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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 ultrafast photodynamics of bacteriochlorophyll with the central Mg2+-ion replaced by Ni2+ ([Ni]-BChl) in toluene and pyridine have been studied by femtosecond time-resolved fluorescence and absorption spectroscopy with a time resolution of 100 fs in the spectral range of 470−900 nm. Excitation of tetracoordinated [Ni]-BChl in toluene in its red-most absorption band leads to four transient states which decay single-exponentially with lifetimes of 100 fs, 450 fs, 4 ps, 25 ps. Except for the 4 ps component these kinetics arise from an internal conversion cascade conserving the overall multiplicity of the singlet configuration. The spectral characteristics of the intermediates indicate a deactivation process via several π- and metal-centered excited states. The 4 ps kinetic results from a relaxation within the metal d-states caused by a change of size of the central nickel ion upon electronic excitation. A different deactivation pattern is observed for [Ni]-BChl in pyridine, where the central metal is additionally coordinated by two axial solvent molecules. On the basis of their dynamics and their spectral positions, three different intermediates have been identified. The fastest decay with 150 fs reflects an internal conversion process. This is followed by a process that can be approximately described by time constants of 6 and 90 ps. This decay pattern, concomitant with the spectral changes, is attributed to the ejection of the axial solvent ligands accompanied by intersystem crossing. The slowest component has a characteristic time of >2 ns and is attributed to the rebinding of the pyridine ligands.