STEREOSELECTIVE SYNTHESIS OF GLYCOSYL AMIDES BY TRACELESS STAUDINGER LIGATION OF GLYCOSYL AZIDES

alpha-Glycosyl amides represent a little known class of non-hydrolysable monosaccharide derivatives that may find useful application as sugar mimics and neo-glycoconjugates. Their stereoselective synthesis has proven difficult because glycosyl amines are not configurationally stable and undergo easy alpha to beta\uf020anomerization. Various methods have been proposed to get around this problem by reducing alpha glycosyl azides in the presence of acylating agents, but only a few have been reported to afford alpha-glycosyl amides, most of which require two steps and/or have been described for a limited number of substrates. Much work has been devoted to the Staudinger reduction/acylation process, which consists in the reduction of glycosyl azides with phosphines in the presence of carboxylic acids or derivatives. In this PhD thesis we have developed a stereoconservative Staudinger ligation of unprotected fluorophenyl esters to afford \u3b1- and \u3b2-N-glycosyl amino acids. The ligation method works reliably well for unprotected \u3b2-azides of the gluco, galacto and fuco series. Lower yields (ca. 50%) were obtained with a \u3b2-glucosyl-Nacetyl azide. The reaction of an \u3b1-glucosyl azide also led to major improvements compared with the use of non-fluorinated phosphines. All the N-glycosyl amino acid products can be isolated and by-products removed from the crude reaction mixtures by simple water extraction. alpha- or beta-glycofuranosyl amides can be synthesized with high stereoselectivity by traceless Staudinger ligation using a non-fluorinated phosphines, starting from unprotected beta-glycofuranosyl azide or tetra-O-acetyl-beta-glycofuranosyl azide, respectively. In particular, the resulting galactofuranosyl amides are hitherto unknown molecules, with interesting potential as inhibitors of mycobacterial growth especially for the alpha compounds

Perylenetetracarboxy-3,4:9,10-diimide derivatives with large two-photon absorption activity

Three new perylenetetracarboxy-3,4:9,10-diimides, bearing 2,6-diisopropylphenyl groups at the imide positions and 4-(R-ethynyl)phenoxy moieties (R = 4,7-di(2-thienyl)benzo[c][1,2,5]thiadiazole (P2), pyrene (P3) or pyrene-CH2OCH2 (P4)) at the four bay positions, were prepared, along with the known related derivative (R = phenyl (P1)), and well characterized. They have large two-photon absorption (TPA) cross-sections (sigma(2)), as determined by the Z-scan technique, the highest values being reached with P2 which bears a planar -delocalized donor moiety. P3 is characterized by higher sigma(2) values than both P1, as expected for the higher -conjugation of the donor pyrene moiety with respect to phenyl, and P4, due to the presence of the flexible and non-conjugated CH2OCH2 bridge between the pyrene and the ethynyl fragment in the latter compound. The molecular geometry of P1-P4 has been optimized by DFT modeling, showing that in P2 and P3 the bay substituents are stacked due to the - interactions of both pyrene and thiophene groups. The LUMO of P1-P4 lies at the same energy and is essentially delocalized on the perylene core whereas the HOMO and HOMO-1 of both P2 and P3 are degenerate and do not show contribution from the perylene core contrarily to that of P1 and P4. The HOMO-LUMO gap is therefore essentially influenced by the HOMO which reflects the electronic charge delocalization on the bay substituents, the lower gaps being observed for P2 and P3, which are characterized by the best TPA properties

Platinum(II) complexes with cyclometallated 5-pi-delocalized-donor-1,3-di(2-pyridyl)benzene ligands as efficient phosphors for NIR-OLEDs

Two new pincer proligands, namely 5-(p-(N,N-diphenylamino)phenylethynyl)-1,3-di(2-pyridyl)benzene (HL1) and trans-5-(p-(N,N-diphenylamino)styryl-1,3-di(2-pyridyl)benzene (HL2) were prepared together with their N^C^N-coordinated cyclometallated platinum(II) complexes PtL1X (X = Cl, NCS) and PtL2Cl. Both ligands are intensely luminescent in solution (quantum yields > 0.8). PtL1X complexes display high quantum yields in solution whereas that of PtL2Cl is very low due to the ease with which trans to cis isomerisation of the diphenylaminostyryl C[double bond, length as m-dash]C bond occurs. Distinct sets of emission bands attributable to the cis and trans forms are observable in glass at 77 K, the assignments being supported by TD-DFT calculations. Organic light-emitting diodes (OLEDs) have been prepared using the new compounds as phosphorescent emitters. Remarkably, despite the inferior quantum yield of PtL2Cl in solution, the best electroluminescence quantum efficiencies are obtained with this complex, which emerges as an excellent candidate for the preparation of NIR-OLEDs

