Synthesis, crystal structure and photoluminescence of phosphorescent copper (I) complexes containing hole-transporting carbazolyl group

Article abstract of DOI:10.1016/j.ica.2011.10.049

Two mononuclear Cu (I) complexes based on a 2-(2-pyridyl)benzimidazolyl derivative ligand containing the hole-transporting ligand 1-[4-(9-carbazolyl) butyl]-2-(2-pyridyl)benzimidazole (L), [Cu(L)(PPh₃) ₂](BF₄) (1) and [Cu(L)(DPEphos)](BF₄) (2), where DPEphos is bis[2-(diphenylphosphino)phenyl]ether, were synthesized and characterized by elemental analysis, ¹H NMR and FT-IR spectra. The structure of complex 2 was determined by X-ray single crystal analysis. It was demonstrated that the central Cu (I) ion in the complex has a distorted tetrahedral geometry and is tetra-coordinated by the two nitrogen atoms from L ligand and two phosphorus atoms from DPEphos ancillary ligand. The photophysical properties of the complexes were examined by using UV-Vis, photoluminescence spectroscopic analysis. The complexes exhibit weak MLCT absorption bands at 410 and 490 nm, and display strong phosphorescence in the solid states at T = 298 K, which is completely quenched in solution.

Full text of DOI:10.1016/j.ica.2011.10.049

Inorganica Chimica Acta 383 (2012) 78–82
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Inorganica Chimica Acta
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Synthesis, crystal structure and photoluminescence of phosphorescent copper
(I) complexes containing hole-transporting carbazolyl group
Tianzhi Yu a, , Ziyi Wang a,b, Yuling Zhao b, Xiaoguang Wen b, Hui Zhang a, Duowang Fan a
⇑
a Key Laboratory of Opto-Electronic Technology and Intelligent Control (Ministry of Education), Lanzhou Jiaotong University, Lanzhou 730070, China
b School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 January 2011
Received in revised form 11 October 2011
Accepted 13 October 2011
Available online 21 October 2011
Two mononuclear Cu (I) complexes based on
a 2-(2-pyridyl)benzimidazolyl derivative ligand
containing the hole-transporting ligand 1-[4-(9-carbazolyl)butyl]-2-(2-pyridyl)benzimidazole (L),
[Cu(L)(PPh3)2](BF4) (1) and [Cu(L)(DPEphos)](BF4) (2), where DPEphos is bis[2-(diphenylphos-
phino)phenyl]ether, were synthesized and characterized by elemental analysis, 1H NMR and FT-IR spec-
tra. The structure of complex 2 was determined by X-ray single crystal analysis. It was demonstrated that
the central Cu (I) ion in the complex has a distorted tetrahedral geometry and is tetra-coordinated by the
two nitrogen atoms from L ligand and two phosphorus atoms from DPEphos ancillary ligand. The photo-
physical properties of the complexes were examined by using UV–Vis, photoluminescence spectroscopic
analysis. The complexes exhibit weak MLCT absorption bands at 410 and 490 nm, and display strong
phosphorescence in the solid states at T = 298 K, which is completely quenched in solution.
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Copper (I) complex
Crystal structure
Benzimidazolyl derivative
Photoluminescence
1. Introduction
green–yellow mononuclear Cu (I) complexes with different phos-
phine and phenanthroline ligands in the solid state, and the best
Since Forrest and co-workers [1] successfully utilized the phos-
phorescent material PtOEP (platinum octaethylporphyrin) to fabri-
cate light-emitting devices, many heavy-metal complexes have
been extensively investigated in highly efficient electrophospho-
rescent organic light-emitting diodes. Most of the complexes are
mainly complexes of noble metal ions such as PtII [1–3], IrIII [4–
10], OsII [11,12] and ReI [13–15]. In these electrophosphorescent
materials, cyclometalated iridium complexes are the most valuable
emitting materials due to their high quantum efficiency, bright-
ness, color diversity and short excited-state lifetime.
Luminescent mono- and multinuclear copper (I) complexes have
been extensively investigated because they have possible applica-
tions in solar energy conversion, luminescence-based sensors, dis-
play devices, and probes of biological systems [16–21]. Generally
Cu (I) complexes have relatively low quantum yields as compared
with rare and noble metal complexes, nevertheless, in view of their
inexpensiveness and non-toxicity, they may constitute a class of
promising phosphorescent emitters for organic light-emitting
diodes (OLEDs) if ligands of suitable design are used in order to
improve their performances [22–24]. Wang and co-workers [22]
reported on the photophysical properties of a series of green to
OLED devices with the complexes as dopants exhibited a current
efficiency of 10.5 cd/A and a maximum brightness of 1663 cd/m2.
