Tuning the Electrochemiluminescence Color by Potential: Design of a

Jun 30, 2015 - A series of heterodinuclear complexes [(bpy)2Ru(bpy)(CH2)n(bpy)Ir(df-ppy)2]3+ (1, where bpy is 2,2′-bipyridyl, df-ppy is 2-(2,4-diflu...
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Tuning the Electrochemiluminescence Color by Potential: Design of a Series of Heterodinuclear Ir/Ru Labels Wei Sun,† Shiguo Sun,*,† Na Jiang,† Hong Wang,† and Xiaojun Peng† †

State Key Laboratory of Fine Chemicals, Dalian University of Technology, 2 Ling gong Road, Dalian 116024, People’s Republic of China S Supporting Information *

ABSTRACT: A series of heterodinuclear complexes [(bpy)2Ru(bpy)(CH2)n(bpy)Ir(df-ppy)2]3+ (1, where bpy is 2,2′-bipyridyl, df-ppy is 2-(2,4-difluorophenyl)pyridine, and n = 10, 12, 14) have been designed and synthesized. Both red (from the Ru moiety) and green (from the Ir moiety) electrochemiluminescence (ECL) can be acquired simultaneously thorough alternation of the scanning potentials; especially, a good linear calibration curve between the ECL intensity ratio (IRu/IIr) and the scanning potential can be reached over a potential range from 0.55 to 1.6 V. All of this provides a general methodology for developing electrochemistry induced light-emitting devices and a ratiometric ECL detection method.

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INTRODUCTION Currently, the word “electrochromism” has been used to describe any device whose color can be adjusted by the application of an electrochemical potential and the related electrochemical conversion of redox species from one color to another, with the development of molecular electrochemistry and modified electrodes.1 A number of inorganic and organic2 electrochromic devices have been developed and widely used, since the first paper dates back to 1968.3 Electrochemiluminescence (ECL) is one type of electrochromism, which involves electron transfer between electrochemically generated radical ions in solution to produce excited species that emit light.4 Typically, the use of ruthenium(II) tris(bipyridyl) ECL labels and tripropylamine (TPrA) coreactant has been found to be a sensitive and versatile detection system.4 Although ruthenium(II)-based systems5 have been dominating applications for many years, recently cyclometalated iridium(I) complexes6 have become a powerful alternative, due to their most efficient electroluminescent performance. As we know, the scanning potential can be easily tuned and applied at multiple levels. A clear change in the color of the emitted light from a mixture of ruthenium(II) and iridium(I) complexes with TPrA as coreactant was visually inspected at the electrode surface with increasing electrode potential conducted in a dark room;7 even more importantly, the potentialcontrolled selective excitation in mixed electrochemiluminescent systems and the on/off switching mechanism has been explained.8 A three-dimensional (intensity versus wavelength and potential) resolution of electrochemiluminophores has been obtained through the repeated acquisition of ECL spectra © XXXX American Chemical Society

during cyclic voltammetry experiments. Meanwhile, using a mixture of ruthenium(II) and iridium(III) complexes in acetonitrile with TPrA as a coreactant, Richter et al. got a considerable ECL spectral overlap.9 Recently, red, green, and blue emitters have been efficiently resolved over the threedimensional space of ECL intensity versus applied potential and emission wavelength.10 Unfortunately, it should be noted that the mixed system utilizes intermolecular interactions to tune the emission color, which is easily affected by different objective factors such as molar ratio between the ECL labels, coreactant, working electrode, solvent system, etc., and the resulting color changes lack any regularity and are not easily controlled; all these factors make it hard to find any real applications for the system. To solve this problem, a potential approach is to utilize the concerted intramolecular and intermolecular interaction ECL synchronously11 to obtain a potential-controlled ratiometric switch on/off system through selective oxidation of the electrochemiluminescent labels. Those kinds of electrochemiluminophores can be selectively addressed through not only their excitation wavelengths but also their characteristic redox energies, by scanning or stepping from low to high electrode potentials to get different colors. In our previous work,12 two ruthenium centers were covalently linked through different carbon chain linkages; the ECL intensity can be significantly increased through combining both intermolecular and intramolecular interaction ECL. Schmittel et al.13 demonstrated that “non-Kekulé-structured” trinuclear Ir(III)−Ru(II)−Ir(III) species were dependent upon Received: May 1, 2015

