Scholarly article on [(5-(diphenylphosphino)-1-pentyl-1H-1,2,4-triazole)(triphenylphosphine)2Cu<sub>2</sub>I<sub>2</sub>] 1435949-30-6 from Chemistry of Materials p. 4471

Article

pubs.acs.org/cm

Heteroleptic, Dinuclear Copper(I) Complexes for Application in

Organic Light-Emitting Diodes

Daniel M. Zink,^†,‡Daniel Volz,^†,‡Thomas Baumann,*^,‡Mathias Mydlak,^‡Harald Flugge,^‡

̈

Jana Friedrichs,^‡Martin Nieger,^§and Stefan Brase*^,†,∥

̈

^†Institute of Organic Chemistry, Karlsruhe Institute of Technology, KIT Campus South, Fritz-Haber-Weg 6, Karlsruhe D-76131,

Germany

^‡cynora GmbH, Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldshafen D-76344, Germany

^§Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55 (A.I. Virtasen aukio 1), Helsinki

FIN-00014, 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: A series of highly luminescent, heteroleptic copper(I)

complexes has been synthesized using a modular approach based on

easily accessible P^N ligands, triphenylphosphine, and copper(I)

halides, allowing for an independent tuning of the emission

wavelength with low synthetic eﬀorts. The molecular structure has

been investigated via X-ray analysis, conﬁrming a dinuclear copper(I)

complex consisting of a butterﬂy shaped metal-halide cluster and two

diﬀerent sets of ligands. The bidentate P^N ligand bridges the two

metal centers and can be used to tune the energy of the frontier

orbitals and therefore the photophysical characteristics, as conﬁrmed

by emission spectroscopy and theoretical investigations, whereas the two monodentate triphenylphosphine ligands on the

periphery of the cluster core mainly inﬂuence the solubility of the complex. By using electron-rich or electron-poor heterocycles

as part of the bridging ligand, emission colors can be adjusted, respectively, between yellow (581 nm) and deep blue (451 nm).

These complexes have been further investigated in particular with regard to their photophysical properties in thin ﬁlms and

polymer matrix as well as in solution. Furthermore, the suitability of this class of materials for being applied in organic light-

emitting diodes (OLEDs) has been demonstrated in a solution-processed device with a maximum current eﬃciency of 9 cd/A

and a low turn-on voltage of 4.1 V using a representative complex as an emitting compound.

KEYWORDS: copper, luminescence, cluster compounds, bridging ligands, P^N ligands, copper complexes, Cu(I) complexes,

heteroleptic complexes, organic light-emitting diodes, dinuclear complexes

After the pioneering work of McMillin and several other

groups, mononuclear cationic copper(I) complexes have

become well-known for their geometry change from a

pseudotetrahedral coordination of the copper ion to a square

planar coordination in the excited state. This ﬂattening

distortion is due to a formal oxidation from Cu(I) to Cu(II)

upon excitation of these kinds of complexes.³⁷⁻⁵¹This large

structural rearrangement is regarded as the major quenching

pathway of the emission and facilitates solvent coordination

and exciplex formation, thus resulting in additional quenching

pathways and leading to low quantum yields in solution.⁵³⁻⁵⁶

By increasing the structural rigidity, these radiationless

deactivation pathways can be reduced in mononuclear

copper(I) complexes, leading to strongly enhanced quantum

1. INTRODUCTION

Organic light-emitting diodes (OLEDs) are highly attractive

candidates for lighting and display applications because they

oﬀer numerous advantages such as enhanced eﬃciency and the

possibility to be manufactured on thin and lightweight

substrates.¹⁻⁶Luminescent metal complexes containing Ir, Pt,

or Os are widely used emitters for OLEDs not only because of

their color tunability but also because of their high

eﬃciencies.⁷⁻²⁶However, these noble metals are rather scarce

and expensive, which complicates their usage in high-volume

productions. To enter mass market applications, luminescent

complexes based on more abundant and cost-eﬃcient metals

with a d¹⁰conﬁguration, such as Cu(I), Ag(I), or Au(I), are

currently in the focus of numerous research studies.²⁷⁻³¹

Among them, copper(I) complexes are favored not only

because of their varied structural and photophysical properties

but also because of the fact that they have already been

successfully tested in OLEDs.³²⁻⁵²

Received: April 27, 2013

Revised: October 17, 2013

A

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yields.^48,50,51Rigid clusters containing more than one copper(I)

atmosphere at room temperature. After stirring for 24 h at room

temperature, the reaction mixture was quenched with water (100 mL)

and extracted with dichloromethane (3 × 80 mL). The organic layers

were combined, washed with brine (80 mL), and dried over MgSO₄×

H₂O, and the solvent was removed under vacuum. The crude products

were used without further puriﬁcation.

ion are supposed to show a similar behavior.^46,57−59

Recently a series of highly luminescent, homoleptic dinuclear

copper(I) complexes was presented bearing three P^N ligands

in the ligand sphere, which allows for a tuning of the emission

color of the complexes by changing the electronic character-

istics of the ligands.³²These complexes feature high quantum

yields³²and are currently in the stage of device testing, resulting

in high-performance OLEDs.⁶⁰

After this analysis of the basic characteristics of these

homoleptic complexes, two studies have been performed to

investigate the inﬂuence of the single- and double-bound

ligands. The ﬁrst study aimed at minimizing the synthetic eﬀort,

by replacing the single-bound bidentate ligands on the

periphery of the complex with commercially available

phosphines, leading to new heteroleptic complexes that allow

for an independent tuning of solubility and color. Various

monodentate phosphines bearing solubility-enhancing groups

were introduced, allowing for a ﬁne-tuning of the solubility in

diﬀerent polar and apolar solvents, which is necessary for the

production of solution-processed OLEDs. This study was

recently published separately by Volz et al.⁶¹

In this consecutive study, the inﬂuence of the bidentate

bridging ligand on the photophysical properties is investigated.

A series of highly luminescent heteroleptic complexes is

presented on the basis of a dinuclear copper(I) halide core, a

bridging P^N ligand, and two triphenylphosphines as ancillary

ligands. The structures of the complexes were revealed by X-ray

structure analysis, conﬁrming that the P^N ligand is bridging

the two copper atoms, whereas the phosphines act as

monodentate ligands. Density functional theory (DFT)

calculations were performed showing that the highest occupied

molecular orbital (HOMO) is mainly located on the copper(I)

halide core, whereas the lowest unoccupied molecular orbital

(LUMO) mainly resides on the bridging P^N ligand, thus

allowing for an easy color tunability of the complexes. By

changing the electronic characteristics of the bridging P^N

ligand, the luminescence properties can be ﬁne-tuned to cover

almost the whole visible spectrum from deep blue (451 nm) to

yellow (581 nm). In addition, the suitability of this modular

emitter system for electronic applications has been demon-

strated in a ﬁrst solution-processed OLED device.

2.2.2. General Procedure for the Phosphorylation of Alkylated

Compounds 19−21, Yielding Compounds 6−8. A solution of the

alkylated triazole precursor (8.67 mmol, 1.00 equiv) in dry

tetrahydrofuran (20 mL) under a nitrogen atmosphere was cooled

to −80 °C. To this solution, lithium diisopropylamine (1.8 M in

tetrahydrofuran, 9.54 mmol, 1.10 equiv) was added dropwise via

cannula. Afterward, the reaction was allowed to warm to 0 °C slowly.

Subsequently, the reaction mixture was cooled to −80 °C again, and

chlorodiphenylphosphane (9.54 mmol, 1.10 equiv) dissolved in

tetrahydrofuran (10 mL) was added dropwise via cannula. The

reaction mixture was allowed to warm to room temperature overnight,

quenched with water (2 mL), and ﬁltered through a plug of silica gel.

Puriﬁcation via ﬂash column chromatography yielded the product.

2.2.3. General Procedure for the Introduction of Alkyl (a) or

Aryl⁶⁷(b) Substituents at Imidazoles or Benzoimidazoles, Yielding

Compounds 22−25. (a) To a solution of the appropriate

benzoimidazole or imidazole (50.0 mmol, 1.00 equiv) and sodium

hydroxide (55.0 mmol, 1.10 equiv) in dimethyl sulfoxide (100 mL)

was added the alkyl halide (50.0 mmol, 1.00 equiv), and the reaction

mixture was stirred at 40 °C overnight. Afterward, the reaction was

quenched with water (200 mL). The aqueous layer was extracted with

ethyl acetate (3 × 100 mL), and the combined organic layers were

washed with water (3 × 100 mL) and dried over MgSO₄× H₂O.

Removing of the solvent under reduced pressure yielded the crude

product, which was used without further puriﬁcation. (b) Benzoimi-

dazole or imidazole (20.0 mmol, 1.00 equiv), 1,10-phenanthroline

(4.00 mmol, 20 mol %), copper(I) iodide (2.00 mmol, 10 mol %),

cesium carbonate (30.0 mmol, 1.50 equiv), and the corresponding aryl

iodide or bromide (20.0 mmol, 1.00 equiv) were dissolved under a

nitrogen atmosphere in dimethylformamide (35 mL) and stirred at

110 °C for 3 h. The reaction mixture was ﬁltered through a plug of

silica gel, and the crude product was further puriﬁed by ﬂash column

chromatography.

2.2.4. General Procedure for the Phosphorylation of Compounds

22−25, Yielding Compounds 12−16.⁶⁸The appropriate alkylated or

arylated benzimidazole or imidazole (15.2 mmol, 1.00 equiv) was

dissolved in tetrahydrofuran (50 mL) under a nitrogen atmosphere

and cooled to −78 °C, and n-butyllithium (2.5 min n-hexane, 16.7

mmol, 1.10 equiv) was added dropwise via cannula. The reaction

mixture was stirred at −78 °C for 30 min followed by stirring at room

temperature for 1 h. Afterward, the reaction mixture was cooled to

−78 °C again, chlorodiphenylphosphane (16.0 mmol, 1.05 equiv)

dissolved in tetrahydrofuran (10 mL) was added dropwise via cannula,

and the reaction mixture was allowed to warm to room temperature

overnight. After quenching with water (3 mL), the mixture was ﬁltered

through a plug of silica gel. Puriﬁcation via ﬂash column

chromatography yielded the product.

2. EXPERIMENTAL SECTION

2.1. General Procedures. Oven-dried glassware was used to

perform all reactions under a nitrogen atmosphere. Solvents as well as

chemicals employed were bought from commercial suppliers and

applied without further puriﬁcation. For thin-layer chromatography

(TLC), silica gel plates (Silica gel 60, F254, Merck) were used with

detection by UV. Preparative chromatography for puriﬁcation was

performed using normal-phase silica gel (Silica gel 60, 230−400 mesh,

Merck).