This efficiency was comparable to that of Ir (III) complexes in similar
device structures. A series of orange–red to red phosphorescent
heteroleptic Cu (I) complexes with biquinoline derivatives and dif-
ferent phosphine ancillary ligands had been investigated as phos-
phorescent materials, and the best OLED devices fabricated with
the complexes as dopants showed efficient red phosphorescence
with a current efficiency of 6.4 cd/A and an external quantum effi-
ciency of 4.5% [23]. The performance was close to that of solution-
processed red-emitting devices based on noble metal complexes.
The OLED device using a halogen-bridged dinuclear complex as a
green emissive dopant exhibited a current efficiency of 10.4 cd/A
and power efficiency of 4.2 lm/W at 93 cd/m2, and maximum exter-
nal quantum efficiency of 4.8% [24]. From the above mentioned
examples, it was indicated that the inexpensive and non-toxic Cu
(I) complexes could be used as promising candidates for OLEDs
applications.
A series of mononuclear and polynuclear Cu (I) complexes con-
taining 2-(2-pyridyl)benzimidazolyl derivative ligands have been
investigated with emission ranging along the visible spectrum, but
the OLED devices using them as dopants displayed very low effi-
ciency due to their long decay lifetimes [25,26]. The mononuclear
Cu (I) complexes contained the 2-(2-pyridyl)benzimidazolylben-
zene ligand, the polynuclear Cu (I) complexes were based on linear
⇑
Corresponding author. Tel.: +86 931 4956935; fax: +86 931 4938756.
E-mail address: yutianzhi@hotmail.com (T. Yu).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.10.049

T. Yu et al. / Inorganica Chimica Acta 383 (2012) 78–82
79
or star-shaped ligands that contained two or more of the 2-(2-pyri-
dyl)benzimidazolyl chelating units linked together by an aromatic
group such as benzene or biphenyl. In order to get better
understanding of how the structural features of the ligands may
control the intrinsic luminescence property of the 2-(2-pyri-
dyl)benzimidazolyl-based Cu (I) complexes, we have synthesized a
new 2-(2-pyridyl)benzimidazolyl derivative ligand which has a con-
nection between the benzimidazolyl ring and a carbazole moiety via
an alkyl linker. Two new mononuclear Cu (I) complexes based on
this ligand, namely [Cu(L)(DPEphos)](BF4) and [Cu(L)(PPh3)2](BF4),
were synthesized and characterized by elemental analysis, 1H
NMR and FT-IR spectra. The photophysical properties of the com-
plexes were examined by using UV–Vis and photoluminescence
spectroscopic analysis.
The organic phase was washed with water and dried over anhydrous
MgSO4. After removal of solvent, the crude was purified by chroma-
tography on silica gel using petroleum ether as eluent to give 9-(4-
bromobutyl)carbazole (5.05 g, 69.8%). m.p. 98–100 °C. IR (KBr pellet
cmꢁ1): 3051 (Aryl-CH), 2960, 2928, 2865 (Alkyl-CH), 1627, 1595,
1483, 1226, 753, 648. 1H NMR: (CDCl3, d, ppm): 8.12 (d, J = 7.6 Hz,
2H, Aryl-H), 7.45 (m, 2H, Aryl-H), 7.41 (d, J = 8 Hz, 2H, Aryl-H),
7.22 (m, 2H, Aryl-H), 4.34 (t, J = 6.8 Hz, 2H, N–CH2), 3.36 (t,
J = 6.4 Hz, 2H, Br–CH2), 2.05 (m, 2H, CH2), 1.92 (m, 2H, CH2).
2.2.3. 1-[4-(9-Carbazolyl)butyl]-2-(2-pyridyl)benzimidazole (L)
Under nitrogen, solid NaH (60% dispersion in mineral oil, 0.50 g)
and 2-(2-pyridyl)benzimidazole (1.00 g, 5.12 mmol) in 20 mL of
anhydrous DMF were stirred at 80 °C for 2 h. The resulting solution
was cooled to room temperature, and 9-(4-bromobutyl)carbazole
(1.55 g, 5.12 mmol) was added. The mixed solution was stirred at
80 °C for 36 h. After completing, the reaction mixture was poured
into 100 mL of cold water, and was extracted with dichlorometh-
ane (3 ꢂ 50 mL). The organic phase was washed with water and
dried over anhydrous MgSO4. After removal of solvent, the residue
was purified by column chromatography using ethyl acetate/petro-
leum ether (1:2, v/v) as eluent to give a white powder. Yield: 88%.
