Influence of large metal cations on the photophysical properties of texaphyrin, a rigid aromatic chromophore

The excited-state properties, including singlet oxygen quantum yields, of a series of metallotexaphyrins (M-Tex), containing coordinated paramagnetic and also diamagnetic lanthanide(III) and other cations, are reported as are the solution-phase magnetic susceptibilities of the paramagnetic species. It is found that the singlet (1.593-1.638 eV) and triplet excited-state (1. 478-1.498 eV) energies are only marginally affected by the choice of coordi nated metal species. By contrast, photophysical parameters that are directl y associated with the intrinsic decay rates of, for example, the singlet ex cited state, such as fluorescence lifetimes, reveal a strong dependence on the nature of the coordinated metal species. In particular, for the series of diamagnetic metals including Y-Tex, In-Tex, Lu-Tex, and Cd-Tex, an incre ase in atomic number leads to notably shorter lifetimes (tau(fluorescence)( Y-Tex) = 1298 ps, tau(fluorescence)(Lu-Tex) = 414 ps), a result that is int erpreted as a heavy atom effect. The paramagnetic species, as a general rul e, give rise to much shorter fluorescence lifetimes (tau(fluorescence) = 99 -515 ps) as compared to their diamagnetic analogues and are seen to fluores ce weakly, with fluorescence quantum yields (Phi(F) = 0.0002-0.0028) that a re at least 1 order of magnitude smaller than those found for the correspon ding diamagnetic species (Phi(F) = 0.015-0.04). Similar trends were also no ted for the intersystem crossing rates and the triplet lifetimes, a finding that is interpreted in terms of an enhanced coupling between the singlet e xcited and triplet states or triplet excited and singlet ground states, res pectively. The magnetic moments of the paramagnetic lanthanide(III) texaphy rin complexes were found to correlate well with the fluorescence lifetimes and the intersystem crossing rates, an observation that, along with other f indings, including analyses of diamagnetic texaphyrin complexes, is conside red consistent with the presence of covalent interactions between the texap hyrin ligand and the various coordinated metal centers.