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Fluoropolymers have useful properties including high thermal stability, chemical resistance, low dielectric constants, and both hydrophobic and oleophobic character, as compared to non-fluorinated analogues. Meanwhile, Diels-Alder polyphenylenes (DAPPs) are known for thermal stability as well as their rigid structure and glassy physical characteristics, which have led to a variety of film and membrane applications. This dissertation merges these two fields by demonstrating a novel and general synthetic approach to highly fluorinated DAPPs. These polymers are expected to retain the physical characteristics of glassy, non-fluorinated DAPPs while also incorporating the desirable attributes of fluoropolymers. The polymer synthesis described herein is based on the well-established polycondensation of bis(cyclopentadienone) (CPD) monomers and dialkynes. Our first main scientific contribution is a general method for preparing CPDs containing both a fluoroaromatic linker and variable fluoroaromatic head-groups. Our CPD synthesis uses nucleophilic aromatic substitution reactions of cyclopentadienyl anions and perfluoroarenes, as well as a new catalytic method of converting cyclopentadiene methylene (CH₂) groups into the corresponding ketones (C=O) that is the primary dissertation subject of Brian S. Hickory in our laboratory. The overall synthesis is notable for its use of inexpensive starting materials, its efficiency, and its structural versatility. Our second main contribution is the synthesis of novel highly fluorinated Diels-Alder polyphenylenes (DAPPs). Fluorinated DAPPs varied in their molecular weight, in the identity of the lateral fluoroaryl substituent (pentafluoro-phenyl or tetrafluoro-4-pyridyl), and in the structure of the aromatic dialkyne monomer. These polymers are glassy materials with high glass transition temperatures and high thermal stability. Since the polyphenylene structure is intrinsically rigid, the polymers form brittle films even at molecular weights of over 30,000 (M_w). Unlike many fluoropolymers, the fluorinated DAPPs are freely soluble in common organic solvents such as tetrahydrofuran and chloroform. An unknown side reaction competes with the polymer propagation and reduces the highest obtainable molecular weights, which limit the ability to form films. However, a stoichiometric imbalance leads to highly fluorinated polyphenylene oligomers terminated with either alkyne or CPD end groups (M_n = 9000). Because preliminary experiments had shown that the desired Diels-Alder propagation reaction was slower than expected, we also undertook a detailed model study of the reaction conditions needed for Diels-Alder reactions of fluorinated CPDs and aromatic alkynes. These experiments showed that protic polar solvents (e.g., m-cresol) and conventional heating at ca. 150 °C optimize reaction rate while minimizing side-reactions that can contribute to lower molecular weight in corresponding polymerization reactions. / Ph. D.

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The cyclopentadienone-fused dihydropyrenes 46 and 47 were synthesized. The internal methyl resonances, the coupling constants, NICS calculations and X-ray results confirmed that the cyclopentadienone displays antiaromatic character resulting in bond localization in the annulene ring consistent with a 4n-pi fused system. The ring current of the dihydropyrene fragment is reduced by fusion of the antiaromatic system by about 80% of that caused by benzene. The syntheses of the methylfulvene fused dihydropyrene 56 and the phenylfulvene fused dihydropyrene 58 have been accomplished. The calculated and experimental NMR data and NICS calculations all demonstrated that the fulvenes had weak diatropic ring currents and caused bond localizations in the DHP rings, in which phenyl fulvene has a larger effect than that of methyl fulvene. A number of bis-dihydropyrene systems, bis-dihydropyrene ketone 117, bis-benzo[e]dihydropyrene ketone 119, benzo[e]dihydropyrene dihydropyrene ketone 122, bis-benzo[e]dihydropyrene methylene 124 and the benzo[e]dihydropyrene- dihydropyrene acetylene 130, have been synthesized, in which 117, 119 and 124 are homo-systems and 122 and 130 are hetero-systems. The multiple photoswitching properties study found that all of these systems except 130 showed multi-states during the photo opening and photo closing processes, which means that each end of the DHP units photo opens or closes separately rather than synchronously. In the homo switches 117, 119 and 124, the two DHP units act independently, but the relative differentiation is not very significant. On the other hand, the hetero-switch 122 showed fully differentiated photo opening process and almost a pure open-closed intermediate 122’ could be achieved. This is the first example which clearly showed four states in the UV closing process. The relative photo opening and closing rates compared to benzoDHP 36 have also been studied. It was found that while the carbonyl linker largely increased the relative photo opening rate (117, 119 and 122), the methylene linker only increased it slightly (124). The photo closing processes were always fast as usual. The studies of the thermal return reactions of these systems showed that while the carbonyl linker substantially slowed down the thermal return reactions of the DHP units (117, 119 and 122), the methylene linker speeded it up slightly (124). The mono-iron tricarbonyl benzo[e]dihydropyrene complex 152, the bis-iron tricarbonyl benzo[e]dihydropyrene complex 153 and the iron tetracarbonyl dihydropyrene complex 151 were synthesized. The structures of 152 and 153 were determined by X-ray crystallography. The coordinations of iron tricarbonyl moieties to the DHP rings caused a distortion of ca. 30 degree away from the central DHP plane. Coordination also increased bond alternation and reduced ring currents in the DHP rings. 1H-NMR and X-ray studies showed that 152 showed a weak paratropic ring current in the DHP ring. Iron coordination of the DHP completely stopped the photochromic properties of the dihydropyrenes.

