Researchers at the Karlsruhe Institute of Technology (KIT) have synthesized a new type of organic light-harvesting supramolecule based on DNA. The DNA double helix acts as a scaffold for organizing chromophores (i.e., fluorescent dyes), which act as electron donors, and buckyballs – electron acceptors – in three dimensions to avoid self-quenching.
Organic molecules that capture photons and convert them into electricity have important uses for green energy production. Light-harvesting complexes require two semiconductors, an electron donor and an acceptor. How well they perform is measured by their quantum efficiency – the rate at which photons convert into electron-hole pairs.
Quantum efficiency is considered suboptimal if self-extinguishing occurs when one molecule, excited by an incoming photon, gives up part of its energy to an identical unexcited molecule, as a result of which the two molecules are in an intermediate energy state, too low for the formation of an electron. But if the electron donors and acceptors are better separated, self-damping is limited, so the quantum efficiency is improved.
“DNA provides an attractive scaffold for light-harvesting supramolecules: its helical structure, fixed distances between nucleotide bases and canonical base pairing precisely control the position of chromophores. Here we show that carbon buckyballs associated with modified nucleosides inserted into the DNA helix significantly increase quantum efficiency. We also show that the three-dimensional structure of the supramolecule is preserved not only in the liquid phase, but also in the solid phase, for example, in future organic solar cells. “Dr. Hans-Achim Wagenknecht, professor of organic chemistry at the Karlsruhe Institute of Technology (KIT)
As a framework, scientists used single-stranded DNA, deoxyadenosine (A) and thymine (T) chains 20 nucleotides long. This length was chosen because theory suggests that shorter DNA oligonucleotides will not assemble in an orderly fashion, and longer ones are insoluble in water. Chromophores were fluorescent pyrene molecules with violet fluorescence and Nile red molecules with red fluorescence, each of which is non-covalently bound to one synthetic uracil-deoxyribose nucleoside. Each nucleoside was base-paired with a DNA scaffold, but the order of pyrene and Nile reds was left to chance during self-assembly.
In terms of electron acceptors, the scientists tested two forms of buckyballs, also called fullerenes, that are known to have excellent quenching (accepting electrons). Each buckyball was a hollow ball made of interlocking rings of five or six carbon atoms for a total of 60 carbon atoms per molecule. The first form of buckyball tested binds non-specifically to DNA through electrostatic charges. The second form – not previously tested as an electron acceptor – was covalently linked via malonic ether to two flanking U-deoxyribose nucleosides, which allowed it to base pair with nucleotide A on DNA.
Researchers have experimentally confirmed that the three-dimensional structure of a DNA-based supramolecule is retained in the solid phase: an essential requirement for solar cell applications. To this end, they tested supramolecules with any form of buckyballs as the active layer of a miniature solar cell. The designs showed excellent charge separation – the formation of a positive hole and negative electron charge in the chromophore and their acceptance by neighboring buckyballs – with any buckyball shape, but especially for the second shape.
The authors explain this by more specific binding through canonical base pairing with the second form of the DNA backbone, which should lead to a smaller distance between the buckyball and the chromophore. This means that the second form is best suited for use in solar cells.
Scientists don’t expect everyone to have solar cells with DNA on their roofs soon. But the chirality of DNA is interesting: solar cells on this basis can perceive light with circular polarization in specialized applications.