Research article Received: 31 July 2013,

Revised: 7 March 2014,

Accepted: 21 March 2014

Published online in Wiley Online Library: 30 May 2014

( DOI 10.1002/bio.2691

Electrogenerated chemiluminescence of tris(2phenylpyridine)iridium(III) in water, acetonitrile and trifluorethanol Wesley D. Robinson and Mark M. Richter* ABSTRACT: The spectroscopic, electrochemical and coreactant electrogenerated chemiluminescence (ECL) properties of Ir (ppy)3 (where ppy = 2-phenylpyridine) have been obtained in aqueous buffered (KH2PO4), 50 : 50 (v/v) acetonitrile–aqueous buffered (MeCN–KH2PO4) and 30% trifluoroethanol (TFE) solutions. Tri-n-propylamine was used as the oxidative-reductive ECL coreactant. The photoluminescence (PL) efficiency (ϕem) of Ir(ppy)3 in TFE (ϕem ≈ 0.029) was slightly higher than in 50 : 50 MeCN–KH2PO4 (ϕem ≈ 0.0021) and water (ϕem ≈ 0.00016) compared to a Ru(bpy)32+ standard solution in water (Φem ≈ 0.042). PL and ECL emission spectra were nearly identical in all three solvents, with dual emission maxima at 510 and 530 nm. The similarity between the ECL and PL spectra indicate that the same excited state is probably formed in both experiments. ECL efficiencies (ϕecl) in 30% TFE solution (ϕecl = 0.0098) were higher than aqueous solution (ϕecl = 0.00092) system yet lower than a 50% MeCN–KH2PO4 solution (ϕecl = 0.0091). Copyright © 2014 John Wiley & Sons, Ltd. Additional supporting information may be found in the online version of this article at the publisher’s web site. Keywords: Electrogenerated Chemiluminescence; ECL; Coreactants; Ir(ppy)3; Trifluoroethanol


Luminescence 2015; 30: 67–71

Experimental Materials Ir(ppy)3 was available from a previous study (37) where it was prepared and purified using methods in the literature by the reaction of Ir(2,4-pentanedione)3 and ppy in glycerol, followed by purification on a silica gel column (45). Purity was determined by comparing electrochemical potentials and molar absorptivity

* Correspondence to: M. M. Richter, Department of Chemistry, Missouri State University, Springfield, MO 65897, USA. E-mail: [email protected] Department of Chemistry, Missouri State University, Springfield, MO, USA

Copyright © 2014 John Wiley & Sons, Ltd.


The electrogenerated chemiluminescence (ECL) of many inorganic and organometallic complexes have been reported, including those containing the main group (e.g., Al, B, Si and Tl), transition metal (e.g., Ag, Au, Cr, Cu, Hg, Ir, Mo, W, Os, Pd, Pt, Re and Ru) and rare earth ions (e.g., Eu and Tb) (1–21). Complexes containing these atoms often possess the electrochemical and spectroscopic qualities required of ECL luminophores, namely, stable oxidative and/or reductive electrochemistry and high photoluminescence (PL) efficiencies. ECL involves the formation of excited states at or near electrode surfaces and is a sensitive probe of electron and energy transfer processes at electrified interfaces. In coreactant ECL the excited state is formed when a species, upon electrochemical oxidation or reduction, produces intermediates that react with luminophores to produce excited states capable of emitting light (22–24). For example, in the Ru (bpy)2+ 3 /tri-n-propylamine (TPrA) system (25), an anodic potential 3+ oxidizes Ru(bpy)2+ 3 to Ru(bpy)3 . The coreactant is also oxidized and decomposes to produce a strong reducing agent (presumably TPrA•) upon deprotonation of an α-carbon from one of the propyl groups (26). This reducing agent can then interact with * 2+ Ru(bpy)3+ 3 to form the excited stated (i.e., Ru(bpy)3 ). 2+ Ruthenium complexes such as Ru(bpy)3 (bpy = 2,2bipyridine) have received particular attention due to their fairly high emission quantum yields (Φem (H2O) ≈ 4.2%) (1–19), long excited state lifetimes (τ ≈ 600 ns) at room temperature, responsiveness to environmental conditions such as solvent, electrolyte and temperature, and their versatility for numerous analytical applications. For example, the PL and ECL emission maxima of Ru(bpy)2+ shifts up to 30 nm in aqueous buffered solution 3 containing fluorinated alcohols such as trifluoroethanol (TFE) compared to aqueous buffered solutions containing no alcohol