The thesis entitled “Addressing subtle physicochemical features exhibited by molecular crystals via Experimental and Theoretical Charge Density Analysis” consists of five chapters. An introductory note provides a brief description of experimental and theoretical charge density methodology, followed by its utilization in obtaining certain physical and chemical properties in molecular crystals. Chapter 1 addresses not so easily accessed molecular property arising due to electron conjugation, highlighting antiaromaticity in tetracyclones. A systematic study of six tetracyclone derivatives with electron withdrawing and electron donating substituents has been carried out using experimental and theoretical charge density analysis. A three pronged approach based on quantum theory of atoms in molecules (QTAIM), nucleus independent chemical shifts (NICS), and source function (SF) has been employed to establish the degree of antiaromaticity of the central five-membered ring in all the derivatives. Electrostatic potentials mapped on the is density surface reveal the finer effects of different electron withdrawing and electron donating substituents on the carbonyl group. Chapter 2 presents a temperature induced reversible first order single crystal to single crystal phase transition (Room temperature Orthorhombic, P22121 to low temperature Monoclinic, P21) in a  hybrid peptide, Boc-γ4(R)Val-Val-OH. The thermal behavior accompanying the phase transition of the dipeptide crystal was characterized by differential scanning calorimetry, visual changes in birefringence of the sample during heating and cooling cycles on a hot-stage microscope with polarized light. Variable-temperature unit cell check measurements from 300 to 100 K showed discontinuity in the volume and cell parameters near the transition temperature, supporting the first-order behavior. The reversible nature of the phase transition is traced to be due to an interplay between enthalpy and entropy. Chapter 3 brings out an unusual stabilizing interaction involving a cooperative -hole and ¬hole character in a short NCS···NCS bond. This chapter describes structural features of four isothiocyanate derivatives, FmocXCH2NCS; X=Leu, Ile, Val and Ala. Among these it is observed that only FmocLeuCH2NCS which crystallizes in a tetragonal space group, P41, (a=b=12.4405(5) Å; c= 13.4141(8) Å) transforms isomorphously to a low temperature form, P41, (a=b=17.4665(1) Å; c= 13.1291(1) Å). The characteristics of the phase transition have been monitored by Differential Scanning Calorimetry, variable temperature IR and temperature dependent unit cell measurements. The short NCS···NCS intermolecular interaction (3.296(1) Å) is analyzed based on detailed experimental charge density analysis which reveals the nature of this stabilizing interaction. Chapter 4 explains a comparative study of syn and anti conformations of carboxylic acids in peptides from both structural aspect and charge density features. Single crystal structures of four peptides having syn conformations [BocLeuγ4(R)Valγ4(R)ValOH, BocLeuγ4(R)ValLeuγ4(R)ValOH, Boc3(S)Leu3(S)LeuOH] and one with anti conformation, BocLeuγ4(R)ValValOH have been analyzed. Experimental charge density analysis has been carried out exclusively on BocLeuγ4(R)ValValOH having anti form, because of its rare occurrence in literature. However, low temperature datasets on the four peptides with syn conformations were collected and theoretical charge density analysis has been carried out on two of these compounds. Electrostatic potentials mapped on is density surface bring out a significant difference at the oxygen atoms of the carboxyl group in the two conformations. However, lone pair orientation of different types of Oxygen atoms in the two forms (urethane, amide, acid) doesn’t exclusively indicate the differences in the corresponding charge density features. Chapter 5 addresses the issue of how sensitive are the charge density features associated with amino acid residues when the backbone conformational angles are varied. Three model systems, 1, L-alanyl–L-alanyl–L-alanine dehydrate; 2, anhydrous L-alanyl–L-alanyl–L¬alanine and 3, cyclo-(D,L-Pro)2(L-Ala)4 monohydrate have been chosen for this evaluation. Compound 1 has ant parallel alignment of tripe tide strands, and compound 2 has parallel alignment. All the alanine residues in compound 1 and 2 are in the -sheet region of the Ramachandran plot, whereas, the four Alanine residues in the cyclic hex peptide 3 span different regions of the Ramachandran plot. Theoretical multipole modelling has been carried out in order to explore the plausibility of transferring multipole parameters across different regions of Ramachandran Plot. Appendix I contains a brief description of charge shift bonding in Ph-CH2-Se-Se-CH2-Ph, as determined based on both experimental and theoretical charge density analysis. Appendix II contains a reprint of a published article on “Conformation-Changing Aggregation in Hydroxyacetone: A Combined Low-Temperature FTIR, Jet, and Crystallographic Study”.

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