Contorted Aromatics from Cyclopentafused Polycyclic Aromatic Hydrocarbons

Kyle Plunkett
Southern Illinois University


Abstract

This presentation will survey recent advances from our lab related to contorted aromatics based on cyclopentafused-polycyclic aromatic hydrocarbons (CP-PAHs). Both small molecule, liquid crystalline derivatives as well as polymeric examples will be provided. The contorted building blocks are fully conjugated but distort from planarity due to steric congestion in cove positions. Our primary work is based on 2,7,13,18-tetraalkoxytetrabenzo[f,h,r,t]rubicene and 2,7,13,18-tetraalkoxydibenzo[4,5:6,7]indeno[1,2,3-cd]dibenzo[4,5:6,7]indeno[1,2 ,3-jk]pyrene building blocks. Similar to the well-known splay angle of benzophenanthrene (e.g., [4]-helicene), these CP-PAHs possess [4]-helicene as well as [5]-helicene like arrangements that induce distortion among the aromatic rings. However, these materials possess a single five-member ring substitution in lieu of one of the six-member ring of traditional helicenes. In their crystalline packing, these materials pi-stack in a columnar lock-and-key arrangements. With the appropriate sized alkyl chain substitutions, thin films can be ordered through a relatively low lying (140 C) liquid crystalline phase. Small-molecule variants are prepared via a two-step reaction pathway involving a palladium-catalyzed cyclopentannulation reaction between aryl-dibromides (e.g., 9,10-dibromoanthracene or 1,6-dibromopyrene) and a diaryl acetylenes followed by a Scholl cyclodehydrogenation reaction. The palladium-catalyzed reaction conditions are efficient enough to be applied to polymerizations between aryl-dibromides and bis-acetylene containing molecules to create macromolecules with MW>20,000 g/mol. Furthermore, the Scholl cyclodehydrogenation applied to these polymers create ladder-type conjugated polymers with reasonable solubility owing to the contorted nature of the backbone. The presented materials possess reasonably low energy lowest unoccupied molecular orbitals and band gaps. The optoelectronic and field-effect transistor performance of these materials will also be discussed.