Photoswitches are molecules that change structure when illuminated, and they underpin emerging technologies ranging from targeted cancer treatments to self-healing materials and microscopic robots. For real-world use, these switches must often operate deep inside materials or biological tissues, where light loses intensity as it scatters. Visible light is especially attractive because it penetrates such environments more effectively than ultraviolet light. However, most conventional azobenzene photoswitches are relatively insensitive to visible wavelengths, and previous efforts to improve their performance often reduced their stability.

The research team solved this problem by equipping azobenzene molecules with dipyrrin complexes that act as highly efficient molecular antennae. These antennae absorb visible light before transferring the captured energy directly to the azobenzene unit, triggering its structural transformation. The team developed two types of hybrid molecules: one linked azobenzene to a simple boron-based dipyrrin complex, while the other incorporated one-dimensional dipyrrin-zinc nanochains that transport energy along the molecular backbone. Through repeated separation processes, the researchers successfully isolated zinc nanochains of ten discrete lengths, enabling them to investigate how molecular size affects performance.

Using UV-visible absorption spectroscopy and proton nuclear magnetic resonance (¹H NMR), the researchers confirmed that the dipyrrin antennae efficiently transferred absorbed light energy to the azobenzene photoswitch. In the zinc nanochain structures, the chains acted as molecular energy highways, transporting excitation energy over long distances to the terminal azobenzene unit. Quantum chemical calculations further confirmed that the dipyrrin antennae efficiently harvest visible light and funnel the energy to the azobenzene core, validating the mechanism observed experimentally.

The new design produced some of the highest visible-light sensitivities reported for azobenzene photoswitches. The boron-based hybrid achieved a sensitivity of 7,500 M⁻¹ cm⁻¹, while the longest zinc nanochain reached 8,100 M⁻¹ cm⁻¹. The researchers also found that sensitivity increased as the nanochains became longer, suggesting that future designs could achieve even greater performance.

Unlike many previous strategies, the new molecular architecture preserved the photoswitches’ thermostable behavior, allowing them to maintain their switched state rather than rapidly reverting. “One of the biggest challenges has been developing photoswitches that respond efficiently to visible light without compromising their thermal stability,” Toyoda said. “Our molecular antenna strategy overcomes this trade-off, allowing us to harvest visible light much more effectively while preserving the switching behavior that practical applications require.”

“This work provides a new design principle for highly sensitive molecular photoswitches,” Toyoda added. “We believe these molecular antennae will accelerate the development of molecular machines, smart materials, photopharmacology, and other technologies that rely on precise light-controlled molecular motion.”

The researchers next plan to integrate these highly sensitive photoswitches into functional systems, including molecular robots, light-responsive smart surfaces, and technologies for manipulating DNA and proteins with light. They also hope the approach will contribute to the development of photopharmacology, in which medicines can be activated only at targeted locations within the body, reducing unwanted side effects. Such technology could one day enable smart materials that respond to their environment and drugs that can be activated only where and when they are needed.

Subject of Research: Development of dipyrrin molecular antennae that enhance the visible-light sensitivity of azobenzene photoswitches without sacrificing thermal stability

Article Title: Molecular antennae make light-activated molecular switches more sensitive

Article References: Molecular antennae make light-activated molecular switches more sensitive. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: azobenzene, photoswitches, molecular antennae, dipyrrin, visible light, zinc nanochains, energy transfer, photopharmacology, smart materials, molecular machines, Tohoku University, Molecular