Photosynthesis captures light and transfers the absorbed energy to a specific energy-accepting site. Natural systems are remarkably efficient at this process, and mimicking this efficiency artificially remains one of the major goals of scientists around the world. Success has been achieved to some degree using specially designed branched polymers, or ‘dendrimers’. Although these branched polymers have been shown to be effective light-harvesting systems, however, their synthesis involves lengthy preparation and purification procedures.

Researchers in Japan lead by Donglin Jiang from the Institute for Molecular Science in Okazaki1 have now synthesized a polymer–dye system with highly efficient light-harvesting ability using a simple preparation procedure.

Fig. 1: Field-emission scanning electron microscopy image of a microporous conjugated polyphenylene polymer framework that when filled with dye molecules can harvest light energy with high efficiency.

The system constructed by Jiang and his colleagues consists of a polyphenylene-based porous scaffold (Fig. 1) filled with a dye known as ‘coumarin 6’. The framework itself emits blue luminescence when exposed to light, but when loaded with the dye, the dye molecules emit a brilliant green luminescence 21 times brighter than they would in isolation. The reason for this intensity multiplication is the rapid and efficient transfer of light energy by the framework to the dye, which quenches the fluorescence from the conjugated scaffold but causes each dye molecule to be excited by energy transfer from approximately 176 phenylene units.

The conjugated framework acts as an electron-donating component, and the encapsulated dye molecules act as energy acceptors, creating a new type of ‘non-covalent’ light-harvesting system. This non-covalent assembly allows the incorporation of different donor–acceptor components, making the strategy more versatile than the covalently assembled molecular light-harvesting systems reported previously.

“The nanoparticles in the scaffold have diameters of several hundred nanometers, resulting in a high density of chromophore units with high absorption capability,” says Jiang. “Site-isolation of the coumarin 6 molecules within the pores makes the system highly luminescent even in the solid state. Other common molecular systems lose luminescence as a result of fluorescence quenching.”

In the future, the researchers hope to incorporate catalytic sites into the light-harvesting structure to create energy-conversion systems that convert absorbed light energy into a chemical energy through a photoinduced chemical reaction.