Copper(I) complexes based on five-membered PaN heterocycles: Structural diversity linked to exciting luminescence properties

Article abstract of DOI:10.1021/ic4019162

Bridging P^aN ligands bearing five-membered heterocyclic moieties such as tetrazoles, 1,2,4-triazoles, oxadiazoles, thiadiazoles, and oxazoles have been investigated regarding their complexation behavior with copper(I) iodide as metal salts. Different complex structures were found, depending either on the ligand itself or on the ligand-to-metal ratios used in the complexation reaction. Two different kinds of luminescent dinuclear complex structures and a kind of tetranuclear complex structure were revealed by X-ray single-crystal analyses and were further investigated for their photophysical properties. The emission maxima of these complexes are in the blue to yellow region of the visible spectrum for the dinuclear complexes and in the yellow to orange region for the tetranuclear complexes. Further investigations using density functional theory (DFT) show that the highest occupied molecular orbital (HOMO) is located mainly on the metal halide cores, while the lowest unoccupied molecular orbital (LUMO) resides mostly in the ligand sphere of the complexes. The emission properties were further examined in different environments such as neat powders, neat films, PMMA matrices, or dichloromethane solutions, revealing the high potential of these complexes for their application in organic light-emitting diodes. Especially complexes with 1,2,4-triazole moieties feature emission maxima in the blue region of the visible spectrum and quantum yields up to 95% together with short decay times of about 1-4 μs and are therefore promising candidates for blue-emitting materials in OLEDs.

Full text of DOI:10.1021/ic4019162

Article
pubs.acs.org/IC
Copper(I) Complexes Based on Five-Membered P∧N Heterocycles:
Structural Diversity Linked to Exciting Luminescence Properties
Daniel M. Zink,† Thomas Baumann,*,† Jana Friedrichs,† Martin Nieger,‡ and Stefan Brase*,§,⊥
̈
†Cynora GmbH, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany
‡Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55 (A.I. Virtasenaukio 1), FIN-00014
University of Helsinki, Finland
§Institute of Organic Chemistry, Karlsruhe Institute of Technology, KIT Campus South, Fritz-Haber-Weg 6, D-76131 Karlsruhe,
Germany
⊥Karlsruhe Institute of Technology (KIT), Institute of Toxicology and Genetics, Hermann-von-Helmholtz-Platz 1, D-76344
Eggenstein-Leopoldshafen, Germany
S
* Supporting Information
ABSTRACT: Bridging P∧N ligands bearing five-membered
heterocyclic moieties such as tetrazoles, 1,2,4-triazoles,
oxadiazoles, thiadiazoles, and oxazoles have been investigated
regarding their complexation behavior with copper(I) iodide as
metal salts. Different complex structures were found, depend-
ing either on the ligand itself or on the ligand-to-metal ratios
used in the complexation reaction. Two different kinds of
luminescent dinuclear complex structures and a kind of
tetranuclear complex structure were revealed by X-ray single-crystal analyses and were further investigated for their
photophysical properties. The emission maxima of these complexes are in the blue to yellow region of the visible spectrum for
the dinuclear complexes and in the yellow to orange region for the tetranuclear complexes. Further investigations using density
functional theory (DFT) show that the highest occupied molecular orbital (HOMO) is located mainly on the metal halide cores,
while the lowest unoccupied molecular orbital (LUMO) resides mostly in the ligand sphere of the complexes. The emission
properties were further examined in different environments such as neat powders, neat films, PMMA matrices, or
dichloromethane solutions, revealing the high potential of these complexes for their application in organic light-emitting diodes.
Especially complexes with 1,2,4-triazole moieties feature emission maxima in the blue region of the visible spectrum and quantum
yields up to 95% together with short decay times of about 1−4 μs and are therefore promising candidates for blue-emitting
materials in OLEDs.
metal ratio used for the complexation reaction.54,61−72 For
example, recently reported 1,2,3-triazole-based ClickPhos-type
INTRODUCTION
■
A great deal of research has focused on the development of new
ligands and copper(I) halides form di-, tri-, and tetranuclear
concepts for light and display applications and for energy-
generating concepts based on organic electronics.1−12 Lumi-
complexes depending on the substitution pattern of the ligand
and on the solvent used for synthesis or for the recrystallization
nescent metal complexes based on Ir, Pt, and Os are promising
candidates for applications as emitting materials in organic
of these kinds of complexes.39
Within a given system, colors can be tuned by changing the
light-emitting devices, because of their color tunability and their
high efficiencies.13−32 However, since these emitting materials
electronic characteristics of the ligands. This has been shown
previously for mononuclear,56,57,73 dinuclear,42,67 trinuclear,39
are based on rare and expensive metals, luminescent complexes
and tetranuclear74 systems and was also recently reported by
based on more abundant d10 metals such as Cu(I), Ag(I), and
our group for a dinuclear complex with a butterfly-shaped
Au(I) have been brought to the forefront of current
research.33−37 Copper complexes offer exciting photolumines-
copper(I) halide core and three ligands in the ligand sphere
with one ligand acting as a bridging unit and two ligands
cence characteristics along with a high structural diversity and,
coordinating via their phosphorus atoms.38 In this system, it
furthermore, have been successfully tested as emitting
was possible to shift the emission maximum from the blue to
the red region of the visible spectrum by changing the
electronic characteristics of the ligands used for complexation.
compounds in organic light-emitting devices during the last
few years.38−60 A large number of mono-, di-, tri-, and
tetranuclear copper(I) complexes are known, and their
formation often depends on the nature of the ligand used for
complexation, i.e. whether it is monodentate, bidentate
(chelating or bridging), or tridentate, as well as the ligand-to-
Received: July 24, 2013
© XXXX American Chemical Society
A
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Article
mL). The aqueous layer was extracted with dichloromethane (3 × 80
mL). Afterward 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.
A direct correlation between the energy of the lowest
unoccupied molecular orbital (LUMO) of the ligand and that
of the complex was found using DFT calculations: the LUMO
energy of the complex is typically localized on the ligand, while
the HOMO is localized on the metal centers. Therefore, by
changing the electronic structure of the ligand to increase its
LUMO energy, the HOMO−LUMO gap consequently
increases. Those complexes were based on P∧N ligands with
substituted pyridine moieties. However, pyridines are known to
be electron-poor heterocycles. Strong electron donor sub-
stituents were needed in order to shift the emission maximum
into the blue region of the visible spectrum. In contrast, five-
membered heterocycles are generally known to be electron-rich
heterocycles with high-lying LUMO energies,75 and therefore
should be well suited for complexes with emission maxima in
the blue range of the visible spectra. The aim of this study was
the investigation of P∧N ligands with different five-membered
heterocycles in order to obtain deep blue emitting complexes.
General Procedure for the Phosphorylation of Alkylated
Compounds 10−13 Yielding Compounds 1−4. The alkylated
triazole precursor (21.6 mmol, 1.00 equiv) was dissolved in dry
tetrahydrofuran (20 mL) under a nitrogen atmosphere and cooled to
−80 °C. Lithium diisopropylamine (1.8 M in tetrahydrofuran, 23.8
mmol, 1.10 equiv) was added to this solution dropwise via cannula.
The reaction mixture was warmed to 0 °C and was subsequently
cooled to −80 °C again. Chlorodiphenylphosphane (22.7 mmol, 1.05
equiv) dissolved in tetrahydrofuran (10 mL) was added dropwise via
cannula, and the mixture was warmed to room temperature overnight.
