A team of Japanese researchers has discovered a new technique for adjusting molecular orientation that could dramatically improve energy transfer when absorbing light. While still in its infancy, this discovery could lead to the development of much improved energy harvesting technologies such as solar cells.

The discovery involves a process called singlet fission (SF). This is a process where an exciton absorbs light, splitting and generating an additional exciton in the process.

For context, excitons are particle-bound pairs of a negatively charged electron and a positively charged “hole”. These pairs are held together by coulombic attraction (attraction of oppositely charged particles) and can move within molecular assemblies.

SF results from the absorption of a single light particle, or photon, in molecules called chromophores (molecules that absorb certain wavelengths of light). Control of molecular orientation and chromophore arrangement is essential to achieve high efficiency in singlet fission materials.

A significant discovery

To date, SF studies have focused on solid materials, and little detailed work has been done to find ways to manipulate the molecular organization to maximize the efficiency of the SF process.

However, a team of Japanese researchers from Kyushu University is set to change that. Led by Professor Nobuo Kimizuka, the team successfully demonstrated that SF can be promoted by introducing chirality into chromophores.

Chirality is a term that refers to a property of molecules that makes them non-superimposable in their mirror images. This happens because of the specific arrangement of atoms in the molecular structure.

Chirality is important in fields such as organic chemistry because different chiral forms, or “enantiomers,” can have distinct properties and behaviors. This difference makes them important in fields such as pharmacology and materials science.

chromophore are parts of a molecule responsible for its color. They absorb light at certain wavelengths, which correspond to certain colors we can see, and this is due to their unique arrangement of electrons.

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

Chirality is the key

The researchers explored the self-assembly characteristics of aqueous nanoparticles derived from ion pairs of tetracene dicarboxylic acid and various chiral or non-chiral amines. They identified that the counterion (an ion with an opposite charge to another ion in solution)—specifically, the ammonium molecule—played a crucial role in this process.

The team found that the counterion influenced several factors, including the molecular orientation of the ion pairs, the structural regularity, the spectroscopic properties, and the strength of the intermolecular coupling between the tetracene chromophores. As a result, it was revealed that the counterion was the key in controlling the alignment of the chromophores and the related singlet fission (SF) process.

When analyzing their technique, the team achieved a remarkable triplet yield (a SF measurement) of 133%, indicating high SF efficiency. Achiral (non-chiral) molecules acting as a control did not show similar results, proving the impact of chirality.

“Our research provides a new framework for molecular design in SF research and will pave the way for applications in energy science, quantum materials, photocatalysis, and life science involving electron spins,” Kimizuka concluded.

“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.”

The study was published in Wiley Online Library.