Colours of dual light emissions can be controlled by design

Molecular ‘butterflies’ that flutter their wings under light can be tuned to glow red or blue, or both.  This discovery points to a simple way of controlling dual phosphorescence with implications for the creation of new LEDs and sensors.

Shining light on certain molecules excites them to a higher energy level and the extra energy can induce a change in molecular structure, like a contraction or a flattening, and phosphorescence emission. So far, scientists have been unable to exercise precise control of the process. But researchers from Florida State University, US, have now designed ‘butterfly’ platinum complexes, consisting of a central body with ligand wings, that can undergo tuned structure changes to ‘flap’ their wings and phosphoresce in one colour when their wings are open and another when they are closed.

This happens because unusually their excited state has two energy minima from which they can relax, instead of one. ‘From each [minimum], they can decay and create a different emission. From one, you get a blue emission and from the other, you get a red emission,’ explains Biwu Ma, one of the paper’s authors. The steric bulk of the butterfly body and wings influences the position of the energy minima, and so by varying the relative bulkiness, the researchers were able to finely adjust the wing-flapping and the colour of the emissions. ‘We can now continuously tune the properties of these molecules,’ says author Chen Huang. The butterfly with the bulkiest body and lightest wings emitted mainly red light and the butterfly with the lightest body and bulkiest wings emitted mainly blue light. All the intermediates displayed dual emission.

The team suggests that the finding can be used to improve white LEDs and create different kinds of sensors. ‘With multiple emissions we can get a single molecule broadband emission. This can be used in [LEDs] – we could get white light out of a single molecule,’ Ma says. ‘We can also use this as a sensor for phase changes … and as a temperature sensor.’ 

‘This is molecular engineering at its best,’ says Philip Coppens from State University of New York at Buffalo in the US. ‘An interesting structural detail is that the “wings” of the “butterfly” fold on the transition between the two differently emitting states. It points the way to developing materials with desired properties.’