2.2. Synthesis of P^N-Type Ligands. Ligands 1−5 were

synthesized according to ref 32, and ligand 9 was synthesized

according to ref 33. Precursors 28 and 29 were synthesized following

the procedures reported by Kawano et al.⁶²Spectroscopic data of

ligands 1,⁶³2,³²3,³²4,³²9,⁶⁴and 10⁶⁵have already been published in

the literature; for spectroscopic data of ligands 5−8 and 11−16, see

the Supporting Information.

2.2.5. General Procedure for the Synthesis of Compounds 10 and

11. To a solution of oxadiazole (28 or 29) (3.61 mmol, 1.00 equiv)

and triethylamine (14.4 mmol, 4.00 equiv) in pyridine (6 mL)

chlorodiphenylphosphane (4.00 mmol, 1.10 equiv) was added

dropwise via cannula under a nitrogen atmosphere, and the reaction

mixture was stirred at room temperature for 24 h. After quenching

with hydrochloric acid (1 M, 50 mL), the aqueous layer was extracted

with dichloromethane (3 × 40 mL). The combined organic layers

were dried over MgSO₄× H₂O, and the solvent was removed under

reduced pressure. Puriﬁcation via ﬂash column chromatography

yielded the product.

2.2.6. General Procedure for the Synthesis of Complexes 1-I−15-

I, 1-Br, and 1-Cl. The copper(I) halide salt (1.00 mmol, 2.00 equiv),

the P^N ligand (0.50 mmol, 1.00 equiv), and the phosphine (1.00

mmol, 2.00 equiv) were suspended in dry dichloromethane (10 mL)

under nitrogen. The reaction mixture was stirred for 12 h at room

temperature, and the appropriate complex was puriﬁed afterward

2.2.1. General Procedure for the Alkylation of 1,2,4-Triazoles,

Yielding Compounds 19−21.⁶⁶To a stirred solution of 1H-1,2,4-

triazole (18) (29.0 mmol, 1.00 equiv) and the appropriate alkyl halide

(30.4 mmol, 1.05 equiv) in tetrahydrofuran (25 mL) was added 1,8-

diazabicyclo[5.4.0]undec-7-ene (34.8 mmol, 1.20 equiv) dissolved in

tetrahydrofuran (5 mL) dropwise via cannula under a nitrogen

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through precipitation with diethyl ether, pentane, or cyclohexane (100

mL). The precipitated solid was ﬁltered oﬀ, further puriﬁed by washing

with the appropriate solvent, and dried under vacuum.

[(1-Benzyl-5-(diphenylphosphino)-1H-1,2,4-triazole)(PPh₃)₂Cu₂I₂]

(8-I). Yield: 854 mg, 0.68 mmol, 78%; white powder. IR (ATR) v_max

:

̅

3050 (vw), 1479 (w), 1433 (m), 1093 (w), 740 (m), 721 (w), 691

(m), 516 (m), 504 (m) cm⁻¹. MS (FAB) m/z (%): 325 (44), 406

(100), 587 (70), 668 (80), 777 (28), 860 (27), 1120 (5). Anal. Calcd

for C₅₇H₄₈Cu₂I₂N₃P₃(1247.0): C, 54.82; H, 3.87; N, 3.36. Found: C,

54.67; H, 3.78; N, 3.28.

[(2-(Diphenylphosphino)pyridine)(PPh₃)₂Cu₂I₂] (1-I). Yield: 969

mg, 0.83 mmol, 87%; yellow powder. IR (ATR) v_max: 3044 (vw),

̅

1478 (w), 1433 (w), 1091 (w), 996 (w), 740 (m), 691 (m), 489 (m),

435 (w), 418 (w) cm⁻¹. MS (FAB): because of the insolubility of the

compound, no mass spectra could be measured. Anal. Calcd for

C₅₃H₄₄Cu₂I₂NP₃(1166.9): C, 54.47; H, 3.79; N, 1.20. Found: C,

54.39; H, 3.79; N, 0.93.

[(4-(Diphenylphosphino)-1,5-diphenyl-1H-1,2,3-triazole)-

(PPh₃)₂Cu₂I₂] (9-I). Yield: 329 mg, 0.25 mmol, 81%; white powder. IR

(ATR) v_max: 3044 (vw), 1480 (w), 1433 (m), 1156 (w), 1094 (m),

̅

1028 (w), 998 (w), 773 (w), 740 (m), 690 (s), 608 (w), 520 (m), 502

(s), 486 (m), 438 (w) cm⁻¹. MS (FAB) m/z (%): 1255 (12), 1065

(1), 920 (5), 850 (11), 730 (5), 658 (12), 587 (10). Anal. Calcd for

C₆₂H₅₀Cu₂I₂N₃P₃(1309.0): C, 56.81; H, 3.84; N, 3.21. Found: C,

56.88; H, 3.85; N, 3.09.

[(2-(Diphenylphosphino)pyridine)(PPh₃)₂Cu₂Br₂] (1-Br). Yield: 977

mg, 0.91 mmol, 80%; yellow powder. IR (ATR) v_max: 1479 (w), 1451

̅

(w), 1434 (w), 1092 (w), 1027 (w), 997 (w), 741 (m), 692 (m), 512

(m), 499 (m), 489 (m), 435 (w), 417 (w) cm⁻¹. MS (FAB): because

of the insolubility of the compound, no mass spectra could be

measured. Anal. Calcd for C₅₃H₄₄Cu₂Br₂NP₃(1071.0): C, 59.23; H,

4.13; N, 1.30. Found: C, 58.91; H, 4.28; N, 1.03.

[(2-(Diphenylphosphino)-5-phenyl-1,3,4-oxadiazole)(PPh₃)₂Cu₂I₂]

(10-I). Yield: 844 mg, 0.68 mmol, 90%; yellow powder. IR (ATR) v_max

:

̅

3051 (vw), 1552 (w), 1480 (w), 1433 (w), 1342 (vw), 1094 (w), 1027

(w), 994 (w), 742 (m), 692 (s), 541 (m), 519 (m), 507 (m), 488 (m),

427 (w) cm⁻¹. MS (FAB): because of the insolubility of the

compound, no mass spectra could be measured. Anal. Calcd for

C₅₆H₄₅Cu₂I₂N₂OP₃(1233.9): C, 54.43; H, 3.67; N, 2.27. Found: C,

54.45; H, 4.07; N, 1.97.

[(2-(Diphenylphosphino)pyridine)(PPh₃)₂Cu₂Cl₂] (1-Cl). Yield: 842

mg, 0.85 mmol, 75%; yellow powder. IR (ATR) v_max: 3045 (vw), 1479

̅

(w), 1434 (w), 1094 (w), 742 (m), 692 (s), 513 (m), 500 (m), 435

(w), 419 (w) cm⁻¹. MS (FAB) m/z (%): 950 (1), 785 (2), 687 (17),

587 (45), 424 (12), 327 (8), 325 (30), 262 (35). Anal. Calcd for

C₅₃H₄₄Cl₂Cu₂NP₃(983.1): C, 64.57; H, 4.50; N, 1.42. Found: C,

64.22; H, 4.51, N, 1.16.

[(2-(Diphenylphosphino)-5-p-tolyl-1,3,4-oxadiazole)(PPh₃)₂Cu₂I₂]

(11-I). Yield: 986 mg, 0.79 mmol, 91%; yellow powder. IR (ATR) v_max

:

̅

[(2-(Diphenylphosphino)-4-methylpyridine)(PPh₃)₂Cu₂I₂] (2-I).

2969 (vw), 1497 (w), 1480 (w), 1434 (w), 1094 (w), 1078 (w), 741

(w), 692 (m), 625 (vw), 520 (w), 507 (m), 487 (w) cm⁻¹. MS (FAB)

m/z (%): 1121 (<1), 943 (1), 861 (2), 777 (2), 669 (8), 597 (10).

Anal. Calcd for C₅₇H₄₇Cu₂I₂N₂OP₃·1.5 CH₂Cl₂: C, 51.02; H, 3.66; N,

2.03. Found: C, 51.31; H, 3.71; N, 2.00.

Yield: 555 mg, 0.87 mmol, 75%; yellow powder. IR (ATR) v_max

:

̅

3047 (vw), 1479 (w), 1433 (w), 1090 (w), 1026 (w), 997 (w), 827

(w), 740 (m), 692 (m), 512 (m), 490 (m), 462 (w) cm⁻¹. MS (FAB):

because of the insolubility of the compound, no mass spectra could be

measured. Anal. Calcd for C₅₄H₄₆Cu₂I₂NP₃(1181.0): C, 54.84; H,

3.92; N, 0.83. Found: C, 54.45; H, 4.04; N, 0.79.

[(2-(Diphenylphosphino)-4,5-dimethylthiazole)(PPh₃)₂Cu₂I₂] (12-

I). Yield: 1.12 g, 0.93 mmol, 92%; yellow powder. IR (ATR) v_max: 3048

̅

(vw), 1478 (w), 1432 (w), 1360 (vw), 1092 (w), 1027 (vw), 998 (vw),

849 (vw), 742 (w), 692 (m), 518 (w), 506 (w), 492 (m), 447 (w), 419

(w) cm⁻¹. MS (FAB): because of the insolubility of the compound, no

mass spectra could be measured. Anal. Calcd for C₅₂H₄₆Cu₂I₂NP₃S·

CH₂Cl₂: C, 50.36; H, 3.76; N, 1.09; S, 2.49. Found: C, 50.31; H, 3.96;

N, 0.86; S, 2.33.

[(2-(Diphenylphosphino)-4-propylpyridine)(PPh₃)₂Cu₂I₂] (3-I).

Yield: 912 mg, 0.75 mmol, 76%; yellow powder. IR (ATR) v_max

:

̅

3049 (vw), 2959 (vw), 1478 (w), 1458 (w), 1433 (w), 1390 (w), 1090

(w), 1027 (w), 997 (w), 742 (m), 692 (m), 508 (m), 486 (m) cm⁻¹.

