Luminophores and Carbon nanostructures: towards new functional materials

In the scenario of depleting fossil fuels, finding new energy technologies and conserving conventional energy resources have become essential to sustain modern civilization. While renewable energies are on the rise, considerable interest has been turned also to reduce energy consumption of conventional devices and appliances, which are often not yet optimized for this purpose. Modern nanotechnology provides a platform to build such devices by using nanomaterials showing exceptional physico-chemical properties. In particular, carbon materials (fullerenes, carbon nanotubes, graphene etc.), which show high thermal and electrical conductivity, aspect ratio, shear strength and chemical/mechanical resistance, are quite promising for a wide range of applications. However, the problem of solubility often hampers their handling and industrial utilization. These limitations can be mitigated by functionalizing carbon nanostructures, either covalently or non covalently, with organic or inorganic compounds. The exo- and endohedral functionalization of carbon nanotubes (CNTs) with organic/inorganic moieties to produce luminescent materials with desired properties are the main focus of this doctoral work. These hybrids have been thoroughly designed and characterized with chemical, microscopic and photophysical analyses. All the materials based on carbon nanostructures described in this thesis are innovative examples of photoactive and luminescent hybrids, and their morphological and photophysical properties help understanding the nature of interactions between the active units. This may prompt the design and fabrication of new functional materials for applications in the fields of optoelectronics and photovoltaics.

Photophysics of Carbon Nanotubes: from Dispersion to Supramolecular Systems

The aim of this PhD thesis is the investigation of the photophysical properties of materials that can be exploited in solar energy conversion. In this context, my research was mainly focused on carbon nanotube-based materials and ruthenium complexes. The first part of the thesis is devoted to carbon nanotubes (CNT), which have unique physical and chemical properties, whose rational control is of substantial interest to widen their application perspectives in many fields. Our goals were (i) to develop novel procedures for supramolecular dispersion, using amphiphilic block copolymers, (ii) to investigate the photophysics of CNT-based multicomponent hybrids and understand the nature of photoinduced interactions between CNT and selected molecular systems such as porphyrins, fullerenes and oligo (p-phynylenevinylenes). We established a new protocol for the dispersion of SWCNTs in aqueous media via non-covalent interactions and demonstrated that some CNT-based hybrids are suitable for testing in PV devices. The second part of the work is focussed on the study of homoleptic and heteroleptic Ru(II) complexes with bipyridine and extended phenanthroline ligands. Our studies demonstrated that these compounds are potentially useful as light harvesting systems for solar energy conversion. Both CNT materials and Ru(II) complexes have turned out to be remarkable examples of photoactive systems. The morphological and photophysical characterization of CNT-based multicomponent systems allowed a satisfactory rationalization of the photoinduced interactions between the individual units, despite several hurdles related to the intrinsic properties of CNTs that prevent, for instance, the utilization of laser spectroscopic techniques. Overall, this work may prompt the design and development of new functional materials for photovoltaic devices.

Safety by design: production of engineering surface modified nanomaterials

This PhD thesis focused on nanomaterial (NM) engineering for occupational health and safety, in the frame of the EU project “Safe Nano Worker Exposure Scenarios (SANOWORK)”. Following a safety by design approach, surface engineering (surface coating, purification process, colloidal force control, wet milling, film coating deposition and granulation) were proposed as risk remediation strategies (RRS) to decrease toxicity and emission potential of NMs within real processing lines. In the first case investigated, the PlasmaChem ZrO2 manufacturing, the colloidal force control applied to the washing of synthesis rector, allowed to reduce ZrO2 contamination in wastewater, performing an efficient recycling procedure of ZrO2 recovered. Furthermore, ZrO2 NM was investigated in the ceramic process owned by CNR-ISTEC and GEA-Niro; the spray drying and freeze drying techniques were employed decreasing NM emissivity, but maintaining a reactive surface in dried NM. Considering the handling operation of nanofibers (NFs) obtained through Elmarco electrospinning procedure, the film coating deposition was applied on polyamide non-woven to avoid free fiber release. For TiO2 NF the wet milling was applied to reduce and homogenize the aspect ratio, leading to a significant mitigation of fiber toxicity. In the Colorobbia spray coating line, Ag and TiO2 nanosols, employed to transfer respectively antibacterial or depolluting properties to different substrates, were investigated. Ag was subjected to surface coating and purification, decreasing NM toxicity. TiO2 was modified by surface coating, spray drying and blending with colloidal SiO2, improving its technological performance. In the extrusion of polymeric matrix charged with carbon nanotube (CNTs) owned by Leitat, the CNTs used as filler were granulated by spray drying and freeze spray drying techniques, allowing to reduce their exposure potential. Engineered NMs tested by biologists were further investigated in relevant biological conditions, to improve the knowledge of structure/toxicity mechanisms and obtain new insights for the design of safest NMs.

Unconventional Catalysis in Organic Chemistry: a Computational Mechanistic Study

Catalysis plays a vital role in modern synthetic chemistry. However, even if conventional catalysis (organo-catalysis, metal-catalysis and enzyme-catalysis) has provided outstanding results, various unconventional ways to make chemical reactions more effective appear now very promising. Computational methods can be of great help to reach a deeper comprehension of these chemical processes. The methodologies employed in this thesis are Quantum-Mechanical (QM), Molecular Mechanics (MM) and hybrid Quantum-Mechanical/Molecular Mechanics (QM/MM) methods. In this abstract the results are briefly summarised. The first unconventional catalysis investigated consists in the application of Oriented External Electric Fields (OEEFs) to SN2 and 4e-electrocyclic reactions. SN2 reactions with back-side mechanism can be catalysed or inhibited by the presence of an OEEF. Moreover, OEEFs can inhibit back-side mechanism (Walden inversion of configuration) and promote the naturally unfavoured front-side mechanism (retention of configuration). Electrocyclic ring opening reaction of 3-substituted cyclobutene molecules can occur with inward or outward mechanisms depending on the nature of substituent groups on the cyclobutene structure (torquoselectivity principle). OEEFs can catalyse the naturally favoured pathway or circumvent the torquoselectivity principle leading to different stereoisomers. The second case study is based on Carbon Nanotubes (CNTs) working as nano-reactors: the reaction of ethyl chloride with chloride anion inside CNTs was investigated. In addition to the SN2 mechanism, syn and anti-E2 reactions are possible. These reactions inside CNTs of different radii were examined with hybrid QM/MM methods, finding that these processes can be both catalysed and inhibited by the CNT diameter. The results suggest that electrostatic effects govern the activation energy variations inside CNTs. Finally, a new biochemical approach, based on the use of DNA catalyst was investigated at QM level. Deoxyribozyme 9DB1 catalyses the RNA ligation allowing the regioselective formation of the 3'-5' bond, following an addition-elimination two-step mechanism.