Organoiridium(III) Complexes as Luminescence Color Switching

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Organoiridium(III) Complexes as Luminescence Color Switching Probes for Selective Detection of Nerve Agent Simulant in Solution and Vapor Phase Sanjoy Kumar Sheet,† Bhaskar Sen,† and Snehadrinarayan Khatua* Centre for Advanced Studies, Department of Chemistry, North Eastern Hill University, Shillong, Meghalaya 793022, India

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S Supporting Information *

ABSTRACT: In this work, cationic organoiridium(III) complex based photoluminescent (PL) probes have been developed to selectively detect the chemical warfare nerve agent mimic, diethyl chlorophosphate(DCP) at nanomolar range by distinct bright green to orange-red luminescence color switching (on−off−on) in solution as well as in the vapor phase. Interference of other chemical warfare agents (CWAs) and their mimics was not observed either by PL spectroscopy or with the naked-eye in solution and gas phase. The detection was attained via a simultaneous nucleophilic attack of two −OH groups of the 4,7-dihydroxy-1,10-phenanthroline ligand with DCP by forming bulkier phosphotriester. The detailed reaction mechanism was established through extensive 1H NMR titration, 31P NMR, and ESI-MS analysis. Finally, a test paper strip and solid poly(ethylene oxide) (PEO) film with iridium(III) complex 1[PF6] were fabricated for the vapor-phase detection of DCP. The solution and vapor-phase detection properties of these luminescent Ir(III) complexes can offer a worthy approach into the design of new metal complex based PL switching probes for chemical warfare agents.

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INTRODUCTION Phosphorus-containing nerve agents, including tabun (GA), sarin (GB), soman (GD), and cyclosarin (GF) (Chart 1) are

attack by organophosphonate (OP) nerve agents are rather untraceable until inhalation since they are colorless gases and a small dose of 0.7 mg for a normal person of 70 kg is incredibly lethal.4 Use of these compounds causes potential risk to humans, other living things, and unwanted side effects to the environment. The reactive phosphate groups of OP nerve agents are capable to bind irreversibly with hydroxyl groups of acetylcholinesterase (AChE); thus, it arrests the decomposition of acetylcholine in synapses, which causes accumulation of acetylcholine in the synaptic junctions, resulting neurological imbalance, organ failure, and rapid death by paralysis due to the damage in central nervous system.4−7 Furthermore, a timedependent intramolecular reaction of AChE incipient adduct involves dealkylation of alkoxy substituent and forms a phosphonate conjugate with the protonated catalytic histidine, termed “aged” AChE. Reactivation of this extremely stable “aged” AChE is practically impossible by antidotes.8,9 Consequently, development of selective, sensitive, troublefree, and reliable techniques for nerve agent detection is immediately required for national security and universal safety. Researchers are utilizing less poisonous nerve agent mimics (Chart 1) in laboratories as model compounds to evade direct contact. Diethyl chlorophosphate (DCP) serves as a harmless simulant of sarin (GB) with comparable reactivity and low

Chart 1. Chemical Structure of Nerve Agents and Less Toxic Simulants

extremely toxic chemical warfare agents (CWAs).1−3 These nefarious volatile liquids and gases are considered the most formidable chemical weapons for mass annihilation. Usage of these deadly nerve agents was started during World War II and is still going on. For example, use during the Iraq−Iran war in 1988, the Tokyo subway terrorist attack in 1995, and very recently the Syrian civil war, the Ghouta chemical attack in 2013, the Khan Shaykhun chemical attack in 2017, and the Douma 2018 event has caused thousands of fatalities. The © XXXX American Chemical Society

Received: October 29, 2018

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DOI: 10.1021/acs.inorgchem.8b03044 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Chemical structures of frequently reported organic-based and the iridium(III) complex based nerve agent probes.