MS (FAB) m/z (%): 1055 (4), 863 (2), 750 (3), 630 (4), 560 (6), 368

(11). Anal. Calcd for C₅₆H₅₀Cu₂I₂NP₃(1209.0): C, 55.55; H, 4.16; N,

1.16. Found: C, 55.53; H, 4.18; N, 0.83.

[(2-(Diphenylphosphino)-1-p-tolyl-1H-imidazole)(PPh₃)₂Cu₂I₂]

(13-I). Yield: 1.31 g, 1.05 mmol, 90%; white powder. IR (ATR) v_max

:

̅

[(2-(Diphenylphosphino)-4-isobutylpyridine)(PPh₃)₂Cu₂I₂] (4-I).

3048 (vw), 1505 (w), 1479 (w), 1432 (m), 1305 (w), 1127 (w), 1092

(w), 1027 (w), 998 (w), 823 (w), 739 (m), 691 (s), 617 (w), 566 (w),

519 (m), 504 (s), 472 (m) cm⁻¹. MS (FAB) m/z (%): 1130 (5), 860

(<1), 787 (5), 595 (5), 405 (16). The crystal structure analysis showed

the presence of one molecule of dichloromethane in the crystal.

Therefore, the complex was precipitated and recrystallized again

leading to a reduction of the incorporated dichloromethane. However,

an elemental analysis without cocrystallized dichloromethane could

not be achieved. Anal. Calcd for C₅₈H₄₉Cu₂I₂N₂P₃·0.25CH₂Cl₂

(1246.0): C, 55.13; H, 3.93; N, 2.21. Found: C, 55.26; H, 4.02; N,

1.88.

Yield: 1.85 g, 1.51 mmol, 96%; yellow powder. IR (ATR) v_max: 3050

̅

(vw), 2954 (vw), 1597 (vw), 1479 (w), 1434 (w), 1385 (vw), 1363

(vw), 1094 (w), 1074 (w), 742 (w), 692 (m), 515 (m), 476 (w) cm⁻¹.

MS (FAB) m/z (%): 1274 (11), 1083 (36), 893 (10), 836 (21), 763

(38), 644 (58), 572 (83), 515 (9), 382 (100), 324 (22). Anal. Calcd

for C₅₇H₅₂Cu₂I₂NP₃(1223.0): C, 55.89; H, 4.28; N, 1.14. Found: C,

55.53; H, 4.22; N, 0.82.

[(2-(Diphenylphosphino)-4-(3-(2-methoxyethoxy)propyl)-

pyridine)(PPh₃)₂Cu₂I₂] (5-I). Yield: 880 mg, 0.68 mmol, 65%; yellow

powder. IR (ATR) v_max: 3048 (vw), 2867 (vw), 1479 (w), 1433 (m),

̅

1093 (m), 1027 (w), 998 (w), 741 (m), 692 (m), 516 (m), 430 (w)

cm⁻¹. MS (FAB) m/z (%): 1202 (7), 894 (7), 824 (2), 704 (5), 632

(10), 587 (18), 442 (10), 380 (2), 325 (8). Anal. Calcd for

C₅₉H₅₆Cu₂I₂NO₂P₃(1283.0): C, 55.15; H, 4.39; N, 1.09. Found C,

54,75; H, 4.34; N, 0.74.

[(2-(Diphenylphosphino)-4-methyl-1-p-tolyl-1H-imidazole)-

(PPh₃)₂Cu₂I₂] (14-I). Yield: 590 mg, 0.47 mmol, 83%; white powder. IR

(ATR) v_max: 3047 (vw), 1508 (w), 1478 (w), 1433 (m), 1398 (w),

̅

1179 (vw), 1150 (vw), 1092 (w), 1028 (w), 997 (w), 821 (w), 742

(m), 693 (m), 503 (m), 490 (m), 424 (w) cm⁻¹. MS (FAB) m/z (%):

1156 (2), 965 (<1), 800 (<1), 419 (4). Anal. Calcd for

C₅₉H₅₁Cu₂I₂N₂P₃: C, 56.16; H, 4.07; N, 2.22. Found: C, 55.78; H,

4.08; N, 2.05.

[(5-(Diphenylphosphino)-1-propyl-1H-1,2,4-triazole)(PPh₃)₂Cu₂I₂]

(6-I). Yield: 1.11 g, 0.93 mmol, 91%; white powder. IR (ATR) v_max

:

̅

3049 (w), 1480 (w), 1432 (w), 1094 (w), 1028 (w), 744 (m), 693 (s),

670 (w), 519 (m), 506 (s), 493 (m), 452 (w) cm⁻¹. MS (FAB):

because of the insolubility of the compound, no mass spectra could be

measured. Anal. Calcd for C₅₃H₄₈Cu₂I₂N₃P₃(1199.0): C, 53.01; H,

4.03; N, 3.50. Found: C, 52.77; H, 4.05; N, 3.11.

[(2-(Diphenylphosphino)-5,6-dimethyl-1-octyl-1H-benzo[d]-

imidazole)(PPh₃)₂Cu₂I₂] (15-I). Yield: 542 mg, 0.34 mmol, 74%; white

powder. IR (ATR) v_max: 3053 (vw), 2927 (vw), 1480 (w), 1434 (w),

̅

1094 (w), 1028 (vw), 998 (vw), 872 (vw), 836 (vw), 742 (w), 693

(m), 519 (m), 490 (w), 463 (w), 433 (w) cm⁻¹. MS (FAB) m/z (%):

1329 (5), 948 (3), 768 (9), 695 (12), 587 (2), 506 (19), 443 (15).

Anal. Calcd for C₆₅H₆₅Cu₂I₂N₂P₃(1346.1): C, 57.91; H, 4.86; N, 2.08.

Found: C, 57.60; H, 4.79; N, 1.93.

[(5-(Diphenylphosphino)-1-pentyl-1H-1,2,4-triazole)(PPh₃)₂Cu₂I₂]

(7-I). Yield: 879 mg, 0.72 mmol, 77%; white powder. IR (ATR) v_max

:

̅

3049 (vw), 2927 (vw), 1479 (vw), 1431 (m), 1093 (w), 1027 (w), 997

(w), 741 (m), 691 (m), 520 (m), 504 (m), 490 (m), 461 (w), 422 (w)

cm⁻¹. MS (FAB) m/z (%): 1414 (8), 1091 (30), 901 (25), 766 (41),

648 (33), 578 (95), 386 (100), 324 (83). Anal. Calcd for

C₅₅H₅₂Cu₂I₂N₃P₃(1227.0): C, 53.76; H, 4.27; N, 3.42. Found: C,

53.52; H, 4.34; N, 3.51.

[(2-(Diphenylphosphino)-5,6-dimethyl-1-p-tolyl-1H-benzo[d]-

imidazole)(PPh₃)₂Cu₂I₂] (16-I). Yield: 1.06 g, 0.80 mmol, 84%; white

powder. IR (ATR) v_max: 3047 (vw), 1513 (w), 1479 (w), 1433 (m),

̅

1400 (w), 1158 (w), 1093 (w), 1027 (w), 995 (w), 887 (w), 850 (w),

C

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811 (w), 740 (w), 690 (s), 520 (m), 506 (s), 488 (m) cm⁻¹. MS

(FAB) m/z (%): 903 (10), 864 (15), 745 (18), 675 (54), 587 (15)

482 (100). Anal. Calcd for C₆₄H₅₅Cu₂I₂N₂P₃(1324.0): C, 57.97; H,

4.18; N, 2.11. Found: C, 57.96; H, 4.32; N, 1.80.

measuring the ferrocene/ferrocenium redox couple. Andersson’s

method was used to obtain values for the absolute HOMO energy

out of the ﬁrst oxidation process.⁸²According to the method of Tauc,

values for the LUMO energy can be approximated by adding the

values for the HOMO and the optical band gaps.⁸³

2.3. X-ray Crystallography. The crystal structure determinations

of 2-I, 11-I, 12-I, 13-I, 14-I, and 16-I were performed on a Bruker-

Nonius Kappa CCD diﬀractometer at 123(2) K, a Bruker SMART

APEXII diﬀractometer at 120(2) K (16-I), and a Bruker Apex Duo

diﬀractometer at 120(K) (13-I) using Mo Kα radiation (λ = 0.71073

Å). Crystal data, data collection parameters, and results of the analyses

are listed in Table S1 in the Supporting Information. Direct methods

were used for structure solution (SHELXS-97),⁶⁹and reﬁnement was

carried out using SHELXL-97 (full-matrix least-squares on F²).⁶⁹

Hydrogen atoms were reﬁned using a riding model. A semiempirical

absorption correction using equivalent reﬂections was applied.

In 2-I, the Cu(2) atom is disordered (s.o.f. = 0.94). In 11-I, there

are four voids around the center of symmetry with 1.5 disordered

molecules CH₂Cl₂in each void. The disorder could not be solved, and

the diﬀuse electron density was treated using SQUEEZE (module in

PLATON).⁷⁰

2.4. Photophysical Measurements. A Horiba Scientiﬁc

FluoroMax-4 spectroﬂuorometer equipped with a 150 W xenon-arc

lamp, excitation and emission monochromators (1200 grooves/nm

blazed at 330 nm (excitation) and 500 nm (emission)), a Hamamatsu

R928 photomultiplier tube, and a TCSPC-option was used to measure

steady-state as well as time-resolved emission spectra. Standard

correction curves were employed to correct the emission and

excitation spectra for source intensity (lamp and grating). Using the

FM-2013 accessory and a Horiba Yvon Jobin TCSPC hub, decay-time

measurements were recorded and detected on the same system. A

NanoLED 370 (λ = 371 nm, 1.5 ns pulse) was used as the excitation

source. The quality of the ﬁt was warranted by minimizing the χ²

function and by visual inspection of the weighted residuals. A

Hamamatsu Photonics absolute PL quantum yield measurement

system (C9920-02G) equipped with a L9799-01 CW Xenon light

source (150 W), monochromator, C7473 photonic multichannel

analyzer, and integrating sphere and employing U6039-05 PLQY

measurement software (Hamamatsu Photonics, Ltd., Shizuoka, Japan)

was employed to measure the photoluminescence quantum yields. A

Thermo Scientiﬁc Evolution 201 UV−vis spectrophotometer was used

for measuring UV−vis absorption spectra. All solvents used were of

spectrometric grade. Degassed samples were prepared by purging with

argon for 30 min.

2.5. Computational Methods. Complexes 2-I, 12-I, 13-I, 14-I,

and 16-I were studied using density functional theory (DFT). Initial

geometries were obtained from single-crystal X-ray diﬀraction data

(see above) and were optimized in the ground state and in the lowest

triplet state using the BP86^71,72functional with the resolution-of-

identity (RI)⁷³⁻⁷⁵approximation. Analytical harmonic vibrational

frequency calculations were performed to verify that the optimized

structures are minima on the potential energy surface. Energies and

frontier orbitals were also computed using the B3LYP^76,77functional.

