article · Journal of the American Chemical Society
Singlet fission is a process in molecular semiconductors where a single light-generated excitation yields two triplet excitations, potentially boosting energy conversion. Organising semiconductor molecules using DNA templates offers precise spatial control over their arrangement, which is vital for efficient fission. Pentacene molecules were assembled into defined, stacked configurations using complementary DNA strands. Molecular dynamics simulations and transient electron spin resonance spectroscopy demonstrated that singlet fission in these structures proceeds through a bound triplet-pair quintet state before forming independent triplets. The spectral properties of this quintet intermediate proved highly sensitive to molecular geometry, showing slight structural strain compared to theoretical models. Despite the dynamic and polar disorder inherent to DNA environments, the assemblies sustained efficient singlet fission, demonstrating that biological scaffolds can reliably control exciton behaviour in organic semiconductors.
Singlet fission can double the electrical charge carriers produced from absorbed light, which could dramatically enhance the performance of solar cells and optoelectronic devices. Using DNA as a programmable scaffold allows precise nanoscale positioning of organic molecules. Proving that efficient exciton generation functions within flexible, disordered biomolecular frameworks validates a versatile approach to engineering functional nanoscale electronics.
This work is at an early experimental stage focused on nanoscale structural and photophysical characterisation. The findings could inform future materials design for optoelectronics, organic photovoltaics, and molecular sensing systems. Developers of light-harvesting technologies and molecular devices may eventually benefit, but substantial applied testing and device integration are still required to translate DNA-templated molecular stacks into commercial hardware.
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Singlet fission (SF), an exciton-doubling process observed in certain molecular semiconductors where two triplet excitons are generated from one singlet exciton, requires correctly tuned intermolecular coupling to allow separation of the two triplets to different molecular units. We explore this using DNA-encoded assembly of SF-capable pentacenes into discrete π-stacked constructs of defined size and geometry. Precise structural control is achieved via a combination of the DNA duplex formation between complementary single-stranded DNA and the local molecular geometry that directs the SF chromophores into a stable and predictable slip-stacked configuration, as confirmed by molecular dynamics (MD) modeling. Transient electron spin resonance spectroscopy revealed that within these DNA-assembled pentacene stacks, SF evolves via a bound triplet pair quintet state, which subsequently converts into free triplets. SF evolution via a long-lived quintet state sets specific requirements on intermolecular coupling, rendering the quintet spectrum and its zero-field-splitting parameters highly sensitive to intermolecular geometry. We have found that the experimental spectra and zero-field-splitting parameters are consistent with a slight systematic strain relative to the MD-optimized geometry. Thus, the transient electron spin resonance analysis is a powerful tool to test and refine the MD-derived structure models. DNA-encoded assembly of coupled semiconductor molecules allows controlled construction of electronically functional structures, but brings with it significant dynamic and polar disorders. Our findings here of efficient SF through quintet states demonstrate that these conditions still allow efficient and controlled semiconductor operation and point toward future opportunities for constructing functional optoelectronic systems.
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DOI: 10.1021/jacs.2c13743
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