Hartmut Yersin

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Organization: Universit?t Regensburg , Germany

Department: Institut für Physikalische und Theoretische Chemie

Title: (PhD)

TOPICS

Physical Chemistry

Material Chemistry

OLED

Organic Chemistry

OLED

Chemicals
References

Synthesis, Structure, and Characterization of Dinuclear Copper(I) Halide Complexes with P^N Ligands Featuring Exciting Photoluminescence Properties

Co-reporter:Daniel M. Zink, Michael Bächle, Thomas Baumann, Martin Nieger, Michael Kühn, Cong Wang, Wim Klopper, Uwe Monkowius, Thomas Hofbeck, Hartmut Yersin, and Stefan Bräse

Inorganic Chemistry 2013 Volume 52(Issue 5) pp:2292-2305

Publication Date(Web):October 12, 2012

DOI:10.1021/ic300979c

A series of highly luminescent dinuclear copper(I) complexes has been synthesized in good yields using a modular ligand system of easily accessible diphenylphosphinopyridine-type P^N ligands. Characterization of these complexes via X-ray crystallographic studies and elemental analysis revealed a dinuclear complex structure with a butterfly-shaped metal-halide core. The complexes feature emission covering the visible spectrum from blue to red together with high quantum yields up to 96%. Density functional theory calculations show that the HOMO consists mainly of orbitals of both the metal core and the bridging halides, while the LUMO resides dominantly on the heterocyclic part of the P^N ligands. Therefore, modification of the heterocyclic moiety of the bridging ligand allows for systematic tuning of the luminescence wavelength. By increasing the aromatic system of the N-heterocycle or through functionalization of the pyridyl moiety, complexes with emission maxima from 481 to 713 nm are obtained. For a representative compound, it is shown that the ambient-temperature emission can be assigned as a thermally activated delayed fluorescence, featuring an attractively short emission decay time of only 6.5 μs at ϕPL = 0.8. It is proposed to apply these compounds for singlet harvesting in OLEDs.

Improving the Performance of Pt(II) Complexes for Blue Light Emission by Enhancing the Molecular Rigidity

Co-reporter:Andreas F. Rausch, Lisa Murphy, J. A. Gareth Williams, and Hartmut Yersin

Inorganic Chemistry 2012 Volume 51(Issue 1) pp:312-319

Publication Date(Web):November 30, 2011

DOI:10.1021/ic201664v

This study highlights the potential benefits of using terdentate over bidentate ligands in the construction of organometallic complexes as organic light-emitting diode (OLED) emitters offering better color purity, and explores in detail the molecular origins of the differences between the two. A pair of closely related platinum(II) complexes has been selected, incorporating a bidentate and a terdentate cyclometallating ligand, respectively, namely, Pt(4,6-dFppy)(acac) (1) {4,6-dFppy = 2-(4,6-difluorophenyl)pyridine metalated at C2 of the phenyl ring} and Pt(4,6-dFdpyb)Cl (2) {4,6-dFdpyb = 4,6-difluoro-1,3-di(2-pyridyl)benzene, metalated at C2 of the phenyl ring}. The emission properties over the range of temperatures from 1.2 to 300 K have been investigated, including optical high-resolution studies. The results reveal a detailed insight into the electronic and vibronic structures of the two compounds. In particular, the Huang–Rhys parameter S that serves to quantify the degree of molecular distortion in the excited state with respect to the ground state, though small in both cases, is smaller by a factor of 2 for the terdentate than the bidentate complex (S ≈ 0.1 and ≈0.2, respectively). The smaller value for the former reflects the greater degree of rigidity induced by the terdentate ligand, leading to a lesser contribution of intraligand Franck–Condon vibrational modes in the green spectral range of the emission spectra. Consequently, an enhanced color purity with respect to blue light emission results. The high rigidity and the short Pt–C bond in Pt(4,6-dFdpyb)Cl also serve to disfavor nonradiative decay pathways, including those involving higher-lying dd* states. These effects account for the greatly superior luminescence quantum yield of the terdentate complex in fluid solution, amounting to ϕPL = 80% versus only 2% found for the bidentate complex.

