Novel dibenzo[a,e]pentalene-based conjugated polymers

Dibenzo[a,e]pentalene (DBP), a ladder-type fused-ring system with 16π anti-aromatic nature, is integrated into conjugated polymers with oligothiophene building blocks to examine the potential of DBP as a new building block for semiconducting polymers. Depending on the incorporation manner of the DBP unit in the polymer backbone, via the 5,10- or 2,7-positions, the polymers show distinct colours, reflecting the different electronic structures, though the HOMO and LUMO energy levels estimated from cyclic voltammograms are almost the same. Interestingly, the impact of the incorporation manner was observed in the characteristics of their field-effect transistors (FETs). For PDTDBP2Ts, in which the DBP units are integrated into the polymer backbone via the 5,10-positions, the DBP units behave like a "dibenzo-annulated 1,3-butadiene" moiety, and their FET characteristics are strongly affected by orientational ordering and crystallinity, similar to ordinary "donor-only" polymers such as P3HT. On the other hand, iPDTDBP2Ts, in which the whole 16π DBP unit is integrated into the polymer backbone via the 2,7-positions, behave like a certain kind of donor-acceptor polymers, and the FET characteristics are independent of orientational order: the field-effect mobilities are higher than 0.1 cm² V^-1 s^-1 regardless of the polymer orientation in the thin film. From these results, we can recognize the 16π anti-aromatic DBP unit as a useful building block with transmutable nature for the development of new conjugated polymers.