BODIPY, an abbreviation for boron-dipyrromethene, is a family Organoboron compounds of interest as fluorescent dyes. BODIPY is composed of dipyrromethene complexed with a disubstituted boron center, typically BF2. The IUPAC name for the BODIPY core is 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene. Reflecting its instability, the unsubstituted BODIPY dye had not been prepared until 2009.
Properties
BODIPY dyes have been well investigated. Most BODIPY dyes have small Stokes shift and are relatively chemically inert. Fluorescence is quenched in a solution, which limits application. This problem has been handled by synthesizing asymmetric boron complexes and replacing the fluorine groups with phenyl groups. BODIPY dyes are notable for their uniquely small Stokes shift, high, environment-independent fluorescence quantum yields, often approaching 100% even in water, sharp excitation and emission peaks contributing to overall brightness, and high solubility in many organic solvents. The combination of these qualities makes BODIPY fluorophores promising for imaging applications. The position of the absorption and emission bands remain almost unchanged in solvents of different polarity as the dipole moment and transition dipole are mutually orthogonal.
Synthesis
BODIPY are prepared by treating a dipyrromethene precursor with boron trifluoride etherate in the presence of a tertiary amine. Dipyrromethenes are accessed from a suitable pyrrole by several methods. Normally, one alpha-position in employed pyrroles is substituted and the other is free. Condensation of such pyrrole, often available from Knorr pyrrole synthesis, with an aromatic aldehyde in the presence of TFA gives dipyrromethane, which is oxidized to dipyrromethene using a quinone oxidant such as DDQ or p-chloranil. Alternatively, dipyrromethenes are prepared by treating a pyrrole with an activated carboxylic acid derivative, usually an acyl chloride. Unsymmetrical dipyrromethenes can be obtained by condensing pyrroles with 2-acylpyrroles. Intermediate dipyrromethanes may be isolated and purified, but isolation of dipyrromethenes is usually compromised by their instability.
Structures and reactivity
The BODIPY core has a rich chemistry and variety of substitutions due to the high tolerance for pyrrole and aldehyde starting materials. The core carries a formal negative charge on the boron atom, and a formal positive charge on one of the nitrogen atoms. The boron atom can also be post-functionalised to exchange the fluorine atoms for strong nucleophilic reagents, such as lithiated alkyne or aryl species. Phenol subsituent on the pyrrole unit can "strap" the BODIPY structure by replacing the fluorine atoms. The reaction is catalysed by BBr3 or SnCl4. Hydrogen atoms at the 2 and 6 positions of the cyclic core can be displaced by halogen atoms using succinimide reagents such as NCS, NBS and NIS - which allows for further post-functionalisation through palladium coupling reactions with boronate esters, tin reagents etc.
Potential applications
BODIPY conjugates are widely studies as potential sensors and for labelling by exploiting its highly tunable optoelectronic properties.
