Tetrahedral boron chemistry for the preparation of highly efficient "Cascatelle" devices

[graphics] The replacement of the two fluorine atoms on the boron center of the well-known Bodipy fluorophore by functionalized acetylenic groups opens the way to a new family of highly luminescent, redox active, and stable fluorophores termed "E-Bodipy" species. The substitution is effective for ethynyl-lithium reagents incorporating tolyl, naphthyl, pyrenyl, fluorenyl, and terpyridinyl units. The protocol also tolerates the presence of various functional groups in the dipyrromethene meso position such as pyrene, phenylethynylpyrene, 4'-terpyridine, and iodophenyl. The last of these is particularly useful for a further coupling reaction enabling introduction of a flexible arm bearing a succinimidyl unit reactive toward primary amines. X-ray structure determinations of two E-Bodipy compounds confirm the introduction of the ethynyl units and show the boron atoms to have a distorted tetrahedral environment, with B-C(ethynyl) similar to 1.59 angstrom and both boron atoms lying essentially in the mean planes of the dipyrromethene units. All the new compounds show intense electronic absorption bands (epsilon 60 000-70 000 M-1 cm(-1)), high quantum yields (> 80%), and slow rates of nonradiative decay. Absorption by the aromatic substituents results in a "cascatelle" process leading to emission exclusively through the boradiazaindacene entity and thus large virtual Stokes' shift (> 10 000 cm(-1)). The new compounds are also redox active, with the formation of both Bodipy(+center dot) and Bodipy(-center dot) occurring more readily than for F-Bodipy species. The molecules in their excited states are strong reducing agents.