Singlet Oxygen Generation and Triplet Excited-State Spectra of Brominated BODIPY

The excited triplet-, singlet-, and ground-state properties as well as singlet oxygen generation capability of four brominated BODIPY dyes were measured in toluene with laser flash photolysis, fluorescence spectroscopy, time-correlated single-photon counting, and absorption spectroscopy. The triplet-triplet (T1-Tn) absorption spectra were identified for four dyes 1B, 2B, 4B, and 6B substituted with one, two, four, and six Br atoms, respectively. The triplet quantum yield (ΦT) of a usual BODIPY dye is negligible and has rarely been studied. So is the case for the parent compound 0B (8-phenyl boron-dipyrromethene), in which no Br atom is present. The substitution of the first Br atom into the π ring of BODIPY allowed a dramatic increase of ΦT from 0.0 of 0B to 0.39 for 1B. The further addition of Br number increased ΦT to 0.46, 0.50, and 0.66 for 2B, 4B, and 6B, respectively. The triplet lifetimes τT are also fairly long, which is 43, 39, 36, and 26 μs, for 1B, 2B, 4B, and 6B, respectively. The brominated BODIPY dyes are therefore efficient singlet oxygen photosensitizers with the formation quantum yield of 0.39, 0.45, 0.49, and 0.64 for 1B, 2B, 4B, and 6B, respectively. The result indicates their potential application in photodynamic therapy of cancer. The fluorescence properties of the dyes were also measured.