"3,3'-Bis(acylamino)-2,2'-bipyridine discotics: desymmetrization and functionalization

Abstract

In the field of supramolecular chemistry control over self-assembly is one of the main targets. This might be accomplished by adaptation of the environment of organic assemblies by allowing their interaction with other molecules or substrates. To do so, the assembling component has to be equipped with a functional site, which requires derivatization. Also, derivatization in itself may result in enhanced, beneficial supramolecular behavior. In this thesis, derivatization of disc-shaped molecules at their periphery is described with the goal of introducing functionality into the discotic systems and of allowing the discotics to perform desirable, programmed interactions with other molecules. The discotics are composed of a central trimesic core and radially equipped with three 2,2'-bipyridinyl-3,3'-diamine moieties that in turn are linked to three gallic moieties decorated with peripheral alkyl tails. In Chapter 1 an overview of functionalized and desymmetrized discotics is given, with the focus on single-core discotics like triphenylenes, hexabenzocoronenes, phthalocyanines, porphyrins and benzene-1,3,5-tricarboxamides. The synthesis of the desymmetrized derivatives as well as their enhanced supramolecular and material properties is described. It is clear that desymmetrization and functionalization of the originally symmetrical discotics allows programmed interaction with other molecules or gives rise to functional materials besides the original focus on one-dimensional assembly and columnar liquid crystals. In Chapter 2, two synthetic strategies to replace one of the 3,4,5-trialkoxyphenyl units of bis(acylamino)-2,2'-bipyridine based discotics with a phenyl (disc 1, Figure 1) or 4-pyridyl (disc 2, Figure 1) unit are proposed. The first synthetic strategy is based on a statistical approach and the second one on a step-wise approach involving protective-group chemistry. Both strategies have afforded the desired non-symmetrical discotics but the second strategy has many advantages over the first one, like easier purification steps and accessibility to multigram amounts of the desired discs and their valuable precursors. Importantly, the desymmetrization does not affect significantly the preorganized hydrogen-bonded structure of the discotics. The self-assembly properties of non-symmetrical discotics 1 and 2 are reported in Chapter 3. Both discotics display helical self-assembly in the mesophase and in apolar solution. Importantly, this assembly of discs 2 is similar to that of their C3-symmetical analogues showing that desymmetrization and functio-nalization of the discotics is feasible without undoing their self-assembly capabilities. In Chapter 4, the interaction of disc 2, possessing a peripheral 4-pyridyl group, with chiral acids is described as well as the supramolecular transfer of chirality. First several acids had to be screened to reveal which acids bind selectively with the discotic without disrupting its supramolecular properties. Apparently, acids of intermediate strength like phosphonic and tartaric acids satisfy this requirement. The appropriate chiral acids have then been used to induce chirality into the helical assemblies of disc 2 in solution. Apparently, the efficiency of the transfer of chirality is not only determined by the strength of the chiral acid, but also by steric effects. The stability of the chiral complex is highly sensitive and depends on the helix stability, the strength of the acid-base complex, and the solubility of the components. Chapter 5 deals with the incorporation of a functionalized discotic in methacrylate based polymers. Desymmetrized discotic 3 (Figure 1) carrying a dangling hydroxy group is synthesized that may act as a starting point for a wide variety of functionalized discotics. This is illustrated by transforming disc 1 into a polymerizable disc carrying a methacrylate group. This disc is then copolymerized under ATRP conditions to afford a disc-functionalized poly(butyl methacrylate) copolymer. The latter may serve as a novel material for supramolecular, fluorescent polymeric nanoparticles. In Chapter 6, a novel C3-symmetrical, heavily fluorinated disc 4 (Figure 1) is introduced. Replacing the originally hydrophobic hydrocarbon periphery by a fluorophilic fluorocarbon periphery (disc 4, Figure 1) allows helical self-assembly in fluorinated media. Teflon star 4 forms very stable columnar mesophases in which helicity may be present. Surprisingly, a proper choice of solvent combination allows the formation of mixed assemblies in which discotics possessing both a chiral, hydrocarbon periphery and fluorinated discotics 4 are present. This allows transfer of chirality from the former to the latter with the expression of amplification of chirality. Figure 1: Discotics described in this thesis. Non-symmetrical discotics 1 and 2 figure as the main topics in Chapters 2, 3 and 4. Hydroxy-disc 3 is applied in Chapter 5 and teflon disc 4 is the key molecule in Chapter 6.