IR (KBr pellet cmꢁ1): 3048 (Aryl-CH), 2926 (Alkyl-CH), 1596 (C@N),
1456, 1328, 1150, 750. 1H NMR(CDCl3, d, ppm): 8.54 (d, 1H, J = 4.8,
Aryl-H), 8.40 (d, 1H, J = 7.6, Aryl-H), 8.09 (t, 2H, J = 8.0, Aryl-H),
7.86–7.80 (m, 2H, Aryl-H), 7.44 (t, 2H, J = 8.8, Aryl-H), 7.34–7.26
(m, 4H, Aryl-H), 7.25–7.21 (m, 4H, Aryl-H), 4.8 (d, 2H, J = 8.0,
N–CH2–), 4.34 (d, 2H, J = 7.6, N–CH2–), 1.99–1.93 (m, 4H, –CH2–).
Anal. Calc. for C28H24N4: C, 80.74; H, 5.31; N, 13.45. Found: C,
80.58; H, 5.35; N, 13.37%.
2. Experimental
2.1. Materials and methods
Cu(BF4)2ꢀ6H2O, bis[2-(diphenylphosphino)phenyl]ether (DPE-
phos) and carbazole were purchased from Aldrich. Triphenylphos-
phine (PPh3) was obtained from Acros Organics. Polyphosphoric
acid, o-phenylenediamine and 2-picolinic acid were obtained from
commercial purchase, used without further purification. 1,4-Dibro-
mobutane and tetrabutylammonium bromide (TBAB) was bought
from Tianjin BASF chemical reagent Co., Ltd. Copper powder was
from Shenyang Keda Chemical Reagent Factory (China). All other
chemicals were analytical grade reagent.
[Cu(NCCH3)4](BF4) was obtained by reaction of Cu(BF4)2ꢀ6H2O
and copper powder in acetonitrile according to the method
reported by Kubas [27].
2.3. Synthesis and characterization of [Cu(L)(PPh3)2](BF4) (1) and
[Cu(L)(DPEphos)](BF4) (2)
IR spectra (400–4000 cmꢁ1) were measured on a Shimadzu
IRPrestige-21 FT-IR spectrophotometer. 1H NMR spectra were
obtained on Unity Varian-500 MHz. C, H, and N analyses were
obtained using an Elemental Vario-EL automatic elemental analy-
sis instrument. UV–Vis absorption and photoluminescent spectra
were recorded on a Shimadzu UV-2550 spectrometer and on a Per-
kin–Elmer LS-55 spectrometer, respectively. Melting points were
measured by using an X-4 microscopic melting point apparatus
made in Beijing Taike Instrument Limited Company, and the ther-
mometer was uncorrected.
1 and 2 were synthesized by following the procedures described
in the literature [29,30]. All manipulations were performed under a
nitrogen atmosphere.
A mixture of [Cu(NCCH3)4](BF4) (0.32 g, 1.00 mmol) and either
PPh3 (0.53 g, 2.00 mmol) or DPEphos (0.54 g, 1.00 mmol) in anhy-
drous dichloromethane (10 mL) was stirred at room temperature
for 1 h. L (0.42 g, 1.00 mmol) in dichloromethane solution was
added to the reaction mixture dropwise and stirring was continued
for 12 h at room temperature to result in a clear yellow solution.
The reaction mixture was concentrated to 5 mL, and hexane
(15 mL) was then introduced to afford a yellow precipitate. The
crude product was crystallized from toluene/hexane at ambient
temperature to give air-stable, bright yellow crystals.
2.2. Synthesis and characterization of 1-[4-(9-carbazolyl)butyl]-2-(2-
pyridyl)benzimidazole (L)
The synthetic routes are shown in Scheme 1.
