Application of five-membered ring products of cyclometalation reactions as sensing materials in sensing devices

Journal of Organometallic Chemistry 823 (2016) 50e75

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Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem

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Application of five-membered ring products of cyclometalation reactions as sensing materials in sensing devices Iwao Omae Omae Research Laboratories, 335-23 Mizuno, Sayama, Saitama, 350-1317, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 June 2016 Received in revised form 4 September 2016 Accepted 12 September 2016 Available online 14 September 2016

Some organometallic compounds, especially five-membered ring products of cyclometalation reactions, can be utilized as sensing materials in sensors for detecting materials of interest, such as gases; metal ions; halogens; cyanic compounds; sugars; glycoproteins, including lectins and avidins; explosives; and DNA. The compounds used for the detection of the presence of materials are often cyclometalated fivemembered ring compounds. These compounds comprise metallic elements, mostly iridium and platinum compounds, and substrate moieties, such as 2-phenylpyridines, 2-phenylbenzothazoles and 2phenylqunolines. Commonly used ancillary ligands include acetyl acetonate (acac), bipyridines and 1,10-phenanthrolines. Detection by sensors is performed by watching the color changes of the emissions and the increased emission intensity or by quenching emission intensity of the compounds used for detection. The reactions of the cyclometalated compounds with the materials to be detected are usually reversible, so the sensors can be repeatedly used for detection. © 2016 Elsevier B.V. All rights reserved.

Keywords: Sensor Cyclometalation Five-membered ring Photoluminescence Phenylpyridine

Contents 1. 2.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 2.2. Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 2.3. Sulfur dioxide gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 2.4. Other gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Metal ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 3.2. Hg2þ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 3.3. Zn2þ and other metal ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Halogen anions, CN, CH3COO and other compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 4.2. Halogen anions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 4.3. CN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 4.4. CH3COO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 4.5. Other compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Sugar, glycoproteins and amino acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 5.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 5.2. Sugar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 5.3. Glycoproteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 5.4. Amino acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 6.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

E-mail address: [email protected]. http://dx.doi.org/10.1016/j.jorganchem.2016.09.008 0022-328X/© 2016 Elsevier B.V. All rights reserved.

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6.2. Explosives and surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 6.3. Free radicals, pH and temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 6.4. DNA and cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

1. Introduction When cyclometalation reactions are performed using metal compounds with substrates that contain heteroatoms with a strong donor ability, such as amines, phosphines, alcohols and sulfur compounds, the reactions show a high reactivity of the substrate to the metal compounds because compounds with these heteroatoms are usually used as catalysts. Therefore, many articles, reviews [1e55] and books [56e61] on this subject have been published since the 1960s, and cyclometalation reactions are considered representative reactions in organic synthesis. Commonly, cyclometalation reactions with conventional substrates that have a g-carbon atom as the heteroatom proceed very easily via agostic interactions with CH activation, as shown in Eq. (1.1) [49]. Then, the metal atom is coordinated by the heteroatom and becomes the active center in the five-membered ring product [62]. Many compounds easily afford derivatives from reactions such as alkylation, alkenylation, alkynylation, carbonylation, isocyanation, halogenation, chiral reactions with amino acids, enantioselective rearrangements, asymmetric Diels-Alder reactions and dehydrogenation [45,47,61].

Various five-membered compounds obtained by the cyclometalation reactions are synthesized with all types of combinations of substrates with heteroatoms, conventional substrates, unconventional substrates, ancillary ligands, metal atoms and metal compounds, wherein the metals are coordinated by a heteroatom and many ligand groups, and the compounds are used as catalysts [45,47,61] and in the fields of OLEDs [52], pharmaceuticals [50], CO2 utilization [53], dye sensitizers for solar cells [54], sensors, and other applications [61]. These five-membered ring products of cyclometalation reactions are also used as sensors, as already briefly described in a previous monograph [61]. Some of these compounds are used as organic light-emitting diodes (OLEDs). In comparison with liquid crystal displays (LCDs) and plasm a displays (PDs), OLEDs display exhibit better characteristics, such as low power consumption, fast response time, wide color range, better contrast, better viewing angles, light weight, flexibility, transparency and durability [52]. OLEDs are required to exhibit high photoluminescent quantum

yields and to have short lifetimes for the displays of animation. Therefore, iridium compounds are commonly used for OLEDs. In the OLED applications of five-membered ring compounds obtained by utilizing cyclometalation reactions, iridium compounds are typically used because they possess the following four characteristics [52]: 1. They are chemically, electrically and thermally stable. 2. They emit highly efficient phosphorescence at room temperature. 3. Their phosphorescent emissions last for only a short time. 4. Many phosphorescent compounds with red to blue colors can be relatively easily synthesized using various types of substrates and ancillary ligands. However, not only iridium compounds but also platinum compounds, which exhibit a longer lifetime, may be used for the application of these products as sensors because the sensors are the object of detection. Other metal compounds such as Pd, Ni, Au and B compounds are used for the sensors. Polymer films such as polystyrene and PMMA may be used for protecting the compounds to provide high sensor durability.

The substrates of detection used for producing sensor compounds detecting metal compounds are mainly 2-phenylpyridines 1.1, 2-phenylbenzothiasoles 1.2 and 2-phenylqunolines 1.3, and the ancillary ligands are mainly acetylacetonate (acac) 1.4, bipyridines (bpy) 1.5 and 1,10-phenanthroline 1.6., as shown in Fig. 1. Many detection methods based on these sensors utilize the change in emission colors of the sensors. For example, the detection of sulfur oxide utilizes the color change of the emission of the sensor from colorless to orange or from green to blue. Other detections utilize the quenching of the emission from the sensor materials, and sometimes, on the contrary, the emission is highly enhanced by the detected objects. Sometimes the absorption spectra or emission lifetimes also change. This article presents the following materials to be detected: 1. Gases such as oxygen, sulfur oxide, carbon monoxide and carbon dioxide.

