Phosphorescent Platinum(II) Complexes with Câ§C* Cyclometalated

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Phosphorescent Platinum(II) Complexes with C∧C* Cyclometalated NHC Ligands Thomas Strassner* Physikalische Organische Chemie, Technische Universität Dresden, D-01069 Dresden, Germany CONSPECTUS: This Account describes our achievements toward the development of a new class of platinum(II) complexes with interesting photophysical properties. The general motif of a strongly donating N-heterocyclic carbene with a cyclometalating phenyl group attached to the nitrogen atom together with β-diketonate based counterligands enabled us to synthesize a new class of phosphorescent emitters for use in organic light-emitting diodes (OLEDs). This Account is divided into sections and introduces imidazolium based as well as triazolium based structures and discusses the effects of structural changes on the photophysical properties. Starting from the basic methylated (substituted) phenylimidalium presursors, we initially extended the π-system of the phenyl ring to the dibenzofuran ligand, its regioisomer, and thio-derivative. As the substituents of the β-diketonate ligands turned out to have a strong influence on the photophysical properties (higher quantum yields as well as shorter decay times) a series of dibenzofuranyl-3-methylimidazol as well as diphenylbenzimidazol platinum complexes were synthesized to investigate the different steric and electronic effects, which are described in a separate section. The next section of the Account then describes other extensions of the π-system. Exchange of the methyl group against a phenyl ring, as well as the extension of the π-system in the backbone of the NHC-ligand lead to a significant improvement of the photophysical properties, which reached a maximum for the diphenylbenzimidazole (DPBIC) system. Further extension of the π-system to the diphenylnaphthylimidazol then lead to a unfavorable long decay time. The effect of substitution is discussed for cyano groups, which change the electronic situation and lead to highly emissive complexes. We are currently working on studying the effect of other substituents on the photophysical properties, as well as the introduction of additional heteroatoms into the general motif. Our initial work in that area had been on 1,2,4-triazole complexes. For the basic phenyl/methyl substituted system, two different isomers are accessible, the 4-phenyl-4H-1,2,4-triazoles as well as the 1-phenyl-1H-1,2,4 triazoles. It was interesting to note that the photophysical properties of the corresponding complexes are strongly dependent on the substituent R of the β-diketonate ligand. For R = methyl, the properties are significantly different, while we found almost identical photophysical results for R = mesityl for both 1,2,4-triazole isomers. The last section describes the synthesis of bimetallic complexes. To investigate whether it is possible to cyclometalate twice into the same phenyl ring, we synthesized dicationic NHC precursors from para- and metadisubstituted bis(imidazole)benzenes. The bimetallic complexes show interesting photophysical properties with quantum yields of up to 93%. All experimental work was accompanied by quantum chemical calculations, which turned out to be very useful for the prediction of the emission wavelengths as well as the interpretation of the emissive states of the platinum complexes.

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play a key role for the design of these new lighting materials.4−6 OLEDs are built from different thin layers as shown in Figure 1. The formation of excitons by recombination of electrons and holes in the recombination layer leads to phosphorescent emission if suitable transition metal complexes are doped into this layer. After the initial report7 of the green phosphorescent tris(2phenylpyridine)iridium complex, Ir(ppy)3, cyclometalated 2phenylpyridines and derived motifs have been investigated in detail in the past decade.8−12 One example, the bidentate 2(4,6-difluorophenyl)pyridyl (N∧C) ligand, is shown in Figure 2, together with an acetylacetonate (acac) ligand at a platinum center. Variations of the general motif include more rigid terdentate, N∧C∧N,13 and tetradentate, N∧C∧C∧N,14 ligands.

INTRODUCTION The development of more energy efficient lighting is one of the important tasks to reduce the worldwide energy consumption, which just for lighting is close to 20% of the total energy demand. By improving the efficiency of the light sources, a significant reduction of the global energy demand can be envisioned. During the past decade, extremely energy efficient light-emitting diodes (LEDs) have been developed, which has recently been honored by the 2014 Nobel Prize in physics.1 But contrary to the incandescent light from light bulbs, the directional light of LEDs is limited when it comes to lighting applications. One concept that is currently seen as the next generation lighting is the development of organic light-emitting diodes (OLEDs).2 They also allow for new designs of lighting in the form of wallpapers as they can be produced in larger areas. In 1998,3 it was recognized that the incorporation of transition metal complexes allows for quantum yields beyond the fluorescence limit of 25%. These phosphorescent emitters © 2016 American Chemical Society

Received: May 20, 2016 Published: December 2, 2016 2680

DOI: 10.1021/acs.accounts.6b00240 Acc. Chem. Res. 2016, 49, 2680−2689

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Accounts of Chemical Research

Table 1. Photoluminescence (PL) Data (2% in PMMA)21 λexca H Br NO2 OMe Me

320 310 355 355 315

CIE x, yb 0.190, 0.171, 0.362, 0.167, 0.161,

0.190 0.163 0.469 0.157 0.131

λemc

ϕd

τ0e

416, 441, 464 418, 446 546 456 419, 445

7 5 11 32 20

91 24 25

a

Excitation wavelength [nm]. bCIE coordinates at rt. cEmission wavelength [nm]. dQuantum yield in %, excited with λexc, N2 atmosphere. eDecay lifetimes [μs] (excited by laser pulses (355 nm, 1 ns)) given as τ0 = τv/ϕ.

