Photosensitizer Design Absorbs Low-Energy Light, Transfers Energy Efficiently

Researchers at Hokkaido University, working with colleagues in Japan to develop a photosensitizer design that could use low-energy light effectively, developed a design that made the rare earth element europium (Eu) shine five times more brightly than any previous design. Their discovery could lead to more efficient photosensitizers with a wide variety of applications.

Photosensitizers are molecules that become excited when they absorb light. They then transfer this excited energy to another molecule. They are used in photochemical reactions, energy conversion systems, and photodynamic therapy, which uses light to kill some early-stage cancers.

The europium Eu(III) complex with nanocarbon antenna emitting fine red light. Courtesy of WPI-ICReDD, Hokkaido University.

To improve the light absorption and energy transfer capabilities of photosensitizers, the Hokkaido team extended the lifetime of the molecule’s triplet excited state and reduced gaps between the energy levels within the molecule. This approach enabled more efficient use of photons and reduced energy loss.

The researchers designed nanocarbon antennas made of coronene, a polycyclic aromatic hydrocarbon containing six benzene rings. They stacked two nanocarbon antennas, one on top of the other, and then connected the antennas to Eu, adding connectors to strengthen the bonds between the antennas and the Eu. When the nanocarbon antennas absorbed light, they transferred this energy to the Eu, causing it to emit red light. Experiments showed that the europium (Eu(III)) complex was most successful at absorbing light at wavelengths of 450 nm.

The Eu(III) complex containing the stacked nanocarbon structure. The nanocarbon structure works as an antenna to harvest light and transfer the energy to europium efficiently, which then emits red light. Courtesy of Kitagawa Y., Hasegawa Y., et al., Communications Chemistry, Jan. 3, 2020.

When the researchers shined a blue LED on the Eu(III) complex, it shined more than five times brighter than the photosensitizer design that until now had exhibited the strongest reported emission under blue light. The researchers also demonstrated that the complex could withstand temperatures above 300 °C, thanks to its rigid structure.

“This study provides insights into the design of photosensitizers and can lead to photofunctional materials that efficiently utilize low-energy light,” researcher Yuichi Kitagawa said. The new design could be applied to fabricate molecular LEDs, among other applications, the researchers said.

The research was published in Communications Chemistry (www.doi.org/10.1038/s42004-019-0251-z).