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M = Ir, Pt Fig. 1. Representative substrates and ancillary ligands in the sensors.

2. Metal ions such as Hg2þ, Zn2þ, Cr3þ, Cu2þ, Ca2þ, Pb2þ, alkali and alkali earth metal ions. 3. Halogen anions, CN, CH3COO and other compounds. 4. Sugars, glycoproteins such as lectins and avidins, amino acids and other compounds such as BrCN, CH3CH and CH2Cl2. 5. Other targets, such as explosives, surfactants, free radicals, pH, temperature, DNA and cells. 2. Gases 2.1. Introduction Gases detected by sensors are mainly oxygen [63e83] and other dangerous gases such as sulfur oxide [84e93], carbon monoxide [94], and carbon dioxide [95]. 2.2. Oxygen Substrates for producing oxygen sensors are mainly 2phenypyridines [63e72,74e76,79e82], although 2phenylbenzothiazoles, etc., can also be utilized [77,78]. The utilized metals are both iridium and platinum, except for one case of gold, as shown in Figs. 2e4. Some photophysical and electrochemical properties are shown in Tables 1e3. For example, fac-tris(2-phenylpyridine)iridium 2.2.1 is a representative green color-emitting compound used in OLEDs [52]. This compound was also used as the oxygen sensor in a poly((n-butylamine)thionylphosphazene matrix. This phosphorescent dye exhibits a long lifetime and high photoluminescent quantum yields (PLQYs) in degassed toluene solution and when included in the polymer matrix. Compound 2.2.1 is 2.7 times and 5 times more sensitive to quenching caused by oxygen than a widely used porphine oxygen sensor (PtOEP) and a chelate ruthenium tris(diphenylphenanthroine chloride ([Ru(ddp)3]Cl2) sensor, respectively [63]. The fluorophenyl substituent at the para position of the 2phenylpridine 2.2.2a provided higher oxygen sensitivity than that at the meta position 2.2.2b. Oxygen sensing films show excellent operational stability in 4000 s saturation O2/N2 cycles that meets

the requirement of monitoring molecular oxygen in real time [64]. In trifluoromethyl-substituted platinum compounds 2.2.3a2.2.3c, the electron-donating 4-methoxyphenyl-containing compound 2.3.3b provides the most sensitive film (KappSV 1 ¼ 0.1251 Torr1). All polystyrene-immobilized sensing materials are sensitive to O2 in different concentrations. The optical signals of oxygen sensing show a high operational stability in 13e16 cycles in 4000 s. An extremely fast response time and recovery time of oxygen sensing films were obtained in 4.0 s and 6.0 s, respectively [68]. Bis(2-phenyl-5-methylpyridine)platinum 4,4’-dimethoxy-2,2’bipyridine 2.2.4, with its long-lived triplet states (lifetimes as long as 260 ms), demonstrates the highest sensitivity to oxygen quenching, with KSV values up to 2.12 mbar -1 (1.59 Torr1) [69]. 2-(4-Diphenylaminophenyl)pyridine platinum compounds 2.2.5a-g exhibited intense phosphorescence emissions at room temperature. The oxygen sensitivity of the compounds was quantitatively evaluated in polymer film. The results of the O2-sensing sensitivity of the Pt(II) compounds 2.2.5g demonstrate that the compounds with electron-withdrawing cyano ligands exhibited the highest sensitivity (KappSV ¼ 0.102 Torr1) [70]. 2-Phenylpridine iridium compounds with electronwithdrawing trifluoromethyl ligands (2.2.6a-d) exhibit high photoluminescence quantum yields, high oxidation potential and good photostability against continuous irradiation. The best polymer matrix for oxygen sensing is ethyl cellulose, and the optimum loading level of the compound is 0.5 mg/g, given the widespread demand for oxygen-sensing devices with high reliability and longterm stability [74]. In 2-(4,6-difluorophenyl)pyridine iridium bipyridine 1-3(oligo(para-phenylene)) compounds 2.2.7a-d, the presence of dioxygen reduces the lifetime dramatically, suggesting their potential use as oxygen sensors because the lifetime can vary from nano- to several microseconds, and their properties strongly depend on the number of phenylene units attached on the bipyridine [75]. The 2-(4,6-difluorophenyl)pyridine iridium picolate derivatives 2.2.8a-c were physically immobilized into a nanostructure metal oxide matrix, AP200/19, and a polystyrene membrane that is totally quenched for oxygen concentration over 10%. In particular, the compounds supported in the AP200/19 provide the best sensitivity to oxygen concentration, with the possibility of detecting oxygen levels below 1% in gas (0.01 bar) [80]. 2-Phenylpyridine iridium 2(4,4’-bis(2-(4-N,N-methylhexylaminophenyl)ethyl)-2,2’-bipyridine 2.2.9 is incorporated in a polystyrene and nanostructured metal oxide support. Compound 2.2.9 exhibits a high quantum yield (58%) and an extremely long phosphorescence lifetime (102 ms). The sensing film shows longterm stability (up to 12 months), the completely reversibility of the signal quenched by oxygen and a quick response time to various oxygen concentrations [81]. In 2-phenylpyridine iridium pyridine derivatives 2.10a-c, the polydimethylsiloxane derivative 2.10b is blended with polystyrene to improve the physical properties of the sensor film. The blend sensors exhibit increased sensitivity relative to films of 2.10b alone and maintained short response times to rapid changes in air pressure [65]. 2-Phenylpyridine platinum N-butylnaphthalimide 2.2.11b, with disrupted p-conjugation between the naphthalimide and 2-phenyl subunits, shows emissions at a much shorter wavelength

1 KappSV The oxygen-sensing properties were evaluated by the reduction in emission intensity when different quencher (oxygen) concentrations are present. The relationship between the fluorescence (luminescence) intensity or lifetime of the photo-excited triplet state of oxygen-sensitive probes and the partial pressure of oxygen is reflected by the Stern-Volmer equation [68].

