Benzodithiophene-Fused Cyclopentannulated Aromatics Via A Palladium-Catalyzed Cyclopentannulation And Scholl Cyclodehydrogenation Strategy

We report the synthesis of a new class of cyclopenta-fused polyaromatic hydrocarbon (CP-PAH) incorporating fused benzodithiophene subunits. These CP-PAHs were prepared utilizing a two-step process involving a palladium catalyzed cyclopentannulation followed by a Scholl cyclodehydrogenation. This work broadens the scope of annulation chemistry by employing 1,2-bis(5-hexylthiophen-3-yl)ethyne and dibromoaryl derivatives based on anthracene, pyrene, and perylene to give 4,4',4 '',4'''-(cyclopenta[hi]aceanthrylene-1,2,6,7-tetrayl)tetrakis(2-hexylthiophene), 4,4',4 '',4'''-(dicyclopenta[cd,jk]pyrene-1,2,6,7-tetrayl)-tetrakis(2-hexylthiophene), and 1,2,7,8-tetrakis(5-hexylthiophen-3-yl)-1,2,7,8-tetrahydrodicyclopenta [cd,lm]perylene. Scholl cyclodehy- drogenation of the pendant thiophene units provided access to the Jr-extended polyaromatic systems 2,5,11,14-tetrahexylrubiceno[5,4- b:6,7-b:12,11-b '': 13,14-b''']tetrathiophene, 2,5,11,14-tetrahexyldithieno-[4,5:6,7] indeno [1,2,3-cd] dithieno [4,5:6,7] indeno-[1,2,3-jk]pyrenes, and 2,9,12,19-tetrahexyldithieno[4,5:6,7]indaceno[1,2,3-cd]dithieno[4,5:6,7]indaceno[1,2,3-lm]perylene that possess helicene-like fragments. The anthracene-based CP-PAH was contorted owing to [5]helicene-like arrangements, while the pyrene- and perylene-based systems were essentially planar. The fully conjugated small molecules give low optical gaps (1.7-2.1 eV) with broad light absorption. The HOMO and LUMO energies of the CP-PAHs were found to be in the range of -5.48 to -5.05 eV and -3.48 to -3.14 eV, respectively. Finally, the anthracene-based CP-PAH was found to be a p- type semiconductor when tested in an organic field effect transistor.