A

DOI: 10.1021/acs.organomet.5b00353 Organometallics XXXX, XXX, XXX−XXX

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Organometallics the applied potential, enabling the wavelength of maximum ECL intensity to be tuned from 649 to 611 nm with increasing anodic scan range, although only the red color can be acquired. All these findings provide further support for our suggestion. Considering the well-known redox properties of ruthenium and iridium ECL labels, it would be interesting to observe a combination of Ir+ (green color) and Ru2+ (red color) centers in a single molecule. Thus, a series of bimetallic ruthenium− iridium complexes, [(bpy)2Ru(bpy)(CH2)n(bpy)Ir(df-ppy)2]3+ (1, where bpy is 2,2′-bipyridinyl, df-ppy is 2-(2,4difluorophenyl)pyridine, and n = 10, 12, 14 for 1a−c, respectively; Scheme 1), were designed by connecting typical Scheme 1. Structures of the Bimetallic Complexes 1 and the Corresponding Reference Molecules [Ru(bpy)3]2+ and [Ir(df-ppy)2(bpy)]+

Figure 1. UV−vis absorption (a) and emission spectra (b) of complexes [Ru(bpy)3]2+, [Ir(df-ppy)2(bpy)]+, [Ru(bpy)3]2+:[Ir(dfppy) 2(bpy)]+ (1:1), and 1a−c in acetonitrile. The complex concentration was 10−5 M.

(bpy)3]2+ and [Ir(df-ppy)2(bpy)]+. All of these bands can be ascribed to the coexistence of the ruthenium and iridium activating centers. Steady state emission spectra for 10−5 M acetonitrile solutions of [Ru(bpy)3]2+, [Ir(df-ppy)2(bpy)]+, a 1:1 monometallic mixed solution of [Ru(bpy) 3 ] 2+ and [Ir(dfppy)2(bpy)]+, and 1a−c were recorded with excitation of the MLCT band at 365 nm (Figure 1b). The profiles are consistent with the MLCT emission commonly observed in Ru(II) and Ir(I) polypyridyl complexes. As shown in Figure 1b, the complexes 1a−c exhibit dual emission maxima at 522 and 611 nm, respectively, similar to the corresponding metallic [Ir(dfppy)2(bpy)]+ emission wavelength (527 nm) and [Ru(bpy)3]2+ emission wavelength (615 nm). The integrated emission intensities, measured as the total number of emitted photons between 450 and 620 nm, showed that complexes 1a−c exhibit a similar emission efficiency, which is slightly higher than that of [Ru(bpy)3]2+ (Figure 1b) and much lower than that of [Ir(df-ppy)2(bpy)]+. As for the 1:1 mixture of [Ru(bpy)3]2+ and [Ir(df-ppy)2(bpy)]+, the intensity of [Ru(bpy)3]2+ is higher than that of the corresponding mononuclear [Ru(bpy)3]2+, but the intensity of [Ir(df-ppy)2(bpy)]+ is lower than that of the corresponding mononuclear [Ir(df-ppy)2(bpy)]+. All these bands can be ascribed to the phosphorescence resonance energy transfer occurring from [Ir(df-ppy)2(bpy)]+ to [Ru(bpy)3]2+.14 As reported, several organic or inorganic processes are markedly dependent on the surrounding polarities, which greatly control the reaction processes.15 1a−c have two metal centers; therefore, they may be sensitive to the polarity. To investigate the behavior of 1 in different polar environments, 1b was taken as an example and its ECL performance was measured in an ensemble of solvents covering a large polarity interval at room temperature. The results are illustrated in Figure S1 in the Supporting Information. It is noted that the

red-emitting ruthenium and green-emitting iridium ECL labels through a flexible saturated carbon chain, to investigate the effect of dual ECL performance aroused by the different carbon chain linkage number n. A good linear calibration curve between the ECL intensity ratio (IRu/IIr) and the adjusted scanning potentials can be observed, and the ECL color can be tuned from green to red by alternating the scanning potential; all of these findings demonstrated that the electrochromism efficiency can be improved and tuned by using the bimetallic complexes, leading to some fundamental understanding for developing electrochemistry-induced light-emitting devices, dual electrochromism, and ratiometric ECL detection methods.

