Exciton Evolution by Singlet Fission and Excimer Formation in DNA-Semiconductor π-Stacks

Natural photosystems control sub-nm interchromophore coupling using a protein scaffold to evolve remote singlet excitons into free charges at the reaction centre. Nature must achieve this with a limited set of pigments, necessitating exquisite control over excitonic coupling. In contrast, traditional organic electronic devices lack such nano-structural fidelity. This has limited our understanding over electronic coupling and our ability to optimize condense-phase organic semiconducting aggregates. To address this disparity, we have developed efficient methods for the insertion of perylene diimides or pentacenes into DNA.^1,2 DNA’s selective self-assembly yields size-defined aggregates of pseudo-1D π-stacked organic semiconductors. We selectively program size and exciton delocalization to vary from isolated monomers up to strongly coupled pentamers.<br/><br/>We characterize excited evolution using transient absorption and spin resonance spectroscopy, we track the evolution of singlet excited states in our aggregates, showing results are well-matched to traditional—but disordered—thin films. In size-controlled pentacene monomers, the excited singlet state remains localized to single chromophore. In larger pentacene assembly size we observe singlet exciton evolution &lt;0.5 ps triplet formation by singlet fission. We interrogate this process further using cryogenic transient electron spin resonance spectroscopy. This reveals the initial singlet exciton evolves into a bound triplet pair quintet state (ca. 0.5 μs), which subsequently converts into free triplets. Interestingly, we can tune the rate of singlet fission through DNA programmed assembly alone. In the case of perylene diimides, we can controllably tune the extent of excimer formation as a function of DNA-directed aggregate size. These results point towards a more rational design process for organic semiconductor clusters, where kinetics can be tuned through extrinsic assembly rather than chemical change to the semiconductor skeleton. Our modular DNA-based assembly offers real opportunities for the rapid development of bespoke semiconductor architectures with molecule-by-molecule precision, a material 'toolbox' for spectroscopic study.<br/><br/>1. <i>J. Am. Chem. Soc.</i> 2023, 145, 9, 5431–5438<br/>2. <i>J. Am. Chem. Soc.</i> 2022, 144, 1, 368–376