Thioetherification Inducing Efficient Excited Triplet State and Singlet Oxygen Generation: Heavy Atom-Free BODIPY Photosensitizer Based on the S ₁ (n,π*) State

We have examined the ability of the newly synthesized thioalkyl and thiophenyl (SR) BODIPYs to generate the excited triplet state (T 1 ) using the laser flash photolysis technique. Compared to the unsubstituted BODIPY parent, SR BODIPYs are much more efficient in generating T 1 . We have also determined the capability of the SR BODIPYs to yield singlet oxygen O 2 ( 1 Δ g ) using two methods: (1) measuring the steady state and time-resolved NIR phosphorescence that peaked at 1270 nm; (2) recording DPBF decomposition caused by the reaction with singlet oxygen. The results show that both thioalkyl and thiophenyl very remarkably increase the quantum yield of singlet oxygen formation up to 0.83 from 0.10. To understand the mechanism, we have studied the related photophysics using steady state and time-resolved fluorescence and UV–vis absorption, as well as quantum chemical calculations based on Density Functional Theory. Both the thioalkyl and thiophenyl remarkably decrease the quantum yield of fluorescence and shorten the fluorescence lifetime of BODIPYs, they also cause the occurrence of a new S 1 (n,π*) with a slightly lower energy than S 1 (π,π*). Since intersystem crossing (ISC) from S 1 (n,π*) to T 1 (π,π*) is symmetrically allowed, while ISC from S 1 (π,π*) to T 1 (π,π*) is symmetrically forbidden, this explains why SR presence renders BODIPY efficient photosensitizer for excited triplet state and singlet oxygen generation. SR induced efficient T 1 formation is different from the cases of carbonyl (C═O) or thiocarbonyl (C═S). This finding provides a new method to get efficient heavy atom free photosensitizers, which is important in developing new generation photosensitizers for photodynamic therapy, photochemical up conversion of near IR sunlight in solar cell, and photoredox catalysis.