SUBSTITUENT EFFECTS ON VALENCE IONIZATION POTENTIALS OF FREE BASE PORPHYRINS - LOCAL DENSITY FUNCTIONAL CALCULATIONS AND THEIR RELEVANCE TO ELECTROCHEMICAL AND PHOTOELECTRON SPECTROSCOPIC STUDIES [Review]

摘要

Local density functional (LDF) calculations with polarized double-xi basis sets have been used to calculate the two lowest one-electron ionization potentials (IPs) of a variety of free base porphyrins with multiple alkyl, aryl, fluoro, chloro, bromo, trifluoromethyl, cyano, and nitro substituents. For unsubstituted porphyrin (PH2), beta-octaalkylporphyrins, and meso-tetraphenylporphyrin (TPPH2), LDF-Delta SCF calculations reproduce the lowest ultraviolet photoelectron spectral (UPS) IPs to within similar to 0.2 eV. The two lowest one-electron IPs of TPPH2 are predicted to be lower than those of PH2 by similar to 0.5 eV, in agreement with UPS experiments. Ab initio Hartree-Fock (HF) theory seriously underestimates this electronic effect of meso-tetraphenyl substitution. Both LDF and HF theories predict that substituents on the phenyl rings of TPPH2 should strongly affect the lowest one-electron IPs of TPPH2. Substitution of the para positions of the phenyl groups of TPPH2 with -CF3, -CN, or -NO2 substituents should raise the lowest IP by 0.7-0.8 eV, while perfluorination of the phenyl rings should increase the lowest IP by similar to 0.95 eV. A comparison of the IPs of ortho-, meta-, and para-fluorinated TPPH2 derivatives suggests that electron-withdrawing substituents at meta and para positions should exert significantly stronger electronic effects on the porphyrin ring than ortho substituents. Ab initio HF theory seriously overestimates the electron-withdrawing effect of peripheral halogen substituents. Provided there is no macrocycle buckling due to steric interactions among peripheral substituents, LDF theory predicts that meso-tetrahalogenation (with F, Cl, or Br) should have almost no influence on the lowest IP of PH2, while beta-octachlorination or octabromination should increase the lowest IP by similar to 0.4-0.5 eV. Electrochemical measurements support these LDF predictions. beta-Octafluorination is predicted to exert an electronic effect of similar to 0.85 eV on the lowest IPs of PH2, which is significantly larger than the electronic effect of beta-octachlorination or octabromination. Substituents such as -CF3, -CN, and -NO2 exert far higher electronic effects than the halogens. For instance, LDF theory predicts that the lowest IP of meso-tetrakis(trifluoromethyl)porphyrin should exceed that of TPPH2 by similar to 1.74 eV and that of meso-tetrakis(pentafluorophenyl)porphyrin by similar to 0.68 eV. Unlike the halogens, meso-tetrasubstitution with cyano groups should drastically increase the lowest IP of PH2, by a margin of similar to 1.2 eV. beta-Octacyano-substitution is predicted to have an electronic effect of > 2.0 eV on the lowest IP of PH2. Mesotetranitrosubstitution is predicted to elevate the lowest IP of PH2 by similar to 0.9-1.1 eV, depending on the conformation of the nitro groups. In general, meso-substituents exert stronger electronic effects than the same substituents placed at beta-positions. [References: 164]