Abstract Alessio Piermattei

In this thesis different aspects of functional hydrogen-bonded (double and tetrarosette) assemblies are described. The functions were inspired by naturally occurring phenomena such as self-organization, supramolecular chirality, and the correct folding of proteins. The studies presented in this thesis are focused on the synthesis of supramolecular materials by these noncovalent assemblies. The emphasis is on liquid crystals (and gels), which are able to interact with the environment and adapt to it.

Double and tetrarosette assemblies are formed upon mixing calix[4]arene dimelamines and calix[4]arene tetramelamines in apolar solvents with barbiturate and cyanurate derivatives in a 1:2 and 1:4 ratio, respectively. The assembly process is driven by the formation of 36 (double rosette) or 72 (tetrarosette) hydrogen bonds between the complementary hydrogen bonding arrays of different building blocks, leading to assemblies with high thermodynamic stability.

Chapter 2 reviews hydrogen-bonded liquid crystalline materials that have been described in the literature in the past decades. The concepts and the principles for the formation of these materials are discussed, and examples are given that illustrate the control over these systems.

Chapter 3 describes the formation of liquid crystalline materials by hydrogen-bonded double rosettes achieved by the functionalization of the melamine units of the calix[4]arene with octadecyl chains. The liquid crystalline phases of these assemblies have been determined by polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The double rosette assemblies organized in columnar fashion in the liquid crystalline phase, showing a remarkable thermal stability considering their non-covalent nature. Furthermore, the strength of the hydrogen bonds and the spatial disposition of the side groups of the barbituric and cyanuric derivatives are crucial factors for the stability of the mesophases.

In Chapter 4 the supramolecular chirality of mesogenic double rosette assemblies in solution and the liquid crystalline state is described. The bulkiness and the position of the chiral group of the barbiturate and cyanurate building blocks play an important role in the formation of double rosette assemblies and the liquid crystalline phases. The chirality of the double rosettes in solution was transferred to the liquid crystalline state, as demonstrated by circular dichroism measurements. In the mesophase different arrangements and orientation of the columns along the stacking direction have been obtained depending on the nature of the building blocks of the assembly. Formation of a gel phase is described for these chiral double rosettes in n-alkane solvents.

Chapter 5 describes the complexation of a molecular guest (alizarin) by mesogenic chiral self-assembled double rosettes in solution and in the liquid crystalline state. Alizarin was encapsulated in barbiturate-based assemblies and intercalated between cyanurate-based assemblies. In solution, the presence of aggregates is a necessary requirement for the intercalation of alizarin molecules between the double rosette floors as confirmed by ¹H NMR DOSY, UV-Vis and CD experiments. In the liquid crystalline state, the alizarin complexation by the double rosettes led to highly stable mesophases. In case of barbiturate-based assemblies, the complexation did not alter the type of phase, while for cyanurate-based assemblies the complexation led to the formation of a crystalline phase. Furthermore, the in-situ exchange of building blocks in the liquid crystalline state led to the modulation of the optical activity in these assemblies.

In Chapter 6, the investigation by AFM and STM of the supramolecular organization of mesogenic double rosettes on surfaces is described. The self-organization of these assemblies on highly ordered pyrolytic graphite (HOPG) depends on the concentration and polarity of the solvent. For example, the double rosette organization on HOPG went from 1D (fibers) to 2D (hexagonal columnar lattice) order by decreasing the concentration. Furthermore, the presence of double rosette aggregates in solution at low concentration (25 mM) leads to the formation of micrometer-long ordered structures on graphite. The chirality in the double rosettes allowed the organization of these assemblies in ordered structures when deposited onto mica. Moreover, STM measurements allowed the visualization of individual mesogenic double rosette on HOPG.

In Chapter 7 the noncovalent synthesis of cyanurate-based tetrarosettes, in which barbiturate derivatives were employed as a catalytic chaperone, is described. In the noncovalent synthesis of tetrarosette assemblies, the barbiturate acts simultaneously as chaperone inhibiting the formation of kinetically stable ill-defined assemblies and as catalyst accelerating the formation reaction. Furthermore, the enantioselective synthesis of cyanurate-based tetrarosette assemblies was achieved via amplification of chirality by using substoichiometric amounts of chiral barbiturates as a chaperone. Moreover the synthesis of barbiturate-based tetrarosette assemblies have been performed and their liquid crystalline behavior have been studied after the proper functionalization of tetramelamine calix[4]arene with octadecyl chains.

The results presented in this thesis illustrate the ability of well-defined nanometer-size self-assembled structures to form supramolecular materials. The noncovalent nature of the assemblies allows to obtain different materials properties by the simple replacement of one building block with another leading toward the synthesis of responsive materials.