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Fig. 2. 2-Pheylpyridine iridium and platinum compounds for oxygen sensors.

(lnm ¼ 538 nm). The drastically red-shifted emission of 2.2.11b was rationalized by the DF/TDDFT calculations. The luminescent lifetimes of the complexes were tuned from 0.86 ms (2.2.11c) to 25.5 ms (2.2.11b), and the luminescent O2 sensing can correspondingly be improved 177-fold [72]. Both 2.2.12a and 2.2.12b have an electron-withdrawing a-diketo moiety attached to a 2-phenylpyridine (ppy) ligand, while 2.2.12c, with an electron-donating carbazole substituent, shows an extended phosphorescence lifetime (t ¼ 12.4 ms), enhanced phosphorescence quantum yield (PLQY ¼ 66), and the highest quenching constant (KappKV ¼ 0.0238 Torr1) [76]. The other 2-phenylpyridine metal compounds discussed in published papers were pyrene platinum compound 2.2.13 [71], triazole iridium derivative 2.2.14 [66] and bis(N-heterocyclic carbene)gold compound 2.2.15 [82]. For the 2-phenylbenzothiazoles that are shown in Fig. 4, their photophysical and electrochemical properties are described in Table 3. The quenching rates for all of the complexes with b-dikonate ancillary ligands were found to be small. For compound 2.2.16a, the singlet oxygen quenching rate (5  105 M1 s1) is in fact smaller than those of many standard singlet oxygen sensitizers such as

tetraphenyl porphyrin (6  107 M1 s1), while the quantum yield is nearly unity [78]. Compounds 2.17a and 2.17c displayed the longest luminescence lifetimes and largest quantum efficiencies in solution and, as a result, are the most promising candidates for future luminescence quenching-based oxygen-sensing studies [79]. Benzothiazole-coumarine platinum compound 2.2.18b shows a longer phosphorescence lifetime (t ¼ 20.3 ms), a high phosphorescence quantum yield (PLQY ¼ 37), and a very high O2 sensitivity. A Stern-Volmer quenching constant KappSV ¼ 0.026 Torr 1 was observed for 2.2.18b, ca. 89-fold of that of 2.2.18c [77]. In comparison to the most commonly used ruthenium(II) polypyridyl dyes and porphyrin for optical oxygen sensing, benzothiazole and benzoxazole coumarine iridium compounds (2.2.19a, 2.2.19b) possess much more efficient visible absorption and higher quantum yields, which result in much higher brightnesses. When incorporated in a model polystyrene film, the probes show the optical dynamic of luminescence decay for oxygen monitoring in the range from 0% to 100% air saturation. However, the probes exhibit significantly lower photostability, which restricts their application [73]. Bis(2-benzothienylpyridine)iridium triphenylphosphonium

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Fig. 3. 2-Phenylpyridine platinum, iridium and gold compounds for oxygen sensors.

cation 2.2.20 exhibited selective mitochondria localization in HeLa cells. The phosphorescence of this compound can be used in a mitochondrion-specific oxygen sensor [67].

2.3. Sulfur dioxide gas

gradually decompose, probably due to the low stability of the penta-coordinated nickel adducts. These pincer platinum compounds are presumed to have low stability because attempts to use pincer monometallic platinum compounds similar to compound 2.3.3 as receptors failed because they slowly sublime at an operating temperature of 50  C [91].

G. van Koten et al. [84], thirty years ago in 1986, already published on the interaction of pincer metal compounds with SO2 by Eq. (2.3.1).

These pincer platinum halides (2.3.1a-c) and nickel halides (2.3.2a-c) are air-stable neutral compounds, and they react readily with toxic gaseous SO2 to form orange platinum and red nickel compounds, respectively. However, on exposure to air at room temperature, they slowly lose SO2, with the reformation of these compounds. These reactions (Eqs. (2.3.1)) occur in the solid state as well as in solution and are fully reversible for the platinum compounds (2.3.1a-c), while the corresponding nickel compounds (2.3.2a-c)

2.3.3

Fig. 4. 2-Phenylbenzothiazole, 2-naphtylbenzothazole and other iridium and platinum compounds for oxygen sensors.

After twelve years, they mainly published on the use of platinum dendrimers as a SO2 sensor, starting in 1998 [84e93]. The studied NCN pincer dendrimers are compounds 2.3.4, 2.3.5 and 2.3.6. The most commonly used SO2 sensors of the pincer platinum compounds are the dendrimers, as shown in Fig. 5 (2.3.4e2.3.6). Titration measurements have shown that concentrations of SO2 in the ppm range may be detected qualitatively and quantitatively [30]. The molecular recognition process between the platinum center and SO2 is very selective and not disturbed by the pressure of other atmospheric gases such as CO and HCl or humidity (H2O). The reversible binding of SO2 is an extremely fast process at ambient temperature, and reaction rates on the order of nanoseconds have been established (k ¼ 2  108 s1) [90].

Efficient sensor devices have been developed that are very selective and robust and can quantify SO2 in the ppm concentration range. 2.4. Other gases Gas sensors for oxygen and sulfur dioxide were described in the previous section. Other gases whose detection has been reported include carbon monoxide and carbon dioxide. Because the oxygen and sulfur dioxide gas sensing reactions are reversible, these sensors may be used any number of times, but the following sensors are not reversible. For example, the pincer CCC carbene platinum acetonitrile

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Table 1 Photophysical and electrochemical properties of iridium and platinum compounds for oxygen sensors.

