Solid-state light-emitting devices, based on ruthenium(II) complexes and operating at low voltages, do not have the high brightness and efficiency of electrogenerated chemiluminescence cells, also based on Ru(II) complexes. This is due to a slow electrochemical “charging” mechanism, which redistributes the counterions to create redox states for charge transport and light emission, and delays the device response after an applied potential bias. If solid-state devices are to be used in flat-panel displays, clearly shorter “charging” times (to brightness) are required.

Now, a team from Massachusetts Institute of Technology has produced single-layer, spin-cast films of small-molecule Ru(bpy)₃(PF₆)₂ (1) complexes (bpy = 2,2′-bipyridine) with high-brightness at low voltage, and no need of “charging” or reactive cathode materials (E. S. Handy, A. J. Pal and M. F. Rubner, J. Am. Chem. Soc ., 1999, 121, (14), 3525–3528).

Thin films (∼ 1000 Å) of the Ru(II) complex were spin-cast onto an indium tin oxide (anode) patterned glass from pyridine solutions. An aluminium cathode completed the devices. All devices had luminance levels of 1000 cd m⁻² at 5 V and 200 cd m ⁻² at 3 V and external quantum efficiencies of 1 per cent at low voltage. The emitted red light could be shifted to a more useful red with increased device stability on replacing ligands in (1) by esterified bpy ligands. Device response times can be shortened by using short, high-voltage pulses, and low voltage operation.