Andreas F. Rausch

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

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 lambda(max) in the range 417-546 nm, which are assigned to states of metal-to-ligand charge-transfer ((1)MLCT) (1)[Pt(5d)-->pi*(Schiff base)] character mixed with (1)[lone pair(phenoxide)-->pi*(imine)] charge-transfer character. The platinum(II) Schiff base complexes are thermally stable, with decomposition temperatures up to 495 degrees C, and show emission lambda(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).

Organometallic Pt(II) and Ir(III) Triplet Emitters for OLED Applications and the Role of Spin-Orbit Coupling: A Study Based on High-Resolution Optical Spectroscopy

High-resolution optical spectroscopy of organometallic triplet emitters reveals detailed insights into the lowest triplet states and the corresponding electronic and vibronic transitions to the singlet ground state. As case studies, the blue-light emitting materials Pt(4,6-dFppy)(acac) and Ir(4,6-dFppy)2(acac) are investigated and characterized in detail. The compounds’ photophysical properties, being markedly different, are largely controlled by spin–orbit coupling (SOC). Therefore, we study the impact of SOC on the triplet state and elucidate the dominant SOC and state-mixing paths. These depend distinctly on the compounds’ coordination geometry. Relatively simple rules and relations are pointed out. The combined experimental and theoretical results lead us towards structure-efficiency rules and guidelines for the design of new organic light emitting diode (OLED) emitter materials.

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

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 C(2) 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 C(2) 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 φ(PL) = 80% versus only 2% found for the bidentate complex.

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

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 t(2g)-manifold, and their splitting patterns are modified differently. This influences spin-orbit coupling (SOC) of the emitting triplet state to higher-lying (1,3)MLCT 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.

Probing the excited state properties of the highly phosphorescent Pt(dpyb)Cl compound by high-resolution optical spectroscopy.

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--C(2)--N-coordinated 1,3-di(pyridylbenzene)}. The results reveal a total zero-field splitting of the emitting triplet state T(1) of 10 cm(-1) and relatively short individual decay times for the two higher lying T(1) substates II and III, while the decay time of the lowest substate I is distinctly longer. Further evidence for the assignment of the T(1) 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 approximately 0.1 can be determined, which are characteristic of very small geometry rearrangements between the singlet ground state and the triplet state T(1). 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.

Spin-orbit coupling routes and OLED performance -Studies of blue-light emitting Ir(III) and Pt(II) complexes

In this study, detailed spectroscopic investigations of the blue emitting compounds Ir(4,6-dFppy)2(pic) and Pt(4,6- dFppy)(acac) are presented. Due to spin-orbit coupling (SOC) of the emitting triplet state with higher lying singlet states both complexes show an intense phosphorescence and are utilized as emitters in organic light emitting diodes (OLEDs). Distinct differences with respect to important photophysical properties are found for the two compounds. For example, the (distorted) octahedral Ir(4,6-dFppy)2(pic) complex exhibits a shorter emission decay time and shows a larger zero-field splitting (ZFS) than the (distorted) square planar Pt(4,6-dFppy)(acac) complex (τ(Ir) = 0.4 μs and τ(Pt) = 3.6 μs of the respective shortest-lifed triplet substate; Δ(ZFS, Ir) = 67 cm-1, ΔE(ZFS, Pt) = 8 cm-1). This behaviour is connected with the extent of metal-to-ligand charge transfer (MLCT, dπ*) character in the emitting triplet state. High MLCT character usually results in a high emission decay rate and indicates a good suitability as OLED emitter material. Of crucial importance in this respect is the effectiveness of SOC. In this study it is shown that the SOC routes depend on the coordination geometry of the emitter compound. In particular, the couplings can be more effective in (distorted) octahedral than in (distorted) square planar compounds. Hence, the photophysical differences of Ir(4,6-dFppy)2(pic) compared to Pt(4,6-dFppy)(acac) can be rationalized. Moreover, this investigation shows that the analysis of SOC paths provides general guidelines for the design of efficient emitters for OLED applications.