Complex 1: Anal. Calc. for C64H54N4P2CuBF4: C, 70.43; H, 4.99; N,
5.13. Found: C, 70.56; H, 4.91; N, 5.22%. 1H NMR(CDCl3, d, ppm): 8.19
(d, 1H, J = 4.8, Aryl-H), 8.12 (t, 2H, J = 8.4, Aryl-H), 8.05 (d, 2H, J = 7.6,
Aryl-H), 7.49 (d, 2H, J = 8.0, Aryl-H), 7.42 (t, 2H, J = 8.0, Aryl-H), 7.34–
7.03 (m, 37H, Aryl-H), 7.06–6.86 (m, 16H, Aryl-H), 6.82 (d, 6H, J = 4.5,
Aryl-H), 4.52 (t, 2H, J = 6.4, N–CH2–), 4.44 (t, 2H, J = 8.0, N–CH2–),
2.32 (t, 2H, J = 7.6, –CH2–), 1.95 (t, 2H, J = 6.4, –CH2–). IR (KBr pellet,
cmꢁ1): 3049 (Aryl-CH), 2923, 2860 (Alkyl-CH), 1589 (C@N), 1445
(C–N), 1327, 1058 (Aryl-C-P).
Complex 2: Anal. Calc. for C64H52N4OP2CuBF4: C, 69.54; H, 4.74;
N, 5.07. Found: C, 69.83; H, 4.82; N, 5.14%. 1H NMR(CDCl3, d, ppm):
8.18 (d, 1H, J = 4.8, Aryl-H), 8.04 (d, 2H, J = 8.0, Aryl-H), 7.99 (d, 2H,
J = 4.0, Aryl-H), 7.48 (d, 2H, J = 8.0, Aryl-H), 7.40 (t, 2H, J = 7.6, Aryl-
H), 7.23 (t, 1H, J = 8.8, Aryl-H), 7.20–7.10 (m, 12H, Aryl-H), 7.06–
6.86 (m, 16H, Aryl-H), 6.82 (d, 6H, J = 4.5, Aryl-H), 4.52 (t, 2H,
J = 6.4, N–CH2–), 4.39 (t, 2H, J = 8.0, N–CH2–), 2.30 (t, 2H, J = 7.2,
–CH2–), 1.94 (t, 2H, J = 7.0, –CH2–). IR (KBr pellet, cmꢁ1): 3050
(Aryl-CH), 2939 (Alkyl-CH), 1596 (C@N), 1438 (C–N), 1225, 1216
(Aryl-C–O–C-Aryl), 1209, 1066 (Aryl-C-P).
2.2.1. 2-(2-Pyridyl)benzimidazole
About 8.77 g (0.081 mol) of o-phenylenediamine, 10.00 g
(0.081 mol) of picolinic acid and 115.00 g of polyphosphoric acid
were mixed into a 500 mL three-necked bottle. The mixture was
stirred under nitrogen at 180 °C for 8 h. The resulting viscous solu-
tion was cooled and poured into 1000 mL of ice water. The pink
precipitate was collected by filtration, washed with water and
dried under vacuum. The crude product was recrystallized from
alcohol subsequent to treatment with a small amount of activated
charcoal. Yield: 85%, m.p. 223–225 °C (222–224 °C [28]).
2.2.2. 9-(4-Bromobutyl)carbazole
To a mixture of TBAB (0.32 g, 1.00 mmol) and aqueous 50% (w/w)
sodium hydroxide (25 mL) was added a solution of carbazole (4.00 g,
23.94 mmol) and 1,4-dibromobutane (15.56 g, 72.00 mmol) in
30 mL of toluene. The mixture was stirred at room temperature for
24 h. After the reaction was completed, the mixture was poured into
50 mL of water and extracted with dichloromethane (3 ꢂ 100 mL).

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T. Yu et al. / Inorganica Chimica Acta 383 (2012) 78–82
Br
N
N
N
N
NH2
NH2
PPA
N
+
N
N
NH
NaH / DMF
N
COOH
P
P
N
N
[Cu(CH3CN)4]BF4
PPh3
Cu
BF4
N
N
N
N
N
1
N
P
[Cu(CH3CN)4]BF4
DPEphos
N
N
Cu
BF4
O
P
N
N
2
Scheme 1. Synthetic routines to the ligand (L) and the Cu (I) complexes.
2.4. Crystallography
N(1) [N(1)–Cu(1)–O(1) = 167.87°]. Similar Cu–O separations were
also observed in DPEphos Cu (I) complexes that containing 1,10-
phenanthroline derivative ligands [29] and 2-(2-pyridyl)benzimi-
dazolyl derivative ligands [26], but the Cu–O distance and the angle
of N(1)–Cu(1)–O(1) in complex 2 are all smaller than those reported
in the literature.
