Singlet fission (SF) is a process where light absorption leads to the creation of two triplet excitons from a single singlet exciton in chromophores. Researchers have found that introducing chirality and manipulating the orientation and arrangement of chromophores can enhance this process. Their research holds promise for advancements in fields such as energy science, quantum materials, photocatalysis, solar energy, and life sciences.
In the realm of organic molecules, an exciton consists of a pair made up of an electron (which carries a negative charge) and its corresponding hole (which carries a positive charge). These particles are kept together due to Coulombic attraction, allowing them to move through molecular assemblies. Singlet fission (SF) amplifies the exciton by converting a singlet exciton into two triplet excitons. This transformation occurs when a single photon is absorbed by chromophores, which are molecules that capture specific light wavelengths. The alignment and configuration of these chromophores are vital for maximizing SF efficiency, particularly in materials with great potential for optical devices.
Up until now, most SF research has focused on solid samples, and there are still no thorough design principles for the molecular organization necessary to achieve effective SF.
Professor Nobuo Kimizuka and his team at Kyushu University have successfully shown that introducing chirality (which refers to molecules that cannot be superimposed on their mirror images) into chromophores and achieving a chiral arrangement in self-assembled structures can enhance SF. Their findings, published in Advanced Science, revealed the presence of SF-based triplet excitons in self-assembled nanoparticles made of chiral π-electron chromophores, a result not seen in similar racemic nanoparticles (which consist of equal amounts of mirror-image molecules).
Kimizuka mentions, “We have uncovered a new approach to boost SF by achieving chiral molecular orientation of chromophores in self-assembled structures.”
The researchers explored the SF properties of aqueous nanoparticles that self-assembled from ion pairs of tetracene dicarboxylic acid together with various chiral or non-chiral amines. They discovered that the counterion (an ion with an opposite charge to another ion) played a crucial role. Specifically, the ammonium molecule was influential in determining the molecular orientation of the ion pairs, their structural consistency, their spectroscopic characteristics, and the intensity of the intermolecular interactions among tetracene chromophores. Thus, the counterion was pivotal in controlling how the chromophores aligned and the SF process associated with them.
After conducting extensive experiments with chiral amines, the research team achieved an impressive triplet quantum yield of 133% and a rate constant of 6.99 × 109 s−1. In contrast, nanoparticles with achiral counterions did not show any signs of SF.
Additionally, the racemic ion pair formed an intermediate correlated triplet pair state through SF; however, in these triplet pairs, triplet-triplet annihilation was more dominant, preventing any formation of free triplets.
“Our findings provide a fresh perspective for designing molecules in SF research and will facilitate developments in energy science, quantum materials, photocatalysis, and life sciences that involve electron spins. Moreover, it encourages us to keep investigating SF in chiral molecular assemblies within organic media and thin film systems, which are essential for the advancement of solar cells and photocatalysts,” Kimizuka concluded with optimism.
