Anal. Chem. 2002, 74, 1340-1342
Green Electrochemiluminescence from Ortho-Metalated Tris(2-phenylpyridine)iridium(III) David Bruce and Mark M. Richter*
Department of Chemistry, Southwest Missouri State University, Springfield, Missouri 65804-0089
The electrochemiluminescence (ECL) of Ir(ppy)3 (ppy ) 2-phenylpyridine) is reported in acetonitrile (CH3CN), mixed CH3CN/H2O (50:50 v/v), and aqueous (0.1 M KH2PO4) solutions with tri-n-propylamine as an oxidative-reductive coreactant. ECL efficiencies (Oecl, photons emitted per redox event) of 0.000 92 in aqueous, 0.0044 in mixed, and 0.33 in CH3CN solutions for Ir(ppy)3 were obtained using Ru(bpy)32+ (bpy ) 2,2′-bipyridine) as a relative standard (Oecl ) 1). Photoluminescence (PL) efficiencies of 0.039, 0.050, and 0.069 were obtained in aqueous, mixed, and acetonitrile solutions, respectively, compared to Ru(bpy)32+ (Oem ) 0.042). The ECL spectra were identical to photoluminescence spectra (λmax = 517 nm), indicating formation of the same metal-toligand (MLCT) excited states in both ECL and PL. The ECL is linear over several orders of magnitude in mixed and acetonitrile solution with theoretical detection limits (blank plus three times the standard deviation of the noise) of 1.23 nM in CH3CN and 0.23 µM in CH3CN/ H2O (50:50 v/v). Photoluminescent polypyridyl complexes of Ru(II) (e.g., Ru(bpy)32+, bpy ) 2,2′-bipyridine), Os(II), and Re(I) have been extensively studied due to their low-lying metal-to-ligand chargetransfer (MLCT) excited states.1,2 Ruthenium complexes have received particular attention due to their relatively high emission quantum yields (e.g., φem (H2O) = 4.2% for Ru(bpy)32+)3 and long excited-state lifetimes (τ = 600 ns) in fluid solution at room temperature. Ortho-metalated complexes of Ir(III) and Rh(III) also show interesting photophysical properties. For example, excitation into the visible MLCT band of Ir(ppy)3 (ppy ) 2-phenylpyridine) results in an excited-state lifetime of ∼100 ns in CH2Cl2 and 5 µs in toluene.4 They also display strong visible absorptions and groundand excited-state redox potentials that, like their Ru(II) counterparts, make them potentially useful in applications such as solar energy conversion, molecular sensing, and photocatalysis. The recent demonstration5 of highly efficient green electrophosphorescence from Ir(ppy)3 doped into a dicarbazole-biphenyl host, * Corresponding author: (e-mail)
[email protected]. (1) Juris, A.; Balzani, V.; Barigelleti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85-277. (2) Meyer, T. Acc. Chem. Res. 1989, 22, 163-170. (3) (a) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583. (b) Van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 98, 4853. (4) King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem. Soc. 1985, 107, 1431. (5) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4.
1340 Analytical Chemistry, Vol. 74, No. 6, March 15, 2002
and subsequent work on a series of ortho-metalated iridium complexes with fluorinated aromatic ligands that emit over a wide color range,6 opens up the possibility of using these types of systems in organic light-emitting devices (OLEDs) such as flat panel displays. The ability to tune the luminescence of these systems also makes them of interest for electrochemiluminescence (ECL). ECL involves the formation of excited states at or near electrode surfaces and, as such, is a sensitive probe of electron- and energytransfer processes at electrified interfaces.7,8 ECL is also being commercially developed for use in clinical analyses (e.g., immunoassays, DNA probes) using Ru(bpy)32+ and a coreactant to generate an ECL signal.9 ECL coreactants are species that, upon electrochemical oxidation or reduction, produce intermediates that react with other compounds to produce excited states capable of emitting light.10-12 For example, in the Ru(bpy)32+/tri-n-propylamine (TPrA) system,13 an anodic potential oxidizes Ru(bpy)32+ to Ru(bpy)33+. The coreactant is also oxidized and decomposes to produce a reducing agent (presumably TPrA•) upon deprotonation of an R-carbon from one of the propyl groups.13 This strong reducing agent can then interact with Ru(bpy)33+ to form the excited state (i.e., *Ru(bpy)32+). Although Ru(bpy)32+ has many properties that make it an ideal ECL luminophore for sensitive and selective analytical methods, it would be useful to have other ECL labels that span a wide range of wavelengths so that simultaneous determination of several analytes in a single sample is possible. For example, Ru(bpy)32+ has a broad emission spectrum stretching from about 500 to 700 nm (λmax ∼ 620 nm) and this can be a disadvantage in applications where an ECL internal standard or multianalyte determinations are desired. In this work, the electrochemiluminescence from fac-tris(2phenylpyridine)iridium(III) (Ir(ppy)3) is reported, opening up a new class of systems for fundamental and applied investigations. The use of Ir(ppy)3 and similar compounds as ECL labels in diagnostics is also proposed. (6) Grushin, V. V.; Herron, N.; LeCloux, D. D.; Marshall, W. J.; Petrov, V. A.; Wang, Y. Chem. Commun. 