Molecular Construction Kit for Tuning Solubility, Stability and

Article pubs.acs.org/cm

Molecular Construction Kit for Tuning Solubility, Stability and Luminescence Properties: Heteroleptic MePyrPHOS-Copper IodideComplexes and their Application in Organic Light-Emitting Diodes Daniel Volz,†,‡ Daniel M. Zink,†,‡ Tobias Bocksrocker,§ Jana Friedrichs,‡ Martin Nieger,∥ Thomas Baumann,*,‡ Uli Lemmer,§ and Stefan Bras̈ e*,†,# †

Institute of Organic Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany cynora GmbH, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany § Light Technology Institute, Karlsruhe Institute of Technology (KIT), Engesserstraße 13, 76131 Karlsruhe, Germany ∥ Laboratory of Inorganic Chemistry, University of Helsinki, P.O. Box 55, A.I. Virtasen aukio 1, FIN-00014 University of Helsinki, Finland # Institute of Toxicology and Genetics, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany ‡

S Supporting Information *

ABSTRACT: Organic light-emitting diodes (OLEDs) are currently being commercialized for lighting and display applications, but more work has to be done. In addition to the ongoing optimization of materials and devices in terms of efficiency and lifetime, the substitution of processing steps involving vacuum deposition for solution processing techniques is favorable. To reach this aim, good soluble materials are required. A modular family of highly emissive PyrPHOS-copper iodide complexes featuring various ancillary phosphine ligands has been synthesized. Photoluminescence spectroscopy, TCSPC (time-correlated single photon counting), cyclic voltammetry, X-ray diffraction, and DFT calculations were performed to gain a broad understanding of the complexes. While the photophysical properties are consistent within the family, thermal stability and solubility depend on the ligands. The materials showed very high photoluminescence quantum efficiencies up to 99% in powders and 85% in thin films. Selected examples were tested in devices, confirming the suitability of heteroleptic PyrPHOS-complexes for OLEDs. KEYWORDS: copper complexes, organic light-emitting diode, structure−property relationships, photoluminescence

E

processing of molecular, polymer host systems are chlorobenzene and toluene. Ir(ppy)3, one of the standard emitters used in high-performance OLEDs,9,10 has a solubility of 1 mg mL−1 in chlorobenzene,4 even less in toluene. The preparation of emitting layers (usually a mixture of a host and the emitting compound as dopant) with a suitable thickness for OLEDs, typically in the order of 50 nm, is hindered by this low solubility. Morphological defects like crystalline grains in functional layers act as charge traps,11 while aggregation, especially for triplet-harvesting emitters, causes emission quenching.12 Such morphology defects can be avoided by using materials with a low crystallization tendency, which corresponds to a low lattice energy, a good solubility, or by immobilizing the relevant molecules, for example, by attaching them to a polymeric backbone.13−19 To address this issue, common materials may be substituted with solubility-enhancing groups. Because of the steric demand of these substituents, the

ven after the commercial launch of organic light-emitting diodes (OLED) as a technology1 for lighting and display applications some years ago, essential questions remain unsolved: 14 years after the introduction of transition-metal compounds as efficient emitting materials by Forrest et al.,2,3 the fabrication of solution-processed OLEDs with metal complexes as emitting materials still has to be developed further into a truly reliable, industrial process. The three main problems preventing the use of solution processing are (i) poor solubility of many OLED-materials in common solvents,4,5 (ii) morphological inhomogeneity due to aggregation and crystallization of small molecules,6 and (iii) blending of the functional layers during or after deposition.5,7 The latter is also relevant even for vacuum-processed OLEDs, but is often neglected.7 Comparing solution- to vacuum-processing, in most cases both efficiency and device-lifetime are lower, while the turn-onvoltage is higher for the former technique, even when identical materials are used. This is expected to be a result of defects at the interfaces.8 Grave problems arise from the insolubility of many functional materials known from vacuum-deposited OLEDs in common organic solvents. Common solvents used for the © 2013 American Chemical Society

Received: April 3, 2013 Revised: July 2, 2013 Published: July 16, 2013 3414

dx.doi.org/10.1021/cm4010807 | Chem. Mater. 2013, 25, 3414−3426

Chemistry of Materials

Article

Table 1. Crystallographic Data, Summary of Data Collection, Refinement for Complexes 1, 2, 4, and 7b 136 formula formula weight (g mol−1) crystal size crystal system, space group unit cell dimensions

C54H46Cu2I2NP3 1182.71 0.20 × 0.15 × 0.10 mm monoclinic, P21/n (No. 14) a = 14.083(1) Å b = 18.455(2) Å c = 18.371(2) Å β = 96.90(1)°

volume (Å3) Z, ρcalc μ (mm−1) F(000) T (K) 2θmax (deg) completeness of data (%) limiting indices

reflections collected/unique absorption correction max. and min. transmission data/restraints/params goodness-of-fit on F2 R1 [I > 2σ(I)] wR2 (all data) largest diff. peak and hole (e Å−3)

4740.1(8) 4, 1.657 Mg m−3 2.339 2344 123(2) 55.0 99.8 −18 ≤ h ≤ 18 −23 ≤ k ≤ 23 −23 ≤ l ≤ 23 71265/10853 [Rint = 0.046] semiempirical from equivalents 0.8014 and 0.7002 10853/0/569 1.04 0.029 0.062 0.845 and −0.397

2

4

C60H58Cu2I2NP3 1266.86 0.30 × 0.15 × 0.10 mm triclinic P1̅ (No. 2) a = 14.239(1) Å b = 14.311(2) Å c = 15.759(2) Å α = 80.39(1)° β = 63.33(1)° γ = 68.86(1)° 2676.5(5) 2, 1.572 Mg m−3 2.077 1268 123(2) 55.0 99.8 −18 ≤ h ≤ 18 −18 ≤ k ≤ 18 −20 ≤ l ≤ 22 69229/12273 [Rint = 0.066] semiempirical from equivalents 0.8372 and 0.5554 12273/0/620 1.05 0.040 0.111 1.603 and −0.927

