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

A synthetic approach to two regioisomeric π-electron extended [1,4]thiaborins anellated with two benzothiophene units was developed. Obtained boracycles exhibit different electronic properties of the central thiaborin ring 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 1LE and 1CT transitions. Strong near infrared phosphorescence in Zeonex thin films at 77 K indicates efficient intersystem crossing and formation of triplet states. Separation of HOMO and LUMO orbitals between boracyclic and BODIPY moieties facilitates the electron transfer to a 1CT state followed by a transition to the 3LE 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.