The triplet state of organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient OLEDs

Co-reporter:Hartmut Yersin, Andreas F. Rausch, Rafał Czerwieniec, Thomas Hofbeck, Tobias Fischer

Coordination Chemistry Reviews 2011 Volume 255(21–22) pp:2622-2652

Publication Date(Web):November 2011

DOI:10.1016/j.ccr.2011.01.042

Based on a very comprehensive set of experimental data and on theoretical models, an understanding of the triplet state properties of organo-transition metal compounds is worked out. Important trends and guidelines for controlling photophysical properties are revealed. In this respect, we focus on spin–orbit coupling (SOC) and its importance for radiative as well as for nonradiative transitions between the lowest triplet state and the electronic ground state. Moreover, as is discussed on the basis of an extensive data set, summarized for the first time, the efficiency of SOC also depends on the geometry of a complex. The investigations are exemplified and supported by instructive case studies, such as efficient blue and very efficient green and red emitters. Additionally, trends being important for applications of these compounds as emitters in OLEDs are worked out. In particular, the properties of the emitters are discussed with respect to the harvesting of singlet and triplet excitons that are generated in the course of the electroluminescence process. The well-known triplet harvesting effect is compared to the recently discovered singlet harvesting effect. This latter mechanism is illustrated by use of a blue light emitting Cu(I) complex, which represents an efficient fluorescent emitter at ambient temperature. By this mechanism, 100% of the generated singlet and triplet excitons can, at least in principle, be harvested by the emitting singlet state. Potentially, this new mechanism can successfully be applied in future OLED lighting with a distinctly reduced roll-off of the efficiency.A large number of data sets allows us to develop a detailed understanding of the triplet state properties of organo-transition metal compounds. Especially, the singlet harvesting effect, a new principle for the improvement of OLED performance, is presented here for the first time.

Blue-Light Emission of Cu(I) Complexes and Singlet Harvesting

Co-reporter:Rafał Czerwieniec, Jiangbo Yu, and Hartmut Yersin

Inorganic Chemistry 2011 Volume 50(Issue 17) pp:8293-8301

Publication Date(Web):August 3, 2011

DOI:10.1021/ic200811a

Strongly luminescent neutral copper(I) complexes of the type Cu(pop)(NN), with pop = bis(2-(diphenylphosphanyl)phenyl)ether and NN = bis(pyrazol-1-yl)borohydrate (pz2BH2), tetrakis(pyrazol-1-yl)borate (pz4B), or bis(pyrazol-1-yl)-biphenyl-borate (pz2Bph2), are readily accessible in reactions of Cu(acetonitrile)4+ with equimolar amounts of the pop and NN ligands at ambient temperature. All products were characterized by means of single crystal X-ray diffractometry. The compounds exhibit very strong blue/white luminescence with emission quantum yields of up to 90%. Investigations of spectroscopic properties and the emission decay behavior in the temperature range between 1.6 K and ambient temperature allow us to assign the emitting electronic states. Below 100 K, the emission decay times are in the order of many hundreds of microseconds. Therefore, it is concluded that the emission stems from the lowest triplet state. This state is assigned to a metal-to-ligand charge-transfer state (3MLCT) involving Cu-3d and pop-π* orbitals. With temperature increase, the emission decay time is drastically reduced to e.g. to 13 μs (Cu(pop)(pz2Bph2)) at ambient temperature. At this temperature, the complexes exhibit high emission quantum yields, as neat material or doped into poly(methyl methacrylate) (PMMA). This behavior is assigned to an efficient thermal population of a singlet state (being classified as 1MLCT), which lies only 800 to 1300 cm–1 above the triplet state, depending on the individual complex. Thus, the resulting emission at ambient temperature largely represents a fluorescence. For applications in OLEDs and LEECs, for example, this type of thermally activated delayed fluorescence (TADF) creates a new mechanism that allows to harvest both singlet and triplet excitons (excitations) in the lowest singlet state. This effect of singlet harvesting leads to drastically higher radiative rates than obtainable for emissions from triplet states of Cu(I) complexes.

