Singlet-oxygen photosensitizers with a tetrad structure and a single BODIPY chromophore: An evidence for transition state stabilization of intersystem crossing

Eleven tetrad Boron dipyrromethene (BODIPY) derivatives were synthesized and examined as singlet-oxygen photosensitizers (PSs). It was clearly shown that a charge shifted state (CSS) with a twisted π-bond makes a considerable contributed to the transition state (TS) stabilization at the minimum energy seam of crossing (MESX) for intersystem crossing (ISC). An EC50 of less than 10 nM was achieved for the G361 human melanoma cell line using one of the investigated PSs, which were only composed of 1st− and 2nd-period elements with compact structures.