Preparation and Electrochromic Performances of Structural Derivatives and Hybrid Homo-/Co-Polymers of Poly(3,4-ethylenedioxythiophene)s
Authors: Chen Shuai , Xu Jingkun 
Published: February 2020

Electrochromic (EC) conducting polymers (CPs) have been one of the primary focuses of organic optoelectronic materials since their discovery forty years ago. Poly(3,4-ethylenedioxythiophene) (PEDOT) and its derivatives are named as one of the most important series in this field which have gained significantly attention benefiting to their outstanding advantages including excellent thermal and electrochemical stability, high contrast ratios, high coloration efficiencies, low switching voltages, fast response times, excellent color persistence, and so on. Moreover, PEDOT is quite easy to tune colors (and band gaps) by structural modifications.

In recent years, our efforts in this field has been devoted to tune the electrochromic properties of PEDOTs, a series of novel CPs derived from PEDOT via structure variation and/or copolymerization strategies. On one hand, much research has laid on the design and synthesis of PEDOT analogs by varying ring size, replacing one or both oxygen in the peripheral ring with other heteroatoms such as sulfur and nitrogen, and/or insteading the sulfur in the thiophene core by heteroatom selenium. The analogs derived include poly(3,4-propylenedioxythiophene) (PProDOT), poly(thieno[3,4-b]-1,4-oxathiane) (PEOTT), poly(3,4-ethylenedithiathiophene) (PEDTT), poly(N-methyl-3,4-dihydrothieno[3,4-b][1,4]oxazine) (PMDTO), poly(N-ethyl-3,4-dihydrothieno[3,4-b][1,4]oxazine) (PEDTO), poly(3,4-ethylenedioxyselenophene) (PEDOS), poly(3,4-propylenedioxyselenophene) (PProDOS), poly(seleno[3,4-b]-1,4-oxathiane) (PEOTS), poly(3,4-ethylenedithioselenophene) (PEDTS), etc. Their derivatives via attaching functional groups (e.g., alkyl, hydroxyl, amino acid groups, etc.) at the ethylene or propylene rings were also paid special attentions. Generally, the electrochromic investigation implied that these analogs had strikingly different electronic, optical and redox properties in comparison with PEDOT.

On another hand, the delicate tuning and moderate control of electronic properties could be realized by the fabrication of donor−acceptor (D−A) type copolymer films from homopolymerization of 3,4-ethylenedioxythiophene (EDOT)-contained hybrid monomers or copolymerization of electron-rich EDOT unit with other electron-poor heterocyclic groups. Structure–property relationships of the oligomers and hybrid polymers, including electrochemical, electronic and optical properties, quantum chemistry calculations and morphology were systematically explored. Results indicate that the incorporation of EDOT into other CPs (e.g., polyfuran, polythiophenes, polyselenophenes, etc.) backbones can combine their photoelectronic and structural advantages. For example, hybrid poly(selenophene–EDOT) present promising electrochromic performances due to the combination of the lower band gap and better planarity of polyselenophene, and the high conductivity, transparency and excellent stability of PEDOT. Others include poly(3-thienylethanol-EDOT), poly(dibenzothiophene-EDOT) and poly(dibenzofuran-EDOT), poly(benzothiadiazole-EDOT), poly(thiadiazolo[3,4-c]pyridine-EDOT), etc. Overall, these hybrid copolymers provide more plentiful electrochromic colors with high quantum yields while vary the ranges of oxidation-reduction potentials. Specially, the design D−A type copolymers can realize the green electrochromic polymers besides more common blue or red, for instance, blue-green poly(furan-EDOT).

In addition, PEDOTs/poly(styrenesulfonate) (PEDOTs/PSS) aqueous dispersions have been also paid particular attentions due to their flexibility, excellent film processability, etc. They can be easily integrated within a wide range of systems and substrates, thus, much facile to fabrication commercial EC devices.

No matter what structures, electrochemical and chemical oxidation polymerization strategies have been feasibly conducted to fabricated PEDOTs films from the monomers. Systematically investigations of the effect of mediums (solvents, supporting electrolytes, oxidants, content ratios, etc.) and reaction conditions on the structures, morphologies, polymerization behaviors, electronic and spectroscopic properties, and electrochromic performances of the as-prepared polymer films have been taken. It is believed that these PEDOT analogs and copolymers may meet a competitive future to be commercial electrochromic materials and optoelectronic devices.

Figure 1