Palladium(II)- and platinum(II)phenyl-2,6-bis(oxazole) pincer complexes: Syntheses, crystal structures, and photophysical properties

Co-reporter:Gang Xu, Qunli Luo, Stefan Eibauer, Andreas F. Rausch, Sabine Stempfhuber, Manfred Zabel, Hartmut Yersin and Oliver Reiser

Dalton Transactions 2011 vol. 40(Issue 35) pp:8800-8806

Publication Date(Web):15 Jun 2011

DOI:10.1039/C1DT10369E

Phenyl-2,6-bis(oxazole) ligands have been explored for the synthesis of novel palladium(II) and platinum(II) pincer complexes. The materials were characterized by spectroscopic methods and by X-ray crystallography. Investigations of the photophysical properties revealed that the lowest triplet states of the materials are largely centred at the bis(oxazole) ligands. The platinum(II) compounds are moderately emissive in fluid solution at ambient temperature. Introduction of both strong donors and strong acceptors leads to a significant red shift of the emission. Due to the facile synthesis of bis(oxazole) based complexes with electronically tuneable oxazole moieties, these materials might be promising alternatives to the well-established phenyl-2,6-bipyridyl systems.

The Lowest Excited State of Brightly Emitting Gold(I) Triphosphine Complexes

Co-reporter:Rafał Czerwieniec ; Thomas Hofbeck ; Olga Crespo ; Antonio Laguna ; M. Concepción Gimeno

Inorganic Chemistry 2010 Volume 49(Issue 8) pp:3764-3767

Publication Date(Web):March 16, 2010

DOI:10.1021/ic902325n

The strongly luminescent neutral gold(I) triphosphine complexes [Au(dipnc)(PPh3)] and [Au(dppnc)(PPh3)] with dipnc = 7,8-bis(diisopropylphosphino)-nido-carborane ([(PiPr2)2B9H10C2)]−) and dppnc = 7,8-bis(diphenylphosphino)-nido-carborane ([(PPh2)2B9H10C2]−) are studied in a wide range of temperatures of 1.5 ≤ T ≤ 300 K. Analysis of the emission decay dynamics provides detailed information about the lowest triplet state. In particular, the magnitude of zero-field splitting of the emitting state is determined to be 47 and 29 cm−1 for [Au(dipnc)(PPh3)] and [Au(dppnc)(PPh3)], respectively. The emission decay times of the individual triplet substates lie in the range of 4 to 130 μs. The observed photophysical properties suggest that the molecular orbitals involved in the lowest electronic transitions exhibit, beside gold orbitals, considerable contributions from nonmetallic ligand orbitals. OLED or sensor application of these complexes is suggested.

Bright Sky-Blue Phosphorescence of [n-Bu4N][Pt(4,6-dFppy)(CN)2]: Synthesis, Crystal Structure, and Detailed Photophysical Studies

Co-reporter:Andreas F. Rausch, Uwe V. Monkowius, Manfred Zabel and Hartmut Yersin

Inorganic Chemistry 2010 Volume 49(Issue 17) pp:7818-7825

Publication Date(Web):July 30, 2010

DOI:10.1021/ic100851b

This work describes the synthesis, crystal structure, and detailed photophysical studies of [n-Bu4N][Pt(4,6-dFppy)(CN)2] (n-Bu = n-butyl, 4,6-dFppy = (4′,6′-difluorophenyl)pyridinate). The material can easily be prepared in high yield and purity by the reaction of [Pt(4,6-dFppy)(H-4,6-dFppy)Cl], [n-Bu4N]Cl, and KCN in CH2Cl2. Because of the bulky counterion [n-Bu4N]+, Pt−Pt interactions, which frequently lead to aggregate formation, are suppressed in the solid state. Thus, monomer emission is observed. The phosphorescence quantum yield of the neat powder amounts to ϕPL = 60% at ambient temperature and decays with 19 μs. In tetrahydrofuran (THF) solution, on the other hand, the emission decay time is with 0.26 μs distinctly shorter, and the quantum yield is very low. By means of emission decay time studies in frozen THF and investigations of the highly resolved single crystal emission at 1.2 K, we can assign the emitting T1 state of the compound as being largely of ligand centered (3LC, 3ππ*) character. The observed differences of the emission properties of the neat powder compared to the fluid solution are rationalized with an energy stabilization of quenching dd* states in solution because of molecular distortions and/or bond elongations.