After quenching with water (2 mL), the mixture was filtered through a
plug of silica gel. The product was obtained by purification via flash
column chromatography.
General Procedure for the Synthesis of Compounds 5−7. The
appropriate oxadiazole or thiadiazole precursor (20.5 mmol, 1.00
equiv) and triethylamine (205 mmol, 10.0 equiv) were dissolved in
pyridine (40 mL) under a nitrogen atmosphere. Chlorodiphenylphos-
phane (30.8 mmol, 1.50 equiv) was added dropwise to this solution at
room temperature. The reaction mixture was stirred for 24 h and
subsequently quenched with hydrochloric acid (1 M, 700 mL). The
aqueous layer was extracted with dichloromethane (3 × 150 mL),
washed with brine (200 mL), and dried over MgSO4·H2O, and the
solvent was removed under vacuum. The product was obtained by
purification via flash column chromatography.
Synthesis of 1-Benzyl-5-(diphenylphosphino)-1H-tetrazole (8).
Under a nitrogen atmosphere, 1-benzyl-1H-tetrazole (3.12 mmol,
1.00 equiv) was dissolved in dry tetrahydrofuran (8 mL) and cooled to
−100 °C, and n-butyllithium (2.5 M in n-hexane, 3.43 mmol, 1.10
equiv) was added dropwise via cannula. The reaction mixture was
stirred at −100 °C for 30 min, and subsequently a solution of
chlorodiphenylphosphane (3.43 mmol, 1.10 equiv) in dry tetrahy-
drofuran (2 mL) was added slowly via cannula. The reaction mixture
was warmed to room temperature overnight. After quenching with
water (3 mL), the mixture was filtered through a plug of silica gel. The
product was obtained by purification via flash column chromatog-
raphy.
General Procedure for the Syntheses of Complexes 1-I−5-I, 5-
I(a), 6-I(a), and 8-I. Copper(I) iodide (0.40 mmol, 2.00 equiv) and
the P∧N ligand (0.60 mmol, 3.00 equiv) were suspended under
nitrogen in dry dichloromethane (10 mL) and stirred for 12 h at room
temperature, resulting in a clear solution. Afterward the complex was
purified through precipitation with diethyl ether, pentane, methyl tert-
butyl ether, or cyclohexane (100 mL). The solid was filtered off,
washed with the appropriate solvent, and dried under vacuum.
General Procedure for the Syntheses of Complexes 5-I(b)−7-I(b).
Copper(I) iodide (0.40 mmol, 1.00 equiv) and the P∧N ligand (0.40
mmol, 1.00 equiv) were suspended under nitrogen in dry dichloro-
methane (10 mL) and stirred for 12 h at room temperature. The
complexes were purified through repeated precipitation in diethyl
ether or methyl tert-butyl ether (100 mL). The solid was filtered off,
washed with diethyl ether, and dried under vacuum.
EXPERIMENTAL SECTION
■
General Procedures. All reactions were performed in oven-dried
glassware under a nitrogen atmosphere. Solvents and chemicals used
were purchased from commercial suppliers. Solvents were dried under
standard conditions. All materials were used without further
purification. Thin-layer chromatography (TLC) was carried out on
silica gel plates (silica gel 60, F254, Merck) with detection by UV.
Purification was performed with preparative chromatography using
normal-phase silica gel (silica gel 60, 230−400 mesh, Merck). 1H
NMR, 13C NMR, and 31P NMR spectra were recorded on a Bruker
AM 400 or Bruker DRX 250 instrument, respectively. Chemical shifts
are reported as δ values (ppm). The signal abbreviations are as follows:
s, singlet; bs, broad singlet; d, doublet; t, triplet; q, quartet; quin,
quintet; m, multiplet; Ar−H, aromatic proton. The signals of 13C
NMR spectra were assigned through DEPT (distortionless enhance-
ment by polarization transfer) technology. The signal abbreviations are
as follows: C−Ar, aromatic carbon; +, primary or tertiary carbon; −,
secondary carbon; Cquat, quaternary carbon. MS (EI) (electron impact
mass spectrometry), FAB (fast atom bombardment mass spectrom-
etry), and HRMS (high resolution mass spectrometry) were carried
out on a Finnigan MAT 90 instrument (70 eV). The molecular
fragments are quoted as the relation between mass and charge (m/z)
and the intensities as percentage values relative to the intensity of the
base signal (100%). The abbreviation [M+] refers to the molecular ion.
Descriptions without nominated temperature were done at room
temperature, and the following abbreviations were used: calcd,
theoretical value; found, measured value. Information is given in
mass percent. IR (infrared spectroscopy) was carried out on a Bruker
IFS 88 FT-IR instrument. IR spectra of solids were recorded in KBr
and as thin films on KBr for oils and liquids. The deposit of the
−1
̃
absorption band was given in wavenumbers ν
̃
in cm . The forms and
intensities of the bands were characterized as follows: s, strong (10−
40% transmission); m, medium (40−70% transmission); w, weak
(70−90% transmission); vw, very weak (90−100% transmission); br,
broad.
Synthesis of P∧N-Type Ligands. Spectroscopic data of ligand 576
have already been reported; for spectroscopic data of ligands 1−4 and
6−8 refer to the Supporting Information. Precursors 14 and 15 were
synthesized following the procedures reported by Kawano et al.77
Precursor 16 was synthesized according to a reaction sequence
reported by Gierczyk et al.,78 and the tetrazole precursor 18 was
obtained following a reaction reported by Vorobiov et al.79
General Procedure for the Alkylation of 1,2,4-Triazoles Yielding
Compounds 10−13.80 A solution of diazabicycloundecene (52.1
mmol, 1.20 equiv) in tetrahydrofuran (10 mL) was added dropwise to
a solution of 1H-1,2,4-triazole (43.4 mmol, 1.00 equiv) and the
appropriate alkyl halide (47.7 mmol, 1.10 equiv) under a nitrogen
atmosphere at room temperature. The reaction mixture was stirred for
24 h at room temperature and subsequently quenched with water (50
[(5-(Diphenylphosphino)-1-propyl-1H-1,2,4-triazole)3Cu2I2] (1-I).
Yield: 519 mg, 0.41 mmol, 81%; white powder. IR (ATR): ν 2965
̃
(vw), 1481 (w), 1435 (w), 1272 (w), 1174 (w), 1096 (w), 1028 (w),
998 (w), 971 (vw), 742 (m), 692 (m), 671 (w), 509 (m), 473 (m)
cm−1. MS (FAB), m/z (%): 1224 (1), 1034 (21), 843 (35), 740 (21),
653 (43), 548 (79), 358 (94), 297 (100). Anal. Calcd for
C51H54Cu2I2N9P3 (1266.9): C, 48.35; H, 4.30; N, 9.95. Found: C,
48.10; H, 4.27; N, 10.06.
[(5-(Diphenylphosphino)-1-butyl-1H-1,2,4-triazole)3Cu2I2] (2-I).
