Carbon nanodots and molecular machines as bottom-up approaches to nanotechnology

The field of nanotechnology, a broad discipline committed to the design, control and manipulation of matter at the nanoscale, has advanced tremendously in the last decades. The final goal of nanotechnology is the development of novel materials, possessing at least one dimension between 1-100 nm. The controlled manipulation of materials at the nanoscale allows conveying new features, which can be dramatically different from those of their corresponding bulk counterparts. Two strategies can be adopted for the fabrication of nanomaterials. Top-down approach consists in the miniaturization of larger materials into nanomaterials; while bottom-up method relies on controlled reactions of atoms and molecular precursors to assemble complex nanomaterials. Interest in the latter strategy is growing due to the possibility to fabricate tailored nanomaterials with fine-tuned properties from molecularly engineered “building blocks”. The fil rouge of this thesis is the use of a rational bottom-up approach to build novel structures at the nanoscale, using two different strategies: a traditional bottom-up approach to obtain materials with nanometric dimensions, and a supramolecular-based approach to build molecular systems performing tasks at the nanoscale. Chapter 1 provides, initially, a general overview of nanotechnology and of respective top-down and bottom-up strategy. An emphasis on the bottom-up methods will be given. After, the synthetic bottom-up approaches for the synthesis of carbon nanomaterials, and in particular carbon nanodots, will be described, and some notable examples will be discussed. Lastly, a complementary approach to the fabrication of nanomaterials will be described, based on supramolecular chemistry. A considerable attention will be given to the synthesis and functioning of one of the most common family of systems used in this domain: artificial molecular machines. Through some specific examples, their operation will also be discussed. Chapter 2 presents the synthesis, purification, and characterization of a family of atropoisomeric carbon nanodots, via bottom-up microwave-assisted method. Contrary to other hydrophilic carbon nanodots, these nanoparticles display solubility in organic solvents. Depending on the chirality of the enantiomer employed, these chiral particles show specular profile in the UV-Visible region, as detected by electronic circular dichroism. Remarkably, one class of these carbon nanodots shows circularly polarized luminescence. Contrary to the literature precedents on carbon nanodots, this advanced optical property is observed in solution, without needing any chiral external matrix. As evidenced by morphological and chiroptical experiments, the axial chirality is transferred from molecular to the nanoscale. In this way, this property is directly encoded within nanomaterial structure, without the need for post-functionalization steps. Chapter 3 presents the synthesis and characterization of a symmetric and an asymmetric molecular axle. Both bear a recognition site for a macrocycle, allowing the formation of pseudorotaxane. Its terminal part can be dynamically stoppered, gaining control on the pseudorotaxane formation. The threading/dethreading operation in response to acid-base inputs was studied, confirming machine operation according to a ratchet mechanism, leading to the energetically-demanding trapping of the macrocycle in a high-energy state. The peculiar choice of molecular stopper allowed controlling the dethreading kinetics, ranging from obtaining kinetically-trapped out-of-equilibrium state to rapid equilibration. The solvent in which the operation occurs is fundamental in controlling the dethreading kinetics. Tuning this parameter, the machine can experience either a rapid equilibration or an observable dissipative relaxation, revealing the directional exploration of a square reaction network underlying machine operation, which can be repeated multiple times in situ.

The field of nanotechnology, a broad discipline committed to the design, control and manipulation of matter at the nanoscale, has advanced tremendously in the last decades. The final goal of nanotechnology is the development of novel materials, possessing at least one dimension between 1-100 nm. The controlled manipulation of materials at the nanoscale allows conveying new features, which can be dramatically different from those of their corresponding bulk counterparts. Two strategies can be adopted for the fabrication of nanomaterials. Top-down approach consists in the miniaturization of larger materials into nanomaterials; while bottom-up method relies on controlled reactions of atoms and molecular precursors to assemble complex nanomaterials. Interest in the latter strategy is growing due to the possibility to fabricate tailored nanomaterials with fine-tuned properties from molecularly engineered “building blocks”. The fil rouge of this thesis is the use of a rational bottom-up approach to build novel structures at the nanoscale, using two different strategies: a traditional bottom-up approach to obtain materials with nanometric dimensions, and a supramolecular-based approach to build molecular systems performing tasks at the nanoscale. Chapter 1 provides, initially, a general overview of nanotechnology and of respective top-down and bottom-up strategy. An emphasis on the bottom-up methods will be given. After, the synthetic bottom-up approaches for the synthesis of carbon nanomaterials, and in particular carbon nanodots, will be described, and some notable examples will be discussed. Lastly, a complementary approach to the fabrication of nanomaterials will be described, based on supramolecular chemistry. A considerable attention will be given to the synthesis and functioning of one of the most common family of systems used in this domain: artificial molecular machines. Through some specific examples, their operation will also be discussed. Chapter 2 presents the synthesis, purification, and characterization of a family of atropoisomeric carbon nanodots, via bottom-up microwave-assisted method. Contrary to other hydrophilic carbon nanodots, these nanoparticles display solubility in organic solvents. Depending on the chirality of the enantiomer employed, these chiral particles show specular profile in the UV-Visible region, as detected by electronic circular dichroism. Remarkably, one class of these carbon nanodots shows circularly polarized luminescence. Contrary to the literature precedents on carbon nanodots, this advanced optical property is observed in solution, without needing any chiral external matrix. As evidenced by morphological and chiroptical experiments, the axial chirality is transferred from molecular to the nanoscale. In this way, this property is directly encoded within nanomaterial structure, without the need for post-functionalization steps. Chapter 3 presents the synthesis and characterization of a symmetric and an asymmetric molecular axle. Both bear a recognition site for a macrocycle, allowing the formation of pseudorotaxane. Its terminal part can be dynamically stoppered, gaining control on the pseudorotaxane formation. The threading/dethreading operation in response to acid-base inputs was studied, confirming machine operation according to a ratchet mechanism, leading to the energetically-demanding trapping of the macrocycle in a high-energy state. The peculiar choice of molecular stopper allowed controlling the dethreading kinetics, ranging from obtaining kinetically-trapped out-of-equilibrium state to rapid equilibration. The solvent in which the operation occurs is fundamental in controlling the dethreading kinetics. Tuning this parameter, the machine can experience either a rapid equilibration or an observable dissipative relaxation, revealing the directional exploration of a square reaction network underlying machine operation, which can be repeated multiple times in situ.