The Triplet State of fac-Ir(ppy)3

Co-reporter:Thomas Hofbeck

Inorganic Chemistry 2010 Volume 49(Issue 20) pp:9290-9299

Publication Date(Web):September 20, 2010

DOI:10.1021/ic100872w

The emitting triplet state of fac-Ir(ppy)3 (fac-tris(2-phenylpyridine)iridium) is studied for the first time on the basis of highly resolved optical spectra in the range of the electronic 0−0 transitions. For the compound dissolved in CH2Cl2 and cooled to cryogenic temperatures, three 0−0 transitions corresponding to the triplet substates I, II, and III are identified. They lie at 19 693 cm−1 (507.79 nm, I → 0), 19 712 cm−1 (507.31 nm, II → 0), and 19 863 cm−1 (503.45 nm, III → 0). From the large total zero-field splitting (ZFS) of 170 cm−1, the assignment of the emitting triplet term as a 3MLCT state (metal-to-ligand charge transfer state) is substantiated, and it is seen that spin−orbit couplings to higher lying 1,3MLCT states are very effective. Moreover, the studies provide emission decay times for the three individual substates of τI = 116 μs, τII = 6.4 μs, and τIII = 200 ns. Further, group-theoretical considerations and investigations under application of high magnetic fields up to B = 12 T allow us to conclude that all three substates are nondegenerate and that the symmetry of the complex in the CH2Cl2 matrix cage is lower than C3. It follows that the triplet parent term is of 3A character. Studies of the emission decay time and photoluminescence quantum yield, ΦPL, of Ir(ppy)3 in poly(methylmethacrylate) (PMMA) in the temperature range of 1.5 ≤ T ≤ 370 K reveal average and individual radiative and nonradiative decay rates and quantum yields of the substates. In the range 80 ≤ T ≤ 370 K, ΦPL is as high as almost 100%. The quantum yield ΦPL drops to ∼88% when cooled to T = 1.5 K. The investigations show further that the emission properties of Ir(ppy)3 depend distinctly on the complex’s environment or the matrix cage according to distinct changes of spin−orbit coupling effectiveness. These issues also have consequences for optimizations of the material’s properties if applied as an organic light-emitting diode (OLED) emitter.

Triplet state properties of a red light emitting [Pt(s-thpy)(acac)] compound

Co-reporter:Tobias Fischer, Rafał Czerwieniec, Thomas Hofbeck, Maria M. Osminina, Hartmut Yersin

Chemical Physics Letters 2010 Volume 486(1–3) pp:53-59

Publication Date(Web):5 February 2010

DOI:10.1016/j.cplett.2009.12.042

Abstract

A photophysical characterization based on optical high-resolution spectra and emission decay properties at low temperatures and at high magnetic fields is carried out for [Pt(s-thpy)(acac)] (s-thpy = 5,2-bis(2-thienyl)pyridinate and acac = acetylacetonate). The electronic 0–0 transition between the lowest triplet state and the ground state lies at 16 150 cm⁻¹ (619 nm). The zero-field splitting (ZFS) of T₁ is smaller than 1 cm⁻¹. Thus, the emitting excited state is mainly of ligand-centered ³LC (³ππ∗) character and experiences only weak spin–orbit couplings to higher lying singlet states. The compound does not fulfill important requirements for OLED applications, but strategies for improvements are pointed out.

Magnetic field effects on the phosphorescence of Pt(4,6-dFppy)(acac) – Tunability of the vibrational satellite structure

Co-reporter:Andreas F. Rausch, Hartmut Yersin

Chemical Physics Letters 2010 Volume 484(4–6) pp:261-265

Publication Date(Web):7 January 2010

DOI:10.1016/j.cplett.2009.11.049

Abstract

Effects of high magnetic fields on the properties of the lowest triplet state of the OLED emitter Pt(4,6-dFppy)(acac) are studied by site-selective high-resolution optical spectroscopy. Application of high fields strongly alters the pattern and magnitude of splitting of the triplet term, the oscillator strengths of the purely electronic 0–0 transitions between the T₁ substates and the singlet ground state, and the vibrational satellite structures. In particular, the mechanism of radiative vibrational deactivation of the lowest triplet substate I changes with increasing field from a vibronically induced (Herzberg–Teller) to a Franck–Condon induced mechanism.

