Bis[1]benzothieno[1,4]thiaborins as a Platform for BODIPY Singlet Oxygen Photosensitizers

A synthetic approach to two regioisomeric pi-electron extended [1,4]thiaborins annulated with two benzothiophene units has been developed. The central thiaborin rings of the boracycles obtained exhibit different electronic properties; this is reflected in their different aromatic characters, boron Lewis acidity and UV-vis spectroscopic behavior. Thiaborins were converted to boron dipyrromethene (BODIPY) complexes. Their emission spectra exhibit two distinct bands resulting from (LE)-L-1 and (CT)-C-1 transitions. Strong near-infrared phosphorescence in Zeonex thin films at 77 K indicates efficient intersystem crossing and the formation of triplet states. Separation of HOMO and LUMO orbitals between boracyclic and BODIPY moieties facilitates electron transfer to a (CT)-C-1 state followed by a transition to the (LE)-L-3 triplet state located on the ligand. These unique properties of spiro thiaborin-BODIPY complexes were explored for their application as singlet-oxygen photosensitizers. They show excellent photocatalytic performance with singlet oxygen quantum yields reaching 77 % and full conversion of the model organic substrate achieved after 1.5 h with only 0.05 % mol catalyst load.

Automated summary

A synthetic approach to two regioisomeric pi-electron extended [1,4]thiaborins annulated with two benzothiophene units has been developed. The obtained boracycles exhibit different electronic properties reflected in their aromatic characters, boron Lewis acidity, and UV-vis spectroscopic behavior. The spiro thiaborin-BODIPY complexes show excellent photocatalytic performance as singlet oxygen photosensitizers with high singlet oxygen quantum yields and full conversion of organic substrates.