Véronique Wintgens

Spectroscopic properties of aromatic dicarboximides. Part1.—N—H and N-methyl-substituted naphthalimides

The photophysical properties of the N—H and N-methyl derivatives of 1,2-, 2,3- and 1,8-naphthalimides have been studied. The shift of the fluorescence emission position as a function of the solvent polarity indicates only a weak variation of dipole moment for the excited state compared with the corresponding value in the ground state (5.7 D for 2b, 2.8 D for 3b and <2 D for 4b, 1 D ≈ 3.335 64 × 10–30 C m, and 2b, 3b and 4b are N-methyl-1,2- naphthalimide, N-methyl-2,3-napthalimide and N-methyl-1,8-naphthalimide). However, important modifications of the photophysical properties are observed which depend on the relative position of the dicarboximide moiety on the naphthalene ring: the intersystem crossing rate constant of 4b increases dramatically by three orders of magnitude compared with that of 2b; simultaneously, the fluorescence quantum yield decreases from 0.77 to 0.03, although the corresponding rate constant, kf, increases. This difference is found to arise from the energy gap between the lowest1(π, π*) singlet excited state and the upper 3(n,π*) triplet state, which is of the order of 9 kcal mol–1 for 2b and less than 2 kcal mol–1 for 4b in acetonitrile solution. Protic solvents increase the energy difference between the n,π* and π,π* states thus decreasing the mixing of the two levels; as a consequence, the lifetime of 4b is increased, i.e. from <60 ps in hexane to 2.1 ns in trifluoroethanol. A triplet–triplet annihilation process occurs with the N-methyl derivatives 3b and 4b which leads to a monomer delayed fluorescence with the former, and mainly to a delayed excimer emission with the latter.

A new fluoroionophore derived from 4-amino-N-methyl-1,8-naphthalimide

A new azacrown ether derived from 1,8-naphthalimide was prepared by a two steps reaction from 4-bromo-1,8-naphthalic anhydride. The spectroscopic properties of this fluoroionophore are described both in the absence and in the presence of cations; the compound shows a strong affinity with Ca2+ and Ba2+.

Aqueous Polysaccharide Associations Mediated by β-Cyclodextrin Polymers

Macromolecular assemblies were elaborated by mixing in water hydrophobically modified dextrans (MDC(n)) and beta-cyclodextrin polymers (pbetaCD) interacting by inclusion complexation between the hydrophobic moieties of MDCn and the beta-cyclodextrin cavities of pbetaCD. Dextrans have been modified by grafting alkyl groups (C(n)) of varying chain lengths (n = 8-16) and grafting ratio (3-6 mol%). Different pbetaCD polymers were synthesized by polycondensation of beta-cyclodextrin and epichlorohydrin. The polymer-polymer interactions have been studied by fluorimetry, isothermal titration microcalorimetry, phase diagrams, and viscosimetry. The viscoelastic properties of the temporary networks (in the semidilute range) have been studied by rheology. The interaction mechanisms between the MDCn and pbetaCD can be understood taking into account the strength of the interaction between the alkyl group and the beta-cyclodextrin cavity (mainly controlled by the alkyl chain length), the density of junctions between the chains (depending on the alkyl grafting density and the pbetaCD molecular weight), and additional cooperative effect (arising for high alkyl grafting density).

Photoinduced spontaneous and stimulated emission in sexithiophene single crystals

Abstract Temperature dependent steady-state and transient photoluminescence (PL) measurements were carried out on sexithiophene (6T) single crystals. As the temperature is lowered, three different sets of equally spaced peaks appear on the steady state PL spectrum. This behavior is interpreted in the frame of the molecular exciton theory. The first energy set is attributed to transitions to the lower level of the Davydov splitting of the 1B u excited state of the isolated molecule, whereas the two other sets are ascribed to the crystal defects. An analysis of the PL emission and excitation spectra allowed us to establish a general exciton energy scheme of sexithiophene crystal. At low excitation levels, the transient PL follows a single exponential decay, with a decay time of 1.8 ± 01 ns. When the excitation energy is increased, a second, much faster component is added. Concurrently, the PL spectrum narrows to a single line centered at the origin of the low energy set of the steady state PL. The width of the emission line is 13 cm −1 at 10 K, and tends to widen and shift towards lower energies when the temperature is raised. All these observations are accounted for in term of stimulated emission.

