A new monophthalocyaninato ytterbium(III) complex 5 with eight 3,5-di-t-butylphenoxyl substituents at peripheral positions is synthesized. X-ray structural analysis of 5·CHCl₃·MeOH reveals that the Yb³⁺ ion is seven-coordinate, surrounded by four nitrogen atoms from the phthalocyaninate dianion and three oxygen atoms from the anionic tripodal L_OMe⁻ ligand {L_OMe⁻ = [(cyclopentadienyl)tris(dimethylphosphito) cobaltate(III)]}. The effects of substituents on the relative singlet oxygen (¹O₂) quantum yields of monophthalocyaninato ytterbium(III) complexes are investigated through comparison. It is found that the monophthalocyaninato ytterbium(III) complex has higher ¹O₂ quantum yield than its corresponding phthalocyaninate ligand. The introduction of a 3,5-di-t-butylphenoxy substituent on the microcycle can enhance the yield of singlet oxygen. Due to the heavy-atoms of both iodine and lanthanide ions, the ytterbium(III) complex 4 based on tert-butyl and iodine substituted phthalocyanine ligands has the highest ¹O₂ quantum yield (0.82).