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RESULTS AND DISCUSSION The UV−visible spectra of [Ru(bpy)3]2+, [Ir(df-ppy)2(bpy)]+, a 1:1 monometallic mixed solution of [Ru(bpy)3]2+ and [Ir(dfppy)2(bpy)]+, and 1a−c measured in acetonitrile at a concentration of 10−5 M are shown in Figure 1a, and the spectral data are summarized in Table S1 in the Supporting Information. As shown in Figure 1a, the absorption spectra for 1a−c exhibit broad overlapping MLCT (metal to ligand charge transfer) bands at ca. 450 nm due to dπ−π* transitions from metal center to the mixed π* orbitals of the ligands. More intense absorption bands aroused by the intraligand π−π* transitions are observed between 250 and 300 nm. The absorption of complexes 1 and a 1:1 monometallic mixed solution of [Ru(bpy)3]2+ and [Ir(df-ppy)2(bpy)]+ are nearly the overlap of the corresponding absorptions for metallic [RuB

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Organometallics ECL of 1b was very sensitive to the solution polarity. When we use more strongly polar solvents, such as water, 1b exhibited a much more perfect dual emission at 543 and 626 nm. In lower polarity solvents, more positive oxidation potentials of the ruthenium moiety and more negative oxidation potentials of the iridium moiety can be observed, which provided an even larger driving force for the electron transfer (ET) and energy transfer processes from the iridium moiety to the ruthenium moiety.16 It is difficult to find the emission of the iridium moiety at 543 nm along with a decrease of the solvent polarity; thus, all of the following experiments were carried out in phosphate buffer solution (PBS, 0.1M, pH 7.4), which not only gives an ideal dual emission result but also can have potential application in living systems. To check the ideal carbon chain linkage number n for achieving satisfactory dual emission, ECL properties for bimetallic iridium and ruthenium complexes 1 at a Pt electrode have been studied in PBS (0.1M, pH 7.4) and 2(dibutylamino)ethanol (DBAE) as the coreactant. The result demonstrated that 1b exhibited the most obvious dual emission, having two well-resolved emission bands centered at 545 and 615 nm (Figure 2), which can be assigned to the

emission of iridium and ruthenium moieties, respectively. The observation is well explained by a mechanism which involves the formation of an excited state upon reaction of DBAE•+ with [Ru(bpy)3]2+ and [Ir(df-ppy)2(bpy)]+, and this mechanism dominates when the coreactant is in large excess.8 The mechanism of ECL spectral emissions of 1b is proposed as follows: (1)

DBAE•+ − H+ → DBAE•

(2)

+

1b − e → 1b

(3)

1b+ + DBAE• → 1b* + P

(4)

1b* → 1b + hv

(5)

In addition, 1b affords two excited states, namely *Ir−Cn− Ru and Ir−Cn−*Ru (eqs 6 and 7). The reductive quenching of *Ir−Cn−Ru by an intramolecular electron transfer process (eq 9) and the building of Ir−Cn−*Ru by an intramolecular energy transfer process (eq 10) are also taken into account; all of these could be responsible for the aforementioned results.17 Ir−Cn−Ru → *Ir−Cn−Ru

(6)

Ir−Cn−Ru → Ir−Cn−*Ru

(7)

*Ir−Cn−Ru → Ir−Cn−Ru + hv

(8)

(9)

*Ir−Cn−Ru → Ir−Cn−*Ru

(10)

Ir−Cn−*Ru → Ir−Cn−Ru + hv

(11)