C42H34Cu2I2NO6P3 1122.49 0.40 × 0.20 × 0.10 mm triclinic P1̅ (No. 2) a = 10.352(1) Å b = 14.870(2) Å c = 16.080(2) Å α = 110.88(1)° β = 97.09(1)° γ = 110.31(1)° 2080.9(4) 2, 1.792 Mg m−3 2.669 1100 123(2) 55 99.7 −13 ≤ h ≤ 13 −19 ≤ k ≤ 19 −20 ≤ l ≤ 20 28681/9514 [Rint = 0.022] semiempirical from equivalents 0.7456 and 0.5829 9514/0/506 1.06 0.024 0.052 1.101 and −0.683

7b C33H48Cu2I2NO6P3 1028.51 0.24 × 0.12 × 0.06 mm triclinic P1̅ (No. 2) a = 10.1765(7) Å b = 20.1801(11) Å c = 20.5535(17) Å α = 80.748(5)° β = 82.002(6)° γ = 87.858(4)° 4125.0(5) 4, 1.656 Mg m−3 2.684 2040 123(2) 55 99.6 −13 ≤ h ≤ 12 −26 ≤ k ≤ 26 −26 ≤ l ≤ 26 56294/18855 [Rint = 0.039] semiempirical from equivalents 0.8372 and 0.7299 18855/2040/824 1.01 0.055 0.126 1.481 and −1.067

N,P-ligands, such as diphenylphosphinepyridine-derivatives (PyrPHOS),36−38 which can readily be modified to satisfy, for example, solubility requirements. Recently, we demonstrated how emissive, alkyne-bearing Cu(I)-complexes can be crosslinked in an OLED with azide-substituted host polymers to form the emissive layer of an OLED.14 In addition to well accessible chemical modifications in combination with very high photoluminescence quantum yields in solid state (PLQY ϕ), many Cu(I)-complexes have a particularly low energetical separation between the T1 and S1-states, allowing for thermally activated delayed f luorescence (TADF). This process is based upon a pseudoequilibrium between T1 and S1, caused by a very fast intersystem crossing from S1 to T1 and a quick thermal repopulation from T1 to S1.12,30,39,40 As a result, the effective emission decay kinetics are in the range of microseconds, differing from pure fluorescence (nanosecond range) or phosphorescence (millisecond range). Accordingly, Cu(I) complexes are considered to be able to harvest both singlet and triplet excitons generated in OLEDdevices, but via a different process to known phosphorescent heavy-metal compounds. This beneficial property, also referred to as singlet harvesting, has been successfully employed in solution and vacuum processed OLEDs.14,40−43 In this study, we report on heteroleptic Cu(I)-complexes based on a butterfly shaped Cu2I2 core bridged by a N,P-ligand, with two ancillary phosphine ligands on the periphery coordinated to the Cu(I)-ions. This structure originates from a series of previously reported homoleptic complexes. The reason for this deviation from the usual planar Cu2X2-geometry

solubility is improved, while the glass transition temperature is raised.20 Blending of functional layers can also occur when a solvent used for the deposition of further layers is able to dissolve already deposited layers. In this context, there exists also the effect of interlayerdiffusion, that is, the gradual blending of adjacent layers by slow diffusion of one or more components through the stack also exists; an effect of interlayer-diffusion. This process is temperature dependent21 and could even be initiated by a slight heating of a vacuum-processed OLED-stack.7 Processinginduced blending can be prevented with orthogonal solvent strategies, that is, using a solvent which is not able to dissolve previously cast layers. This strategy requires the tuning of the solubility. Another approach is the cross-linking5,22−29 of each layer after deposition, which prevents the dissolving of the layer during or after processing and stabilizes it against diffusion during device operation. Known approaches cover photochemical,18 chemical,25,29 thermal,26 and electrochemical techniques.27,28 The broad structural variability of one- or multinuclear Cu(I) complexes make them ideal candidates to address all these criteria, while rare metals, such as iridium, require much effort in order to substitute ligands;9 Cu(I) complexes can often be synthesized in one-pot reactions at low temperatures by mixing ligands and Cu(I)-sources, while the stoichiometry controls the products.30−33 Common ligands used in Cu(I) complexes are N,N-ligands, such as phenanthroline or neocuproine,34 P,Pligands, such as bis(diphenyl-phosphino)-diphenyl-ether,35 or 3415



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Chart 1. All P-Ligands for Heteroleptic PyrPHOS-Complexes Used in This Studya

a

R is 4-Me for most cases except 7b, where R = 4-butinyl, and 12b, where R = 4-iBu.

also are discussed there.30 These homoleptic compounds have two free coordination sites that might lead to undesired reactions in the excited state and were difficult to modify for improved solubility without changing the photophysical characteristics, which are mainly determined by the bridging ligand. We have exchanged the ancillary bidentate ligands for monodentate phosphines. Therefore, this approach allows us to modify the solubility without affecting the emission color. While we discuss the properties of heteroleptic complexes with PPh3 as ancillary ligand in combination with different bridging N,P-ligands in a separate publication,36 this report focuses on the influence of different phosphines on the properties of the complexes (Chart 1, Scheme 1). Using commercially available phosphines, it is possible to adjust both the solubility of the complexes and to minimize synthetic effort at the same time. PyrPHOS-complexes prepared based on this construction kit

are soluble in a wide range of solvents, such as ethanol or hexane, making them good candidates for the orthogonal solvent strategy. In addition, the suitability of these materials for OLED applications has been confirmed by preparing solution-processed devices.

