| Abstract
| - The well-known red emitting complex Ir(btp)2(acac) (bis(2-(2‘-benzothienyl)-pyridinato-N,C3‘)iridium(acetylacetonate)),frequently used as emitter material in OLEDs, has been investigated in a polycrystalline CH2Cl2 matrix. The studieswere carried out under variation of temperature down to 1.2 K and at magnetic fields up to B = 10 T. Highlyresolved emission and excitation spectra of several specific sites are obtained by site-selective spectroscopy. Forthe preferentially investigated site (I → 0 at 16268 cm-1), the three substates I, II, and III of the T1 triplet state areseparated by ΔEII-I = 2.9 cm-1 and ΔEIII-I = 25.0 cm-1, respectively. ΔEIII-I represents the total zero-fieldsplitting (ZFS). The individual decay times of these substates are τI = 150 μs, τII = 58 μs, and τIII = 2 μs,respectively. The long decay time of the lowest substate I indicates its almost pure triplet character. The time forrelaxation from state II to state I (spin−lattice relaxation, SLR) is as long as 22 μs at T = 1.5 K, while thethermalization between the two lower lying substates and substate III is fast. Application of a magnetic field inducesZeeman mixing of the substates of T1, resulting in an increased splitting between the two lower lying substatesfrom 2.9 cm-1 at zero field to, for example, 6.8 cm-1 at B = 10 T. Further, the decay time of the B-field perturbedlowest substate IB decreases by a factor of about 7 up to 10 T. The magnetic field properties clearly show that thethree investigated states belong to the same triplet parent term of one single site. Other sites show a similarbehavior, though the values of ZFS vary between 15 and 27 cm-1. Since the amount of ZFS reflects the extentof MLCT (metal-to-ligand charge transfer) parentage, it can be concluded that the emitting state T1 is a 3LC (ligandcentered) state with significant admixtures of 1,3MLCT (metal-to-ligand charge transfer) character. Interestingly, theresults show that the MLCT perturbation is different for the various sites. An empirical correlation between theamount of ZFS and the compound's potential for its use as emitter material in an OLED is presented. As a ruleof thumb, a triplet emitter is considered promising for application in OLEDs, if it has a ZFS larger than about 10cm-1.
- The OLED emitter Ir(btp)2(acac) has been investigated in a CH2Cl2 matrix by highly resolving emission and excitation spectroscopy. Variation of temperature and magnetic field have been utilized to characterize the lowest triplet T1. The zero-field splitting amounts to 15−27 cm-1, depending on the specific site. The results show that the emitting state is a ligand centered state with significant MLCT perturbation, which is sensitive to the environment. These properties are discussed with regard to the material's usability in OLEDs.
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