Chiral 2,3‐Dihydro‐1H‐1,3,2‐diazaboroles

A series of differently substituted chiral 2,3-dihydro-1H-1,3,2-diazaboroles has been prepared by various methods. 2Bromo-1-tert-butyl-3-[(S)-1-phenylethyl]-2,3-dihydro-1H1,3,2-diazaborole (3a), 2-bromo-1,3-di[(S)-1-phenylethyl]-2,3-dihydro-1H-1,3,2-diazaborole (3b) and 2-bromo-1,3di[(S)-1-cyclohexylethyl]-2,3-dihydro-1H-1,3,2-diazaborole (3c) were formed from the reaction of the corresponding 1,4-diazabutadienes and boron tribromide and the subsequent reduction of the resulting borolium salts [R1Na = CH−CH=Nb(R2)BBr2]Br(Na−B) [2a: R1 = tBu, R2 = CH(Me)Ph; 2b: R1 = R2 = CH(Me)Ph; 2c: R1 = R2 = CH(Me)(cC6H11)] with sodium amalgam. Treatment of (S)-3a with LiAlH4 or methyllithium afforded 1-tert-butyl-2-hydro-3-[(S)-1-phenylethyl]-2,3-dihydro-1H-1,3,2-diazaborole [(S)-6] and 1-tert-butyl-2-methyl-3[(S)-1-phenylethyl]-2,3-dihydro-1H-1,3,2-diazaborole [(S)-7], respectively. Aminolysis of the BBr bond of (S)-3a by tert-butylamine or (S)-1-phenylethylamine gave the corresponding 2-tert-butylamino- and 2-[1-phenylethylamino]-2,3-dihydro-1H-1,3,2-diazaboroles (S)-8 and (S,S)-9, respectively. Similarly, (S,S)-3b and (S,S)-3c were reacted with tert-butylamine to furnish the 2-tert-butylamino-2,3-dihydro-1H-1,3,2-diazaborole derivatives (S,S)-11 and (S,S)-12, respectively. The 2-trimethylstannyl-2,3-dihydro-1H-1,3,2-diazaboroles (S,S)-10 and (S,S)-14 were accessible from 3b or 3c and trimethylstannyllithium. The transformation of achiral 2bromo-1,3-di-tert-butyl-2,3-dihydro-1H-1,3,2-diazaborole into the chiral (S)-2-[1-phenylethylamino]- and (S)-2-[1-cyclohexyl-ethylamino] derivatives (S)-15 and (S)-16 was effected by aminolysis with enantiomerically pure (S)-1-phenylethylamine or (S)-1-cyclohexylethylamine. The novel compounds were characterized by 1H, 11B, 13C, and 119Sn NMR spectroscopy as well as mass spectrometry and determination of the optical rotation. The molecular structure of compound 3c was confirmed by X-ray structural analysis. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Lithium alkyl assisted coupling of a 2-cyano-2,3-dihydro-1H-1,3,2-diazaborole to give tBuNCH=CHN(tBu)BC(iPr)=N-BN(tBu)CH=CHNtBu.

Reaction of 2-cyano-1,3,2-diazaborole tBuNCH=CHN(tBu)BCN (2) with half an equivalent of isopropyllithium afforded compound tBuNCH=CHN(tBu)BC(iPr=N-BN(tBu)CH=CHNtBu (7). In contrast to this, a 1:1 stoichiometry of the reactants led to tBuNCH=CHN(tBu)BiPr (6) as the product of a nucleophilic substitution process at the boron atom. Similarly, regardless of the molar ratio of reactants employed, treatment of 2 with cyclopropyllithium, isobutyllithium or phenyllithium afforded solely substitution products tBuNCH=CHN(tBu)BR [R =cPr (12); Ph (13); iBu (14)].

Synthese, Struktur und Reaktivität von 2-Amino- und 2-Imino-2,3-dihydro-1H-1,3,2-diazaborolen / Synthesis, Structure and Reactivity of 2-Amino- and 2-Imino-2,3-dihydro-1H-1,3,2-diazaboroles

The 2-halo-2,3-dihydro-1H-1,3,2-diazaboroles (1a′: R = tBu, X = Br; 1b: R = 2,6-Me2C6H3; X = I) were converted into the 2-amino-2,3-dihydro-1H-1,3,2- diazaboroles (2a: R = tBu; 2b: 2,6-Me2C6H3) by treatment with dry gaseous ammonia. Similarly reaction of 1a′ with 2,6-dimethylaniline or tBuNH2 afforded the corresponding derivates (3; R1 = 2,6-Me2C6H3; 4; R1 = tBu). The treatment of 1a′ with the ethylene diamine adduct of lithium acetylide led to the formation of (5). Lithiation of 2 a and subsequent silylation gave 6 (R1 = SiMe3), which was transformed to the diborolylamine (7) upon exposure to 1a′. Borolylketimine (8 ) and borolylcarbodiimide (9) resulted from 1a′ and Ph2C=NSiMe3 or Me3SiN=C=NSiMe3, respectively. All the new compounds were characterized by elemental analyses as well as spectroscopic data (IR, 1H, 11B, 13C NMR, MS). Heterocycle 5 was also subjected to an X-ray diffraction analysis.