Comparison study on the influence of the central metal ions in palladium(II)- and gadolinium(III)-porphyrins for phosphorescence-based oxygen sensing

The luminescence and oxygen sensing properties of a series of palladium(ii)- and gadolinium(iii)-porphyrins were evaluated and compared. Spectral analysis indicates that absorption and luminescence of Gd-porphyrins are red-shifted compared to those of Pd-porphyrins. This demonstrates that the energy levels of excited states in Gd-porphyrins are lower than those in the corresponding Pd-porphyrins. Phosphorescence quantum yield of Pd-hematoporphyrin monomethyl ether (HMME) was about 3-fold higher than that of Gd-HMME due to the decreased non-radiative decay rate of triplet states in Pd-HMME, which was confirmed by the longer phosphorescence lifetime of Pd-HMME. This is attributed to the higher first excited triplet state (T₁) in Pd-HMME as compared to that of Gd-HMME. However, the phosphorescence intensity and lifetime responses of Gd-HMME to oxygen were larger than those of Pd-HMME. To understand this difference, oxygen quenching constants (k_q) of Gd- and Pd-HMME were evaluated and were found to be 4972.9 and 26.4 s^-1, respectively. The huge difference in k_q between the two species is responsible for their disparate oxygen responses. We suggest that the greater k_q of Gd-HMME results from the better energy matching between its T₁ (12&658-14&006 cm^-1) and the second excited state (13&123 cm^-1) of oxygen, which was demonstrated by the higher singlet oxygen quantum yield of Gd-HMME. The same phenomena (spectral red-shifts, lower phosphorescence quantum yields, shorter lifetimes, and larger oxygen responses) were also observed upon substituting Pd(ii) by Gd(iii) in other porphyrins studied in this work.