Using Biological Photophysics to Map the Excited‐State Topology of Molecular Photosensitizers for Photodynamic Therapy

This study employs TLD1433, a RuII‐based photodynamic therapy (PDT) agent in human clinical trials, as a benchmark to establish protocols for studying the excited‐state dynamics of photosensitizers (PSs) in cellulo, in the local environment provided by human cancer cells. Very little is known about the excited‐state properties of any PS in live cells, and for TLD1433, it is terra incognita. This contribution targets a general problem in phototherapy, which is how to interrogate the light‐triggered, function‐determining processes of the PSs in the relevant biological environment, and establishes methodological advances to study the ultrafast photoinduced processes for TLD1433 when taken up by MCF7 cells. We generalize the methodological developments and results in terms of molecular physics by applying them to TLD1433’s analogue TLD1633, making this study a benchmark to investigate the excited‐state dynamics of phototoxic compounds in the complex biological environment. A methodology to investigate the excited‐state dynamics of phototoxic compounds in cellulo using transient absorption spectroscopy is established and exemplified by two RuII photosensitizers (PSs) showing relevancy in photodynamic therapy. The kinetic signatures recorded for the PSs in cellulo are distinct from those in water, indicating a complex interplay of either PS‐biomolecular interaction or interaction between PSs or both.