Unexpectedly high second-order nonlinear optical properties of simple Ru and Pt alkynyl complexes as an analytical springboard for NLO-active polymer films

The unexpectedly high quadratic hyperpolarizability values of simple Ru and Pt alkynyl complexes have been measured by the EFISH technique, and this prompted us to investigate their potential as molecular building blocks for composite films with second harmonic generation properties

An acido-triggered reversible luminescent and nonlinear optical switch based on a substituted styrylpyridine : EFISH measurements as an unusual method to reveal a protonation-deprotonation NLO contrast

Diphenyl-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]-vinyl}-phenyl)-amine (DPVPA) constitutes a novel acido-triggered reversible luminescent and nonlinear optical switch. Remarkably, for the first time the Electric-Field Induced Second Harmonic generation (EFISH) technique is used to reveal a protonation-deprotonation NLO contrast

Highly efficient acido-triggered reversible luminescent and nonlinear optical switch based on 5-π-delocalized-donor-1,3-di(2-pyridyl)benzenes

The acidochromic behaviour of trans-5-(p-(N,N-diphenylamino)styryl)-1,3-di(2-pyridyl)benzene (1) and 5-(p-(N,N-diphenylamino)phenylethynyl)-1,3-di(2-pyridyl) benzene (2) was investigated, both in solution and doped into films, studying their linear and nonlinear optical properties. Remarkably, the efficient luminescence of 1 drops drastically upon protonation and is restored upon deprotonation - both in solution and in the solid state - leading to an excellent on/off emissive acido-triggered reversible switch. In parallel, protonation leads to a large enhancement of the second-order NLO response, as determined by the EFISH technique, which is reversible upon deprotonation. The related methylated salts of 1 and 2 are similarly characterized by a high NLO response

Novel terthiophene-substituted fullerene derivatives as easily accessible acceptor molecules for bulk-heterojunction polymer solar cells

Five fulleropyrrolidines and methanofullerenes, bearing one or two terthiophene moieties, have been prepared in a convenient way and well characterized. These novel fullerene derivatives are characterized by good solubility and by better harvesting of the solar radiation with respect to traditional PCBM. In addition, they have a relatively high LUMO level and a low band gap that can be easily tuned by an adequate design of the link between the fullerene and the terthiophene. Preliminary results show that they are potential acceptors for the creation of efficient bulk-heterojunction solar cells based on donor polymers containing thiophene units

Neutral N^C^N terdentate luminescent Pt(ii) complexes: their synthesis, photophysical properties, and bio-imaging applications

An emerging field regarding N^C^N terdentate Pt(II) complexes is their application as luminescent labels for bio-imaging. In fact, phosphorescent Pt complexes possess many advantages such as a wide emission color tunability, a better stability towards photo- and chemical degradation, a very large Stokes shift, and long-lived luminescent excited states with lifetimes typically two to three orders of magnitude longer than those of classic organic fluorophores. Here, we describe the synthesis and photophysical characterization of three new neutral N^C^N terdentate cyclometallated Pt complexes as long-lived bio-imaging probes. The novel molecular probes bear hydrophilic (oligo-)ethyleneglycol chains of various lengths to increase their water solubility and bio-compatibility and to impart amphiphilic nature to the molecules. The complexes are characterized by a high cell permeability and a low cytotoxicity, with an internalization kinetics that depends on both the length of the ethyleneglycol chain and the ancillary ligand

Tuning the optical emission of MoS2 nanosheets using proximal photoswitchable azobenzene molecules

We report photoluminescence measurements performed on monolayer- and two-layer-MoS2 placed on two types of mixed self-assembled monolayers (mSAMs) of photoswitchable azobenzene molecules. The two mSAMs differ via the electronegative character of the azobenzene derivatives. Thin layers of a transition metal dichalcogenide - MoS2 - were mechanically exfoliated on mSAM to allow for direct interaction between the molecules and the MoS2 layers. When the MoS2 nanosheet is in contact with the electropositive azobenzene molecules in trans configuration, an emission side band at lower energies and at low excitation powers suggest n-type doping. The photoisomerization of the molecules from trans to cis configuration lowers the doping, quenching the side band and enhancing the overall PL efficiency by a factor of 3c3. Opposite results were observed with the chlorinated, more electronegative molecules, exhibiting a reversed trend in the PL efficiency between trans and cis, but with an overall larger intensity. The type of doping induced by the two types of mSAMs was determined by Kelvin probe force microscopy technique