The transient effect in fluorescence quenching observed in a viscous nonpolar solvent, liquid paraffin, was measured and analyzed in order to study the mechanism of electron transfer fluorescence quenching in such solvents. The method of analysis is similar to that adopted in previous papers (J. Phys. Chem. 1995, 99, 5354; 1996, 100, 4064) to study electron transfer in polar solvents. The difference from the previous method is that a function of the form A exp[− b(r − r 0)] was used as the electron transfer rate constant k(r) instead of the Marcus equation and that the dependence of the steady-state fluorescence intensity on the quencher concentration was also analyzed in addition to the fluorescence decay at high quencher concentrations (≥50 mM). The parameters A and b and the diffusion coefficient D were determined from these analyses. The distribution of quenching distance was calculated using these parameter values. The distribution shows that the quenching occurs at distances longer than the contact distances of the fluorescer and quencher molecules. It was concluded that, even in nonpolar liquid paraffin, quenching occurs by long-range electron transfer rather than by exciplex formation as long as the ΔG of electron transfer is negative and far from 0 eV. The relation between electron transfer and exciplex formation in nonpolar solvents was discussed.

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Quenching of singlet oxygen luminescence at 1.27 µm by PhS • , PhSO • , and peroxyl radicals PhOO • , t-BuOO • , PhCH 2 OO • , Ph 2 CHOO • , and Ph 3 COO • was studied in liquid solution. The quantum yields of decomposition of different initiators which lead to the formation of free radicals were measured by using nanosecond transient absorption. This allowed determination of singlet oxygen O 2 ( 1 ∆ g ) quenching rate constants by the radicals. They are <2 × 10 8 M -1 s -1 for the sulfur-centered radicals and (2-7) × 10 9 M -1 s -1 for peroxyl radicals in acetonitrile. The rapid quenching is attributed to energy transfer quenching by the peroxyls, which have an n f π* transition leading to a low-lying 2 A′ state above their 2 A′′ ground state. PhSO • is shown computationally not to have such a low-lying 2 A′ state. There may be a very low-lying 2 B 1 state, for PhS • , but it is apparently not an efficient acceptor of electronic energy from O 2 ( 1 ∆ g ).

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International audienceLipo-phosphoramidates constitute an interesting class of synthetic vectors for gene delivery. A structure-activity relationship study points out that the incorporation of the linoleyl polyunsaturated lipid chains produces efficient vectors for gene delivery in the lung. In addition, neutral lipo-phosphoramidates possessing an imidazole motif in the polar head region have been successfully employed in association with a cationic lipophosphoramide to improve in vitro transfection efficiencies. Our recent achievements on this topic are briefly summarized here