Balls on DNA to collect light

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Organic molecules that capture photons and convert them into electricity have important applications for green energy production. Light-gathering complexes need two semiconductors, an electron donor and an acceptor. How well they work is measured by their quantum efficiency, the speed at which photons are converted into pairs of electron holes.

Quantum efficiency is lower than optimal if there is “self-extinguishing”, where one molecule excited by an incoming photon donates part of its energy to an identical unexcited molecule, giving two molecules in the intermediate energy state too low to produce an electron-pair hole. But if electron donors and acceptors are better apart, self-extinguishing is limited, so quantum efficiency improves.

In a new paper in Limits in chemistry, researchers from the Karlsruhe Institute of Technology (KIT) are synthesizing a new type of organic supramolecule for collecting DNA-based light. The double-stranded DNA acts as a scaffold for arranging chromophores (i.e., fluorescent colors) – which function as electron donors – and “buckyballs” – electron acceptors – in three dimensions to avoid self-extinguishing.

“DNA is an attractive scaffold for building light-collecting supramolecules: its helical structure, fixed distance between nucleobases, and canonical base pairing precisely control the position of chromophores. Here we show that carbon buckyballs bound to modified nucleosides inserted into helical DNA “We also show that the 3-D structure of the supramolecule survives not only in the liquid phase but also in the solid phase, for example in future organic solar cells,” says the lead author, Dr. Hans-Achim Wagenknecht, Professor of Organic Chemistry at the Karlsruhe Institute of Technology (KIT).

DNA provides the correct structure, like a bead on a helix

As a scaffold, Wagenknecht and colleagues used single-stranded DNA, deoxyadenosine (A) and thymine (T) strands 20 nucleotides long. This length was chosen because the theory suggests that shorter DNA oligonucleotides could not be assembled properly, while longer ones would not be soluble in water. The chromophores were purple-fluorescent pyrine and red-fluorescent Nile molecules, each of which is non-covalently linked to a single synthetic nucleoside uracil (U) -deoxyribose. Each nucleoside was paired with the base of the DNA scaffold, but the order of pyrene and nile reds was left to chance during self-assembly.

For electron acceptors, Wagenknecht et al. tested two forms of “buckyballs” – also called fullerenes – which are known to have excellent “quenching” capacity. Each ball was a hollow sphere built of interconnected rings of five or six carbon atoms, for a total of 60 carbons per molecule. The first form of backyball tested binds nonspecifically to DNA electrostatic charges. The second form – which had not previously been tested as an electron acceptor – was covalently linked via a malonic ester to two side nucleosides of U-deoxyribose, allowing it to be paired essentially with nucleotide A to DNA.

High quantum efficiency, including solid phase

The researchers confirmed experimentally that the 3-D structure of the DNA-based supramolecule remains in a solid phase: a crucial requirement for application in solar cells. For this purpose, they tested supramolecules with any of the forms of backyballs as the active layer in a miniature solar cell. The constructs showed excellent charge separation – creating a positive hole and a negative electron charge in the chromophore and accepting them by nearby backyballs – with any of the backyball shapes, but especially for the other shape. The authors explain this by a more specific binding, through canonical base pairing, to the DNA scaffold of another shape, which should result in a smaller distance between the backyball and the chromophore. This means that the second form is a better science for use in solar cells.

Importantly, the authors also show that the DNA-color-backyball supramolecule has strong circular dichroism, i.e. it is much more reactive to left than right polarized light, due to its complex three-dimensional helical structure – even in the solid phase.

“I don’t expect everyone to have solar cells with DNA on the roof any time soon. But the chirality of DNA will be interesting: DNA-based solar cells could sense circularly polarized light in specialized applications,” Wagenknecht concludes.

Small molecules could be the key to increasing the efficiency of organic solar cells

More information:
Sara Müller et al., Molecular chromophore-DNA architecture with fullerenes: optical properties and solar cells, Limits in chemistry (2021). DOI: 10.3389 / fchem.2021.645006,… 2021.645006 / abstract

Citation: Buckyballs on DNA to collect light (2021, February 24) retrieved February 24, 2021 from

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