Synthetic strategies of new molecular paramagnetic mechanically-interlocked complexes

Supramolecular chemistry is a multidisciplinary field which impinges on other disciplines, focusing on the systems made up of a discrete number of assembled molecular subunits. The forces responsible for the spatial organization are intermolecular reversible interactions. The supramolecular architectures I was interested in are Rotaxanes, mechanically-interlocked architectures consisting of a "dumbbell shaped molecule", threaded through a "macrocycle" where the stoppers at the end of the dumbbell prevent disassociation of components and catenanes, two or more interlocked macrocycles which cannot be separated without breaking the covalent bonds. The aim is to introduce one or more paramagnetic units to use the ESR spectroscopy to investigate complexation properties of these systems cause this technique works in the same time scale of supramolecular assemblies. Chapter 1 underlines the main concepts upon which supramolecular chemistry is based, clarifying the nature of supramolecular interactions and the principles of host-guest chemistry. In chapter 2 it is pointed out the use of ESR spectroscopy to investigate the properties of organic non-covalent assemblies in liquid solution by spin labels and spin probes. The chapter 3 deals with the synthesis of a new class of p-electron-deficient tetracationic cyclophane ring, carrying one or two paramagnetic side-arms based on 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) moiety. In the chapter 4, the Huisgen 1,3-dipolar cycloaddition is exploited to synthesize rotaxanes having paramagnetic cyclodextrins as wheels. In the chapter 5, the catalysis of Huisgen’s cycloaddition by CB[6] is exploited to synthesize paramagnetic CB[6]-based [3]-rotaxanes. In the chapter 6 I reported the first preliminary studies of Actinoid series as a new class of templates in catenanes’ synthesis. Being f-block elements, so having the property of expanding the valence state, they constitute promising candidates as chemical templates offering the possibility to create a complex with coordination number beyond 6.

Design, realization, and characterization of complex reaction networks in dynamic mechanically interlocked molecules

The experimental projects discussed in this thesis are all related to the field of artificial molecular machines, specifically to systems composed of pseudorotaxane and rotaxane architectures. The characterization of the peculiar properties of these mechano-molecules is frequently associated with the analysis and elucidation of complex reaction networks; this latter aspect represents the main focus and central thread tying my thesis work. In each chapter, a specific project is described as summarized below: the focus of the first chapter is the realization and characterization of a prototype model of a photoactivated molecular transporter based on a pseudorotaxane architecture; in the second chapter is reported the design, synthesis, and characterization of a [2]rotaxane endowed with a dibenzylammonium station and a novel photochromic unit that acts as a recognition site for a DB24C8 crown ether macrocycle; in the last chapter is described the synthesis and characterization of a [3]rotaxane in which the relative number of rings and stations can be changed on command.

Novel prototypes of electro-activated molecular machines

The work presented in this thesis deals with the investigation of new prototypes of molecular machines, based on rotaxane and pseudorotaxane architectures, by means of voltammetric and spectroscopic techniques. The discussion is divided in two parts. Part I concerns the investigation of electro-switchable molecular muscles, based on mechanically interlocked molecules. This study is performed on systems of increasing complexity, starting from [2]rotaxanes and arriving to polymers. In Chapters 3 and 4, [2]- and [3]rotaxanes, characterized by the presence of three stations for the macrocycle(s), are investigated. In both systems, the macrocycle(s) movement can be controlled through a combination of stimuli, resulting in a processive and directional motion. In Chapter 5, daisy chain rotaxanes, dimers of the [2]rotaxanes discussed in Chapter 3, are investigated. These systems can be switched between an extended and a contracted conformation, and they represent the monomeric units for the realization of polymeric molecular muscles. In Chapter 6, the properties of electro-switchable polymeric molecular muscles, composed by the daisy chains investigated in Chapter 5, are discussed. The repeating units of these poly-daisy chains contract and extend upon electrical stimulation, and this motion is expected to be transmitted to the polymer itself, resulting in an amplification of the effect. Part II concerns the investigation of rotaxanes and pseduorotaxanes based on heteroditopic calix[6]arenes and cationic guests. In Chapters 8 and 9, novel calix[6]arene macrocycles, functionalized with thiourea or dansyl units, and their related pseudorotaxanes are investigated. In both cases, the calix[6]arene functionalization adds new features to the pseudorotaxane. In Chapters 10 and 11, the influence of orientational isomerism on the properties of [2]- and [3]rotaxanes is investigated. The [3]rotaxanes discussed in Chapter 10 display similar properties, while the [2]rotaxanes described in Chapter 11, characterized by a calix[6]arene and a stilbazolium unit, exhibit distinct photophysical and photochemical properties.