Phosphorescence energies were calculated using the following

approaches: (1) time-dependent DFT (TDDFT) with the B3LYP

functional, (2) energy diﬀerence between the closed-shell singlet

ground state and the lowest (unrestricted) triplet state (ΔSCF), and

(3) using the spin-ﬂip Tamm−Dancoﬀ⁷⁸approximation (SF-TDA).

Ground-state geometries of ligands 1−16 were optimized using the

B3LYP functional. In all calculations, the def2-SV(P) basis set^79,80and

the m4 grid for numerical integration were employed. All calculations

were performed with the Turbomole program package, version 6.4.⁸¹

2.6. Electrochemistry. The measurements were performed on a

model 600D series electrochemical analyzer with workstation, CH

Instruments. At room temperature and under an argon atmosphere,

experiments were carried out in a three-electrode cell, with a Pt-wire as

counter electrode and Pt-disk electrodes as reference as well as

working electrodes. As electrolyte, NBu₄PF₆was used at a

concentration of 10⁻¹M in dichloromethane. The concentration of

the complex was 10⁻³M. The potential of the quasi-reference

electrode (Pt disk) was corrected to the Ag/AgCl reference by

2.7. Device Fabrication and Characterization. OLED devices

have been prepared starting from indium−tin-oxide (ITO)-covered

glass substrates with a sheet resistance of 16 Ω/m². The substrates

were cleaned in a 2% aqueous solution of Helmanex, washed with

deionized water, and treated with ozone for 10 min prior to spin-

coating with PEDOT:PSS (Heraeus AL4083) ﬁltered through a 0.45

μm syringe ﬁlter. This layer was dried at 120 °C for 1 h in air. The

following organic layers were deposited under an inert atmosphere

from solutions passed through 0.45 μm syringe ﬁlters. A 17 nm poly-

TPD ﬁlm (Lumtec, Taiwan) was spin-coated from a chlorobenzene

solution and annealed at 110 °C for 60 min. Subsequently, the

emitting layer was deposited, consisting of compound 4-I:TPBi:PVK

(45:45:10), from a toluene solution, resulting in a 30 nm thick layer.

After annealing at 80 °C for 10 min, the following layers were

deposited by evaporation under vacuum: TPBi (30 nm), Ca (8 nm),

and Ag (100 nm). The active area of the resulting device was 24 mm².

Finally, the device was encapsulated using 3 M barrier foil. Current (J),

voltage (V), and luminance (L) characteristics were measured with a

BoTest System.

3. RESULTS AND DISCUSSION

3.1. Synthesis. Copper(I) complexes with either phos-

phane, pyridine, or bridging P^N ligands are well-known for

their rich coordination chemistry including mono-, di-, tri-,

tetra-, or polynuclear complexes.^46,84−91These complexes can

be prepared systematically by controlling the reaction

stoichiometry.^46,92On the basis of our experiences with

dinuclear complexes containing three identical P^N ligands

with one of them acting as a bridging ligand and the other as

ancillary ligands, coordinated solely via their phosphorus atom,

our approach was to substitute these two monodentate ligands

with two commercially available triphenylphosphine ligands.

This approach not only simpliﬁes the synthetic eﬀort

dramatically but also is also very modular because the emission

and the solubility can be tuned independently via the bridging

ligand and the ancillary ligands, respectively. Herein, we report

on the eﬀects of the bridging ligand regarding the luminescence

properties of the resulting complexes, and a detailed

investigation of a variety of diﬀerent ancillary ligands and

their eﬀects on the solubility and processability has been

published separately.⁶¹

The P^N ligands studied in this work can be divided into

diphenylphosphines with ﬁve- and six-member heterocycles, as

shown in Chart 1. These ligands were chosen not only because

of their diﬀerent electronic characteristics arising from diﬀerent

heterocycles but also because of the possible facile introduction

of substituents on the heterocycles to ﬁne-tune either the

solubility or the photophysical properties. Substituents with

alkyl chains of diﬀerent lengths and branching can be used to

ﬁne-tune solubility in nonpolar organic solvents such as

chlorobenzene, toluene, xylene, or tetrahydrofuran, which are

potential solvents for device mass-production because of their

relatively high boiling points. The investigated complexes bear

diﬀerent heterocycles as part of the bridging ligand, namely,

pyridine (1−5), 1,2,4-triazole (6−8), 1,2,3-triazole (9),

oxadiazole (10 and 11), thiazole (12), imidazole (13 and

14), and benzimidazole (15 and 16) along with two

triphenylphosphines as ancillary ligands (Chart 1). In addition,

the introduction of alkyl and phenyl substituents on the

heterocycles has been analyzed for the majority of the groups.

D

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a

Chart 1. Synthesized P^N Ligands Bearing Diﬀerent

Heterocycles to Tune the Emission Spectra of the

Corresponding Complexes through Diﬀerent Electronic

Scheme 1. Synthesis of P^N-Type Ligands 1−16

a

Characteristics

a

Reagents and conditions: (a) LiPPh₂, THF, rt, 8 h; (b) (1) LDA,

THF, −78 °C, (2) R-X, −78 °C to rt; (c) DBU, R-X, THF, rt; (d) (1)

LDA, THF, −78 °C, (2) Ph₂PCl, −78 °C to rt; (e) NaOH, DMSO, rt;

R-X; (f) (1) n-BuLi, −78 °C, (2) Ph₂PCl, −78 °C to rt; (g) (1)

EtMgBr, THF, rt to 50 °C, (2) PhN₃, rt to 50 °C, (3) Ph₂PCl, rt; (h)

NEt₃, Ph₂PCl, pyridine, rt.

a

Additional alkyl substituents were introduced for better solubility and

processability.

The syntheses of the ligands were carried out in one- or two-

step reactions according to literature-known processes. Ligands

1 and 2 can be synthesized starting from the appropriate 2-

chloropyridine precursor through the reaction with lithium

diphenylphosphanide. In the case of ligands 3 and 4, the

extension of the alkyl chain was conducted after the

introduction of the phosphine moiety through deprotonation

of the methyl group with lithium diisopropylamide and reaction

with the appropriate alkyl halide. Ligands with triazole,

imidazole, and benzimidazole moieties are accessible by

introducing the alkyl substituents in a substitution reaction

followed by the introduction of the diphenylphosphino group

via deprotonation of the corresponding precursor with n-

butyllithium and reaction with chlorodiphenylphosphane. 1,2,3-

Triazoles can be synthesized in a one-pot procedure out of

phenylazide, phenylacetylene, and chlorodiphenylphosphane,

whereas oxadiazoles and thiazoles are accessible via one-step

reactions starting from literature-known precursor molecules

(Scheme 1).^32,62All P^N ligands were obtained in moderate to

good yields and were fully characterized by standard chemical

analysis (NMR, MS, and IR). It should be noted that both the

synthesis of the chromophoric ligands and their subsequent

complexation (vide infra) can be performed on multigram

scales at constantly high reaction yields.

Because of its increased stability against oxidation processes,

copper(I) iodide was used for complexation reactions instead of

copper(I) bromide and copper(I) chloride. However, to

investigate additionally the inﬂuence of the ligand ﬁeld strength

of the diﬀerent halides on the photophysical properties,

complexes with the full set of the respective copper halides

(iodide, bromide, and chloride) have been synthesized for one

representative ligand. To synthesize the complexes, the

appropriate P^N ligand was mixed together with triphenyl-

phosphine and the metal salt in a 1:2:2 ratio in dichloro-

methane at room temperature and stirred for several hours

(Scheme 2). A completed formation of products was indicated

by a clearing of the reaction mixture. Pure compounds were

obtained by repeated precipitation of the ﬁltered reaction

mixture with diethyl ether or hexane and subsequent washing

with the solvent used for precipitation. Crystallization of the

complexes for X-ray analysis was achieved by slow diﬀusion of

diethyl ether into the reaction mixture as well as into a

E

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Scheme 2. General Procedure for the Synthesis of

Heteroleptic, Dinuclear Copper(I) Complexes

saturated dichloromethane solution. Characterization of the

complexes was carried out by mass spectroscopy, elemental

analysis, and single-crystal X-ray diﬀraction (for complexes 2-I,

11-I, 12-I, 13-I, 14-I, and 16-I), which conﬁrmed a general

1:2:2 stoichiometry for all compounds.

3.2. Structural Studies. X-ray structure analyses conﬁrm a

binuclear complex structure containing a Cu₂X₂metal-halide

core, one P^N ligand, and two phosphine ligands, as indicated

by elemental analysis. The Cu₂X₂core shows a butterﬂy shaped

structure surrounded by the three ligands. The P^N ligand acts

as bridging ligand between the two copper centers, whereas the

phosphine ligands coordinate via their phosphorus atom to

obtain an approximately tetrahedral coordination for both

copper ions. The crystal structure of complex 2-I is shown in

Figure 1.

The general structure of the complexes presented in this

study is not aﬀected by the nature of the heterocycle in the

bridging ligand. Therefore, a replacement of the six-member

heterocycles (e.g., pyridyl species) by ﬁve-member heterocycles

(e.g., triazoles, benzimidazoles, or thiazoles) is possible without

changing the butterﬂy shape of the copper-halide core (Figure

1, complexes 12-I, 13-I, 14-I, and 16-I).

The tetrahedral coordination of the copper(I) atom is not

surprising because many mononuclear copper(I) complexes as

well as polynuclear copper(I) clusters do show this type of

geometry.⁴⁶However, the butterﬂy shaped metal-halide core is

in contrast to the typical planar Cu₂X₂geometry observed for

many halogen-bridged Cu₂X₂-type structures.^33,90,93,94

Figure 1. Molecular structure of complexes 2-I, 11-I, 12-I, 13-I, 14-I,

and 16-I (hydrogen atoms and minor disordered parts are omitted for

clarity, and displacement parameters are drawn at the 50% probability

level).