In contrast, although the emission band of ruthenium at 615 nm can be observed, the emission band of iridium at 545 nm is not clear enough in the case of complexes 1a,c. The shorter carbon chain linkage of 1a could have a relatively greater steric hindrance, exhibiting a strong impact on the dual ECL intensity; meanwhile, the efficiency of the intramolecular energy transfer is related to the distance between Ru(II) and Ir(I) in the bimetallic complexes, and 1c has a longer carbon chain linkage than 1b; all of these findings can be responsible for 1b having a higher efficiency of intramolecular energy transfer and ECL intensity.17 The same trend can be seen in the case of TPrA coreactant (Figure S2 in the Supporting Information). All these findings demonstrated that, with the presence of an amine coreactant, the synergistic effect on ECL between the two intramolecularly linked ruthenium and iridium activating centers depends largely on the length of the flexible saturated carbon chain linkage, and the best linkage number among the three tested numbers (n = 10, 12, 14) for the bimetallic system is 12. Figure 3 shows the results of ECL experiments conducted using 1b dissolved in PBS containing 5 mM DBAE. When the potential was swept from 0 V to values between 0.55 and 1.6 V, different ECL can be emitted from the electrode surface, with a brightness that depended on the potential. The ECL spectrum at 0.55 V began to display a weak wave centered at 543 nm (Figure 3a), which is close to the photoluminescence (PL) peak wavelength, 522 nm in acetonitrile. The discrepancy between the ECL and PL wavelengths is probably due to the different solvent and the self-absorption aroused by the higher concentrations used in the ECL study.18 Still in this system, the intensity of the red ECL from the ruthenium moiety at 618 nm increases steadily and slowly over the range 0.55−0.95 V before rising sharply (Figure 3a), which is basically consistent with the differential pulse voltammetry (DPV) curve in Figure S3 in the Supporting Information. As shown in Figure 2b, the ECL intensity at 543 and 618 nm increased gradually along with the scanning potential from 0.55 to 0.95 V and a good linear calibration curve between the ECL intensity ratio (I618 nm/I543 nm) and the scanning potential can be established over a potential range from 0.55 to 0.95 V, providing a general methodology to control ECL color changes much more easily and precisely; this would be helpful in building a ratio ECL detection method. When the applied scanning potential was further increased from 1.0 to 1.6 V (Figure 3c), the intensity of the red ECL from the ruthenium moiety at 618 nm increased sharply, while the intensity of the green ECL from the iridium moiety at 543 nm grew little, and the peak of the iridium moiety was overlapped with that of ruthenium moiety, due to the highly efficient phosphorescence resonance energy transfer from the iridium moiety to the ruthenium moiety. The ECL intensity of the ruthenium moiety increased and reached a maximum at 1.25 V, which is in good accordance with the first maximum potential appearing in the ECL−voltage curve. More importantly, the apparent ECL peak of the ruthenium moiety at 618 nm showed two good linear calibration curves (Figure 3d) between the ECL intensity and the scanning potential at a potential range

Figure 2. ECL spectra of 5 × 10−5 M 1 and 5 mM DBAE in PBS (0.1 M, pH 7.4).

DBAE − e → DBAE•+

*Ir−Cn−Ru → Ir−Cn−Ru

C

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Figure 3. (a) ECL spectra generated using an electrode potential sweep at 0.01 V s−1 from 0 to 0.95 V for 5 × 10−5 M 1b and 5 mM DBAE as coreactant in PBS (0.1 M, pH 7.4). (b) Linearity of the ECL intensity ratio (I618 nm/I543 nm) and the scanning potential. (c) ECL spectra generated using an electrode potential swept from 0.95 to 1.60 V. (d) Linearity of the ECL intensity at 618 nm and the scanning potential.

from 1.0 to 1.25 V and from 1.25 to 1.6 V, respectively. Utilization of intra- and intermolecular ECL performance and ratiometric ECL will be certainly attractive for multiplexed analysis. The ability to distinguish between different colored emitters (at different scanning potential) would increase the potential applicability of this approach for applications such as low-cost point of care diagnostics and light-emitting devices. The efficient phosphorescence resonance energy transfer occurred from iridium to ruthenium, making the ECL intensity of ruthenium much higher than that of iridium, which somehow overlapped with that of the iridium emission due to the gap of 75 nm between them. This provides the possibility to control the ECL color through a change in scanning potential. At lower potentials (less than 0.6 V), the green emission from the iridium moiety can be selectively addressed for 1b (Figure 3a). Along with an increase in the scanning potentials, the green ECL was replaced gradually by a red emission from the ruthenium moiety. Importantly, unlike the previously reported dual-emission experiments19 where high potential resulted in simultaneous emission of light from each luminophore, here the green light was effectively “switched off” at high potentials, leaving only a red emission in the system. The switching process was immediate and reversible, requiring only the selection of appropriate potentials. The ability to switch the ECL emission color of 1b between green and red is realized through alternating electrode potential, and the on−off switching phenomena are quite dependent on the scanning potential. As a demonstration of the use of digital cameras to discriminate between electrochemiluminophores, we analyzed photographs of ECL (Figure 4) using cyclic voltammetry experiments that switched the color from green to red at each potential. Because of the efficient phosphorescence resonance energy transfer occurring from iridium to ruthenium, making the ECL intensity of ruthenium much higher than that of iridium with an

Figure 4. Photographs of electrogenerated chemiluminescence from (a) a single solution of 10−3 M 1b at the four electrode potentials: 0.95, 1.05, 1.2, and 1.4 V and (b) 10−3 M 1b together with 3 × 10−3 M reference molecules of [Ir(df-ppy)2(bpy)]+ at the five electrode potentials 0.5, 0.6, 0.95, 1.2, and 1.4 V, with 100 mM DBAE as coreactant in 0.1 M (n-Bu)4NPF6 acetonitrile.