Scheme 1. Synthesis of Different para-Substituted C∧C*Cyclometalated Platinum(II) Complexes17

Figure 1. Layered structure of a typical OLED device.

Figure 2. Cyclometalated 2-phenylpyridine based motifs.

Based on an efficient spin−orbit coupling they show good quantum yields, short decay times, and high stability, but the majority emit in the green to red part of the spectrum, and the need for a phosphorescent blue emitter has become obvious. To shift the color coordinates of the N-coordinated complexes, stronger donating ligands have been developed. Maintaining the concept of cyclometalation, the nitrogen atoms were replaced by stronger donating N-heterocyclic carbene carbon atoms (C∧C*).

Frequently we could also get crystals suitable for solid state structure determination20 and studied their photoluminescence (PL) data. With the exception of the nitro-substituted complex, all complexes show very similar absorption spectra. The emission spectra show structured emission bands in the blue-green part of the spectrum with maxima around 450−460 nm, again with the exception of the nitro-substituted complex, which shows a pronounced bathochromic shift of the emission with an unstructured band at around 550 nm. As examples, the very different emission spectra of the methyl- and the nitrosubstituted compound are given in Figure 4. We believe that this is due to aggregate formation and a MMLCT (metal metalto-ligand charge transfer) and used concentration-dependent measurements, but the origin of the shift could not be determined without a doubt. To investigate their photophysical properties, the absorption and emission spectra were generally measured in amorphous poly(methyl methacrylate) (PMMA) films at room temperature with 2% emitter concentration. We found that, with the exception of the electron-withdrawing nitro group, all complexes showed emissions in the deep blue part of the spectrum. Changing the substituent in the 4-position had a significant effect on the observed quantum yield as well as on the decay lifetime (Table 1). Compared to the unsubstituted complex (QY = 7%), one methoxy group changed the observed quantum yield to 32%.

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CYCLOMETALATED C∧C* PLATINUM COMPLEXES The first published example of a platinum(II) complex with a cyclometalated N-heterocyclic carbene (NHC) ligand15,16 and acac ligand is shown in Figure 3. We17 and others18,19 started to study this new class of complexes in 2009 and investigated the effect of different substituents in the para-position.

Figure 3. C∧C*-cyclometalated platinum complex.

We observed green-blue phosphorescence with emission wavelengths of 416−546 nm and moderate quantum yields of 5−32% (Table 1). These cyclometalated (NHC)PtII(acac) complexes can be synthesized by reaction of the corresponding imidazolium salts with silver(I) oxide. Without isolation, the resulting intermediate is transmetalated onto dichloro(1,5cyclooctadiene)-platinum(II). After changing the solvent (from dioxane/butanone to DMF), an excess of 2,4-pentanedione and potassium-tert-butanolate is added (Scheme 1). These complexes with electron-donating and -withdrawing substituents could be synthesized in yields between 19% and 57% and have been characterized by standard techniques including 195Pt NMR and two-dimensional NMR methods.17

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DIBENZOFURAN AND -THIOPHENE LIGANDS When we used a ligand with a different skeleton and extended π-system, the quantum yield improved to 90% for the [1(dibenzo[b,d]furan-4-yl)-3-methylimidazolin-2-ylidene]acetylacetonato platinum(II) complex shown in Figure 5. The dibenzofuran (DBF) NHC complex showed a strong emission at 463 and 497 nm with a decay time of 23 μs, which turned out to be independent of the temperature of the measurement. The emission spectra measured at 5, 77, 100, 200, and 300 K are given in Figure 5. 2681


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Figure 6. ORTEP representation of the [1-(dibenzo[b,d]-thiophen-4yl)-3-methyl-1H-imidazolin-2-ylidene-κC,κC′](2,4-pentanedionatoκO,κO′)platinum(II) complex. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å) and angles and dihedral angles (deg): Pt(1)−C(1) 1.954(4); Pt(1)−C(5) 1.975(4); Pt(1)−O(1) 2.091(3); Pt(1)−O(2) 2.056(3); O(1)−Pt(1)−O(2) 89.60(13); C(1)−Pt(1)−C(5) 80.47(18); C(4)−N(1)−C(1)-Pt(1) 1.3(5); N(1)−C(1)−Pt(1)-O(1) 176.3(3).

Scheme 2. Synthesis of Different DBF Isomersa

Figure 4. Absorption spectra of cyclometalated (NHC)PtII(acac) complexes together with the emission spectra of the methyl (Me) and nitro (NO2) substituted complexes.21

Conditions: (i) nBuLi, C2H4Br2; (ii) BTMA·ICl2·ZnCl2; (iii) Cu2O, KOH, DMSO, (benz)imidazole; (iv) CH3I, THF, reflux. a

Bromination in 4-position is the result of a deprotonation/ lithiation at −40 °C followed by metal−halogen exchange at −78 °C using 1,2-dibromoethane.17 The use of benzyl trimethylammonium dichloroiodate (BTMA·ICl2) and zinc chloride results in the iodination in 2-position of the dibenzofuran. The halogenated dibenzofurans can be coupled in a copper(I) catalyzed Ullmann-type reaction to generate the corresponding (benz)imidazoles, which can be methylated using an excess of iodomethane in tetrahydrofuran (THF) at elevated temperatures (Scheme 2). The platinum complexes shown in Figure 7 were synthesized following our previously reported general route. The photophysical properties of the two isomeric complexes turned out to be very different. Especially

Figure 5. C∧C*-cyclometalated (DBF-NHC)PtII(acac) complex and its emission spectra at different temperatures.

