Electrochemically regulated luminescence of europium complexes with β-diketone in polyether matrices

Abstract

This study investigates the electrochemical modulation of luminescence color, i.e., electrofluorochromism, of an Eu complex in a polyether solvent. The electrofluorochromic (EFC) reaction of the Eu complex occurred via a reversible redox reaction between Eu³⁺ and Eu²⁺. Initially, the intrinsically stable Eu³⁺ complex showed intense red photoluminescence (PL) induced by f–f transitions. After the electrochemical reduction of Eu³⁺ to Eu²⁺, broad blue PL was observed attributed to the d–f transitions in the Eu²⁺ complex. This distinct blue luminescence from the Eu²⁺ complex was attributed to the effective stabilization of the Eu²⁺ state by the polyether solvent. The dynamic EFC reaction that changes the valence state of the Eu ion can be potentially applied to novel chemical sensors, security devices, and display devices.

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Introduction

Chromogenic materials exhibit optical properties including luminescence and absorption that can be altered using external stimuli such as light, heat, and electricity. They show potential applications in chemical sensors,^1,2 biochemical labels,³ molecular memory,⁴ and display devices.^5,6 Electrofluorochromic (EFC) materials display a change in luminescence color regulated via electrochemical redox reactions. They are innovative materials as they can convert electrical inputs into visual signals rapidly and repeatedly,^4,7–10 and include molecules,^11–15 conjugated polymers,^16–20 inorganic compounds,^21,22 and metal complexes.^23–29

Ln(III) complexes are composed of a luminescent core of Ln(III) ions surrounded by antenna ligands possessing a light-absorbing capability. The luminescence of Ln(III) complexes is enhanced via efficient intramolecular energy transfer from the antenna ligands to the Ln(III) center, resulting in brilliant luminescence.^30–33 Furthermore, Ln(III) complexes show attractive photoluminescence (PL) properties such as characteristic and narrow emission bands in the visible-near infrared (vis-NIR) region attributed to Ln(III), long emission lifetimes, and high transparency in the visible region due to the large pseudo-Stokes shift.³⁴ In particular, Eu complexes have excellent emission properties, and the stabilities of their trivalent (Eu³⁺) and divalent (Eu²⁺) states are higher than those of other Ln(III) complexes. Eu³⁺ is characterized by intense and long-lived red luminescence induced by f–f transitions and is extensively utilized in the development of biosensors, light-emitting materials, etc. Eu²⁺ is characterized by broad and blue luminescence induced by d–f transitions from the excited state of 4f₆ 5d₁ to the ground level state of 8S_7/2 (4f₇),³⁵ and the emission from Eu²⁺ in inorganic matrices could be applied to various potential luminescent devices.^36–39 The reversible control of red luminescence from Eu³⁺ and blue luminescence from Eu²⁺via electrochemical redox reactions can contribute to the development of a novel display device. However, the observation of luminescence from Eu²⁺ is challenging because it is unstable in air and commonly used solutions. Therefore, previous studies reported high temperature and extended time requirements for Eu²⁺ formation; for instance, Eu²⁺-doped zeolite derivatives were synthesized via annealing of Eu-containing zeolites in a reducing atmosphere at 800 °C for 3 h.³⁹ Additionally, the electrochemical control of trivalent and divalent ions is difficult, therefore, this study focuses on regulating the luminescence between Eu³⁺ and Eu²⁺via electrochemical redox reactions. It is well-known that the blue luminescence of Eu²⁺ can be stabilized by polyethers, such as crown ethers and polyethylene glycols.^35,40 Upon stabilization, the Eu²⁺ state could be produced recurrently in a facile manner via the electrochemical reduction of Eu³⁺ in polyethylene glycol solutions. Considering the EFC reaction of Eu compounds, the β-diketonate Eu complex exhibiting superior red luminescence properties in the Eu³⁺ state was explored in the present study.

The electrochemical redox reactions of polyether solutions containing Eu ions and a β-diketonate Eu complex were investigated. The studies revealed that the electrochemically generated Eu²⁺ state was stable in both the solutions containing Eu ions and the Eu complex. A distinct change in luminescence color, i.e., electrofluorochromism, was observed via the electrochemical reduction of the Eu complex in the two-electrode device using luminescence spectroscopy and the naked eye.

