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

Abstract

Time-resolved and steady-state electron paramagnetic resonance (EPR) spectra have been observed for the lowest excited triplet (T 1) 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 T 1 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 T y 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 T 1 state of phenazinium is the 3A 1(ππ *) state.