The standard device built for this emitter (DBF-NHC)PtII(acac) is described in detail in the literature.17 Different measurements have been carried out which can be summarized in the following way: the electroluminescence spectra are showing their maxima at 480 nm and a pronounced progression of the emission band independent of the dopant concentration, while the maximum luminance is strongly concentration dependent and can be found between 4900 cd m−2 (13 V, 6% doping) and 6750 cd m−2 (13.2 V, 12% doping). At the highest doping concentration of 12% also the maximum external quantum efficiency (EQE) of 6.2% is reached. The corresponding dibenzothiophene (DBT) complex (Figure 6) was synthesized in an analogous way but showed inferior photophysical properties (QY = 63%, τ = 31 μs).22 Depending on the halogenation procedure of the dibenzofuran, two different isomers can be synthesized (Scheme 2).

Figure 7. (DBF_BIM) and iso-(DBF_BIM) platinum(II) complexes with sterically demanding mesacac ligands. 2682


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Accounts of Chemical Research in combination with a variation of the substituents at the βdiketonate, which will be described in the next paragraph in detail, emitters with high quantum yields and comparatively short decay times could be synthesized (Figure 7). The isomers differ significantly in the decay time, which is lower by more than 50% for the iso-(DBF_BIM)PtII(mesacac) complex, while emission wavelengths and quantum yields are comparable (Table 2). Table 2. PL Data (2% in PMMA) of the Cyclometalated Benzimidazole Mesacac Platinum Complexes21 DBF_BIM iso-DBF_BIM

λexca

CIE x, yb

λemc

ϕd

τ0e

370 340

0.180, 0.427 0.175, 0.268

475 479

84 80

22.4 10.8

Figure 9. Emission spectra of the (DBF_BIM)Pt(mesacac) and iso(DBF_BIM)Pt(mesacac) isomers (2% in PMMA).21

a

Excitation wavelength [nm]. bCIE coordinates at rt. cEmission wavelength [nm]. dQuantum yield in %, excited with λexc, N2 atm. e Decay lifetimes [μs] (excited by laser pulses (355 nm, 1 ns)) given as τ0 = τv/φ.

Scheme 3. Influence of the β-Diketonate Counterligand

Both isomers (Figure 7) could be characterized by solid state structures.23 Figure 8 shows the solid state structure of the iso(DBF_BIM) complex.

Table 3. PL Data (2% in PMMA) of Different β-Diketonate Complexes21 R

λexca

CH3 t Bu Ph Mes

355 355 370 335

CIE x, yb 0.162, 0.160, 0.336, 0.165,

0.314 0.318 0.565 0.333

λemc

Φd

τ0e

463, 497 465 530 466

90 83 51 91

23 27.4 4.7 18.9

a Excitation wavelength [nm]. bCIE coordinates at rt. cEmission wavelength [nm]. dQuantum yield in %, excited with λexc, N2 atmosphere. eDecay lifetimes [μs] (excited by laser pulses (355 nm, 1 ns)) given as τ0 = τv/ϕ.

Figure 8. ORTEP representation of the iso-DBF complex. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å) and angles and dihedral angles (deg): C(1)−Pt(1), 1.926(8); C(9)−Pt(1), 1.970(7); O(2)−Pt(1), 2.082(5); O(3)−Pt(1), 2.059(5); C(1)−Pt(1)−C(9), 80.4(3); O(2)−Pt(1)−O(3), 89.5(2); C(8)−N(1)−C(1)−Pt(1), −0.8(8).

emission profile with three distinct peaks, the phenyl groups (Ph) led to a significant red shift of the emission with a single broad band. According to our DFT calculations, the phenyl ring rotates into the plane of the acac core in the triplet state, thereby extending the π-system. This allows for an increased delocalization and a red shift of the emission. The emission profile of the complex with mesacac ligand also changed to one broad band, reduced the decay time to 19 μs, and kept the wavelength of the emission and the quantum yield (Table 3). We therefore studied the influence of the β-diketonate ligands with aromatic groups also for other ligand motifs and could see an overall improvement of the photophysical properties when methyl-substituted aromatic β-diketonates were used. DFT calculations (B3LYP/6-31G*) confirm that substituents in the ortho-positions force the aryl group in an orthogonal position, while the unsubstituted phenyl ring shows a dihedral angle of 30° between the acetylacetonate plane and the phenyl ring. The calculated singlet state geometries of the (DBF-NHC)PtII(phacac) and (DBF-NHC)PtII(mesacac) are given in Figure 10. Another example is the series of complexes we synthesized using the 1,3-diphenylbenzo[d]imidazolin-2-ylidene (DPBIC) ligand with five different β-diketonates (Scheme 4, Figure

As can be seen from the emission spectra in Figure 9, the maximum emission wavelength is very similar for both isomers, but the profile is significantly different. The different electronic structure of both isomers also had an influence on the decay time of the emission (Table 2).

