The judicial placement of main group elements in conjugated structures is emerging as a key route to novel functional hybrid materials. We demonstrate here that the formation of B-N Lewis pairs at the periphery of anthracene leads to buckling of the backbone while also dramatically lowering the LUMO energy. The resulting BN-substituted contorted polycyclic aromatic hydrocarbons show large bathochromic shifts in the absorption and emission relative to all-carbon analogs. In the presence of light, they rapidly react with O without the need for an external photosensitizer, resulting in selective and reversible formation of the corresponding endoperoxides.

We demonstrate that the modification of anthracene with B ← N Lewis pairs at their periphery serves as a highly effective tool to modify the electronic structure with important ramifications on the generation and reactivity toward singlet oxygen. A series of BN-fused dipyridylanthracenes with Me groups in different positions of the pyridyl ring have been prepared via directed electrophilic borylation. The steric and electronic effects of the substituents on the structural features and electronic properties of the isomeric borane-functionalized products have been investigated in detail, aided by experimental tools and computational studies. We find that BDPA-2Me, with Me groups adjacent to the pyridyl N, has the longest B–N distance and shows overall less structural distortions, whereas BDPA-5Me with the Me group close to the anthracene backbone experiences severe distortions that are reflected in the buckling of the anthracene framework and dislocation of the boron atoms from the planes of the phenyl rings they are attached to. The substitution pattern also has a dramatic effect on the self-sensitized reactivity of the acenes toward O2 and the thermal release of singlet oxygen from the respective endoperoxides. Kinetic analyses reveal that BDPA-2Me rapidly reacts with O2, whereas BDPA-5Me is converted only very slowly to its endoperoxide. However, the latter serves as an effective singlet oxygen sensitizer, as demonstrated in the preferential formation of the endoperoxide of dimethylanthracene in a competition experiment. These results demonstrate that even relatively small modifications in the substitution of the pyridyl ring of BN-fused dipyridylanthracenes change the steric and electronic structure, resulting in dramatically different reactivity patterns. Our findings provide important guidelines for the design of highly effective sensitizers for singlet oxygen on one hand and the realization of materials that readily form endoperoxides in a self-sensitized manner and then thermally release singlet oxygen on demand on the other hand.

The functionalization of triarylboranes with pyridalthiadiazole (PT) acceptor moieties gives Ar2B–π–PT dyads and PT–π–B(Ar)–π–PT triads with low-lying PT-centered LUMO orbitals. Addition of a fluoride source results in competing anion binding to boron and PT reduction, depending on the steric and electronic structure of the B-aryl group.