2.2.1 2.2.2a 2.2.2b 2.2.3a 2.2.3b 2.2.3c 2.2.4 2.2.5a 2.2.5b 2.2.5c 2.2.5d 2.2.5e 2.2.5f 2.2.5g 2.2.6a 2.2.6b 2.2.6c 2.2.6d 2.2.7a 2.27b 2.2.7c 2.2.7d 2.2.8a 2.2.8b 2.2.8d 2.2.9 2.2.10a 2.2.10b 2.2.10c

lem (nm)

t (ms)

PLQYs (%)

510 525,559 499,531 506,535 510,538sh 491,523 513,551 540,578sh 533,572sh 542,585sh 541,580sh 549,590sh 568 595 554 551 553 522 534 536 541 543 470 463 459 665 506 509 509

2.0 4.11 2.23 4.11 6.83 2.35 259 5.11 12.22 13.90 6.96 6.43 9.08 10.94 1.53 1.47 1.13 1.72 1.50 1.30 12 26 1.78 1.94 2.29 102 0.06 0.19 0.43

50 18 14 23 22 20 3.5 11 9 4 19 11 14 3 48 49 40 32 17.8 20 28.4 12.1 39 44 8 58 6 32 7

KappSV (Torr1)

0.00547 0.1251 0.0615 1.59(21.2 mbar1) 0.05714 0.08202 0.05072 0.05752 0.06929 0.06726 0.10167 0.00755 0.00710 0.00583 0.00866

Table 2 Photophysical and electrochemical properties of iridium and platinum compounds for oxygen sensors.

Ref [63] [64] [64] [68] [68] [68] [69] [70] [70] [70] [70] [70] [70] [70] [74] [74] [74] [74] [75] [75] [75] [75] [80] [80] [80] [81] [65] [65] [65]

2.2.11a 2.2.11b 2.2.11c 2.2.12a 2.2.12b 2.2.12c 2.2.13 2.2.14 2.2.15

lem (nm)

t (ms)

PLQYs (%)

KappSV (Torr1)

Ref

455 538 560 554 556 540 485

0.0032 25.5 0.86 1.86 2.82 12.40 0.0024 1.80 14.8

1.1 18.2 4.5 13 20 66 65

0.016 0.234 0.002 0.0058 0.0032 0.0238

[72] [72] [72] [76] [76] [76] [71] [66] [82]

498

3

Table 3 Photophysical and electrochemical properties of iridium and platinum compounds for oxygen sensors.

lem (nm) 2.2.16a 2.2.16b 2.2.17a 2.2.17b 2.2.17c 2.2.17d 2.2.18a 2.2.18b 2.2.18c 2.2.19a 2.2.19b 2.2.20

532 532 586,613 520 514,542,580 535,572 543,585 596,646 597,650 563 552 480

t (ms)

PLQYs (%)

6.0 0.1 0.4 0.2 0.34 20.3 1.59 11.3 10.7 5.5

100 77 22 2 20 3 36 37 3 54 34 29

KappSV (Torr1)

0.00190 0.02583 0.00029

Ref [78] [78] [79] [79] [79] [79] [77] [77] [77] [73] [73] [67]

compound 2.4.1a easily reacts with carbon monoxide to form the carbon monoxide derivative 2.4.1b. The pincer CCC carbene platinum acetonitrile compound 2.4.1a has lem: 536 nm, PLQY is 26%.

Then, carbon monoxide can be easily detected by the color change from green to blue or by the intensity change of the green color [94]. Bis(2-phyenylpridine)iridium hydrazine compound 2.4.2a (lem ¼ 501 nm, t ¼ 1.56 ms, PLQY ¼ 42%) with CO2 undergoes a red

The carbon monoxide derivative 2.4.1b can be obtained as crystals in one of two polymorphic forms by slow the diffusion of diethyl ether into either an acetone (form B) or acetonitrile solution (form G) of the complex. Form B has an intermolecular Pt/Pt distance of 5.123 Å, lem: 467, 493 nm, with a PLQY of 24%, while form G has an intermolecular Pt/Pt distance of 3.362 Å, lem: 529 and a PLQY of 71%.

shift (2.4.2b: lem ¼ 524 nm, t ¼ 1.80 ms, PLQY ¼ 45%), accompanied by a change in the peak shape upon exposure to CO2 in solution. The spectral changes observed are attributed to the formation of the corresponding neutral carbazate species Ir(ppy)2(H2NNHCOO) 2.4.2b and are not consistent with the protonation of the ligated hydrazine. The conversion of the hydrazine species to the carbazate species is solvent-dependent and irreversible [95a].

I. Omae / Journal of Organometallic Chemistry 823 (2016) 50e75

Fig. 5. Some examples of pincer platinum dendrimers 2.3.4e2.3.6 [85,86,89,91].

57

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Recently, W.-Y. Wong [95b] CH3COO published another sensor 2.4.3 that a phosphorescent CO2 gas probe based on an iridium compound with 2-phenylimidazo-[4,5-f] [1,10] phenanthroline ligand has been developed. Its phosphorescence is quenched by the addition of CH3COO. The quenched phosphorescence can be recovered by bubbling CO2 into the detecting solution. The compound 2.4.3 can be repeatedly switched on and off by treatment with CH3COO followed by CO2 bubbling.

3. Metal ions 3.1. Introduction Among metal ions, the detection of the toxic mercury ion, Hg2þ, has been much reported. Subsequently, the detection of Zn2þ, Cu2þ,

Cd2þ, Cr2þ, Pb2þ, Ca2þ, Mg2þ, Ba2þ and alkali metal ions such as Naþ followed. The metal ions in the cyclometalated compounds used as sensing materials are mostly iridium compounds, although many platinum compounds are also reported. The most common substrates are 2-phenylpyridines, and other substrates are 2-phenylbenzothazoles, 2-(benzo[b]thiophen-2-yl) pyridines, 1-phenylpyrazoles and CNN pincer compounds. As ancillary ligands, many acetylacetonates (acac), bipyridines and 1,10-phenanthrolines are used.