Figure 2. (a) Schematic representation of DCP detection by 1[PF6]. (b) Luminescence switching is shown by the PL of 1[PF6] (10 μM) (green, λex = 392 nm) and in the presence of DCP (orange-red, λex = 392 nm) and their digital photographs under UV (at 365 nm) illumination.

toxicity.10,11 The existing nerve agent mimic probes are mainly based on small organic molecules, such as pyrene,12 fluorescein,13,14 coumarin,15,16 cyanine/hemicyanine, 17,18 rhodamine, 19−21 pyronin, 22 BODIPY, 23−25 naphthalimide,26−28 and squaraine29 fluorogenic/chromogenic system (Figure 1). Most of them used (i) direct phosphorylation reaction by oximes,15,28,30 alcohols,25 and amines,13,22 (ii) phosphorylation and rapid n-alkylation using acid groups,17 carbonyl,16 and intramolecular cyclization,11,12,17,18,24,31−34 and (iii) phosphorylation and successive protonation of pyridine35 and quinoline10,30,34 groups for nerve agent detection. In addition, small chromo/fluorogenic probes other than the above moieties have been used recently for the detection nerve agent.36−40 Furthermore, several nano,41,42 polymer,43−45 and gel materials46 and devices47 were also constructed for the fluorometric and colorimetric detection of nerve agent gas mimics for in-field application. During the past two decades, scientists have been working extensively for the selective and sensitive detection of nerve agent gases, using metal complexes for direct detection of nerve agent gas, yet this has not been explored aptly.4c The metal complex based probes reported to date are typically based on lanthanides [Eu(III) and La(III)], along with few Fe(II) and Zn(II) metals.48−52 In most cases, nerve agent detection occurs through the sequestration of metal ion and luminescence quenching.51 Therefore, vapor-phase detection was not accomplished for most of the metal complex based probes except for a few metallopolymer films where at least 1−

2 h was required.51 Even though an Ir(III)/Eu(III) based in situ formed dyad also has been reported for the V-type nerve agent simulant detection based on metal ion displacement approach, a 1:1 ensemble for the sensing and gas-phase detection was not achieved.53 Luminescent d6-metal complexes as chemosensors offer certain advantages over many organic probes, including large Stokes shift, long excited−state lifetimes, tunable photo physical/chemical properties, and solubility and stability in aqueous media.54−59 Cationic iridium(III) polypyridine complexes are famed for their excellent tunable photophysical properties. In addition to singlet excited states, due to the involvement of triplet metal-to-ligand charge transfer (3MLCT), triplet ligand-to-ligand charge transfer (3LLCT), and triplet of intraligand state (3IL), they achieved tunable photophysical properties in a predictable way.54,60,61 The alteration of PL band position and intensity due to the specific reactions of analyte with the complex encourage researchers to skillfully design iridium(III) complex based on analyte approach via the modification in analyte targeting ligand substituents.54,62−65 With great anticipation of OP nerve agent mimic detection, two new iridium(III) complexes, 1[PF6] and 2[PF6], of 4,7dihydroxy-1,10-phenanthroline ligand were synthesized (Figure 1, inset). As expected, 1[PF6] and 2[PF6] preferably interacted with nerve agent surrogate DCP in organic solvent as well as in aqueous buffer solution over a wide pH range. Distinct PL switching from bright green to orange-red through B


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Inorganic Chemistry

Figure 3. (a) UV−vis absorption spectra of 1[PF6] (10 μM) with various analytes (150 μM) in CH3CN. (b) UV−vis absorption titration of 1[PF6] (10 μM) with DCP (0−150 μM) in CH3CN at 25 °C.

