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The lowest excited singlet (S1) and triplet (T1) states of bacteriochlorophyll a (BChl) and its central metal derivatives [Zn]- and [Pd]- BChl were investigated by femtosecond time-resolved absorption and fluorescence spectroscopy. In contrast to previously reported dynamic solvation effects on the picosecond time scale for BChl, no short kinetic components were observed for BChl and [Zn]-BChl provided that photochemical transformations of the pigments caused by multiple excitation are avoided. The S1 lifetimes of 2.6 ns, 2.1 ns, and 65 ps for BChl, [Zn]-BChl and [Pd]-BChl, respectively, are dominated by intersystem crossing (ISC) to the triplet state T1. The respective triplet quantum yields amount to 76%, 85%, and >99%. The data support that [Pd]-BChl is a highly efficient photosensitizer for photodynamic tumor therapy.

The recombination dynamics of a transition metal redox system monitored by femtosecond pump-probe spectroscopy are shown to be sensitive to high magnetic fields at times shorter than 10 picoseconds. The effect, based on coherent population beats of different spin states, is quantitatively accounted for and allows direct access to spin relaxation rates far beyond the time resolution of the fastest electron paramagnetic resonance technique. The presence of this ultrafast magnetic field effect helps in understanding complex reaction schemes in transition metal chemistry, which occur in a wide range of fields, such as bioinorganic chemistry and catalysis.