Dynamics of the Excited States of [Ir(ppy)2bpy]+ with Triple Phosphorescence

Abstract

We investigated the relaxation dynamics of bis(2-phenylpyridinato-)(2,2′-bipyridine)iridium(III), [Ir(ppy)₂bpy]⁺ using the technique of time-resolved spectroscopy. In the visible emission spectra this molecule exhibits triple phosphorescence: displaying blue, green, and orange bands. From the dependence of spectral shifts with polarity of solvent, decay lifetimes, and the results of calculations using time-dependent density functional theory, we assigned these three emitting states to be triplet interligand charge-transfer (³LLCT), metal-to-ligand ppy charge transfer (³MLCT_ppy), and metal-to-ligand bpy charge transfer (³MLCT_bpy) states. The blue states were formed promptly after excitation at wavelength 355 nm; the one lying at higher energy decaying with a time coefficient 0.79−2.56 ns is assigned to be a triplet MLCT, and the other at lower energy decaying in 1.5−2.8 μs is assigned to ³LLCT(A), A symmetry. This decay time coefficient of ³LLCT(A) decreases with increasing dielectric constant of the solvent indicating this state mixing of some MLCT character. The green state ³MLCT_ppy decays in 0.13−4.8 ns to a nearby intermediate state either ³MLCT_ppy or ³MLCT_bpy. The orange state ³MLCT_bpy is coupled to the intermediate state to have a rise time about 0.36−0.84 ns and decays in 425−617 ns. Although many triplet states exist in a small energy range, they couple weakly to display triple emission. All ³LLCT and ³MCLT states are coupled to the singlet ¹LLCT manifold directly and/or indirectly and contribute to the emission in the visible range.