Optically active photoresponsive multifunctional polymeric materials

In the last year [1], Angiolini and co-workers have synthesized and investigated methacrylic polymers bearing in the side chain the chiral cyclic (S)-3-hydroxypyrrolidine moiety interposed between the main chain and the trans-azoaromatic chromophore, substituted or not in the 4’ position by an electron-withdrawing group. In these materials, the presence of a rigid chiral moiety of one prevailing absolute configuration favours the establishment of a chiral conformation of one prevailing helical handedness, at least within chain segments of the macromolecules, which can be observed by circular dichroism (CD). The simultaneous presence of the azoaromatic and chiral functionalities allows the polymers to display both the properties typical of dissymmetric systems (optical activity, exciton splitting of dichroic absorptions), as well as the features typical of photochromic materials (photorefractivity, photoresponsiveness, NLO properties). The first part of this research was to synthesize analogue homopolymers and copolymers based on bisazoaromatic moiety and compare their properties to those of the above mentioned analogue derivatives bearing only one azoaromatic chromophore in the side chain. We focused also the attention on the effects induced on the thermal and chiroptical behaviours by the insertion of particulars achiral comonomers characterized by different side-chain mobility and grown hindrance (MMA, tert-BMA and TrMA). On the other hand carbazole containing polymers [2] have attracted much attention because of their unique features. The use of these materials in advanced micro- and nanotechnologies spreads in many different applications such as photoconductive and photorefractive polymers, electroluminescent devices, programmable optical interconnections, data storage, chemical photoreceptors, NLO, surface relief gratings, blue emitting materials and holographic memory. The second part of the work was focused on the synthesis and the characterization polymeric derivatives bearing in the side chain carbazole or phenylcarbazole moieties linked to the (S)- 2-hydroxy succinimide or the (S)-3-hydroxy pyrrolidinyl ring as chiral groups covalently linked to the main chain through ester bonds. The last objective of this research was to design, synthesize, and characterize multifunctional methacrylic homopolymers and copolymers bearing three distinct functional groups (i.e. azoaromatic, carbazole and chiral group of one single configuration) directly linked in the side chain. This polymeric derivatives could be of potential interest for several advanced application fields, such as optical storage, waveguides, chiroptical switches, chemical photoreceptors, NLO, surface relief gratings, photoconductive materials, etc.

De Novo Design of secondary structures: Synthesis and conformational studies

Chemists have long sought to extrapolate the power of biological catalysis and recognition to synthetic systems. These efforts have focused largely on low molecular weight catalysts and receptors; however, biological systems themselves rely almost exclusively on polymers, proteins and RNA, to perform complex chemical functions. Proteins and RNA are unique in their ability to adopt compact, well-ordered conformations, and specific folding provides precise spatial orientation of the functional groups that comprise the “active site”. These features suggest that identification of new polymer backbones with discrete and predictable folding propensities (“foldamers”) will provide a basis for design of molecular machines with unique capabilities. The foldamer approach complements current efforts to design unnatural properties into polypeptides and polynucleotides. The aim of this thesis is the synthesis and conformational studies of new classes of foldamers, using a peptidomimetic approach. Moreover their attitude to be utilized as ionophores, catalysts, and nanobiomaterials were analyzed in solution and in the solid state. This thesis is divided in thematically chapters that are reported below. It begins with a very general introduction (page 4) which is useful, but not strictly necessary, to the expert reader. It is worth mentioning that paragraph I.3 (page 22) is the starting point of this work and paragraph I.5 (page 32) isrequired to better understand the results of chapters 4 and 5. In chapter 1 (page 39) is reported the synthesis and conformational analysis of a novel class of foldamers containing (S)-β3-homophenylglycine [(S)-β3-hPhg] and D- 4-carboxy-oxazolidin-2-one (D-Oxd) residues in alternate order is reported. The experimental conformational analysis performed in solution by IR, 1HNMR, and CD spectroscopy unambiguously proved that these oligomers fold into ordered structures with increasing sequence length. Theoretical calculations employing ab initio MO theory suggest a helix with 11-membered hydrogenbonded rings as the preferred secondary structure type. The novel structures enrich the field of peptidic foldamers and might be useful in the mimicry of native peptides. In chapter 2 cyclo-(L-Ala-D-Oxd)3 and cyclo-(L-Ala-DOxd) 4 were prepared in the liquid phase with good overall yields and were utilized for bivalent ions chelation (Ca2+, Mg2+, Cu2+, Zn2+ and Hg2+); their chelation skill was analyzed with ESI-MS, CD and 1HNMR techniques and the best results were obtained with cyclo-(L-Ala-D-Oxd)3 and Mg2+ or Ca2+. Chapter 3 describes an application of oligopeptides as catalysts for aldol reactions. Paragraph 3.1 concerns the use of prolinamides as catalysts of the cross aldol addition of hydroxyacetone to aromatic aldeydes, whereas paragraphs 3.2 and 3.3 are about the catalyzed aldol addition of acetone to isatins. By means of DFT and AIM calculations, the steric and stereoelectronic effects that control the enantioselectivity in the cross-aldol addition of acetone to isatin catalysed by L-proline have been studied, also in the presence of small quantities of water. In chapter 4 is reported the synthesis and the analysis of a new fiber-like material, obtained from the selfaggregation of the dipeptide Boc-L-Phe-D-Oxd-OBn, which spontaneously forms uniform fibers consisting of parallel infinite linear chains arising from singleintermolecular N-H···O=C hydrogen bonds. This is the absolute borderline case of a parallel β-sheet structure. Longer oligomers of the same series with general formula Boc-(L-Phe-D-Oxd)n-OBn (where n = 2-5), are described in chapter 5. Their properties in solution and in the solid state were analyzed, in correlation with their attitude to form intramolecular hydrogen bond. In chapter 6 is reported the synthesis of imidazolidin-2- one-4-carboxylate and (tetrahydro)-pyrimidin-2-one-5- carboxylate, via an efficient modification of the Hofmann rearrangement. The reaction affords the desired compounds from protected asparagine or glutamine in good to high yield, using PhI(OAc)2 as source of iodine(III).