(27). Furthermore, dramatic increases in the ECL efficiency (photons emitted per redox event, ϕecl) ranged from six- to 270-fold in mixed alcohol/water solutions with 30% TFE displaying the greatest increases. Like their Ru(II) counterparts, Ir(III) complexes such as Ir(ppy)3 (ppy = 2-phenylpyridine) also show promise for ECL (28–37). The PL and ECL emission maxima can be synthetically “tuned” from blue to red by changing the ligand identity or composition (38–42), they have relatively high PL efficiencies, long excited state lifetimes and are sensitive to environmental conditions (43). In several instances, the ECL efficiency of Ir(III) complexes is higher than that of Ru(bpy)2+ 3 under identical conditions (44). Therefore, to understand better the electrochemical and ECL properties of transition metal complexes in TFE, the spectroscopic, electrochemical and coreactant ECL properties of Ir (ppy)3 in buffered aqueous, mixed 50 : 50 (v/v) MeCN–KH2PO4, and 30% TFE solution are reported.

W. D. Robinson and M. M. Richter values from previous studies (45). Ru(bpy)3Cl2•6 H2O (98%; Strem Chemical, Newburyport, MA, USA), potassium phosphate monobasic hydrate (KH2PO4; 99%, EM Science, Gibbstown, NJ, USA), TPrA (98%; Sigma-Aldrich, Milwaukee, WI, USA) and acetonitrile (MeCN; Optima, Fisher Scientific, Fair Lawn, NJ, USA) were used as supplied. The pH of solutions was adjusted using concentrated NaOH or H2SO4 and the reported values are accurate to within ± 0.1 pH units. Deionized water was filtered using a Barnstead/Thermolyne triple filtration system (Thermo Scientific, USA).

where I is the intensity in photons per second, i is the current in amperes (coulombs/s), F is Faraday’s constant and NA is Avogadro’s constant.

Results and discussion Absorbance and PL data for Ir(ppy)3 in each solvent is presented in Table 1. Ir(ppy)3 shows characteristic absorption peaks around 380 nm assigned as metal-to-ligand charge transfer transitions, with ligand-based transitions in the UV (43). TFE 30% does not change the wavelength maximum or shape of the absorption peak compared to aqueous buffered (KH2PO4) and 50 : 50 KH2PO4–MeCN solution. Typical PL spectra in each solvent are shown in Fig. 1. Excitation into the absorption maximum (~ 380 nm) produces room temperature PL under all conditions with an emission maximum (λem) around 512 nm (Table 1). PL efficiencies (Φem) of Ir(ppy)3 in TFE (Φem ≈ 0.029) were slightly higher than those in 50 : 50 MeCN (Φem ≈ 0.0021), and water (Φem ≈ 0.0016). This suggests a stronger interaction between the luminophore and solvent molecules in 30% TFE, possibly due to hydrogen bonding interactions and greater polarity in the alcohol–water mixture (27). Φem of Ir(ppy)3 was also lower than Ru(bpy)2+ 3 in both water (Φem ≈ 0.042) and 30% TFE solution (Φem = 0.032) (27). ECL was observed for Ir(ppy)3 in aqueous, 50% CH3CN–H2O and 30% TFE solutions at a Pt working electrode by sweeping to positive potentials in the presence of 0.05 M TPrA. An example

Methods Cyclic and square wave voltammetry experiments without photon detection utilized an electrochemical analyzer (CH Instruments, Austin, TX, USA) with a glassy carbon, working electrode (to prevent adsorption of compound on the electrode surface). The glassy carbon electrode was cleaned after each run by polishing with 0.5 μm alumina, followed by rinsing with deionized water. ECL was obtained using a conventional three-electrode system described previously (46), consisting of the electrochemical analyzer (CH Instruments), a HC 135 photomultiplier tube (Hamamatsu Corporation, Middlesex, NJ, USA) (Hamamatsu) contained in a “light-tight” box, and a platinum mesh working electrode. The working electrode was cleaned before each run by repeated cycling (+ 2.0 to – 2.0 V) in 6.0 M sulfuric acid followed by rinsing with deionized water. All electrochemical and ECL experiments were referenced with respect to an Ag–AgCl electrode (0.20 V vs. Normal Hydrogen Electrode (NHE)) (47). A schematic of the ECL instrumental setup is provided in the supplementary material. A Cary-100 ultraviolet (UV)-visible spectrophotometer (Varian Inc., Palo Alto, CA, USA) was used for absorption spectroscopy. PL spectra were obtained using a Shimadzu RF-5301 spectrofluorophotometer (Shimadzu Corporation, Japan) using excitation and emission slit widths of 3 nm. The spectra are uncorrected for photomultiplier tube response. Excitation for PL experiments was at the lowest energy absorption maximum obtained via UV-visible spectroscopy, with detection between 400 and 700 nm. PL efficiencies (ϕem; photons emitted per photons absorbed) were obtained relative to Ru(bpy)2+ 3 (ϕem (H2O) = 0.042) (48,49) and ECL efficiencies (ϕecl = photons generated per redox event) were obtained by methods found in the literature, relative to Ru(bpy)2+ 3 –TPrA (ϕecl = 1) using the following equation (50,51):