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VARIATION OF THE COUNTERLIGAND MOTIF The [1-(dibenzo[b,d]furan-4-yl)-3-methylimidazolin-2-ylidene] platinum complex (Figure 5) was then used to investigate the effect of different β-diketonates (Scheme 3).24 Depending on the auxiliary ligand, the complexes emit either in the deep-blue or the orange-red part of the spectrum (Table 3). We found that by changing the CH3 groups (QY 90%) to CF3 groups (QY 0%), the emission was completely shut off. Obviously the CF3 groups deactivate the complex by shifting the electronic levels so thoroughly that no emission could be observed any more. While the sterically demanding tert-butyl groups did not significantly change the emission data and the structured 2683


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Figure 10. Calculated singlet state geometries (B3LYP/6-31G*) of two aryl substituted DBF-NHC complexes. Hydrogen atoms have been omitted for clarity.

Scheme 4. Synthesis of C∧C*-Cyclometalated (DPBIC)PtII Complexes with Different β-Diketonates

Figure 12. Emission spectra of DPBIC platinum(II) complexes with different β-diketonates.21

Table 4. PL Data (2% in PMMA) of Different DPBIC βDiketonate Complexes21

11).25 Next to the regular acetylacetonate (R = Me), we used the 2,2,6,6-tetramethylheptane-3,5-dione (R = tBu), 1,3-

R

λexca

CH3 t Bu Ph Mes Duryl

340 355 375 330 355

CIE x, yb 0.158, 0.157, 0.300, 0.178, 0.171,

0.147 0.161 0.559 0.256 0.224

λemc

Φd

τ0e

457 461 520 473 465

41 54 78 81 81

9.2 7.8 4.8 4.2 4.4

a


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EXTENSION OF THE π-SYSTEM Obviously there are various extensions of the π-systems possible, starting with using benzimidazole instead of imidazole or a naphthyl ligand instead of the phenyl system (Figure 13).27

Figure 11. DPBIC platinum(II) complexes with different substituted β-diketonates. Figure 13. C∧C*-cyclometalated (NHC)PtII(acac) complex.

diphenylpropane 1,3-dione (R = Ph), 1,3-dimesitylpropane1,3-dione (R = Mes), and 1,3-bis(2,3,5,6-tetramethylphenyl)propane-1,3-dione (R = Duryl) in the synthesis of the corresponding platinum DPBIC complexes. The mesacac ligand was synthesized by a Friedel−Crafts-reaction using malonyl dichloride, AlCl3, and 1,3,5-trimethylbenzene following a reported procedure.25 The Duryl ligand was synthesized in a similar reaction with 1,2,4,5-tetramethylbenzene.26 Figure 12 shows the resulting emission spectra of the DPBIC complexes, which clearly demonstrate the influence of the βdiketonate ligand on the emission process. Also in this case, the emission profile changes from a structured emission for the alkyl substituted β-diketonates to a single broad emission band for the aromatic auxiliary ligands. As can be seen from the data in Table 4, the aromatic βdiketonate ligands together with the DPBIC ligand significantly reduce the decay lifetimes to 4 μs, while the measured quantum yields are only slightly lower than in the DBF case and emit in the same region around 470 nm.

The synthesis of the precursor salts could be accomplished by different synthetic strategies described in Scheme 5. For the benzimidazole and naphthyl ligands with two phenyl rings in 1,3-position (DPBIC, DPNIM), we used the amines as starting material and closed the ring using triethylorthoformiate and ammonium tetrafluoroborate. For the MPBIM ligand, Cu(I) catalyzed coupling chemistry was followed by methylation using iodomethane, while for the MNBIM ligand, Pd(0) catalysis followed by the above-described ring closure reaction with HC(OCH2CH3)3 were successful (Scheme 5). Using the established synthesis, the corresponding platinum complexes (Figures 13 and 14) could be synthesized in good yields.27 But not only this extension of these π-systems is possible: a delocalized system could also be added in the 3-position of the imidazole core, examples are given in Figure 14. 2684


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Accounts of Chemical Research Scheme 5. Synthesis of Ligand Precursors with Extended πSystemsa

Table 5 summarizes the photophysical results for the measurements in PMMA films. The highest quantum yield Table 5. PL Data (2% in PMMA) of Different Acetylacetonate Complexes21 λexca DPIM MPBIM MNBIM DPBIC DPNIM

355 330 355 340 370

CIE x, yb 0.155, 0.197, 0.235, 0.158, 0.323,

0.110 0.238 0.458 0.147 0.628

λemc

Φd

τ0e

446 454 476 457 515

17 40 58 41 81

18.3 8.0 183.0 9.2 404.6

a


was found for DPNIM with the largest π system accessible, but the emission maximum is shifted to 515 nm and exhibits an extremely long decay time of more than 400 μs. But even the addition of only one phenyl ring (DPIM) compared to the basic methyl substituted phenyl imidazole increases the quantum yield from 7%17 for the methyl substituted complex to nearly 17% for DPIM. The addition of a phenyl ring in the backbone of the N-heterocyclic carbene (MPBIM) leads to an even stronger effect by increasing the quantum yield to 40%.

