Ionization potentials of porphyrins and phthalocyanines. A comparative benchmark study of fast improvements of Koopman's Theorem

Abstract

The vertical ionization potentials (IP's) of a variety of free-base and zinc porphyrins and free-base and zinc phthalocyanine, including all those for which experimental ultraviolet photoelectron spectral (UPS) data are presently known, are computed using six semiempirical molecular orbital methods. Koopman's Theorem (KT), second order outer valence Green's function methods with a large number of active orbitals (OVGF), and explicit computation of the relative energies of neutral species and vertically ionized radical cations (ΔSCF IP) are used in combination with both PM3 and AM1 parameterizations, and the results are compared to experimental data. On average, both the OVGF and ΔSCF IP approximations reproduce the first vertical IP's, as determined by UPS, far more accurately than KT at minimal extra computational costs. Over the full set of available experimental data, the average error for the lowest IP with both OVGF and ΔSCF IP is only ca. 40% of that of KT (AM1 data, AM1 being generally more accurate than PM3). Inclusion of higher order terms in the OVGF treatment (third order truncation or full expression of the self-energy part) does not affect the computed IP's significantly, but inclusion of a large number of active orbitals in the OVGF technique is shown to be essential for this class of molecules. In agreement with the experimental data, zinc porphyrins and zinc phthalocyanines are computed to be better electron donors than their free-base analogues. Conformational differences of the peripheral substituents have no significant effects on the valence IP's.