Fluorescent Modular Boron Systems Based on NNN- and ONO-Tridentate Ligands: Self-Assembly and Cell Imaging

We
have synthesized a series of new fluorescent boron systems 1a–c and 2a–d based
on nitrogen (NNN) or nitrogen and oxygen (ONO)-containing
tridentate ligands. These novel dyes are characterized by high thermal
and chemical stability. They show large Stokes shifts (mostly above
3200 cm–1) and quantum yields in solution and in
the solid state up to 40%. The easy, modular synthesis facilitates
the convenient variation of the axial substituent on the central boron
atom, allowing the functionalization of this dye for biochemical use.
Introducing a long alkyl chain with a phenyl spacer at this axial
position enables the self-assembly of the boron compound 2d to form a fluorescent vesicle, which is able to encapsulate small
molecules such as sulforhodamine. Additionally, boron compound 2d was found to serve as a dye for cell imaging since it has
the capability of binding to the nuclear membranes of HeLa cells.
With phospholipids such as DOPC, giant unilamelar vesicles (GUV) are
formed. These results demonstrate the wide applicability of this new
boron system in supramolecular and medicinal chemistry.