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EXPERIMENTAL SECTION

General Method for the Synthesis of Heteroleptic PyrPHOS Complexes. The complex was synthesized by stirring CuI (1.00 mmol, 190 mg, 1.00 equiv.), a PyrPHOS-derivative (0.50 mmol, 0.50 equiv.), a suitable P-donor-ligand (1.00 mmol, 1.00 equiv.) in 10 mL dichloromethane (method a) or acetonitrile (method b) in the dark under a nitrogen atmosphere at room temperature. The mixture was stirred for two hours. During this time, the suspension turned into a clear, yellow solution. The reaction mixture was filtrated over a 0.45 μm syringe filter and precipitated from an appropriate solvent (hexane, methanol or diethylether, Scheme 2). See the Supporting Information for details, experimental data for all complexes. Materials. CuI (99.999% trace metals basis) was purchased by Sigma Aldrich and used without further purification. MePyrPHOS, iBuPyrPHOS, and butynylPyrPHOS were synthesized according to recently published procedures.14,30 To preclude any effects resulting

Scheme 1. Substitution of the Two Monodentate Ligands in PyrPHOS-Complexes with Arbitrary Monodentate PLigands Leads to Heteroleptic Complexes

Scheme 2. General Procedure for the Synthesis of Heteroleptic PyrPHOS Complexes

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Table 2. Selected Bond Lengths and Angles for the Structurally Characterized Complexes 1, 2, 4, and 7b 136major

2

4

2.8164(5)

2.7135(7)

2.6978(6)

CuP−I

2.6864(5) 2.6878(4)

2.6871(6) 2.6535(7)

2.6872(4) 2.6764(6)

CuP−PP,N

2.2353(8)

2.2503(11)

2.2647(7)

CuN−N

2.090(2)

2.105(3)

2.099(2)

CuN−PP

2.2533(7)

2.2523(11)

2.2303(8)

CuP−PP

2.2493(7)

2.2504(12)

2.2506(7)

lengths Cu−Cu

angles Cu−I−Cu

P−Cu−P

63.953(13) 63.357(12)

123.90(3)

60.84(2) 61.21(2)

121.20(4)

from trace impurities, we distilled MePyrPHOS, triphenylphosphine, triethylphosphite twice prior to use. All solvents were purchased by Aldrich and VWR with at least 99.5% purity. All other materials and ligands were used as received. Photophysical Measurements. Absorption spectra were measured on a Thermo Scientific (Evolution 201) UV−visible spectrophotometer in acetonitrile. Emission and excitation spectra in the solid state were measured with an Horiba Scientific FluoroMax-4 spectrofluorometer using a JX monochromator and a R928P PMT detector. Fluorescence lifetime measurements were recorded and detected on the same system using the TCSPC (time-correlated single photon counting) method with the FM-2013 accessory, a TCSPC hub from Horiba Yvon Jobin. For this, a NanoLED 370 was used as excitation source (λ = 370 nm, 1.5 ns pulse). Decay curves were analyzed with the software DAS-6, DataStation provided by Horiba Yvon Jobin. The quality of the fit was determined by the Chi-Squaremethod by Pearson.44 The quality factor χ2 should be as close to unity as possible. The definition for the factor can be written as follows: i

χ2 =

∑ k=1

60.648(14) 60.850(13)

114.33(3)

7b 2.766(1) 2.785(1) 2.722(1) 2.657(1) 2.695(1) 2.640(1) 2.253(2) 2.266(2) 2.076(5) 2.083(6) 2.195(2) 2.205(2) 2.228(2) 2.231(2) 61.51(3) 62.35(3) 62.49(3) 62.97(2) 118.74(7) 121.98(7)

geometries, we used the B3LYP functional. Phosphorescence energies were also calculated using the spin-flip Tamm−Dancoff Approximation (SF-TDA).54 All calculations were done using the Turbomole program package (version 6.4).55 X-ray Diffraction.56,57 The crystal structure determinations of complex 1, complex 2, complex 4, and complex 7b were performed on a Bruker-Nonius Kappa CCD diffractometer at 123(2) K using Mo Kα radiation (λ = 0.71073 Å). Crystal data, data collection parameters, and results of the analyses are listed in Table 2. Direct Methods (complex 1a and complex 7b) or Patterson Methods (complex 2 and complex 4) were used for structure solution (SHELXS-97),57 refinement was carried out using SHELXL-97 (full-matrix leastsquares on F2).57 The parameters for hydrogen atoms were refined using a riding model. A semiempirical absorption correction using equivalent reflections was applied. In complex 1a, the Cu2-atom is disordered (s.o.f. = 0.94). In complex 7b, the OEt-groups were disordered (s.o.f.= 0.689(4)) (see the cif file). CCDC-906844 (complex 1a), CCDC-910261 (complex 2), CCDC-910262 (complex 4), and CCDC-910263 (complex 7b) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre. Device Preparation. Poly-TPD was provided by American Dye Source (ADS254BE). PEDOT:PSS was provided from Heraeus (Al4043). All other chemicals and solvents used for device preparation were purchased from Sigma-Aldrich and used as received. Prior to the device preparation, the ITO-coated glass substrates were cleaned by subsequent sonication in acetone and isopropanol for 15 min, followed by an UV ozone treatment for 10 min. The layer thickness of all spincast layers was determined with a thin film profiler by Veeco (Dektak 150). A 20 nm anode buffer layer of PEDOT:PSS was spin-cast on the ITO substrates and dried by baking the substrates at 110 °C for 30 min. The hole transporting layer (30 nm) was made by casting polyTPD from toluene. The HTL was annealed at 110 °C for 60 min. The emitting layer consisted of 45% mCPy as a host material, 10% PVK to prevent crystallization and 45% of emitting material. The layer thickness was 30 nm. The emitting layer was dried at 80 °C for 2 min. TPBi (25 nm) was evaporated, the layer thickness was monitored by a quartz-crystal thickness/ratio monitor. LiF (1 nm) and Al (200 nm) were evaporated successively. The pixel size was 25 mm2. Except for the spin coating of the PEDOT-layer, all processes were carried out in the controlled atmosphere of a nitrogen glovebox. Physical character-

(oi − ei)2 ei

with ei = value proposed by the fit and oi = actual value For the determination of PLQY ϕ, an absolute PL quantum yield measurement system from Hamamatsu Photonics was used. The system consisted of a photonic multichannel analyzer PMA-12, a model C99200-02G calibrated integrating sphere and a monochromatic light source L9799−02 (150 W Xe-, Hg−Xe-lamps). Data analysis was done with the PLQY measurement software U6039-05, provided by Hamamatsu Photonics. Computational Details. Density functional theory (DFT) calculations were performed using the BP86,45,46 and B3LYP47,48 functionals, and the def2-SV(P)49,50 basis set. For numerical integration, the m4 grid was employed. In the BP86 calculation, the resolution-of-identity approximation51−53 was used. The initial structures were obtained from single-crystal X-ray diffraction data and were optimized using the BP86 functional. The initial structure of complex 7a was obtained from diffraction data of complex 7b. Triplet structures were optimized using unrestricted DFT. Analytical harmonic vibrational frequency calculations were performed to verify that the optimized structures are minima on the potential energy surface. For time-dependent (TD) DFT calculations at these 3417



