X-ray structural investigations towards the design of non-linear optical organic materials

Abstract

The thesis entitled “X?Ray Structural Investigations Towards the Design of Nonlinear Optical Organic Materials” is divided into five chapters and two appendices.

Chapter I Chapter I briefly describes the various aspects of nonlinear optics (NLO) with special emphasis on Second Harmonic Generation (SHG) in organic/organometallic compounds. Conventional as well as recent examples related to developments in this area are presented. The plan of the present investigations is outlined at the end of this chapter. The necessary and sufficient conditions for an organic molecule to exhibit SHG are: (i) the occurrence of charge transfer, (ii) extensive delocalization of ??electrons, (iii) a large difference in dipole moment between ground and excited states, (iv) a non?centrosymmetric crystal structure, and (v) favourable molecular packing within the crystal lattice. X?ray crystallographic studies on a large number of “push–pull” ethylenes have shown appreciable ??electron delocalization between donor and acceptor groups across the double bond (D. Adhikesavalu et al., Proc. Ind. Acad. Sci. (Chem. Sci.), 1983, 92 (4 and 5), 449). Therefore, molecules of this class are expected to possess significant molecular hyperpolarizability ?, which is essential for SHG. Many such molecules crystallised in centrosymmetric space groups and therefore did not show SHG activity. To overcome this, several chiral derivatives were prepared and their X?ray structures and SHG properties were studied (D. Kanagapushpam et al., Acta Cryst., 1988, C44, 337). These derivatives nonetheless showed poor SHG activity due to unfavourable molecular packing. To search for newer organic materials with enhanced SHG activity and to better understand structure–property correlations, a “push–pull” ethylene containing a cyano acceptor and a dioxolane donor was synthesised (Scheme 1).

Chapter II Chapter II describes the X?ray crystal structure of 2?dicyanomethylene?1,3?dioxolane (I) and analyses the correlation between SHG activity and molecular packing. Compound (I) crystallises in the non?centrosymmetric space group Cc (Z = 8). The charge?transfer axis (owing to approximate mm2 symmetry) passes through the ethylenic double bond and makes an angle of 28.4° with the crystallographic b?axis. This differs significantly from the theoretically predicted optimum angle of 54.74° for maximum SHG activity (J. Zyss and J. L. Oudar, Phys. Rev., 1982, A26, 2028). Also, the presence of two molecules in the asymmetric unit and a non?crystallographic 2?fold axis along a reduce the SHG response to about half that of urea. Achieving only one molecule in the asymmetric unit is a challenge involving crystal engineering.

Chapter III To obtain larger ? values, four “push–pull” butadienes were synthesised and their structures determined by X?ray diffraction: I. Ethyl 2?cyano?5?dimethylamino?3?methyl?2,4?pentadienoate II. 4,4?Bis(methylthio)?2?phenyl?1,3?butadiene?1,1?dicarbonitrile III. Ethyl 2?cyano?5?dimethylamino?3?phenyl?2,4?pentadienoate IV. 4?Dimethylamino?4?methylthio?3?phenyl?1,3?butadiene?1,1?dicarbonitrile (Scheme 2) All four compounds crystallised in centrosymmetric space groups and hence exhibited no SHG activity. Nevertheless, X?ray analysis revealed strong ??electron delocalization, and theoretical ? values were large. These systems could thus become useful SHG materials if crystallised in non?centrosymmetric lattices through substitution with chiral groups or via crystal?engineering strategies.

Chapter IV Because SHG performance is influenced not only by molecular hyperpolarizability but also by crystal packing, a family of acid–base salts derived from L?tartaric acid (TA) and substituted pyridines (Scheme 3) were prepared and characterised for SHG activity. All salts exhibited SHG intensities comparable to urea. The crystal structure of L?tartaric acid : 4?dimethylaminopyridine (1:1) dihydrate was determined to analyse packing and derive design principles for improved materials. A rationale for the SHG activity based on molecular alignment and hydrogen?bond patterns is provided in this chapter.

Chapter V Motivated by the strong propensity of TA–amine salts to form well?directed hydrogen?bond networks, a salt of D(+)-dibenzoyltartaric acid (DBT) and 4?aminopyridine (4?AP) was prepared (Scheme 4). Recrystallisation yielded two crystal types—needles and prisms. X?ray analysis revealed:

The prismatic crystals correspond to a 1:1 DBT/4?AP acid–base salt. The needle crystals correspond to a non?stoichiometric 1.5:1 DBT/4?AP monohydrate.

This unusual stoichiometry arises because two conformations of DBT coexist within the crystal lattice—one in a general position and the other on a crystallographic 2?fold axis, contributing 1.5 equivalents to the unit cell. Both salts displayed SHG intensities between 1.4–1.6 times that of urea. Chapter V discusses the structural features of both salt forms and their implications for designing new NLO materials.