Regioisomeric control of charge transport polarity for indigo-based polymers†
In this work we report the opposite charge transport polarity observed for two regioisomeric polymer semiconductors. Previously, we observed n-type electron transport behaviour for a polymer semiconductor, 6,6′-PIDBDT, which contains 6,6′-indigo units. Electron distributions in the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of indigo calculated by density functional theory (DFT) could explain the electron transport behaviour since electrons can be delocalized along the polymer backbone through the LUMO rather than the HOMO via the 6- and 6′-positions. Serendipitously, we found that the 5- and 5′-positions of indigo are occupied by electrons in the HOMO but empty in the LUMO, opposite to the 6- and 6′-positions. This suggests that 5,5′-PIDBDT containing the 5,5′-indigo units, a regioisomer of 6,6′-PIDBDT, may exhibit the opposite p-type charge transport behaviour. To prove our assumption, we synthesized 5,5′-PIDBDT and found that this polymer indeed showed p-type semiconductor behaviour. Hence we demonstrated a novel approach to control the electron or hole charge transport polarity by simply varying the main chain regiochemical connections.