Raymond Ziessel

BODIPY-bridged push-pull chromophores for nonlinear optical applications.

A set of linear and dissymmetric BODIPY-bridged push-pull dyes are synthesized. The electron-donating substituents are anisole and dialkylanilino groups. The strongly electron-accepting moiety, a 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) group, is obtained by insertion of an electron-rich ethyne into tetracyanoethylene. A nonlinear push-pull system is developed with a donor at the 5-position of the BODIPY core and the acceptor at the 2-position. All dyes are fully characterized and their electrochemical, linear and nonlinear optical properties are discussed. The linear optical properties of dialkylamino compounds show strong solvatochromic behavior and undergo drastic changes upon protonation. The strong push-pull systems are non-fluorescent and the TCBD-BODIPY dyes show diverse photochemistry and electrochemistry, with several reversible reduction waves for the tetracyanobutadiene moiety. The hyperpolarizability μβ of selected compounds is evaluated using the electric-field-induced second-harmonic generation technique. Two of the TCBD-BODIPY dyes show particularly high μβ (1.907 μm) values of 2050 × 10(-48) and 5900 × 10(-48) esu. In addition, one of these dyes shows a high NLO contrast upon protonation-deprotonation of the donor residue.

The first comparative study of the ability of different hydrophilic groups to water-solubilise fluorescent BODIPY dyes

A green-emitting BODIPY dye carrying a carboxylic acid at the meso-phenyl ring was derivatised with a set of eight different hydrophilic groups neutral, negatively- or positively-charged at physiological pH. The determination of both partition coefficient (log P) and fluorescence quantum yield (ΦF) for each polar BODIPY dye allows building the first water-solubility scale for such fluorescent organic materials. This valuable tool can be used to select the most suitable molecular candidate for converting a hydrophobic BODIPY scaffold into a water-soluble and brightly fluorescent dye, and for tuning its net electric charge/polarity according to the biolabelling application targeted for this specific fluorescent marker.

Synthesis of luminescent BPh2-coordinated 2-(2′-hydroxyphenyl)benzoxazole (HBO)

Triphenylborane (BPh3) has been successfully coordinated to five π-conjugated aromatic systems containing a N∧O bidentate 2-(2′-hydroxyphenyl)benzoxazole (HBO) ligand. Complexes 6–8 are directly substituted on the phenolic side of the HBO core while the structure of derivatives 9–10 includes a 4-dibutylaminophenyl module linked through an ethynyl fragment, at position 4 or 5. The crystal structure of complexes 8 and 10 reveals different solid-state molecular packing depending on the substitution, a herringbone molecular packing being observed for complex 8 while the dibutylamino fragment present in complex 10 is in favour of a lamellar structure. The optical properties are highly dependent on the nature and the position of substituents. Solvatochromic charge-transfer emissions are observed for substitution at position 3 or 5 while singlet emission is favoured when position 4 is functionalized. Solid-state fluorescence reveals that complex 8 possesses red-shifted emission when dispersed in a KBr matrix.

Substituent and Solvent Effects on the Excited State Deactivation Channels in Anils and Boranils

Differently substituted anils (Schiff bases) and their boranil counterparts lacking the proton-transfer functionality have been studied using stationary and femtosecond time-resolved absorption, fluorescence, and IR techniques, combined with quantum mechanical modelling. Dual fluorescence observed in anils was attributed to excited state intramolecular proton transfer. The rate of this process varies upon changing solvent polarity. In the nitro-substituted anil, proton translocation is accompanied by intramolecular electron transfer coupled with twisting of the nitrophenyl group. The same type of structure is responsible for the emission of the corresponding boranil. A general model was proposed to explain different photophysical responses to different substitution patterns in anils and boranils. It is based on the analysis of changes in the lengths of CN and CC bonds linking the phenyl moieties. The model allows predicting the contributions of different channels that involve torsional dynamics to excited state depopulation.

Synthesis of Water-Soluble Red-Emitting Thienyl-BODIPYs and Bovine Serum Albumin Labeling

The synthesis of three red-emitting and water-soluble thienyl-BODIPYs has been achieved. The trimethyl(propargyl)ammonium group was chosen as a vector for water solubility. One or two cationic arms were introduced either on the 2-position of the thienyl unit or on the 4-position on the boron atom. These dyes have pronounced absorption around 600 nm and intense emission at 650 nm with quantum yield of about 60% in water. Grafting of such BODIPYs via a flexible arm to BSA is very efficient, allowing attachment of 1 to 30 labels in a controlled manner. Very strong fluorescence (quantum yield 56%)without aggregation of the dye at a low loading ratio (1:5 BSA/label) in PBS buffer is measured.

Photoinduced Proton Transfer Promoted by Peripheral Subunits for Some Hantzsch Esters

It is noted that, for a small series of 3,5-diacetyl-1,4-dihydrolutidine (DDL) derivatives and the corresponding Hantzsch esters, the presence of methyl groups at the 2,6-positions serves to extinguish fluorescence in solution but not in the solid state. Emission is weakly activated and affected by changes in solvent polarity. The latter situation arises because the optical transition involves intramolecular charge transfer. Calculations, both semiempirical and DFT, indicate that, in all cases, rotation of the carbonyl function is facile and that the dihydropyridine ring is planar. These calculations also indicate that the 2,6-methyl groups do not affect the generic structure of the molecule. It is proposed that illumination increases the molecular dipole moment and pushes electron density toward the carbonyl oxygen atom. Proton transfer can now occur from one of the methyl groups, leading to formation of a relatively low-energy, neutral intermediate, followed by a second proton transfer step that forms the enol. Reaction profiles computed for the ground-state species indicate that this route is highly favored relative to hydrogen transfer from the 4-position. The barriers for light-induced proton transfer are greatly reduced relative to the ground-state process but such large-scale structural transformations are hindered in the solid state. A rigid analogue that cannot form an enol is highly emissive in solution, supporting the conclusion that proton transfer is in competition to fluorescence in solution.