An Efficient Probe of Cyclometallated Phosphorescent Iridium Complex for Selective Detection of Cyanide.

A cyclometallated phosphorescent iridium-based probe to detect CN- was prepared through a cyanide alcoholize reaction based on the C^N type main ligand and N^N type ancillary ligand (2-phenyl pyridine and 1,10-phenanthroline-5-carboxaldehyde, respectively). The efficient probe exhibited good sensitivity in response to CN- in an CH3CN and H2O (95/5) mixture within a 1.23 μM detection limit. The response of PL is directly in line with the concentration of CN- from 0 to 2.0 equiv. The PL investigation of other reactive anions proved the great selectivity to CN-. Additionally, upon adding 1.0 equiv. of cyanide, the formation of cyanohydrin was correctly elucidated in 1H NMR, FT-IR, and mass spectra studies. The conspicuous results indicate that the iridium complex has the potential possibility of application in other biosystems related to CN-.

Introduction

Cyclometallation is a studied reaction in coordination and organometallic chemistry used to synthesize organometallic complexes normally.[1−9] Recently, there is an extensive concern utilizing diversified transition-metal complexes as fluorescent molecular probes to selectively detect anions.[10−14] Cyclometallated iridium(III) complexes have exciting photophysical properties because of the listed key advantages: (i) the precursors, in which the main ligand and ancillary ligand are facilely synthesized with higher productivity; (ii) the high stability, visible excitation wavelengths, large Stokes shifts, long lifetimes, and lower self-quenching in contrast to traditional organic materials, which are applied in some latent areas such as luminescent devices, detecting probes, and chemosensors.[15−28] Therefore, studying the composition and adhibition of cyclometallated phosphorescent iridium(III) complex probes has a significant meaning in many areas.

Cyanide is a seriously toxic inorganic anion that is harmful to the human health and environment.[29,30] Despite its extreme toxicity, chemical compounds including cyanide were extensively applied in gold mining, electroplating technology, and multiple diverse industrial areas, they yet bring the unavoidable environmental issues.[31,32] Cyanide’s particular affinity can be applied in the design of fluorescent probes that have the electron-deficient carbonyl unit (C=O), finally forming cyanohydrin.[33] Consequently, many works were performed to discover specifically susceptive probes on the basis of iridium(III) complexes to detect and sense cyanide and apply in the lighting area.[34] For example, in a previous study, based on the Michael addition reaction and FRET mechanism, a battery type of “turn-on” probe was investigated and adapted to dual channels to test cyanide anions.[35]

These specific factors have aroused our interest to develop the electron-deficient carbonyl unit iridium(III) complex probe to be applied in the rapid response and high-sensitivity sensing of CN–. We herein report a cyclometallated phosphorescent iridium complex probe based on 2-phenyl pyridine as the C^N type main ligand and 1,10-phenanthroline-5-carboxaldehyde as the N^N type ancillary ligand, which is widely used in the selective detection of cyanide. The simple model of ligands assists us to elucidate the effects of the formation of cyanohydrin.[36−40] The response of PL is directly in line with the concentration of CN– from 0 to 2.0 equiv. In addition, the great plane geometry of ligands is conducive to the geminated of single crystals of the phosphorescent iridium complex. The primary research shows that the complex can act as a probe for cyanide detection in great sensitivity and selectivity.

Results and Discussion

Synthesis of the Iridium Complex

The target product was synthesized based on the C^N type main ligand and N^N type ancillary ligand (2-phenyl pyridine and 1,10-phenanthroline-5-carboxaldehyde) (Scheme ). The target iridium complex was confirmed by 1H NMR, 13C NMR spectra, and molecular structure represented in X-ray diffraction analysis, demonstrating that the target complex is successfully synthesized. In comparison to the iridium complex, after adding the CN–, there was a remarkable shift disappearance at δ 10.56 ppm belonging to the aldehyde proton, suggesting the occurrence of the cyanide alcoholize reaction at the aldehyde group. Meanwhile, a new peak at δ 8.60 ppm appeared, belonging to the formation of the cyanohydrin proton signal (−CH(CN)OH). In FT-IR spectra studies, a characteristic absorption band in the 1701 cm–1 of the iridium complex was referred to the remarkable vibration of the C=O bond. Meanwhile, the C=O absorption band unsurprisingly disappeared after adding the CN–. This is consistent with the formation of cyanohydrin through the cyanide alcoholize reaction.

