Z-scan studies and quantum chemical calculations of meso-tetrakis(p-sulfonatophenyl)porphyrin and meso-tetrakis(4-N-methyl-pyridiniumyl)porphyrin andtheir Fe(III) and Mn(III) complexes

Optical self-defocusing, characterized by the non-linear refractive index n (2), was investigated by the Z-scan technique in water solutions of two por phyrins (PPhs), negatively charged meso-tetrakis (p-sulfonatophenyl)porphyr in (TPPS4) and positively charged meso-tetrakis(4-N-methyl-pyridiniumyl)por phyrin (TMPyP), in their free base forms and as Fe(III) and Mn(III) complex es. Significant n(2) values were observed only for the TMPyP metal complexe s, while for the other porphyrins the n(2) values were negligible. The effe ct is explained by the reorientation of the porphyrin molecule due to inter action of its permanent dipole moment perpendicular to the molecular ring p lane with the electromagnetic field of the exciting light pulse. The perman ent dipole moment is due to the shift of the metal atom out of the molecula r plane. The electrostatic interaction between the metal atom and charged s ubstituents increases (repulsion) or decreases (attraction) the shift of th e metal atom and consequently affects the dipole moment value. Ligand bindi ng to the metal atoms also increases the out-of-plane metal shift and hence the dipole moment and n(2) value. pH changes were shown to modify the Fe(I II)-ligand structure, thus changing nz. The experimental data correlate wel l with the metal shift and dipole moment values calculated for simplified P Ph structures by the ZINDO method. Copyright (C) 2001 John Wiley & Sons, Lt d.