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ization was carried out at ambient conditions at room temperature using a BoTest measurement unit. The devices were tested within 2 h of fabrication.

iodide, monodentate phosphine ligands complete the typical tetrahedral coordination geometry of Cu(I)-complexes. The bend Cu2I2 core differs from the usually observed, planar geometry found for other copper complexes.63,64 From a structural point of view, changing the P-donor-ligands does neither affect the complexation behavior, nor the major structural motif (see Tables 1 and 2). Judging from the copper−copper distances (2.70 to 2.82 Å), which are in the range of the van der Waals radii (2.80 Å), no metallophilic interactions occur. The different P-donors seem to have no big influence on typical bond lengths such as the distance between the pyridineN and Cu (2.08 to 2.09 Å), the P atoms of either PyrPHOS or the monodentate ligands (2.20 to 2.25 Å) and Cu or between I and Cu (2.66 to 2.72 Å). The size of the P-donors does, however, influence the angles between the phosphor atoms of PyrPHOS, copper and the P-donor (114.3 to 123.9°). This seems to be a result of steric repulsion. For complex 1, an interesting case of distortion in the crystalline sample has been observed: the copper atom Cu2′, which is coordinated by triphenylphosphine, two iodide atoms and the pyridine-nitrogen (N1), slips from a tetrahedral to a trigonal-planar coordination situation along with an elongation of the Cu2′−N distance from 2.09 to 2.86 Å and a shortening the Cu1−P-bond from 2.25 to 2.03 Å. The ratio between the main structure and the distorted isomer is approximately 94:6 in solid state. Based on the lack of any copper-distortions whatsoever for the other heteroleptic PyrPHOS complexes in the presented or related36 crystal structures, this results does not seem representative for other heteroleptic PyrPHOScomplexes. Solubility of the Complexes. The solubility of the studied complexes has been investigated with a range of solvents of different polarities. Toluene and chlorobenzene have been chosen as two common solvents used for OLEDs in addition to hexane (low polarity) and ethanol (high polarity). The latter two are rather uncommon due to the insolubility of most OLED-materials in these solvents, therefore a good choice for an orthogonal solvent strategy. Most complexes show an excellent solubility of up to 20 mg mL−1 in toluene and chlorobenzene. Some of the complexes (5, 8) bearing unpolar ligands feature decent solubility even in hexane, while even more examples (2, 3, 5, 7a, 7b, 8, 9, 11, 12a, 12b) could be dissolved in ethanol. This means, that in combination with wellsoluble host materials, these materials can be deposited as emissive layer (EML) on top of hole-transporting layers (HTL), which are only soluble in chlorobenzene or toluene. To add another solution-processed layer on top of such an EML, a cross-linking process should be used to enhance the solvent-stability of the layer. This concept has been previously reported by our group. Compound 7b can be used for this process because of its free butynyl group.14 Table 3 shows the complete results of a solubility study. DFT Calculations. Density functional theory (DFT) calculations of complexes 1, 2, 4, and 7a were carried out using data from X-ray analyses as starting structures. Geometrical parameters for the optimized ground state geometries are in fair agreement with the corresponding values determined by crystallography. The frontier orbitals are similar to the orbitals of homoleptic complexes with the same butterfly shaped Cu2I2 core: The highest occupied molecular orbital (HOMO) is localized mainly on the Cu2I2-core, while the

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RESULTS AND DISCUSSION Synthesis. As mentioned earlier, Cu(I) complexes can be prepared in a one-pot synthesis (Scheme 2). Synthetic details, e.g. suitable solvents for precipitation, and spectroscopic data are collected in the Supporting Information. The formation of the expected complexes has been confirmed by elemental analysis, FAB-MS, IR- and NMR-spectroscopy. Unlike the homoleptic complexes, where the information content is limited because of dynamic processes in solution,37 the stoichiometric relation of the N,P and P-ligands of heteroleptic complexes can be analyzed by 1H and 31P NMR spectroscopy. For some representative examples, X-ray structures could be measured, confirming the proposed structure of the complexes. Most examples herein contain MePyrPHOS as a bridging N,Pligand. We additionally used butynylPyrPHOS in complex 7b and iBuPyrPHOS in 12b (Scheme 3) to prove that this approach is not limited to MePyrPHOS but can also be extended to functionalized PyrPHOS-derivatives.14,30 Scheme 3. PyrPHOS Derivatives Used in This Study

2,6-Biscarbazoyl-pyridine (mCPy) is a well-known host material for OLEDs58−61 and has been used for OLEDs with Cu(I)-complexes before.41,61 A new synthetic route to mCPy has been developed (see Scheme 4 and the Supporting Scheme 4. Synthetic Route to mCPy

Information for details). Carbazole and 2,6-dibromopyridine were coupled using a Cu(I)-catalyst (CuI/DACH) and gave mCPy in good yield. According to the literature, the frontier orbital energies of the host material are 5.7 eV for the HOMO and 2.2 eV for the LUMO.61 Bond lengths are given in angstroms and angles are in degrees; for 7b, the values of the second independent molecule are in italic. Structural Studies. Crystals suitable for X-ray diffraction revealed the butterfly shaped Cu2I2 core unit, which is already known from homoleptic PyrPHOS complexes,14,30,62 for compounds 1, 2, 4, and 7b (Figure 1). PyrPHOS acts as a bridging unit connecting the two copper atoms. Bridging 3418



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Figure 1. Molecular structures of complex 1, 2 (first row), 4, and 7b (second row). For complex 1, the major disordered parts are shown, while only one of the two crystallographic independent molecules 7b is shown (hydrogen atoms, minor disordered parts are omitted for clarity; displacement parameters are drawn at 50% probability level).

Table 3. Solubility of Complexes 1−12b in Nonpolar and Polar Solventsa solvent hexane 1 2 3 4 5 6 7a 7b 8 9 10 11 12a 12b

Figure 2. Molecular structure of the minor disordered part (6%) of complex 1 (hydrogen atoms are omitted for clarity; displacement parameters are drawn at 50% of probability level).

