Time-Resolved Spectral Investigation of Bacteriochlorophyllaand Its Transmetalated Derivatives [Zn]-Bacteriochlorophyllaand [Pd]-Bacteriochlorophylla

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.