Electrofluorochromic Devices Based on Electrochemical Valance Change of Europium Complex in Polyether Matrices
Authors: Yabuta Ryoto , Kobayashi Norihisa , Nakamura Kazuki 
Published: December 2024

Chromogenic materials, in which optical properties such as luminescence and absorption are altered by external stimuli such as light, heat, and electricity, have potential applications in chemical sensors, biochemical labels, molecular memory, and display devices. Electrofluorochromic (EFC) materials, in which wavelength and/or intensity of their photoluminescence are controlled by electrochemical redox reactions, are innovative materials because they can rapidly and repeatedly convert electrical inputs into visual signals. These EFC phenomena were reported by using small molecules, conjugated polymers, inorganic compounds, and luminescent metal complexes. Among the luminescent metal complexes, the lanthanide(III) complexes are composed of a luminescent center of Ln(III) ions and surrounding antenna ligands with sufficient light-absorbing ability. They have attractive photoluminescence properties such as characteristic and narrow emission bands in the visible-near infrared (Vis-NIR) region, long emission lifetimes, and high transparency in the visible region due to the large pseudo-Stokes shift. In particular, Eu complexes have relatively stable trivalent (Eu3+) and divalent (Eu2+) states. The Eu3+ state is characterized by intense and long-lived red luminescence induced by f–f transitions and is extensively applied to biosensors, light-emitting materials, etc. On the other hand, Eu2+ is known to show broad and blue luminescence induced by d–f transitions, from the excited state of 4f65d1 to the ground level state of 8S7/2 (4f7), and the emission of Eu2+ in inorganic matrices is applicable to various phosphor materials. However, Eu2+ is unstable in common solutions, which complicates the observation of its luminescence in solution state. Therefore, this study aims to control the luminescence between Eu3+ and Eu2+ by electrochemical redox reactions. It is recognized that the blue luminescence of Eu2+ can be stabilized by polyethers, such as crown ethers and polyethylene glycols[1]. By this stabilization, the Eu2+ state was expected to produce stalely and repeatedly by the electrochemical reduction of Eu3+ in polyethylene glycol solutions. Considering the EFC reaction of Eu compounds, this study focuses on the β-diketonate Eu complex that exhibits superior red luminescence properties of the Eu3+ state.
The solution was prepared by dissolving the lithium trifluoromethanesulfonate (LiCF3SO3) (500 mM) as the supporting electrolyte, β-diketone type Eu(III) complex (Eu(hfa)3(H2O)2) (10 mM) as the emitting material in polyethylene glycol (MW = 400, PEG400). A three-electrode electrochemical cell with ITO electrode as the working electrode, Pt wire as the counter electrode, and Ag/Ag+ as the reference electrode was prepared using the Eu complex solution, and the electrochemical reaction behavior of the Eu(III) complex and the changes in luminescence properties during the reaction were measured. Then two-electrode electrochemical device was fabricated by sandwiching Eu complex solution between the two ITO electrodes (ITO-ITO device) to evaluate photoluminescence (PL) properties under the application of reduction voltage.
When the CV measurement using the constructed three-electrode cell structure, stable Eu2+/3+ reactions were not observed in the common electrochemical solvents (dimethyl sulfoxide, acetonitrile, and propylene carbonate). In contrast, relatively stable Eu2+/3+ changes were observed in the PEG400 electrolyte solution. This indicates that the electrochemically generated Eu2+ is more stable in the PEG400 solution than in the other solvents.
The figure shows the emission spectra of the Eu(hfa)3(H2O)2 solution in the two-electrode ITO-ITO device under the application of reduction voltage. When a reduction voltage was applied for working electrode for 60 minutes, the emission bands of Eu2+ (430 nm) in the device significantly increased, and the red emission from Eu3+ (616 nm) simultaneously decreased as electrochemical reduction proceeded. Considering luminescence color, the intensity ratio of Eu2+ emission (430 nm, I
430) to Eu3+ emission (616 nm, I
616) 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 for the two-electrode devices. Thus, changes in the luminescence color can be recognized by the naked eye after electrochemical reduction (Figure photos). These results show that electrochemical redox reaction of Eu complexes leads to a distinct change in luminescence color in PEG electrolyte by stabilizing Eu2+ state in liquid matrix.

Figure 1