− − − − + − − − − − − − − −

− − − − − − − − − − −

toluene

ethanol

chlorobenzene

−− − + − ++ −− ++ ++ ++ ++ ++ ++ ++ ++

−− + − −− ++ −− − − + − −− − − −

− ++ ++ ++ ++ −− ++ ++ ++ ++ ++ ++ ++ ++

Code: a concentration of 20 mg mL−1, ++/ 10 mg mL−1, +/ 1 mg mL−1, −/Lower solubility, − −. a

lowest unoccupied molecular orbital (LUMO) resides mainly on the bridging N,P-ligand.30 In this study, we varied the monodentate ligands, which influences neither the HOMO nor the LUMO directly. Our

TD-B3LYP calculations indicate that, both at the ground state geometry and at the triplet geometry, the major contribution to the first excited singlet and triplet states is an excitation from 3419



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the HOMO to the LUMO. Therefore, the influence of the monodentate ligands on the emission properties is supposed to be small. The monodentate ligands can, however, change, for example, the packing of the complexes and can thus influence properties like PLQY indirectly. Substitution of the methyl group of the bridging ligand (7a) by a butynyl group (7b) scarcely influences the electronic properties of the ligand and of the resulting complex. Vertical absorption energies were calculated using TD-B3LYP with structures that were optimized in the ground state using BP86. To get emission energies, we optimized the structures in the lowest triplet state with the BP86 functional. Using these geometries we calculated TD-B3LYP excitation energies, which are similar for all compounds, but are red-shifted by up to 1 eV. While the quantitative results of our calculations deviate from the experiment, the qualitative force of expression is high, since the influence of the monodentate donors was predicted correctly. Better absolute values for phosphorescence energies could be obtained using the spin-flip (SF) TDA method,54 where the lowest triplet state is used as a reference state. The data suggests that all compounds have similar photophysical properties (Table 4). Experimental values are given for comparison; they are discussed in more detail below (see Tables 5 and 6).

Table 6. Results from Absorption Spectroscopy λabs [nm] a

1 2

3 272 298 310 4

5

7a 8

9

Table 4. Calculated Measured Absorption and Emission Energies in eV

1 2 4 7a

ΔEexc TDB3LYP

ΔEabs exp.

ΔEem TDB3LYP

S1 = 2.31, T1 = 2.29 S1 = 2.20, T1 = 2.18 S1 = 2.35, T1 = 2.33 S1 = 2.43, T1 = 2.41

3.55

S1 = 1.51, T1 = 1.46 S1 = 1.41, T1 = 1.35 S1 = 1.56, T1 = 1.52 S1 = 1.53, T1 = 1.48

4.00 4.00 4.00

ΔEem SFTDA

ΔEem exp.

2.37

2.43

2.34

2.25

2.48

2.48

2.49

2.45

10

11

12a

12b

Table 5. Physical Propertiesa 1 2 3 4 5 6 7a 7b 8 9 10 11 12a 12b

Tdec (°C)

λem (nm)

ΔEopt (eV)

⟨τ⟩ (μs)

ϕPL

295 272 310 200 300 348 169 b 264 252 227 263 261 255

510 552 555 500 555 547 507 510 534 537 548 555 545 555

2.7 2.5 2.6 2.7 2.6 2.6 2.7 b 2.7 2.6 2.6 2.7 2.6 2.6

1.92 3.73 0.66 1.76 0.62 3.69 2.34 b 0.85 0.96 1.42 1.74 3.00 1.17

0.99 0.74 0.63 0.84 0.40 0.28 0.80 0.56 0.84 0.77 0.45 0.48 0.89 0.58

256 349 256 290 310

272 298 310 249 264 310 256 310 250 276 303 257 295 320 256 290 330 256 297 334 256 296 326 256 290 322

(sh) (sh) (sh) (sh) (sh) 254 (sh) (sh) (sh) 255 (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh)

ε [M cm−1]

assigned transition

35878 15162 53372 25460 19357 86083 51267 21470 13000 81378 30565 16451 10805 25628 14794 5585 25628 8403 28337 14469 9052 45099 20336 10847 51184 24235 9108 61790 26843 7022 43460 22322 7196 40057 20558 8760

LC, Ph, π−π* (X + M)LCT LC, Ph, π−π* (X + M)LCT* (X + M)LCT LC, Ph, π−π* LC (X + M)LCT (X + M)LCT LC, Ph, π−π* LC (X + M)LCT (X + M)LCT LC, Ph, π−π* LC (X + M)LCT LC, Ph, π−π* (X + M)LCT LC, Ph, π−π* LC (X + M)LCT LC, Ph, π−π* (X + M)LCT (X+M)LCT LC, Ph, π−π* (X + M)LCT (X + M)LCT LC, Ph, π−π* (X + M)LCT (X+M)LCT LC, Ph, π−π* (X + M)LCT (X + M)LCT LC, Ph, π−π* (X + M)LCT (X + M)LCT

a Data for this compound have been published.36 absorption spectra were measured in dichloromethane, ± 3 nm. sh = shoulder. The free MePyrPHOS-ligand shows peaks at 256 nm, ε = 9841 M−1 cm−1, 291 nm (shoulder), ε = 3759 M−1 cm−1.

Thermal Properties. Good thermal stability is highly desired for OLED-materials because it is considered to correspond with the overall-stability of the material, therefore the lifetime of the device.65,66 It should also be verified that no decomposition occurs when processing steps are carried out at elevated temperature, for example, during annealing steps. Because of this, thermogravimetric analyses (TGA) was carried out to evaluate the thermal stability. The results are given in Table 3. Depending on the monodentate P-ligands, the decomposition temperature Tdec, which corresponds to a weight loss of 5%, was varied between 169 and 348 °C. Judging from this, no dissociation is expected at the processing temperatures (the highest temperature amounts to 110 °C during the annealing of the emitting layer) for all tested substances. Decomposition does not seem to correspond to the solubility of the complexes; for example, the ill-soluble complex 1 features a slightly lower decomposition temperature than the well-soluble complex 3 (Figure 3). During thermal decomposition, the ligands were removed continuously, no distinct

a

Tdec = decomposition temperature, corresponding to 5% weight loss upon heating in TGA. λem = emission maximum, ±1 nm (powder, 350 nm exc.). ΔEopt = optical bandgap derived by the method of Tauc,67 see Supporting Information for details. ⟨τ⟩ = average emission decay time (powders, bi- or triexponential fit, see the Supporting Information for details, χ2). ϕPL/PLQY = powder samples, ±0.02. bFor 7b, some measurements are not included.