3.2. Hg2þ 2-Phenylbenzothiazole 3.2.1a responds to Hg2þ with a noticeable change in the phosphorescent emission color from yellow to green, as shown in Eq. (3.2.1). The interaction between Hg2þ and the sulfur atom of the cyclometalated ligand is responsible for the significant variations in the optical and electrochemical signals. Importantly, cell imaging experiments have demonstrated that

compound 3.2.1a is membrane permeable and can readily reveal changes in the intracellular Hg2þ concentration in a ratiometric way [96a]. In 2-(2-thiophenyl)quinolone iridium acetylacetonate 3.2.2a, the addition of Hg2þ induces an evident color change from red (620 nm) to yellow-green (ca. 570 nm) and a pronounced ON/

OFF-type phosphorescent signaling behavior that could be observed by the naked eye. The Hg2þ is coordinated with compound 3.2.2b, forming a 1:1 compound. After the addition of 1 equiv. of Hg2þ, the emission of compound 3.2.2b was obviously quenched, and only weak luminescence at ca. 570 nm with a low quantum yield of ~0.3 was observed. The interaction between Hg2þ and the sulfur atom of the cyclometalated ligand is responsible for the significant variations in the optical and electrochemical signals [96b].

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Bis(2-(benzo[b]thiophe-2-yl)pyridine)iridium acetylacetonate 3.2.3a is colorless (lme 363 nm). Upon the addition of Hg2þ,

mechanism of a highly selective and sensitive phosphorescent sensor for Hg2þ [98].

obvious spectral blue shifts in the phosphorescent emission bands were measured for compound 3.2.3b. Further investigation indicated that the interaction between Hg2þ and the sulfur atom of the cyclometalated ligands is responsible for the significant variation in the optical and electrochemical signals, which was also confirmed by density functional theory (DFT) calculations. It is the charge transfer from the iridium center to cyclometalated ligands that increases the electron density of the latter that makes the coordination of sulfur to Hg2þ easier [97].

In bis(2-(4,6-difluorophenyl)pyridine)iridium chloride 1,2bis(diphenylphosphino)ethane 3.2.5a, the free phosphorous donor atom present on the appended diphosphine is shown to provide selective binding to the Hg2þ. The selective binding ability of the probe molecule towards Hg2þ results in a detectable signal due to the complete quenching of their aggregation-induced emission (AIE2) properties. The detection limit of Hg2þ from solution increased to 170 nM, which demonstrates the efficiency of the probe molecules as Hg2þ sensors [99].

In bis(3,4-diphenylcinnoline)iridium N-carbazolylcarbodithioate 3.2.4a, upon the addition of aqueous Hg2þ, the acrylonitrile solution of compound 3.2.4a exhibited a visual color change and significant phosphorescence enhancement (130-fold) at 580 nm (OFF-ON). The interaction between the S atom of the N-carbazolylcarbodithioate ancillary ligand and Hg2þ was responsible for the

2 AIE: The detection limit of mercury ions using this compound can be further improved if the system used as probe molecules shows aggregation-induced emission activity, the reverse phenomenon of aggregation-caused quenching.

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3.3. Zn2þ and other metal ions In bis(2-pheny-4-methyl(pyridine))iridium di(2-picolyl) appended bipyridine 3.3.1, the lifetime of 67 ms is exceptionally long and has the emission lem ¼ 637 nm. Most strikingly, the coordination of Zn2þ ions induced a unique response: a blue-shifted emission (lem ¼ 610 nm) of the intensity comparable to compound 3.3.1, with a lifetime of 34 ms, was observed, indicative of the formation of a new emissive species [100e102].

The luminescence intensity of bis(2-phenypyridine)iridium alkynyl-2-picoly appended bipyridine 3.3.2 shows selective luminescence recognition to Zn2þ, with a 25-fold luminescence enhancement at 618 nm [103].

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Bis(2-phenylpyridine)iridium 1,10-phentharoline derivative compounds 3.3.3, 3.3.4, and 3.3.5 show phosphorescent emissions of weak green (lem 530 nm), green, blue and green (470e570 nm), respectively. The addition of Zn2þ, Cr3þ and Cu2þ to these compounds caused changes from weak green to strong blue (540 nm), from green to yellow, and from green to red (580e700 nm), respectively, enabling them to be used as sensors for these ions [101,104].

61

Bis(2-phenylpyridine)iridium 2-(2’-benzo[b]thienyl)-4-(di(2picoly)amine pyridine 3.3.6 is reversibly and selectively able to quantify Cu2þ in aqueous media, and it detects the intracellular copper ratiometrically. Intracellular copper ion imaging was also achieved by using the ratio of phosphorescence signals acquired through green (470e570 nm) and red channels (580e700 nm) in the response to Cu2þ [105].

Bis(1-pheny, 3.5-dimethylpyrazole)iridium 3-(4-aza-15-crown5-pheny)pyridyl pyrazole 3.3.7 showed a yellow-green emission at ~560 nm at room temperature and remarkable differences in the spectral properties upon metal cation (e.g., Ca2þ) complexation. The emission is gradually blue-shifted toward 520 nm, accompanied by an increase in the emission intensity that makes 3.3.7 a highly sensitive phosphorescence probe (PLQY: 3.3.7: 3.3.7/ Ca2þ ¼ 0.1: 1.2) [106].

Bis(1-pheny, 3.5-dimethylpyrazole)iridium 3,5-bis(pyridyl)pyrazole 3.3.8a exhibits a 485 nm emission with moderate intensity (PLQY ~ 4, t ¼ ~70 ns). The addition of Pb2þ forms the 1:1 adduct 3.3.8b, and the 485 nm phosphorescence was gradually red-shifted toward 505 nm, accompanied by a drastic decrease in the emission intensity [107].