2001, 1494. (7) Faulkner, L. R.; Bard, A. J. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1977; Vol. 10, pp 1-95. (8) Knight, A. W.; Greenway, G. M. Analyst 1994, 119, 879-890. (9) (a) Yang, H.; Leland, J. K.; Yost, D.; Massey, R. J. Biotechnology 1994, 12, 193. (b) Blackburn, G. F.; Shah, H. P.; Kenten, J. H.; Leland, J.; Kamin, R. A.; Link, J.; Pterman, J.; Powell, M. J.; Shah, A.; Talley, D. B.; Tyagi, S. K.; Wilkins, E.; Wu, T.-G.; Massey, R. J. Clin. Chem. 1991, 37, 1626. (10) Chang, M.-M.; Saji, T.; Bard, A. J. J. Am. Chem. Soc. 1977, 99, 9, 5399-5403. (11) Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 512-516. (12) White, H. S.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 6891-6895. (13) Leland, J. K.; Powell, M. J. J. Electrochem. Soc. 1990, 137, 3127-3131. 10.1021/ac0111513 CCC: $22.00
© 2002 American Chemical Society Published on Web 02/19/2002
EXPERIMENTAL SECTION Ir(ppy)3 was prepared and characterized by a literature method.14 Tetra-n-butylammonium hexafluorophosphate (Bu4NPF6, Aldrich) was used as the electrolyte in the nonaqueous experiments. Ru(bpy)3Cl2 (98%, Strem Chemical Inc., Newbury Port, MA), acetonitrile (Burdick and Jackson Spectroquality), potassium phosphate monobasic hydrate (99.0%, EM Science, Gibbstown, NJ), tri-n-propylamine (98%, Avocado Research Chemicals, Ward Hill, MA), and 2-phenylpyridine (Aldrich) were used as received. Potassium phosphate buffer solutions, 0.1 M KH2PO4, were prepared with deionized water that had been passed through a Barnstead/Thermolyne filtration system. Buffer solutions containing TPrA (0.05 M) were prepared similarly except that it was necessary to stir them vigorously to completely dissolve the amine. The pH of these buffer solutions was adjusted to 8.0 ( 0.1 M with either 6 M H2SO4 or 6 M NaOH. Methods. Electrochemical and ECL instrumentation and experimental methods have been described elsewhere.15 All electrochemical and ECL experiments were referenced with respect to Ag/AgCl gel electrode (0.20 V vs NHE).16 The platinum mesh (27 mm2) working electrode was cleaned prior to each experiment by repeated cycling (+2.0 to -2.0 V) in 6.0 M sulfuric acid, followed by sonication in 2 M nitric acid and rinsing in deionized water. Solutions used to obtain ECL were 0.01 µM-1 mM Ir(ppy)3 and 0.05 M TPrA with 0.2 M potassium phosphate (aqueous and mixed 50:50 (v/v) CH3CN/H2O) or 0.1 M Bu4NPF6 (CH3CN solutions) as electrolyte. Due to the low solubility of Ir(ppy)3 in aqueous solution, the solid was dissolved in a minimum amount (e5 mL) of 95% ethanol followed by addition of 0.1 M KH2PO4 buffer solution. For concentrations of Ir(ppy)3 of g10-6 M in aqueous buffered solution, it was also necessary to sonicate the solutions for 10 min to completely dissolve the compound. Photoluminescence (PL) spectra were obtained with a Shimadzu RF-5301 spectrofluorophotometer (slit widths 3-5 nm). Excitation was at 452 and 383 nm for Ru(bpy)32+ and Ir(ppy)3, respectively, with detection between 450 and 700 nm. ECL efficiencies (φecl, photons generated per redox event) were obtained by the literature methods, using Ru(bpy)32+ (φecl ) 1) as the standard.17,18 Reported values are the average of at least three scans with a relative standard deviation of ( 5% for CH3CN and CH3CN/H2O (50:50 v/v) and ( 8% for H2O solutions. Similarly, relative photoluminescence efficiencies followed published procedures19 using Ru(bpy)32+ (φem (H2O) ) 0.042). RESULTS AND DISCUSSION Electrochemistry. Cyclic voltammetric data for Ir(ppy)3 in all solvents are presented in Table 1. The oxidative wave is assigned to the Ir(ppy)3+/Ir(ppy)30 couple.4,14 The redox chemistry shows ia/ic of 1.19, indicating a quasi-reversible system and a peak-to(14) Dedeian, K.; Djurovich, P. I.; Graces, F. O.; Carlson, G.; Watts, R. J. Inorg. Chem. 1985, 24, 318. (15) (a) Workman, S.; Richter, M. M. Anal. Chem. 2000, 72, 5556-5561. (b) McCall, J.; Alexander, C.; Richter, M. M. Anal. Chem. 1999, 71, 2523. (16) Bard, A. J.; Faulkner, L. R. Electrochemical Methods Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (17) White, H. S.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 6891-6895. (18) Richter, M. M.; Bard, A. J.; Kim, W. K.; Schmehl, R. H. Anal. Chem. 1998, 70, 310. (19) (a) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583. (b) Van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 98, 4853.