Yield: 220 mg, 0.17 mmol, 62%; white powder. IR (ATR): ν 2958
̃
(vw), 2871 (vw), 1480 (w), 1434 (w), 1271 (w), 1174 (w), 1096 (w),
1044 (m), 998 (vw), 971 (vw), 876 (vw), 742 (m), 692 (m), 670 (w),
510 (m), 475 (w) cm−1. MS (FAB), m/z (%): 1252 (3), 1062 (25),
871 (35), 754 (23), 681 (52), 562 (89), 372 (98), 310 (100). Anal.
B
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Article
Calcd for C54H60Cu2I2N9P3 (1308.9): C, 49.55; H, 4.62; N, 9.63.
Found: C, 49.44; H, 4.56; N, 9.58.
[(5-(Diphenylphosphino)-1-hexyl-1H-1,2,4-triazole)3Cu2I2] (3-I).
Bruker-Nonius KappaCCD diffractometer (2-I) at 123(2) K using Mo
Kα radiation (λ = 0.71073 Å). Crystal data, data collection parameters,
and results of the analyses are given in Table S1 (Supporting
Information). Direct methods (for 7-I(b) and 8a-I Patterson
methods) were used for structure solution (SHELXS-97), and
refinement was carried out using SHELXL-97 (full-matrix least
squares on F2).81 Hydrogen atoms were refined using a riding
model. A semiempirical absorption correction using equivalent
reflections was applied (see Table S1).
In 3-I one phenyl group and one n-hexyl group are disordered (sof
= 0.63(1) and 0.54(1), respectively). In 5-I(b) (crystallographic Ci
symmetry) one phenyl group (sof = 0.544(5)) and the solvent MTBE
and water (over the center of symmetry, sof = 0.50) are disordered.
The water hydrogen atoms could not be localized (due to the
disorder) and were not included in the refinement. In 5-I(b)′
(crystallographic C2 symmetry) the solvent Et2O and water are
disordered (over the 2-fold axis, sof = 0.50). The water hydrogen
atoms could not be localized (due to the disorder) and were not
included in the refinement. In 8-I one phenyl group (sof = 0.66(1))
and the copper atoms are disordered (major and minor isomer in the
ratio 95:5).
Photophysical Measurements. Steady-state emission spectra
were recorded on a FluoroMax-4 spectrofluorometer from Horiba
Scientific equipped with a 150 W xenon arc lamp, excitation and
emission monochromators (1200 grooves/nm blazed at 330 nm
(excitation) and 500 nm (emission)), and a Hamamatsu R928
photomultiplier tube. Emission and excitation spectra were corrected
for source intensity (lamp and grating) by standard correction curves.
Decay time measurements were recorded and detected on the same
system using the TCSPC method with the FM-2013 accessory and a
TDSPC hub from Horiba Yvon Jobin. A NanoLED 370 was used as
excitation source (λ 371 nm, 1.5 ns pulse). The quality of the fit was
assured by minimizing the χ2 function and by visual inspection of the
weighted residuals. Luminescence quantum yields were measured with
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 U6039-05 PLQY measurement
software was used (Hamamatsu Photonics, Ltd., Shizuoka, Japan). All
solvents used were of spectrometric grade. Degassed samples were
prepared by purging with argon for 30 min. UV−vis absorption spectra
were measured on a Thermo Scientific Evolution 201 UV−visible
spectrophotometer.
̃
Yield: 506 mg, 0.36 mmol, 61%; white powder. IR (ATR): ν 2923
(vw), 2853 (vw), 1481 (w), 1435 (w), 1272 (w), 1175 (w), 1096 (w),
1050 (vw), 1027 (vw), 998 (vw), 971 (vw), 876 (vw), 741 (w), 691
(m), 508 (m), 466 (w) cm−1. MS (FAB), m/z (%): 1308 (2), 1119
(15), 927 (18), 782 (18), 737 (44), 590 (75), 400 (100), 338 (90).
Anal. Calcd for C60H72Cu2I2N9P3 (1393.1): C, 51.73; H, 5.21; N, 9.05.
Found: C, 51.47; H, 5.09; N, 8.80.
[(5-(Diphenylphosphino)-1-benzyl-1H-1,2,4-triazole)3Cu2I2] (4-I).
Yield: 480 mg, 0.34 mmol, 88%; white powder. IR (ATR): ν 3050
̃
(vw), 1481 (vw), 1435 (w), 1328 (vw), 1272 (w), 1173 (vw), 1097
(w), 1028 (vw), 999 (vw), 971 (vw), 743 (w), 719 (w), 691 (m), 669
(w), 584 (vw), 516 (m), 467 (w) cm−1. MS (FAB), m/z (%): 1283
(2), 1130 (22), 940 (49), 749 (21), 595 (85), 405 (62), 343 (100).
Anal. Calcd for C63H54Cu2I2N9P3 (1409.0): C, 53.63; H, 3.86; N, 8.93.
Found: C, 53.25; H, 3.81; N, 8.73.
[(2-(Diphenylphosphino)-5-phenyl-1,3,4-oxadiazole)3Cu2I2] (5-
I(a)). Yield: 683 mg, 0.50 mmol, 78%; yellow powder. IR (ATR): ν
̃
3051 (vw), 2921 (vw), 2847 (vw), 1550 (w), 1481 (w), 1435 (w),
1340 (vw), 1159 (vw), 1095 (w), 1068 (w), 1027 (vw), 990 (vw), 956
(vw), 925 (vw), 778 (vw), 742 (w), 688 (m), 526 (w), 510 (w) cm−1.
MS (FAB), m/z (%): 1242 (3), 1104 (3), 915 (17), 775 (5), 583 (31).
Anal. Calcd for C60H45Cu2I2N6O3P3 (1371.9): C, 52.53; H, 3.31; N,
6.13. Found: C, 52.64; H, 3.68; N, 5.73.
[(2-(Diphenylphosphino)-5-phenyl-1,3,4-oxadiazole)4Cu4I4] (5-
I(b)). Yield: 155 mg, 0.07 mmol, 25%; yellow powder. IR (ATR): ν
̃
3053 (vw), 1551 (w), 1481 (w), 1435 (w), 1341 (vw), 1095 (w), 1068
(vw), 1027 (vw), 988 (vw), 956 (vw), 778 (vw), 742 (w), 688 (vw),
525 (w), 507 (w) cm−1. MS (FAB), m/z (%): 1434 (21), 1244 (2),
1104 (43), 915 (18), 774 (61), 723 (25), 583 (51). Anal. Calcd for
C80H60Cu4I4N8O4P4 (2083.1): C, 46.13; H, 2.90; N, 5.38. Found: C,
46.08; H, 3.02; N, 5.10.
[(2-(Diphenylphosphino)-5-(p-tolyl)-1,3,4-oxadiazole)3Cu2I2] (6-
I(a)). Yield: 236 mg, 0.17 mmol, 29%; yellow powder. IR (ATR): ν
̃
1613 (vw), 1582 (vw), 1554 (vw), 1495 (w), 1434 (w), 1333 (vw),
1182 (vw), 1095 (w), 1077 (w), 1021 (vw), 989 (vw), 955 (vw), 823
(w), 733 (w), 690 (w), 621 (w), 505 (m), 464 (w) cm−1. MS (FAB),
m/z (%): 1287 (2), 1133 (9), 943 (38), 789 (11), 751 (35), 597
(100), 407 (28), 345 (18). Anal. Calcd for C63H51Cu2I2N6O3P3
(1413.9): C, 53.53; H, 3.64; N, 5.94. Found: C, 53.90; H, 3.68; N,
5.86.