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Other metal ion sensors have been reported on alkali and alkali earth metal ions. Bis(2-phenylpyridine)iridium 4,4’-dimethylbipyridine 3.3.9 was adsorbed by colloidally dispersed montmorillonite clay, with luminescence appearing at 560 nm. The emission quantum yield (PLQY) decreased from 6 to 1 on adsorption, while the PLQY recovered to ca. 12 by adding alkali or alkali earth metal ions at a concentration of ca. 2  102 or 2  105 M, respectively [108].

4. Halogen anions, CN¡, CH3COO¡ and other compounds 4.1. Introduction Anions such as halogens (e.g., F and I), CN, CH3COO and

anions of other compounds such as BCN and MeCN are described in this chapter. 4.2. Halogen anions Among the halogen ions, this section describes sensors for F or I. Bis(2-phenyl(4-(2,4,6-trimethylphenylboryl)pyridine)iridium acetylacetonate 4.2.1 has a strongly electro-accepting B(Mes)2 to the pyridine ligand in the 2-phenylpyridine, and more stabilized metalto-ligand charge-transfer (MLCT) can be obtained by transferring electron density from the pyridyl moiety to the boron atom of the B(Mes)2 group in the metallophosphors to give red phosphorescence by adding F ions, as shown in Table 4. Taking advantage of the binding effect between the boron atom and F ions, the phosphorescent emission color of the iridium cyclometalated compound can be dynamically changed by the sequential addition of external F ions from red to yellow and green through the modulation of the charge-transfer emitting states, representing very unique F ion sensing behavior. The present system is promising in light of its good reversibility and selectivity, as well as its good response to the naked eye in the higher-energy region [109]. 2-(5-(2,4,6-Trimethylphenylboryl)phenyl)pyridine iridium acetylacetonate 4.2.2a exhibits both extremely high selectivity toward F ions (Sensitivity: > 1000) and high-efficiency two-color phosphorescence behavior at room temperature, enabling colorimetric as well as ratiometric fluoride ion sensing. Aqueous fluoride ions could be successfully detected by applying the solution to PMMA films doped with 4.2.2a, thus enabling the realization of a practical sensing system. This system shows that the bluish-green phosphorescence (512 nm) of 4.2.2a immediately changed to the orange phosphorescence (567 nm) of 4.2.2b. Compound 4.2.2a almost exclusively reacts with fluoride ions over other halide ions (Cl, Br, I) and  multi-atomic anions (CN, SCN, NO 3 , OH ), resulting in ca. 1000 times greater selectivity [110].

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Bis(2-phenyl(4-(2,4,6-trimethylphenylboryl)quinoline iridium bipyridine 4.2.3 shows an intense emission band at 592 nm with a shoulder at 637 nm, and the photoluminescent color is orange-red at room temperature in an acetonitrile solution. With the addition of F ions, a decrease in the emission intensity of 4.2.3 was observed and almost completely quenched, as could be observed by the naked eye. Hence, 4.2.3 could be used as an ON-OFF-type phosphorescent probe for F ions [111].

63

In bis(2-phenylpyridine)iridium 2,4,6-trimethylboryl (B(Mes)) derivatives 4.2.5a,b, under aqueous conditions with I ions at a concentration of ca. 1.0  105 M, the phosphorescence from the sensing paper is almost totally quenched to the detection limit of 107 M. Furthermore, these phosphorescent chemosensors show good selectivity to I ions [113].

4.3. CN

Upon the addition of F, CH3COO and H2PO 4 , the emissions of the three compounds 4.2.4a-c were quenched completely, and the solution colors of 4.2.4a-c changed from yellow-green to brown. 4.2.4a-c prefer to bind F and CH3COO over H2PO 4 . 4.2.4a especially prefers to bind F over the other anions, suggesting that it can act as an ideal phosphorescent chemosensor for F [112].

For bis(2-phenylpyridine)irdium (Z)-4-(4-(dimethylamino)benzylidene)-3-methyl-1-H-pyrazole-5-(4H)-one 4.3.1a, the addition of CN to the solution induces a color change in the solution from pink to colorless [114].

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While 4.3.2a was almost dark in photoluminescence, the addition of tetra-n-butylammonium cyanide (200 equiv.) caused the absorption at 238 nm to decrease, while the intensity of the photoluminescence at 579 nm increased drastically by over 350fold. The addition of excess amounts of other anions (F, Cl,      Br, I, H2PO 4 , PF6 , BF4 , HSO4 , AcO , ClO4 ) with tetra-n-butylammonium salt showed no significant effect on the photoluminescence [115].

N-(2-Anthryl)-2-[bis(pentafluorophenyl)boryl]benzylineamine 4.3.3a-4.3.5a changed its fluorescence color from yellow to green upon the addition of an equimolar amount of CN ions, in contrast to the N-phenyl derivatives 4.3.3b-4.3.5b, which showed the quenching of the emissions [116].

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Table 4 Photophysical data for compound 4.2.1 with F ions.

1 1 1

F

lem (nm)

PLQY (%)

0 0.875 5

604 556 505

84 40 63

4.4. CH3COO

4.5. Other compounds

In addition to CN detection, bis(2-phenylquinoline)iridium chloride 4.3.2a can also be used for the detection of acetate, with the former (CN) undergoing nucleophilic attack, and the latter (acetate) Kolbe electrolysis and radical-metal combination. The electrochemiluminesecence was detected for acetate at 609 nm, as shown in Eq. (4.4.1) [115].

