Matrix Effects on the Triplet State of the OLED Emitter Ir(4,6-dFppy)2(pic) (FIrpic): Investigations by High-Resolution Optical Spectroscopy

The sky-blue emitting compound Ir(4,6-dFppy)(2)(pic) (iridium(III)bis[2-(4',6'-difluorophenyl)pyridinato-N,C-2']-picolinate), commonly referred to as FIrpic and representing a well-known emitter material for organic light emitting diodes (OLEDs), has been investigated in detail by optical spectroscopy. Studies at temperatures from T = 1.5 K to T = 300 K were carried out in CH2Cl2 and tetrahydrofuran (THF). In CH2Cl2, two discrete sites were observed at cryogenic temperatures and studied by site-selective, high-resolution spectroscopy. The investigations reveal that the molecules located at the two sites exhibit distinctly different photophysical properties. For example, the three substates I, II, and III of the emitting triplet state T, of the low-energy site A show a distinctly larger zero-field splitting (ZFS) and exhibit shorter individual decay times than observed for the high-energy site B. The vibrational satellite structures in the emission spectra of the substates I(A) and I(B) exhibit clear differences in the ranges of metal-ligand (M-L) vibrations. For the compound studied in a polycrystalline THF host, giving only strongly inhomogeneously broadened spectra, the ZFS parameters and substate decay times vary in a similar range as observed for the two discrete sites in the CH2Cl2 matrix. Thus, the amount of ZFS, the emission decay times, and also the intensities of the M-L vibrational satellites are affected by the matrix cage, that is, the host environment of the emitting complex. These properties are discussed with respect to variations of spin-orbit coupling routes. In particular, changes of d-orbital admixtures, that is, differences of the metal-to-ligand charge transfer (MLCT) character in the emitting triplet, play an important role. The matrix effects are expected to be also of importance for FIrpic and other Ir(III) compounds when applied as emitters in amorphous OLED matrixes.