General simulations of excited quartet spectra with electron-spin polarizations: The excited multiplet states of (tetraphenylporphinato)zinc(II) coordinated by p- or m-pyridyl nitronyl nitroxides

The excited multiplet states of (tetraphenylporphinato)zinc(II) (ZnTPP) coordinated by p-pyridyl nitronyl nitroxide (p-nitpy), ZnTPP-p-nitpy, and of ZnTPP coordinated by m-nitpy, ZnTPP-m-nitpy, were: studied by time-resolved electron paramagnetic resonance (TREPR). The TREPR spectra observed at 20 K and 0.5 μs after laser excitation were assigned to the lowest excited doublet (D₁) and quartet (Q₁) states for both the para and meta complexes. The TREPR spectra of the Q₁ state with electron-spin polarization (ESP) were well simulated for the first time. From the spectral simulation, it was established in general that the ESP in the Q₁ state was interpreted by selective intersystem crossing (ISC), which was generated by spin-orbit coupling (SOC) between the excited doublet states and the eigenfunctions of the Q₁ state in zero magnetic field. The TREPR spectra of the ZnTPP-nitpy systems were interpreted by selective ISC to the |± 1/4 > spin sublevels of the Q₁ state, which originated from SOC due to the zinc ion. The ESP in the D₁ state was interpreted by the difference between the internal conversion rate to the |+ 1/4 > spin sublevel and that to the |- 1/4 > spin sublevel.