Highly Systematic and Efficient HOMO–LUMO Energy Gap Control of Thiophene-Pyrazine-Acenes

We report a series of unique and simple donor–acceptor–donor molecular systems that provide compelling insights into the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) energy gap. In this system, thiophene and pyrazine-acene were employed as a donor and an acceptor, respectively. More specifically, acceptor moieties are a combination of phenazine, bisphenazine, and thiadiazole. These donor and acceptor units are well-known primarily for generating low band gap polymers, however a systematic study on why these donor–acceptor combinations are successful has yet to be investigated. In this study, we attempt to gain a fundamental understanding about how these specific donor and acceptors impact the energetics of the resulting molecular systems. A combination of theoretical and experimental methods was used to explore the impact of geometry on the HOMO and LUMO energies. One of the most critical findings is that the dihedral angle between the donor and acceptor influences the HOMO energy predominantly while having very little impact on the LUMO energy, verified experimentally with a perfectly planar system and a closely related nonplanar counterpart. With this collaborative approach, we effectively produced a simple molecular system with an unusually small energy gap of 1.43 eV. The donor–acceptor–donor molecules showed an exceptional ability to produce one-dimensional self-assembled clusters by solution casting, with one in particular exhibiting bundles of ca. 100–200 nm width which is composed of ca. 10 nm width nanotapes. Finally, bulk heterojunction organic solar cell performance is tested using the donor–acceptor–donor molecule as an interlayer between P3HT/PC 60 BM blend and Al cathode. Power conversion efficiency was improved by 13% compared to the bulk heterojunction without the interlayer.