Planar vs non-planar pi-conjugated semiconductive molecole. On the potential of anthracene-based aryl-acetylenes and helically-shaped quinone derivatives in organic solar cells

Extended pi-conjugated polycyclic hydrocarbons are of tremendous interest because of their potential use in optical and electronic device applications. Currently, pi-conjugated triple-bond containing molecular systems, i.e. p-phenyleneethynylenes (also referred to as aryl-acetylenes), are a subject of ever intense research efforts because of their flexible HOMO/LUMO energetics, which is known to be broadly tunable via proper functionalization. Moreover, the availability of efficient synthetic protocols allows the easy modification of the effective p-conjugation length of these shape-persistent rod-like structures by controlling the number of the aryl-acetylene repeating units. Note also that an alkyne between aromatic building blocks is quite adaptable to conformational and steric constraints owing to its quasi-cylindrical electronic symmetry. Recently we proposed a new family of soluble extended arylacetylene structures as p-type semiconducting systems with promising application prospects in BHJ solar cells. Here we report on typical examples to illustrate the design rationale of such systems, and synthetic efforts to face the issues of material properties control and performance optimization. Besides, we report on the preparation of semiconducting materials based on molecules featuring [n]helicene quinone skeleton, along with the initial exploration of their properties/device response. Indeed, to date a majority of studies have focused on acenes and their derivatives, although these systems are often susceptible to oxidative and photolytic degradation; thus, there is a pressing need for alternative, acene-like topologies. We have shown that helically-shaped quinone derivatives can be prepared gram quantities in fair good overall yields and in excellent purity. In addition to being challenging synthetic targets, helicenes feature unique helical, nonplanar π-electron system. These molecules show extraordinary electronic and optical properties, and their polarizability provides a propensity to undergo pi-stacking, thereby making them interesting candidates as active materials in organic photovoltaics.