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Excited electronic states in extended molecular systems

Periodic Reporting for period 4 - EXMOLS (Excited electronic states in extended molecular systems)

Reporting period: 2020-04-01 to 2021-09-30

The directed movement of electronic excitations in molecular materials lies at the heart of photosynthesis and also in nanoscale synthetic materials systems used for electronic applications. Efficient materials systems must span many length scales; from nm molecular dimensions, to the 10 nm length scale of Coulomb interactions at 300 K in molecular systems, to the macroscopic dimensions of biological structures and of synthetic electronic devices. There is now tantalising evidence that efficient biological and synthetic systems use ultrafast coherent electronic state evolution to couple molecular and macroscopic length scales, which requires special structural arrangements over intermediate length scales of 10 nm and more.

Within the EXMOLS project have developed a new platform to study and control electronic excitations in extended molecular systems using DNA assembly methods to construct functional molecular semiconductor stacks. In contrast to current synthetic molecular systems that have little control beyond simple heterojunctions, these DNA-assembled structures can allow for the precise placement of molecules within stack and allow for the definition of precise electronic couplings and energetic landscapes, within extended artificial molecular systems.

The principal tool to track these electronic energy landscapes is time-resolved optical spectroscopy that can monitor wavefunction evolution from 10fs. These can be used to study a range of emergent electronic phenomena on the 5-100nm length scale including, charge delocalisation, coherent electron-hole separation, singlet exciton fission, resonant energy transfer across the organic-inorganic interface and topologically protected electronic excitations.

EXMOLS is a fundamental science project, but has also delivered real design rules for practical molecular-scale devices, particular for solar cells and to LEDs.
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.
Key outputs include:
(1) Deoxyribonucleic Acid Encoded and Size-Defined π‑Stacking of Perylene Diimides, JACS (2021), DOI 10.1021/jacs.1c10241
(2) Efficient energy transport in an organic semiconductor mediated by transient exciton delocalization, Science Advances (2021)DOI 10.1126/sciadv.abh4232
(3) The role of charge recombination to triplet excitons in organic solar cells, Nature (2021) DOI 10.1038/s41586-021-03840-5
(4) Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors, Nature Communications (2021) DOI 10.1038/s41467-021-26689-8
(5) Singlet exciton evolution to spin quintet coupled triplet excitons in DNA-assembled pentacene assemblies (preprint)

Outputs 1 and 5 depend on our breakthroughs in the control on DNA assembly of molecular semiconductor structures. Though this was a core objective for the project, the actual pathways we develop extended way beyond the ideas included in the original project proposal, and there were several surprises along the way. In particular, we have found that DNA-coupling does work, but it does require careful design to ensure that other ordering interactions can contribute to give a hierarchy of interactions. We think this does generalise and does set out a pathway for future research. Achievements 2,3 and 4 all bring real advances to understanding of the energetics and kinetics of photoexcited state evolution in molecular semiconductor assemblies.
scheme for DNA assembly of organic semiconductor