Experimental

Materials

Europium nitrate hexahydrate (Eu(NO₃)₃·6H₂O), tetra-n-butylammonium perchlorate (TBAP), silver nitrate (AgNO₃), propylene carbonate (PC), and acetonitrile (MeCN) were purchased from Kanto Chemical Industry Co., Japan. Dimethyl sulfoxide (DMSO) was purchased from Sigma Aldrich Japan. Hexafluoro acetylacetone (hfa) was procured from Tokyo Chemical Industry Co., Japan. Lithium trifluoromethanesulfonate (CF₃SO₃Li), lithium nitrate (LiNO₃), and polyethylene glycol 400 (PEG₄₀₀) were purchased from FUJIFILM Wako Pure Chemical Co., Japan. CF₃SO₃Li was used as the supporting electrolyte without further purification. Indium tin oxide (ITO) (10 Ω sq⁻¹; Yasuda, Japan) was used as an electrode after washing and ozonation.

Synthesis of the luminescent Eu(III) complex

Tris (hexafluoro acetylacetonate) europium (Eu(hfa)₃(H₂O)₂) was prepared according to a method previously reported in the literature.⁴¹ Europium acetate n-hydrate was dissolved in deionized water at room temperature. Subsequently, 3 equiv. of the liquid hfa-H₂ was added dropwise into the solution. After stirring for 3 h, the obtained white precipitate was filtered and purified by recrystallization using methanol/water. Yield: 80%. Anal. Calcd for C15H7O8F18Eu: C, 22.48; H, 0.88%. Found: C, 21.52; H, 1.04%.

Preparation of the electrolyte solutions

The electrochemical redox behaviour of Eu ions was investigated using a Eu³⁺ solution containing Eu(NO₃)·6H₂O (10 mmol L⁻¹) and LiCF₃SO₃ (500 mmol L⁻¹) in PEG₄₀₀. For comparison, two types of electrolyte solutions without Eu ions, namely LiCF₃SO₃ (500 mmol L⁻¹) in PEG₄₀₀, and LiNO₃ (60 mmol L⁻¹) and LiCF₃SO₃ (500 mmol L⁻¹) in PEG₄₀₀, were prepared. The electrochemical redox behaviour of the β-diketonate Eu complex was investigated using a PEG₄₀₀ solution containing Eu(hfa)₃(H₂O)₂ (10 mmol L⁻¹) and LiCF₃SO₃ (500 mmol L⁻¹). For comparison, a solution composed of hfa (30 mmol L⁻¹) and LiCF₃SO₃ (500 mmol L⁻¹) in PEG₄₀₀ was prepared. The solutions for optical measurements were purged with nitrogen gas for 20 min before each measurement.

Construction of EFC devices

A two-electrode cell was constructed with ITO glass electrodes or carbon-modified ITO electrodes (CM electrodes) using a silicon spacer (300 μm; Mitsubishi Chemical, Japan). CM electrodes were prepared according to a previously reported method.⁴² Carbon paste was directly coated onto the ITO substrate via blade coating, and the gap between the blade and the ITO substrate was 180 μm. The obtained porous CM electrode was heated to 250 °C for 1 h on a hot plate. The Eu complex solution was sandwiched between the ITO electrodes (ITO–ITO device) or between the ITO and CM electrodes (ITO–CM device) to evaluate their electrochromic and PL properties.

Electrochemical measurements

For the electrochemical measurements, a three-electrode cell comprising an ITO electrode as the working electrode, Pt wire (φ = 1 mm) or a CM electrode as the counter electrode, and an Ag/Ag⁺ electrode as the reference electrode were used. The reference electrode was prepared by injecting an Ag⁺ solution (10 mmol L⁻¹ AgNO₃ and 100 mmol L⁻¹ TBAP in MeCN) into a sample holder with ion-permeable glass (BAS, Japan), and sealing with a cap containing an Ag wire (φ = 1 mm). Cyclic voltammetry (CV) measurements of the three-electrode cell were conducted using an electrochemical analyzer (ALS 440A, CH Instruments Inc., USA).

Photophysical measurements

The PL spectra of Eu(NO₃)₃·6H₂O and Eu(hfa)₃(H₂O)₂ in the electrolyte solution were obtained using a JASCO FP-6600 spectrofluorometer. Quartz optical cells with an optical path length of 1 cm and the three-electrode electrochemical cell were used for the measurements. For the two-electrode EFC devices, the luminescence properties were characterized using an epifluorescence unit (JASCO EFA-133). Excitation wavelengths of 272 and 365 nm were used for the Eu(NO₃)₃·6H₂O and Eu(hfa)₃(H₂O)₂ solutions, respectively. The excitation spectra at a detection wavelength of 450 nm were obtained by subtracting the Raman scattering of PEG₄₀₀.