Intensity (a.u.)

500 400 300 200 100 0 450





Wavelength (nm)


ϕecl ¼

∫0 I dt t ∫0 i



Figure 1. Photoluminescence spectra of 1 μM Ir(ppy)3 in (—) 30% trifluoroethanol (  ) 50% CH3CN:H2O and (•••) 0.1 M potassium phosphate buffer. pH 8.0 ± 0.2.

Table 1. Spectroscopic and electrochemical data for Ir(ppy)3 Solvent

E1/2 (+1/0), Va

λabs (nm)

λem (nm)



– 0.49 0.46

383 383 383

507, 532(sh) 507, 532(sh) 507, 532(sh)

0.029 0.0021 0.0016

0.0048 0.0091 0.00092

30% Trifluoroethanold CH3CN:H2O (50 : 50 v/v)d H2Od

a All electrochemical and electrogenerated chemiluminescence experiments were referenced with respect to Ag–AgCl gel electrode (0.20 V vs. Normal Hydrogen Electrode (NHE)) (42). b Photoluminescence efficiency with respect to Ru(bpy)2+ 3 (ϕem = 0.042) (48,49). c Relative electrogenerated chemiluminescence efficiency with respect to Ru(bpy)2+ 3 (ϕecl = 1) (50,51). Reported values are the average of at least three scans with a relative SD of ± 5%. d 0.1 M KH2PO4 as electrolyte.


Copyright © 2014 John Wiley & Sons, Ltd.

Luminescence 2015; 30: 67–71

ECL of tris(2-phenylpyridine)iridium(III)

9,000,000 7,000,000 6,000,000 5,000,000 4,000,000 3,000,000 2,000,000

ECL intensity (c.p.s.)


1,000,000 0 2





Potential (V)

the sensitivity of these excited states to solution composition. Interestingly, while Φem is slightly higher for the 30% TFE solution compared to the 50% MeCN solution, ΦECL is lower for the 30% TFE solution. The reasons for this are unclear and may be due to electrode and solution dynamics in the production of electroactive products and during excited state formation. ECL is linear with respect to Ir(ppy)3 in water when [Ir(ppy)3] is varied from 1 to 0.01 μM in 50% MeCN, and 1 μM to 1 nM in the other solvents with determination coefficients (r2) from 0.9985 to 0.9657 (SD ± 5%). Typical ECL intensity versus time transient for Ir(ppy)3 in all three solvents are in Fig. 4. The potential of the working electrode was poised at + 2.0 V while measuring ECL intensity. As expected in the absence of TFE or MeCN, there was an immediate increase in light intensity due to the higher concentrations of coreactant and luminophore near the electrode surface that then decreased over time and repeated cycling. This behavior has been observed for Ru(bpy)2+ in 3 other solvent systems and indicates that ECL generation is diffusion controlled. When TFE or MeCN is present in solution, the ECL continues to rise over the course of the experiment as was also observed with Ru(bpy)2+ 3 in TFE (27). The reasons for this are unclear, but suggests that more ECL reaction events are occurring in the presence of the fluorinated solvent, which may also explain the enhanced ECL intensities of the mixed solvent systems. Analogous to Ru(bpy)2+ 3 and other transition metal systems, a likely mechanism for the production of the excited state in the Ir(ppy)3/TPrA system is the following (25,26,58):


35,000 30,000

intensity (c.p.s.)