Mass transport in polymers

The study of mass transport in polymeric membranes has grown in importance due to its potential application in many processes such as separation of gases and vapors, packaging, controlled drug release. The diffusion of a low molecular weight species in a polymer is often accompanied by other phenomena like swelling, reactions, stresses, that have not been investigated in all their aspects yet. Furthermore, novel materials have been developed that include inorganic fillers, reactive functional groups or ions, that make the scenery even more complicated. The present work focused on the experimental study of systems where the diffusion is accompanied by other processes; suitable models were also developed to describe the particular circumstances in order to understand the underlying concepts and be able to design the performances of the material. The effect of solvent-induced deformation in polymeric films during sorption processes was studied since the dilation, especially in constrained membranes, can cause the development of stresses and therefore early failures of the material. The bending beam technique was used to test the effects of the dilation and the stress induced in the polymer by penetrant diffusion. A model based on the laminate theory was developed that accounts for the swelling and is able to predict the stress that raise in the material. The addition of inorganic fillers affects the transport properties of polymeric films. Mixed matrix membranes based on fluorinated, high free volume matrices show attractive performances for separation purposes but there is a need for deeper investigation of the selectivity properties towards gases and vapors. A new procedure based on the NELF model was tested on the experimental data; it allows to predict solubility of every penetrant on the basis of data for one vapor. The method has proved to be useful also for the determination of the diffusion coefficient and for an estimation of the permeability in the composite materials. Oxygen scavenging systems can overcome lack of barrier properties in common polymers that forbids their application in sensitive applications as food packaging. The final goal of obtaining a membrane almost impermeable to oxygen leads to experimental times out of reach. Hence, a simple model was developed in order to describe the transport of oxygen in a membrane with also reactive groups and analyze the experimental data collected on SBS copolymers that show attractive scavenging capacity. Furthermore, a model for predicting the oxygen barrier behavior of a film formed as a blend of OSP in a common packaging material was built, considering particles capable of reactions with oxygen embedded in a non-reactive matrix. Perfluorosulphonic acid ionomers (PFSI) are capturing attention due to a high thermal and chemical resistance coupled with very peculiar transport properties, that make them appropriate to be used in fuel cells. The possible effect of different formation procedure was studied together with the swelling due to water sorption since both water uptake and dilation can dramatically affect the fuel cells performances. The water diffusion and sorption was studied with a FTIR-ATR spectrometer that can give deeper information on the bonds between water molecules and the sulphonic hydrophilic groups and, therefore, on the microstructure of the hydrated ionomer.