Figure 4. (a) PL spectra of 1[PF6] (10 μM) and upon addition of DCP and other analytes (700 μM). (b) PL titration of 1[PF6] (10 μM) with DCP (0−700 μM) in CH3CN (λex = 392 nm). (c) Luminescence of 1[PF6] (10 μM) under UV light (365 nm) in the presence of different amount of DCP in CH3CN, (i) 0 μM, (ii) 140 μM, (iii) 280 μM, (iv) 420 μM, (v) 560 μM, (vi) 700 μM after a 20 min incubation.

results in PL enhancement (Figure 2). Additionally, a solid paper strip and PEO film with 1[PF6] have been equipped for visual detection of DCP in vapor phase. To the best of our knowledge, this is the first report of transition metal complex for the luminescent detection of nerve agent mimics through ligand based phosphorylation reaction.4c

a rapid quenching path (on−off−on) was recorded for both the probes in the presence of DCP (Figure 2). In the 1,10phenanthroline ligand, the 5,6-C−H (herein Hd) unit acting as a steric abutment; therefore, the restriction of intramolecular rotation (RIR) around C(phen)-O(phos) after phosphorylation and inhibition of nonradiative decay from excited state C


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Inorganic Chemistry

Figure 5. (a) pH effect on PL spectra of 1[PF6] (10 μM) and 1[PF6] (10 μM) with DCP (120 equiv) in mixed buffer solution (buffer/CH3CN; 1:1; v/v) (λex = 392 nm). (b) Time course of the PL response of 1[PF6] (10 μM) upon addition of 700 μM of DCP in CH3CN (λex = 392 nm). (c) Luminescence of 1[PF6] (10 μM) in the presence 700 μM of DCP with time (0−20 min) under UV light (365 nm).

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(0−150 μM), both the absorption bands at 359 and 376 nm for 1[PF6] decreases in regular manner (see Figure 3b). Likewise, for 2[PF6], absorbance of both the bands at 359 and 376 nm decreases with increasing DCP amount in acetonitrile (see Figure S17). The clear isosbestic point at 392 nm is detected for 1[PF6] which implies existence of single reaction equilibrium between probe and 1-(DCP)2 adduct. Both 1[PF6] (10 μM) and 2[PF6] (10 μM) show bright green luminescence with a band centered at 518 (Φ = 0.023) and 512 nm (Φ = 0.032), respectively (λex = 392 and 396 nm) in CH3CN at 25 °C (Figure S19). Upon addition of 70 equiv of DCP, however, a significant PL quenching was documented at 518 and 512 nm and a new band was appeared at 565 nm (Φ = 0.004) and 576 nm (Φ = 0.006), respectively (Figure 4a and Figure S20). The PL intensity was not significantly influenced by the addition of other competitive analytes, CWAs and their mimics, although the diethyl cyanophosphonate (DECP) exhibits minute PL quenching (Figures 4a and S20) which can be explained by the better leaving of Cl− ion of DCP compared to CN− of DECP. The orange-red luminescence in the presence of DCP was clearly observed by the naked eye under UV light (see Figures 2b and 4c). Akin to the previous reports,23 here it is expected that the nucleophilic attack at phosphoryl halide (DCP) by the phenoxides (O−) of 1[PF6] and steric perturbation by the formation of two bulkier phosphotriesters is the key reason for “on−off−on” PL switching. The PL intensity of 1[PF6] largely depends on the pH of the media. At basic pH, 1[PF6] shows very bright green luminescence, whereas, at acidic pH, the PL is quenched (Figure S21). The phenoxides (O−) of the ligand L in basic media (pH ∼ 8−12) might be converted into hydroxyl groups (−OH) at lower pH (∼2−5) (Figure S21). At higher pH (8−12) in the deprotonated state, the emissive state of the probe 1[PF 6 ] is mainly 3 MLCT (Ir→ppy) and

