Intrinsic Characterization Method on the Heavy Atom Effect of Metalloporphyrins

The presence of room temperature phosphorescence emission in metalloporphyrin, via the transition from the excited triplet state (T₁) to the ground state (S₀), relies on the chelated heavy metal ions, which is known as the heavy atom effect (HAE). Despite the HAE being a reliable method to tune the phosphorescence process widely, the realization of the HAE nature is a tough task as the induced phosphorescence process is sensitive to not only the specie of bonded heavy atoms but also chemical environments such as the oxygen quenching and solvent effect. In this study, we have aimed at a quantitative determination of the intrinsic phosphorescent transition rate (k_P) in metalloporphyrin gadolinium-labeled hematoporphyrin monomethyl ether (Gd-HMME). After the theoretical analysis based on the rate equation model to remove the nonintrinsic contribution and the experimental results of phosphorescence, the k_P is calculated to be ∼2.4 × 10^-4 μs^-1. This study enables us to approach the intrinsic energy characteristic of metalloporphyrins; moreover, our work provides an effective pathway for the further optimization of the varied functional metalloporphyrin.