Photo- and redox-active supramolecular systems containing metal complexes

The aim of this Ph.D. project has been the photophysical and photochemical characterization of new photo- and redox-active supramolecular systems. In particular we studied two different classes of compounds: metal complexes and dendrimers. Two different families of bis-cyclometalated neutral Ir(III) complexes are presented and their photophysical properties are discussed. The first family of complexes contains two 2-phenylpyridyl (ppy) or 2-(4,6-difluorophenyl)pyridyl (F2ppy) cyclometalated ligands and an ancillary ligand constituted by a phenol-oxazoline (phox), which can be substituted in the third position with a fluorine group (Fphox). In the second part of this study, we present another family of bis-cyclometalated Ir(III) complexes in which the ancillary ligand could be a chiral or an achiral bis-oxazoline (box). We report on their structural, electrochemical, photophysical, and photochemical properties. Complexes containing phox and Fphox ancillary ligands show blue luminescence with very high quantum yield, while complexes with box ligands do not show particularly interesting photophysical properties. Surprisingly these complexes give an unexpected photoreaction when irradiated with UV light in presence of dioxygen. This photoreaction originates a stable, strong blue emitting and particularly interesting photoproduct. Three successive generations of a family of polyethyleneglycol (PEG)-coated Pd(II) tetrabenzoporphyrin (PdTBP)-based dendritic nanoprobes are presented, and their ability to sensitize singlet oxygen and inflict cellular photodamage are discussed. It was found that the size of the dendrimer has practically no effect on the singlet oxygen sensitization efficiency, that approximate the unity, in spite of the strong attenuation of the triplet quenching rate with an increase in the dendrimer generation. Nevertheless, when compared against a commonly used singlet oxygen sensitizer, as Photofrin, the phosphorescent probes were found to be non-phototoxic. The lack of phototoxicity is presumably due to the inability of PEGylated probes to associate with cell surfaces and/or penetrate cellular membranes. The results suggest that protected phosphorescent probes can be safely used for oxygen measurements in biological systems in vivo. A new family of two photoswitchable (G0(Azo) and G1(Azo)) dendrimers with an azobenzene core, two cyclam units as coordination sites for metal ions, and luminescent naphthalene units at the periphery have been characterized and their coordination abilities have been studied. Because of their proximity, the various functional groups of the dendrimer may interact, so that the properties of the dendrimers are different from those exhibited by the separated functional units. Both the naphthalene fluorescence and the azobenzene photoisomerization can be observed in the dendrimer, but it has been shown that (i) the fluorescent excited state of the naphthalene units is substantially quenched by excimer and exciplex formation and by energy transfer to the azobenzene units, and (ii) in the latter case the fluorescence quenching is accompanied by the photosensitized isomerization of the trans → cis, and, with higher efficiency, the cis → trans reaction. Complexation of these dendrimers, both trans and cis isomers, with Zn(II) ions shows that complexes of 1:1 and 2:1 metal per dendrimer stoichiometry are formed showing different photophysical and photochemical properties compared to the corresponding free ligands. Practically unitary efficiency of the sensitized isomerization of trans → cis and cis → trans reaction is observed, as well as a slight increase in the naphthalene monomer emission. These results are consistent with the coordination of the cyclam amine units with Zn(II), which prevents exciplex formation. No indication of a concomitant coordination of both cyclam to a single metal ion has been obtained both for trans and cis isomer.

Sistemi adesivi con applicazioni in campo biomedicale

Polymeric adhesives have been used for many applications like suture and embolization, instead of classic surgical methods or as for dental uses. In this work both subjects have been investigated and the results separated in two parts. In the first, new dentinal adhesives with different polymerizable groups (methacrylic or vinyl-ethereal) were synthesized. A low sensitivity to hydrolysis and equal or enhanced properties, compared to existing commercial products, were considered essentials. Moreover, these monomers need to polymerize by radical photopolymerization and functional groups of different characteristics were tested. All these products were characterized by microtensile bond strength test to determine the bonding strength between the adhesive and tooth. Concerning embolization, cyanoacrylates are nowadays the most-used adhesives in surgery. Thus, they must respond to several requirements. For instance, polymerization time and adhesive strength need to be low, to avoid diffusion of the products in the body and adhesion to the catheter. In order to overcome these problems we developed new cyanoacrylates, which practically instantly polymerize upon contact with blood but do not demonstrate strong adhesion to the catheter, thank to the presence of fluorine atoms, linked to the ester chain. The synthesis of these products was carried out in several steps, such as the depolymerization of the corresponding oligomers at high temperature in acid conditions. Two types of adhesion strengths were determined. Bonding strength between human veins and a microcatheter was determined in vitro by using organic materials as the most realistic model. Another test, on two layers of skin, was conducted to verify the possible use of these new cyanoacrylates as a glue for sutures. As a conclusion, we were able to demonstrate that some of the prepared monomers posses adhesive strength and polymerization time lower than the commercial product Glubran2.