Photophysical Properties and OLED Applications of Phosphorescent Platinum(II) Schiff Base Complexes

Co-reporter:Chi-Ming Che Dr.;Chi-Chung Kwok Dr.;Siu-Wai Lai Dr.;AndreasF. Rausch Dipl.-Chem.;WalterJ. Finkenzeller Dr.;Nianyong Zhu Dr. Dr.

Chemistry - A European Journal 2010 Volume 16( Issue 1) pp:233-247

Publication Date(Web):

DOI:10.1002/chem.200902183

Abstract

The syntheses, crystal structures, and detailed investigations of the photophysical properties of phosphorescent platinum(II) Schiff base complexes are presented. All of these complexes exhibit intense absorption bands with λ_max in the range 417–546 nm, which are assigned to states of metal-to-ligand charge-transfer (¹MLCT) ¹[Pt(5d)π*(Schiff base)] character mixed with ¹[lone pair(phenoxide)π*(imine)] charge-transfer character. The platinum(II) Schiff base complexes are thermally stable, with decomposition temperatures up to 495 °C, and show emission λ_max at 541–649 nm in acetonitrile, with emission quantum yields up to 0.27. Measurements of the emission decay times in the temperature range from 130 to 1.5 K give total zero-field splitting parameters of the emitting triplet state of 14–28 cm⁻¹. High-performance yellow to red organic light-emitting devices (OLEDs) using these platinum(II) Schiff base complexes have been fabricated with the best efficiency up to 31 cd A⁻¹ and a device lifetime up to 77 000 h at 500 cd m⁻².

Probing the Excited State Properties of the Highly Phosphorescent Pt(dpyb)Cl Compound by High-Resolution Optical Spectroscopy

Co-reporter:Andreas F. Rausch ; Lisa Murphy ; J. A. Gareth Williams

Inorganic Chemistry 2009 Volume 48(Issue 23) pp:11407-11414

Publication Date(Web):October 30, 2009

DOI:10.1021/ic901523u

Detailed photophysical studies of the emitting triplet state of the highly phosphorescent compound Pt(dpyb)Cl based on high-resolution optical spectroscopy at cryogenic temperatures are presented {dpyb = N∧C2∧N-coordinated 1,3-di(pyridylbenzene)}. The results reveal a total zero-field splitting of the emitting triplet state T1 of 10 cm−1 and relatively short individual decay times for the two higher lying T1 substates II and III, while the decay time of the lowest substate I is distinctly longer. Further evidence for the assignment of the T1 substates is gained by emission measurements under high magnetic fields. Distinct differences are observed in the vibrational satellite structures of the emissions from the substates I and II, which are dominated by Herzberg−Teller and Franck−Condon activity, respectively. At T = 1.2 K, the individual spectra of these two substates can be separated by time-resolved spectroscopy. For the most prominent Franck−Condon active modes, Huang−Rhys parameters of S ≈ 0.1 can be determined, which are characteristic of very small geometry rearrangements between the singlet ground state and the triplet state T1. The similar geometries are ascribed to the high rigidity of the Pt(N∧C∧N) system which, unlike complexes incorporating bidentate phenylpyridine-type ligands and exhibiting similar metal-to-ligand charge transfer admixtures, cannot readily distort from planarity. The results provide new insight into strategies for optimizing the performance of platinum-based emitters for applications such as organic light-emitting diode (OLED) technology and imaging.

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

Co-reporter:Andreas F. Rausch, Mark E. Thompson and Hartmut Yersin

Inorganic Chemistry 2009 Volume 48(Issue 5) pp:1928-1937

Publication Date(Web):January 27, 2009

DOI:10.1021/ic801250g

The sky-blue emitting compound Ir(4,6-dFppy)2(pic) (iridium(III)bis[2-(4′,6′-difluorophenyl)pyridinato-N,C2′]-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 T1 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.

