BODIPY-based regioisomers and a donor–acceptor rotor as organic photosensitizers for maximizing singlet oxygen quantum yields and for the photooxidation of thioanisole

Abstract

The efficient production of singlet oxygen using organic photosensitizers (PSs) is highly attractive for photocatalytic applications. Herein, we designed and synthesized regioisomeric biphenyl-BODIPYs such as pp-BODIPY, mp-BODIPY, mm-BODIPY and T-BODIPY (thiophene at the meso-position) and a triad T-ADA (based on benzodithiophene as the donor and BODIPY as the acceptor) as organic PSs. Singlet oxygen quantum yields reached up to 77% in T-BODIPY due to the heavy atom effect (bromine substitution in the thiophene ring) and charge transfer (CT)-mediated intersystem crossing (ISC) and ∼35% in T-ADA due to CT-mediated ISC. The variation in connectivities of spacers in regioisomeric BODIPYs and the type of spacer in T-BODIPY significantly altered the photophysical properties. Among all the PSs, T-ADA showed a charge transfer band at ∼650 nm. Detailed insights into the ultrafast dynamics and excited state processes were obtained using femtosecond transient absorption spectroscopy (fs-TAS). The technique allowed for a thorough investigation of these systems, indicating the participation of charge-transfer (CT) states in the observed dynamics of triplet state formation. Owing to the efficient generation of ¹O₂, T-BODIPY and T-ADA as well as regioisomers pp-BODIPY, mp-BODIPY and mm-BODIPY were utilized in the aerobic photooxidation of thioanisole to methyl phenyl sulfoxide with high selectivity towards sulfoxide formation.