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

We investigated the relaxation dynamics of bis(2-phenylpyridinato-)(2,2'-bipyridine)iridium(III), [Ir(ppy)(2)bpy](+) 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 ((LLCT)-L-3), metal-to-ligand ppy charge transfer ((MLCTppy)-M-3), and metal-to-ligand bpy charge transfer ((MLCTbpy)-M-3) 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 mu s is assigned to (LLCT)-L-3(A), A symmetry. This decay time coefficient of (LLCT)-L-3(A) decreases with increasing dielectric constant of the solvent indicating this state mixing of some MLCT character. The green state (MLCTppy)-M-3 decays in 0.13-4.8 ns to a nearby intermediate state either (MLCTppy)-M-3 or (MLCTbpy)-M-3. The orange state (MLCTbpy)-M-3 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 (LLCT)-L-3 and (MCLT)-M-3 states are coupled to the singlet (LLCT)-L-1 manifold directly and/or indirectly and contribute to the emission in the visible range.