[(2-(Diphenylphosphino)-5-(p-tolyl)-1,3,4-oxadiazole)4Cu4I4] (6-
Computational Methods. Density functional theory (DFT)
calculations of complexes 2-I, 5-I(b), and 7-I(b) as well as ligands
1−8 were performed using BP8682,83 and B3LYP84,85 as functionals
and the def2-SV(P) basis set.86,87 For numerical integration, the m4
grid was employed. In the BP86 calculations the resolution of identity
(RI) approximation was used.88−90 The initial structures of the
complexes were obtained from single-crystal X-ray diffraction data and
were optimized using the BP86 functional. Triplet structures were
optimized using unrestricted DFT. Analytical harmonic vibrational
frequency calculations were performed to verify that the optimized
complex structures are minima on the potential energy surface.
Frontier orbitals depicted in Figures 6 and 10 were calculated using
B3LYP at BP86 optimized geometries. All calculations were done
using the Turbomole program package (version 6.4).91
I(b)). Yield: 763 mg, 0.36 mmol, 82%; yellow powder. IR (ATR): ν
̃
1615 (w), 1556 (w), 1495 (m), 1481 (w), 1435 (w), 1097 (w), 1077
(w), 988 (w), 827 (w), 742 (m), 735 (m), 691 (m), 623 (w), 527 (m),
514 (m), 503 (m), 478 (w), 459 (w), 441 (w), 428 (w) cm−1. MS
(FAB), m/z (%): 1476 (25), 1287 (3), 1133 (25), 941 (10), 789 (32),
597 (22). Anal. Calcd for C84H68Cu4I4N8O4P4 (2139.2): C, 47.16; H,
3.20; N, 5.24. Found: C, 47.35; H, 3.29; N, 5.21.
[(2-(Diphenylphosphino)-5-phenyl-1,3,4-thiadiazole)4Cu4I4] (7-
I(b)). Yield: 347 mg, 0.16 mmol, 84%; orange powder. IR (ATR): ν
̃
1480 (w), 1456 (w), 1434 (w), 1425 (w), 1097 (w), 764 (w), 740
(m), 689 (m), 641 (w), 512 (m), 501 (m), 490 (m), 444 (w) cm−1.
MS (FAB), m/z (%): because of the insolubility of the compound, no
mass spectra could be measured. Anal. Calcd for
C80H60Cu4I4N8O4P4·1/4MTBE (2147.3): C, 44.98; H, 2.93; N, 5.17;
S, 5.91. Found: C, 45.38; H, 2.85; N, 5.02; S, 6.07.
RESULTS AND DISCUSSION
[(1-Benzyl-5-(diphenylphosphino)-1H-tetrazole)3Cu2I2] (8-I).
Yield: 219 mg, 0.15 mmol, 18%; white powder. IR (ATR): ν 1482
̃
■
Synthesis and Structural Studies. The P∧N ligands
studied in this work are tetrazoles, 1,2,4-triazoles, oxadiazoles,
and thiadiazoles (Chart 1). These ligands are accessible through
one- or two-step reactions following processes known from the
literature.
(w), 1436 (w), 1184 (vw), 1159 (vw), 1099 (w), 1027 (vw), 999 (w),
781 (vw), 747 (m), 720 (m), 693 (s), 592 (w), 516 (m), 469 (m)
cm−1. MS (FAB), m/z (%): 1284 (2), 1132 (2), 942 (32), 786 (2),
751 (10), 599 (35), 379 (72). Anal. Calcd for C60H51Cu2I2N12P3
(1414.0): C, 50.97; H, 3.64; N, 11.89. Found: C, 50.81; H, 3.57; N,
11.68.
For 1,2,4-triazole-based ligands 1−4, the introduction of an
additional alkyl substituent at the 1-position was possible in a
one-step reaction followed by the introduction of diphenyl-
phosphine via deprotonation of the appropriate alkyl-
X-ray Crystallography. The crystal structure determinations of 2-
I, 3-I, 5-I(b), 5-I(b)′, 7-I(b), and 8a-I were performed on a Bruker-
Nonius APEXII diffractometer (3-I, 5-I(b), 5-I(b)′, 7-I(b), 8a-I) or a
C
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Chart 1. Different P∧N Ligands Investigated for Their
Complexation Behavior
pyridylphosphine ligands, it has been shown that copper(I)
iodide complexes feature better photoluminescence character-
istics.38 The complexes were synthesized by mixing the
appropriate ligands with copper(I) iodide in different molar
ratios. After they were stirred in dichloromethane at room
temperature for several hours, the reaction mixtures cleared up,
indicating the end of the reaction. Pure complexes were
available after precipitation with diethyl ether, methyl tert-butyl
ether, or hexane from the filtered reaction mixture and
subsequent washing with the solvent used for precipitation.
Single crystals were obtained by slow diffusion of diethyl ether
or methyl tert-butyl ether into the reaction mixture or into a
saturated dichloromethane solution of the corresponding
complex. Complexes were characterized using mass spectros-
copy, elemental analysis, and, where possible, single-crystal X-
ray diffraction.
X-ray structure analyses reveal three different complex
structures using these five-membered heterocycles as coordi-
nating P∧N ligands: two dinuclear complexes with different
structures and one tetranuclear complex structure. Complexes
1-I−4-I, 5-I(a), and 6-I(a) using 1,2,4-triazoles or oxadiazoles
as bridging ligands were synthesized by reacting the appropriate
P∧N ligand together with copper(I) iodide in a 3:2 molar ratio.
These compounds feature a dinuclear complex structure with a
Cu2I2 metal halide core surrounded by three ligands (see Figure
1).
One ligand acts as a bridging unit, while the other two
ligands coordinate only via their phosphorus atoms. Selected
bond distances and angles are shown in Table S2 in the
Supporting Information. This structural motif is also found for
the corresponding complexes based on 2-pyridylphosphane
ligands.38 Changing the metal to ligand ratio from 3:2 to 1:1
results in the formation of tetranuclear complexes of L4Cu4I4
stoichiometry in the case of ligands based on oxadiazoles (5-
I(b) and 6-I(b)), while ligands containing 1,2,4-triazoles still
yield dinuclear complexes of L3Cu2I2 stoichiometry. Because of
structural similarities, thiadiazole 7 was used to synthesize the
tetranuclear complex 7-I(b) in order to investigate the effect of
different electronic characteristics of the P∧N ligands.
substituted triazole precursor with butyllithium and addition of
chlorodiphenylphosphine. In the case of 1,2,4-triazoles, long
alkyl groups were introduced in order to enhance the solubility
of these compounds in nonpolar solvents such as chlor-
obenzene, toluene, xylene, and tetrahydrofuran.
Ligands with oxadiazole or thiadiazole moieties (5−7) can be
synthesized by deprotonation of the appropriate heterocyclic
precursors (14−16) with triethylamine followed by the
addition of chlorodiphenylphosphine. Ligand 8 with a tetrazole
moiety is accessible starting at benzylamine, which can be
reacted with sodium azide and triethyl orthoformate to the
benzyl-substituted tetrazole precursor. Deprotonation of this
molecule at −100 °C followed by the addition of
chlorodiphenylphosphine results in the target compound
(Scheme 1).