Synthesis of Cyclometallated Platinum Complexes with Substituted Thienylpyridines and Detailed Characterization of Their Luminescence Properties

Co-reporter:Dmitry N. Kozhevnikov, Valery N. Kozhevnikov, Maria M. Ustinova, Amedeo Santoro, Duncan W. Bruce, Burkhard Koenig, Rafał Czerwieniec, Tobias Fischer, Manfred Zabel and Hartmut Yersin

Inorganic Chemistry 2009 Volume 48(Issue 9) pp:4179-4189

Publication Date(Web):April 1, 2009

DOI:10.1021/ic802401j

Synthesis of various derivatives of 2-(2-thienyl)pyridine via substituted 3-thienyl-1,2,4-triazines is reported. The final step of the synthesis is a transformation of the triazine ring to pyridine in an aza-Diels−Alder-type reaction. The resulting 5-aryl-2-(2-thienyl)pyridines (HL1-HL4) and 5-aryl-2-(2-thienyl)cyclopenteno[c]pyridines (HL5-HL8) (with aryl = phenyl, 4-methoxyphenyl, 2-naphtyl, and 2-thienyl) were used as cyclometallating ligands to prepare a series of eight luminescent platinum complexes of the type [Pt(L)(acac)] (L = cyclometallating ligand, acac = acetylacetonato). X-ray single crystal structures of three complexes of that series, [Pt(L5)(acac)] = [Pt(5-phenyl-2-(2-thienyl)cyclopenteno[c]pyridine)(acac)], [Pt(L6)(acac)] = [Pt(5-(4-methoxy)-2-(2-thienyl)cyclopenteno[c]pyridine)(acac)], and [Pt(L7)(acac)] = [Pt(5-(2-naphtyl)-2-(2-thienyl) cyclopenteno[c]pyridine)(acac)] were determined. Photoluminescence and electronic absorption spectra of the new [Pt(L)(acac)] complexes are reported. For two representative compounds of that series, [Pt(L4)(acac)] and [Pt(L5)(acac)], a detailed photophysical characterization based on highly resolved emission and excitation spectra, as well as on emission decay properties, was carried out. The studies down to low temperature (T = 1.2 K) and up to high magnetic fields (B = 10 T) allowed us to characterize the three individual substates of the emitting triplet state. In particular, it is shown that the lowest triplet states of [Pt(L4)(acac)] and [Pt(L5)(acac)] are largely ligand-centered (LC) of 3ππ* character, which experience only weak spin−orbit couplings to higher lying singlet states.

Triplet state relaxation processes of the OLED emitter Pt(4,6-dFppy)(acac)

Co-reporter:Andreas F. Rausch, Mark E. Thompson, Hartmut Yersin

Chemical Physics Letters 2009 Volume 468(1–3) pp:46-51

Publication Date(Web):13 January 2009

DOI:10.1016/j.cplett.2008.11.075

Abstract

The emitting triplet state of Pt(4,6-dFppy)(acac) doped into n-octane is studied at cryogenic temperatures by site-selective high-resolution optical spectroscopy. The investigations reveal a very specific zero-field splitting (ZFS) pattern of the emitting T₁ state and the individual deactivation times of the substates to the singlet ground state S₀. Spin-lattice relaxation (SLR) processes occurring between the T₁ substates are ascribed to a combination of the direct and the Raman process. Due to the relatively long SLR time at low temperature, a Boltzmann distribution is not established directly after the excitation pulse below ≈2 K.

Photophysical properties of Re(pbt)(CO)4 studied by high resolution spectroscopy

Co-reporter:Rafał Czerwieniec, Walter J. Finkenzeller, Thomas Hofbeck, Alexander Starukhin, Armin Wedel, Hartmut Yersin

Chemical Physics Letters 2009 Volume 468(4–6) pp:205-210

Publication Date(Web):22 January 2009

DOI:10.1016/j.cplett.2008.11.086

Photophysical properties of Re(pbt)(CO)4 are investigated at cryogenic temperatures and high magnetic fields. The highly resolved spectra show that the zero field splitting of the lowest triplet T1 into three substates is smaller than 2 cm−1. With this result, the T1 state can be classified as only slightly MLCT-perturbed 3LC (3ππ∗) state. Consistently, spin-lattice relaxation times are slow at T = 1.2 K and emission decay times with τI = 960 μs, τII = 320 μs, and τIII = 24 μs are long due to only small singlet admixtures. The vibrational satellite structures observed reflect different vibrational deactivation mechanisms and reveal similar geometries of the emitting triplet and the ground state.Photophysical properties of Re(pbt)(CO)4 are investigated at cryogenic temperatures and high magnetic fields.

