Copying nature's elaborate surface chemistries
In nature, photosystems are highly efficient in converting sunlight into the chemical energy of life, thanks to their multi-chromophoric structures. Similarly, highly organised reduction–oxidation (redox) pathways allow the electrons and holes generated by the photosynthetic process to travel through separate pathways and thus suppress charge recombination. Phthalocyanine (PC) is an intensely blue-green–coloured aromatic macrocyclic compound. However, while pure PC-based dyes have been widely used in organic electronics, the synthesis of structurally modified PCs has proved a challenge. Preparing these molecular assemblies was the aim of the 'Supramolecular active layer, self-assembly on surface' (SUPRAL_SAS) project, funded by the EU. During the project, these assemblies were prepared by the self-organising and surface-initiated polymerisation of PC to form separate pathways for hole and electron transport. To obtain the desired structures, the PCs were arranged into two types of assemblies. One type, the conductive, was used to introduce the positive or p-type transport channels. The other, which showed poor photoactivity, produced the negative or n-type charge–transport channels. Two approaches were engineered and investigated to minimise the recombination between the charges in the conductive pathways: an antiparallel redox gradient and a lateral multiple channel. Now complete, the project's outcomes will increase the prospects of using structurally modified PCs in more practical applications. These results will not only benefit the scientific community, but also companies and industries that design and fabricate optoelectronic materials. Moreover, the molecular assemblies on a surface produced and studied in this project will also be of interest for making advanced organic-based transistors. These have a multitude of applications, including displays, sensors and electronic bar codes.