Molecular Design, Synthetic Strategies, and Applications of Cationic Polythiophenes

This review delves into the synthesis of cationic-functionalized oligothiophenes and polythiophenes, their properties, and their diverse applications from the year 2002 to 2019. Pristine polythiophene chains are hydrophobic and are not easily converted into a form that is useful for broad applications or for device fabrications. Functional modification of the polymer side chain especially with cationic pendant groups imparts favorable properties to the system, such as solubility in aqueous medium and affinity with anions and negatively charged species, alongside the extensive π conjugation that provides unique optoelectronic characteristics. This review provides a detailed account of the design and the different synthetic strategies to access cationic polythiophenes (CPTs) via chemical oxidation using iron(III) chloride (FeCl₃), via metal catalyzed and initiated polymerization, and via postpolymerization functionalization approaches. CPTs have been traditionally used in sensing and as a component in optoelectronic devices; the utility of these systems has been extended to biomedical applications, such as nonviral gene delivery, cell targeting and imaging, anticancer, antifungal, and bactericidal actions, and biofilm formation, and to use as a molecular probe. A summary and outlook are also presented, discussing the remaining challenges and the future direction for this field.