Sensors for other compounds, such as BrCN, MeCN, PrCN, volatile halogenated solvents, MeOH, water, acetone and acid vapors, have also been reported. (2-Thiopheny-2-yl)pyridine platinum derivatives (4.5.1a-e) show orange-red phosphorescence in room temperature oxygenfree solution, and the addition of BrCN to the metal complexes in solution or as dispersed in poly(methyl methacrylate) gave blueshifted emissions of the Pt compounds [117].

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2-Phenylpyridine iridium quinoxaline-2-carboxylate 4.5.2 has a black form and can be transformed into a red form upon exposure to acetonitrile or propionitrile vapor. The behaviors are evident and fast, whereas no response was observed when it was exposed to other volatile organic compounds. On the basis of crystallographic and DFT studies, the 2-phenylpyridine compound 4.5.2 has a porous packing structure showing the black form, and when it absorbs acetonitrile vapor, it forms the red form by weak intermolecular interactions such as hydrogen bonding and pp interaction [118].

In four 2-phenylpyridine derivative 4.5.3a-d (n ¼ 3,2,1,0), the fac-2-phenylpyridine. 4.5.3a (n ¼ 3) was prepared for constructing a luminescent Langmuir-Blodgett(LB) film. A glass substrate was modified with a single-layered LB film and placed into a quartz cell. The luminescence was monitored under atmospheres of various types of gas. As a result of exposure to, especially, MeOH, water and acetone, they were found to quench excited Ir compound 4.5.3a efficiently [119].

Bis-CNC pincer compound 4.5.4 shows a fast and reversible vapoluminescent response, which is most intense with volatile halogenated solvents such as CH2Cl2, CHCl3, 1,2-C2H4Cl2, CH3I, 1,2C2H4Br2 and CH2Br2, except CCl4. The emission intensity of desolvated 4.5.4 is dramatically enhanced in the presence of the vapor of halogenated solvents [120].

The 2-(3-pyridylphenyl)pyridine bis-carbenene compound 4.5.5 displays an emission, lem, at ca. 510 nm. The compound can be used as a solid-state sensor for acidic vapor. Thus, a film of 4.5.5, drop-cast from a chloroform solution onto a quartz plate, is subjected to a panel of volatile acid vapors in a sealed quartz cuvette, and the emission intensity was monitored. The emission maximum remained constant at 510 nm, but the emission intensity was enhanced 10-, 25- and 5-fold in the presence of HCl, formic acid and acetic acid vapors, respectively [121].

CNN pincer platinum compound 4.5.6a immobilized in Nafion film exhibits a solvatochromic shift in the emission maximum from 530 to 650 nm upon immersion in ethanol, but no effect is detected with aprotic organic solvents, whereas the emission of the 4.5.6b anchored in silica materials shows a blue shift from ~655 to 550 nm upon exposure to pentane vapor, but no shift is observed for ethanol vapor [122].

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5. Sugar, glycoproteins and amino acids

5.3. Glycoproteins

5.1. Introduction

5.3.1. Lectins Lectins are carboxylate-binding proteins that mediate important biological process such as cell growth, the inflammatory response and viral infections. The emission intensity of bis(2-phenylquinoline)iridium bipyridyl derivative 5.3.1 upon the addition of N,N-bis(benzl-3-boronic acid)-4,4’-bipyridium dibromide (BBV) was increased upon the addition of concanavaline A (Con A). The strong binding affinity between glucose-iridium compound 5.3.1 and lectin probably results in separating the quencher (BBV) from the iridium compound, and thus increasing the luminescent signal. These results confirmed that the water-soluble iridium compound 5.3.1 could act as a lectin biosensor [124].

This chapter describes sensors for sugar, glycoproteins such as lectins and avidins, and amino acids.

5.2. Sugar Boronic acids are widely used for sugar sensors because they react with diol moieties of sugar to form boronate esters. For example, for the diazo cyclometalated compounds 5.2.1a, in the pH range from 7 to 10, an absorption maximum was observed at 505 nm (5.2.1b), which is significantly red-shifted compared to that of 4-aminoazobenene 5.2.1a (356 ppm) [123].

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biotin that can be measured by this assay was from ca. 1  105 to 1  107 M [125].

5.3.2. Avidins Avidins specially bond with biotins. Upon binding to avidin, those compounds showed a significant emission enhancement and lifetime extension. The use of various arylbenzothiazoles (5.3.2a - d) enabled the emission of the compounds to occur in a wider wavelength range from 528 to 712 nm in fluid solutions at 298 K. The naphthyl compound (5.3.2d) showed remarkably high emission enhancement factors upon biding to avidin, rendering them useful probes for this protein. The concentration range of

The cyclometalated bipyridine biotin derivatives 5.3.3a-f display intense and long-lived emission in fluid solution at 298 K as shown in Table 5. The concentration range of biotin that can be determined by the assay using 5.3.3a is between ca. 1  107.5 and 1  105.5 M [126].

I. Omae / Journal of Organometallic Chemistry 823 (2016) 50e75 Table 5 Photophysical properties of cyclometalated bipyridine biotin derivatives 5.3.2a-f.

5.3.3a 5.3.3b 5.3.3c 5.3.3d 5.3.3e 5.3.3f

lem (nm)

t (ms)

PLQYs (%)

556 587 560 574 577 554

0.48 0.35 0.92 0.39 0.47 2.95

16 8.1 19 9.3 12 37

2-Phenylpyridine iridium biotin derivative 5.3.4 displayed a significant change of the emission profile upon binding to avidin, rendering it a useful luminescent probe to the protein [127].

The iridium diamine derivatives 5.3.5a,b showed an emission intensity enhancement and excited-state lifetime extension upon binding to protein and can cross-link avidin to give dimers and trimers [128].

69

The neutral sensing system for biotin-avidin assays offers remarkable sensitivity over traditional transition metal based probes, due to the intramolecular energy transfer and increased hydrophobicity associated with the avidin binding site and neutral probe 5.3.6 and can be used as a homogeneous competitive assay for biotin [129].

