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 T 3 ← T 1 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 T 7 ← T 1 , T 6 ← T 1 , and T 6 ← T 1 transitions, respectively. For Ir(ppy) 3 , the most intense band involved transitions between different triplet metal-to-ligand charge transfer ( 3 MLCT) states, while for Ir(piq) 3 and FIrpic, they involved a metal center to 3 MLCT transition. These T 1 states were assigned to 3 MLCT.