A Highly Selective Chemosensor for Cyanide Derived from a Formyl- Functionalized Phosphorescent Iridium(III) Complex K. S. Bejoymohandas, †,‡,∥ Ajay Kumar, †,∥ S. Sreenadh, † E. Varathan, § S. Varughese, † V. Subramanian, § and M. L. P. Reddy* ,†,‡ † Materials Science and Technology Division, CSIR-Network of Institutes for Solar Energy, CSIR-National Institute for Interdisciplinary Science & Technology (CSIR-NIIST), Thiruvananthapuram 695 019, India ‡ Academy of Scientific and Innovative Research (AcSIR), New Delhi 110025, India § Chemical Laboratory, CSIR-Central Leather Research Institute, Chennai 600 020, India * S Supporting Information ABSTRACT: 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 (Φ PL = 0.16) and faint emission in acetonitrile solution (Φ PL = 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 × 10 2 -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 1 H 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. ■ INTRODUCTION Cyanide is a strikingly toxic inorganic anion that adversely affects human health and the environment. 1 The maximum acceptable level of cyanide in drinking water is 1.9 μM, as per the requirements of the World Health Organization. 2 In spite of their extreme toxicity, various cyanide-containing compounds are still widely used in different industrial processes such as in gold mining, electroplating, and various metallurgical industries, causing their leakage into the aquatic environment. 3 Con- sequently, there is considerable interest in the selective detection of aqueous cyanide at submicromolar levels by using simple and visual methods. 4 As of today, various colorimetric and fluorometric receptors are known for the sensing of cyanide ions, in which many transition-metal complexes have been utilized. 5 Nevertheless, fluorescent chemodosimeters based on specific chemical reactions always show better selectivity, and some of them display a detection limit at the submicromolar level for cyanide in organic solvents. 6 It is well documented that phosphorescent iridium(III) complexes have attractive features such as long lifetime, large Stokes shift, high stability, excellent color tuning, and lower self-quenching in comparison to conventional organic fluorophores. 7 Due to these unique photophysical properties, iridium(III) complexes have been recognized as potential luminescent materials for applications in light-emitting devices and chemosensors. 8 Li and co-workers have reported a ratiometric upconversion luminescence (UCL) probe for the selective detection of cyanide anion in aqueous solution with a detection limit of 0.18 μM based on a chromophoric iridium(III) complex coated nanosystem (NaYF 4 : 20%Yb, 1.6% Er, 0.4% Tm nanocrystals). 9 Silica nanoparticles doped Received: December 14, 2015 Published: March 23, 2016 Article pubs.acs.org/IC © 2016 American Chemical Society 3448 DOI: 10.1021/acs.inorgchem.5b02885 Inorg. Chem. 2016, 55, 3448−3461