Selected bond lengths and angles of the investigated

complexes are given in Tables 1 and 2. The copper−copper

distance of all analyzed complexes is in the range of 2.76−2.89

Å, which is on the order of the sum of the van der Waals radii

(2.80 Å).⁹⁵Accordingly, these measured distances are too large

for eﬀective metallophilic interactions.³³Obviously, the

copper−copper distances are only negligibly aﬀected by the

nature of the bridging heterocycles (i.e., ﬁve-member or six-

member ring systems). The copper-halide distances diﬀer only

slightly within each particular complex, with values in the range

of 2.63−2.78 Å. The copper−phosphorus distance Cu(1)−

P(1) is slightly longer than Cu(2)−P(2), which can be

attributed to the electronegativity of the nitrogen atom.³²In

addition, the copper−nitrogen distance is shorter on average by

0.2 Å compared to the corresponding copper−phosphorus

distance.

Each complex shows a slightly distorted tetrahedral geometry

around the copper atoms, as is revealed by analyzing the

relevant bond angles. All measured bond distances and angles

are comparable to the corresponding values found in the

homoleptic dinuclear complexes, indicating that the exchange

of the two monodentate P^N ligands in the homoleptic

complex for two triphenylphosphine ligands inﬂuences only

slightly the complex structure.³²The steric inﬂuence of a

methyl substituent in the ortho position to the nitrogen atom

can be seen by comparing the N(1)−Cu(2)−P(3) angle of

complexes 13-I and 14-I. This angle is widened in complex 14-I

(125.9°) compared to complex 13-I (115.19°) without a

methyl substituent. Bond distances of 13-I and 14-I, however,

are hardly aﬀected by such a substitution pattern.

3.3. Photophysical Characterization. Absorption meas-

urements for representative examples of each heterocycle (i.e.,

complexes bearing pyridylphosphine (2-I), triazolylphosphine

(8-I and 9-I), oxadiazolylphosphine (10-I), thiazolylphosphine

(12-I), and imidazolylphosphine (13-I and 16-I) as the

bridging ligand) were recorded in dichloromethane solution

at room temperature. The spectra of complex 2-I together with

the free ligand 2 and PPh₃is shown in Figure 2 and is discussed

here as a representative example, whereas the spectra of

complexes 8-I, 9-I, 10-I, 12-I, 13-I, and 16-I along with those of

their corresponding free ligands can be found in Figures S2−S7

in the Supporting Information. Electronic absorption maxima

and extinction coeﬃcients of compounds 2, 2-I, 8, 8-I, 9, 9-I,

10, 10-I, 12, 12-I, 13, 13-I, 16, 16-I, and PPh₃are summarized

in Table 3.

Complex 2-I shows a rather unstructured spectrum, whereas

the corresponding spectra of ligand 2 and PPh₃are more

F

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Table 1. Selected Bond Lengths (Å) of Structurally Characterized (PPh₃)₂(P^N)Cu₂I₂Complexes

a

2-I

11-I

12-I

13-I

14-I

16-I

Cu(1)−Cu(2)

2.816(1)

[2.882(6)]

2.877(1)

2.886(1)

2.759(1)

2.818(1)

2.787(1)

Cu(1)−I(1)

Cu(1)−I(2)

Cu(2)−I(1)

2.686(1)

2.688(1)

2.671(1)

2.674(1)

2.668(1)

2.677(1)

2.704(1)

2.694(1)

2.689(1)

2.700(1)

2.724(1)

2.647(1)

2.678(1)

2.783(1)

2.654(1)

2.640(1)

2.681(1)

2.631(1)

[2.534(6)]

Cu(2)−I(2)

2.675(1)

[2.526(6)]

2.690(1)

2.681(1)

2.628(1)

2.677(1)

2.686(1)

P(2)−Cu(1)

P(1)−Cu(1)

P(3)−Cu(2)

2.249(1)

2.235(1)

2.255(1)

2.258(1)

2.215(2)

2.268(1)

2.252(2)

2.243(1)

2.271(1)

2.278(1)

2.225(1)

2.267(1)

2.273(1)

2.240(1)

2.245(1)

2.251(1)

2.232(1)

2.253(1)

[2.025(6)]

N(1)−Cu(2)

2.090(2)

[2.861(7)]

2.052(4)

2.073(3)

2.049(3)

2.050(2)

2.052(2)

a

Data for the minor isomer (6%) is shown in brackets. For the crystal structure, see Supporting Information Figure S1.

Table 2. Selected Bond angles (Degrees) of Structurally Characterized (PPh₃)₂(P^N)Cu₂I₂Complexes

a

2-I

11-I

12-I

13-I

14-I

16-I

I(1)−Cu(1)−I(2)

I(1)−Cu(2)−I(2)

106.24(1)

106.88(2)

106.47(3)

108.50(2)

108.67(2)

109.19(2)

110.28(2)

111.38(1)

107.30(1)

108.92(1)

106.73(1)

108.21(2)

[116.3(2)]

Cu(1)−I(1)−Cu(2)

Cu(1)−I(2)−Cu(2)

63.95(1) [66.9(1)]

65.22(2)

64.87(2)

64.99(2)

64.79(2)

61.28(2)

62.35(2)

62.45(1)

63.49(1)

62.97(1)

63.09(1)

63.36(1)

[67.0(1)]

P(1)−Cu(1)−I(1)

P(1)−Cu(1)−I(2)

P(2)−Cu(1)−P(1)

P(1)−Cu(1)−Cu(2)

105.02(2)

103.09(2)

123.90(3)

105.15(4)

104.57(4)

121.22(5)

89.62(4)

97.77(3)

103.79(3)

121.12(4)

85.56(3)

105.90(3)

103.16(3)

128.97(5)

89.13(3)

99.33(2)

106.78(2)

121.12(2)

87.31(2)

110.77(2)

99.39(2)

122.30(2)

88.62(2)

87.16(2)

[102.9(1)]

N(1)−Cu(2)−P(3)

120.09(6)

[99.3(3)]

119.9(1)

51.6

129.08(8)

46.6

115.2(1)

53.6

125.87(5)

50.9

123.78(5)

54.9

b

golding of the Cu₂I₂moiety

54.5

[24.3]

b

a

Data for the minor isomer (6%) is shown in brackets. The angle between the planes Cu(1), I(1), I(2) and Cu(2), I(1), I(2).

M⁻¹cm⁻¹) and 261 nm (ε = 9874 M⁻¹cm⁻¹), respectively.

Beyond that, complex 2-I shows a shoulder around 299 nm (ε

= 15 162 M⁻¹cm⁻¹), which is assigned to π−π* transitions

within the pyridine moiety. Furthermore, additional tails are

present in the spectrum of complex 2-I ranging from

approximately 349 to 400 nm (ε = 2034 M⁻¹cm⁻¹), which

are not visible in the free-ligand spectra. These low-energy

absorption bands are assigned to metal-to-ligand charge-

transfer (MLCT) mixed with halogen-to-ligand charge transfer

(XLCT) transitions and can therefore be described as (X +

M)LCT states,³²which is in agreement with density functional

theory calculations (vide infra). The absorption spectra of

complexes 8-I, 9-I, 10-I, 12-I, 13-I, and 16-I exhibit slight

diﬀerences in the region of 250−300 nm because of diﬀerent

bridging heterocycles and the energetically diﬀerent π and π*

orbitals. However, the general trends are similar to complex 2-I

and are therefore not discussed in detail.

Emission spectra of neat complex powders were measured at

room temperature. For complexes 1-I, 3-I, and 4-I with pyridyl-

phosphane ligands, 6-I and 9-I with triazolyl phosphine ligands,

10-I with oxadiazolylphosphine ligands, and 12-I, 13-I, 14-I,

and 15-I with imidazolyl or benzoimidazolyl ligands, the

corresponding emission spectra are shown in Figure 3. The

spectra of complexes 2-I, 5-I, 7-I, 8-I, and 16-I are similar in

shape to the spectra shown in Figure 3 and are therefore

omitted for clarity, but they can be seen in Figure S8 in the

Supporting Information. For a collection of photophysical

Figure 2. Absorption spectrum of complex 2-I along with the spectra

of the corresponding P^N ligand 2 and PPh₃at room temperature

measured in CH₂Cl₂solution.

deﬁned. Compound 2-I exhibits a broad shoulder around 256

nm (ε = 35 878 M⁻¹cm⁻¹), which is assigned to ligand-

centered (LC) π−π* transitions on the bridging P^N ligand as

well as on the monodentate PPh₃. This assumption is

supported by the occurrence of corresponding peaks in the

free-ligand spectra of 2 and PPh₃around 256 nm (ε = 9841

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Table 3. Extinction Coeﬃcients of Compounds 2, 2-I, 8, 8-I,

9, 9-I, 10, 10-I, 12, 12-I, 13, 13-I, 16, 16-I, and PPh₃

Table 4. Photoluminescence Characteristics

a

b

c

compound λ_em(nm)

Φ_PL

τ_av(μs)

k_r(10⁵s⁻¹

)

k_nr(10⁵s⁻¹

)

compound

λ_abs(nm)

ε (M⁻¹cm⁻¹

)

assigned transition

LC, Ph, π−π*

1-I

514

545

579

515

522

540

552

498

499

506

556

558

545

468

451

478

481

0.86

0.70

0.42

0.88

0.99

0.91

0.75

0.99

0.72

0.50

0.99

0.85

0.86

0.93

0.72

0.36

0.87

0.91

2.75

1.93

2.32

3.78

3.05

2.67

2.19

2.96

2.11

2.17

2.85

1.62

3.67

2.52

2.82

1.09

4.22

3.13

3.63

1.81

2.33

3.25

3.41

3.42

3.34

3.41

2.30

3.47

5.25

2.34

3.69

2.55

3.30

2.06

0.51

1.55

2.50

0.32

0.03

0.34

1.14

0.03

1.33

2.30

0.04

0.93

0.38

0.28

1.00

5.87

0.31

0.36

2

256

9841

1-Br

1-Cl

2-I

291 (sh)

256 (sh)

299 (sh)

349 (sh)

249

3750

LC, pyridine, π−π*

LC, Ph, π−π*

LC, pyridine, π−π*

(X + M)LCT

2-I

35 878

15 162

2034

3-I

4-I

8

11 835

46 814

18 400

23 380

13 384

22 619

49 024

7414

LC, triazole, π−π*

(X + M)LCT

5-I

8-I

250 (sh)

303 (sh)

248 (sh)

305 (sh)

257

6-I

7-I

9

LC, triazole, π−π*

(X + M)LCT

8-I

9-I

10

10-I

9-I

LC, oxadiazole, π−π*

(X + M)LCT

10-I

11-I

12-I

13-I

14-I

15-I

16-I

262

320

12

273

9647

LC, Ph, π−π*

284

9634

LC, thiazole, π−π*

LC, Ph, π−π*

LC, thiazole, π−π*

(X + M)LCT

12-I

260 (sh)

286 (sh)

334 (sh)

255

33 280

24 960

4010

2.49

3.65

a

b

c

Neat powders at rt, λ_exc= 350 nm. 5% error. Because the decay is

13

15 122

10 477

50 316

18 904

11 315

15 499

17 154

15 391

30 729

6500

LC, Ph, π−π*

LC, imidazole, π−π*

LC, Ph, π−π*

LC, imidazole, π−π*

(X + M)LCT

best-ﬁtted by bi- or triexponentials, a weighted average lifetime (τ_av) is

given, which is calculated by the equation τ_av= ∑A_iτ_i/∑A_i, where A_iis

the pre-exponential for lifetime τ_i; the respective values can be seen in

Table S2 in the Supporting Information.

