Time-resolved electron paramagnetic resonance of the lowest excited triplet state of phenazinium cation

Time-resolved and steady-state electron paramagnetic resonance (EPR) spectra have been observed for the lowest excited triplet (T1) states of phenazine (Phz) and its singly protonated cation (phenazinium) in sulfuric acid−ethanol mixtures at 77 K. The single protonation appears to have little effect on the anisotropic sublevel populating rates of the T1 state of phenazine. However, the zero-field splitting (ZFS) parameter D decreases on the protonation, reflecting the increase of delocalization of the two unpaired electrons. The sublevel preferentially populated by intersystem crossing (ISC) is Ty in both phenazine and phenazinium (the y-axis is parallel to the in-plane long axis). From the analysis of the observed anisotropy in the ISC rates and the semi empirical molecular orbital calculations of the ZFS parameters, we concluded that the T1 state of phenazinium is the 3A1(ππ*) state.