Skip to main content

Excited electronic states in extended molecular systems

Periodic Reporting for period 3 - EXMOLS (Excited electronic states in extended molecular systems)

Reporting period: 2018-10-01 to 2020-03-31

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 we are developping 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 will allow for the precise placement of molecules within stack-structures of dimension 5 nm or more, which will 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 for the study of 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 will also deliver real design rules for practical molecular-scale devices, from solar cells, to LEDs, to spintronics, to solar fuels. The principal objectives are to develop the methodologies to build these new structures through chemical synthesis and to use these to reveal the length scales required to deliver processes such as photogenerated separation of electronic charges.
The principle aim of the EXMOLS project is to build unique, well-defined energy landscapes made from multiple different organic semiconductors and study the movement and evolution of their photoexcited states. To achieve this, two different single-strands of DNA need to be covalently attached to opposing ends of a semiconductor. This asymmetric coupling is a challenging task and we have followed the route of using Solid Phase Oligonucleotide Synthesis in combination with semiconductor phosphoramidites. This allows the sequential growth of the first DNA sequence, the functional organic semiconductor molecule, and finally, the second DNA sequence. At present we have succeeded in producing perylene di-imides, PDIs, by this method, and have successfully coupled pairs of these to form dimers. Optical spectroscopy has been used to characterise these dimer systems. Changes in ground and excited state changes upon dimerization of two PDIs are consistent with the expected interactions between the coupled PDI molecules and show that the EXMOLS design concept is successful in a dimer system. Extensions to trimer and pentamer structures are currently underway.
One of the key questions is whether there is a fundamental limitation to the size of stacks that can be produced, given the larger size of the DNA chains (2 nm) than the inter-molecular spacings targeted for the molecular semiconductors (0.5 nm). Recent progress with simulations of possible packing structures as controlled by the length of the spacer molecules between the DNA sequence and the organic semiconductor molecule allows confidence that we can progress to pentamer structures. New methodologies for extending further are being explored, including the use of 3- semiconductor molecule 'foldamers' that can then be stacked through the DNA method.