Highly Active Chromium Complexes Supported by Constrained Schiff-Base Ligands for Cycloaddition of Carbon Dioxide to Epoxides.

Novel constrained Schiff-base ligands (inden) were developed based on the well-known salen ligands. Chromium complexes supported by the constrained inden ligands were successfully synthesized and used as catalysts for the synthesis of cyclic carbonates from epoxides and carbon dioxide (CO2). The catalyst having tert-butyl (tBu) groups as substituents in combination with tetrabutylammonium bromide (TBAB) as a cocatalyst exhibited very high catalytic activity with a turnover frequency of up to 14800 h-1 for the conversion of CO2 and propylene oxide into propylene carbonate exclusively at 100 °C and 300 psi of CO2 under solvent-free conditions. The catalyst was found to be highly active for various epoxide substrates to produce terminal cyclic carbonates in 100% selectivity.

New water soluble terphenylene diethynylene fluorophores.

Inspired by our earlier works on sensors from dendritic phenyleneethynylenes, two new star-shaped water-soluble fluorophores containing terphenylene diethynylene units and anionic carboxylate peripheries are successfully synthesized. The convergent synthesis relies on Sonogashira cross-coupling reactions between tris-(4-ethynylphenyl)amine and the iodophenyleneethynylene branches. All of the compounds are characterized by (1)H, (13)C NMR, and mass spectrometry. In aqueous solution, the less polar fluorophore 1 shows lower quantum yield than 2 (18 vs 33%) as a result of hydrophobic induced aggregation. One of these anionic water-soluble fluorophores exhibits a selective fluorescence quenching by Fe(3+) ion in phosphate buffer pH 8.

A highly selective turn-on fluorescent sensor for glucosamine from amidoquinoline-napthalimide dyads.

Three amidoquinoline-naphthalimide dyads are designed and synthesized in 67-73% overall yields in 3 steps from commercially available starting materials. Compounds with unsubstituted and nitro naphthalimide (1 and 2) show excellent selective fluorescent responses towards glucosamine with the enhancement of fluorescence quantum yields by 14 folds. The determination of HOMO-LUMO levels by linear sweep voltammetry suggests that the sensing mechanism likely involves the inhibition of photo-induced electron transfer (PET) between the aminoquinoline and naphthalimide moieties by glucosamine. The association constants of 1.55×10(4) and 1.45×10(4)M(-)(1), along with the glucosamine detection limits of 1.06 and 0.29µM are determined for 1 and 2, respectively. The application of 2 as a fluorescent probe for real-time detection of cellular glucosamine at micromolar level in living Caco-2 cells is also demonstrated.

FRET detection of DNA sequence via electrostatic interaction of polycationic phenyleneethynylene dendrimer with DNA/PNA hybrid.

Interaction between a polycationic compound and DNA is a useful phenomenon for development of a new DNA sensing system. In this work, dendritic polycationic phenyleneethynylene fluorophores are investigated as a Förster resonance energy transfer (FRET) donor for the detection of DNA hybridization in conjunction with a fluorescein-labeled pyrrolidinyl peptide nucleic acid (Fl-acpcPNA) probe. The first generation dendrimer is an efficient energy donor for the fluorescein acceptor but also shows non-specific FRET signal with Fl-acpcPNA. The addition of N-methyl 2-pyrrolidone can virtually completely remove the non-specific interaction between Fl-acpcPNA and the dendrimer. Under the optimal condition, the complementary DNA gives a distinctively high FRET ratio (1.42) comparing with those of the non-complementary (0.26) and singly mismatched (0.51) DNAs. The FRET ratio responses linearly with the DNA concentration with the detection limit lower than 1nM. The FRET ratio is even higher for the complementary target DNAs with extra hanging nucleotide sequences, which is a more frequently encountered scenario in real applications.