Table 1. Spectroscopic and Electrochemical Data for Ir(ppy)3 solvent CH3CNd CH3CN/H2O (50:50 v/v)e H2Oe
E1/2(+1/0)a λabs (V) (nm) 0.670 0.490 0.462
380 380 380
λem (nm)
φemb
φeclc
517 0.069 0.33 507, 532 (sh) 0.050 0.0044 507, 532 (sh) 0.039 0.00092
a All electrochemical and ECL experiments were referenced with respect to an Ag/AgCl gel electrode (0.20 V vs NHE).16 b Photoluminescence efficiency with respect to Ru(bpy)32+ (φem ) 0.042).19 c Relative ECL efficiency with respect to Ru(bpy) 2+ (φ 17,18 3 ecl ) 1). Reported values are the average of at least three scans with a relative standard deviation of (5% for CH3CN and CH3CN/H2O (50:50 v/v) and (8% for H2O solutions. d 0.1 M Bu4NPF6 as electrolyte. e 0.1 M KH2PO4 as electrolyte.
Figure 1. Absorption spectrum of Ir(ppy)3 (10 µM) in CH3CN.
Figure 2. Photoluminescence spectra of Ir(ppy)3 in (A) CH3CN, (B) mixed and (C) aqueous solution and (D) photoluminescence spectrum of Ru(bpy)32+ (10 µM) in CH3CN. Excitation wavelengths were at 380 nm for Ir(ppy)3 and 452 nm for Ru(bpy)32+ with slit widths of 10 nm.
peak separation (∆Epp) of 0.086 mV. ∆Epp is less than or equal to that observed for ferrocene+/0 under similar conditions, indicating a one-electron process. Absorption and Photoluminescence. An absorption spectrum for Ir(ppy)3 in CH3CN is shown in Figure 1. The absorption bands centered at 380 nm have been assigned as MLCT transitions4,14 and are not solvent dependent. Excitation into this broad visible absorption band produces room-temperature photoluminescence for Ir(ppy)3 in CH3CN, mixed, and aqueous solution. An emission band is observed at 517 nm in acetonitrile and at 507 and 532 nm in 50:50 CH3CN/H2O (v/v) and aqueous solution (Figure 2). The photoluminescent emission spectra match well with those obtained for Ir(ppy)3 in ethanol/methanol gas at 77 K and in toluene at 295 K and have been assigned as MLCT transitions.4 The green photoluminescence is readily visible to the nondark adapated eye using a hand-held UV light in both the solid state and in solution. The wavelength maximum for emission (λem) Analytical Chemistry, Vol. 74, No. 6, March 15, 2002
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Figure 3. ECL spectra of (A) a 10 µM Ir(ppy)3 and 10 µM Ru(bpy)32+ solution in CH3CN containing 0.05 M TPrA (0.1 M Bu4NPF6 as electrolyte), (B) 10 µM Ir(ppy)3 (0.05 M TPrA) in CH3CN (0.1 M Bu4NPF6), and (C) 10 µM Ir(ppy)3 (0.05 M TPrA) in CH3CN/H2O (50:50 (v/v), 0.1 M KH2PO4).
Figure 4. pH dependence of ECL using 0.05 M TPrA and 10 µM Ir(ppy)3 in (9) mixed solvent (50:50 (v/v) CH3CN/H2O (0.1 M KH2PO4)) and (b) aqueous buffered (0.1 M KH2PO4) solution. ECL in mixed solvent has been normalized to that of Ir(ppy)3 in CH3CN. Each point is the average of at least three scans with error bars at (5%.
