A Highly Selective Chemosensor for Cyanide Derived from a Formyl-Functionalized Phosphorescent Iridium(III) Complex

A new
phosphorescent iridium­(III) complex, bis­[2′,6′-difluorophenyl-4-formylpyridinato-N,C4′]­iridium­(III) (picolinate)
(IrC), was synthesized, fully characterized by various
spectroscopic techniques, and utilized for the detection of CN– on the basis of the widely known hypothesis of the
formation of cyanohydrins. The solid-state structure of the developed IrC was authenticated by single-crystal X-ray diffraction.
Notably, the iridium­(III) complex exhibits intense red phosphorescence
in the solid state at 298 K (ΦPL = 0.16) and faint
emission in acetonitrile solution (ΦPL = 0.02). The
cyanide anion binding properties with IrC in pure and
aqueous acetonitrile solutions were systematically investigated using
two different channels: i.e., by means of UV–vis absorption
and photoluminescence. The addition of 2.0 equiv of cyanide to a solution
of the iridium­(III) complex in acetonitrile (c =
20 μM) visibly changes the color from orange to yellow. On the
other hand, the PL intensity of IrC at 480 nm was dramatically
enhanced ∼5.36 × 102-fold within 100 s along
with a strong signature of a blue shift of the emission by ∼155
nm with a detection limit of 2.16 × 10–8 M.
The cyanohydrin formation mechanism is further supported by results
of a 1H NMR titration of IrC with CN–. As an integral part of this work, phosphorescent test strips have
been constructed by impregnating Whatman filter paper with IrC for the trace detection of CN– in the contact
mode, exhibiting a detection limit at the nanogram level (∼265
ng/mL). Finally, density functional theory (DFT) and time-dependent
density functional theory (TD-DFT) calculations were performed to
understand the electronic structure and the corresponding transitions
involved in the designed phosphorescent iridium­(III) complex probe
and its cyanide adduct.