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).
