Twenty-fold Enhancement of Gadolinium-Porphyrin Phosphorescence at Room Temperature by Free Gadolinium Ion in Liquid Phase

The influence of free gadolinium ion (Gd³⁺) on room-temperature phosphorescence (RTP) of gadolinium labeled hematoporphyrin monomethyl ether (Gd-HMME) was studied. Twenty-fold enhancement of Gd-HMME RTP by the titration of free Gd³⁺ was achieved. We found that the absorption from the ground (S₀) to singlet excited states of Gd-HMME did not change with the addition of Gd³⁺, which means that the phosphorescence quantum yield has been tuned from 1.4% to 28%. According to the excitation spectra, the transition possibility from S₀ to the lowest triplet excited state (T₁) is increased by 1.2-fold because of the heavy atom effect of free Gd³⁺. The phosphorescence lifetime of Gd-HMME with free Gd³⁺ is 7.0-fold greater than that of Gd-HMME itself, which demonstrates that the nonradiative processes of Gd-HMME is decreased by Gd³⁺ with the rate constant of nonradiative processes decreasing from 2.2(2) × 10⁵ to 2.8(2) × 10⁴ s^-1. This sharp decrease is mainly responsible for the huge enhancement of Gd-HMME RTP. The reason for the decrease of nonradiative processes is due to the formation of a rigid microenvironment, which protects Gd-HMME from being quenched.