Analysis of the triplet-state kinetics of a photosensitizer for photoimmunotherapy by fluorescence correlation spectroscopy

[Display omitted] •Intersystem crossing quantum yield (ΦISC) of a silicon phthalocyanine (compound 1) derivatized from IRDye700DX (IR700) which is used as a photosensitizer for photoimmunotherapy (PIT) was calculated to be 0.030 ± 0.010. Since IR700 is almost same structure as compound 1, the ΦISC of IR700 is expected to be similar.•The intersystem crossing of IR700 was a minor relaxation process, although the triplet state of IR700 would be involved in the photoreaction which induces cell damage in PIT.•The cytotoxic effect of PIT can be improved by applying a light-sensitive photosensitizer with a high ΦISC to PIT reagents. Herein, we evaluated the intersystem crossing quantum yield (ΦISC) of a silicon phthalocyanine derivatized from IRDye700DX (IR700) which is used as a photosensitizer for photoimmunotherapy (PIT), using fluorescence correlation spectroscopy (FCS). The calculated ΦISC was 0.019 ± 0.002. The FCS measurement was validated by experiment in the presence of potassium iodide, which can change the kinetics of the relaxation process in the excited state.