occurs at ∼517 nm, clearly distinct from that of Ru(bpy)32+ (Figure 2). Photoluminescence efficiencies (φem, photons emitted per photons absorbed) for Ir(ppy)3 are reported in Table 1 relative to Ru(bpy)32+ (φem (H2O) ) 0.042)19 and are solvent dependent, showing the sensitivity of these compounds to microenvironmental effects of the solvent media. It is interesting that the efficiencies of Ir(ppy)3 in acetonitrile and mixed CH3CN/H2O are higher than Ru(bpy)32+ under identical conditions. Electrochemiluminescence. The higher photoluminescence efficiencies of Ir(ppy)3, coupled with stable oxidative redox chemistry and the ability to distinguish between Ir(ppy)3 and Ru(bpy)32+ based on photoluminescence emission spectra, make this system of interest in fundamental and applied ECL studies. TPrA was used as “oxidative-reductive” coreactant11-13 to generate ECL due to the reversible to quasi-reversible nature of the Ir(ppy)30/+ anodic redox couple. ECL was observed for Ir(ppy)3 in aqueous (H2O), nonaqueous (i.e., CH3CN), and mixed solvent (i.e., 50:50 (v/v) CH3CN/H2O) solutions containing 0.05 M TPrA at a Pt interface. The ECL intensity peaks at potentials of ∼+0.8 V. At these potentials, oxidation of both TPrA (Ea ∼ +0.5 V vs Ag/AgCl) and Ir(ppy)3 (E° ∼ +0.7 V) has occurred. ECL emission spectra were obtained in each solvent (Figure 3) and are identical to photoluminescence spectra, indicating the same MLCT excited state is formed in both experiments. As in PL, the ECL emission of Ir(ppy)3 is distinct from that of Ru(bpy)32+, allowing the determination of both species in the same sample solution (Figure 3A). (20) Palecek, E. In Topics in Bioelectrochemistry and Bioenergetics; Milazzo, G., Ed.; Wiley: London, 1983; Vol. 5, p 65.
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While photoluminescence efficiencies for Ir(ppy)3 in CH3CN and mixed-solvent solutions are higher than Ru(bpy)32+, the ECL efficiencies (φecl) are lower (Table 1). The differences may be due to the interactions of the excited state (e.g., *Ir(ppy)3) with solvent molecules or decomposition upon electrolysis and interaction with the strong reducing agent (TPrA•). However, in aqueous buffered solution, solubility also becomes an issue. Even using a minimum amount of ethanol to initially dissolve Ir(ppy)3, at [Ir(ppy)3] g 0.1 µM, a fine precipitate could be observed in solution and it was necessary to sonicate the solution ∼10 min prior to running an experiment. The poor solubility of Ir(ppy)3 at higher concentrations limited the range over which the ECL emission was linear. However, the ECL is linear with respect to [Ir(ppy)3] in acetonitrile and 50:50 (v/v) CH3CN/H2O with the concentration of Ir(ppy)3 being varied from 10-8 to 10-4 M. Correlation coefficients (r2) of 0.9996 and 0.9963 were obtained in CH3CN and 50:50 (v/v) CH3CN/H2O, respectively, for six data points with theoretical detection limits (blank signal plus three times the standard deviation of the noise) of 1.2 nM and 0.23 µM. The ECL emission is pH dependent in both aqueous and 50: 50 (v/v) CH3CN/H2O solutions, with maximum intensities observed between pHs 8 and 9 (Figure 4). Similar trends are observed for Ru(bpy)32+ using TPrA as a coreactant and indicate that deprotonation of the TPrA radical cation (i.e., TPrA•+) is critical to the generation of ECL in both iridium and ruthenium systems. This is important for potential applications since the pH of environmental and biological systems is ∼7.4 and would require less sample preparation prior to analysis. It also improves the likelihood that both ruthenium- and iridium-based polypyridyl compounds can be present in the same sample solution for multianalyte ECL determination. CONCLUSIONS This study illustrates that Ir(ppy)3 (and, by analogy, other ortho-metalated iridium(III) complexes) exhibits electrochemiluminescence in aqueous and nonaqueous solutions. Although the ECL emission quantum efficiency is weaker than Ru(bpy)32+ under similar conditions, the green ECL emission maximum of Ir(ppy)3 and the red/orange emission of Ru(bpy)32+ are far enough removed that it is possible to distinguish both signals in a single ECL experiment. This ability may prove useful in applications where an ECL internal standard or multianalyte determination is desired. The lower potentials required to excite Ir(ppy)3 (i.e., e1 V) compared to Ru(bpy)32+ (i.e., ∼1.2-1.6 V) may also prove useful in DNA diagnostic applications since it has been well documented that oligonucleotide sequences undergo irreversible oxidative damage at potentials of g1 V.20 Work on the ECL of Ir(LL)3 (LL ) phenylpyridine with covalently attached fluorine groups) that have been shown to photoluminescence over a range of wavelengths6 is in progress and will be the subject of a forthcoming report. ACKNOWLEDGMENT This research was supported by an award from Research Corp.. Acknowledgment is also made to Southwest Missouri State University for partial support of this work. Received for review November 5, 2001. Accepted January 18, 2002. AC0111513
