Abstract A series of 1-methylpyridinium-substituted brominated distyryl-BODIPY dyes, PyB X I ( X = H, M, or Br), was synthesized to achieve cooperative singlet oxygen ( 1 O 2 ) production through qualitatively different dual intersystem crossing (ISC) pathways: spin–orbit charge-transfer ISC (SOCT-ISC) and heavy-atom-induced ISC. Upon photoexcitation of the PyB X I dyes, charge-transfer states were preferentially formed through photoinduced electron transfer from the distyryl-BODIPY core to the 1-methylpyridinium moiety, however, followed by nonradiative charge recombination rather than the desired SOCT-ISC. This resulted in negligible fluorescence and 1 O 2 quantum yields in the non-brominated dye PyBHI. The introduction of bromine atoms improved 1 O 2 quantum yield from 0.0034 for the mono-brominated dye PyBMI to 0.0061 for the di-brominated dye PyBBrI, attributable to the heavy atom effect. Nonetheless, the 1 O 2 production efficiency of these dyes remained limited, as photoinduced electron transfer was considered to occur nearly two orders of magnitude faster than singlet-to-triplet ISC. In vitro assays using MCF-7 and HeLa cells demonstrated that PyBBrI induced significant cell death, with IC 50 values of ca. 95 and 220 nM, respectively, confirming its potential for use in cancer therapy.
Kim et al. (Sat,) studied this question.