Cyclopentannulation and Cyclodehydrogenation of Isomerically Pure 5,11-Dibromo-Anthradithiophenes Leading to Contorted Aromatics

Isomerically pure 5,11-dibromo-2,8-dihexylanthra[2,3-b:7,6-b']dithiophene, a brominated analog of anthracenedithiophene (ADT), was prepared and utilized for a palladium catalyzed cyclopentannulation reaction with 3,3’-dimethoxyphenylacetylene. The resulting cyclopentannulated-ADT (CP-ADT) was found to be more photo-oxidatively stable than isoelectronic CP-pentacene analogs previously prepared. In addition, the CP-ADT was able to undergo an additional Scholl cyclodehydrogenation reaction to create a contorted aromatic, an incapable feat for previous CP-pentacene analogs. The resulting compound, 4-dihexyl-5,10,17,22-tetramethoxytetrabenzo[4,5:6,7:11,12:13,14]rubiceno[2,3-b:10,9-b']dithiophene, was significantly contorted out of planarity owing to four [5]helicene-like arrangements. The density functional theory (DFT) energy minimized structures suggests splay angles of 41.80 and 40.90 for the cove regions, which are significantly larger than previously published anthracene cyclopentannulated analogs. The contorted aromatic possessed a moderately low optical gap (1.50 eV) and relatively low Lowest Occupied Molecular Orbital energy (-3.70 eV).