Photophysical study of iridium complexes by absolute photoluminescence quantum yield measurements using an integrating sphere

We have developed an apparatus (Hamamatsu C9920-02) for measuring the absolute photoluminescence quantum yield. This system consists of an excitation light source, a sample holder mounted in an integrating sphere and a multi-channel CCD spectrometer. Using this apparatus, the absolute phosphorescence quantum yields were measured for several iridium complexes doped in organic thin films or dissolved in deaerated solutions. The iridium complexes fac-tris(2-phenylpyridine)iridium(III) (Ir(ppy)₃) and its derivatives, showed high phosphorescence quantum yields (>90 %), while bis(2-(2'-benzo(4,5-a)thienyl)pyridinato-N,C^3')iridium(III) (acetylacetonate) (Btp₂Ir(acac)) gave a lower phosphorescence quantum yield (less than 40 %). To reveal the mechanism of nonradiative decay of the excited iridium complexes, we made time-resolved photoacoustic measurements. It was found that all of the iridium complexes undergo S₁-T₁ intersystem crossing with efficiencies of close to 100 % after photoexcitation. This indicates that the lower phosphorescence quantum yield for Btp ₂Ir(acac) is due to involvement of the T₁-S₀ intersystem crossing process.