Scheme 1

Synthesis of the Iridium Complex

Reaction conditions: (i) 2-ethoxyethanol/water (v/v = 3:1) mixture, 120 °C, 24 h; (ii) DCM and CH3OH (1/1) mixture, rt, 24 h. The detailed descriptions of experiments are in the Experimental Section.

Crystal Structure of the Iridium Complex

To elucidate the plane structure of the cyclometallated iridium(III) complex, the single-crystal X-ray analysis was employed. Through the method of addition of n-hexane in the saturated dichloromethane complex solution, the single crystal was obtained for X-ray diffraction analysis. The iridium complex crystallized in the orthorhombic space group Pna21 (Figure ). The crystallographic data and the structure refinement parameters are collected in Table . Partly bond lengths (Å), bond angles (°), and torsion angles (°) are collected in Table . The structure shows a typical distorted-octahedral geometry environment within the iridium(III) center coordinated by chelating the cyclometallated 2-phenyl pyridine ligand and C and N atoms and ancillary ligand. The formation of the cyclometallated iridium complex makes it have the higher air and thermal stability. The bond lengths of Ir–C, in the range of 1.987 (11) to 2.015 (12) Å, and Ir–N of the complex, in the range of 2.024 (9) to 2.157 (9) Å, are both within the value at the foregone scope of the semblable complex.[41] Compared to the Ir–N, the Ir–C bond length is shorter, revealing the forceful interaction in the metal Ir center and C atom. Within the complex, the pyridine rings of the cyclometallated ligand are approximately coplanar with a dihedral angle of 8.7°. Noncovalent intermolecular interaction in the crystal stacking structure of the complex is observed.

Figure 1

Plane structure of the cyclometallated iridium(III) complex with thermal ellipsoids drawn at the 30% level. All hydrogen atoms are omitted for clarity. Partly bond lengths (Å) and angles (°): Complex: Ir(1)–N(1), 2.036(9); Ir(1)–C(1), 2.023(10); Ir(1)–C(12), 1.987(11); Ir(1)–N(2), 2.024(9); Ir(1)–N(3), 2.150(9); Ir(1)–N(4), 2.146(9); C(12)–Ir(1)–C(1), 89.9(4); C(12)–Ir(1)–N(2), 94.4(4); N(2)–Ir(1)–N(1), 172.5(3); C(12)–Ir(1)–N(4), 173.5(4); C(1)–Ir(1)–N(4), 96.1(4).

Table 1

Crystallographic Data and Structure Refinement Parameters for the Complex

	complex
chemical formula	C38H31F6IrN4OP
FW	896.84
T (K)	173(2)
λ (Å)	1.34138
crystal system	orthorhombic
space group	Pna21
a (Å)	23.9369(6)
b (Å)	9.0172(2)
c (Å)	33.4311(8)
α (°)	90
β (°)	90
γ (°)	90
V (Å3)	7215.9(3)
Z	8
ρ (Mg m–3)	1.651
μ (mm–1)	5.404
F(000)	3528
θ range (°)	3.212–53.021
reflections collected	40,569
data/restraints/param.	12,391/15/921
goodness-of-fit on F2	1.014
final R indices [I > 2σ(I)a]	R1 = 0.0393 wR2 = 0.0933
largest diff. peak/hole (e Å–3)	0.799/–0.744

R1 = Σ||F0| – |Fc||/Σ|F0| (based on reflections with F02 > 2σF2). wR2 = [Σ[w(F02 – Fc2)2]/Σ[w(F02)21/2; w = 1/[σ2(F02) + (0.095P)2]; P = [max(F02, 0) + 2Fc2]/3 (also with F02 > 2σF2).[45]