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Figure 4. Emission spectra of all complexes. All spectra were measured from powder samples at 350 nm excitation wavelength.

All compounds show broad charge-transfer emission. Our DFT calculations suggest (X+M)LCT-transitions between the Cu2I2-core and the bridging PyrPHOS-ligands with almost no contribution from the monodentate P-donors. This is supported by the fact that the emission wavelength is influenced by changing the bridging ligand, as can be seen comparing 7a, 7b, 12a, and 12b, respectively. Different monodentate P-ligands probably have no direct influence on the photophysical properties, but can lead to less close-packing or different nonradiative decay properties and can thus affect the emission properties indirectly. The emission color can be modified by changing the N,P-ligand, though. This is discussed in a corresponding publication.36 TCSPC revealed a multiexponential emission decay for all complexes at room temperature. However, a calculation of radiative, nonradiative decay rates from averaged decay times is not accurate and is therefore not done in this work. From the excitation spectra, we derived approximate values for the optical band gap, following the method of Tauc et al.66 (Table 5) The values for ΔEopt vary between 2.5 and 2.7 eV for all compounds. The respective spectra as well as the Tauc plots are given in the Supporting Information in full detail. The average emission decay times τ range between 0.6 and 3.7 μs. It is noteworthy that none of the decay profiles showed monoexponential decay; this is often found for amorphous transition metal compounds showing CT-emission.68 The individual decay times and the fitting parameters can be found in the Supporting Information. The photoluminescence quantum yield varied from 0.28 (complex 6) to 0.99 (complex 1), while most complexes showed efficiencies greater than 0.5 (Table 5). As stated above, the PLQY-values of the pure substances may vary due to different packing of the substituents which are attached to the complex. In a more diluted environment, for example, if the complexes are doped into a host layer, these differences are evened out to a certain degree, assuming that the quenching mechanisms are sufficiently comparable. Table 6 shows the absorption bands of all complexes with values of the extinction coefficients and suggestions for the corresponding transitions. Several absorption bands can be observed in solution, mostly in form of broad, structureless shoulders. All absorption spectra are shown in Figure 5. From the relative extinction coefficient it is apparent that the absolute values of ε depend to a certain

unstructured (∼150 nm full width at half-maximum), which is typical for charge-transfer emission, in agreement with our calculations, vide supra. The deviation of the peak emission seems to be a result of more or less dense packing in amorphous solid state rather than profound electronic effects.62 Recently, we reported how variations of the processing solvent can have a serious impact on properties like the emission maximum, emission decay time and PLQY for homoleptic PyrPHOS-complexes.62 This effect can be explained with a distortion of the rather long-living excited states (τ is in μs range) by interactions with the environment and has been observed for Cu(I) complexes as well as some other metal− organic compounds with long-lived charge transfer emission.30,38 The combined emission spectra are compiled in Figure 4, while separate spectra for each compound are given in the Supporting Information.

Figure 5. Absorption spectra in dichloromethane. All spectra were measured at concentrations of 5 × 10−5 M under ambient conditions.

Figure 3. TGA-traces for complexes 1−12b. The scan rates were 5 °C min−1. The black line marks 95% of the original sample weight.

intermediates were formed, which is mirrored by an illstructured thermogram with shoulders. All thermograms, as well as the experimental details, are given in the Supporting Information. Photophysical Properties. In their solid state, the powder samples of all complexes 1−12b emit in the yellow-to-green region (500−555 nm) of the visible spectrum upon excitation (Figure 4 and Table 5). The emission bands are broad,

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Figure 6. (left) Excitation and emission spectra of complex 9 in solid state. (right) Excitation, absorption, and emission spectra of complex 9 in diluted dichloromethane solution. Solid state spectra were obtained by measuring powder samples. Solution spectra were measured in dichloromethane at approximately 10−5 M under ambient conditions. Excitation spectra were measured by observation of the emission peak wavelength. λexc= 350 nm.

Scheme 5. TADF of Compound 1a

degree on the number of π-systems attached to the monodentate ligands. This issue is discussed in detail in the Supporting Information. The state of aggregation influences on the photophysical behavior of heteroleptic PyrPHOS-complexes. Figure 6 clarifies this effect for complex 9. The emission is red-shifted from 537 nm in neat powder to 608 nm in solution. This can be explained by the less confining environment, that is, the emission decay time is in the chronological order of proteinfolding,69 so the structural relaxation of the excited molecules is likely to be influenced by the greater degree of freedom in solution. Dynamic processes lead to an even broader emission band for the solvated complex. The effects on the excitation spectra are even more distinctive. In the solid state, the spectra are very broad with several shoulders, while the band is narrow in solution with a main band at 320 nm and a small shoulder at 295 nm. A comparison with the absorption spectrum (Table 4 and Figures 5 and 6) reveals that the shape of the emission of the absorption bands is not related with their extinction coefficients. Excitation of the phenyl rings at 256 nm does not contribute to the emission for the solvated molecules, probably due to energy transfer to the solvent molecules. The band and shoulder in the excitation spectra correspond to the shoulders at 320 and 295 nm in the absorption spectra. In the more restricted environment of the solid state, energy loss of the phenyl rings is less likely, which explains why excitation at 254 nm is still resulting in MXLCT-emission. To verify if the aforementioned TADF-mechanism is relevant for our complexes, we measured the emission properties (τ, λem) of compound 1 as an representative example at low temperatures as well as time-resolved emission spectra at room temperature. The details of these experiments and their interpretation are presented in the Supporting Information, while the conclusion is presented in Scheme 5. Upon cooling to 77 K, the emission is red-shifted (about 480 cm−1), because the thermal energy is not high enough for a thermal repopulation of the S1-state. Instead, the emission occurs from the T1 state, which corresponds to an increased emission decay time. These first results prove that compound 1 as a representative example for the other complexes herein in deed shows TADF.

a

The red arrow corresponds to the emission behavior at room temperature, while the blue arrow corresponds to 77 K.

