Time-Resolved EPR Study on the Photoexcited Triplet State of the Electron–Donor–Acceptor Complex Formed in the System of <i>fac</i>-Tris[2-(4-octyl-phenyl) pyridine] Iridium(III) and Tetracene

The physical properties of the phosphorescent organic light-emitting diode material <i>fac</i>-tris­(phenylpyridine) iridium­(III), Ir­(ppy)₃, have been reported with experimental and theoretical studies. Here, the photochemical properties of the excited triplet state of partially modified <i>fac</i>-tris­[2-(4-octyl-phenyl) pyridine] iridium­(III), Ir­(C₈ppy)₃, were investigated using time-resolved electron paramagnetic resonance (tr-EPR) and optical methods by adding tetracene in the toluene solution. The tr-EPR observation at 77 K revealed the following two species: the excited triplet state of tetracene and another triplet species with zero field splitting parameters of |<i>D</i>| = 0.088 cm^–1 and |<i>E</i>| = 0.018 cm^–1 with characteristic spin polarization. The latter species was assigned to the electron–donor–acceptor (EDA) complex formed between Ir­(C₈ppy)₃ and tetracene. The mechanism of formation and the properties of this EDA complex, including the information on the principal axes of ³Ir­(C₈ppy)₃*, are discussed