3,6-Dithiophen-2-yl-diketopyrrolo[3,2- b ]pyrrole (isoDPPT) as an Acceptor Building Block for Organic Opto-Electronics

The electron acceptor building block for π-conjugated copolymers, 3,6-dithiophen-2-yl-diketopyrrolo[3,2-b]pyrrole (isoDPPT), was synthesized following two routes. The comparison between isoDPPT and widely investigated 3,6-dithiophen-2-yl-diketopyrrolo[3,4-c]pyrrole (DPPT) in terms of molecular orbital computations, single crystal X-ray diffraction, optical absorption and cyclic voltammogram was utilized to elucidate structural and electronic structure differences between the two cores. Both units are found to be planar in the solid state, exhibit similar LUMO energy, however, isoDPPT exhibits a much deeper HOMO energy. Six isoDPPT-based polymers with optical bandgaps spanning from 1.44 to 1.76 eV were synthesized by copolymerizing isoDPPT with the following building blocks: 2,2′-bithiophene (for P1), 4,4′-bis(2-ethylhexyl)-dithieno[3,2-b:2′,3′-d]silole (for P2), 3,3‴-didodecylquaterthiophene (for P3), 4,8-didodecylbenzo[1,2-b:4,5- b′]dithiophene (for P4), 4,8-didodecyloxybenzo[1,2-b:4,5-b′] dithiophene (for P5) and 3,3′-bis(dodecyloxy)-2,2′-bithiophene (for P6). Field-effect transistors and bulk heterojunction solar cells based on isoDPPT copolymers were fabricated and the response compared vis-a-vis to those of some DPPT-based polymers. Hole mobility (μ_h) of 0.03 cm ²/(V·s) and solar cell power conversion efficiency (PCE) of 5.1% were achieved for polymer P2.