A new discovery by Japanese scientists has overturned the perceived wisdom that photoreactions in crystals follow classical photoreaction kinetics

Stimuli-responsive materials, whose physical properties change in response to external stimuli such as light and heat, are being widely studied as next-generation functional materials.

Photo-responsive materials especially are attracting much attention because their physicochemical properties can be modulated remotely without any physical contact.

As one of the photo-responsive materials, photomechanical molecular crystals, which consist of molecules that undergo photochemical reactions upon light irradiation, are being intensively investigated because of their unique photo-responsive behaviours.

Unlike in solutions where molecules exist independently, the molecules are arranged densely and regularly in crystals, so the unique internal photoreaction dynamics must be considered.

However, previous studies on photoreactive molecular crystal materials have usually assumed that photoreactions proceed like those in solutions and follow classical photoreaction kinetics. So, understanding of physicochemical property changes based on the photochemical reaction kinetics in crystals has been an issue for further development of this research field.

See also: First single-crystal perovskite optical fibres developed


A research group led by Kohei Morimoto, a third-year doctoral student at the Osaka City University Graduate School of Engineering, and Dr Daichi Kitagawa and Professor Seiya Kobatake at the Osaka Metropolitan University Graduate School of Engineering has discovered that photoreactions in 2,5-distyrylpyrazine (DSP) crystals propagate from each of the crystal’s edges to its centre.

Normally, when light shines evenly on a crystal, the colour change caused by the photoreaction also proceeds evenly. However, the research group found that in DSP crystals, the colour change started at the crystal’s edge and proceeded to its centre, propagating the photoreaction like a wave.

The research group determined that this edge-to-centre propagation of the colour change happens for two reasons: a surface effect, which makes reactivity extremely high at the edges of the crystal; and a co-operative effect, which also elevates reactivity for molecules whose neighbours have already changed colour.

Kitagawa said: “This phenomenon, which deviates from the conventional conception of photochemistry, is hugely significant for how we understand the basic fundamental science of photoreactions.

“In the future, we would like to clarify the conditions necessary for this unique photoreaction to occur and explore how they can be used to create new functional materials that take advantage of this phenomenon.”

The research is published in Angewandte Chemie International Edition.

Image: Photochemical reactions were thought to occur uniformly so that when light shines on crystals evenly, they change colour evenly. But a research group at Osaka Metropolitan University has discovered that a photomechanical molecular crystal, 2,5-distyrylpyrazine (DSP), undergoes photochemical reactions differently, with the colour change spreading from the edges and propagating to its centre. © Daichi Kitagawa/ Osaka Metropolitan University.