Organic electronics are becoming increasingly popular as promising alternatives of their inorganic counterparts. Long-term operational stability of such electrochemical devices relies on the reversible switching between neutral and doped states of the functional material. Conductive doped states of conjugated molecules and polymers consist of radical and/or charged species, which can be particularly vulnerable towards irreversible side reactions. Therefore, it is critical to evaluate possible degradation pathways and establish structural features enabling the design of more robust redox-active organic materials.<br/>Renewed interest in anodically-coloring conjugated molecules for transparent-to-colored electrochromic switching, has inspired investigation of their structure-property relationships. The high reactivity of small molecule radical cations can cause dimerization, disproportionation, polymerization or degradation, which can result in unexpected color variations, formation of residual colors and irreversibility in electrochromic devices. In this work, we evaluate the opto-electronic properties of a library of propylenedioxythiophene-based small molecules, with variations in substituent functional groups, substituent positions and end-capping groups. Chemical and electrochemical doping experiments reveal significant deviations from theoretically predicted optical transitions of the radical cation state, indicating instability. Interestingly, the number of substituents as well as their positional isomers were found to influence electrochemical reversibility as well as the degradation pathway, with loss of different functional moieties in the doped state. Isolation of degraded products reveal radical loss, with consequent formation of degraded cations and dimerized dications. Conditions favoring radical cation dimerization and degradation were identified through a combined experimental and theoretical approach. These findings provide new insights into molecular design features that govern radical cation stability of conjugated systems.

Degradation pathways of conjugated radical cations

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ACS Fall 2022

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