The diffraction data were collected with a Bruker Smart Apex
CCD area detector with graphite-monochromatized Mo K radia-
tion (k = 0.71073 Å) at 185(2) K. The structure was solved by using
the program SHELXL and Fourier difference techniques, and refined
by full-matrix least-squares method on F2. All hydrogen atoms
were added theoretically.
a
In the chelated ligand L, the pyridyl ring is not coplanar with the
benzimidazolyl ring, the dihedral angle between them is 24.19°.
As shown in Supplementary Fig. 2, there is no intermolecular
p–
p
stacking interaction between cations of 2 in the crystal lattice.
3. Results and discussion
3.1. X-ray crystal structure of complex 2
3.2. UV–Vis absorption and photoluminescence of 1 and 2
Crystals of complex 2 suitable for X-ray analysis were obtained
as the solvate species 2ꢀ2C6H5CH3, whose crystal structure and
packing diagram are given in Fig. 1 and Supplementary Fig. 2,
respectively. The crystal and experimental data of complex 2 are
shown in (Table 1). Selected bond lengths and angles are listed in
Table 2.
Within complex 2 the Cu (I) ion is tetracoordinated by two N
atoms from L and two P atoms from the ancillary ligand DPEphos,
displaying a distorted tetrahedral coordination geometry. The two
CuAP bond lengths are 2.2310 (11) and 2.2478 (11) Å, respectively.
The Cu(1)–N(1) (pyridyl) bond length (2.113 (3) Å) is longer than the
Cu(1)–N(2) (imidazolyl) bond length (2.049 (3) Å), indicating that
the imidazolyl nitrogen atom is a strongerdonor thanthat of thepyr-
idyl ring. The phenomenon is consistent with those observed in
mononuclear and polynuclear Cu (I) complexes based on the 2-(2-
pyridyl)benzimidazolyl derivative ligands [25,26]. The dihedral an-
gle between the N–Cu–N plane and the P–Cu–P plane is 79.71°. The
N–Cu–N and P–Cu–P bond angles are 79.64(14) and 111.96(4),
respectively. The unbound ether oxygen of the DPEphos ligand lies
at a distance of 3.109 Å from the Cu(I) center and opposite that of
The UV–Vis absorption spectra of dilute dichloromethane solu-
tions of the Cu (I) complexes and the free ligands are shown in
Fig. 3. The absorption spectra of free DPEphos and PPh3 are similar,
exhibiting two absorption bands at 234 and 260 nm. The free
ligand L has four intensive absorption bands at 238, 263, 295 and
312 nm, and one weak absorption band at 346 nm. The Cu (I) com-
plexes exhibit two absorption bands at 275 and 335 nm, which are
attributed to the
p ?
p⁄ transitions of the ligands. In addition, two
weak absorption bands at 410 and 490 nm are observed for all
complexes, which are characteristic of metal-to-ligand charge-
transfer (MLCT) transitions.
Fig. 4 shows the photoluminescence spectra of the complexes
measured at 298 K in PMMA films and at 77 K in frozen dichloro-
methane solutions. Complex 1 emits a yellow–orange light whose
spectrum at 298 k shows a broad band at 550 nm, which shifts to
566 nm at 77 K. At 298 K complex 2 exhibits a strong green–yellow
emission with a broad band at 520 nm, which moves to 534 nm at
77 K. The kmax values of the complexes are in the order 1 > 2, indicat-
ing the effect of the ancillary phosphine ligands on the luminescence
properties of the complexes. The emission peak in PMMA at room

T. Yu et al. / Inorganica Chimica Acta 383 (2012) 78–82
81
Fig. 1. Crystal structure of complex 2. Hydrogen atoms, counter-ion and solvents are removed for clarity.
Table 1
Crystal data and structure refinement for [Cu(L)(DPEphos)](BF4).
1.5
1.0
0.5
0.0
L
0.12
0.10
0.08
0.06
0.04
[Cu(L)(DPEphos)](BF)
4
DPEphos
[Cu(L)(PPh) ](BF
)
Formula
Formula weight
C78H68BCuF4N4OP2
1289.65
3 2
4
PPh
3
[Cu(L)(DPEphos)](BF
)
4
T (K)
185(2)
0.71073
0
[Cu(L)(PPh ) ](BF
)
4
3 2
k (AÅ)
Crystal system
Space group
Monoclinic
P2(1)/n
Unit cell dimensions
350
400
450
500
550
0
Wavelength (nm)
12.6073(9)
14.3848(11)
36.248(3)
a (AÅ)
0
b (AÅ)
0
c (AÅ)
b (°)
V (ÅA3)
92.9490(10)
6565.0(8)
0
Z
q
l
4
250 300 350 400 450 500 550 600
Wavelength (nm)
(g cmꢁ3
(mmꢁ1
)
)
1.305
0.443
F (000)
2688
Crystal size (mm)
0.31 ꢂ 0.22 ꢂ 0.13
Fig. 3. Absorption spectra of free ligands and the Cu (I) complexes in dichloro-
methane solutions at room temperature. The inset shows a magnified view of the
absorption edges for the Cu (I) complexes.