THE FOUR-LEVEL STIMULATED EMISSION IN SEXITHIOPHENE SINGLE CRYSTALS

Single crystal of conjugated sexithiophene oligomer shows stimulated emission when excited with a low-energy photonic pulse. This phenomenon is interpreted in terms of the excitonic energy diagram of the crystal, which presents a four-level pathway for the photoexcitation and emission, similar to the one observed in classical inorganic-based laser materials.

Associative network based on cyclodextrin polymer: a model system for drug delivery.

Associative networks have been elaborated by mixing in aqueous media a cyclodextrin polymer to a dextran bearing adamantyl groups. The two polymers interact mainly via inclusion complexes between adamantyl groups and cyclodextrin cavities, as evidenced by the high complexation constants determined by isothermal titration microcalorimetry (approximately 10(4) L mol(-1)). Additional interaction mechanisms participating in the strength of the network, mainly hydrogen bonding and electrostatic interactions, are sensitive to the pH and ionic strength of the medium, as shown by pH-dependent rheological properties. The loading and release of an apolar model drug, benzophenone, has been studied at two pH values and different cyclodextrin polymer content. Slow releases have been obtained (10-12 days) with slower kinetics at pH 2 than at pH 7. Analysis of the experiments at pH 7 shows that drug release is controlled both by diffusion in the network and by inclusion complex interactions with cyclodextrin cavities.

Development and Characterization of New Cyclodextrin Polymer-Based DNA Delivery Systems

In this study, we investigated whether a cyclodextrin polymer (polybetaCD) complexed with cationic adamantyl derivatives (Ada) could be used as a vector for gene delivery. DNA compaction as a function of adamantyl/DNA phosphate ratio (A/P) by this new class of vector was demonstrated using surface enhanced Raman spectroscopy, zeta potential measurements, and DNA retardation assays. Transfection data highlight the relationship between in vitro gene delivery efficiency and the combination of several physical properties of the polybetaCD/Ada/DNA polyplexes, including cationic polar headgroup valency and chemical structure of the spacer arm of Ada connectors, the adamantyl/DNA phosphate ratio (A/P) of the polybetaCD/Ada/DNA polyplexes, and the ionic strength of the medium. Finally, when associating the best formulation with a fusogenic peptide, we reached transfection levels which were of the same order as those obtained with DOTAP.

Transient electroluminescence of monolayer and bilayer sexithiophene diodes

Abstract The transient electroluminescence of monolayer and bilayer sexithiophene-based diodes has been measured. The delay time of the luminescence onset of the monolayer diode corresponds to a hole mobility of 5 × 10−6 cm2 V−1 s−1 which is considerably lower than that obtained by field-effect measurements. This is interpreted in terms of strong transient trapping. The bilayer diode presents a twofold time-resolved response which is attributed to the different mobility of its constituent layers.

Spectroscopic properties of aromatic dicarboximides part 3: Substituent effect on the photophysical properties of N-phenyl-2,3-napthalimides

Abstract In the present article we describe the effect of the substitution on the photolysis properties of a series of N -phenyl-2,3-napthalimides. It is found that the decrease in the electron-donating character of the substituent on the N -phenyl ring changes the fluorescence emission from a weak, broad and short-lived long-wavelength (LW) emission into a structured and long-lived fluorescence localized at the short wavelengths (SW) which behaves like that of the unsubstituted 2,3-napthalimide. Both SW and LW emissions can be observed in some cases. The different results can be explained on the basis of a three-level scheme where the vibrationally relaxed Franck-Condon state populates two different excited states, the one (SW) emitting at short wavelengths, and the other (LW) emitting at long wavelengths. The substitution on the phenyl ring influences mainly the energy of the LW excited state. The nature of the substituent on the N -phenyl ring has a determining influence also on the internal conversion process by virtue of the solvent- and rotation-induced pseudo-Jahn-Teller coupling of the two excited states.

Laser effect of a series of variously substituted pyrylium and thiopyrylium salts

Abstract The laser effect of 47 pyrylium salts and 6 thiopyrylium salts has been investigated by excitation with a pulsed (5 Hz) nitrogen laser (337 nm) and compared to that of rhodamine 6G. The thiopyrylium derivatives as well as the pyrylium salts which contain nitro substituents show a rather poor effect if any. Out of the pyrylium series, 19 compounds have an important response as laser dyes. For most of them, the lasing range is larger than that of rhodamine 6G and for several of them (19, 20, 21, 32, 35, 39, 47 and 46) this range covers more than 80 nm. The behaviour of 35 and 47 is particularly striking.