The design, synthesis and assembly of new organised semiconductor structures required new designs and synthesis of the target organic semiconductors, and we developed new designs for the organic semiconductor, including several based on perylene bisdiimide moieties. One key innovation, developed in collaboration with the Stulz group at the University of Southampton, was to integrate the semiconductor molecule within the DNA chains using phosphoramidite coupling chemistry, allowing selection of the DNA sequence to either side. This provided the core for the successful construction of DNA-assembled structures, publication 9, and two further papers to be submitted.
The development of transient optical spectroscopy techniques allowed us to study the spatial evolution of photoexcitations in our well-defined molecular structures, covering excitons and charges. For excitons key questions are the natural size of excitons in molecular assemblies which we were able to observe in DNA-constructed assemblies and other self-organised assemblies. Larger structures also revealed very surprising long range exciton diffusion which we consider may well allow better light harvesting structures that may find application in photodetectors and solar cells. The dissociation of excitons to form separated electrons and holes, and their later recombination to either spin singlet or spin triplet states is central to the operation of organic solar cells, and we have made important discoveries on the role of (unwanted) triplet exciton formation and on schemes to avoid these processes. We have also successfully developed DNA-coupled assemblies of pentacenes. Pentacene demonstrates unusual electronic structure causing a photogenerated spin singlet exciton to split into a pair of spin triplet excitons, each at about one half of the energy of the singlet exciton. We complemented our optical spectroscopy with light-induced electron spin resonance through collaboration with the Behrends group at Freie Universität Berlin. This process is critically dependent on intermolecular packing, and we found that the DNA-assembled structures provided a new regime of weaker intermolecular interactions that still allowed triplet pair formation, but allowed also their evolution to an overall spin quintet state formed by adjacent triplet excitons.
