Pyrenoimidazolyl-Benzaldehyde Fluorophores: Synthesis, Properties, and Sensing Function for Fluoride Anions

Two
structural isomers of (9H-pyreno­[4,5-d]­imidazol-10-yl)-benzaldehyde, with para and meta substitution
patterns, were synthesized by condensation of 4,5-pyrenedione with
terephthalaldehyde and isophthalaldehyde, respectively. These new
pyrenoimidazole derivatives were characterized by single-crystal X-ray
crystallography, UV–vis absorption spectroscopy, fluorescence
spectroscopy, and cyclic voltammetry to elucidate their structural,
solid-state packing, and electronic properties. Interactions of these
compounds with fluoride anions in polar organic solvents (acetone
and dimethyl sulfoxide) were investigated by NMR, UV–vis, and
fluorescence techniques in conjunction with density functional theory
calculations. UV–vis analysis showed that the binding of the
two pyrenoimidazolyl benzaldehydes with fluoride anions resulted in
significant colorimetric responses, while fluorescence studies showed
that the para-pyrenoimidazolyl benzaldehyde behaved
as an intramolecular charge transfer fluorescent probe, exhibiting
ratiometric sensing performance to efficiently detect and quantify
fluoride anions at the sub-millimolar level.