Triplet State Properties of the OLED Emitter Ir(btp)2(acac): Characterization by Site-Selective Spectroscopy and Application of High Magnetic Fields
- 9 May 2007
- journal article
- research article
- Published by American Chemical Society (ACS) in Inorganic Chemistry
- Vol. 46 (12), 5076-5083
- https://doi.org/10.1021/ic0622860
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\ud 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\ud splitting (ZFS). The individual decay times of these substates are τ(I) = 150 μs, τ(II) = 58 μs, and τ(III) = 2 μs,\ud respectively. The long decay time of the lowest substate I indicates its almost pure triplet character. The time for\ud relaxation from state II to state I (spin-lattice relaxation, SLR) is as long as 22 μs at T = 1.5 K, while the\ud thermalization between the two lower lying substates and substate III is fast. Application of a magnetic field induces\ud Zeeman mixing of the substates of T₁, resulting in an increased splitting between the two lower lying substates\ud 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\ud lowest substate I(B) decreases by a factor of about 7 up to 10 T. The magnetic field properties clearly show that the\ud 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\ud of MLCT (metal-to-ligand charge transfer) parentage, it can be concluded that the emitting state T₁ is a ³LC (ligand\ud 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\ud 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\ud cm⁻¹.\uKeywords
This publication has 49 references indexed in Scilit:
- Narrow-Line and Broadband Spectra of Iridium(III) Complexes in a Shpol'skii Matrix and an Amorphous HostThe Journal of Physical Chemistry A, 2006
- Photoemission and X-ray absorption spectroscopies of phosphorescent organic iridium complexesSynthetic Metals, 2005
- White organic light-emitting devices with a solution-processed and molecular host-employed emission layerApplied Physics Letters, 2005
- Electrogenerated chemiluminescence from polymer-boundortho-metallated iridium(III) systemsLuminescence, 2005
- Sprayed Organic Electrophosphorescent Devices with Small Organic MoleculesJapanese Journal of Applied Physics, 2005
- Tetradentate Schiff base platinum(ii) complexes as new class of phosphorescent materials for high-efficiency and white-light electroluminescent devicesElectronic supplementary information (ESI) available: synthesis and spectroscopic, thermal (TGA), photophysical, electrochemical and EL characterization; CIF. See http://www.rsc.org/suppdata/cc/b4/b402318h/Chemical Communications, 2004
- High-efficiency phosphorescent polymer light-emitting devicesOrganic Electronics, 2003
- Phosphorescent light-emitting electrochemical cellApplied Physics Letters, 2002
- Electronic Spectra of the Cyclometalated Complexes M(2-thienylpyridine)2with M = Pd, Pt: A Theoretical StudyThe Journal of Physical Chemistry A, 2000
- Characterization of excited electronic and vibronic states of platinum metal compounds with chelate ligands by highly frequency-resolved and time-resolved spectraPublished by Springer Science and Business Media LLC ,1997