RESULTS AND DISCUSSION Synthesis and Characterization. The ligand, 4,7dihydroxy-1,10-phenanthroline (L), was synthesized according to the reported literature procedure66 and characterized by 1H and 13C NMR spectroscopy and electrospray ionization-mass spectrometry (ESI-MS) (Figures S1−S3). The synthetic routes of two new organoiridium(III) complexes, 1[PF6] and 2[PF6], are illustrated in Scheme S1. Probes 1[PF6] and 2[PF6] have been synthesized by heating a mixture of 4,7-dihydroxy-1,10phenanthroline with the iridium(III) starting materials in ethylene glycol under N2. The pure products, obtained by silica gel column chromatography, were thoroughly characterized by 1D (1H and 13C) and 2D (1H−1H COSY, 1H−13C HSQC, and 1 H−13C HMBC) NMR spectroscopy, ESI-MS, and elemental analysis. The solution structures of both cationic organoiridium(III) probes 1[PF6] and 2[PF6] have been confirmed by ESI-MS, and all proton and carbon signals were assigned by 1D and 2D NMR spectroscopy (Figures S4−S15). Detection of Nerve Agent Simulant DCP in Solutions. To find the ability of 1[PF6] and 2[PF6] for selective OP nerve agent mimics detection, several analytes were first tested in UV−vis spectroscopy which showed noticeable absorbance change only in the presence of DCP (Figures 3a,b, S16, and S17). Probe 1[PF6] shows absorption band centered at 359 and 376 nm in acetonitrile at room temperature. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were performed to assign those absorption bands (Figure S18 and Tables S1 and S2). The absorption bands at 359 and 376 nm are ascribed to spinallowed, ligand-to-ligand charge transfer (1LLCT) and metalto-ligand charge transfer (1MLCT) characters, as previously reported for other iridium(III) complexes (see the Supporting Information).67 Upon addition of increasing amount of DCP D


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Inorganic Chemistry

Figure 6. (a) Likely reaction pathway for DCP detection. Partial 1H NMR spectra of 1[PF6] (9.3 mM) upon addition of DCP (0−56 mM) in DMSO-d6/D2O (25:1; v/v; containing 0.25 mM NaOH). (b) Aromatic and (c) aliphatic regions. (d) Plot of the shifting of proton signals with equiv of DCP. 3

PL is quenched. Likewise, after phosphorylation reaction, immediate quenching was observed as two phenoxides (O−) were converted to phosphotriester, and the quantum efficiency of the 1-(DCP)2 adduct is somewhat inferior than that of

ILCTppy(π→π*) in character which is obtained from the computational study (Figure S22 and Table S3). However, after protonation at lower pH (∼2−5), the nonradiative decay occurs from 3LLCT(L→ppy)/3IL(L→L) based excited state and the E


