Structural, spectroscopic and charge density investigations of organic molecular crystals

Abstract

The thesis pertains to investigations on several organic crystal systeM.S. employing X-ray diffraction, photophysical measurements as well as the experimental charge density method. It consists of six chapters. Chapter 1 provides an overview on supramolecular assemblies in relation to the solid state properties with few examples from experimental charge density. Chapter 2 deals with the supramolecular assemblies formed by pyrimidine bases, uracil and cytosine with various hydrogen bond donor and acceptor compounds. Another related molecule, 5-nitrouracil forM.S. supramolecular assemblies with variety of compounds including solvents. The study brings out various hydrogen bonded architectures and patterns including the proton transfer interactions in these assemblies. Understanding the weak interactions present in 1,4-bis(phenylethynyl)-2,5-bis(ndialkyloxy) benzenes and their influence on molecular geometry, packing as well as on the solid state fluorescence properties is the subject matter of Chapter 3. The molecule in the crystal structure becomes increasingly planar with longer alkyl chains. The molecules pack to form J-aggregates and the band position is observed to vary linearly with the distance between the interacting molecules in the aggregate. The quantum yield lies in the range, 0.3 – 0.8. Interestingly, the fluorescence behavior of the methoxy substituted molecule depends crucially on the method of crystallization. Melt cooled filM.S. of this molecule fluoresce in the blue region with four distinct bands, the relative intensities depending on the rate of cooling. The crystallites formed show preferred orientations sensitive to the wetting behavior of the substrate. The experimental charge density analysis of two phenyleneethynylene prototypes- 1,2-diphenylacetylene and 1,4-diphenylbutadiyne, and the use of various charge density descriptors such as density, Laplacian and ellipticity in describing the conjugation in these molecules is the content of Chapter 4. The non-zero ellipticity value associated with the triple bond in 1,2-diphenylacetylene is a clear indication of the extent of conjugation present in the molecule. For 1,4-diphenylbutadiyne, the deviation from the ideal value is small. Chapter 5 presents the results of the experimental charge density analysis of two proton transfer complexes- melaminum L-tartrate monohydrate and Lasparginium picrate. The study brings out the role of hydrogen bonding in proton transfer and also the enhancement dipole moments in the non-centrosymmetric crystal field. Experimental charge density analysis on antiepileptic drugs- carbamazepine and oxcarbazepine, is presented in Chapter 6. Of the two, oxcarbazepine exhibits a higher dipole moment. The study gives some insight into the improved bioavailability and specificity of oxcarbazepine over carbamazepine.