These ligands were investigated with regard to their
complexation abilities with copper(I). Copper(I) iodide was
used as the metal salt due to its better oxidation stability in
comparison to copper(I) bromide or copper(I) chloride. In
addition, in the case of dinuclear complexes based on
a
Scheme 1. Synthesis of P∧N-type Ligands 1−8
The structures of complexes 5-I(b) and 7-I(b) are shown in
Figure 2; selected bond lengths and angles are given in Tables
S3 and S4 in the Supporting Information.
The tetranuclear copper halide core can be seen as a dimer of
Cu2I2 dimers, each showing a butterfly-shaped structure. The
copper atoms of each subunit are coordinated by one bridging
P∧N ligand. Two other P∧N ligands act as monodentate ligands
coordinating one copper atom of each subunit via their
phosphorus atoms. One of the bridging halide atoms in each
Cu2I2 core coordinates a copper atom of the other subunit and
thus links the two subunits. As mentioned before, ligand 6
behaves similarly to ligand 5 and forms the corresponding
complexes 6-I(a) and 6-I(b), depending on the exact molar
ratio between P∧N ligand and copper salt used for synthesizing
the corresponding complex.
Another different structure of a dinuclear complex was
obtained using 1-benzyl-5-(diphenylphosphino)-1H-tetrazole
(8) as ligand. Again, the synthesis was carried out as described
above and the obtained complex powder was analyzed via
elemental analysis, confirming a structure with a 3:2
stoichiometry for complex 8-I. However, single crystals were
analyzed, revealing a dinuclear complex with different ligands as
shown in Figure 3 (complex 8a-I). Obviously, a rearrangement
of the ligands according to Scheme 2 took place, which resulted
a
Reagents and conditions: (a) DBU, R−X, THF, room temperature;
(b) (1) n-BuLi, THF −78 °C, (2) Ph2PCl, −78 °C to room
temperature; (c) NEt3, Ph2PCl, pyridine, room temperature; (d)
NaN3, HC(OEt)3, AcOH, 80 °C; (e) (1) n-BuLi, THF, −100 °C, (2)
Ph2PCl, −100 °C to room temperature.
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Figure 2. Molecular structure of complexes 5-I(b) (top) and 7-I(b)
(bottom). Hydrogen atoms and minor disordered parts are omitted
for clarity; displacement parameters are drawn at the 50% probability
level.
Figure 1. Molecular structure of complexes 2-I (top) and 3-I
(bottom). Hydrogen atoms and minor disordered parts are omitted
for clarity; displacement parameters are drawn at the 50% probability
level.
in the transfer of a diphenylphosphino group from one ligand
to the benzyl carbon atom of another ligand with substitution
of a hydrogen atom. Tetrazoles are known for their wide range
of rearrangement reactions, and keeping in mind that copper is
a metal known for its multifaceted catalytical activities, a kind of
copper-mediated rearrangement reaction seems to take
place.92−96 However, since this is a nontrivial task to clarify,
detailed studies on the exact reaction mechanisms will be
necessary and are currently in the focus of our research work.
As shown in Figure 3, ligand 8a acts as chelating ligand via its
two phosphorus atoms. Each copper atom is tetrahedrally
coordinated by two iodides and two phosphines, and the
resulting complex structure features a copper−copper distance
of 3.16 Å (Table S5, Supporting Information).
Figure 3. Molecular complex structure of complex 8a-I. Hydrogen
atoms and minor disordered parts are omitted for clarity; displacement
parameters are drawn at the 50% probability level.
Scheme 2. Structural Rearrangement in the Ligand Sphere of
Complex 8-I Yielding Complex 8a-I
Such a complex structure with a butterfly-shaped copper
halide core and two chelating bisphosphines ligands is rare in
the literature, but has been reported by Tsuboyama et al. with
1,2-bis(diphenylphosphino)benzene as chelating ligand.42
In summary, copper(I) complexes based on five-membered
heterocycles exhibit a high structural diversity. Dinuclear
complexes 1-I−4-I, 5-I(a), and 6-I(a) with the stoichiometry
L3Cu2I2 are obtained using ligands with 1,2,4-triazole or
oxadiazole moieties and a 3:2 molar ratio between P∧N ligand
and metal salt. Additionally tetranuclear complexes 5-I(b), 6-
I(b), and 7-I(b) of L4Cu4I4 stoichiometry can be synthesized
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by reacting oxadiazole- or thiadiazole-containing P∧N ligands in
a 1:1 stoichiometry. Finally, by using 1-benzyl-5-(diphenyl-
phosphino)-1H-tetrazole (8) as ligand, complex 8-I of L3Cu2I2
stoichiometry is accessible. However, a rearrangement in the
ligand sphere leading to the dinuclear complex 8a-I of
stoichiometry L2Cu2I2 took place, as revealed by single-crystal
X-ray measurements. Investigations on the detailed reaction
mechanism are ongoing. The results of the different
aforementioned complexation reactions are also summarized
in Table 1.
Table 1. Summary of the Syntheses of Different Copper(I)
Complexes in This Study
a
b
c
ligand
complex
L:M
stoichiometry
structure
1
2
3
4
5
5
6
6
7
8
1-I
3:2
3:2
3:2
3:2
3:2
1:1
3:2
1:1
1:1
3:2
L3Cu2I2
L3Cu2I2
L3Cu2I2
L3Cu2I2
L3Cu2I2
L4Cu4I4
L3Cu2I2
L4Cu4I4
2-I
L3Cu2I2
L3Cu2I2
3-I
4-I
5-I(a)
5-I(b)
6-I(a)
6-I(b)
7-I(b)
8-I
97
L4Cu4I4
98
L4Cu4I4
L4Cu4I4
d
L3Cu2I2
L2Cu2I2
a
b
Ligand-to-metal ratio used for the synthesis of the complex. Based
on elemental analysis of the isolated complex powder after
c
precipitation and washing of the complex. Result of single-crystal
d
X-ray structure analysis if existent. Due to rearrangement in the
ligand sphere of the complex in solution.
Figure 4. Absorption spectra of complexes 4-I, 5-I(a), and 5-I(b) as
well as their corresponding ligands 4 and 5 at room temperature in
dichloromethane solution.
Photophysical and Theoretical Characterization. Ab-
sorption spectra of complexes 1-I, 4-I, 5-I(a), and 5-I(b) were
measured in dichloromethane solution at room temperature.99
These spectra are shown in Figure 4 along with those of the
free ligands 4 and 5. Spectra of 1 and 1-I are omitted for clarity,
since they are similar in shape to the spectra of 4 and 4-I, but
they are available in Figure S2 in the Supporting Information.
Dinuclear triazolylphosphane-based complexes 1-I and 4-I
show a shoulder at 248 nm (ε = 43790 and 44630 M−1 cm−1),
which can be correlated to an absorption peak at 248 nm (ε =
13420 and 11830 M−1 cm−1) in the free ligand spectra of 1 and
4, respectively. These are assigned to π−π* transitions within
the phenyl rings on the phosphane moieties. In addition, these
complexes feature additional tails, which are not present in the
corresponding free ligand spectra, ranging from 300 to 420 nm
and from 300 to 380 nm (ε = 11990−9820 M−1 cm−1) for
complexes 1-I and 4-I, respectively. These low-energy bands
are assigned to charge transfer transitions.
