New Lewis-acidic anion keeps photocatalysis efficient even in dilute solutions

Researchers at Kyushu University, working with collaborators at The University of Tokyo, have developed a photocatalytic system that can selectively capture reacting molecules and hold them close to the active sites that absorb energy from light. The innovation addresses a long-standing challenge: keeping the photocatalyst’s reaction rate high even when the reacting molecules are present at very low concentrations. The work, led by Professor Takanori Iwasaki of Kyushu University’s Faculty of Engineering, was published in the journal Chemical Science on October 8, 2026.

Photocatalysis uses light to drive chemical reactions, reducing the need for the high temperatures that other conventional catalytic reactions may require. These catalysts are designed to absorb light and transfer its energy to the reactants. Because energy transfer depends on collisions between the excited photocatalyst and the reacting molecules, such collisions become less frequent when the reactants are dilute, causing the reaction to slow down and lose efficiency.

To overcome this limitation, the team paired a light-absorbing ruthenium complex with a recently developed counteranion called BBcat. Counteranions are negatively charged ions that accompany positively charged ions in ionic chemical compounds. Although they can influence catalytic reactions, their role in photocatalysis has traditionally focused on their ability to donate electrons. BBcat is unusual in that it acts as a Lewis acid, meaning it can accept electron pairs. As a result, basic molecules are bound by the anion instead of being repelled by it.

In this way, the proposed system captures specific reacting molecules and holds them near the light-absorbing component, increasing the likelihood of energy transfer to the reacting molecules without requiring them to be present at high concentrations. According to the researchers, the unique molecule acts as both a Lewis acid and an anion at the same time, capturing basic substrates and keeping them close to the photocatalytic cation.

The team tested the photocatalyst in a reaction called [2+2] photocycloaddition, in which light helps two molecules form a four-membered carbon ring. The BBcat-based catalyst produced the desired product with a 76% yield. With a conventional catalyst carrying a different counteranion, the yield was only 24%. Under more diluted conditions, the difference widened further: the BBcat-based system maintained a product yield of 75%, while the conventional catalyst’s yield fell to 9%.

The findings show that the counteranion can influence selectivity toward specific reactants and enhance activity. “Imparting strong acidity to an anion makes it possible to move beyond the conventional view of anions as bases and use the photocatalyst’s counteranion itself as a substrate-recognition site,” Iwasaki said. He noted that the approach could offer a new way to design photocatalysts for reactions that require selective substrate activation or operate at low concentrations.

The researchers also point out that BBcat’s function may be incorporated into other catalytic systems. “Because cationic transition metal catalysts are widely used in laboratories and industry, the introduction of BBcat to these catalysts as a counteranion can be achieved via simple mixing,” Iwasaki explained. “Thus, the selective transformation and reaction rate acceleration functions enabled by BBcat—selective capturing of basic substrates—can be readily combined with other transition metal catalysis.”

Overall, the work could help establish counteranion design as a practical strategy for enhancing selectivity and efficiency in photocatalysis, improving the production of valuable compounds. The study, “A Lewis-Acidic Anion Enhances Energy Transfer from Excited Cationic Photocatalysts to Basic Substrates,” by Ryo Mandai, Yuri Oyama, Takanori Iwasaki, and Kyoko Nozaki, was described as an experimental study in the Chemical Science publication.

Subject of Research: A Lewis-acidic counteranion that captures basic substrates to improve photocatalytic energy transfer and reaction yields under dilute conditions

Article Title: BBcat: A novel anion for more selective and efficient light-driven catalysis

Article References: BBcat: A novel anion for more selective and efficient light-driven catalysis. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: photocatalysis, BBcat, counteranion, Lewis acid, ruthenium complex, energy transfer, Chemical Science, Kyushu University, photocycloaddition, selective catalysis, dilute conditions, substrate recognition