Table 2

Partly Bond Lengths (Å), Bond Angles (°), and Torsion Angles (°) for the Complex

bond lengths
Ir(1)–N(1)	2.036(9)	Ir(1)–N(6)	2.059(9)
Ir(1)–N(2)	2.024(9)	C(7)–C(8)	1.397(16)
C(2)–C(3)	1.389(16)	N(1)–C(7)	1.376(14)
Ir(1)–C(1)	2.023(10)	O(1)–C(35)	1.221(14)
N(3)–C(27)	1.381(14)	Ir(1)–C(12)	1.987(11)
bond angles
C(12)–Ir(1)–C(1)	89.9(4)	C(12)–Ir(1)–N(2)	80.3(4)
C(1)–Ir(1)–N(1)	80.5(4)	N(2)–Ir(1)–N(4)	96.8(3)
N(5)–Ir(2)–N(7)	97.0(4)	C(7)–N(1)–Ir(1)	115.5(7)
C(18)–N(2)–Ir(1)	116.9(7)	C(28)–N(4)–C(32)	118.7(10)
C(4)–C(5)–C(6)	120.7(12)	N(1)–C(7)–C(8)	118.2(11)
C(9)–C(8)–C(7)	121.6(12)	N(1)–C(11)–C(10)	122.0(11)

Detection of CN– in Aqueous Acetonitrile Solution

First, we studied the PL response of the probe to CN– including variable proportion of acetonitrile and water, and detailed information is attached in (Figure ). The probe displayed the most obvious fluorescence enhancement in the CH3CN and H2O (95/5) mixture, while the fluorescence response to the probe increased with increasing proportion of water in the mixture solution and decreased after reaching the maximum value. Owing to the obvious change, we described in detail the conditions of the probe in the CH3CN and H2O (95/5) mixture.

Figure 2

(A) Fluorescence intensity variation of the probe (20 μM) and the probe (20 μM) in the presence of 20 μM CN– in 0, 1, 3, 5, 10, 50, and 99% acetonitrile at 298 K (λex = 380 nm). The green columns show the intensity of the probe. The red columns show the intensity of adding CN–. (B) Changes of the fluorescence intensity at 570 nm of the probe with [CN–] (0–2.0 equiv.) in 0, 1, 3, 5, 10, 50, and 99% aqueous acetonitrile.

Detection of CN– by the Probe in UV–vis and PL Emission Spectra

The iridium complex displays remarkable color variation in the presence of CN–. The UV–vis absorption and phosphorescence titration spectra of the iridium complex were explored. Reaction condition optimization of the probe (20 μM) was performed by using the incremental addition of CN– (0–2.0 equiv.) as the model substrate in an CH3CN and H2O (95/5) mixture at ambient temperature. From the UV–vis absorption spectra (Figure ), the strong absorption at 267 nm was ascribed to the singlet transitions, and a main absorption band at 380 nm was attributed to a metal ligand charge transfer (MLCT). Upon the addition of CN–, the absorption peak at 380 nm gradually emerged with a slight increase from the original absorption. This phenomenon is reasonable because cyanide can strongly attack the electron-deficient carbonyl unit (C=O) to form cyanohydrin. Therefore, this reaction would affect the state of MLCT, which resulted in the changes of emission properties.

Figure 3

(A) UV–vis titration spectra of the probe (20 μM) in the presence of CN– (0–2.0 equiv.) in an CH3CN and H2O (95/5) mixture at 298 K. Inset: changes of the absorbance ratio between 380 and 267 nm of the probe with [CN–]. (B) Linear calibration curve of A380/A267 vs CN–.

The fluorescence titration experiments for CN– were as well used in the same condition of 20 μM solution of probe in CH3CN/H2O (v/v = 95:5). In the PL emission titration experiment, the probe exhibits weak fluorescence (ΦPL = 6.6%) in the absence of CN– at an excitation of 380 nm, referring to aldehyde quenching through photoinduced electron transfer (PET).[42] After adding CN– (0–2.0 equiv.), the probe displayed an obvious fluorescence enhancement with increasing concentrations of CN– at 570 nm. The fluorescence quantum yield (Φ) of the probe was increased to 28.0% in the presence of CN– (1.0 equiv.), while the fluorescence intensity reached its maximum at 2 equiv. of CN–, with a 15-fold enhancement. Moreover, once the CN– was added, the color of probe solution turned from pale yellow to bright orange under a 365 nm UV lamp. The phenomenon indicated that the probe could easily detect CN– by the “naked eye”. Notably, the fluorescence response is directly in line with the concentration of CN– in the range of 0 to 40 μM, in which I570 = 95.57CCN + 88.80 (the unit of C is μM, R2 = 0.984) (Figure ), revealing that the probe can be used in the quantitative testing of CN–. Furthermore, the calculated detection limit was 1.23 μM (S/N = 3, N = 10),[43] significantly lower than the allowable limit (1.9 mM) set by WHO. The detection limits of different types of probes are listed in Table .

