Synthesis Of New Viologen Derivatives And Their Applications In Electrochromic Devices

Electrochromic devices(ECDs)can adjust absorption or reflection of light under a small applied voltage to change their own colors,which are widely used in glass curtainwalls,automobile rearview mirrors and aircraft windows.As a kind of typical electrochromic material,viologen has attracted numerous attentions of researchers around the world because of its clear redox mechanism and relatively comprehensive electrochromic performance.However,the synthesis of viologen derivatives generally involves modification on the nitrogen atom of the pyridine ring,which limits the development of viologen-based electrochromic materials,and it is difficult to obtain electrochromic devices with excellent properties.In this dissertation,synthesis of viologen compounds with excellent properties are selected as one of the main research object.To improve the stability of ECD and obtain various colors,two strategies including establishing intermolecular weak interactions and designing new viologen compounds by using the electron donor-acceptor theory(D-A theory)are proposed.A series of highly stable,multi-colors electrochromic devices were fabricated.The internal electrochromic mechanism and structure-activity relationship were discussed to explain electrochromic processes and performance of the devices.In addition,in order to broaden the application range of ECDs,investigations in the field of self-powered ECDs and electrochromic supercapacitors were carried out.Two alkynyl substituted viologen compounds DPV and MPV were synthesized by introducing alkynyl groups with electron donor properties.The fabricated DPV-based ECD showed excellent cyclic stability(stable cycle was over 70000 times),robust optical modulation ability and short response time.The reason for the high stability of DPV-based ECD is that the alkynyl groups at both ends of the molecule will interact with the other radical cations of DPV molecules,respectively,which will establish intermolecular weak donor-acceptor interactions,thereby"locking"the molecule itself and the probability of face-to-face contact of bipyridine ring units was greatly reduced.Then,the probability of the formation of free radical dimer was greatly reduced,which benefits to the improvement of stability of ECD.It was also found that the deep colored ECDs could be used as electrochromic supercapacitors to drive a LED bulb.In addition,DPV or MPV-based ECDs were combined with perovskite solar cells(PSCs)to form self-powered ECDs that could automatically adjust light transmittance according to surrounding light intensities.An asymmetric viologen compound of DVB was synthesized by introducing styryl and sterically hindered phosphate groups.And a transparent-gray liquid-type ECD was prepared based on single viologen molecular chromophore.The gray colored state of ECD was attributed to the weak interaction between the styryl group in the DVB molecule and radical cation,which changed the electron cloud distribution of the DVB molecule,and finally formed an extended-conjugated viologen intermediate state.The coloring and bleaching time of ECD were 2.2 s and 1.9 s,respectively,and the coloring efficiency was127.9 cm~2/C.According to Electron-Donor-Acceptor(D-A)theory,a new type of viologen compound DPB containing pyridine salt(A)-thiophene(D)-pyridine salt(A)skeleton was synthesized,and a liquid-type ECD that could perform the electrochromic process stably in air was prepared.The transmittance difference(?T)between the colored state and the bleached state of the ECD at 517 nm was 72.2%,the coloration efficiency value was 102.9cm~2/C,and the?T value of 69.6%was maintained after 5000 s of cycling work in air.A viologen compound TMP containing the pyridine salt(A)-thiophene(D)-pyridine salt(A)skeleton was synthesized,and the fabricated gel ECD could simultaneously adjust the visible and infrared light.The optical contrast of the ECD at 570 nm,835 nm,910 nm,and 1046 nm were 81.7%,68.6%,62.8%and 55.3%,respectively,and the ECD showed excellent stability(?T maintained 70.6%after 25000 cycles at 570 nm).By comparing the electrochromic performances of methyl viologen(DMV)-based gel ECD and utilizing DFT theoretical calculation and analysis,it was proved that the reduction of band gap(E_g)was the key factor for the TMP-based ECD to adjust infrared light.Furthermore,under the guidance of DFT theoretical calculation,another viologen compound DEV containing pyridine salt(A)-thiophene(D)-pyridine salt(A)skeleton was synthesized,and a DEV-based gel ECD with better stability was fabricated(?T maintained 78.6%after 60000cycles at 570 nm),and the deeply colored DEV device could be used as an electrochromic supercapacitor to drive a LED bulb.In addition,DEV-based ECD were combined with PSCs to form a self-powered ECD that could automatically adjust light and heat according to surrounding light intensities.