Picosecond spectroscopy of some metalloporphyrins

The energy relaxation process of metal-free protoporphyrin IX dimethyl ester and its compounds with Cu(II), Ag(II), Ni(II), Pd(II), Pt(II), Zn(II), Fe(III) and the heme proteins myoglobin, hemoglobin and cytochrome c were studied by dual-beam picosecond spectroscopy. Excitation at 530 nm, which is in the β visible absorption band for these compounds, revealed that both the metal-free porphyrin and the Zn(II) porphyrin have lifetimes for S₁ which are longer than 1 ns, in keeping with their known fluorescence properties. From the time-resolved spectra after excitation, absorption due to S₁, T₁ and S₀ could be identified. In addition, a low-lying singlet d-d level was seen for the Ni(II) porphyrin. While the Pt(II) and Pd(II) compounds have intersystem crossing times of less than 8 ps and 20 ps respectively, the Ni(II) compound relaxes from S₁ to S_d in about 10 ps and from S_d to S₀ in 260 ps. The paramagnetic Cu(II) and Ag(II) species have intersystem crossing from ²S₁ to ²T₁ in less than 8 ps followed by an equilibrium of the ²T₁ and ⁴T₁ states, requiring about 450 ps for the Cu(II) compound and about 12 ps for the Ag(II) compound. An anomalous lengthening of the decay time with increasing excitation intensity was observed for the copper but not for the silver porphyrin. The Fe(III) porphyrins relax with rates faster than 6 ps, which was the resolving time of this experiment. No differences between heme proteins and Fe(III) protoporphyrins in solution were observable within this time resolution.