Blue Light Emitting Ir(III) Compounds for OLEDs - New Insights into Ancillary Ligand Effects on the Emitting Triplet State

Co-reporter:Andreas F. Rausch, Mark E. Thompson and Hartmut Yersin

The Journal of Physical Chemistry A 2009 Volume 113(Issue 20) pp:5927-5932

Publication Date(Web):April 28, 2009

DOI:10.1021/jp902261c

The sky-blue emitting phosphorescent compound Ir(4,6-dFppy)2(acac) (FIracac) doped into different matrices is studied under ambient conditions and at cryogenic temperatures on the basis of broadband and high-resolution emission spectra. The emitting triplet state is found to be largely of metal-to-ligand charge transfer (MLCT) character. It is observed that different polycrystalline and amorphous hosts distinctly affect the properties of the triplet. Moreover, a comparison of FIracac with the related Ir(4,6-dFppy)2(pic) (FIrpic), differing only by the ancillary ligand, reveals obvious changes of properties of the emitting state. These observations are explained by different effects of acac and pic on the Ir(III) d-orbitals. In particular, the occupied frontier orbitals, strongly involving the t2g-manifold, and their splitting patterns are modified differently. This influences spin−orbit coupling (SOC) of the emitting triplet state to higher-lying 1,3MLCT states. As a consequence, zero-field splittings, radiative decay rates, and phosphorescence quantum yields are changed. The important effects of SOC are discussed qualitatively and are related to the emission properties of the individual triplet substates, as determined from highly resolved spectra. The results allow us to gain a better understanding of the impact of SOC on the emission properties with the aim to develop more efficient triplet emitters for OLEDs.

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

Co-reporter:Reinhard Bauer, Walter J. Finkenzeller, Udo Bogner, Mark E. Thompson, Hartmut Yersin

Organic Electronics 2008 Volume 9(Issue 5) pp:641-648

Publication Date(Web):October 2008

DOI:10.1016/j.orgel.2008.04.001

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.

Phosphorescence dynamics and spin-lattice relaxation of the OLED emitter Ir(btp)2(acac)

Co-reporter:Walter J. Finkenzeller, Mark E. Thompson, Hartmut Yersin

Chemical Physics Letters 2007 Volume 444(4–6) pp:273-279

Publication Date(Web):27 August 2007

DOI:10.1016/j.cplett.2007.07.022

The lowest triplet state T1 of Ir(btp)2(acac) doped into CH2Cl2 is studied by site-selective and time-resolving emission and by excitation spectroscopy at cryogenic temperatures. Different sites due to different environments of Ir(btp)2(acac) show dissimilar intensity distributions of the substate emissions, zero-field splittings, emission decay times, and spin-lattice relaxation (SLR) times. Temperature dependent investigations of the SLR processes reveal the importance of the direct process and the Orbach process. Moreover, the emitting T1 state is assigned as 3LC (ligand-centered) state with significant 1,3MLCT (metal-to-ligand charge transfer) perturbation. This classification is related to the usability of phosphorescent emitters in OLEDs.The triplet state of the OLED emitter Ir(btp)2(acac) is characterized with regard to emission decay and spin-lattice relaxation (SLR) dynamics.

Emission and absorption of Ir(ppy)2(CO)(Cl) – temperature dependence, phosphorescence decay dynamics, and assignment of excited states

Co-reporter:W.J. Finkenzeller, P. Stößel, H. Yersin

Chemical Physics Letters 2004 Volume 397(4–6) pp:289-295

Publication Date(Web):21 October 2004

DOI:10.1016/j.cplett.2004.08.074

Abstract

Emission spectra and decay times of Ir(ppy)₂(CO)(Cl) dissolved in THF were recorded for 1.2 K ⩽ T ⩽ 300 K. The emission stems from a triplet state T₁ which is split into three substates by less than 1 cm⁻¹. We classify this state as ³LC(ligand centered, ppyππ*) state. At T = 1.2 K, the substates emit independently with three individual decay times (τ_I = 300 μs, τ_II = 85 μs, τ_III = 9 μs) due to slow equilibration processes, i.e. slow spin-lattice-relaxation, between the substates. With increasing temperature, the emission decay becomes monoexponential as a result of fast equilibration. The results are compared to the spectroscopic behavior of Ir(ppy)₃ which represents a metal-to-ligand charge transfer ³MLCT emitter.