Triplet state properties of the OLED emitter Ir(btp)₂(acac) - Characterization by site-selective spectroscopy and application of high magnetic fields

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 studies were carried out under variation of temperature down to 1.2 K and at magnetic fields up to B = 10 T. Highly resolved emission and excitation spectra of several specific sites are obtained by site-selective spectroscopy. For the preferentially investigated site (I ―› 0 at 16268 cm⁻¹), the three substates I, II, and III of the T₁ triplet state are
separated by ΔE(II-I) = 2.9 cm⁻¹ and ΔE(III-I) = 25.0 cm⁻¹, respectively. ΔE(III-I) represents the total zero-field
splitting (ZFS). The individual decay times of these substates are τ(I) = 150 μs, τ(II) = 58 μs, and τ(III) = 2 μs,
respectively. The long decay time of the lowest substate I indicates its almost pure triplet character. The time for
relaxation from state II to state I (spin-lattice relaxation, SLR) is as long as 22 μs at T = 1.5 K, while the
thermalization between the two lower lying substates and substate III is fast. Application of a magnetic field induces
Zeeman mixing of the substates of T₁, resulting in an increased splitting between the two lower lying substates
from 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 perturbed
lowest substate I(B) decreases by a factor of about 7 up to 10 T. The magnetic field properties clearly show that the
three investigated states belong to the same triplet parent term of one single site. Other sites show a similar behavior, though the values of ZFS vary between 15 and 27 cm⁻¹. Since the amount of ZFS reflects the extent
of MLCT (metal-to-ligand charge transfer) parentage, it can be concluded that the emitting state T₁ is a ³LC (ligand
centered) state with significant admixtures of ¹,³MLCT (metal-to-ligand charge transfer) character. Interestingly, the results show that the MLCT perturbation is different for the various sites. An empirical correlation between the
amount of ZFS and the compound’s potential for its use as emitter material in an OLED is presented. As a rule of thumb, a triplet emitter is considered promising for application in OLEDs, if it has a ZFS larger than about 10
cm⁻¹.