DFT calculations suggest that these transitions have metal to
ligand charge transfer (MLCT) character mixed with halogen
to ligand charge transfer (XLCT) character and are therefore
assigned to (M+X)LCT states. The same trends can be
observed in the spectra of 5, 5-I(a), and 5-I(b), with peaks at
256, 261, and 262 nm (ε = 22632, 77510, and 107930 M−1
cm−1, respectively) assigned to π−π* transitions within the
phenyl rings on the phosphane moieties and additional tails at
320 nm to approximately 400 nm (ε = 6941−4322 M−1 cm−1),
which were assigned to (M+X)LCT transitions according to
DFT calculations (see below). The absorption spectra of
complexes 5-I(a) and 5-I(b) are very similar in shape and peak
position. Keeping in mind that complex 5-I(a) has a dinuclear
structure and complex 5-I(b) features a tetranuclear complex
structure, a possible explanation for the similarity would be that
the photophysical properties of both complex structures are
related to each other. This issue will be further discussed below
on the basis of emission and lifetime measurements in solution.
The photophysical properties of neat complex powders of 1-
I−4-I, 5-I(a), 5-I(b), 6-I(a), 6-I(b), 7-I(b), and 8-I were
investigated. The emission spectra of complexes 2-I, 3-I, 5-I(a),
5-I(b), 6-I(b), 7-I(b), and 8-I are displayed in Figures 5 and 9.
In addition, the photophysical characteristics of all investigated
complexes can be seen in Table 2. Emission spectra of
complexes 1-I, 4-I, and 6-I(a) are shown in Figure S3 in the
Supporting Information for completeness, since they are similar
in profile to the spectra shown in Figures 5 and 9.
The spectra of neat powder measurements of triazole
complexes 1-I−4-I, oxadiazole complexes 5-I(a), 5-I(b), 6-
I(a), and 6-I(b), thiadiazole complex 7-I(b), and tetrazole
complex 8-I are broad and unstructured, indicating charge
transfer processes, which is in agreement with the results gained
from DFT calculations (see below). Complexes 1-I−4-I with
1,2,4-triazole moieties exhibit luminescence maxima in the blue
range of the visible spectrum at 485, 485, 458, and 496 nm
(complexes 1-I−4-I, respectively), while complexes 5-I(a), 6-
I(a), and 8-I with oxadiazole or tetrazole moieties have
emission maxima in the green-yellow region at 544, 548, and
531 nm, respectively. All of these complexes have the same
structure, namely L3Cu2I2, and are therefore comparable with
each other.
The assumption of (M+X)LCT transitions for the dinuclear
complexes 1-I−4-I, 5-I(a), 6-I(a), and 8-I is in accordance with
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identical structures featuring substituted pyridylphosphanes as
P∧N ligands: ligands with high LUMO energies lead to
increasing LUMO energies of the resulting complexes.38 The
energy of the HOMO stays roughly the same, and thus a higher
LUMO energy leads to a blue shift of the emission maximum.
Using the LUMO energy of the ligand to predict the
emission wavelength is based on the assumption that there is
not only a correlation between the complex LUMO and the
ligand LUMO regardless of the complex structure but also a
correlation of the complex LUMO and the emission energy,
thus neglecting that the structural relaxation in the excited state
might be different for different compounds. Considering the
enormous structural variety of the complexes presented in this
study, it is almost surprising that there is indeed a relationship
between the energy of the ligand LUMO and the emission
wavelength (see Figure 7).
Figure 5. Neat powder measurements of complexes 2-I, 3-I, 5-I(a),
and 8-I (λexc 350 nm). For clarity, emission spectra of complexes 1-I,
4-I, and 6-I(a) can be found in Figure S3 in the Supporting
Information.
a
Table 2. Photoluminescence Characteristics
b
c
d
d
compd
λem (nm)
ΦPL
τav (μs) kr (105 s−1
)
knr (105 s−1
)
1-I
485
485
458
496
544
550
548
541
617
531
0.19
0.95
0.10
0.26
0.70
0.48
0.68
0.26
0.45
0.47
1.09
3.69
1.61
1.77
1.81
2.46
1.19
1.38
1.27
1.05
1.74
2.57
0.62
1.47
3.87
1.95
5.71
1.88
3.54
4.48
7.43
0.14
5.59
4.18
1.66
2.11
2.69
5.36
4.33
2-I
3-I
4-I
5-I(a)
5-I(b)
6-I(a)
6-I(b)
7-I(b)
8-I
5.05
Figure 7. Correlation between ligand LUMO energies and emission
maxima of the corresponding complexes (for exact values of the
LUMO energies see Table S9 in the Supporting Information).
a
b
c
Neat powders at room temperature, λexc 350 nm. Error 5%. Since
the decay is best fitted by two or three exponentials, a weighted
average lifetime (τav) is given, calculated by the equation τav = ∑Aiτi/
∑Ai, where Ai is the pre-exponential for the lifetime τi. The respective
values can be seen in Table S6 in the Supporting Information. Decay
times at 512 and 549 nm, respectively.
d
Measurements in dichloromethane solution, neat films, or
PMMA matrices have been conducted in order to investigate
the influence of different environments.99 The emission spectra
of complex 2-I are shown in Figure 8 as a representative
example for complexes with a 1,2,4-triazole moiety, while the
emission spectra of complex 4-I can be seen in Figure S4 in the
the results gained from DFT calculations of 2-I, showing that
the HOMO resides on the copper iodide core and the LUMO
is located in the ligand sphere (see Figure 6).
As can be seen from the data (Table 2), the emission maxima
of the dinuclear complexes 1-I−4-I, 5-I(a), 6-I(a), and 8-I are
red-shifted on going from triazole to tetrazole to oxadiazole
moieties, indicating a decrease in the HOMO−LUMO gap. It
has been previously reported that there is a correlation between
ligand LUMO energy and the LUMO energy of complexes with
Figure 8. Photoluminescence spectra of complex 2-I in different
environments: powder, solution, neat film, and PMMA doped with
50% emitting compound (λexc 350 nm).
Figure 6. HOMO (left) and LUMO (right) of the dinuclear complex
2-I.
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Table 3. Luminescence Properties of Complexes 2-I and 4-I in Different Environments
2-I
4-I
a
b
a
b
complex
powder
λmax (nm)
ΦPL
τav (μs)
kr (105 s−1
)
knr (105 s−1
)
λmax (nm)
ΦPL
τav (μs)
kr (105 s−1
)
knr (105 s−1
)
486
492
492
532
0.95
0.42
0.29
0.03
3.69
1.40
1.74
0.10
2.57
3.00
3.79
3.00
0.14
4.14
1.95
9.70
495
495
495
541
0.26
0.43
0.66
0.03
1.77
1.62
1.72
0.21
1.47
2.65
3.84
1.42
4.18
3.52
1.98
46.2
neat film
50% PMMA
CH2Cl2 solution
a
b
Error 5%. Since the decay is best fitted by using two or three exponentials, a weighted average lifetime (τav) is given, calculated by the equation
τav = ∑Aiτi/∑Ai, where Ai is the pre-exponential for the lifetime τi. The respective values can be seen in Table S7 in the Supporting Information.
Supporting Information. In Table 3, photophysical character-
istics of these complexes are summarized. The emission maxima
in dichloromethane solution are red-shifted in comparison to
powder, film, or matrix measurements. This red shift is also
present when comparing powder with film or matrix measure-
ments, but it is less pronounced with the latter.
