Band structure engineering for low band gap polymers containing thienopyrazine

In this research, we demonstrated that the energy levels, including highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and the optical absorptions of low band gap conjugated copolymers, consisting of 3-hexylthiophene (3HT) as electron-donating units and 2,3-diethylthieno[ 3,4-b]pyrazine (ETP) as electron-accepting units, could be systematically tuned by adjusting the composition and the geometric structure of the copolymers. Four new copolymers comprising of 3HT and ETP in different molar ratios (from 1 : 1 to 4 : 1), named P1-P4, were designed and synthesized via Suzuki coupling. The positions of the hexyl side chains on 3HT were varied to adjust the co-planarity of the copolymers. The LUMO, ranging from -2.94 eV to -3.11 eV, was lowered monotonically with increasing ETP content, and the break of co-planarity along the main chain showed a trivial effect on the LUMO. By contrast, the HOMO (-4.74 eV to -4.88 eV) was controlled by both the composition and the geometric structure of the copolymer. P2, having a twisted geometric structure, possessed a lower HOMO and a larger band gap compared to the planar P3. Bimodal optical absorptions with a relatively stronger absorption at long wavelengths were observed for all polymers which are due to intramolecular charge transfer. Optical band gaps in solution, ranging from 1.27 eV to 1.76 eV, decreased with increasing ETP content in the copolymer except P2 and the trend was consistent with the HOMO-LUMO gaps. The optical absorptions and the energy levels are further confirmed by theoretical calculations. Good co-planarity was also found to benefit the electrical conductivity of p-doped thin films. Our results suggested that tuning the composition and the geometric structure would be an effective molecular design strategy toward desired band structure for low band gap conjugated polymers based on thiophene and thienopyrazine derivatives.