Switching Between Fluorescence and Phosphorescence - Synthesis and Optical Properties of Metallacyclopentadienes

• Applicant: Professor Dr. Todd B. Marder, Ph.D.
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2015 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 279197767

The development of photoactive transition metal complexes is of great importance, as such materials have shown immense potential in the last few years for many applications including biological imaging, non-linear optics, photocatalysis, solar cells and OLEDs. Selected organo-transition metal compounds are now employed commercially in OLEDs for mobile phones and other flat panel displays. Despite the progress in the commercial field of display technology, there are still many challenges and additional fundamental studies are required prior to the development of photoactive transition metal compounds for other applications. One key issue is the formation of singlet and triplet excited states and the ability of a system to cross between these two spin states, a process which is normally forbidden, but can become allowed in the presence of a metal atom. These two states have very different properties, including fluorescence from singlets and phosphorescence and singlet oxygen sensitization from triplets. Their different lifetimes also allow for other applications which, for example, involve charge separation or recombination. It is thus of considerable important to understand intersystem crossing between these states.We have recently discovered that a class of organometallic compounds, namely metallacyclopentadienes, which are easy to prepare in excellent yield from simple diyne precursors, can display unexpectedly efficient fluorescence rather than phosphorescence. In addition, preliminary results show that by changing the ligands on the metal precursor, the reactions can simultaneously generate isomeric dibenzo-metallacyclopentadienes which display amongst the highest phosphorescence quantum yields for this class of compounds.We envisage several potential applications of our compounds, but to realize them, fundamental studies are required in order to understand and modulate their highly unusual optical properties in a controlled fashion. In addition, the surprising and very exciting formation of two isomers which display completely different optical properties, one fluorescent and one phosphorescent, with excited state lifetimes differing by 5 orders of magnitude, requires a systematic study to gain full control of the reaction pathway making this a general route to access highly interesting dibenzo-metallacyclopentadienes. We may also discover entirely new types of products formed by other novel diyne coupling processes under certain conditions. We will prepare a wide range of these types of compounds and make detailed measurements of their optical properties which, in combination with theoretical studies, will provide insights into the nature of their excited states, allowing us to tune their properties in a controlled manner.

• DFG Programme: Research Grants