Acid-Sensitive BODIPY Dyes: Synthesis through Pd-Catalyzed Direct C(sp3 )-H Arylation and Photophysics.

A novel palladium-catalyzed direct C(sp3 )-H arylation of the methyl group at the 8-position of BODIPY by bromoarenes was established. A deprotonative cross-coupling process was supposed to be involved in the reaction. This approach allowed us to attach electron-donating/withdrawing, halogen substituted aryls and a heteroaryl with a yield running from 55 to 99 %. Novel pH sensors, which in the absence of acid showed the occurrence of photoinduced electron transfer, were synthesized by attaching dimethylaniline to the methyl at the C8-position of BODIPY. The reference compounds with dimethylaniline directly attached to the C8-position were also synthesized and besides photoinduced electron transfer also showed a charge-transfer emission. Their photophysical properties were investigated by steady-state fluorescence, time-correlated single-photon counting and femtosecond fluorescence up-conversion. Time-dependent density functional (TD-DFT) electronic-structure calculations on the properties of the excited states corresponding to local excitation of the BODIPY core and to charge transfer were conducted. Upon addition of trifluoroacetic acid in toluene and ethanol, the partial fluorescence intensity recovery was at least an order of magnitude more efficient with the newly synthesized sensors compared to the traditional reference sensors. The improved sensitivity of these novel BODIPY-based pH sensors was attributed to less efficient proton-coupled electron transfer of the protonated species.

A Blue-Light-Emitting BODIPY Probe for Lipid Membranes.

Here we describe a new BODIPY-based membrane probe (1) that provides an alternative to dialkylcarbocyanine dyes, such as DiI-C18, that can be excited in the blue spectral region. Compound 1 has unbranched octadecyl chains at the 3,5-positions and a meso-amino function. In organic solvents, the absorption and emission maxima of 1 are determined mainly by solvent acidity and dipolarity. The fluorescence quantum yield is high and reaches 0.93 in 2-propanol. The fluorescence decays are well fitted with a single-exponential in pure solvents and in small and giant unilamellar vesicles (GUV) with a lifetime of ca. 4 ns. Probe 1 partitions in the same lipid phase as DiI-C18(5) for lipid mixtures containing sphingomyelin and for binary mixtures of dipalmitoylphosphatidylcholine (DPPC) and dioleoylphosphatidylcholine (DOPC). The lipid phase has no effect on the fluorescence lifetime but influences the fluorescence anisotropy. The translational diffusion coefficients of 1 in GUVs and OLN-93 cells are of the same order as those reported for DiI-C18. The directions of the absorption and emission transition dipole moments of 1 are calculated to be parallel. This is reflected in the high steady-state fluorescence anisotropy of 1 in high ordered lipid phases. Molecular dynamic simulations of 1 in a model of the DOPC bilayer indicate that the average angle of the transition moments with respect to membrane normal is ca. 70°, which is comparable with the value reported for DiI-C18.

Radical C-H alkylation of BODIPY dyes using potassium trifluoroborates or boronic acids.

A one-step synthetic procedure for the radical CH alkylation of BODIPY dyes has been developed. This new reaction generates alkyl radicals through the oxidation of boronic acids or potassium trifluoroborates and allows the synthesis of mono-, di-, tri-, and tetraalkylated fluorophores in a good to excellent yield for a broad range of organoboron compounds. Using this protocol, multiple bulky alkyl groups can be introduced onto the BODIPY core thus creating solid-state emissive BODIPY dyes.

Thiocyanation of BODIPY dyes and their conversion to thioalkylated derivatives.

A high-yielding method for the direct thiocyanation of BODIPY dyes is described. In 1,3-dimethyl BODIPYs, the thiocyanato group adds at position 2, whereas the insertion occurs at position 5 in 3-amino BODIPYs. The transformation of the thiocyanato group enables the synthesis of thioalkylated BODIPYs. 2-Thioalkylated BODIPYs and 3-thiocyanato-5-piperidino BODIPYs exhibit interesting spectroscopical features. Hence, the described synthetic methodology can be used for the photophysical tuning of BODIPY dyes.

Radical C-H arylation of the BODIPY core with aryldiazonium salts: synthesis of highly fluorescent red-shifted dyes.

We describe herein the first radical C-H arylation of BODIPY dyes. This novel, general, one-step synthetic procedure uses ferrocene to generate aryl radical species from aryldiazonium salts and allows the straightforward synthesis of brightly fluorescent (Φ>0.85) 3,5-diarylated and 3-monoarylated boron dipyrrins in up to 86 % yield for a broad range of aryl substituents. In this way, new and complex dyes with red-shifted spectra can be easily prepared.