Bio-Inspired Luminescent Transition Metal Complexes: from Nature to Iridium Based OLED Devices

In recent years, transition metal complexes have gained new interest for their electrochemical and photo-physical properties making them suitable to be used as phosphorescent emitters in organic electroluminescent devices, i.e. organic light-emitting diodes (OLEDs).1 Particular attention has been paid to iridium-based complexes because: a) the photo-physical properties of the complexes can be tuned in a predictable way; b) they possess stable and accessible redox states; c) they exhibit a good stability and environmental inertness when compared to other metal-ligand complexes; d) they display shorter triplet lifetimes; e) they possess strong spin-orbit coupling so that excited single states can undergo efficient inter-system crossing (ISC) into the triplet state resulting in high phosphorescent quantum yields.2,3 Overall, these data pointed out the high efficiency of iridium-based light-emitting devices making them very appealing and competitive with respect to the typically used inorganic emitters. Recently the research on OLED has been focused also on nature-inspired materials with the aim of developing novel prototype of eco-friendly and bio-compatible devicesfor integration with biological tissues.4 In this regard, a series of experiments have been carried out to test the potentiality of two heterocyclic backbones, the 6,7-dihydroxy-3,4-dihydroisoquinoline I and the 5H-pyridophenoxazin-5-one II, as nature-inspired ligands for the assembly of iridium(III) complexes. In particular I has been obtained from the catecholic neurotransmitter dopamine, whereas II, a novel class of powerful antitumor intercalating derivatives, contains the phenoxazinone ring system occurring in arthropods, cephalopods and other invertebrates.