272 (sh)

248 (sh)

293 (sh)

314 (sh)

265 (sh)

300

13-I

16

Ph, π−π*

complexes can be easily tuned by changing the electronic

properties of the bridging P^N ligand.

LC, benzimidazole, π−π*

(X + M)LCT

308 (sh)

296 (sh)

330 (sh)

261

Complex 1-I has been chosen as reference compound for the

following discussion because it is based on a pyridine

heterocycle without any substitution. The emission spectra of

compounds with solubilizing alkyl groups (2-I−5-I) on the

pyridine, oxadiazoles (10-I and 11-I), or thiadiazole (12-I)

ligands are red-shifted relative to 1-I. On the contrary,

compounds with imidazole- (13-I−16-I) or triazole-based (6-

I−8-I) heterocycles as the bridging ligand show a blue-shift

relative to the reference compound. This behavior can be

rationalized on the basis of the relative energy content of the

heterocycles and the electron-donating or -withdrawing

character of the substituents.

16-I

PPh₃

9874

LC, Ph, π−π*

a

In dichloromethane solution. sh = shoulder.

The emission wavelength is mainly dependant on the nature

of the heterocycle of the bridging P^N ligand. Complexes 1-I−

5-I with pyridylphosphane ligands show emission maxima in

the range of 514−552 nm, whereas complexes with lower

LUMO energies because of their electron-poor character, such

as oxadiazoles (10-I and 11-I) or thiadiazole (12-I), are red-

shifted with emission maxima of 556, 558, and 545 nm,

respectively.⁹⁶Accordingly, complexes with high LUMO

energies like imidazoles (13-I and 14-I), benzimidazoles (15-

I and 16-I), or triazoles (6-I, 7-I, 8-I, and 9-I) show blue-

shifted emission maxima in the range of 451−506 nm. More

precisely, complexes 6-I, 7-I, and 8-I, bearing diﬀerent alkyl

substituents at the nitrogen atom of the triazolyl moiety, show

emission maxima at 498, 499, and 506 nm, respectively. By

comparing a 1,2,4-triazolyl with a 1,2,3-triazolyl moiety, only

small diﬀerences can be seen because both kinds of complexes

show emission maxima roughly around 500 nm, such as

complex 6-I (1,2,4-triazolyl) and complex 9-I (1,2,3-triazolyl)

with emission maxima at 498 and 506 nm. Complexes bearing

an imidazolyl moiety feature emission maxima in the range of

451−481 nm. Again, the inﬂuence of the nature of the

heterocycle is obvious because for complexes 13-I or 14-I with

Figure 3. Powder emission spectra of complexes 1-I, 3-I, 4-I, 6-I, 9-I,

10-I, 12-I, 13-I, 14-I, and 15-I (λ_exc= 350 nm, rt).

properties, see Table 4. The emission colors range between 451

nm for compound 14-I and 558 nm for compound 11-I.

The spectra of all complexes are broad and unstructured, as

observed for their homoleptic counterparts.³²This is also in

accordance with the assumption of (M + X)LCT states as

suggested by DFT calculations (i.e., the HOMO being on the

metal-halide core and the LUMO on the bridging P^N ligand

(vide infra)). Accordingly, the emission characteristics of the

H

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Table 5. Calculated and Experimental Excitation and Emission Energies (in eV) of Compounds 2-I, 12-I, 13-I, 14-I, and 16-I

compound

ΔSCF_{exc, BP86}

ΔE_{exc, TD‑B3LYP}

ΔE_{abs, exp}

ΔSCF_{em, BP86}

ΔE_{em, TD‑B3LYP}

SF-TDA

2.37

ΔE_{em, exp}

2-I

2.29

S₁: 2.31

T₁: 2.29

3.55

1.57

S1: 1.51

T₁: 1.46

2.44

12-I

13-I

14-I

16-I

2.38

2.52

2.57

2.43

S₁: 2.37

T₁: 2.35

3.71

3.95

a

1.52

1.64

1.79

1.60

S₁: 1.42

T₁: 1.36

2.31

2.39

2.47

2.37

2.27

2.64

2.75

2.59

S₁: 2.79

T₁: 2.78

S₁: 1.65

T₁: 1.60

S1: 2.90

T₁: 2.88

S₁: 1.85

T₁: 1.81

S₁: 2.54

T₁: 2.53

3.76

S₁: 1.62

T₁: 1.58

a

Not determined.

electron-rich imidazolyl heterocycles, emission maxima at 468

and 451 nm can be found, respectively, whereas complexes with

benzimidazoyl moieties show emission maxima at 478 nm (15-

I) and 481 nm (16-I). This can be explained with the extended

aromatic system of the bridging ligand in the case of complexes

15-I and 16-I, which results in a lower LUMO energy of the

corresponding complexes. The diﬀerence in the luminescence

maxima of complexes 13-I (468 nm) and 14-I (451 nm) can be

explained by steric inﬂuences because complex 14-I has a

methyl substituent in the ortho-position to the nitrogen atom,

leading to a slight distortion in the complex structure (Tables 1

and 2). In detail, the copper−copper distance increase from

2.759 (13-I) to 2.818 Å (14-I), whereas the P(2)−Cu(1)−P(1)

bond angle decreases (128.97° for 13-I and 121.12° for 14-I),

and the N(1)−Cu(2)−P(3) bond angle increases (115.2° for

13-I and 125.87° for 14-I). This observed trend in the emission

maxima is also supported by DFT investigations (vide infra)

with a calculated distance between the corresponding emission

maxima of 17 nm, which is comparable to the value obtained

from the experimental measurements (17 nm) (for exact values

see Table 5).

DFT investigations of complexes 2-I, 12-I, 13-I, 14-I, and

16-I using the B3LYP functional indicate that the observed

transitions in fact have mainly (M + X)LCT character. As can

be seen in Figure 4, the HOMO is located on the Cu₂I₂core,

whereas the LUMO resides mainly on the bridging P^N ligand.

Therefore, modiﬁcations of the bridging P^N ligand inﬂuence

the LUMO energy of the complex to a larger extent than the

HOMO energy.³²The calculated orbital energies show that for

all complexes with ﬁve-member rings (i.e., 12-I, 13-I, 14-I, and

16-I) a lower energy of the ligand LUMO (−0.89, −0.72,

−0.72, and −0.88 eV, respectively) results in a lower energy of

the LUMO of the complex (−1.54, −1.06, −0.99,and −1.24 eV,

respectively), whereas the HOMO of the complex remains at

approximately the same level (−4.58, −4.41, −4.45, and −4.43

eV, respectively). For details, see Table S4 in the Supporting

Information.

To gain better insight into the photophysical behavior of

these complexes, TD-DFT and spin-ﬂip TDA calculations of 2-

I, 12-I, 13-I, 14-I, and 16-I were performed and compared to

the experimental values (Table 5). Excitation energies were

calculated at the BP86-optimized ground-state structure. The

BP86-optimized triplet geometry was used to calculate emission

energies following the assumption that the molecule relaxes in

the excited state after vertical excitation. Excitation and

emission energies were calculated both as energy diﬀerences

between the closed-shell singlet state and the lowest triplet state

(ΔSCF approach) and by using TD-DFT. Because of the

charge-transfer character of the excitations, the TD-B3LYP

Figure 4. Spin density and frontier orbital plots of complexes 14-I and

16-I. Left: (a) Spin density, (b) LUMO, and (c) HOMO of 16-I.

Right: (a) LUMO + 1, (b) LUMO, and (c) HOMO of 14-I.

excitation energies are much lower than the experimental

values. It has been shown previously that the spin-ﬂip Tamm−

Dancoﬀ approximation (SF-TDA) yields phosphorescence

energies that correlate much better with the experimentally

observed emission wavelengths.⁷⁸For the compounds studied

in this work, the deviation between the computed and

experimental emission wavelength is less than 50 nm, whereas

the energies obtained using TD-B3LYP and the ΔSCF

approach can be red-shifted even into the infrared region of

the spectrum. Despite these limitations, to gain accurate

absolute values, the computed absorption and emission

wavelengths follow the experimental trend regardless of the

particular approach used.

TD-B3LYP calculations in this work and in previous studies

indicate that the lowest excited singlet and triplet state of the

I

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Table 6. Character of Lowest TD-B3LYP Singlet and Triplet Excitations

compound

state

E_exc(eV)

character

E_em(eV)

character

2-I

S₁

T₁

S₁

T₁

S₁

2.31

2.29

2.37

2.35

2.79

98.7% HOMO → LUMO

98.1% HOMO → LUMO

98.5% HOMO → LUMO

97.7% HOMO → LUMO

82.4% HOMO → LUMO

9.5% HOMO → LUMO + 2

79.7% HOMO → LUMO

11.7% HOMO → LUMO + 2

71.0% HOMO → LUMO

23.5% HOMO → LUMO + 1

67.8% HOMO → LUMO

25.1% HOMO → LUMO + 1

98.3% HOMO → LUMO

97.8% HOMO → LUMO

1.51

1.46

1.42

1.36

1.65

98.6% HOMO → LUMO

97.3% HOMO → LUMO

98.2% HOMO → LUMO

96.8% HOMO → LUMO

98.4% HOMO → LUMO

12-I

13-I

T₁

S₁

2.77

2.90

2.88

1.60

1.85

1.81

97.4% HOMO → LUMO

98.6% HOMO → LUMO

97.6% HOMO → LUMO

14-I

16-I

T₁

S₁

2.54

2.53

1.62

1.58

98.8% HOMO → LUMO

98.1% HOMO → LUMO

T₁

present kind of dinuclear copper complexes can usually be

described as a HOMO − LUMO excitation.³²Only for

complexes 13-I and 14-I in their respective ground-state

structure was a notable inﬂuence of other orbitals was found:

the excitation from the HOMO to the LUMO is mixed with the

excitation from the HOMO to the LUMO + 1 or the LUMO +

2 for 14-I and 13-I, respectively (Table 6). Because these

higher virtual orbitals are also located on the bridging P^N

ligand, this does not change the (M + X)LCT character of the

excitation (Figure 4, right).

