Design, synthesis and characterizations of novel electrochromic polymers
Cho, Ching Mui
Date of Issue2016-12-30
School of Materials Science and Engineering
Institute of Materials Research and Engineering
Electrochromic (EC) materials and their devices have been extensively researched over the years especially those based on conjugated polymers. Although these materials are now able to meet the key performance indicators such as high optical contrast and fast switching, there are still challenging problems faced by current EC technologies, particularly the stability of EC devices. This thesis is structured into different sections, starting from the motivation of this work, which is to explore the structural design of conjugated polymers to improve their EC properties (Chapter 1), followed by a literature review on the different types of EC materials and devices (Chapter 2) and the different experimental methodologies used (Chapter 3). In the subsequent sections, three different approaches for the design of new electrochromic polymers are proposed, and their electronic and EC properties are discussed. First, in chapter 4 and 5, a new synthetic methodology based on the inverse electron demand Diels-Alder (iEDDA) reaction has been created for the synthesis of new electron acceptors. Using this new synthetic methodology, novel electron acceptors such as pyrrolo[3,4-d]pyridazine-5,7-dione (PPD) and pyrrolo-acenaphtho-pyridazine-diones (PAPD) have been successfully synthesized and found to be highly electron deficient with very low lying lowest unoccupied molecular orbital (LUMO) energy levels. Their LUMO energy levels are lower than most of the commonly used electron acceptors. As such, this new synthetic methodology can be used to synthesize novel electron acceptors and contribute in increasing the existing library of electron acceptors available in the preparation of donor-acceptor polymers. These newly prepared electron acceptors moieties, PPD and PAPD, are then used in the synthesis of electrochromic polymers PPD1 – PPD3 and PAPD1 – PAPD5 respectively. All the polymers obtained exhibit very high solubility in common organic solvents. The PPD series of polymers displays purple to transmissive blue electrochromism while the PAPD series shows red to grey electrochromism. In particular, PAPD3 possesses very high optical contrast of 81 % in the near infra-red (NIR) region and attains high redox stability with only a 0.6 % drop in optical contrast after 800 cycles of redox cycling. Secondly, in chapter 6, the approach of utilizing 4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene (IDT) as a building block for EC conjugated polymers has been demonstrated. Due to its good performances, IDT is frequently used to prepare organic electronics, but they are rarely reported in EC materials. Herein, the IDT units are co-polymerized with dioxythiophene-based units to test its feasibility as electrochromic building block. The resultant polymers are found to exhibit a red to black electrochromism with high optical contrasts in both visible and NIR regions. As such, IDT units can be considered as potential building blocks in electrochromic materials. In chapter 7, the use of triphenylamine (TPA) as a tool to improve the redox stability of the electrochromic polymers has been demonstrated. A series of EC polymers with varying amount of TPA incorporated has been synthesized. The polymers are obtained in medium yields and highly soluble in organic solvents. High molecular weights ranging from 59 to 81 KDa are achieved and the polymers display multi-colored electrochromism. These polymers are able to transit from purple/red at reduced state, to grey at intermediate state, and then to transmissive blue at oxidized state. Notably, polymer TPA3 is able to achieve a six-fold improvement in redox stability with the incorporation of just 15 % of TPA into its polymer chains. With such significant improvement, this simple yet effective design strategy could be potentially used to modify EC polymers to enhance their redox stabilities. Lastly, a summary of my thesis is presented in chapter 8, where the three proposed design strategies are proven to be effective in creating good performance electrochromic materials. Some significant contributions made in this thesis are also being highlighted with a discussion on the suggestions for future work.