Degradation Pathways of Conjugated Radical Cations

Abstract

Organic electrochemical devices and their operational stability depend on the reversible switching between the neutral and doped states of redox-active materials. The charged, radical state of doped π-conjugated molecules can be reactive toward radical and/or ionic side reactions that degrade the functionality of the material and device. In this work, we focus on a group of thiophene-based conjugated molecules and investigate how substituents, commonly used to modulate optical and redox properties, can influence radical cationic stability. Subtle changes in the number and position of electron-donating substituents on the conjugated moiety were shown to affect the localization of the unpaired electron in the radical cation, leading to distinct degradation pathways. Using a combination of electrochemistry, mass spectrometry, UV−visible spectroscopy, single-crystal Xray diffraction analysis, and theoretical calculations, we identify degradation pathways involving either demethylation or σdimerization coupled with end-group loss, and correlate to the electronic structure of the radical cation. We explore the chemistry and conditions favoring these degradation events and provide new insights into understanding and improving the stability of redoxactive conjugated systems.

Transparent Electrochromic Polymers with High Optical Contrast and Contrast Ratio

“…When we further increase the applied potential, particularly the second oxidation peak, the intensity of the peak in the visible region begins to decrease with the continued growth of a broad peak in the NIR region indicative of a dication species. For the BP and BNP polymers, a further increase of the potential would also cause a new absorption peak to form at shorter wavelength (comparing with the NIR absorption), indicating the formation of a dimer dication . It is worth noting that all polymers exhibit relatively low oxidation onset potential, which is attributed to the conjugation from aromatic moieties.…”

“…For the BP and BNP polymers, a further increase of the potential would also cause a new absorption peak to form at shorter wavelength (comparing with the NIR absorption), indicating the formation of a dimer dication. 34 It is worth noting that all polymers exhibit relatively low oxidation onset potential, which is attributed to the conjugation from aromatic moieties. The low oxidation onset potential is beneficial for the electrochemical stability of polymers since undesirable side reactions such as water oxidation could be avoided.…”