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Inorganic Chemistry

nm (1[PF6]) and 512 nm (2[PF6]) which completed within 5 min. After that enhancement was observed at 565 nm (1[PF6]) and 576 nm (2[PF6]) which took additional ∼15 min to reach a plateau (Figures 5b and S33). The digital images at different time interval were captured under laboratory UV-lamp (365 nm), which clearly endorsed “on− off−on” bright green (0−5 min) to orange-red (6−20 min) PL switching through rapid quenching pathway (Figure 5c). Mechanistic Investigation and DCP Binding Evidence by ESI-MS, NMR Spectroscopy, and Theoretical Study. The ESI-MS spectra of the reaction mixture of 1[PF6] with DCP authenticate the product formation and show a peak at m/z = 985.24, which corresponds to the diphosphorylated (1:2) product (calcd for [C42H42IrN4O8P2]+ = 985.21) (Figures 6a and S34). The two hydroxyl groups at 4,7positions of ligand L in the C2 symmetric Ir(III) complex, 1[PF6] can react with DCP stepwise or simultaneous manner.55a For advance assessment of this nucleophilic substitution reaction, 1H NMR titration experiments were conducted, which portrayed an unambiguous outlook of sensing mechanism (Figure 6b−d). Upon addition of DCP to 1[PF6], the proton signals of 4,7-dihydroxy-1,10-phenanthroline, assigned by 1H−1H COSY (Figure S35) change drastically (Figure 6b). The doublet at δH = 5.97 ppm for Hb proton is shifted downfield (Δδ = 1.39 ppm) after reaction with DCP. Likewise, the signals for Ha and Hd at δH = 7.02 and 7.71 ppm, respectively, also undergo downfield shifting and appear at δH = 7.78 ppm (Δδ = 0.76 ppm) and 8.24 ppm (Δδ = 0.53 ppm) (Figure 6b). In addition, slight upfield shifting (Δδ = 0.2 ppm) of H2 proton signal of 2-phenylpyridine is recorded in the presence of DCP (Figure 6b). Interestingly, the ethyl protons of DCP were appeared slowly at δH 1.19 ppm (−CH3) and 3.94−3.86 ppm (−CH2) and finally showed a slight downfield shift (Δδ = 0.12 and 0.26 ppm, respectively) after addition of 6 equiv of DCP (Figure 6c). The gradual peak position shifting in 1H NMR titration strongly suggests reversible reaction equilibrium and clearly supports the formation of diphosphorylated product 1-(DCP)2 through the concurrent nucleophilic addition reaction with DCP. If the nucleophilic reaction occurs stepwise (Figure 6a) after 1:1 product formation, then the complex must lose its symmetry,55a and in any case ∼22 aromatic proton signals might be observed which is absent in the present case (Figure 6b). In the 31P NMR spectra, a peak appeared at −0.82 ppm for DCP was shifted upfield and arrived at −2.26 ppm with compound 1[PF6], which further infers phosphorylation in 1[PF6] (see Figure S36). Ground-state optimized geometry unveils that the HOMO for both 1[PF6] and the diphosphorylated product, 1-(DCP)2, is based on Ir(dπ) and ppy ligand, and the LUMO is phenanthroline ligand (L) based. Both HOMO and LUMO are stabilized in diphosphorylated 1-(DCP)2 adduct and the ΔEHOMO−LUMO in diphosphorylated product 1-(DCP)2 (3.11 eV) is significantly decreased (0.22 eV) from 1[PF6] (3.33 eV) supports red-shifting of λem and corresponding luminescence color switching (see Figures S37 and S38).68 Vapor-Phase DCP Detection with the Probe Immobilized on Solid-State Paper Strip and Film. It is extremely important and realistic to detect nerve agents under an actual threat scenario. To achieve this goal, 1[PF6] immobilized on low-cost filter paper strips were first constructed. This simple solid-state device can be employed as portable chemosensor kit. Similar to solution-phase