of ECL intensity versus potential is shown in Fig. 2. The ECL intensity peaks at potentials between + 1.1 and + 1.3 V, indicating oxidation of both TPrA (Ea ~ + 0.9 V vs. Ag–AgCl) (25,26) and Ir(ppy)3 (EA½ ~ + 0.500 V) (27) has occurred. It is well known that transition metal complexes such as Ru(bpy)2+ 3 can react with hydroxyl groups to produce ECL. This occurs upon oxidation in aqueous alkaline solutions at pH ≥ 10 (52–56). To be certain that the solvents were not acting as coreactants, all experiments in this study were run at pH 8.0 ± 0.2. Background studies also confirmed that very little ECL was emitted at this pH when TPrA was absent from solution. A representative ECL emission spectrum is shown in Fig. 3. Spectra in each solvent are similar to PL spectra in both shape and λmax (wavelength of maximum emission) within experimental error (± 10 nm). Clearly, the same metal-toligand charge transfer excited state is formed in both experiments. This also indicates that any effects from the solvent sphere observed in PL experiments are also present in ECL and that TPrA and electrolytes in solution do not interfere with these processes. Ir(ppy)3 demonstrated dramatically lower ECL efficiencies (ΦECL) than Ru(bpy)2+ 3 under identical conditions (Table 1). This was also observed in previous studies for Ir(ppy)3 in aqueous and partially aqueous solution (37,57). Both PL and ECL emission efficiencies were higher than aqueous solutions for both the 50 : 50 MeCN–H2O and 30% TFE solutions showing

25,000 20,000 15,000 10,000 5,000

Figure 2. ECL intensity versus potential for 1 μM Ir(ppy)3 (0.05 M TPrA) in (—) 50 : 50 (v/v) MeCN:KH2PO4 and (  ) 30% trifluoroethanol. ECL, electrogenerated chemiluminescence.


ECL intensity (c.p.s.)

3.2 3 2.8 2.6 2.4 2.2


20,000,000 18,000,000 16,000,000 14,000,000 12,000,000 10,000,000 8,000,000 6,000,000 4,000,000 2,000,000 0



580 630 Wavelength (nm)





Time (s) Figure 4. ECL versus time for 1 μM Ir(ppy)3, and 0.05 M TPrA at pH 8.0 ±0.1 in (A) 0.2 M potassium phosphate buffer, and (B) 50 : 50 (v/v) KH2PO4:MeCN and 30% TFE. ECL, electrogenerated chemiluminescence; TFE, trifluoroethanol.

Copyright © 2014 John Wiley & Sons, Ltd.


Luminescence 2015; 30: 67–71


30% TFE


Figure 3. ECL spectrum of 10 μM Ir(ppy)3 and 0.05 M TPrA in 50 : 50 (v/v) 0.1 M KH2PO4:MeCN (pH 8.0 ±0.2). ECL, electrogenerated chemiluminescence.


50:50 MeCN:KH2PO4




Time (s)



ECL intensity (a.u.)


W. D. Robinson and M. M. Richter – e– → IrðppyÞ 1þ  þ 3 TPrA – e– → TPrA· → TPrA· þ Hþ









þ TPrA· → IrðppyÞ



þ products

→ IrðppyÞ 0 þ hν 3

ECL is produced upon concomitant oxidation of Ir(ppy)03 and TPrA. Upon oxidation, the short-lived TPrA radical cation (TPrA•+) is believed to lose a proton from an α-carbon to form the strongly reducing intermediate TPrA• (25,26,58). 0* TPrA• can then reduce Ir(ppy)1+ 3 to Ir(ppy)3 (28–37).

Conclusions This study illustrates the effects of various solvent mixtures, most notably TFE, on the electrochemical, spectroscopic and coreactant ECL of Ir(ppy)3. The most dramatic effects are in PL and ECL quantum yields for emission where, in all but the 50 : 50 (v/v) MeCN–KH2PO4 mixed solvent, lower values are observed during ECL, demonstrating the sensitive nature of Ir(ppy)3 excited state to solvent medium. However, both ϕem and ϕecl are higher in 30% TFE compared to aqueous buffered solution, which when coupled to the same observation observed in the Ru(bpy)2+ 3 /TPrA systems, shows that fluorinated alcohols are promising solvents for ECL. Acknowledgments Many thanks to Missouri State University for funding.



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Supporting information Additional supporting information may be found in the online version of this article at the publisher’s web site.


Luminescence 2015; 30: 67–71

Copyright © 2014 John Wiley & Sons, Ltd.

Electrogenerated chemiluminescence of tris(2-phenylpyridine)iridium(III) in water, acetonitrile and trifluorethanol.

The spectroscopic, electrochemical and coreactant electrogenerated chemiluminescence (ECL) properties of Ir(ppy)3 (where ppy = 2-phenylpyridine) have ...
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