Electroacid/Electrobasc Theory And Their Applications In Organic Electrochromic Materials

Electrochromic materials have been intensively investigated over the pastdecades due to their broad applications in displays, data recording, sensing, opticalcommunication, etc. However, some intrinsic problems, such as the single mechanism,limited color tunability, the long switching time and the unstable performance, hindertheir applications. pH-sensitive materials possess the advantages of the excellentoptical properties, such as the wide varity and the low-priced raw materials. However,their applications in photoelectric devices are limited by the means of the chemicalbase/acid stimuli. Thus, these two kinds of materials may complement each other.To enhance the worth of pH-sensitive materials, the electric field considered asthe simplest and easiest method can be used to replace traditional chemical base/acidstimuli to switch the properties of pH-sensitive materials. In our work, a moreconvenient in-situ ‘‘electrobase” and “electroacid’’ has been demonstrated to switch thepH-sensitive materials reversibly. Electroacid is defined as an electroactive moleculethat their acidity can be reversibly changed upon electrical field. Electrobase is definedas an electroactive molecule that their alkalinity can be reversibly changed uponelectrical field. This newly-demonstrated stimulus of “electro-acid/base” in-situ formolecular switch is far more convenient than conventional chemical stimulus of acidsor bases for practical applications in IT devices, not only because the electric field isthe simplest and easiest method for implementation, but also it can control both thereaction progress and direction quickly and precisely in a closed system based on theneeds of the devices.In this dissertation, methyl ketone has been designed as a switching unit forelectrically addressable molecular colour switches. A new mechanism of the reversible colour switch via single-electron coupled proton transfer or triple-electron coupledproton transfer is proved clearly by our group through cyclic voltammetry (CV), X-rayphotoelectron spectroscopy (XPS), infrared spectroscopy (IR), and in-situ UV-Visspectroscopy. Based on this mechanism, the newly-demonstrated concept of theelectrobase and electroacid theory is proposed.Based on the electrobase theory, p-benzoquinone (BQ) is introduced as a newsource of convenient in-situ base for the color and emission switching of fluorescein.And benzoquinone derivatives have been synthesized to investigate the substituenteffect on the intensity of electrobase. A simple electrochemical method has beenreported to determine the pKa values of radical anion and dianion of the benzoquinonederivatives to predict quantitatively the strength of electrobase. The pKa values ofconjugate acids of benzoquinone derivatives can be regulated from7.0(pKa(Dichloroaceticacid)=6.4) to30.0(pKa(tert-Butanol)=32.2) by changing theelectron-donating ability and steric effect of substituents on benzoquinone. Comparingthe theoretical pKa value of radical anion and dianion with experimental value,PBEPBE/6-311++g(2df,p)//PBEPBE/6-31g(d) could predict the absolute pKa valueswith a precision of±4pKa units.Based on our electroacid theory, we have proved that hydroquinone can be usedas electroacid to switch the color and emission of a pH sensitive oxazolidine derivateFXF. The mechanism of double-electron-coupled proton transfer in FXF/hydroquinonesystem has been demonstrated by CV and in-situ absorption spectroscopy. The methodfor determining efficiency of proton transfer induced by electron transfer has beendeveloped to quantitatively predict the intensity of electroacid. The efficiency of protontransfer induced by electron transfer can be increased by the increasing the amount ofwater and the ratio of FXF/hydroquinone.Then, methyl ketone-bridged derivatives as electrobase, benzoquinonederivatives as electrobase and Urea-N as electroacid are used to develop the newelectrochromic devices with low-priced raw materials and excellent property. In thiswork, we have developed an unconventional electrochromic approach to achieve the long-awaited bistable display material by Urea-N/Rh-M system. This new system hassuperb switch-ability with rapid switching time shorter than30ms, amazingreversibility more than1.0×104cycles, excellent coloration efficiency of150cm2/C,and the highest fluorescent contrast ratio of254.Most importantly, this display materialexhibits stable color and fluorescence between each switch and can save up to99.9%energy compared with LCD and LED. To our knowledge, this is the only demonstrationso far to achieve such good bistable display properties with electrochromic mechanism.Finally, methyl ketone-bridged derivatives as electrobase, benzoquinone derivatives aselectrobase and Urea-N as electroacid are used to develop the new multicolorelectrochromic devices. And based on the electrochromism, solvatochromism andbasochromism of a novel methyl ketone bridged molecule TM1, a multicolorsingle-molecule electrochromic prototype device (colorless, green, blue and magenta)has been developed by integrating medium engineering/in situ “electro base”/laminatedelectrode technologies. This multicolor electrochromic device is durable and has a highcoloration efficiency (350cm2/C), a fast responsive time (50ms), and superiorreversibility.In one word, the electro-acid/base can replace the conventional chemical stimulusof acids or bases to switch the properties of pH-sensitive materials for practicalapplications in photoelectric devices. The electroacid/base will certainly inspire andaccelerate the further development of pH-sensitive materials and their applications inultrathin flexible displays, data recording, sensing and optical communication, etc.