Synthetic strategies, photophysical and first biological applications of new glycoamino OPEs

Oligo(phenyleneethynylenes) (OPEs) and the corresponding polymers, Poly(phenyleneethynylenes) (PPEs), are organic molecules with π‐conjugated backbone, which could be designed to display high electrical conductivity, outstanding photophysical properties, and excellent biocompatibility. While they have been extensively explored for a wide range of applications, from electronics and optoelectronic devices, to energy harvesting and nanotechnology, they attracted attention in the biomedical field only in the past decades. Specifically, they have found innovative applications in a variety of biotechnologies, as biosensors, probes for cell imaging, biocides and many others. In the last years, my research group has widely studied the photophysical and biological properties of a new class of end-only glucose functionalized oligo(phenyleneethynylenes): thanks to the modulation of substituents and length of the central conjugated chain, some of the obtained OPEs have demonstrated to be promising dyes in bioimaging and superficial Photodynamic Therapy (PDT). On the basis of these assumptions, my PhD work consisted in the synthesis of differently modified glycosilated OPEs with the aim to better modulate their photophysical features and biological behaviour, studying new possible applications in the biomedical field. Figure A1 In particular, Chapter 1 consists in a literature survey and gives an overview on general principles and previous works upon which this research draws. In paragraph 1.1, carbohydrates conjugation, as an efficient strategy for delivering and targeting drugs, is discussed. Paragraph 1.2 deals with the main applications of luminescent dyes in the biomedical field: at first, in 1.2.A, a general view on the principles of bioimaging and the design of suitable fluorescent probes is reported; in 1.2.B, the mechanisms that are the basis of photodynamic therapy (PDT) and a brief literature overview on different classes of dyes and nanoparticles, which have found application as photosensitizers in this type of treatments, are described; 1.2.C deals with drugs interactions with nucleic acids, with particular attention to the principal binding modes and to the possible uses of luminescent dyes for DNA labelling; finally, in 1.2.D, applications of specific kinds of fluorescent dyes, whose luminescence is pH dependent, as intracellular pH indicators are reported. To conclude, paragraph 1.3 provides an overall picture of Oligo(phenyleneethynylenes) (OPEs) features and biomedical applications, mainly focusing on the previous works of my research group. Chapter 2 deals with the discussion of the synthetic pathways used for the obtainment of the new glycosylated OPE systems and the structural modifications introduced during the PhD work. In fact, in paragraph 2.1 the synthetic approach and the optimization of yields and methods for the obtainment of the already reported OPE_Glucose 1 and OPE_Glucose 2 (Figure A1), which had shown the best results in terms of cell internalization and applications as photosensitizers in PDT, are described. In paragraph 2.2, the synthetic routes which led to the elongation of the OPE chain are reported: the extension of the π-conjugated system is meant to further study the influence of chain length on the photophysical behaviour. The synthesis of three new amino-OPE systems, bearing different sugar terminations (monosaccharide galactose and mannose, and disaccharide maltose) are described in paragraph 2.3: the presence of different carbohydrate residues could, in fact, influence the biological behaviour of the new compounds, with respect to the glucosylated systems. Furthermore, the synthesis of several tetralkylammonium OPEs is the subject of paragraph 2.4: the disappearance of the lone pair on the amino nitrogen and the formation of a positive charge on the OPE chain could, in fact, strongly influence the photophysical OPE features but also the water solubility, opening the way to other biological applications. Finally, the last modifications reported in paragraph 2.5 are focused on the desymmetrization of the OPE chain. In part 2.5A the introduction of electron withdrawing fluorinated aromatic groups as chain termination is discussed: the new moiety, which is able to interact with the methoxy and amino electron donating groups already present as substituents on the OPE conjugated chain, could influence the photophysical features, leading to the obtainment of new push-pull systems. In part 2.5B the synthesis of OPEs, bearing suitable termination for the functionalization of upconverting nanoparticles (UCNPs), is described. These nanoparticles, after excitation with NIR light (980 nm), emit in a region of UV that matches the absorption of our OPEs (~380 nm): an energy transfer process from the NPs to our conjugated system could extend their applications in NIR-PDT. Chapter 3 deals with the preliminary photophysical and biological studies performed on some selected compounds. In particular, in paragraph 3.1 the effects of the different modification on our OPE systems (chain elongation, nitrogen quaternarization and desymmetrization) on the spectroscopic features are discussed, in comparison with the already reported OPE_Glucose 1 and OPE_Glucose 2. In fact, it has been demonstrated that each structural change in the OPE chain causes different variations in the photophysical properties; moreover, preliminary studies on the fluorinated compounds show that the emission of one of these derivatives is pH-sensitive. On the other hand, the first biological tests, carried out on some of the synthesized compounds, are described in paragraph 3.2. It has been found that the two positive charged tetralkylammonium OPEs, analogues of OPE_Glucose 1 and OPE_Glucose 2, strongly bind the double helix of DNA: this interaction is also responsible for their absorption and emission switching-off and reduction of cancer cells proliferation. Moreover, the first internalization and PDT tests on healthy and cancer cells performed with the synthesized compound bearing two maltosidic residues are also reported. Finally, in Chapter 4 experimental synthetic methods, spectral and analytical characterizations of the obtained compounds, equipment, materials and methods for the photophysical and biological experiments are reported.