probe 1[PF6]. The bulky tetrahedral phosphotriester group encountered excessive hindrance by Hd protons of the phen ligand and as a result the rigidity is attained. Further, the accumulation of adequate amount of 1-(DCP)2 adduct causes slow PL enhancement (Figures 2 and 4c). During the PL titration of 1[PF6] (10 μM) with DCP, the emission maxima, λem at 518 nm (512 nm for 2[PF6]) gradually decreases with the incremental addition of DCP (0− 250 μM) and ∼11-fold (∼15-fold for 2[PF6]) PL quenching was observed (Figures 4b and S23−S25). Afterward, a new PL band centered at 565 nm (576 nm for 2[PF6]) was developed and ∼3-fold (∼3-fold for 2[PF6]) intensity enhancement was recorded up to 700 μM DCP additions (see Figures 4b and S23−S25) which is imitated in the digital photographs, taken under UV-lamp (Figure 4c). The emission maxima (λem) were ∼47 and ∼64 nm red-shifted for 1[PF6] and 2[PF6], respectively, after phosphorylation. The initial Stokes shifts for 1[PF6] (142 nm, 7290 cm−1) and 2[PF6] (136 nm, 7064 cm−1) further increase to 189 nm (8896 cm−1) and 200 nm (9234 cm−1), respectively, after reaction with DCP which are superior to those of many reported nerve agent mimic probes (see Table S4). The detection limits of 1[PF6] and 2[PF6] for DCP were calculated as low as 7.9 and 8.7 ppb, respectively, from PL titration data (see Figures S26 and S27) which is somewhat lower than many other reported nerve agent mimic probes (see Table S4). The reaction of 1[PF6] with DCP is a typical pseudo-first-order process which is obtained from the reaction between 1[PF6] (10 μM) and DCP (700 μM) in CH3CN at 25 °C and the rate constant (kobs) for the 1(DCP)2 adduct is 0.004 s−1(see Figure S28). The free 4,7dihydroxy-1,10-phenanthroline ligand (10 μM) shows blue emission at 432 nm (λex = 364 nm) in DMSO (1 μM Et3N) (see Figure S29). With increasing amounts of DCP (0−80 equiv), the regular quenching of blue emission at 432 nm was observed rather than any color switching like that of 1[PF6] (Figure 4c). It is expected that the π−π* excited state (ES) contributes in the ligand emission, while in 1[PF6], the PL is primarily dominated by the 3MLCT(Ir→ppy) and 3ILCTppy(π→π*) excited state (see Figure S22 and Table S3) which can easily be transformed by analyte interaction and instigate luminescence color switching. The nucleophilic substitution reaction with OP nerve agent simulants widely depends on pH value and frequently favored basic condition.28 To investigate the pH effects on the probe 1[PF6] (10 μM), we operated PL study in mixed aqueous buffer with wide pH range (Figure 5a). In the absence of DCP, the PL intensity of 1[PF6] at 518 nm regularly increases with increase in pH. While 1[PF6] can detect DCP almost equally at a wide range of pH from 7 to 13, better luminescence switching was observed at pH 8.0 in the presence of 120 equiv of DCP (Figure 5a). Furthermore, PL titration of 1[PF6] with DCP in acetone, CH3CN/HEPES buffer solution (0.1 M, v/v, 1:1; pH 7.4), methanol, and DMF were accomplished to inspect the effect of polar protic and aprotic solvents in the nucleophilic substitution reaction (Figures S30 and S31). More DCP is required for completion of the reaction in mixed aqueous buffer solution (120 equiv) than for in acetone (70 equiv), and green to orange-red luminescence switching. In water, DCP probably decomposed and started to form phosphoric acid and demands higher amount of analytes (Figure S32). The time course analysis for the real-time DCP detection (700 μM) showed that the rapid quenching occurred at 518 F


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Inorganic Chemistry

Figure 7. (a) Selectivity study of various analyte vapors (100 μL) by test paper strip: (1) Blank, (2) DCP (3) DECP, (4) acetic anhydride, (5) acetyl chloride, (6) chloroacetic acid, (7) chloroacetyl chloride, (8) oxalyl chloride, (9) phosgene, (10) phosphoryl chloride, (11) sulfuryl chloride, (12) SOCl2, (13) HCl, and (14) Et3N. (b) Luminescence color switching of 1[PF6] immobilized test paper strips upon exposure to increasing amounts of DCP vapor (i) 0, (ii) 100, (iii) 250, (iv) 350, and (v) 500 μM. (c) View of the real-time vapor-phase DCP detection by 1[PF6]-coated PEO solid film through luminescence color switching.

effective detection of DCP in vapor phase than in solution. The result demonstrated the aptitude of our probe-coated test paper and PEO strip toward nerve agent gas for practical application.

