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 coordinated metal species. By contrast, photophysical parameters that are directly associated with the intrinsic decay rates of, for example, the singlet excited 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 increase in atomic number leads to notably shorter lifetimes (tau(fluorescence)(Y-Tex) = 1298 ps, tau(fluorescence)(Lu-Tex) = 414 ps), a result that is interpreted as a heavy atom effect. The paramagnetic species, as a general rule, give rise to much shorter fluorescence lifetimes (tau(fluorescence) = 99-515 ps) as compared to their diamagnetic analogues and are seen to fluoresce weakly, with fluorescence quantum yields (Phi(F) = 0.0002-0.0028) that are at least 1 order of magnitude smaller than those found for the corresponding diamagnetic species (Phi(F) = 0.015-0.04). Similar trends were also noted for the intersystem crossing rates and the triplet lifetimes, a finding that is interpreted in terms of an enhanced coupling between the singlet excited and triplet states or triplet excited and singlet ground states, respectively. The magnetic moments of the paramagnetic lanthanide(III) texaphyrin complexes were found to correlate well with the fluorescence lifetimes and the intersystem crossing rates, an observation that, along with other findings, including analyses of diamagnetic texaphyrin complexes, is considered consistent with the presence of covalent interactions between the texaphyrin ligand and the various coordinated metal centers.