Figure 4

(A) PL titration experiments of the probe (20 μM) with CN– (0–2.0 equiv.) in an CH3CN and H2O (95/5) mixture at 298 K (λex = 380 nm). Inset: changes of the intensity at 570 nm of the probe with [CN–] and color change from pale yellow to bright orange upon addition of 0–2.0 equiv. of CN–, under a 365 nm handheld UV lamp. (B) Linear calibration curve of I570 vs CN– (0–40 μM).

Table 3

Listed Detection Limits of CN– for Diverse Types of Probes

compound	detection limit (μM)
Azo-1[44]	1.10
S1[45]	3.60
2[46]	1.41
probe[47]	1.70
GSB48	0.88
Probe 149	0.67
ESIPT50	1.60
probe51	5.00
probe (this work)	1.23

Fluorescence Quantum Efficiency

In order to enable both the probe and reference material to be compared in the same situation that all the substances have parallel absorption, we chose 9,10-diphenylanthracene as the method of comparison at 380 nm excitation light to measure the fluorescence quantum efficiency. Photophysical characterization data for the probe are listed in Table . The high quantum efficiency showed that the complex has excellent fluorescence properties and has potential applications in biosystems.

Table 4

Photophysical Characterization Data for the Probe

compounda	quantum yieldb	λex (nm)	λem (nm)	UV–vis absorption λabs (nm)	DRc
probe	0.066	380	570	267, 380	4.24
probe + CN–	0.28	380	570	267, 380

20 μM probe in CH3CN/H2O (v/v = 95:5) at 298 K.

Relative fluorescence quantum yield (9,10-iphenylanthracene; Φ = 0.9).

Dynamic range: DR = Φ/Φ0; Φ and Φ0 are the fluorescence quantum yields of the probe-CN– and probe-free.

Effect of Reaction Time

Inspired by the excellent results of the probe to detect CN–, we began to turn the observation to the measurement of reaction time. The optimization results are shown in Figure . When the CN– was not added to the system, the PL intensity of the probe remained unchanged (curve a). When added with varying concentrations of CN–, the response time became orderly accelerated (curves b, c, and d). When added with a 1.0 equiv. concentration of CN–, the PL response increased from 20 to 120 s and eventually attained its maximum at 130 s (curve e), indicating that the probe has a great timely response to CN–.

Figure 5

Reaction process of PL intensity of the probe (20 μM) before and after adding varying concentrations of CN– (a: blank, b: 0.2 equiv., c: 0.5 equiv., d: 0.8 equiv., e: 1.0 equiv. in an CH3CN and H2O (95/5) mixture at 298 K (λex = 380 nm).

Selectivity of the Probe for CN–

To estimate the selectivity conditions of the probe for CN–, a range of possible competitive anions, like Br–, Cl–, I–, NO3–, OAc–, CO32–, NO2–, SO42–, S2O32–, H2PO4–, OH–, SCN–, SO32–, HSO3–, S2–, Cys, HS–, and N3–, were also examined. Even at a higher concentration (100 μM), there was no significant emission intensity change after adding a range of competitive anions covering more nucleophilic ones like SCN–. Moreover, after the cyanide was added, the color change was observed by the naked eye (Figure ). A remarkable fluorescence enhancement was observed after addition of 40 μM CN– (Figure ). These results suggested that the phosphorescence changes of the probes were selectively induced by CN–, which could be used to detect CN– without interference from another biological anion.

