Studies on Monomers and Dimers of Y(III) and La(III) Porphyrin Complexes by Time-Resolved Electron Paramagnetic Resonance

EPR parameters such as zero-field splittings (zfs) and anisotropic g values of rare-earth metal centered porphyrin monomers, Y(TPP) + and Y(OEP) +, and double-decker dimers, [Y(TPP) 2 ] -, [Y(OEP) 2 ] -, and [La(TPP) 2 ] -, were measured in the lowest excited triplet (T 1 ) states by a time-resolved electron paramagnetic resonance (TREPR) technique. The zfs parameter D of Y(OEP) + was smaller than that of ZnOEP, and its g z z was smaller than that of Y(TPP) + . From the analyses of these data and the temperature dependence of the steady state EPR spectra, it was found that the decreases in the D and g z z values of Y(OEP) + were due to a spin−orbit coupling between the Jahn−Teller split T 1 and T 2 states. By comparing the D values of the dimers with those of the corresponding monomers, we estimated a charge resonance character, b 2, as 0.34, 0.26, and <0.25 for [Y(OEP) 2 ] -, [Y(TPP) 2 ] -, and [La(TPP) 2 ] -, respectively. The b 2 value was larger for the OEP dimer than those for the TPP dimers. We evaluated the b 2 values using a semiempirical calculation and showed that b 2 was larger for the 3 (a 1u e g ) electronic configuration than that for the 3 (a 2u e g ) configuration.