A mechanism for the enhanced room-temperature phosphorescence (RTP) of gadolinium-coordinated hematoporphyrin monomethyl ether (Gd-HMME) in the presence of imidazole and free gadolinium ions (Gd3+) is revealed. Imidazole, the molten solvent used in the synthesis of Gd-HMME, was effective in enhancing the Gd-HMME RTP. In the presence of imidazole, further enhancement of the Gd-HMME RTP was observed upon adding Gd3+. Overall, a 40-fold enhancement of Gd-HMME RTP intensity was achieved by adding both imidazole and Gd3+. In addition, there was an increase in the RTP lifetime. Through spectroscopic analysis, we deduced that a protective medium is formed by the imidazole and the degree of this protection is further increased by Gd3+. The protective medium enhances the Gd-HMME RTP by partially inhibiting energy transfer from the lowest triplet state of Gd-HMME to oxygen. This was demonstrated by the presence of lower levels of singlet oxygen in the Gd-HMME solution after the addition of imidazole. These results indicate that imidazole could have potential application as an RTP enhancer or triplet state protector.
