Discovering light in darkness: activating luminescent materials with environmental tuning

Some metal compounds quickly lose their ability to emit light because they rapidly go into a state where they cannot emit light. Understanding how and why certain molecules do not produce light can advance new and more energy-efficient lighting technologies, such as OLED. Since the 1970s fundamental research has been performed on the mechanisms underlying the non-luminescence of such molecules. Researchers from the Institute for Molecules and Materials (IMM) of Radboud University now are able to make individual nickel phthalocyanine (NiPc) molecules light up by transferring energy from similar glowing molecules, at low temperatures. The results have recently been published in the prestigious Journal of the American Chemical Society.

NiPc is a prototypical example of some metal compounds that absorb energy but do not release it as fluorescent light like we might expect. Instead, they quickly change to a “dark” state and release the energy as heat. 

From dark to light

In a recent research study, a research team led by Daniel Wegner from the IMM’s Scanning Probe Microscopy department, in collaboration with Cristian Strassert and Nikos Doltsinis from the University of Münster, was able to shed light on the mysteries of luminescent laws of nature of metal phthalocyanines. By using the state-of-art techniques of Scanning Tunneling Microscopy-induced luminescence (STML), they found out with unprecedented detail why some metal compounds do not emit light which, however, led them to realize that fluorescence of such “dark” molecules can actually be activated by a specific type of excitation. “To that end, we moved another metal phthalocyanine close to NiPc, by means of STM manipulation,” Wegner explains. “That other complex serves as a donor transferring just the right amount of energy - not too much! - to NiPc. This way, we forced NiPc to stay in its emissive state, eventually leading to fluorescence.”

Luminescent materials

These fundamental findings stress the importance of the molecule's environment in influencing its luminescent properties. Moreover, the study introduces an innovative approach to tune the optical properties of molecules, through environmental adjustments. The established approach is to tune molecular optical properties is by chemical design of the molecules themselves. Wegner: “But in a device such as an OLED, the molecules are embedded in a solid environment. Knowing how this changes the optoelectronic properties is the stepping stone toward rationally designing the local environment around the emitter molecules.” This could open routes for more energy-efficient and sustainable alternatives for metal compounds currently used in organic light-emitting diodes (OLEDs).

Schematic view of the experiment: a donor molecule is excited by electrons tunneling from the tip of a scanning tunneling microscope. The excitation energy is transferred to the NiPc, allowing it to emit light

Activating the Fluorescence of a Ni(II) Complex by Energy Transfer

Literature reference

Activating the fluorescence of a Ni(II) complex by energy transfer
Tzu-Chao Hung, Yokari Godinez-Loyola, Manuel Steinbrecher, Brian Kiraly, Alexander A. Khajetoorians, Nikos L. Doltsinis, Cristian A. Strassert, Daniel Wegner
Journal of the American Chemical Society (2024)

Contact information

For more information, please contact
Daniel Wegner, d.wegner [at] (d[dot]wegner[at]science[dot]ru[dot]nl)
Alex Khajetoorians, a.khajetoorians [at] (a[dot]khajetoorians[at]science[dot]ru[dot]nl)
IMM Communications: imm-communication [at] (imm-communication[at]ru[dot]nl)

Sustainability, Innovation, Molecules and materials, Laws of nature