Synthesis and Properties of an Acetal-bridged Ladder Poly(p-phenylene)

Abstract

Ladder π-conjugated polymers are promising materials for broad applications in organic-based devices, including light-emitting diodes, thin film transistors, and solar cells. Their rigid coplanar structures promise resistance to deformation and enhanced π-conjugation, which lead to a set of desirable properties, such as intense luminescence, high carrier mobility and environmental stability. Most of the studies in this field have been focused on poly(p-phenylene)based polymers such as step ladder-type poly(phenylenes) and fully ladder-type poly(phenylenes). There are, however, only limited reports of ladder poly(p-phenylenes) with heteroatom bridges. This is mainly due to a lack of useful synthetic routes in terms of efficacy as well as accessible structural diversity. The exploration of conceptually new methodology is thus a compelling subject in this chemistry. We have recently developed a BBr3-promoted cyclization to produce an ether and ester-bridged ladder-type poly(pphenylenes). This method constitutes significant improvements over other routes to produce ladder polymers because the cyclization is fast and quantitative. Versatility of the reaction is another advantage of our method to prepare ladder polymers. As a new entry into this class of polymers, we introduce herein a novel ladder poly(p-phenylene) with an acetalbridge. The strategy to construct an acetal-bridged ladder system is based on our chemical cyclization method. Thus, phenoxide, generated by BBr3-promoted demethylation, acts as a nucleophile for nucleophilic addition with the neighboring ketone, causing cyclization (Scheme 1). This constructs a hemiacetal-bridged triphenylene skeleton 2 in one spot. The subsequent methylation using methanol in TFA produces the desired acetal-bridged ladder structure 3, as a model for the corresponding poly(p-phenylene) 6, also in a quantitative yield. The photophysical and electrochemical properties of the acetal-bridged poly(p-phenylene) 6 are further described in this communication.