Electrode potential measurement

Electrode potential measurements were performed to characterize the redox behavior of the Eu(hfa)₃(H₂O)₂ solution using two electrochemical analyzers, namely HZ-7000 (Hokuto Denko Co., Japan) and ALS 440A, as shown in Scheme 1, following the method in our previous report.⁴³ An ITO electrode was used as the working electrode, ITO or CM electrodes as the counter electrodes (the distance between the electrodes was 1 cm), and an Ag/Ag⁺ electrode placed between the counter electrodes was used as the reference electrode.

Scheme 1 The structure of a two-electrode cell system for the measurement of electrode potential using two electrochemical analyzers.

Results and discussion

Redox and luminescence properties of Eu ions and the Eu complex in PEG₄₀₀

The CV curves of Eu(NO₃)₃·6H₂O in PEG₄₀₀ using the constructed three-electrode cell structure are shown in Fig. 1, displaying the redox current corresponding to the reduction and oxidation reactions of Eu ions dissolved in PEG₄₀₀.

Fig. 1 Cyclic voltammograms of Eu(NO₃)₃·6H₂O or LiNO₃ in PEG₄₀₀. ([LiCF₃SO₃] = 500 mM, [Eu(NO₃)₃·6H₂O] = 10 mM, [LiNO₃] = 60 mM).

Scanning from the negative potential direction, a reduction current appeared from −0.75 V, reaching a maximum at −1.65 V. Scanning backward in the positive direction, an oxidation current corresponding to the reverse reaction was observed at −0.50 V. In contrast, no distinct redox current was observed for the electrolyte solution without Eu ions. Therefore, it is inferred that the redox current observed for the PEG₄₀₀ solution containing Eu ions was due to the redox reaction of Eu³⁺/Eu²⁺. The ratio of the oxidation charge to the reduction charge of the Eu ion (Q_a/Q_c) in PEG₄₀₀ was calculated to be approximately 54% whereas the ratios measured in common electrochemical solvents were relatively low (14% for DMSO, 16% for MeCN, and 13% for PC) (Fig. S1, ESI†). This indicates that the electrochemically generated Eu²⁺ is more stable in the PEG₄₀₀ solution as compared to that in the other solvents. Fig. 2 shows the change in the emission spectrum of the Eu(NO₃)₃·6H₂O solution. When a reduction potential at −1.65 V was applied for 60 min, a new emission band was observed at approximately 430 nm.

Fig. 2 Emission spectra of the Eu(NO₃)₃·6H₂O solution in a three-electrode electrochemical cell with/without an applied reduction potential (−1.65 V). Excitation wavelength = 272 nm.

It is proposed that Eu²⁺ is electrochemically generated and a broad emission band owing to the d–f transition in Eu²⁺ appeared. Simultaneously, the red emission intensity owing to f–f transitions in excited Eu³⁺ slightly decreased. Using the solvents, such as DMSO, MeCN, or PC, blue luminescence from Eu²⁺ was not observed (Fig. S2, ESI†), indicating that among all the solvents used in this study, the electrochemically generated blue luminescent Eu²⁺ is stable only in the PEG solution. However, the change in luminescence owing to both Eu³⁺ and Eu²⁺ in Eu(NO₃)₃·6H₂O was weak and could not be detected with the naked eye because the excited state of the free Eu ions without ligands undergoes luminescence quenching owing to the organic solvent. Moreover, the excitation efficiency (i.e., the light absorption ability) of Eu ions was low due to the parity forbidden f–f transition.⁴⁴ To improve the luminescence intensity of the developed EFC system, an Eu complex with β-diketonate ligands was used, which exhibited strong red luminescence in organic solutions. Thus, the redox and PL properties of the β-diketonate Eu complex were determined.

Fig. 3 shows the CV curves of Eu(hfa)₃(H₂O)₂ in PEG₄₀₀ using the constructed three-electrode cell. When the potential was scanned in the negative direction, a reduction current flowed from −0.85 V, and the corresponding oxidation reaction was observed when the potential was scanned back in the positive direction.

Fig. 3 Cyclic voltammograms of Eu(hfa)₃(H₂O)₂ or hfa in PEG₄₀₀. ([LiCF₃SO₃] = 500 mM, [Eu(hfa)₃(H₂O)₂] = 10 mM, [hfa] = 30 mM).

