General Simulations of Excited Quartet Spectra with Electron-Spin Polarizations: The Excited Multiplet States of (Tetraphenylporphinato)zinc(II) Coordinated by <i>p</i>- or <i>m</i>-Pyridyl Nitronyl Nitroxides

Abstract

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 |±1/2〉 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 |+1/2〉 spin sublevel and that to the |−1/2〉 spin sublevel.