Molecular tuning in highly fluorescent dithieno[3,2-b:2',3'-d]pyrrole-based oligomers: effects of N-functionalization and terminal aryl unit.

A series of eight conjugated oligomers consisting of central dithieno[3,2-b:2',3'-d]pyrroles (DTPs) end-capped with either thienyl or phenyl groups have been prepared from N-alkyl-, N-aryl-, and N-acyl-dithieno[3,2-b:2',3'-d]pyrroles via Stille and Suzuki cross-coupling. The DTP-based quaterthiophene, N-phenyl-2,6-bis(2-thienyl)dithieno-[3,2-b:2',3'-d]pyrrole was characterized via X-ray crystallography and was found to crystallize in the orthorhombic space group Pna2(1) with a = 10.8666(3) Å, b = 22.8858(6) Å, c = 7.4246(2) Å, and Z = 4. The full oligomeric series was thoroughly investigated via photophysical, electrochemical, and DFT calculations in order to correlate the cumulative effects of both aryl end-groups and N-functionalization on the resulting optical and electronic properties. Through such molecular tuning, it was found to be possible to modulate the HOMO energy by as much as 0.32 V and to generate highly fluorescent oligomers with solution fluorescence efficiencies as high as 92%.