction data some of its characteristics (atomic charges, dipole and higher multipole moments) responsible for NLO properties. These opportunities of the method have been demonstrated in the charge density study of the o-methoxy-dicyanovinylbenzene (DIVA) - the well known organic NLO material. The dipole moment and components of the second-order susceptibility tensor for DIVA were estimated from the X-ray data using a multipole model via molecular octopole moments, and were found to be close to the experimental values. A topological analysis of the charge density distribution in this molecule showed the absence of the "quinoid" contribution to its electronic structure that is usually assumed in the discussion of the molecular geometry and electronic characteristics of similar NLO chromophores.