selectivity study, likely interference of hydrochloric acid vapor, Et3N, and other CWAs was investigated in gas phase with the paper strip as shown in Figure 7a. The initial bright green luminescence of the strip under the UV irradiation (λex = 365 nm) was switched to orange-red only in the presence of DCP vapor. This study clearly indicates that the paper strip with 1[PF6] is capable to selectively detect nerve agent mimic in gas phase. Furthermore, the luminescence of 1[PF6] immobilized on paper strips were checked at various concentration of DCP vapor. Like solution-phase quantification (Figure 4c), “on− off−on” luminescence color switching was observed with increasing DCP concentration. Upon exposure to lower amount of DCP vapor (100 and 250 μM with equimolar Et3N), the intensity of initial green luminescence of paper strip was decreased gradually and changed to orange-red with increased DCP amount (350 and 500 μM with equimolar Et3N) (Figure 7b). A paper strip of 4,7-dihydroxy-1,10phenanthroline ligand (L) also was prepared which showed blue emission under UV light, but vapor-phase DCP detection was not practically useful as the fluorescence quenching was not remarkable (Figure S39). A poly(ethylene oxide) PEO test strip was also prepared by dissolving 1[PF6] (50 μM) and poly(ethylene oxide) in CH3CN/DCM to form a fine sheath after the solvent evaporation. The prepared thin film was kept hanging from inside glass vial and showing bright green luminescence under the UV irradiation (λex = 365 nm). Interestingly, parallel result was monitored in solid state PEO film (Figure 7c). Under exposure of DCP (0.5 mM) vapor (in the presence of 0.5 mM Et3N), the luminescence color of PEO strip changes from green to dark and finally to bright orange with time under UV light. The reaction between probe-immobilized PEO-film and DCP was started within 1 min, and the green luminescence was switched to orange-red within 4 min. Finally, at ∼6 min, the reaction was completed. A time-lapse video evidently demonstrated the time course of the gas phase DCP detection (Movie S1). These experiments confirm that both the paper and polymer test strips can achieve comparatively fast and

