JACS: a new strategy for color control of electrochromic molecules in anodizing

Electrochromism is essentially an electrochemical oxidation-reduction reaction After the reaction, the material shows reversible color change in appearance Electrochromic materials are widely used in energy-saving colored glass and goggles Electrochromism can be divided into two types according to different electrochemical reaction modes: cathodic and anodic The electrochromic polymer with cathode coloring is widely concerned by researchers because of its processability, mechanical flexibility and other advantages Although the advantages of cathode colouring are obvious, the cathode colouring materials will absorb red light in the process of oxidation delivery, which leads to the interference light of light blue, which limits the development of electrochromic polymers with cathode colouring The electrochromic polymer with anode coloring is colorless in neutral state and colorless in oxidation state, which has become a new research hotspot Recently, Professor John R Reynolds of Georgia Institute of technology in the United States constructed a series of electrochromic molecules based on the structure of 2-thiomethylethylene dioxothiophene based on the anodizing coloring strategy, providing a new research example for the field of electrochromism A series of electrochemiluminescence molecules ace1-ace4 (Figure 1) were constructed by coupling 2-thiomethylvinyldioxythiophene with 4-methoxybenzene derivatives Ace molecules regulate the absorption in neutral state by changing the electron donor / acceptor groups between benzene rings, which leads to the increasing trend of the electron density of ACE1 - ace4 The para substituents are kept constant to increase the electron density and maintain a neutral state similar to the optical slit, so as to effectively change the absorption of cation free radicals without depending on the neutral absorption, and finally present a colored molecule different from the colorless neutral state The results were published in J am Chem SOC (DOI: 10.1021 / JACS 9b01507) (source: J am Chem SOC.) after the synthesis of target molecules, the theoretical spectra of four molecules in neutral state and cation free radical state are simulated by time-dependent density functional theory (Figure 2) The results show that, compared with the neutral state, the emission wavelength of the molecule in the cationic free radical state has a significant red shift (dotted line in Figure 2) The author studies this phenomenon (Figure 3), and finds that the maximum transition energy is generally consistent with the excitation of SOMO α → LUMO α (A and C in Figure 3) In the electron poor molecular structure, the excitation transition from somo-1 β to LUMO β results in lower energy absorption (B in Figure 3) After increasing the electron cloud density of the chromophore, the main excitation type of the electron becomes SOMO β → LUMO β Compared with the barren structure of the electron, the energy required to excite the electron is lower, and the excess energy is converted into the emission wavelength, so the emission wavelength has a red shift (D in Figure 3) (source: J am Chem SOC.) (source: J am Chem SOC.), then, the author constructed a light permeable thin layer electrode (ottle), in which platinum is the working electrode to allow light to pass through the battery Four polymers were coated on ottle by the author After electric activation, ace1-ace4 showed four different colors of green, green, yellow and red respectively The results of spectrogram confirmed that the emission wavelengths of four molecules had obvious red shift, and the maximum red shift was 1013 nm (Figure 4) The red shift of the emission wavelength of ACE1 - ace4 is closely related to the formation of double cation and free radical cation (source: J am Chem SOC.) finally, the author mixed ACE2 and ace4 in the proportion of 1:1 and then applied them on ottle (Figure 6) In the state of electric activation, the mixture changes from a neutral transmission solution to an opaque black, which provides a direct support for the development of synthetic color using ACE mixture (source: J am Chem SOC.) Summary: the author designed and synthesized a series of electrochromic molecules ACE1 - ace4, and proposed a new electrochromic example, that is to control the color change by controlling the absorption band in the oxidation state The design principles of these polymers can also be applied to other conjugated organic systems to regulate the electronic structure of free radical cations This study provides a new idea for the development of color changing materials.