Conditions: (i) Cu2O, KOH, DMSO, Ar, 120 °C, 24 h; (ii) MeI, THF, 100 °C, 24 h; (iii) Pd(0), base, toluene, reflux, 15 h; (iv) HC(OCH2CH3)3, NH4BF4, reflux, 6 h.

a

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SUBSTITUTION BY CYANO GROUPS There are many aromatic systems with electron-donating and -withdrawing groups accessible via different synthetic routes. We initially started with electron-withdrawing cyano groups; the first C∧C*-cyclometalated complex with good photophysical properties carried a cyano group in 4-position (Figure 3). Phenylimidazolium salts with cyano substituents, the 1-(4cyanophenyl)-3-isopropyl-1H-benzo[d]imidazolium tetrafluoroborate (IPrBICN) and the 1-(4-cyanophenyl)-3-methyl-4-(1methylprop-2-yl)-1H-imidazolium iodide (TBuICN) were synthesized; the synthetic pathways are given in Scheme 6.

Figure 14. C∧C*-cyclometalated (NHC)PtII(acac) complexes.

As expected, the change of the π-system was reflected in the absorption and emission spectra. The complexes shown in Figures 13 and 14 have been measured in 2% PMMA films and exhibit at least three distinct maxima (Figure 15).

Scheme 6. Synthesis of Ligand Precursors with Cyano Groupsa

Figure 15. Emission spectra of C∧C*-cyclometalated (NHC)PtII(acac) complexes with extended π-systems.21 Conditions: (i) acetone, DCM, AcOH; 0 °C, BH3SMe2; (ii) Pd/C, H2, MeOH, rt; (iii) 4-fluorobenzonitrile, Pd2(dba)3, Xantphos; base, toluene, 80 °C; (iv) HC(OCH2CH3)3, NH4BF4, reflux; (v) 4fluorobenzonitrile, NaH, DMF; (vi) MeI, THF, 2 d, rt. a

For the complexes with the smallest π extension, DPIM and MPBIM, the first observed emission peak lies in the deep blue region at around 450 nm, with vibronic progressions toward the green region of the spectrum. When the π-systems become larger, the corresponding emission maxima are red-shifted with the exception of DPBIC, which remains a true blue emitter (CIE 0.158, 0.147). With increasing size of the π system the color changes to higher wavelengths can be explained by the resulting stabilization of the LUMO and thus a smaller energy gap between the singlet ground state and the first excited emissive state.

For the synthesis of the IPrBICN ligand, we started from the 2nitroaniline and introduced the isopropyl group by reduction of the imine, while for the benzonitrile, Pd(0) coupling chemistry was used followed by standard ring closure conditions. In the case of TBuICN, the benzonitrile was introduced via substitution of the fluoro derivative and quaternization by 2685


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Accounts of Chemical Research methyl iodide, which allowed for the synthesis of the ligand in two steps. To investigate the effect of the aromatic β-diketones on this class of compounds, we synthesized two complexes for each of the ligand systems, one with the regular acetylacetonate and one with the mesityl derivative. Three of the complexes could also be characterized by solid state structures (Figure 16). The

Figure 17. Emission spectra of cyano substituted platinum βdiketonate complexes.21

aryl halides functionalized in 4-position was found to be the most convenient route29 (Scheme 7).

Figure 16. Solid-state structures of platinum complexes with cyano substituents. Thermal ellipsoids are drawn at 50% probability. Selected bond lengths [Å] and angles [deg]: (IPrBICN)Pt(acac): Pt1−C1 1.940(4); Pt1−C9 1.969(4); Pt1−O1 2.087(3); Pt1−O2 2.059(3); C1−Pt1−C9 80.46(18); O1−Pt1−O2 89.35(13); Pt1−C1−N1−C8− 1.5(5); N1−C1−Pt1−O1 177.1(3); (IPrBICN)Pt(mesacac): Pt1−C1 1.954(4); Pt1−C9 1.986(4); Pt1−O1 2.083(3); Pt1−O2 2.047(3); C1−Pt1−C9 80.26(15); O1−Pt1−O2 90.22(11); Pt1−C1−N1−C8 3.5(4); N1−C1−Pt1−O1 171.1(3); TBuICN)Pt(acac): Pt1−C1 1.957(7); Pt1−C9 1.999(7); Pt1−O1 2.082(5); Pt1−O2 2.030(4); C1−Pt1−C9 80.4(3); O1−Pt1−O2 89.26(17); Pt1−C1−N1−C8 4.2(8); N1−C1−Pt1−O1 171.2(5).

Scheme 7. Synthesis of 1-Phenyl-1H-1,2,4-triazole (i) and 4Phenyl-4H-1,2,4-triazole (ii)a

Conditions: (i) Cu2O, K2CO3, phen, DMF, Ar, 100 °C, 48 h; (ii) 180 °C, 3 h.

a

Table 6. PL Data (2% in PMMA) of Cyano Substituted Complexes21 λexca (IPrBICN)Pt(acac) (TBuICN)Pt(acac) (IPrBICN)Pt(mesacac) (TBuICN)Pt(mesacac)

