Nanotechnology

Study shows self-assembly of light-absorbing chiral molecules facilitates singlet fission process

TEM image of aqueous NPs. a) Tc_NP, scale, 100 nm. b) Tc_R-NEA_NP, scale, 200 nm. c) High-resolution TEM of Tc_R-NEA_NPs, scale, 100 nm. Credit: Advanced Science (2024). DOI: 10.1002/advs.202405864

In organic molecules, an exciton is a pair of an electron (negative charge) and its hole (positive charge) bound to a particle. They are held together by Coulombic attraction and can move within molecular assemblies. Singlet fission (SF) is a process in which an exciton is amplified and two triplet excitons are generated from a singlet exciton.

This is caused by the absorption of single particles of light, or photons, in molecules called chromophores (molecules that absorb light at specific wavelengths). Controlling the molecular orientation and arrangement of chromophores is important to achieve high SF efficiency in materials with great potential for optical device applications.

So far, studies on SF have been performed on solid samples, but there is still no comprehensive design guideline regarding the molecular organization required for efficient SF.

Professor Nobuo Kimizuka of Kyushu University and his colleagues now demonstrate that science fiction can be promoted by introducing chirality (molecules that cannot be superimposed into mirror images) into chromophores and achieving chiral molecular orientation in the self-assembled molecular structure. succeeded in doing so.

In a paper published in Advanced Science, the research team demonstrated SF-based triplet excitons in self-assembled aqueous nanoparticles containing chiral pi-electron chromophores. This phenomenon is not observed with similar racemic nanoparticles (mixtures of equal amounts of molecules that are mirror images of each other). .

Professor Kimizuka said, “We have discovered a new way to enhance SF by achieving chiral molecular orientation of chromophores in self-assembled structures.”

Researchers investigated the SF properties of aqueous nanoparticles self-assembled from ion pairs of tetracene dicarboxylic acid and various chiral or nonchiral amines. They identified an important role for counterions (ions with an opposite charge to other ions in the solution), particularly ammonium molecules.

The counterion influenced the molecular orientation, structural regularity, spectroscopic properties of the ion pair, and the strength of the intermolecular bond between the tetracene chromophores. Therefore, counterions played an important role in controlling the chromophore alignment and related SF processes.

Through extensive experiments with chiral amines, the team achieved a triplet quantum yield of 133% and a rate constant of 6.99 × 109 s−1. In contrast, they observed that nanoparticles with achiral counterions did not exhibit SF.

Racemic ion pairs also produced intermediate correlated triplet pair states by SF. However, triplet-triplet annihilation was dominant for triplet pairs. Therefore, no dissociation into free triplets was observed.

“Our research provides a new framework for molecular design in science fiction research and will pave the way for applications in life sciences, including energy science, quantum materials, photocatalysis, and electron spin. Furthermore, it This provides us with the motivation to continue exploring science fiction in chiral molecular assemblies, “organic media and thin film systems that are important for solar cell and photocatalytic applications,” concludes Kimizuka.

Further information: Ilias Papadopoulos et al, Chirality of singlet fission: Control of singlet fission in aqueous nanoparticles of tetracenedicarboxylic acid ion pairs, Advanced Science (2024). DOI: 10.1002/advs.202405864

Provided by Kyushu University

Citation: Study Demonstrates Light-Absorbing Chiral Molecular Self-Assembly Facilitates Singlet Fission Process (November 1, 2024) https://phys.org/news/2024-11-chiral-molecular-absorb Retrieved November 1, 2024 from -boost- singlet.html

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