The redox reaction of Eu(hfa)₃(H₂O)₂ occurred at approximately the same potential as that of Eu ions in PEG₄₀₀ as depicted in Fig. 1. Moreover, the solution containing only the hfa ligand did not exhibit a significant redox reaction in this potential range, indicating that the Eu complex exhibited the same Eu³⁺/Eu²⁺ redox reaction as that of Eu ions in the PEG₄₀₀ solution. Fig. 4 shows the change in the emission spectra of the Eu(hfa)₃(H₂O)₂ solution. When a reduction potential of −1.80 V was applied for 30 min, a broad emission band of Eu²⁺ was observed at approximately 430 nm. As the Eu²⁺ emission increased, the red emission from Eu³⁺ at 615 nm slightly decreased, indicating that the emission shapes of the Eu complex can be electrochemically controlled between the Eu³⁺ and Eu²⁺ states for the β-diketonate Eu complex. This emission control was not observed for other solvents, such as MeCN (Fig. S3, ESI†), as depicted for Eu ions without ligands. Therefore, the Eu²⁺ state in the β-diketonate complex is stabilized in the PEG solution.

Fig. 4 Emission spectra of the Eu(hfa)₃(H₂O)₂ solution in a three-electrode electrochemical cell with/without an applied reduction potential (−1.80 V). Excitation wavelength = 365 nm.

To elucidate the luminescence mechanism of the Eu complex, the excitation spectra of the Eu(hfa)₃(H₂O)₂ solution during the redox reaction were obtained by monitoring the Eu²⁺ emission at 450 nm and Eu³⁺ emission at 616 nm (Fig. 5).

Fig. 5 Excitation spectra of the Eu(hfa)₃(H₂O)₂ solution before and after the application of a reduction voltage. ((a) excitation wavelength = 615 nm), (b) excitation wavelength = 450 nm).

The luminescence from the Eu³⁺ state without potential application (Fig. 5(a), black line), resulted in a large excitation band and a small band with a maximum at 365 and 396 nm being observed. The large excitation band corresponds to light absorption by the hfa ligand, and the small excitation band is attributed to the f–f transition of Eu³⁺. Upon effective energy transfer from the ligand to the Eu ion, the light absorption of the hfa ligand enhanced the luminescence intensity of Eu³⁺ compared to that of Eu³⁺ without hfa ligands (Fig. 5(a), dotted line). When a reduction potential of −1.80 V was applied for 30 min (Fig. 5(a), red line), the intensity of the excitation band of the Eu³⁺ state was slightly decreased with decreasing Eu³⁺ emission (Fig. 4(b)). For the blue emission of the Eu²⁺ state (Fig. 5(b)), a broad excitation band with a maximum at 365 nm was observed as the reduction of Eu³⁺ proceeded. This indicates that Eu²⁺ in Eu(hfa)₃(H₂O)₂ could be photo-excited via light absorption by the hfa ligands and subsequent energy transfer to the central Eu²⁺ ion.

However, in contrast to the increase in the Eu²⁺ luminescence intensity, a slight decrease in the Eu³⁺ luminescence intensity was observed. Eu²⁺ is generated at the electrode surface, whereas Eu³⁺ primarily occupies the solution in the cell (1 × 1 cm). To improve the contrast of the electrochemical control, two-electrode EFC devices were fabricated.

Electrofluorochromism of the Eu complex in the two-electrode devices

Fig. 6 shows the CV curves of Eu(hfa)₃(H₂O)₂ in PEG₄₀₀ using the constructed two-electrode devices.

Fig. 6 Cyclic voltammograms of the Eu(hfa)₃(H₂O)₂ solution. (The black line represents the ITO–ITO device, and the red line represents the ITO–CM device).

Two-electrode devices were prepared by sandwiching the Eu complex solution between two ITO electrodes (the ITO–ITO device) or between the ITO and carbon-modified ITO electrodes (ITO–CM device). When the potential is scanned in the negative direction, a redox current flowed from −2.3 and −1.5 V in the ITO–ITO and ITO–CM devices, respectively. Thus, the ITO–CM device exhibited better electrochemical properties with a lower redox voltage as compared to that of the ITO–ITO device. The difference in the redox behaviour of these two-electrode devices was investigated using electrode potential measurements. Fig. 7 shows the electrode potential (vs. Ag/Ag⁺) measurement results for the ITO–ITO and ITO–CM devices.

Fig. 7 Electrode potential measurement using the two-electrode devices (a) ITO–ITO and (b) ITO–CM.