In addition to these luminescence characteristics concerning

diﬀerent emission maxima, the emission quantum yields and

excited-states lifetimes of these complexes were also inves-

tigated. Emission quantum yields of neat powders are

remarkably high, reaching values between 75 and 99% for

pyridylphosphane-based complexes (1-I−5-I), 50 and 99% for

triazole-based complexes (6-I−9-I), and 36 and 91% for

imidazole-based complexes (13-I−16-I). Oxadiazole-based

complexes (10-I and 11-I) show quantum yields of 85 and

86%, respectively, and the thiazole-based complex 12-I features

a quantum yield of 93% (Table 4). These quantum yields are

comparable to the values reported for similar dinuclear

complexes.^32,36,90

In detail, pyridylphosphine-based complexes 1-I, 2-I, 3-I, 4-I,

and 5-I show quantum yields of 86, 88, 99, 91, and 75%,

respectively, which are, in general, higher than the values

reported for the corresponding homoleptic, dinuclear com-

plexes.^32,97The emission decay times are in the range of 1.93−

3.78 μs, which is somewhat shorter compared to their

homoleptic counterparts, resulting in higher radiative rate

constants (Table 4). For the homoleptic complexes, an increase

of emission quantum yield with increasing size of the alkyl

chains used as substituents on the pyridine ring was tentatively

found to be due to a more distinct hindrance with respect to

geometry changes upon excitation as a result of the more space-

demanding alkyl substituents.³²This trend can also be seen for

the heteroleptic complexes 1-I−5-I; however, because these

compounds bear two ancillary triphenylphosphine ligands

instead of two ancillary P^N ligands, these eﬀects seem not

to be inﬂuenced by the nature of the ancillary ligands.⁶¹

Complexes with a 1,2,4-triazolyl moiety (complexes 6-I, 7-I,

and 8-I) show quantum yields of 99, 72, and 50%, respectively,

together with excited state lifetimes of 2.96, 2.11, and 2.17 μs.

The radiative rate constants stay roughly at the same value

(3.34 × 10⁵, 3.41 × 10⁵, and 2.30 × 10⁵s⁻¹, respectively),

whereas the nonradiative rate constants increase by 1 or 2

orders of magnitude when going from alkyl (6-I and 7-I) to

benzyl (8-I) substituents (0.03 × 10⁵, 1.33 × 10⁵, and 2.30 ×

10⁵s⁻¹, respectively). Obviously, structural changes upon

excitation leading to a higher nonradiative rate constant and

therefore to a lower quantum yield can occur to a larger extent

in complex 8-I as compared to complexes 6-I and 7-I because

of the diﬀerent substitution pattern.³²

Complexes 13-I and 14-I, both bearing an imidazolylphos-

phane as bridging ligand, show signiﬁcantly diﬀerent emission

quantum yields of 72 and 36% and excited-state lifetimes of

2.82 and 1.09 μs, respectively. Although the radiative rate

constants of both complexes are comparable (2.55 × 10⁵and

3.30 × 10⁵s⁻¹, respectively), the nonradiative rate constants

diﬀer by a factor of 6 (1.00 × 10⁵and 5.87 × 10⁵s⁻¹,

respectively). Therefore, the additional methyl group ortho to

the coordinating nitrogen atom on ligand 14 (cf. ligand 13) not

only inﬂuences the emission maxima of the complex but also

the emission quantum yields and the excited-state lifetimes.

Benzimidazoylphosphane complexes 15-I and 16-I show

high quantum yields of 87 and 91% together with decay times

of 4.22 and 2.49 μs, respectively. The inﬂuence of the two

diﬀerent substituents on the emission maxima is negligible

because both are almost identical; however, complex 16-I with

a p-tolyl substituent shows a higher radiative rate constant (3.65

× 10⁵s⁻¹) compared to complex 15-I with an octyl substituent

(2.06 × 10⁵s⁻¹). Obviously, a more rigid substituent, such as p-

tolyl, enhances the radiative rate constant and reduces the

decay time.

To summarize, the excited-state lifetimes of all complexes

measured as neat powders are in the range of 1−4 μs with rate

constants for the radiative decay between 2 × 10⁵and 5 × 10⁵

s⁻¹and for the nonradiative decay between 0.03 × 10⁵and 5.8

× 10⁵s⁻¹(Table 4). These values are comparable to those

found for the corresponding homoleptic complexes and

moreover these decay times are in the range of Pt(II)

complexes, which have already been used with good eﬃciencies

in OLEDs.^19,22,32

To examine the eﬀect of diﬀerent environments, the

photoluminescence properties of complexes 2-I, 3-I, 4-I, 6-I,

13-I, and 16-I were measured in a degassed dichloromethane

solution, as a neat ﬁlm, and as 50% dopant in a PMMA matrix

in addition to the powder measurements mentioned above.

Figure 5 shows the emission spectra of complex 4-I in these

diﬀerent environments as a representative example. The

J

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the range of 11−35 nm for ﬁlm or matrix measurements and

46−78 nm for solution measurements compared to the

respective emission maxima of neat powders. Thus, the

inﬂuence of the substituent in the 4-position of the pyridine

ring seems to decrease in less rigid environments like neat ﬁlms,

PMMA matrices, and solutions. Compounds 3-I and 4-I with

longer alkyl chains compared to complex 2-I show higher

quantum yields in powder (99, 91, and 88%, respectively), ﬁlm

(78, 70, and 53%, respectively) and matrix (56, 84, and 75%,

respectively) measurements, whereas the emission quantum

yields of these complexes are equal in solution (8, 7, and 7%,

respectively). The radiative rate constants of complexes 2-I, 3-I,

and 4-I are comparable to each other in powder (2.33 × 10⁵,

3.25 × 10⁵, and 3.41 × 10⁵s⁻¹, respectively) and decrease by 1

order of magnitude in solution (0.59 × 10⁵, 0.63 × 10⁵, and

0.80 × 10⁵s⁻¹, respectively), whereas the nonradiative rate

constant increases by 1 order of magnitude for complex 2-I

(0.32 × 10⁵to 7.82 × 10⁵s⁻¹) and 2 orders of magnitude for

complex 3-I and 4-I (0.03 × 10 to 7.30 × 10⁵s⁻¹and 0.34 ×

10⁵to 10.7 × 10⁵s⁻¹). Obviously, the inﬂuences of alkyl chains

depend on the environment. In less rigid environments such as

solutions, the probability of radiationless deactivation is

enhanced in an equal way for all complexes (2-I, 3-I, and 4-

I) independently from the nature of the substituent (i.e., low

quantum yields and high nonradiative rate constants), whereas

in more rigid environments such as crystal powders, the

inﬂuence depends on the nature of the alkyl chain: long alkyl

chains lead to higher quantum yields and higher radiative rate

constants. This is assigned to a more distinct hindrance with

respect to geometry changes upon excitation because of the

alkyl substituents.³²

Figure 5. Photoluminescence spectra of complex 4-I in diﬀerent

environments: powder, CH₂Cl₂solution, neat ﬁlm, and PMMA (50%)

(λ_exc= 350 nm).

corresponding spectra of complexes 2-I, 3-I, 6-I, 13-I, and 16-I

can be seen in Figures S9−S13 in the Supporting Information.

Emission maxima, quantum yields, lifetimes, and rate constants

are summarized in Table 7.

The emission maxima of all measured complexes are red-

shifted in neat ﬁlms and PMMA matrix compared to powder

measurements. The largest red shifts are observed when the

emission is measured in solution. Complexes 2-I, 3-I, and 4-I

diﬀer only in the kind of substituent in the 4-position of the

pyridine moiety and can therefore be compared directly to each

other. As neat powders, these complexes feature emission

maxima of 515, 522, and 540 nm, respectively.⁹⁶In neat ﬁlms as

well as doped in PMMA matrix, these emission maxima cover a

smaller range of 548−551 nm. The same trends are observed in

solution, where the emission maxima are 593, 587, and 586 nm

for complexes 2-I, 3-I, and 4-I, respectively. The red-shifts of

the emission maxima of the complexes 2-I, 3-I, and 4-I are in

In addition, the emission quantum yields of complex 2-I are

comparable in powder and higher in ﬁlm, matrix, or solution

measurements compared to its corresponding homoleptic

counterpart, indicating that the substitution of the two ancillary

P^N ligands with triphenylphosphine ligands leads to a more

rigid complex structure, which is less vulnerable to structural

changes upon excitation and consequently shows an increase of

Table 7. Luminescence Properties of Complexes 2-I, 3-I, 4-I, 6-I, 13-I, and 16-I in Diﬀerent Environments

2-I

3-I

a

b

a

b

complex

powder

λ_max(nm)

Φ_PL

τ_av(μs)

k_r(10⁵s⁻¹

)

k_nr(10⁵s⁻¹

)

λ_max(nm)

Φ_PL

τ_av(μs)

k_r(10⁵s⁻¹

)

k_nr(10⁵s⁻¹

)

515

550

548

593

0.88

0.53

0.75

0.07

3.78

2.35

2.28

1.19

2.33

2.26

3.29

0.59

0.32

2.00

1.10

7.82

522

548

551

587

0.99

0.78

0.56

0.08

3.05

2.57

2.37

1.26

3.25

3.04

2.36

0.63

0.03

0.86

1.86

7.30

neat ﬁlm

50% PMMA

CH₂Cl₂solution

4-I

b

6-I

a

b

complex

λ_max(nm)

Φ_PL

τ_av(μs)

k_r(10⁵s⁻¹

k_nr(10⁵s⁻¹

λ_max(nm)

Φ_PL

τ_av(μs)

k_r(10⁵s⁻¹

k_nr(10⁵s⁻¹

powder

540

551

586

0.91

0.70

0.84

0.07

2.67

2.79

2.03

0.87

3.41

2.51

4.14

0.80

0.34

1.08

0.79

10.7

498

515

513

562

0.99

0.72

0.11

2.96

2.14

2.49

0.99

3.34

3.36

2.89

1.11

0.03

1.31

1.12

8.99

neat ﬁlm

50% PMMA

CH₂Cl₂solution

13-I

b

16-I

a

b

complex

λ_max(nm)

Φ_PL

τ_av(μs)

k_r(10⁵s⁻¹

k_nr(10⁵s⁻¹

λ_max(nm)

Φ_PL

τ_av(μs)

k_r(10⁵s⁻¹

k_nr(10⁵s⁻¹

powder

468

506

499

544

0.72

0.34

0.44

0.35

2.82

1.65

2.09

1.73

2.55

2.06

2.11

2.02

1.00

4.00

2.68

3.76

481

522

519

554

0.91

0.31

0.69

0.21

2.49

1.35

2.04

1.20

3.65

2.30

3.38

1.75

0.36

5.11

3.24

6.58

neat ﬁlm

50% PMMA

CH₂Cl₂solution

a

b

5% error. Because the decay is best-ﬁtted by bi- or triexponentials, a weighted average lifetime (τ_av) is given calculated by the equation τ_av

=

∑A_iτ_i/∑A_i, where A_iis the pre-exponential for the lifetime τ_i; the respective values can be seen in Table S3 in the Supporting Information.

