Magnetic field dependence of ultrafast intersystem-crossing:: A triplet mechanism on the picosecond time scale?

The influence of strong magnetic fields up to 9 T on the decay kinetics of Ni2+-bacteriochlorophyll a was studied by femtosecond pump-probe spectroscopy. The appearance of a magnetic field effect depends on the initial electron spin state of the molecule, which can be switched by using suitable solvents. In toluene, where the central Ni2+ is tetracoordinated, the ground state has singlet multiplicity, while in pyridine, where in addition two solvent molecules bind to the Ni2+ ion, it is a paramagnetic tripler state. In toluene, no field dependence of the excited-slate decay kinetics is observed, whereas in pyridine the decay is accelerated by strong magnetic fields, on a time scale of 10 ps. The field-dependent process has been identified as a loss of axial pyridine ligands which is accompanied by intersystem crossing to a singlet state. The field dependence is discussed in terms of a triplet mechanism, which is applicable if an electron spin relaxation time of greater than or equal to 100 ps and a large zero-field splitting of similar to 20 cm(-1) are assumed.