Correlation between the Triplet-State Energy of Metalloporphyrins and Oxygen Response of Their Phosphorescence

Metalloporphyrins are suitable for medical oxygen sensing without disturbance to local tissue environment. To understand the correlation between the first triplet-state (T1) energy of metalloporphyrins and their phosphorescence responses to oxygen, the luminescence and oxygen sensing properties of two porphyrins (hematoporphyrin monomethyl ether, HMME; tetraphenylporphyrin, TPP) coordinated to gadolinium(III) (Gd) were studied comparatively. The absorption and luminescence of Gd-TPP are red-shifted relative to that of Gd-HMME, indicating that Gd-TPP has a lower T1 energy. The phosphorescence quantum yield (ΦP) of Gd-TPP is ∼10% of that of Gd-HMME. The large difference in ΦP is due to the increased nonradiative decay rate of T1 in Gd-TPP, as demonstrated by a shorter phosphorescence lifetime. The oxygen response (KSV) is related to the unquenched phosphorescence lifetimes (τ0) and oxygen quenching constants (kq) according to the equation KSV = τ0kq. KSV of Gd-HMME phosphorescence is larger than that of Gd-TPP. The larger kq of Gd-HMME stems from a better energy overlap between its T1 and the second excited state of oxygen, as demonstrated by its higher singlet oxygen quantum yield. Thus, the difference between the kq values of metalloporphyrins is another factor contributing to their diverse oxygen responses.