K

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the radiative rate constants and a decrease of the nonradiative

rate constants.³²

The red shift of the emission maxima of neat powder

measurements for complexes bearing a triazolylphosphane (6-

I), an imidazolylphosphane (13-I), and a benzimidazoylphos-

phane (16-I) as bridging ligand are in the range of 11−35 nm

for neat ﬁlm measurements and 60−70 nm for solution

measurements. These complexes show comparable emission

quantum yields in powders (i.e., 99 (6-I), 72 (13-I), and 91%

(16-I)) as well as radiative rate constants (i.e., 3.34 × 10⁵(6-I),

2.55 × 10⁵(13-I), and 2.49 × 10⁵s⁻¹(16-I)) to the values

obtained for complexes 2-I, 3-I, and 4-I. However, emission

quantum yields (11, 35, and 21%) as well as the radiative rate

constants (1.11 × 10⁵, 2.02 × 10⁵, and 1.75 × 10⁵s⁻¹) in

solution are an order of magnitude higher compared to the

values of pyridylphosphane-based complexes mentioned above.

In addition, the nonradiative rate constants are lower, with

values of 8.99 × 10⁵(6-I), 3.76 × 10⁵(13-I), and 6.58 × 10⁵s⁻¹

(16-I). Therefore, complexes based on ﬁve-member hetero-

cycles seem to be less vulnerable to structural changes upon

excitation that would lead to an enhanced radiationless

deactivation compared to the corresponding complexes based

on six-member heterocycles. This characteristic is expressed in

higher quantum yields and higher radiative rate constants in

solution

Figure 6. Emission spectra of neat powder of complexes 1-I, 1-Br, and

1-Cl (λ_exc= 350 nm).

energies, however, behave inversely and increase from 2.29

eV (2-I) to 2.39 eV (2-Cl).

3.4. Electroluminescence Studies. Complex 4-I was used

for a test device because of its photophysical properties,

excellent quantum yields, and high solubility in the solvent used

for device fabrication, namely, toluene. Therefore, approximate

HOMO and LUMO energies of complex 4-I were determined

experimentally to choose the appropriate supporting layers.

From cyclic voltammetry in dichloromethane with NBu₄PF₆as

the supporting electrolyte and Ferrocene as the internal

standard, the energy of the HOMO was determined. For this,

we measured the oxidation potential (E_p,a= +0.63 V) and

calculated the HOMO energy with Andersson’s method, CV-

E_HOMO= −(E_p,a/V + 4.6) eV, to be −5.2 eV.⁸²This is

comparable to the values of related heteroleptic, dinuclear

complexes.⁶¹The LUMO energy was determined using the

method of Tauc⁸³because of the occurrence of an irreversible

reduction process during the cyclic voltammetry measurements,

which led to a dissociation of the complex and the deposition of

metallic copper on the working electrode. According to the

method of Tauc, values for the LUMO energy can be

approximated by adding the values for the HOMO and the

optical band gaps. Therefore, the energy of the LUMO was

calculated to be −2.5 eV.

To estimate the performance of the emitting compound

upon electrical excitation, an OLED test device was prepared

with the following setup: glass/ITO/PEDOT:PSS (30 nm)/

poly-TPD (17 nm)/compound 4-I:TPBi:PVK 45:45:10 (30

nm)/TPBi (30 nm)/Ca (8 nm)/Ag (100 nm) (Figure 7). The

HOMO and LUMO energies of these layers ﬁt well with the

corresponding HOMO and LUMO energies of 4-I. Of these

layers, the hole-injection layer (HIL), consisting of PE-

DOT:PSS, the hole-transport layer (HTL), consisting of

poly-TPD, and the emitting layer (EML), consisting of the

emitter blend, were deposited from solution by spin-coating.

After preparation, the devices were encapsulated by a barrier

foil, which was laminated on the top of the bottom-emissive

device.

In conclusion, both the emission wavelength and the

emission quantum yield strongly depend on the environment

of the complexes as well as on the substitution pattern and on

the nature of the heterocycle of the bridging ligand. By

comparing rigid media like crystals with less rigid environments

such as neat ﬁlms, polymer matrices, or solutions, drastic eﬀects

can be observed. These changes can be explained with the

possibility of larger structural changes upon excitation in these

media. In more rigid environments like crystals, the probability

of radiationless deactivation is reduced, resulting in signiﬁcantly

higher emission quantum yields.⁴⁸Solutions, in which

rearrangements upon excitations can occur more easily,

enhance radiationless deactivation processes, leading to a

considerable decrease in emission quantum yields.³²Con-

sequently, in less rigid media, k_nrvalues of the complexes are

higher than they are in more rigid environments (Table 7).

The eﬀect of diﬀerent halides on the photoluminescence

properties was studied in the case of the reference structure 1-I

and its analogous structures 1-Br and 1-Cl, which all contain 2-

diphenylphosphinopyridine as the bridging ligand. The

corresponding emission spectra can be seen in Figure 6. A

strong red-shift can be observed going from iodide (1-I, 514

nm) to bromide (1-Br, 545 nm) to chloride (1-Cl, 579 nm),

which was also observed for comparable dinuclear complexes

and was related to the order of the ligand ﬁeld strength.^32,36

A

similar trend is also predicted by DFT calculations for the

homologous row of complexes 2-I, 2-Br, and 2-Cl. The

calculations were performed on the basis of the crystal structure

of complex 2-I followed by an exchange of the appropriate

halides in the copper-halide core because a corresponding

crystal structure with ligand 1 was not available for calculations.

Calculating the phosphorescence energy of these compounds as

the energy diﬀerence between the lowest triplet state and the

singlet ground state in each case at the optimized triplet

geometry (ΔSCF method), the calculated phosphorescence

energy decreases from 1.57 eV for 2-I to 1.48 eV for 2-Cl. This

corresponds well with the experimentally observed red-shift of

complexes 1-I, 1-Br, and 1-Cl. The calculated excitation

The I−V−L characteristics are presented in Figure 8. The

device shows a low turn-on voltage of 4.1 V and a steep

increase in luminance, reaching 1800 cd/m²at 10 V, which was

chosen as the maximum voltage for the measurement. The

maximum current eﬃciency is 9.0 cd/A at 6.3 V and a current

L

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Article

Figure 7. Schematic device structure along with the molecules used for

device preparation.

density of 1.6 mA/cm². These test-device results demonstrate

the outstanding potential of this class of emitting compounds

for OLED applications. The high current eﬃciency of 9.0 cd/

m²and the turn-on voltage of 4.1 V are a result of the

unoptimized device structure, where the triplet energies of host

and guest are not well matched. Many copper complexes

feature a triplet energy close to 3 eV, whereas the triplet energy

of most host- and charge-transport materials is lower. For

complex 4-I, the triplet energy has been estimated to be 2.8 eV

from the onset of phosphorescence spectra at 77 K, as

proposed by Adachi.⁹⁸This decreases the device eﬃciency

because of an energy transfer from triplet excitons to the host

materials.⁹⁸⁻¹⁰¹The full potential of these materials is still

under investigation in other device architectures but is already

comparable to solution-processed OLEDs based on copper(I)-

emitters, whereas the performance of established Ir(III)- or

Pt(II)-systems has not yet been achieved by our sys-

tem.^{35,36,98−102}For Cu(I), solution-based OLEDs have been

reported to achieve eﬃciencies up to 50 cd/A,⁹⁸which is in

same range as solution-processed devices with Ir(III)-

emitters.^103−106In work by the group of Adachi,⁹⁸special

materials with high triplet energies were used to conﬁne all

triplet excitons in the EML. Our goal is to apply this concept to

our materials. Currently, our group is working on developing an

optimized device architecture to achieve better performance at

lower current densities. This extensive work will be published

separately.

Figure 8. I−V−L characteristics of the test device using complex 4-I as

the emitting compound.

4. CONCLUSIONS

A series of highly luminescent heteroleptic copper(I)

complexes was synthesized using a modular approach based

on easily accessible P^N ligands, triphenylphosphine, and

copper(I) halides. The butterﬂy shaped structure of the

copper(I) halide core is surrounded by one P^N ligand,

bridging the two metal centers, and two triphenylphosphine

ligands, coordinating via their phosphorus atoms to the copper

centers, fulﬁlling a tetrahedral coordination geometry for each

copper atom as revealed by X-ray structure analysis. The

photoluminescence characteristics have been investigated,

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Article

revealing a dependency of the emission maxima and the

electronic characteristics of the bridging P^N ligand.

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AUTHOR INFORMATION

Corresponding Authors

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*E-mail: baumann@cynora.com (T.B.).

*E-mail: braese@kit.edu (S.B.). Fax: (+ 49) 721 608 48581.

(32) Zink, D. M.; Bachle, M.; Baumann, T.; Nieger, M.; Kuhn, M.;

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Notes

Wang, C.; Klopper, W.; Monkowius, U.; Hofbeck, T.; Yersin, H.;

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dx.doi.org/10.1021/cm4018375 | Chem. Mater. XXXX, XXX, XXX−XXX

Article Doi

Heteroleptic, dinuclear copper(i) complexes for application in organic light-emitting diodes

DOI: 10.1021/cm4018375

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Article abstract of DOI:10.1021/cm4018375

Full text of DOI:10.1021/cm4018375

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Article Doi

Heteroleptic, dinuclear copper(i) complexes for application in organic light-emitting diodes

DOI: 10.1021/cm4018375

Source and publish data:

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Article abstract of DOI:10.1021/cm4018375

Full text of DOI:10.1021/cm4018375

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