Matrix influence on the OLED emitter Ir(btp)2(acac) in polymeric host materials – Studies by persistent spectral hole burning

Fundamental photophysical properties of the phosphorescent organometallic complex Ir(btp)2(acac) doped in the polymeric matrices PVK, PFO, and PVB, respectively, are investigated. PVK and PFO are frequently used as host materials in organic light emitting diodes (OLEDs). By application of the laser spectroscopic techniques of phosphorescence line narrowing and persistent spectral hole burning – improved by a synchronous scan technique – we studied the zero-field splitting (ZFS) of the T1 state into the substates I, II, and III. Thus, we were able to probe the effects of the local environment of the emitter molecules in the different amorphous matrices. The magnitude of ZFS is determined by the extent of spin–orbit coupling (SOC) of the T1 state to metal-to-ligand charge transfer (MLCT) states. Only by mixings of MLCT singlets, a short-lived and intense emission of the triplet state to the singlet ground state becomes possible. Thus, sufficiently large ZFS is crucial for favorable luminescence properties of emitter complexes for OLED applications. The analysis of the spectral hole structure resulting from burning provides information about the ZFS values and their statistical (inhomogeneous) distribution in the amorphous matrices. For Ir(btp)2(acac), we found a significant value of ≈18 cm−1 for the splitting between the substates II and III for all three matrices. Interestingly, for PVK the width of the ZFS distribution is found to be ≈14 cm−1 – almost twice as large as for PFO and PVB. Consequently, for a considerable fraction of Ir(btp)2(acac) molecules in PVK, the ZFS is relatively small and thus, the effective SOC is weak. Therefore, it is indicated that a part of the emitter molecules shows a limited OLED performance.