(Invited) Electrochemically Controllable Emission and Coloration Using Luminescent Europium(III) Complex and Electrochromic Viologen Derivatives
Authors: Nakamura Kazuki , Kanazawa Kenji , Kobayashi Norihisa 
Published: February 2020

A dual-mode display (DMD) device which can operate in both emissive and reflective modes, having advantages of both modes, is considerable promise as a next generation display. Using the dual-mode display, we can choose either reflective or emissive mode as the situation demands. However, only a few studies have been reported on the DMD devices because it should include two different mechanisms in one device. For creation of novel materials enabling electrochemical modulation of both emission and coloration, we focused on the concept of “emission control based on EC reactions”. By combination of luminescent material and EC material, the energy transfer between the two materials can be controlled by EC reactions. When the EC molecule is colorless state, the energy gap between ground state and excited state of the EC molecule is larger than that of the luminescent material. Therefore, under the photo- excitation of the luminescent material, the transfer of excitation energy from luminescent material to EC material is not allowed, leading to bright emission from the luminescent material, i.e. “emissive mode”. When the EC molecule is colored, on the other hand, the energy gap of the EC molecule became smaller than that of luminescent material. In this state, the excitation energy is expected to transfer to the colored EC molecule from luminescent material. Since the energy transfer will quench the luminescence, only coloration of EC molecule can be obtained without photoluminescence, i.e. “reflective mode”.
Here, we have tried to control coloration as well as emission based on electrical stimuli by combination of photoluminescent and EC materials to achieve a novel DMD material. In this paper, the electrochemical switching systems enabling the control of the both modes were investigated. Furthermore, a prototype DMD device with emissive and coloration based on electrochromism were fabricated.
We first demonstrated both luminescence and coloration control by using a composite material containing a luminescent Eu(III) complexes and electrochromic viologen derivatives as the luminescent and coloration materials, respectively. The coloration of the composite material was controlled by the electrochromic reaction of viologen derivatives. Modulation of red emission from Eu(III) complex was also achieved by the electrochromism of viologen derivatives via intermolecular energy transfers from the excited states of Eu(III) ions to colored viologens. The 2-electrode cell containing the luminescent complexes and electrochromic molecules functioned as an electrochromic device when bias voltage was applied and as a light-emitting device if excitation lights are irradiated. When bias voltage was not applied to the cell, the outer appearance of the cell was clear transparent, the strong red emission from Eu(III) complex was observed under UV excitation. This can be considered as ‘emission on’ state. On the other hand, after application of voltage ca. 2 V, the cell changed from clear transparent state to colored state by electrochromic reaction of the viologen derivatives, resulting in ‘refection on’ state. As the coloration of the cell, red emission of the Eu(III) complex was completely quenched under colored state.
In order to achieve a rapid fluorescence modulation response, it is necessary to immobilize both luminescent and electrochemically active molecules on the same electrode because response times of modulation of both emission and coloration are dependent on the response time of electrochromism and energy transfer efficiency. We finally tried to fabricate photo-electro functional modified electrode incorporating the Eu(III) complex and viologen derivative on the electrode. The novel modified electrode was fabricated by covalently- connecting the Eu(III) complex and viologen derivatives on porous TiO2 electrode through carboxyl groups or phosphate groups. The immobilizing of these materials on porous TiO2 electrode is expected to achieve absorption and fluorescence intensity, because large surface area of the TiO2 film allows the increase of adsorption amount of the materials, leading to high emission contrast of modulation. The adsorption amount of the Eu(III) complex and viologen derivative were analyzed by FT-IR and EDX measurements. The modified electrode showed strong red photoluminescence from the Eu(III) complex at transparent state. The EC reaction of the viologen derivative was also observed with rapid response of few seconds by applying -0.8 V. The photoluminescence from Eu(III) complex was successfully modulated by EC reaction of the viologen derivatives within few seconds.