Oxidation of sp- and sp2-hybridised molecular semiconductors with iron(iii) chloride in organic media

Abstract

Access to the charged states of emerging organic molecular semiconductors is crucial to understand their electronic properties. In the case of materials optimised for hole transport, oxidation can be challenging for molecules with deep HOMO levels. Beyond some of the more common sp²-molecules, such as acenes and nanographenes, molecules based on sp-hybridised carbon, namely cumulenes, are emerging as materials with unique electronic properties, and good hole transport in field-effect devices based on polycrystalline thin-films. Very little is known, however, about the nature of the charged states contributing to charge transport in cumulenes. In this study, we demonstrate a successful chemical oxidation strategy of two model systems using sp- and sp²-hybridised molecular semiconductors with deep HOMO levels using iron(III) chloride in optimised organic solvent mixtures. The optimisation process relies on controlling the solvent environment. In the case of the model cumulene tetraphenylbutatriene ([3]Ph), the formation of the oxidised species [3]Ph⁺ exploits the sp-carbon backbone to accommodate the excess positive charge, and the charge delocalisation dramatically increases the bond length alternation leading to a polyynic structure. We extend this approach to the sp² semiconductor C8-BTBT, validating its general applicability. Our findings offer insights into the redox behavior of sp-carbon systems and provide a robust chemical route to access charged states in sp- and sp²-hybridised small molecules.