Electroswitchable Emission and Coloration Based on Single-Molecular Fluoran Derivatives

Multifunctional molecular switches exhibiting changes in both emission and coloration in response to various external triggers such as thermal, photo, and electric stimuli have attracted much attention for potential application as chemical sensors, biochemical labels, molecular logic gates, molecular memories, and display devices. In this study, we demonstrated reversible electrochemically induced high-contrast switching of the emission and coloration of fluoran molecules associated with changes in the intramolecular structure. In order to investigate changes in the photophysical properties induced by electrical stimuli, we measured cyclic voltammogram, absorption spetra, emission spectra, and molecular orbital calculation. The mechanism for this electroswitching was attributed to the reversible electrochemically induced closing and opening of the lactone ring in the fluoran molecule. All neutral fluoran molecules were colorless and did not exhibit any fluorescence, while the oxidized (lactone ring-opened form) molecule was yellow and displayed a green fluorescence as a consequence of the extended, planar, conjugated system. Furthermore, this fluoran molecule achieved reversible electroswitchable emission and coloration with high on/off contrast[1]. In order to achieve a quick response time and high reversibility of the electroswitching optical device for modulating both emission and coloration, a 1,4-benzoquinone (BQ) molecule was introduced as a bifunctional material. The inclusion of a redox-active moiety on the auxiliary electrode maintained the charge balance and acted as an EC molecule (transparent/yellow in neutral/reduced state, respectively). BQ acted as a bifunctional molecule, leading to quick and stable switching of both emission and coloration[2]. Reference [1] K. Kanazawa, K. Nakamura and N. Kobayashi, J. Phys. Chem. A, 118, 6026–6033 (2014). [2] K. Kanazawa, K. Nakamura and N. Kobayashi, Sol. Ener. Mater. Solar Cell., 145, 42-53 (2016). Figure 1