Benzobisoxazole Cruciforms: A Cross-Conjugated Platform For Designing Tunable Donor/Acceptor Materials

Four cross-conjugated molecules based on the benzo[1,2-d:4,5-d']bisoxazole (BBO) moiety have been synthesized from a common synthon. Theoretical studies indicated that these cruciforms had highly segregated HOMO and LUMO levels enabling semi-autonomous tuning of the LUMO level from the HOMO through substitution along the 2,6-axis. The experimental data confirms that the HOMO levels within these systems varied by 0.3 eV, whereas the LUMO levels varied by over 1.6 eV when the electron-density along the 2,6-axis was increased. The introduction of relatively electron-deficient moieties along the 2,6-axis resulted in a bathochromic shift in the absorption profiles concurrent with the stabilization of the LUMO. These substituents also prolonged the photoluminescent lifetimes owing to improved intramolecular charge transfer states between the 4,8- and 2,6- axis. The BBO cruciforms were evaluated as donor materials in organic solar cells (OSC)s, but the energy-level mismatches and poor thin film morphology led to poor performance. These results indicate that benzobisoxazole cruciforms are a promising platform for the development of tunable materials for use in organic semiconductors, but improvements in the optical, electronic and film-forming properties are needed to enable their use in efficient OSCs.