330 355 370 370

CIE x, yb 0.158, 0.164, 0.164, 0.166,

0.161 0.216 0.212 0.229

λemc 444, 443, 442, 443,

473 474 471 471

Φd

τoe

63 57 80 85

15.5 17.0 8.9 8.4

Quaternization, for example, with methyl iodide then gives the corresponding 1,2,4-triazolium salts in high yields followed by the synthesis of the platinum(II) complexes, which were accessible via a similar route as described for the imidazolin-2ylidene complexes. Two different series of complexes based on the two different motifs were reported in the literature.30 The additional nitrogen atom caused significant changes in the photophysical properties of the two different triazole complexes compared to the imidazole based complexes shown before (Scheme 1). For the unsubstituted 1-phenyl-3methylimidazolin-2-ylidene platinum(II) acetylacetonate a very low quantum yield of 7% (λmax = 441 nm) was measured (Table 1), while for the 4-phenyl-1-methyl-1,2,4-triazol-5ylidene platinum(II) acetylacetonate (Figure 18, R2 = CH3, R3 = H), a quantum yield of 11% (λmax = 431/456 nm) was observed. For the asymmetric 1-phenyl-4-methyl-1,2,4-triazol5-ylidene platinum(II) acetylacetonate (Figure 18, R1 = H, R2 =

a

Excitation wavelength [nm]. bCIE coordinates at room temperature. Emission wavelength [nm]. dQuantum yield in %, excited with λexc, N2 atmosphere; eDecay lifetimes [μs] (excited by laser pulses (355 nm, 1 ns)) given as τo = τv/ϕ. . c

photophysical data given in Table 6 again confirm the beneficial effect of the mesityl groups as the quantum efficiency improved by 20% and also the decay time was significantly shorter while the emission wavelengths and emission profile (Figure 17) did not change.

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TRIAZOLE-BASED COMPLEXES28 The C∧C*-cyclometalated systems described above are all based on different 1-arylimidazolin-2-ylidene ligands. We also changed the electronic structure in the direct vicinity of the metal by introducing additional nitrogen atoms in the backbone of the five-membered ring. This can be achieved either via 1aryl-1,2,4-triazol-5-ylidene or 4-aryl-1,2,4-triazol-5-ylidene ligands. The 4-aryl triazoles are accessible from the respective anilines by reaction with diformyl hydrazine as described in the literature,29 while for the synthesis of the 1-aryl triazoles a modified Ullmann reaction protocol from 1H-1,2,4-triazole and

Figure 18. Platinum(II) β-diketonate complexes based on 4-phenyl4H-1,2,4-triazoles (left) and 1-phenyl-1H-1,2,4-triazoles (right). 2686


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Accounts of Chemical Research Table 7. PL Data (2% in PMMA) of Triazole Based Complexes21 R1

R3

λexca

H Cl Me OMe

330 325 340 335 325 330 330

H Cl OMe

λemc

CIE x, yb 0.169, 0.192, 0.174, 0.166, 0.193, 0.169, 0.165,

0.194 0.272 0.200 0.180 0.261 0.227 0.201

405, 411, 421, 428, 432, 470 435,

431, 436, 445, 455, 459,

456 465, 493 468 483 488

462, 489

Φd

τ0e

11 12 14 30 41 38 38

15 9 18 13 11 16 14

Excitation wavelength [nm]. CIE coordinates at room temperature. Emission wavelength [nm]. dQuantum yield in %, excited with λexc, N2 atmosphere. eDecay lifetimes [μs] (excited by laser pulses (355 nm, 1 ns)) given as τ0 = τv/ϕ. . a

b

c

Scheme 8. Synthesis of Dicationic NHC Precursorsa

CH3), we even observed a QY of 30% (λmax = 428/455/483 nm) and a comparable decay time (Table 7). For the acetylacetonate (Figure 18, R2 = CH3) complexes, it was obvious that the asymmetric (red colored) triazole ligand led to significantly better results. Independent of the substitution (Figure 18, R3 = Cl, OCH3, CH3) quantum yields between 30% and 40% were measured, more than twice the quantum yield found for the complexes with 4-phenyl triazole ligands. The emission spectra of the acetylacetonate complexes show a well-structured emission with three maxima (Table 7), which indicates strong contributions of ligand centered states to the emission process. When we extended our research program to triazole complexes with mesityl substituted acetylacetonates (Figure 18, R2 = Mes) the differences disappeared.31 As expected the photophysical properties improved significantly, quantum yields more than doubled to 82%, and the decay time was reduced by a factor of 3 to 4 μs. But to our surprise the difference between the two triazole motifs disappeared. Both complexes gave almost identical results with emission wavelengths of 477 and 478 nm. The emission spectra of the mesacac complexes reveal a different emission behavior. In contrast to the vibronic structure observed for the acac complexes, we now see an unstructured emission with only one maximum. According to our DFT calculations another transition contributes to the emission process in the case of the mesacac ligand. The chargetransfer transition involves the metal center, which explains the improved quantum yields and reduced decay times. We also looked at different substituents in the 4-position of the aromatic ring, but they showed only minor effects on the emission wavelengths (H 478 nm; OCH3 477 nm; CH3 475 nm) with 82% quantum yield and decay times between 3 and 5 μs. This confirms the change of the emission process to a transition with reduced contributions of ligand centered states.

