BF₂- and PO₂-Chealted Oxasmaragdyrins – Excited state properties, singlet oxygen generation, and imaging potential in mice bearing colon tumor

Free-base oxasmaragdyrin (SmFb), a 22-π electron aromatic macrocycles containing four pyrroles and a furan ring connected via three methine bridges and two direct bonds, and its BF₂- (SmBF₂) and PO₂-chealted (SmPO₂) derivatives, peripherally modified at the meso-aryl rings with polar and nonpolar groups, have been evaluated for their singlet oxygen producing ability and fluorescence imaging potential in mice bearing colon cancer. The oxasmaragdyrin derivatives investigated in this study exhibited absorption into the visible/near-IR region, fluorescence between 700 and 750 nm, and phosphorescence in the range of 875–1000 nm. The measured fluorescence lifetimes and quantum yields were close to those of the tetrapyrrole analogs. Using both femtosecond and nanosecond transient absorption spectral studies, the excited state properties of representative SmBF₂ and SmPO₂ were investigated in toluene. The triplet lifetimes of SmBF₂ and SmPO₂ were found to be 72 and 38 ns, respectively. Both SmBF₂ and SmPO₂ derivatives showed singlet oxygen-producing ability with quantum yields comparable to the free-base tetraphenylporphyrin. An energy-level diagram was prepared to illustrate the generation of singlet oxygen from the Sm derivatives. The tumor-specificity and fluorescence imaging capabilities and biodistribution of these analogs were compared in mice bearing Colon26 tumors using the ‘In Vivo Imaging System’ (IVIS) spectrum. Such studies revealed the significant potential for tumor imaging of BF₂-Sm among the investigated Sm compounds.