Influence of Large Metal Cations on the Photophysical Properties of Texaphyrin, a Rigid Aromatic Chromophore

Abstract

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 (τ fluorescence (Y-Tex) ) 1298 ps, τ 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 (τ fluorescence ) 99-515 ps) as compared to their diamagnetic analogues and are seen to fluoresce weakly, with fluorescence quantum yields (Φ F ) 0.0002-0.0028) that are at least 1 order of magnitude smaller than those found for the corresponding diamagnetic species (Φ 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.

Visible and Near-Infrared Luminescence of Lanthanide-Containing Dimetallic Triple-Stranded Helicates: Energy Transfer Mechanisms in the SmIII and YbIII Molecular Edifices

“…We have performed a variable analysis to investigate the dependence of the quantum yield upon the rate constants k 21 , k 31 , and k 32 ( 46 This means that second sphere interactions of water molecules with the carboxylic groups not only depopulate the emitter level of the Sm III ion, but also the triplet state level, consistent with the quantum yields for the ligand-centered luminescence discussed above. The large k 32 value is typical of a heavy-atom effect.…”

Photosensitized singlet oxygen and its applications

“…Also, the atomic periphery consists of a 22p-electron system, instead of the 18p-electron system of the porphyrins, causing a red-shift in absorption. A number of complexes of texaphyrins containing paramagnetic and diamagnetic metal ions have recently been investigated [34]. Once again, the paramagnetic complexes of this macrocycle have shorter fluorescence lifetimes than their diamagnetic counterparts and this difference is reflected in their singlet oxygen quantum yields (Table 4).…”

Metallation of pentaphyrin with Lu(III) dramatically increases reactive-oxygen species production and cell phototoxicity

“…In contrast, the two metal complexes 5 and 6 displayed, after irradiation, a stronger cytotoxic effect proportional to the atomic weight of the metal ion (Lu(III)-PCRed 6 > Zn(II)ePCRed 5) [10,13]. These experiments were carried by treating the cells for 24 h with increasing amounts of photosensitizer, followed by irradiation and 24 h-incubation in the dark before a resazurin proliferation assay was performed.…”

“…Energy transfer to triplet oxygen to yield highly reactive singlet oxygen occurs when the triplet state of the photosensitizer is significantly populated. The efficiency of singlet oxygen production [13] is directly related to the quantum yield of triplet state of the photosensitizer and its lifetime. In this mechanism ISC is a key parameter.…”

“…For the series of diamagnetic metals including Y, In, Lu and Cd, the production of singlet oxygen was found proportional to the increase in atomic number: a result that is interpreted as a heavy atom effect [14]. Instead, paramagnetic species deactivate the excited state, reduce the excited-state lifetime and prevent photochemical reactions from taking place [13]. In fact, it has been demonstrated that photophysical parameters that are directly associated with the intrinsic decay rates of the singlet excited state or fluorescence lifetimes, are enhanced by coordination of paramagnetic metal ions.…”