Tetrahedral Boron Chemistry for the Preparation of Highly Efficient “Cascatelle” Devices

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)
∼1.59 Å and both boron atoms lying essentially in the mean planes of the dipyrromethene units. All the
new compounds show intense electronic absorption bands (ε 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+• and Bodipy-• occurring more readily than for F-Bodipy species. The molecules in their
excited states are strong reducing agents.