Curved Aromatic Molecules and Anti-aromatic Molecules

Dr. Michael Pittelkow, University of Copenhagen

Host: Dr. Lyle Isaacs

Abstract: In this presentation I will discuss the synthesis and properties of a range of aromatic-, anti-aromatic- and helical aromatic molecules. The talk will feature molecules with 'weird' magnetic properties, helical chirality and abnormal reactivity due to close proximity. I will discuss some of the unusual properties (and some of the very trivial and unsurprising properties) of these large well-defined conjugated molecules.

               I will describe the journey from fundamental studies of the acid-mediated oligomerization of simple 1,4-benzoquinones to the controlled synthesis of heterocyclic [8]circulenes (featuring an antiaromatic planar cyclooctatetraene) and even a larger planar [9]helicene. In the simplest picture two units of benzoquinone gives a dihydroxy-dibenzofuran + water, thus forming a new furan ring. This sets up a 1+1=3 ‘logic’ for elongation of the p-system. The synthetic methodology has allowed us to prepare a range of fully conjugated helicenes, including the longest known optically resolved chiral [13]helicenes. I will also discuss strategies to prepare longer [oligo]helicenes based on the synthetic logic.

               Access to these rather complex molecules allows us to study the properties of conducting helicenes in break-junction experiments. All the helicenes and circulenes prepared exhibit significant fluorescence (mostly blue or blue/green), and they perform well in OLED devices. Preliminary data correlating circular polarized luminescence (CPL) output of the optically pure helicenes show a correlation of CPL intensity with pitch length, and these heterocyclic helicenes exhibits some of the brightest chiral fluorescence observed. 

Organic/Inorganic Seminar