Investigation of Electronic Structures of Triplet States Using Step-Scan Time-Resolved Fourier-Transform Near-Infrared Spectroscopy

Triplet transitions of light-emitting materials, including rose bengal, tris(2-phenylpyridine)iridium(III) [Ir(ppy)₃], tris(1-phenylisoquinoline)iridium(III) [Ir(piq)₃], and bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic), were studied using step-scan time-resolved Fourier-transform near-infrared spectroscopy. The samples were excited to their singlet excited states by a 355 nm laser and then underwent efficient conversions/crossings to their triplet manifolds. For rose bengal, a transient absorption band appeared at 9400 cm^–1, attributed to the T₃ ← T₁ transition based on the corresponding time evolution and the theoretical calculations. For Ir(ppy)₃, Ir(piq)₃, and FIrpic, the most intense bands were observed at 7700, 7500, and 7500 cm^–1 and assigned to T₇ ← T₁, T₆ ← T₁, and T₆ ← T₁ transitions, respectively. For Ir(ppy)₃, the most intense band involved transitions between different triplet metal-to-ligand charge transfer (³MLCT) states, while for Ir(piq)₃ and FIrpic, they involved a metal center to ³MLCT transition. These T₁ states were assigned to ³MLCT.