Boron-Dipyrromethene Based Reversible and Reusable Selective Chemosensor for Fluoride Detection

We synthesized benzimidazole
substituted boron-dipyrromethene 1 (BODIPY 1) by treating 3,5-diformyl BODIPY 2 with o-phenylenediamine under mild acid catalyzed conditions and characterized
by using various spectroscopic techniques. The X-ray structure analysis
revealed that the benzimidazole NH group is involved in intramolecular
hydrogen bonding with fluoride atoms which resulted in a coplanar
geometry between BODIPY and benzimidazole moiety. The presence of
benzimidazole moiety at 3-position of BODIPY siginificantly altered
the electronic properties, which is clearly evident in bathochromic
shifts of absorption and fluorescence bands, improved quantum yields,
increased lifetimes compared to BODIPY 2. The anion binding
studies indicated that BODIPY 1 showed remarkable selectivity
and specificity toward F– ion over other anions.
Addition of F– ion to BODIPY 1 resulted
in quenching of fluorescence accompanied by a visual detectable color
change from fluorescent pink to nonfluorescent blue. The recognition
mechanism is attributed to a fluoride-triggered disruption of the
hydrogen bonding between BODIPY and benzimidazole moieties leading
to (i) noncoplanar geometry between BODIPY and benzimidazole units
and (ii) operation of photoinduced electron transfer (PET) from benzimidazole
moiety to BODIPY unit causing quenching of fluorescence. Interestingly,
when we titrated the nonfluorescent blue 1-F– solution with TFA resulted in a significant enhancement of fluorescence
intensity (15-fold) because the PET quenching is prevented due to
protonation of benzimidazole group. Furthermore, the reversibility
and reusability of sensor 1 for the detection of F– ion was tested for six cycles indicating the sensor 1 is stable and can be used in reversible manner.