Discrete Donor–Acceptor Conjugated Systems in Neutral and Oxidized States: Implications toward Molecular Design for High Contrast Electrochromics

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• 作者：Natasha B. TeranJohn R. Reynolds
• 刊名：Chemistry of Materials
• 出版年：2017
• 出版时间：February 14, 2017
• 年：2017
• 卷：29
• 期：3
• 页码：1290-1301
• 全文大小：765K
• ISSN：1520-5002

文摘

Donor–acceptor systems are ubiquitous redox-active materials in electrochromic devices, making the study of their neutral and charged state characteristics expedient for the design of materials with improved properties. In this paper, we explore the absorption properties of the neutral and oxidized states of two dioxythiophene- and benzothiadiazole-containing penta- and hepta-heterocycles (EPBPE, EPPBPPE) having a monodisperse, well-defined π-conjugated structure, using electrochemistry, optical absorption and electron paramagnetic resonance (EPR) spectroscopy, spectroelectrochemistry, and microscopy. The molecules and their precursors were obtained via a direct (hetero)arylation coupling strategy that exploits stoichiometric control to obtain well-defined ter- and penta-heterocycles from bifunctional heteroarenes. Both molecules show intense and narrow dual-band absorptions in the visible region, reflecting the discrete nature of their π-systems, leading to strongly colored neutral states. The electron-rich dioxythiophene units enable access to their radical cation and dication states at potentials below 5 mV and 260 mV (vs ferrocene/ferrocenium), respectively, and give rise to stability toward repeated oxidative switching (voltammetric cycling). EPR and absorption spectroscopy of their chemically and electrochemically derived oxidized states showed them to be dominated by polaronic, π-dimeric, and, in the case of EPPBPPE, bipolaronic charge carriers. These species exhibited transitions with maxima in the near-IR region, leading to highly transmissive oxidized states and promising structures for high contrast electrochromics. A polymer (Poly-EPBPE) that maintains a discrete conjugated segment along the backbone was also designed using EPBPE as the multi-ring heterocycle linked together with an aliphatic n-decyl chain, to obtain a mechanically robust yet solution processable material. Poly-EPBPE showed narrow optical transitions and well-resolved oxidation waves in solution that correlated strongly with the properties of EPBPE. However, strong intermolecular interactions were observed in the absorption spectroscopy and electrochemistry of its film state. The oxidized state absorption properties of Poly-EPBPE reflected these interactions, with absorption properties dominated by π-dimers and higher order aggregates, leading to irreversibility in its film spectroelectrochemistry. The coupled structural, optical, electrochemical, magnetic, and microscopic studies enabled us to propose potential resonance structures of the charge carriers in these discrete conjugated systems and inform the design of high contrast electrochromic materials.