Figure 6

PL intensity of the probe (20 μM) with CN– (40 μM) and other anions (100 μM) in an CH3CN and H2O (95/5) mixture at 298 K (λex = 380 nm). Inset: photograph showing the corresponding visual fluorescence change of the probe solution for CN– and other anions, under a 365 nm handheld UV lamp.

Figure 7

(A) Fluorescence intensity changes of the probe (20 μM) in the presence of 40 μM CN– and 100 μM other anions (Br–, Cl–, I–, NO3–, OAc–, CO32–, NO2–, SO42–, S2O32–, H2PO4–, OH–, SCN–, SO32–, HSO3–, S2–, Cys, HS–, and N3–) in CH3CN/H2O (v/v = 95:5) at 298 K (λex = 380 nm). (B) The blue columns show the emission of the probe to other anions. The red columns show emission of adding CN–.

Recognition Mechanism of the Probe for CN–

To elucidate the possible PL increasing mechanism, the reaction of the probe with cyanide was investigated. The probe and the probe-CN– adduct (in dry DMSO-d6) were dissolved in an NMR tube and then characterized at 25 °C by 1H NMR. After adding 1.0 equiv. of cyanide, there was a remarkable shift disappearance at δ 10.56 ppm belonging to the aldehyde proton (H*) signal, suggesting the occurrence of the cyanide alcoholize reaction at the aldehyde group. Meanwhile, a new peak at δ 8.60 ppm appeared, belonging to the formation of the cyanohydrin proton signal CH(CN)OH(H*). The aldehyde phenanthroline ring proton peaks of the cyclometallated ligand are shifted upfield. The signals for a (9.76 ppm) and c (9.07 ppm) were shifted to 9.869 and 8.529 ppm, respectively; at the same time, the signal for b (9.13 ppm) gradually disappeared and together with d (8.32 ppm) and e (8.25 ppm) to emerge a new signal at 8.17 ppm. In theory, the conjugation would be reduced after the formation of cyanohydrin. Additionally, the signals of phenyl pyridine aromatic protons did not change basically (Figure ). The mechanism makes clear that one new substance (CH(CN)OH) was created in the reaction.

Figure 8

1H NMR mechanism of the cyanohydrin species formation. Inset: spectra showing the probe and adding 1.0 equiv. of CN– in DMSO-d6. The asterisks represent the specific proton signals.

The probable formation mechanism of the cyanohydrin substance has been lucubrated based on the above experimental findings and former reports. Upon adding 1.0 equiv. of CN–, aldehyde reacted with cyanide to give the cyanohydrin. The process incorporates three main steps; the electron properties of the aldehyde obtained from the probe make it susceptible to CN– attack on the carbonyl group by forming cyanohydrin. The studies of a similar process were reported previously (Figure ). Moreover, the data for ESI-HRMS spectra were coincident with 1H NMR spectra. The mass spectra clearly displayed the characteristic peak of resultant cyanohydrin in a 1:1 molar ratio of probe to CN– (Figure S4).

Figure 9

Possible mechanism for the formation process of cyanohydrin.

DFT Theoretical Studies

To further understand the optical physical properties of the complex, density functional theory (DFT) was employed to study the theoretical electronic structures. The molecular orbital chart of the complex is provided in Figure . The highest occupied molecular orbital (HOMO) resides principally at the Ir center and the pyridyl part of the cyclometallated ligand, while the lowest unoccupied molecular orbital (LUMO) is essentially located in the 1,10-phenanthroline-5-carboxaldehyde ancillary ligand because of the electron acceptability of substituents and conjugation of the aldehyde group to the phenyl portion. In addition, the energy gap between HOMO and LUMO is 3.01 eV; the partial molecular orbital energy values of the complex are summarized in Table .

Figure 10

LUMO and HOMO distribution of the complex.