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5.4. Amino acids Bis(2-(4-formylphenyl)pyridine)iridium acetylacetonate 5.4.1a is a highly selective luminescent chemosensor for homocysteine, with an emission enhancement from deep red (615 nm) to green (525 nm) [130].

Bis(1-alkyl-2-phenylbenzoimidazole)iridium 2triazolepyridine derivative 6.2.2 exhibits an N-alkyl chain length dependency, showing an obviously blue-shifted emission (513 nm) upon extending their chain length. Compound 6.2.2 with the longest chain (n ¼ 8) and moreover with the merit of a high quantum efficiency in the aggregate state can serve as an

6. Others 6.1. Introduction This chapter describes sensors for explosives, surfactants, free radicals, pH, temperature, DNA and cells. 6.2. Explosives and surfactants The bis(2-phenylpridine)iridium 2-triazolepyridne derivative 6.2.1 exhibits selective detection of 2,4,6-trinitrophenol (TNP), even in the presence of a certain amount of anions, cations and nitro compounds, as well as at different pH values. The result demonstrates that both the electron and energy transfer quenching mechanisms are responsible for the emission (ca. 550 nm) quenching of compound 6.2.1 towards TNP [131].

efficient sensor for the sensitive and selective detection of the explosive 2,4,6-trinitrophenol (TNP). The luminescence intensity obviously decreases below 50% at a TNP concentration of 2 ppm. When the TNP concentration reached 9 ppm, negligible emission was observed, with a quenching efficiency of nearly 92% [132].

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Bis(4,6-difluorophenylpyridine)iridium picolate 6.2.3a (lem ~ 470 nm) and bis(2-benzotheinylpyridine)iridim acetylacetonate 6.2.3b (lem ~ 600 nm) were studied in the presence of the nonionic surfactant Triton X-100 (poly(ethylene glycol)t-octylphenyl ether). The photoluminescence intensity of these compounds increased in the presence of the nonionic surfactant up to 6-fold for 6.2.3a and up to 20-fold for 6.2.3b [133].

6.3. Free radicals, pH and temperature Although bis(2-phenylpyridine)irdium with iminonitroxide bipyridine ligand 6.3.1 showed no luminescence due to quenching by the nitronyl radical on the ligand molecule, the luminescence

71

was dramatically enhanced by the photo-generation of isobutyronitrile radicals. The radical concentration dependence of the enhanced luminescence intensity indicated that compound 6.3.1 can quantitatively detect free radicals up to 120 mM [134].

Solutions of the acid-free iridium compounds 6.3.2a-d showed strong green emissions (ca. 500 nm) in dimethylsulfoxide. The protonation of three pyridyl groups of 6.3.2a and 6.3.2c causes a significant red-shift in the emission wavelength (ca. 600 nm) with a decrease in emission intensity [135].

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The compounds 6.3.3a-d showed red emissions, which was enhanced to a considerable extent upon the protonation of their basic groups in aqueous solution. The emission of 6.3.3a is nearly nonexistent at pH > 5, but a red emission is observed at pH < 5. In contrast, the guanidine and iminoimdazoline derivatives 6.3.3b and 6.3.3c exhibit a strong emission at pH < 9, and the emission intensity of 6.3.3d with three CF3 groups is increased at pH < 8 [136a].

6.4. DNA and cells The 2-phenylpyridine iridium phenazine derivative 6.4.1 can be immobilized on an indium tin oxide electrode. This photoelectrochemical sensor detects DNA concentrations ranging from 0.025 to 100 pmol L1 [138e141].

Recently, W.-Y. Wong [136b] also published bis(2phenylpyridine)iridium bipyridine compounds for ratiometric and lifetime imaging of intracellular pH variations. 2-(4-Bromophenyl)-5,6,7,8-tetrahydroquinoline platinum acetylacetonate 6.3.4 displays a strong temperature quenching effect. The distinct response to temperature was additionally calibrated after incorporation in poly(vinylidene chloride-co-acrylonitrile) serving as an oxygen-blocking matrix copolymer. The resulting yellow-greenemitting temperature sensor represents an interesting alternative to the available mostly red emitting temperature-sensitive probes, as it displays a strong temperature-quenching effect 0º- 60  C [137].

2-(2,4-Difluorophenyl)pyridine bipyridine type compounds 6.4.2a-e and 2-naphthylpyridine bipyridine type compound 6.4.3 were prepared as phosphorescent dyes for live cell imaging sensors. The emission colors of these compounds can be tuned from blue to red, while high quantum efficiencies are retained. These compounds exhibited low cytotoxicity, moderate efficiencies, cell membrane permeability and exclusive staining of the cytoplasm of live cells [142].

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7. Concluding remarks Papers on oxygen sensors were published the most. The oxygen sensors are very important because the preservation of foods depends on the oxidation caused by the presence of oxygen in the materials. The representative four oxygen sensors are all 2-phenylpyridine platinum compounds 2.2.3b, 2.2.4, 2.2.5g and 2.2.11b, bearing electron-donating or ewithdrawing ligands. The next important sensors are for sulfur oxide because the burning of fossil fuels containing sulfur compounds leads to grave urban air pollution. The sensors for sulfur dioxide are NCN pincer platinum dendrimers 2.3.4, 2.3.5, 2.3.6. Among the other sensors, papers on toxic metal ions, Hg2þ, F and avidins are very. The sensors used are compounds 3.2.1, 4.2.3 and 5.3.6 for Hg2þ, F and avidins, respectively. Acknowledgments The author wishes to express his sincere appreciation to Dr. Sumio Chubachi for reading the full manuscript, enhancing its accuracy and clarity, and providing much valuable constructive criticism. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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