Novel Synthetic Procedures in Organocatalysis

The main aim of my PhD project was the design and the synthesis of new pyrrolidine organocatalysts. New effective ferrocenyl pyrrolidine catalysts, active in benchmark organocatalytic reactions, has been developed. The ferrocenyl moiety, in combination with simple ethyl chains, is capable of fixing the enamine conformation addressing the approach trajectory of the nucleophile in the reaction. The results obtained represent an interesting proof-of-concept, showing for the first time the remarkable effectiveness of the ferrocenyl moiety in providing enantioselectivity through conformational selection. This approach could be viably employed in the rational design of ligands for metal or organocatalysts. Other hindered secondary amines has been prepared from alkylation of acyclic chiral nitroderivatives with alcohols in a highly diastereoselective fashion, giving access to functionalized, useful organocatalytic chiral pyrrolidines. A family of new pyrrolidines bearing sterogenic centers and functional groups can be readily accessible by this methodology. The second purpose of the project was to study in deep the reactivity of stabilized carbocations in new metal-free and organocatalytic reactions. By taking advantage of the results from the kinetic studies described by Mayr, a simple and effective procedure for the direct formylation of aryltetrafluoroborate salts, has been development. The coupling of a range of aryl- and heteroaryl- trifluoroborate salts with 1,3-benzodithiolylium tetrafluoroborate, has been attempted in moderate to good yields. Finally, a simple and general methodology for the enamine-mediated enantioselective α-alkylation of α-substituted aldehydes with 1,3-benzodithiolylium tetrafluoroborate has been reported. The introduction of the benzodithiole moiety permit the installation of different functional groups due to its chameleonic behaviour.

Biochar in perennial crops: nutritional, agronomical and environmental implications

Biochar is the solid C-rich matrix obtained by pyrolysis of biomasses, currently promoted as a soil amendment with the aim to offset anthropogenic C emissions, while ameliorating soil properties and growth conditions. Benefits from biochar seem promising, although scientific understandings are beginning to be explored. In this project, I performed a suite of experiments in controlled and in field conditions with the aims to investigate the effect of biochar on: a) the interaction with minerals; b) Fe nutrition in kiwifruit; c) soil leaching, soil fertility, soil CO2 emissions partitioning, soil bacterial profile and key gene expression of soil nitrification-involved bacteria; d) plant growth, nutritional status, yield, fruit quality and e) its physical-chemical changes as affected by long-term environmental exposure. Biochar released K, P and Mg but retained Fe, Mn, Cu and Zn on its surface which in turn hindered Fe nutrition of kiwifruit trees. A redox reaction on the biochar surface exposed to a Fe source was elucidated. Biochar reduced the amount of leached NH4+-N but increased that of Hg, K, P, Mo, Se and Sn. Furthermore, biochar synergistically interacted with compost increasing soil field capacity, fertility, leaching of DOC, TDN and RSOC, suggesting a priming effect. However, in field conditions, biochar did not affect yield, nutritional status and fruit quality. Actinomadura flavalba, Saccharomonospora viridis, Thermosporomyces composti and Enterobacter spp. were peculiar of the soil amended with biochar plus compost which exhibited the highest band richness and promoted gene expression levels of Nitrosomonas spp., Nitrobacter spp. and enzymatic-related activity. Environmental exposure reduced C, K, pH and water infiltration of biochar which instead resulted in a higher O, Si, N, Na, Al, Ca, Mn and Fe at%. Oxidation occurred on the aged biochar surface, it decreased progressively with depth and induced the development of O-containing functional groups, up to 75nm depth.