increase in the potential, which somehow made the green color of iridium difficult to observe with the naked eye (Figure 4a), we added 3 equiv of [Ir(df-ppy)2(bpy)]+ as a green control to make the color of iridium display on the screen (Figure 4b). We are now trying to increase the ECL efficiency of iridium (for example, to increase the number of iridium in the molecule), so that both the green and red colors can be ratiometrically adjusted with a single molecule. All these findings create new possibilities for color manipulation in electrochemistry induced light-emitting materials and devices. In the future, we want to fabricate similar solid organic electroluminescent devices (OLEDS) for demonstrating the light modulation.20 To make a comparison, the ECL performance was investigated for the reference metal complexes (Ru(bpy)32+, Ir(F2ppy)2(bpy)+, and a 1:1 mixed solution of Ru(bpy)32+ and Ir(F2ppy)2(bpy)+) under the same conditions. As shown in Figure S4 in the Supporting Information, the ECL intensity is sensitive to the scanning potential from 0.7 to 1.2 V; however, the ECL of Ru(bpy)32+ and Ir(F2ppy)2(bpy)+ cannot avoid the influence of DBAE due to its significant background noise,21 D

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which is overlapped with them. What is more, the two reference complexes can only emit single color under different electrode potentials. For the 1:1 monometallic mixed solution of Ru(bpy)32+ and Ir(F2ppy)2(bpy)+ at a Pt electrode, both the green and red emissions can be observed at different potential, but no linear relation can be found between the ECL intensity and the scanning potential from the result (Figure S4).

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NSF of China (21272030, 21472016, 21421005).

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CONCLUSION In conclusion, the ability to switch the ECL color of 1b between green and red using the electrode potential has been demonstrated; the on−off switching phenomenon is shown to be dependent on the applied scanning potential in the PBS. Even more importantly, by changing the electrode potential from 0.55 to 0.90 V, a good linear calibration curve between the ECL intensity ratio (IRu/IIr) and the scanning potential can be realized. The ability to tune the ECL intensity ratio with alternating electrode potentials creates new possibilities for color manipulation in electrochemistry induced light-emitting devices. The work described herein has important implications for multiplexed ECL-based assays, as it enables the selective excitation of intramolecular luminophores on the basis of scanning potential without interference from intermolecular interactions.

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ASSOCIATED CONTENT

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The absorption spectra were recorded on a HP 8453 spectrophotometer. Fluorescence spectra were recorded by a PerkinElmer LS 55 luminescence spectrometer. Differential pulse voltammetry (DPV) was measured on a CHI 660D instrument using a three-electrode system consisting of Ag/Ag+ (0.01 M AgNO3 in acetonitrile) as the reference electrode, a platinum wire as the counter electrode, and freshly polished glassy carbon (GC, diameter 3 mm) as the working electrode. The ECL measurements were performed on a homemade joint system of electrochemistry and a fluorescence spectrometer. All experiments were carried out at room temperature. ECL studies were carried out by using the literature method.22 Different volumes of the corresponding amine were added to acetonitrile solution which contained 5 × 10−5 M of the corresponding ECL labeling complex in PF6− salt. An AgNO3saturated Ag/Ag+ electrode and a platinum-wire electrode were used as the reference and auxiliary electrodes, respectively. The working electrodes (Pt wire mesh 1.0 cm in the side of one square) were sonicated and rinsed with deionized water. Cyclic potential sweep experiments were carried out in the potential region from 0 to 1.6 V and then back to 0 V at a scan rate of 0.10 V/s, and then the ECL signals and CV versus time were recorded. The net peak intensities of the recorded ECL signal versus complex 1 concentration were measured repeatedly at least five times, and the averaged readings were used for the creation of plots.

S Supporting Information *

Text, a table, and figures giving experimental details, materials and reagents, synthesis of ruthenium(II) and iridium(II) tris(bipyridyl) complexes 1, detail of PL emission spectra and DPV curve, and ECL spectra generated by [Ir(df-ppy)2(bpy)]+, [Ru(bpy)3]2+, and 1:1 Ru(bpy)32+:Ir(F2ppy)2(bpy)+ using an electrode potential sweep from 0 to 1.20 V. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00353.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail for S.S.: [email protected]. E

DOI: 10.1021/acs.organomet.5b00353 Organometallics XXXX, XXX, XXX−XXX