Organic photovoltaic (OPV) systems are expected to offer an economic and technically attractive alternative to conventional silicon-based solar cells due to their mechanical flexibility, ease of handling and low production cost. Using a self-assembly approach, Hirotaka Ihara and co-workers from Kumamoto University and Kyoto University in Japan have prepared organic materials with outstanding light-induced electron-transfer ability.1

Fig. 1: Schematic illustration showing the coordination of fullerines (gray) to self-assembled small-molecule components (purple) with a central zinc atom (yellow) to form a donor–acceptor supramolecular complex. © 2010 RSC

According to Ihara, most successful synthetic OPV systems rely on intra-linked or polymer-based structures featuring an electron donor and an electron acceptor component linked by strong covalent bonds. Although they display good charge separation properties, these are often difficult to process and show limited possible combinations.

To mimic natural photosynthetic systems such as the light-harvesting antennas found in plants and bacteria, Ihara and his team created non-covalent yet polymer-like phase-separated donor–acceptor assemblies using small molecular components. “Our molecules were designed to favor a head-to-tail orientation of the molecular building blocks through a peptide-assisted self-assembly,” says Ihara.

The researchers prepared the donor component in aqueous solution by attaching an L-glutamide to a porphyrin macrocycle featuring a central zinc atom. The resulting molecule self-assembled into one-dimensional architectures through hydrogen bonding, in good agreement, Ihara notes, with previous observations of the assembly behavior of glutamide units.

These structures were then mixed with fullerenes — ball-shaped electron acceptor molecules — in an organic solvent. The fullerenes were functionalized with a pyridine moiety, which coordinated to the zinc central atom of the porphyrin to form a phase-separated, non-covalently linked donor–acceptor complex.

These structures exhibited fluorescence quenching efficiencies that matched the best covalent systems. In contrast, mixtures containing similar amounts of non-assembled porphyrins and fullerenes only showed weak electron-transfer abilities. “Our low-molecular-weight donor–acceptor supramolecular complex behaves just like a polymeric OPV system,” says Ihara. The combination of glutamide unit self-assembly and zinc–fullerene coordination is considered to be responsible for the unexpectedly high electron-transfer efficiency.

The researchers also found that the assemblies alternated from well-defined to disordered states with variations in temperature, thus allowing the electron-transfer ability to be tuned thermally. Ihara and his team are currently using this donor–acceptor supramolecular complex to manufacture thin films for OPV applications.