Electrochemical Studies and Absolute Energies of HOMO and LUMO. The electrochemical behavior of complexes 1−12b was investigated by cyclic voltammetry. Experimental details are given in the Supporting Information. Ferrocene was used as internal standard. The data are listed in Table 5: The oxidation of the complexes showed at least two processes: (i) a quasi-reversible or irreversible process Cu(I) → Cu(II) and (ii) the irreversible oxidation of the P-ligands from the process P(III) → P(V). The latter can also be found in the voltammograms of the free ligands at nearly the same potential.70 We used the first oxidation process to obtain values for the ionization potential with Andersson’s method (Table 7).71 Following this method,71−73 EHOMO may be approximated using the anodic peak potential Ep,a by the expression CV − E HOMO = − (Ep,a /V + 4.6)eV

During the reduction of the complexes, dissociation occurs, which leads to the formation of Cu(0), the deposition of a copper film on the working electrode.70,74 Since this does not correspond to the sole reduction of the molecule, the calculation of the LUMO energy from this value cannot be regarded as reliable. 3422



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into optimizing our device architecture for these new emitter materials. Also, the variation of VT was rather large (Table 8), which reflects the high doping concentration (45 wt. %) and the resulting influence of the guest on parameters like energy barriers and layer-conductivity.

Table 7. Electrochemical Properties and Frontier Orbital Energies of Complexes 1−12aa Ep,ac (V) 1 2 3 4 5 7ab 8 9 10 11 12aa

+0.48 +0.45 +0.39 +0.42 +0.49 +0.50 +0.49 +0.50 +0.50 +0.63 +0.36

(pr) (ir) (ir) (ir) (ir) (pr) (pr) (ir) (pr) (pr) (ir)

EHOMOd,e (eV)

ΔEopt (eV)

ELUMOe (eV)

−5.1 −5.1 −5.0 −5.0 −5.1 −5.1 −5.1 −5.1 −5.1 −5.2 −5.0

2.7 2.5 2.6 2.7 2.6 2.7 2.7 2.6 2.6 2.7 2.6

−2.4 −2.2 −2.4 −2.3 −2.5 −2.4 −2.4 −2.5 −2.5 −2.5 −2.4

Table 8. Comparison of EL versus PL Features of Selected Complexes in a mCPy/PVK-Host-Systema 2 3 5 7ab 8 9 10 12ab

a

Cyclic voltammograms were recorded in 0.1 M NBu4PF6 in dichloromethane as a supporting electrolyte at concentrations of 10−3 M. Ferrocene was used as the internal standard. All measurements were done under an Ar atmosphere. bData for compounds 7b and 12b are omitted herein. Cyclic voltammograms for complex 6 could not be recorded because of the poor solubility. cAnodic peak potential, corrected versus the standard calomel electrode. pr = pseudo reversible; ir = irreversible. dEstimation of EHOMO was done with the method from Andersson.71 eELUMO was approximated by adding EHOMO, ΔEopt, frontier orbital energies ±0.2 eV.

source of EL

Vonb (V)

λem,devicec (nm)

λem,modeld (nm)

ϕPLe

complex complex complex host host complex host complex

8.5 9.5 8

547 547 558

545 540 544 535 531 540 542 533

0.51 0.85 0.59 0.35 0.65 0.65 0.38 0.82

11

545

11

550

Both the emission layers in the devices and the films used in the PLexperiments consisted of 45 wt. % complex in 45 wt. % mCPy and 10 wt. % PVK and were dried under the same conditions. The layer thickness was 30 nm. bVon was measured at L = 1.0 cd m−2. For the devices showing only host-emission, no values for Von were determined. cEmission-peak in the EL spectra. dEmission peak in the PL spectra. eϕPL = PLQY:thin films with 45 wt. % mCPy and 10 wt. % PVK, ± 0.02. a

Therefore, LUMO energies were approximated by adding the optical band gaps ΔEopt values (Table 5) on top of the HOMO energy. These values are collected in Table 5 and agree well with our DFT calculations: The ionization potential of 1, 2, 4, and 7a was calculated as the energy difference between the ground state with N electrons and the corresponding cation with N − 1 electrons using the BP86 functional at the optimized ground state structure. This quantity can be compared to −CV−EHOMO values (variation from −5.0 to −5.2 eV) and varies between 5.3−5.8 eV. The results are quite consistent with results for other Cu(I)−phosphine-complexes.70,74−76 The HOMO−LUMO gap calculated using B3LYP at the BP86 optimized ground state structures vary between 2.9 and 3.1 eV, which agrees very well with the measured optical gaps (2.5−2.7 eV). Deviations of both CV− EHOMO (5.0−5.2 eV) and CV−ELUMO (2.2−2.5 eV) are in the same order as the experimental error of the approximation. This similar behavior of all studied complexes justified for the testing and comparison of some of the new complexes in OLED-devices consisting of one set of host and charge transport materials, without having to optimize host and charge transport materials for each complex individually. OLED Device Manufacturing. On the basis of our preliminary experiments, we chose complexes 2, 3, 5, 7a, 8, 9, 10, and 12a to be tested in OLED-devices. We used the stack ITO (120 nm)/PEDOT:PSS (20 nm)/poly-TPD (50 nm)/ complex: mCPy:PVK (30 nm)/TPBi (25 nm)/LiF (1 nm)/Al (200 nm) (Scheme 2). Complexes 1, 4, and 6 were omitted because of their relatively low solubility. To see the influence of the P-donors alone, only complexes with MePyrPHOS were chosen. The obtained device showed in general only moderate performance because of their early development stage. The device with complex 5 as a representative example showed a brightness of 20 cd m−2 at 15 V and an efficiency of 0.1 cd A−1. The turn-on-voltage VT at L = 1 cd m−2 was between 8 and 11 V because of poor charge balance. Further work will be directed

