This is driven by the absorption of a photon by molecules known as chromophores, which absorb specific wavelengths of light. Controlling the orientation and arrangement of these chromophores is key for achieving high SF efficiency in materials that hold potential for optical device applications.
Until now, most SF studies have been performed on solid samples, and comprehensive design guidelines for molecular organization aimed at efficient SF have yet to be fully developed.
Researchers led by Professor Nobuo Kimizuka at Kyushu University have successfully demonstrated that singlet fission can be enhanced by introducing chirality into chromophores, leading to chiral molecular orientation in self-assembled molecular structures.
Publishing in *Advanced Science*, the team demonstrated triplet exciton generation through SF in self-assembled aqueous nanoparticles containing chiral p-electron chromophores, a phenomenon not observed in racemic nanoparticles, which are mixtures of equal amounts of molecules that are mirror images of each other.
"We have discovered a novel method to enhance SF by achieving chiral molecular orientation of chromophores in self-assembled structures," explained Kimizuka.
The team examined the SF characteristics of aqueous nanoparticles that self-assembled from ion pairs of tetracene dicarboxylic acid combined with various chiral or non-chiral amines. They identified the vital role of the counterion, specifically the ammonium molecule, in determining the molecular orientation of the ion pairs, affecting the structural regularity, spectroscopic properties, and intermolecular coupling strength between tetracene chromophores. Consequently, the counterion was found to be crucial in influencing the alignment of chromophores and the corresponding SF process.
Through extensive testing with chiral amines, the researchers achieved a triplet quantum yield of 133% and a rate constant of 6.99 + 10^9 s^-1. By contrast, nanoparticles with non-chiral counterions did not exhibit SF.
The racemic ion pairs also resulted in an intermediate correlated triplet pair state via SF. However, triplet-triplet annihilation dominated in these triplet pairs, which meant that dissociation into free triplets was not achieved.
"Our research provides a new framework for molecular design in SF studies, paving the way for advancements in energy science, quantum materials, photocatalysis, and applications involving electron spins in life sciences. It further motivates us to explore SF in chiral molecular assemblies in organic media and thin film systems, which are important for solar cell and photocatalyst applications," concluded Kimizuka.
Research Report:Chirality in Singlet Fission: Controlling Singlet Fission in Aqueous Nanoparticles of Tetracenedicarboxylic Acid Ion Pairs
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