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CONCLUSIONS We have synthesized two cationic organoiridium(III) complexes of 4,7-dihydroxy-1,10-phenanthroline ligand for highly selective, real-time detection of nerve agent simulant DCP at parts per billion level. The DCP detection was achieved via a bright green to orange-red luminescence color switching through a rapid quenching pathway. Interference of other chemical warfare agents (CWAs) and their mimics was not observed either by PL spectroscopy or with the naked-eye in solution and gas phase. For this “on−off−on” probe, the “on− off” is quite fast (300 °C. ESIMS [C12H8N2O2 + H]+: calcd, m/z = 213.06. Found, m/z = 213.02. 1 H NMR (400 MHz, DMSO-d6/D2O; 25:1; v/v; containing 0.25 mM NaOH): δ (ppm) 8.08 (d, J = 5.6 Hz, 1H, Ha), 7.76 (s, 1H, Hd), 6.19 (d, J = 5.6 Hz, 1H, Hb). 13C NMR (100 MHz, DMSO-d6/D2O; 25:1; v/v; containing 0.25 mM NaOH): δ (ppm) = 174.2 (1C), 149.7 (1C), 148.5 (1C), 126.6 (1C), 117.2 (1C), 110.0 (1C). Synthesis of [4,7-Dihydroxy-1,10-phenanthroline-bis(2phenylpyridine)iridium(III) Hexafluorophosphate], 1[PF6]. A mixture of [Ir(ppy)2Cl]2 (0.100 g, 0.093 mmol) (ppy = phenylpyridine) and ligand L (0.049 g, 0.233 mmol) was stirred in ethylene glycol at 90 °C for 20 h in dark under N2. The reaction was monitored by thin-layer chromatography (TLC), and after completion of reaction the reaction mixture was cooled, filtrated, an excess of solid KPF6 in water added, and stirred for another 15 min at room temperature. A dark yellow solid was separated and dissolved in dichloromethane and methanol. The compound was then purified by silica gel column chromatography (DCM/MeOH = 95:5). Dark yellow solid compound 1[PF6] was isolated in 83% (0.066 g) yield. Melting point (MP): >300 °C. Anal. Calcd for C34H24F6N4O2PIr (Mw = 857.76): C, 47.61; H, 2.82; N, 6.53, Found: C, 47.75; H, 2.78; N, 6.51. FTIR in KBr disc (ν/cm−1): 3433, 3060, 1606, 1582, 1477, 1316, 1125, 909, 830, 756, 602.; ESI-MS [C34H24IrN4O2]+: Calcd: m/ z = 713.15; Found: m/z = 713.17. 1H NMR (400 MHz, DMSO-d6/ D2O; 25:1; v/v; containing 0.25 mM NaOH): δ (ppm) = 8.13 (d, J = 8.1 Hz, 1H, H6), 7.81−7.75 (m, 2H, H7,9), 7.71 (m, 2H, Hd,2), 7.07 (t, J = 7.3 Hz, 1H, H8), 7.02 (d, J = 6.5 Hz, 1H, Ha), 6.90 (t, J = 7.6 Hz, 1H, H3), 6.79 (t, J = 7.4 Hz, 1H, H4), 6.30 (d, J = 7.4 Hz, 1H, H5), 5.97 (d, J = 6.5 Hz, 1H, Hb). 13C NMR (100 MHz, DMSO-d6/D2O; 25:1; v/v; containing 0.25 mM NaOH): δ (ppm) = 174.4 (1C, Cf), 168.1 (1C, C11), 156.0 (1C, C12), 148.8 (1C, Cc), 148.3 (1C, C2), 147.5 (1C, Ca), 144.7 (1C, Ce), 137.4 (1C, C7/9), 131.8 (1C, C5), 129.7 (1C, C4), 128.5 (1C, C13), 124.7 (1C, C7/9), 123.0 (1C, C8), 120.9 (1C, C3), 119.4 (1C, C6), 117.9 (1C, Cd), 112.2 (1C, Cb). Synthesis of [4,7-Dihydroxy-1,10-phenanthroline-bis(2-(ptolyl)pyridine)iridium(III) Hexafluorophosphate], 2[PF6]. A mixture of [Ir(2-(p-Tolyl)pyridine)2Cl]2 (0.100 g, 0.089 mmol) and ligand L (0.047 g, 0.222 mmol) was stirred in ethylene glycol at 90 °C for 20 h in dark under N2. After completion of the reaction, the mixture was filtrated, and then an excess of solid KPF6 in water was added and stirred for 15 min at room temperature. A dark yellow solid which was further extracted using dichloromethane and methanol. The compound was then purified by silica gel column chromatography (DCM: MeOH = 95:5). Dark yellow solid compound 2[PF6] was isolated in 85% (0.067 g) yield. Melting point (MP): >300 °C. Anal. Calcd for C36H28F6N4O2PIr (Mw = 885.81): C, 48.81; H, 3.19; N, 6.32. Found: C, 48.85; H, 3.17; N, 6.32. FTIR in KBr disc (ν/ cm−1): 3213, 3040, 1609, 1565, 1477, 1357, 1016, 906, 824, 759, 539.; ESI-MS [C36H28IrN4O2]+: Calcd: m/z = 741.18. Found: m/z = 741.17. 1H NMR (400 MHz, DMSO-d6/D2O; 25:1; v/v; containing

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03044.

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All 1D and 2D NMR, ESI-MS, UV−vis and PL data, DFT calculation related files for 1[PF6] and 2[PF6], associated figures (PDF) Movie S1: Time-lapse video of DCP gass detection (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. ORCID

Snehadrinarayan Khatua: 0000-0003-0992-4800 Author Contributions †

S.K.S. and B.S. contributed equally.

H


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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by DST, India (No. SB/ FT/CS/115/2012). We thank Sophisticated Analytical and Instrumentation Facility (SAIF), North Eastern Hill University for NMR and ESI-MS data. Dr. B. Sarkar (NEHU), Dr. P. N. Chatterjee (NIT Meghalaya) and Dr. D. Samanta (JNCASR, Bangalore) are gratefully acknowledged for useful discussion. S.K.S and B.S thank RGNF and CSIR (SRF), India for their research fellowship. Also, we would like to thank the reviewers for their constructive comments and suggestions. The present work is dedicated to Dr. Sudhir Chandra Pal on the occasion of his 67th birthday.

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Organoiridium(III) Complexes as Luminescence Color Switching

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