Surprisingly, not all of the complexes showed electroluminescence (EL): The devices containing complexes 7a, 8, and 10 gave blue-emitting devices showing only host-emission according to the EL-spectra. All other complexes in this study gave green-emitting OLEDs, with a peak luminance around 540 nm, as expected. The turn-on voltage of these devices ranged from 8 to 11 V. This is similar to other works using solutionprocessed Cu(I)-complexes.35,77,78 To rationalize the differences between complexes 7a, 8, and 10 and the complexes 2, 3, 5, 9, and 12a, additional experiments were performed: PL-spectra of thin films with the composition of the emitting layers (45 wt. % complex, 45 Scheme 6. Schematic Energy Level Diagram for the Device Made of Complex 2a

a

OLED-stack: ITO (120 nm)/PEDOT:PSS (20 nm)/poly-TPD (30 nm)/complex:mCPy:PVK (30 nm)/TPBi (25 nm)/LiF (1 nm)/Al (200 nm). HOMO and LUMO values of the complexes are given in Table 7. All other frontier orbital energies were taken from published data.79−81 Gray line: PVK, black line: mCPy, dashed line: complex 2.

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CONCLUSION Substitution of the monodentate ligands in homoleptic PyrPHOS complexes with various P-donors resulted in a new class of heteroleptic PyrPHOS complexes with interesting photophysical properties. The (X+M)LCT-character of the emission was hardly affected: All compounds show bright yellow-to-green photoluminescence with PLQY-values between 0.28 and 0.99. The electrochemical properties measured using cyclic voltammetry and calculated using density functional theory are quite similar for all complexes. The new compounds show a broad solubility in solvents of different polarity. Most compounds are thermally stable well beyond 250 °C. Also, our previously introduced autocatalytic cross-linking concept may be used with these compounds. The new materials are wellsuited both for orthogonal solvent approaches, as well as for our new autocatalytic cross-linking-procedure.14 Several OLED devices were constructed. The present results indicate that heteroleptic PyrPHOS complexes are suitable for OLED, even though the performance has to be improved. However, the number of P+III−O bonds should not exceed one per monodentate ligand to prevent a radical-induced degradation of the emissive complexes during device operation. It was shown that the complexes show thermally activated delayed fluorescence, potentially also the singlet harvesting effect. A first optimization step has already been done in a related study, where an OLED with a PyrPHOS-complex similar to complex 2 showed an efficiency of 9 cd A−1 and 1800 cd m−2 with a turn-on voltage of 4.6 V is presented.36 Further optimization of the host and charge-transport materials is ongoing, to enable the manufacturing of solution-processed high-performance OLEDs based on PyrPHOS-complexes.

wt. % mCPy, 10 wt. % PVK) used in the devices were recorded to ensure a complete energy transfer from the host system to the complexes. The results are summarized in Table 8. None of these films showed emission of the host material. The emission spectra were quite comparable to the spectra of the neat powders, while the PLQY-values were lower because of the less rigid environment. This reflects the effects of a more rigid environment. The deviation of the emission maxima was smaller than in the powder experiments. λmax in host−guestfilms laid between 531 and 545 nm (Δλ = 0.06 eV), while the peak wavelengths of the powder samples were between 507 and 555 nm (Δλ = 0.22 eV). (The insoluble complex 4 could not be processed into a thin film and is omitted in these considerations.) This emphasizes the effects of the environment: When putting the complexes in an environment of similar rigidity, the emission is more comparable. Since the energy transfer between host and guest seem to be comparable for all tested complexes, the differences are probably a result of a lower stability. The TGA measurements (Table 5, Figure 3) hinted in this direction: complexes 7a, 8, and 10 are the least thermally stable complexes in this study (Tdec < 230 °C), even though the gap between the “instable” and the “stable” complexes was not very high (Tdec > 252 °C). It can be assumed that the main cause for the differences among the emitters is an electrochemical degradation: all devices showing host-emission contained complexes with P+III− O bonds, such as P(OEt)3. Although these systems usually are considered to be more stable than molecules containing P+III− C bonds, for example, in terms of oxidation, they are vulnerable to rearrangement reactions in the presence of stable free radicals.82 In a nonoptimized OLED-device like in our case, an excess of holes/electrons or radical cations/anions, respectively, is expected to occur in the emission layer. The resulting rearrangement of the ligands leads to a rapid degradation of the emissive species. Another aspect is the fact that due to an excess of holes in the device, the emitting complexes might participate in the charge transport, which is expected to accelerate the degradation. First experiment with single-carrier devices hint in this direction and will be published soon. Complex 9 seems to represent the limit in terms of P−O bonds: The phosphinite ligands containing two P+III−C and only one P+III−O bonds, seems to be more stable than phosphites and phosphonites. A general trend of all devices with Cu(I)-complex-emission was the red shift of the electroluminescence of the OLEDs compared to the photoluminescence of the film-models. This effect has been observed before for other Cu(I) complexes by C. Adachi:77 Different devices with identical emitting layers but different electron transfer layers (ETL) showed a shift of up to 13 nm depending on the choice of the ETL. The high triplet energy (ET = ΔE(T1,S0) of Cu(I)-complexes was given as a possible reason for this behavior: When using ETL-materials with a low triplet energy like TPBi, triplet excitons are not confined in the emitting layer (EML), but some are lost at the ETL-interface. A similar effect seems to be relevant for the present work: According to the onset of the emission spectra at 77 K, the triplet energy of the PyrPHOS complexes described herein is as high as 2.7 eV. This is considerably higher than the triplet energy of TPBi. The substitution of TPBi with other ETL-materials is currently under investigation by our group and is expected to be a key factor for efficient devices with PyrPHOS complexes.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic procedures, chemical characterization data, a more detailed discussion of the absorption spectra, a showcase CV curve, PL spectra of the modeled emission layers, fitting parameters for the emission decay times, crystallography, emission and excitation spectra for all complexes, and Tauc plots. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.B.); [email protected] (S.B.) Fax: (+ 49) 721 608 48581 (S.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Verband der Chemischen Industrie (VCI), the Deutsche Forschungsgemeinschaft (DFG) for funding the transregional collaborative research center SFB/ TRR 88 “3MET” are acknowledged. S.B., D.V. acknowledge the support of the Karlsruhe School of Optics and Photonics (KSOP).

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REFERENCES

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