Conditions: (i) Cu2O, imidazole, DMSO, Ar, 48 h, 130 °C; (ii) MeI or BnBr, THF, reflux.

a

Scheme 9. Bimetallic Complexes

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BIMETALLIC COMPLEXES To evaluate the influence of a second platinum atom in the same system, a series of bimetallic complexes with the general formula [Pt(NHC)(L)]2Ph, which are composed of two [Pt(NHC)(β-diketonate)] fragments, were synthesized.32 These fragments are cyclometalated to a central phenyl ring either in meta- or para-position. The synthesis of the dicationic precursors could be accomplished by standard copper coupling conditions in DMSO using the 1,3- or 1,4-diiodobenzene followed by quarternization with methyl iodide or benzyl bromide. Scheme 8 describes the synthesis of the parasubstituted 1,4-bis(imidazole)benzene (p-BIB) as well as the dicationic NHC precursors 1,4-bis(3-methylimidazolium)-

benzene (BMIB) iodide and 1,4-bis(3-benzylimidazolium)benzene (BBzIB) bromide. The established route of the reaction with Ag2O and subsequent transmetalation with Pt(COD)Cl2 followed by treatment with an excess of the β-diketonate ligand and potassium tert-butanolate lead to the formation of the corresponding platinum(II) complexes in relatively low yields 2687


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succeeded in growing single crystals and could unequivocally confirm the structure of the bimetallic compound (Figure 20).

Table 8. PL Data (2% in PMMA) of the Bimetallic Complexes21 Bu2BMIM_Pt2 Mes2BMIM_Pt2 Bu2BBzIM_Pt2

λexca

CIE x, yb

λemc

Φd

τ0e

355 355 340

0.233, 0.537 0.255, 0.544 0.240, 0.552

486, 521 489; 523 487; 524

86 76 93

10.9 8.8 10.4

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QUANTUM CHEMICAL CALCULATIONS The experimental work was accompanied by density functional theory calculations, which have been extremely useful to identify potentially interesting ligand motifs. We developed a methodology to predict the emission wavelengths of the metal−organic complexes.33 Standard density functional theory calculations (B3LYP/6-31G(d), BP86/6-31G(d) together with a Hay−Wadt-ECP for platinum, Gaussian09) allowed us to screen in silico for promising structures. We then synthesized only those complexes that were predicted to emit in the blue/ green part of the spectrum, which was of interest to us. This screening is based on the calibrated vertical energy differences between the fully optimized singlet and triplet states and the singlet state energy of the optimized triplet structure (ΔE = HF(T1 − S0(T1)). We also calculated the various transitions to analyze the contribution of the various states and to characterize the emission.34

a

Excitation wavelength [nm]. bCIE coordinates at room temperature. Emission wavelength [nm]. dQuantum yield in %, excited with λexc, N2 atmosphere. eDecay lifetimes [μs] (excited by laser pulses (355 nm, 1 ns)) given as τ0 = τv/ϕ. c

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Figure 19. Emission spectra of bimetallic C∧C*-cyclometalated platinum complexes with different β-diketonates.21

Notes

The author declares no competing financial interest. Biography Thomas Strassner was born in Nuremberg, Germany. He received his diploma and Ph.D. in organic chemistry from the Friedrich-AlexanderUniversity (FAU) in Erlangen. After two years as a lecturer in Dresden, he went to UCLA to work on metal−organic reaction mechanisms using density functional theory with Prof. Ken Houk. In 1998, he started his independent career in inorganic chemistry at the TU Munich (associated with Prof. W.A. Herrmann) before accepting his current position at the TU Dresden. His research interests cover a wide area from the catalytic activation and oxidation of hydrocarbons, cross-coupling reactions to photophysically active materials, and a new generation of ionic liquids. Quantum chemistry and mechanistic studies are used in all fields of research.

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ACKNOWLEDGMENTS First I acknowledge the students who have contributed to the project over the last years. I am grateful to the OLED group of BASF for the collaboration, the scientific discussions, and the measurement of the photophysical data. Support by the BMBF (FKZ: 13N10477) is gratefully acknowledged.

Figure 20. Solid-state structure of 1,4-bis[(3-benzyl-1H-imidazolin-2ylidene-κC2)(2,2,6,6-tetra-methyl-heptan-3,5-dionato-κO,κO′)platinum(II)](κN,κPt,κN′,κPt′)benzene (Bu2BMIM_Pt2). Thermal ellipsoids are drawn at 50% probability. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Pt1−C1 1.943(3); Pt1−C5 1.987(3); Pt1−O1 2.086(2); Pt1−O2 2.051(2); C1−Pt1−C5 80.40(12); O1−Pt1−O2 89.82(8); C4−N1−C1−Pt1 −4.1(4); N1−C1−Pt1−O1 −174.1(2);.

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ABBREVIATIONS p-BIB, 1,4-bis(imidazole)benzene; BMIB, 1,4-bis(3-methylimidazolium)benzene; BBzIB, 1,4-bis(3-benzylimidazolium)benzene; COD, 1,5-cyclooctadiene; DBF, dibenzofuran; DMF, dimethylformamide; DMSO, dimethyl sulfoxide; DPBIC, 1,3-diphenylbenzo[d]imidazolin-2-ylidene; DPIM, 1,3-diphenylimidazolin-2-ylidene; DPNIM, 1,3-diphenylnaphtho[d]imidazolin-2-ylidene; Duryl, 1,3-bis(2,3,5,6-tetramethylphenyl)propane-1,3-dione; EQE, external quantum efficiency; IPrBICN, 1-(4-cyanophenyl)-3-isopropyl-1H-benzo[d]imidazolin-2-ylidene; mesacac, 1,3-dimesitylpropane-1,3dione; MNBIM, 3-methyl-1-naphthylbenzo[d]imidazolin-2-ylidene; MPBIM, 3-methyl-1-phenylbenzo[d]imidazolin-2-yli-

between 6% and 21%, but the complexes shown in Scheme 9 showed very good photophysical data given in Table 8. The emission spectra of these complexes are given in Figure 19. It is interesting to note that in these cases we see neither different emission profiles for the alkyl- and the mesitylacetylacetonates nor large differences in the photophysical data. Due to the low solubility of the bimetallic complexes, it was very difficult to get crystals suitable for solid state structure determination, but in case of the Bu2BMIM_Pt2 complex, we 2688


