Physical properties of triplet excited states of [Ir(ppy)2bpy]+ in polar solvent and in nonaqueous confined reversed micelle

• Relaxation of triplet excited states of [Ir(ppy)2bpy]+ are studied using time-resolved spectroscopy.
• Multiple phosphorescence bands are from ligand charge transfer and metal-to-ligand charge transfer states.
• In DMF/AOT/hexane reverse micelle, the blue and green bands become intense to yield a broad emission.
• The 3LC and green–blue 3MLCT bands are intense because of encapsulated in confined environment limiting the structural relaxation of the excited states.

The emission properties of the triplet excited states of bis(2-phenylpyridinato-)(2,2′-bipyridine)iridium(III), [Ir(ppy)2bpy](PF6) in various solvents and under constrained environment are investigated using technique of time-resolved spectroscopy. This mixed-chelate/ortho-metalate complex has several triplet ligand center and metal-to-ligand charge transfer states in the near ultraviolet to visible range. After 355 nm excitation this triplet emitter exhibits multiple phosphorescent bands in the visible region. Five emission bands are identified, two blue bands with lifetime 1.2–2.1 ns and 1.4–3.1 μs, respectively assigned to triplet ligand centered charge transfer states 3LC, two blue–green bands 0.2–0.5 ns and ∼6 ns assigned to triplet metal-to-ligand charge transfer 3MLCTbpy (3A) and 3MLCTppy (3B), respectively, and the orange band lying at the lowest energy lifetime 295–620 ns to 3MLCTbpy (3B). These three 3MLCT states interact differently in solvent with varied polarity to yield varied features in emission curves. In N,N-dimethylformamide (DMF)/sodium diethylhexyl sulfosuccinate (AOT)/hexane reverse micelle, cavity 1–3 nm, the 3MLCTbpy (3B) band has shortened lifetime 140–180 ns indicating the polar nature of the environment. In reverse micelle the photoluminescence curve displays broad feature, the blue and green bands with apparent intensity distinct from those in polar solvents. The emission of the orange band 3MLCTbpy (3B) has similar red-shifted feature as that in DMF solution. The other states display large solvatochromic effect in polar cage. Hence, the intense high-energy bands might be because encapsulated in confined environment limits the structural variation of the excited states thus inefficient conversion among them. Relatively less amount of population is cascaded to the lowest energy state 3MLCTbpy (3B).