Reevaluation of the gas-phase valence photoelectron spectra of octaethylporphyrin and tetraphenylporphyrin

Gas-phase valence photoelectron spectra are reported for 2,3,7,8,12,13,17,18-octaethylporphyrin (OEPH₂) and 5,10,15,20-tetraphenylporphyrin (TPPH₂). Comparison of spectra collected with He I and He II sources gives an empirical assignment of the relative order of the low-energy ²A_u and ²B_1u ion states in the gas phase. For OEPH₂, the first ionization at 6.24 eV is assigned to the ²A_u ion state, with the ²B_1u ionization located 0.25 eV to higher energy. The photoelectron spectrum reported here is quite different from the previously reported spectrum of OEPH₂. The first ionization of TPPH₂ is assigned to the ²B_1u ion state, with the 2A_u ionization 0.27 eV to higher energy. The reversal of the ordering of the first two ionizations of OEPH₂ and TPPH₂ is due to the different site of the substituents and the different nodal properties of the porphyrin Orbitals.