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Cyclometalated [Pt(Ĉ C*) (acac)] Complexes. Angew. Chem., Int. Ed. 2010, 49, 10214−10216. (18) Petretto, G. L.; Wang, M.; Zucca, A.; Rourke, J. P. Platinum(ii) N-heterocyclic carbene complexes: coordination and cyclometalation. Dalton Trans. 2010, 39, 7822−7825. (19) Fuertes, S.; Chueca, A. J.; Sicilia, V. Exploring the Transphobia Effect on Heteroleptic NHC Cycloplatinated Complexes. Inorg. Chem. 2015, 54, 9885−9895. (20) Tenne, M.; Unger, Y.; Strassner, T. (Acetylacetonato-κ2 O,O′)[1-(4-bromophenyl-κC2)-3-methylimidazol-2-ylidene-κC2] platinum(II). Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2012, 68, m203−m205. (21) Measured at room temperature. (22) Tronnier, A.; Risler, A.; Langer, N.; Wagenblast, G.; Muenster, I.; Strassner, T. A Phosphorescent C∧C* Cyclometalated Platinum(II) Dibenzothiophene NHC Complex. Organometallics 2012, 31, 7447− 7452. (23) Tronnier, A.; Wagenblast, G.; Muenster, I.; Strassner, T. Phosphorescent platinum(II) complexes with C∧C* cyclometalated NHC dibenzofuranyl ligands: impact of different binding modes on the decay time of the excited state. Chem. - Eur. J. 2015, 21, 12881− 12884. (24) Tronnier, A.; Nischan, N.; Metz, S.; Wagenblast, G.; Muenster, I.; Strassner, T. Phosphorescent C∧C* Cyclometalated PtII Dibenzofuranyl-NHC Complexes - An Auxiliary Ligand Study. Eur. J. Inorg. Chem. 2014, 2014, 256−264. (25) Zhang, C.; Yang, P.; Yang, Y.; Huang, X.; Yang, X.-J.; Wu, B. High-yield synthesis of 1,3-dimesityl-propane-1,3-dione. Isolation of its aluminum complex as a stable intermediate. Synth. Commun. 2008, 38, 2349−2356. (26) Tronnier, A.; Heinemeyer, U.; Metz, S.; Wagenblast, G.; Muenster, I.; Strassner, T. Heteroleptic platinum(II) NHC complexes with a C∧C* cyclometalated ligand - synthesis, structure and photophysics. J. Mater. Chem. C 2015, 3, 1680−1693. (27) Tronnier, A.; Poethig, A.; Metz, S.; Wagenblast, G.; Muenster, I.; Strassner, T. Enlarging the π system of phosphorescent (C∧C*) cyclometalated platinum(II) NHC complexes. Inorg. Chem. 2014, 53, 6346−6356. (28) Tenne, M. Photophysically active platinum(II)-NHC-complexes, Ph.D. thesis, TU Dresden, 2015. (29) Meyer, D.; Strassner, T. 1,2,4-Triazole-Based Tunable Aryl/ Alkyl Ionic Liquids. J. Org. Chem. 2011, 76, 305−308. (30) Tenne, M.; Metz, S.; Muenster, I.; Wagenblast, G.; Strassner, T. Phosphorescent Platinum(II) Complexes Based on C∧C* Cyclometalating Aryltriazol-5-ylidenes. Organometallics 2013, 32, 6257− 6264. (31) Tenne, M.; Metz, S.; Wagenblast, G.; Muenster, I.; Strassner, T. C∧C* cyclometalated platinum(II) N-heterocyclic carbene complexes with a sterically demanding β-diketonato ligand - synthesis, characterization and photophysical properties. Dalton Trans. 2015, 44, 8444− 8455. (32) Tronnier, A.; Strassner, T. (C∧C*) Cyclometalated binuclear Nheterocyclic biscarbene platinum(ii) complexes - highly emissive phosphorescent emitters. Dalton Trans. 2013, 42, 9847−9851. (33) Unger, Y.; Strassner, T.; Lennartz, C. Prediction of the emission wavelengths of metal-organic triplet emitters by quantum chemical calculations. J. Organomet. Chem. 2013, 748, 63−67. (34) Powell, B. J. Theories of phosphorescence in organo-transition metal complexes - From relativistic effects to simple models and design principles for organic light-emitting diodes. Coord. Chem. Rev. 2015, 295, 46−79.

dene; OLED, organic light-emitting diode; phacac, 1,3diphenyl-propane-1,3-dione; Phen, phenanthroline; PL, photoluminescence; PMMA, poly(methyl methacrylate); Pt(COD)Cl2, dichloro(1,5-cyclooctadiene)-platinum(II); TBuICN, 1-(4cyanophenyl)-3-methyl-4-(1-methylprop-2-yl)-imidazolin-2-ylidene; TLC, thin layer chromatography; QY, quantum yield

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