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 (D 1 ) and quartet (Q 1 ) states for both the para and meta complexes. The TREPR spectra of the Q 1 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 Q 1 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 Q 1 state in zero magnetic field. The TREPR spectra of the ZnTPP−nitpy systems were interpreted by selective ISC to the |± 1 / 2 〉 spin sublevels of the Q 1 state, which originated from SOC due to the zinc ion. The ESP in the D 1 state was interpreted by the difference between the internal conversion rate to the |+ 1 / 2 〉 spin sublevel and that to the |− 1 / 2 〉 spin sublevel.