Iron's cool orange glow

The first iron-containing molecule that shows iron-involved photoluminescence has been synthesized by researchers at Lund University in Sweden. This material might find applications in lower-cost and environment friendly materials for light sources and displays and even solar energy conversion.

Chemists have worked on metal-based dye molecules for the best part of half a century for display technology and solar panels. Unfortunately, the best results are often achieved with relatively scarce or expensive metals. Ideally, such materials based on common metals would be optimal in terms of cost and environmental impact. Iron, for instance, is much more abundant and accessible than palladium say. Ruthenium and europium have proven useful, but again, they are not as useful as an iron-based metal dye or ones based on copper would be for many reasons, such as earth abundance, low cost, and lack of toxicity.

Now, through a molecular design approach the Lund team has successfully manipulated the electronic properties of iron-based molecules so that they much better resemble the ruthenium-based substances. They have thus for the first time, created a low-spin, iron(III) -based dye molecule which can absorb light and then emit it at a different wavelength. In their proof of principle they can achieve emission of orange light from their iron compound. There are iron complexes that are photoluminescent however that is due to a photoluminescent ligand, in the present material the iron itself is involved in the photoluminescence.

"Medieval alchemists tried to produce gold from other substances, but failed. You could say that we have succeeded in performing modern alchemy by giving the iron properties which resemble those of ruthenium," muses Kenneth Wärnmark. The team published detail of their research recently [Wärnmark, K et al. Nature (2017) 543, 695-699; DOI: 10.1038/nature21430]. The compound developed by the team is based on the ion [Fe(btz)3]3+ (where btz is 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene)). It shows room temperature photoluminescence and a long charge-transfer lifetime, 100 picoseconds, this lifetime is quite adequate for a range of applications. Indeed, the team explains, "The absence of intersystem crossing, which often gives rise to large excited-state energy losses in transition-metal complexes, enables the observation of spin-allowed emission directly to the ground state and could be exploited as an increased driving force in photochemical reactions on surfaces."

The work was an international collaboration between Lund researchers and colleagues at and at the Ångström Laboratory at Uppsala University, Sweden, the National Institute of Standards and Technology, in Boulder, Colorado, USA, and the University of Copenhagen, Denmark. The researchers concede that much work remains to be done and it may be another five years before a commercial iron-based dye is marketed suggests Lund's Petter Persson. 

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".