Nonconjugated Fluorescent Molecular Cages of Trinuclear Fluoroborate Complexes with Salicylaldehyde-Based Schiff Base Ligands

Fluorescent organic
materials are commonly π-conjugated planar
molecules. In the present work, however, we report a novel class of
nonconjugated fluorescent molecular cages of trinuclear fluoroborate
complexes (nine samples) with salicylaldehyde-based Schiff base ligands.
Owing to the stress from lone pair electrons of N atom in the triethylamine
bridge, these B­(III) complexes exhibit unusual enantiomers with a
tripodlike side-single-opening structure. They emit blue, green, and
red emission with large Stokes shifts (up to 159 nm) and high fluorescence
quantum yields in both solution (up to 0.24) and solid state (up to
0.25), which might contribute to their strong intramolecular hydrogen
bonds and weak intermolecular and intramolecular π–π
interactions. Combining their advantages of nonconjugation and biocompatibility,
these flexible complexes have potential applications in living cell
imaging and anion hosts. We have examined the inherent relationships
between their chemical structures and emission properties and afforded
a new stage for the design of nonconjugated fluorescent fluoroborate
complexes.