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

Professor Nobuo Kimizuka and his colleagues at Kyushu University have now successfully demonstrated that SF can be promoted by introducing chirality (molecules that cannot be superimposed on their mirror images) into chromophores and achieving chiral molecular orientation in self-assembled molecular structures. Publication in Advanced Science, team showed SF-based triplet excitons in self-assembled aqueous nanoparticles containing chiral π-electron chromophores, a phenomenon not observed in similar racemic nanoparticles (a mixture of equal amounts of molecules that are mirror images of each other).

Kimizuka says, “We discovered a new method to improve SF by realizing chiral molecular orientation of chromophores in self-assembled structures.”

The researchers investigated the SF characteristics of aqueous nanoparticles, which were self-assembled from ion pairs of tetracene dicarboxylic acid and various chiral or non-chiral amines. They identified the critical role of the counterion (an ion with an opposite charge to that of another ion in solution), specifically the ammonium molecule. The counterion influenced the molecular orientation of the ion pairs, the structural regularity, the spectroscopic properties, and the strength of the intermolecular coupling between the tetracene chromophores. Thus, the counterion played a key role in controlling the alignment of the chromophores and the associated SF process.

Through extensive experiments with chiral amines, the team obtained a triplet quantum yield of 133% and a rate constant of 6.99 × 10.⁹ S^-1. In contrast, they observed that nanoparticles with achiral counterions did not exhibit SF.

The racemic ion pair also produced a triplet pair intermediate state correlated by SF. However, triplet-triplet annihilation was dominant in triplet pairs; therefore, no dissociation into free triplets was observed.

“Our research provides a new framework for molecular design in sci-fi research and will pave the way for applications in energy science, quantum materials, photocatalysis and life science involving electron spins. Furthermore, it inspires us to continue exploring SF in chiral molecular assemblies in organic media and thin-film systems, which are essential for applications in solar cells and photocatalysts,” Kimizuka concludes with hope.