Table 5

Partial Molecular Orbital Energy of the Complex

orbital	HOMO – 2	HOMO – 1	HOMO	LUMO	LUMO + 1	LUMO + 2
energy (eV)	–8.9812	–8.5008	–8.3299	–5.3199	–5.0689	–3.9819

Fluorescence Imaging of CN– in Living Cells

Besides, we made an inquiry about the ability of the probe to monitor CN– within living cells. Before the research of fluorescence cell imaging, the cytotoxicity of the probe was tested in A549 cells. Cells were dipped to an increasingly proportion of every probe for 24 h, and cell viability was measured through the MTT assay. Results exhibited small cytotoxicity until a concentration of 60 μM incubated for 24 h (Figure ). Moreover, the cytotoxicity of the probe even at a higher concentration of 100 μM incubated for 8 h was studied. The cell viabilities of the probe were mainly over 80% within 100 μM concentration (Figure ). These results made clear that the cytotoxicity of the probe in the concentration scope is negligible and was sufficiently appraised for their potential biological imaging applications. After being incubated with a 10 μM probe for 20 h at 37 °C, as shown in Figure A, the cells dipped with the probe emerged with feeble PL, indicating that the probe had the ability to permeate the cell membrane. Cells were dipped in 10 μM CN– for 0.5 h; after being incubated in the 10 μM probe for 20 h at 37 °C, the cells had an intense PL emission compared to no added cells (Figure B). DIC imaging of the probe expounded that the cells were alive in the whole experiment process. These results suggest that the probe has cellular permeability and could be employed to imaging CN– in living cells.

Figure 11

Cytotoxicity of the probe; (a) variable proportion of the probe for 24 h; (b) 100 μM probe for different times.

Figure 12

Confocal fluorescence imaging for A549 cells. (a) Fluorescence images; (b) DIC images of cells shown in panel (a); (c) overlay images. (A) A549 cells were incubated in the 10 μM probe for 20 h at 37 °C. (B) A549 cells were incubated for 0.5 h with 10 μM CN– and then incubated for 20 h in the 10 μM probe at 37 °C (λex = 405 nm, solvent: DMSO/H2O (v/v 1/1000)).

Detection of CN– in Test Strips of Practical Samples

Inspired by the admirable properties of the complex to detect CN–, we developed test strips to apply for the signaling of CN– of practical samples. Test strips (2.0 cm × 2.0 cm) were impregnated with the probe (1 mM in CH3CN/H2O (v/v = 95:5)), affording a filter-paper-based test strip for the sensing of CN–. A drop of (10 μL) variable proportion of CN– (c = 0.5 to 2.0 mM) was placed in probe-impregnated test strips. As displayed in Figure , spots of pronounced colors were observed that could be easily discernible to the naked eye and under 365 nm UV excitation. This observation implied that the probe could be used as a preliminary sensing kit by variable proportion of CN– in practical samples and demonstrated the pragmatic application value of the complex infiltrated test strips for in situ immediate cyanide visibility detection.

Figure 13

Photos of the test strip image under (A) daylight and (B) 365 nm UV excitation of the probe with varying proportions of CN– (c = 0 to 2.0 mM) in CH3CN/H2O (v/v = 95:5).

Conclusions

In conclusion, we described a new type of probe based on the iridium complex for the detection of cyanide. The probe exhibited good sensitivity to CN– in an CH3CN and H2O (95/5) mixture within the 1.23 μM detection limit and timely response. The response of PL is immediately in line with the concentration of CN– from 0 to 2.0 equiv. The PL investigation of other reactive anions proved the great selectivity of the probe for CN–. In addition, upon adding 1.0 equiv. of cyanide, the formation of cyanohydrin was exactly illuminated by 1H NMR, FT-IR, and mass spectral studies. The conspicuous results indicate that the iridium complex has the potential possibility of application in other biosystems related to CN–.

Experimental Section

General Data

All operations were carried out in standard Schlenk techniques under the nitrogen atmosphere. Chemicals were used as commercial products without further purification. The precursor salt of CN– was tetra-n-butylammonium cyanide ([CH3(CH2)3]4N+CN–). 1H NMR and 13C NMR (500 MHz) spectra were measured in a Bruker DMX-500 spectrometer. UV–vis absorption spectra and fluorescence spectra were measured in a UV 765 spectrophotometer and a Hitachi F-4600, respectively. FT-IR spectra were recorded on a Nicolet FT-IR spectrophotometer. [(C^N)2Ir(μ-Cl)]2 was synthetized based on the literature.[44]

Synthesis of the Iridium(III) Dimer Complex [(C^N)2Ir(μ-Cl)]2

The mixture of 2-phenyl pyridine (2 mmol, 2 equiv.) and IrCl3·3H2O (1 mmol) in 2-ethoxyethanol/water (3/1) mixture (12 mL) in a 50 mL Schlenk tube under a nitrogen atmosphere was heated for 24 h at 120 °C. After cooling, a bright yellow precipitate was filtered and washed with water, ethanol, and n-hexane. The crude product was used in the next reaction without further purification in good yields.

