Giant Rylene Imide-Based Electron Acceptors for Organic Photovoltaics

Rylene imides are oligo(pen-naphthalene)s bearing one or two six-membered carboxylic imide rings. Their flexible reaction sites and unique photoelectronic properties have afforded active research for applications in photovoltaic devices, light-emitting diodes, and fluorescent sensors. Over the past few decades, synthetic flexibility along with the evolution of molecular design principles for novel aromatic imides has rendered these intriguing dyes considerably valuable, especially for organic photovoltaics (OPVs). During the course of molecular evolution, the most difficult criterion to meet is how to modulate the intra- and intermolecular interactions to alter the aggregation behavior of rylene imides as well as their compatibility with donor materials, with the prerequisite that the appropriate molecular energy level is maintained. In the meantime, our group has focused on the precise synthesis of pi-extended rylene imide electron acceptors (RIAs) to rationally alter the molecular chemical and electronic structure, packing arrangement, and photoelectronic properties. These powerful molecular design strategies include the construction of a fully conjugated rigid multichromophoric architecture and successful integration of heteroatoms. Herein, these multichromophoric oligomers are precisely defined as giant rylene imides. Importantly, these strategies provide a vast space for progress in RIAs and present a more comprehensive structure-performance relationship network that can be distinguished from other electron acceptor systems. In particular, the successful acquisition of these fused superhelical architectures provides a meaningful reference for the pluralistic development of OPVs, such as triplet organic solar cells and polarized-light photovoltaic detectors. Meanwhile, the introduction of heteroatoms into the rylene conjugated skeleton provides donor/acceptor interfaces with enhanced electronic interactions and thereby suppresses the polaron-pair binding energy. Nonetheless, much remains to be implemented to broaden the absorption capability of rylene imides as well as to realize full utilization of these meaningful chiral isomers with a wide and strong UV-vis spectroscopic response. In this Account, we provide an overview of our novel approaches toward a supermolecular framework and of the reformed molecular design principle for rylene imide-based electron acceptors since 2012. We begin with a discussion of the rapidly emerging synthesis strategies for giant rylene imides. Then several typical examples with remarkable photovoltaic properties and unique working mechanisms are selected, aimed at providing an in-depth discussion of structure-property-performance relationships. The remaining challenges and newly emerging research information for giant rylene imide-based electron acceptors are further put forward. It is our aspiration that this Account will trigger intensive research interest in these pluralist rylene-based electron acceptors, thereby further accelerating the profound sustainable development of organic solar cells.