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 (OEPH(2)) and 5,10,15,20-tetraphenylporphyrin (TPPH(2)). Comparison of spectra collected with He I and He II sources gives an empirical assignment of the relative order of the low-energy (2)A(u) and (2)B(1u) ion states in the gas phase. For OEPH(2), the first ionization at 6.24 eV is assigned to the (2)A(u) ion state, with the (2)B(1u) ionization located 0.25 eV to higher energy. The photoelectron spectrum reported here is quite different from the previously reported spectrum of OEPH(2). The first ionization of TPPH(2) is assigned to the (2)B(1u) ion state, with the (2)A(u) ionization 0.27 eV to higher energy. The reversal of the ordering of the first two ionizations of OEPH(2) and TPPH(2) is due to the different site of the substituents and the different nodal properties of the porphyrin orbitals.