Antioxidants may affect the outcome of
photodynamic therapy (
PDT) through the inactivation of
reactive oxygen species. Their direct interaction with
photosensitizers excited at the triplet state is also worthy of interest. This process is investigated by
laser flash photolysis of
m-THPC (meso-tetra(3-hydroxyphenyl)chlorin, Foscan) hydroalcoholic solutions added with
Trolox (TrOH), a standard
antioxidant or
Propofol (PfOH,
Diprivan(®)), a common
anesthetic agent also characterized for its
antioxidant properties. Transient UV-visible absorption spectra, kinetics at selected wavelengths and final spectra after extensive
laser irradiation show that both compounds react with the
m-THPC triplet state, (3)m-THPC, to ultimately restore the
photosensitizer in its ground state. For PfOH, this process mainly appears as a single step obeying pseudo-first order kinetics. The bimolecular rate constant for the quenching of (3)m-THPC by PfOH is around 2 × 10(6) M(-1) s(-1), a value increased to some extent by the water content of the
solution. A bimolecular reaction between (3)m-THPC and TrOH is observed with a rate constant of similar magnitude and dependence upon water. However, the reaction leads, at least partly, to intermediate species assigned to the TrO˙ radical and the
m-THPC anion radical. Within a few ms, these species back react to yield
m-THPC in its ground state. A general mechanism involving an intermediate activated complex with some charge transfer character is proposed. Depending on the redox potentials for the oxidation of the
antioxidant, this complex evolves predominantly either toward the formation of radicals (TrOH) or back to the
photosensitizer ground state (PfOH). Notably, the kinetics data suggest that
Propofol may quench (3)m-THPC at concentrations relevant of clinical situation in
PDT involving
anesthesia.