Studies on nonlinear optical materials and photochemical reactions in the crystalline state by X-ray crystallography

Abstract

The thesis entitled "Studies on Nonlinear Optical Materials and Photochemical Reactions in the Crystalline State by X-ray Crystallography" has been divided into two parts. Part I deals with X-ray structural investigations of four polar compounds and the correlation between the molecular packing and nonlinear optical property, namely second harmonic generation (SHG) efficiency in the crystalline state. In Part II, photochemical studies in the solid state of benzoin alkyl ethers and benzylidene-dipiperitone are discussed. The thesis begins with a brief review on various aspects of nonlinear optics. The main emphasis is on second-order nonlinear effect, i.e., second harmonic generation (SHG). Recent developments in this field with particular reference to experimental search for new organic nonlinear optical materials with SHG efficiency are presented. In addition, a brief mention is made of the application of inclusion hosts such as cyclodextrin and deoxycholic acid for achieving good SHG efficiency. For an organic molecule to show SHG, it is necessary that it must satisfy the following properties: (i) asymmetry, (ii) high conjugation, (iii) occurrence of charge transfer, and (iv) large difference in dipole moment between ground and excited state. It has been found from X-ray studies investigated on a large number of push-pull ethylenes (Adhikesavalu, Kamath and Venkatesan, 1983) that there is appreciable delocalization between donor and acceptor groups. However, many of these compounds crystallize in centrosymmetric space groups and, therefore, would not show SHG. Hence chiral derivatives of push-pull ethylenes by attaching menthyl group at the acceptor end, (I)–(V) (Fig. S.1) were synthesized and their second harmonic efficiency measured. Chapters II and III deal with the single crystal X-ray analyses of two push-pull ethylenes (I) and (II). Chapter IV describes the X-ray structures of two heterocyclic compounds (A) and (B) (Fig. S.1). Though these two compounds do not belong to the category of push-pull ethylenes, their nonlinear optical property was studied as they have high ground state dipole moment (9.20 D for (A) and 8.25 D for (B)) and crystallize in noncentrosymmetric space groups. The intensity data were collected on a Nonius CAD-4 diffractometer and all the structures were solved by direct methods using MULTAN80. The SHG efficiency of these compounds was measured by the method of Kurtz and Perry (1968) using powder samples. The crystal data and SHG efficiency of the compounds are recorded below:

Chapter II deals with the crystal and molecular structure of menthyl 2-(bisdimethylamino)-methylene-3-oxobutyrate (I) (Fig. S.1). Routine applications of direct method for structure determination using the program MULTAN80 did not yield the structure. Various attempts leading to the successful solution are discussed. The most significant observation in the geometry of the molecule is the lengthening of the C=C bond (1.447(6) Å) indicating significant ?-electron delocalization between the donor and acceptor groups. The torsion angle about the ethylene bond is as large as 59.3(5)°. The molecular packing is stabilized by hydrogen bonding in the crystal. In Chapter III, the syntheses and characterization of push-pull ethylenes (II)–(V) (Fig. S.1) are described. Details of the crystal and molecular structure of (II) (1-(1,3-dimethylimidazolidinylidene)-2,2-biscarbomenthoxy ethylene) are presented. Significant delocalization of the ?-electrons between the donor and acceptor groups is indicated by the lengthening of the C=C bond which is as long as 1.435(9) Å. A rationalization of the observed low SHG activity in the two push-pull ethylenes (I and II) in terms of the molecular packing is provided. In molecule (I), the charge transfer axis which lies along the C=C bond makes an angle of 43.5° with the crystallographic two-fold axis which differs from the theoretically calculated value of 54.7° favourable for nonlinear interactions belonging to the point group 2. In molecule (II), the charge transfer axis is almost perpendicular (86.9°) to the a-axis of the unit cell which is clearly unfavourable for SHG activity. With the view to improve the SHG efficiency, inclusion complexes of push-pull ethylenes with p-cyclodextrin and deoxycholic acid were prepared. The negligible SHG activity of the inclusion complexes appears to be due to the random orientation of the guest molecules in the host leading to vanishing polarization. Chapter IV discusses details of crystal and molecular structures of 2-(4-ethyl-5-methylthiazoline-2-thione)-propionate (A) (Fig. S.1) and 3-(N-methylpiperidine-2?-thion-3?-yl)-1-methylindole (B) (Fig. S.1) and the structure-SHG correlation of (A) and (B). The molecule (A) is in syn conformation with respect to the bond N(3)-C(10) and molecule (B) is in anti conformation (anti conformation with respect to the bond C(3)-C(10) and thiopiperidonyl ring attached to C(3) is in distorted half-chair conformation) in the solid state. Using the charges calculated by CNDO/2 method, the dipole moments of these molecules were calculated. In molecule (A), the resultant dipole moment vector is approximately perpendicular to c-axis (84.4°) and in (B) it is almost perpendicular to a-axis (87.2°). The observed SHG efficiencies of both (A) and (B) have been interpreted in the light of the observed molecular packing. Chapter V begins with a brief review of unimolecular photochemical reaction, namely intramolecular hydrogen abstraction in the solid state. With the aim of understanding the solid-state photochemical behaviour, the crystal and molecular structures of benzoin alkyl ethers (I) and (II) (Fig. S.2) have been investigated. Molecule (I) crystallizes in the space group P1, a = 8.441(2), b = 10.276(2), c = 15.342(2) Å, ? = 91.02(2)°, ? = 79.26(2)°, ? = 105.88(2)° and Z = 4. Molecule (II) belongs to the space group Pca2?, a = 8.476(1), b = 16.098(3), c = 10.802(3) Å, Z = 4. The crystal structures of (I) and (II) refined to R values of 0.060 (1793 significant reflections) and 0.071 (867 significant reflections) respectively. However, molecules (I) and (II) (Fig. S.2) are photostable. Upon irradiation in the solid state, the photostability of (I) and (II) has been interpreted from a knowledge of relevant geometrical parameters, namely: (i) O...H distance, (ii) C–H...O angle, (iii) C–O...H angle, and (iv) the angle subtended by the O...H vector and its projection on the mean plane of the carbonyl group. In connection with the studies on the role of molecular packing on bimolecular photochemical reactions in crystals, the photobehaviour of ?-benzylidene-dl-piperitone was investigated. Chapter VI is devoted to a discussion of the crystal and molecular structure of ?-benzylidene-dl-piperitone (Fig. S.2) and structure-reactivity correlation. The crystal data are: a = 6.116(2), b = 16.127(3), c = 14.417(5) Å, ? = 96.62(2)°, Z = 4, space group P2?/n. R = 0.062 for 1800 significant reflections. The reactive double bonds in the cyclohexenone ring are related by a centre of inversion with centre-to-centre distance of 3.915(3) Å and are properly oriented for [2+2] photocycloaddition. In spite of the fact that the arrangement of the double bonds satisfies the topochemical criterion for dimerisation reaction (Schmidt, 1964), the crystal is photostable. Lattice energy calculations on ground-state molecules in crystals have been performed to understand the photoinertness of the crystal. A possible explanation for the inertness of the compound in terms of large-scale atomic displacement expected for the styrene group upon excitation in the crystal lattice is provided. The references are listed in alphabetical order at the end of each chapter. The lists of observed and calculated structure factors for all structures have been placed together at the end of the last chapter.