(Invited) Routes to Stabilization of Linearly-Fused Aromatics

Linearly-fused aromatic hydrocarbons have remarkable electronic properties that change substantially as the length of the fused ring chain is increased. Unfortunately, this increased length brings with it worsened solubility and dramatically decreased environmental stability. A number of strategies have been explored to combat these issues. In the case of simple functionalization, the decrease in stability and solubility upon increased chromophore length can be mitigated slightly, but as the materials grow beyond 6 or 7 linearly fused rings, the substituents become so numerous and bulky that intermolecular aromatic-aromatic contacts are eliminated entirely, precluding semiconductor applications for these materials. An alternative approach substitutes strategic carbocyclic rings with heterocycles; these hetero-acenes do show improved stability up to a point, but again once the ~7-ringed systems have been attained, the same problems of solubility and stability again become an issue. Further complicating matters in these systems, issues of isomerism due to the limited synthetic routes to these compounds become an issue. It is becoming clear that while isomeric mixtures yield excellent semiconductor performance, the pure isomers exhibit fewer destructive phase transitions along with slight increases in performance. An alternative strategy involves the off-linear fusion of aromatic rings. In these carbocyclic systems, off-linear fusion allows the construction of linearly-fused aromatic systems of unprecedented length, while maintaining excellent stability and tunable solubility. The off-linear ring fusions allow for additional functionalization to tune the chromophore's optical and electronic properties, and yield the potential to substantially increase intermolecular overlap. The synthesis, structural, and electronic characterization of these new fused-ring chromophores will be discussed.