Annulation of the additional aromatic pi-system to the ligand is an effective method of influencing the luminescence of metal complexes. In this contribution, we show the effect of benzannulation of a ditopic ligand on the photophysical properties of a dinuclear Ir(III) complex and also utilize it to prepare its mono-nuclear analogue. The new dinuclear complex di-Ir in which the iridium centres are linked by benzo[1,2-d : 4,5-d′]bisthiazole shows drastically improved efficiency of phosphorescence compared to the previously reported complex Ir-2 that has thiazolo[5,4-d]thiazole as the linking unit. The new mono-nuclear complex mono-Ir, utilizing the same ditopic ligand as di-Ir, allows tracking the effects of dinuclearity. In degassed dilute dichloromethane solution, the mononuclear complex mono-Ir displays yellow phosphorescence (λem = 552 nm) with a quantum yield of ΦPL = 70% and decay time of τ = 7.85 μs, which correspond to the radiative rate of kr = 0.89 × 105 s−1. The dinuclear complex di-Ir displays slightly red-shifted phosphorescence (λem = 560 nm) with a quantum yield of ΦPL = 85% and decay time of τ = 4.50 μs corresponding to the radiative rate of kr = 1.90 × 105 s−1, which is about two times higher compared to that of the mono-nuclear mono-Ir. The phosphorescence intensity of both complexes is highly sensitive to molecular oxygen, as in the air-equilibrated samples the decay times drop to values τ = 0.73 μs and τ = 0.84 μs for mono-Ir and di-Ir, respectively. Applied as singlet oxygen sensitizers, mono-Ir and di-Ir show very high efficiencies of 78% and 71%, respectively. Such characteristics mean the two complexes are promising materials for optical oxygen sensing and applications associated with singlet oxygen generation.