Synthesis of the Iridium Complex

A mixture of the iridium(III) dimer complex [(C^N)2Ir(μ-Cl)]2 (0.1 mmol) and 1,10-phenanthroline-5-carboxaldehyde (0.22 mmol) was stirred at room temperature in 10 mL of dichloromethane and methanol (1/1) mixture overnight under a nitrogen atmosphere. After that, an excess of KPF6 (0.6 mmol) was stirred for 1 h. The crude product was purified by silica gel column chromatography (CH2Cl2:methanol = 30:1) to get the orange iridium complex 1 in moderate yield. 1: orange solid, 78% yield. 1H NMR (500 MHz, DMSO-d6): δ 10.59 (s, 1H), 9.77 (d, J = 7.6 Hz, 1H), 9.15 (s, 1H), 9.10 (d, J = 7.3 Hz, 1H), 8.33 (d, J = 5.0 Hz, 1H), 8.26 (d, J = 8.2 Hz, 3H), 8.15 (dd, J = 8.1, 5.2 Hz, 2H), 7.96 (d, J = 7.6 Hz, 2H), 7.88 (t, J = 7.7 Hz, 2H), 7.50 (dd, J = 12.3, 5.6 Hz, 2H), 7.07 (t, J = 7.2 Hz, 2H), 7.00–6.93 (m, 4H), 6.28 (dd, J = 6.8, 4.5 Hz, 2H). 13C NMR (100 MHz, DMSO-d6): 193.42, 167.24, 153.43, 151.64, 149.82, 148.52, 144.47, 140.81, 139.24, 136.34, 131.71, 130.73, 130.27, 128.68, 128.46, 125.56, 124.33, 122.95, 120.46. Elemental analysis calcd for C35H24F6IrN4OP: C 49.24, H 2.83, N 6.56; found: C 49.31, H 2.75, N 6.52.

FT-IR Spectra

A solution containing 1.0 equiv. of CN– was dropped to a 20 μM solution of iridium complex in CH3CN and was utilized for the FT-IR measurements.

X-ray Crystal

A Bruker Smart APEX CCD diffractometer was used to extract information on the complex by graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Data were recorded at ambient temperature, and the direct methods were applied for the structure, which was optimized in F2 through SHELXL.[45] Absorption correction was performed using SADABS.[46] All non-hydrogen atoms were refined anisotropically. The positions of hydrogen atoms were computed. The Bruker program Smart was used for all calculations.

Theoretical Calculations

The computational method was applied using the Gaussian 03 program package on account of density functional theory (DFT). The B3LYP was adopted for the optimization of the complex structure. The LANL2DZ and 3-21G* basis sets were adopted to analyze the Ir atom and the rest of the atoms.[47] In order to comprehend the properties of the excited state, the contours of the HOMO and LUMO orbitals were plotted.

Cell Culture

The A549 cells were placed in glass Petri dishes in a density of 80,000 cells per dish and then cultured in the RPMI Medium 1640 replenished with 10% FBS, 100 units/mL penicillin, and 100 μg/mL streptomycin for 20 h at 37 °C in CO2/air (5:95). The probe (10 μM) was appended to the unit milieu and preincubated in DMSO/H2O (v/v = 1:1000) for 4 h, and then tetrabutylammonium cyanide (10 μM) was added and incubated for 0.5 h. After incubation for the corresponding time, the cells were washed with PBS to wipe off the dissociative compound and ions before analysis. Fluorescence imaging was executed by a confocal microscope (LSM 880). For the probe channel, the excited wavelength was 405 nm, while the emission wavelength was collected in the range of 550–650 nm.