As can be seen from the data, complex 2-I with a long alkyl
substituent, i.e. a butyl group, exhibits a stronger red shift of the
emission maxima in film or matrix measurements in
comparison to that in powder measurements (as powder, 486
nm; as neat film or doped in PMMA, 492 nm; in CH2Cl2, 532
nm), while complex 4-I with a benzyl substituent exhibits no
red shift of the emission maxima in film or matrix measure-
ments (as powder, 495 nm; as neat film or doped in PMMA,
495 nm; in CH2Cl2, 541 nm).
quantum yields, and the increase of the nonradiative rate
constants, can be explained by molecular distortions in the
excited state, which can occur more easily in less rigid
environments such as solutions in comparison to environments
of higher rigidity such as crystal powders, films, and matrices.102
The emission maxima of tetranuclear complexes 5-I(b), 6-I(b),
and 7-I(b) are at 550, 541, and 617 nm, respectively. There
seems to be a slight influence of the additional methyl group in
ligand 5 in comparison to ligand 6, leading to a marginal blue
shift in the emission maxima of 6-I(b) in comparison to 5-I(b).
A clearly visible red shift is observed in the case of 7-I(b) in
comparison to 5-I(b) or 6-I(b), as can be seen in Figure 9.
Emission quantum yields of all compounds as neat powders
are shown in Table 2. The measured values for the dinuclear
complexes are moderate to high, reaching values up to ϕPL
=
0.95 for complex 2-I, which is comparable to those found for
dinuclear complexes based on pyridylphosphane ligands and
other comparable copper(I) clusters.38,42,67,101 The lower
quantum yields for complexes 1-I, 3-I, and 4-I might be
explained by the influence of the different substituents,
although the trend is not very clear.38 Complex 1-I and
complex 3-I, which have a shorter and a longer alkyl chain,
respectively, show relatively small quantum yields with ϕPL
=
0.19 and ϕPL = 0.10, and complex 4-I with a benzyl substituent
also features a rather small value of ϕPL = 0.26. Emission
quantum yields of both complexes, 2-I and 4-I, as neat
powders, films, or matrices are in the range of 29−66% but are
significantly lower in dichloromethane solution at 3% (Table
3). Excited-state lifetimes of the dinuclear complexes are in the
range 1.61−3.69 μs. The decay curves were best fitted with
two- and three-exponential decays, which might be due to
different structural environments, as was recently shown by the
Coppens group.100 They are comparable to the values found in
similar complexes, dinuclear as well as tetranuclear in
nature.38,42,67,101 The exact values together with the corre-
sponding radiative and nonradiative rate constants can be seen
in Table 2.
Excited-state lifetimes of neat films or PMMA matrices
doped with 50% of the emitting compound are in the range 1−
2 μs for the investigated complexes 2-I and 4-I and are
therefore slightly shorter than the values of neat complex
powders. Emission decay times measured in dichloromethane
solution are even shorter than those found in measurements of
neat powders, films, or matrices with decay times of 0.1−0.3 μs.
Simultaneously, the nonradiative rate constants increase up to
knr = 9.70 × 10−5 and knr = 46.2 × 10−5, while the radiative rate
constants remain roughly the same for powder, film, matrix, and
solution measurements (Table 3). This behavior, i.e. the red
shift of the emission maxima, the decrease of lifetimes and
Figure 9. Neat powder measurements of complexes 5-I(b), 6-I(b),
and 7-I(b) (λexc 350 nm).
Dinuclear complexes 5-I(a) and 6-I(a) exhibit emission
quantum yields with ϕPL = 0.70 and 0.68. Tetranuclear
copper(I) complexes feature only slightly smaller quantum
yields in comparison to their dinuclear counterparts, with values
of ϕPL = 0.48, 0.26, and 0.45 for 5-I(b), 6-I(b), and 7-I(b). The
emission decay times of the tetranuclear complexes are between
1.27 and 2.46 μs.
Complexes 5-I(b) and 7-I(b) were investigated using DFT.
According to single-crystal diffraction data the complexes 5-
I(b) and 7-I(b) have very similar structures. Because of the
disorder in 5-I(b), the crystal structure of 7-I(b) was used as
the starting structure for the geometry optimization of both
compounds and sulfur was substituted by oxygen to get
complex 5-I(b). Single-crystal diffraction data show that these
tetranuclear complexes can be regarded as dimers of two
dinuclear complexes: both subunits contain a butterfly-shaped
Cu2I2 core, a bridging P∧N ligand, and one monodentate ligand.
In addition, photophysical measurements of the dinuclear
complexes 5-I(a) and 6-I(a) show emission maxima which
differ only slightly from those of tetranuclear complexes 5-I(b)
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and 6-I(b) with values of 544 and 548 nm in comparison to
550 and 541 nm, respectively. This suggests that the dinuclear
and the tetranuclear forms have similar electronic properties.
The DFT calculations reveal that the HOMO resides on the
Cu4I4 core and the LUMO on both bridging ligands (see Figure
10), similar to the frontier orbitals of the dinuclear complexes.
hypothesis, since they are similar: 0.52 μs for 5-I(a) and 0.64 μs
for 5-I(b).
Low-temperature measurements have been performed for
complexes 1-I−4-I, 5-I(a), 5-I(b), 7-I(b), and 8-I. Figure 12
Figure 10. HOMO (left) and LUMO (right) of 7-I.
Thus, an excitation from the HOMO to the LUMO has
pronounced M+X to L charge transfer character. The
phosphorescence energy of 7-I(b), which contains a thiadiazole
ligand, is much smaller than the phosphorescence energy of 5-
I(b) with its oxadiazole ligand. The absolute values (0.96 eV for
complex 7-I(b) and 1.27 eV for complex 5-I(b)) calculated as
the energy difference between the lowest triplet state and the
singlet ground state at the optimized triplet geometry, are red
shifted by approximately 1 eV due to the insufficient
description of charge transfer excitations using BP86. Despite
the large red shift, the difference between the phosphorescence
of 5-I(b) and 7-I(b) is calculated to be 0.3 eV, which agrees
very well with measured emission energies of 2.25 and 2.01 eV,
respectively.
Figure 12. Emission spectra of neat complex powders of complexes 1-
I and 7-I(b) at ambient temperature and at 77 K (λexc 350 nm).
shows luminescence spectra of representative examples for
dinuclear and tetranuclear complexes 1-I and 7-I(b) measured
as neat powders at 77 K and at room temperature. The
corresponding spectra of complexes 2-I−4-I, 5-I(a), 5-I(b),
and 8-I are similar in shape and are therefore omitted for
clarity, but they are available in Figures S5−S10 in the
Supporting Information. The emission maxima of the
investigated complexes 1-I−4-I, 5-I(a), 5-I(b), 7-I(b), and 8-
I display a blue shift in the range 0−17 nm in comparison to
the corresponding ambient-temperature spectra (Table 4).