The electrode potentials of the working and counter electrodes in the EFC devices (vs. Ag/Ag⁺) were monitored during voltage scanning between the two electrodes. In both devices, the reduction current of the Eu complex flowed when the working electrode potential reached −1.0 V (vs. Ag/Ag⁺). For the ITO–ITO device, when a voltage of −2.3 V was applied, and the working electrode potential reached −1.0 V (vs. Ag/Ag⁺), a redox current began to flow. At this voltage, the counter electrode potential was +1.3 V (vs. Ag/Ag⁺). Alternatively, for the ITO–CM device, when a voltage of −1.5 V was applied, the working electrode potential was −1.0 V (vs. Ag/Ag⁺), but the counter electrode potential retained a low value of +0.5 V (vs. Ag/Ag⁺). Using the CM electrode, a significant decrease in the driving voltage of the EFC devices was observed owing to a decrease in the reaction potential of the counter electrode from +1.3 to +0.5 V because the surface area of the CM electrode is significantly larger than that of the ITO electrode.³⁹ Using the CM as the counter electrode, only the working electrode potential was regulated, whereas the counter electrode potential remained the same.

Fig. 8 shows the emission spectra of the Eu(hfa)₃(H₂O)₂ solution in the two-electrode devices under various applied bias voltages.

Fig. 8 Emission spectra of the Eu(hfa)₃(H₂O)₂ solution in PEG₄₀₀ using a two-electrode cell ((a)ITO–ITO or (b) ITO–CM) before and after the application of reducing and oxidizing voltages.

When a reduction voltage was applied to the working electrode in both devices, the emission bands from the Eu²⁺ state (430 nm) increased significantly, and the red emission from the Eu³⁺ state (616 nm) decreased simultaneously as the electrochemical reduction proceeded (Fig. 8(a) and (b), red line). It is observed that the intensity ratio of Eu²⁺ emission (430 nm) to Eu³⁺ emission (616 nm) was significantly improved using the two-electrode devices; the I_{430 nm} : I_{616 nm} ratio was 1 : 100 for the three-electrode cell and 1 : 2.5–3.0 for the two-electrode devices. Therefore, a change in the luminescence color could be detected by the naked eye after electrochemical reduction (Fig. 8 photos).

Furthermore, a decrease in the degree of Eu³⁺ luminescence improves significantly for the two-electrode devices, whereas it remains unaltered in the three-electrode cell, during the electrochemical reaction.

After the oxidation of Eu ions by applying an oxidation voltage for the working electrode, the emission band from the Eu²⁺ state (430 nm) slightly decreased and the emission band from the Eu³⁺ state (615 nm) slightly increased for the ITO–ITO device (Fig. 8(a)). In contrast, the change of luminescence intensity for each band during the redox reaction of Eu ions was enhanced for the ITO–CM device (Fig. 8(b)). This result was corroborated by the CIE chromaticity diagram (Fig. 9).

Fig. 9 The CIE chromaticity diagram of the Eu(hfa)₃(H₂O)₂ solution in PEG₄₀₀ using a two-electrode cell ((a) ITO–ITO or (b) ITO–CM) before and after the application of reducing and oxidizing voltages.

Before voltage application in the initial state, both devices exhibited bright red luminescence; however, after application of the reduction voltage for the working electrode, the color hue of the luminescence changed to bluish magenta. After an oxidation voltage was applied for the working electrode, the luminescence color of the ITO–CM device was similar to its initial state, indicating good luminescence reversibility.

Conclusions

This study focused on regulating the PL color from the β-diketonate Eu complex via electrochemical alteration of the valence state of the Eu ions between Eu³⁺ and Eu²⁺. The redox behaviour and changes in the luminescence properties of Eu ions and Eu complex in polyether solutions were investigated, revealing that the electrochemically generated Eu²⁺ was stable in both solutions. In the two-electrode device, a distinct change in the luminescence color via electrochemical reduction of the Eu complex, i.e., electrofluorochromism of the Eu complex, was observed using luminescence spectroscopy and the naked eye. Initially, intense red emission from the Eu³⁺ complex was observed, and subsequently, blue emission from the Eu²⁺ complex was generated, which increased owing to the electrochemical reduction of Eu³⁺. Furthermore, by changing the counter electrode from a flat ITO to a porous CM electrode, the reversibility of electrofluorochromism was improved due to a decreased driving voltage, which resulted in the suppression of undesired electrochemical side reactions. Therefore, this study successfully detected electrofluorochromism in the strongly luminescent β-diketonate Eu complex for the first time. These findings could be of great significance in the development of novel display devices and chemical and biological sensors.

Author contributions

Ryoto Yabuta: investigation (Lead) and writing – original draft (Lead). Norihisa Kobayashi: writing – review & editing (Equal). Kazuki Nakamura: project administration (Lead) and writing – review & editing (Equal)

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was partly supported by JSPS KAKENHI (17H06377, 20K05641, 22H02154, and 23K04871).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cp02283h

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