The photophysical properties of the dinuclear complex 5-
I(a) and the tetranuclear complex 5-I(b) were investigated
additionally in dichloromethane solution, and the spectra are
shown in Figure 11 along with the powder spectra. The
a
Table 4. Photoluminescence Characteristics
b
λem (nm)
τav (μs)
complex
298 K
77 K
298 K
77 K
1-I
485
485
458
496
544
550
617
531
475
485
441
487
539
538
617
525
1.09
3.69
1.61
1.77
1.81
2.46
1.27
38.7
49.8
18.4
33.8
24.8
30.8
62.3
40.9
2-I
3-I
4-I
5-I(a)
5-I(b)
7-I(b)
8-I
1.05
a
b
Neat powders at room temperature, λexc 350 nm. Since the decay is
best fitted by two exponentials, a weighted average lifetime (τav) is
given, calculated by the equation τav = ∑Aiτi/∑Ai, where Ai is the pre-
exponential for the lifetime τi. The respective values can be seen in
Table S8 in the Supporting Information.
Figure 11. Measurement of complexes 5-I(a) and 5-I(b) as neat
powders and in dichloromethane solution (λexc 350 nm).
In addition, excited-state lifetimes increase by an order of
magnitude into the range of 24.8−62.3 μs, which can be
explained by the suppression of thermally activated non-
radiative processes at low temperature.38 The observed blue
shift of the emission maxima for complexes 1-I−4-I is in
contrast with the reported results of red-shifted emission
maxima for dinuclear complexes of comparable structure, which
were explained by an emission from a lower lying triplet
state.38,42,67 Blue-shifted emission maxima were found, for
example, in Ir(III) or Re(I) complexes at low temperatures and
emission maxima of neat powder measurements of both
complexes (544 and 550 nm) are very similar, and the emission
maxima in solution (589 and 588 nm) are almost identical,
where the latter are red-shifted in comparison to the powder
measurements. The similarity of the emissions suggests that
both complex structures have similar electronic properties
responsible for their photophysical charactertistics, as was also
seen in DFT calculations. Lifetime measurements support this
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are explained by the occurrence of rigidochromic effects.103−105
A progressive blue shift can be observed for the complexes
studied in this work by changing the environment from
nonrigid media such as solvents to rigid media such as crystal
powders at room temperature. This can be explained by
rigidochromic effects, and a further blue shift at low
temperature might be related to similar effects.
spectrum in the range 458−496 nm, with quantum yields
reaching values up to 95%. The dinuclear and tetranuclear
complexes based on oxadiazole ligands show similar emission
spectra measured as powders as well as in dichloromethane
solution because both complexes have similar electronic
properties. This is supported by DFT calculations, which reveal
that the HOMO resides on the Cu4I4 core and the LUMO on
both bridging ligands, similar to the frontier orbitals of the
dinuclear complexes.
Low-temperature measurements at 77 K of both the
dinuclear and the tetranuclear complexes revealed a blue shift
of the emission maxima in comparison to the corresponding
measurements at ambient temperature. This behavior was
ascribed to rigidochromic effects, which were clearly visible
while going from less rigid (i.e., dichloromethane solution) to
more rigid (i.e., crystal powder) environments. In an even more
rigid environment at 77 K the emitting state in these complexes
might be less stabilized, which might lead to a further blue shift
of the emission maxima.
It may be possible that the emitting state in these complexes
is less stabilized due to a more rigid environment at 77 K,
resulting in a blue shift of the emission maxima at low
temperature. It should be noted that the assignment of the
1
3
exact nature of the emitting state, i.e. MLCT or MLCT, is a
nontrivial task. Luminescent copper(I) complexes often show
relatively short lifetimes at ambient temperature and red-shifted
emission maxima along with significantly longer lifetimes at low
temperature. Such a behavior can be attributed to a process
called thermally activated delayed fluorescence.56,106,107 Ac-
cording to this model, the emission of these complexes is
1
assigned to MLCT transitions at room temperature and to
lower lying 3MLCT transitions at low temperature. Therefore, a
red shift of the emission maxima is observed.38,56 The
complexes studied in this work show that two effects might
be present at low temperature: a blue shift due to rigidochromic
effects and a red shift due to emission from lower lying triplet
states. Obviously, a potential red shift is overcompensated by
rigidochromic effects, resulting in a blue shift of the emission
maxima.
In summary, P∧N ligands based on five-membered hetero-
cycles are an intriguing class of ligands, leading to several
different complex structures with interesting photophysical
properties. In particular, dinuclear complexes with a 1,2,4-
triazole moiety exhibit emission maxima in the deep blue region
(458−496 nm) together with high quantum yields up to 95%
and are therefore well suited for potential applications as blue-
emitting materials in organic light-emitting devices.
ASSOCIATED CONTENT
* Supporting Information
CONCLUSION
■
■
Bridging P∧N ligands bearing five-membered heterocyclic
moieties have been investigated with regard to their complex-
ation behavior with copper(I) iodide as the metal salt and with
respect to the photophysical properties of the resulting
complexes. Different structures were found, depending either
on the nature of the ligand itself or on the ligand to metal ratios
used for the complexation reaction. A dinuclear complex of the
general composition L3Cu2I2 was found for 1,2,4-triazole-based
(complexes 1-I−4-I) and oxadiazole-based (complexes 5-I(a)
and 6-I(a)) P∧N ligands, leading to a complex structure in
which one ligand acts as a bridging ligand for the dinuclear,
butterfly-shaped metal iodide core, while the other two ligands
coordinate solely via their phosphorus atoms. In addition, in the
case of oxadiazoles, a tetranuclear complex structure of the
general composition L4Cu4I4 was found by reacting the P∧N
ligand and the metal salt in a 1:1 molar ratio instead of a ratio
of 3:2, leading to a new kind of complex structure.This
structure can be regarded as a dimer of two dinuclear
complexes: both subunits contain a butterfly-shaped Cu2I2
core, a bridging P∧N ligand, and one monodentate ligand.
Furthermore, it was possible to synthesize either the dinuclear
or the tetranuclear complex by controlling the stoichiometry of
ligand to metal salt. In the case of thiadiazoles, only the
tetranuclear form was obtainable. Two different dinuclear
complex structures showing a composition of L2Cu2I2 were
obtained by using a tetrazole ligand. In the case of the tetrazole
complex, elemental analysis confirmed the presence of a
dinuclear complex with L3Cu2I2 stoichiometry. However,
single-crystal X-ray analysis revealed a rearrangement in the
ligand sphere of the complex, leading to a complex of L2Cu2I2
stoichiometry. The exact reaction mechanism is currently under
investigation.
S
CIF files, text, tables, and figures giving crystallographic data for
2-I, 3-I, 5-I(b), 5-I(b)′, 7-I(b), and 8a-I, spectroscopic data of
compounds 2, 3, 7, and 8, additional photophysical spectra, and
Cartesian coordinates (in Å) of the optimized BP86/def2-
SV(P) ground-state and triplet geometries of compounds 2-I
and 7-I(b). This material is available free of charge via the
Internet at http://pubs.acs.org. Crystallographic data have also
been deposited with the Cambridge Crystallographic Database
with the following deposition numbers: CCDC-913209 (2-I),
CCDC-913210 (3-I), CCDC-913511 (5-I(b)), CCDC-913212
(5-I(b)′), CCDC-913213 (7-I(b)), and CCDC-913215 (8a-I).
AUTHOR INFORMATION
Corresponding Authors
*T.B.: e-mail, baumann@cynora.com.
*S.B.: fax, (+ 49) 721 608 48581; e-mail, braese@kit.edu.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge financial support from the KIT. We also thank
the Deutsche Forschungsgemeinschaft (DFG) for support
through project B2 of SFB/TRR 88and the German Federal
Ministry of Education and Research (BMBF) in the funding
program cyCESH.
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