h (o)
1.52–25.03
Index ranges
ꢁ15 6 h 6 14
ꢁ17 6 k 6 16
ꢁ43 6 l 6 40
33540/11580 (Rint = 0.0516)
Semi-empirical from
equivalents
Reflections collected
Absorption correction
Maximum and minimum transmission
Refinement method
0.9447 and 0.8750
Full-matrix least-squares on F2
11580/2/798
1.039
R1 = 0.0655
wR2 = 0.1588
R1 = 0.1035
[Cu(L)(DPEphos)](BF ) (77 K)
4
Data/restraints/parameters
[Cu(L)(DPEphos)](BF ) (298 K)
4
Goodness-of-fit (GOF) on F2
1.0
[Cu(L)(PPh
[Cu(L)(PPh
)
](BF ) (77 K)
4
3 2
Final R indices [I > 2
R indices (all data)
r(I)]
)
](BF ) (298 K)
4
3 2
wR2 = 0.1797
0.873 and ꢁ0.419
0
Largest difference in peak and hole (e ÅAꢁ3
)
0.5
0.0
Table 2
Selected interatomic distances (Å) and angles (°) of [Cu(L)(DPEphos)](BF4).
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–P(1)
Cu(1)–P(2)
2.113(3)
2.049(3)
2.2310(11)
2.2478(11)
N(2)–Cu(1)–N(1)
N(2)–Cu(1)–P(1)
N(1)–Cu(1)–P(1)
N(2)–Cu(1)–P(2)
N(1)–Cu(1)–P(2)
P(1)–Cu(1)–P(2)
79.64(14)
111.95(10)
128.11(10)
115.62(10)
106.19(10)
111.96(4)
450
500
550
650
700
750
Wavele6n0g0th (nm)
Fig. 4. Photoluminescence spectra of the complexes in PMMA (15 wt.%) at 298 K
and 77 K. (kex = 410 nm).

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T. Yu et al. / Inorganica Chimica Acta 383 (2012) 78–82
temperature is considerably blue shifted compared to that at 77 K. It
is suspected that the rigid polymer matrix restricts degrees of free-
dom of the complex resulting in the 15 nm or so blue shift in emis-
sion. Both complexes exhibit strong emissions at 77 K in
dichloromethane solutions when excited by the MLCT energy. How-
ever, at room temperature, 1 and 2 do not display any detectable
emission in air-saturated dichloromethane solutions, clearly indi-
cating the triplet nature of the emission.
Acknowledgements
This work was supported by the Science and Technology Project
of Lanzhou (2009-1-15) and ‘Qing Lan’ talent engineering funds
(QL-05-23A) by Lanzhou Jiaotong University.
Appendix A. Supplementary material
The decay lifetimes of the complexes at 77 K in dichlorometh-
ane were measured by using a time-resolved phosphorescence
spectrometer (Supplementary Fig. 5). The observed emission life-
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.10.049.
times of 1 (335.52 ls) and 2 (206.77 ls) in the microsecond time
References
scale indicate the phosphorescent nature of the emission. The
emission lifetime of 2 is considerably shortened with respect to
that observed for the analogous Cu(I)(DPEphos) complex [26] con-
taining the ligand 2-(2-pyridyl)benzimidazolylbenzene, that dif-
fers from L because the substituent at position 1 is a phenyl
group instead of a carbazolylbutyl group. The reason for this is that
the carbazolylbutyl substituent can sterically prevent structural
relaxation in the MLCT state, which may narrow the energy gap
between excited and ground states, and therefore increase non-
radiative decay [29,30]. The introduction of the bulky substituent
at 1-position of 2-(2-pyridyl)benzimidazolyl ligand effectively
inhibits the structural distortion from the tetrahedral to the flat-
tened structure in the excited state and suppresses the extent of
distortion in the MLCT state. On the contrary, the decay lifetime
of 1 is much longer than that observed for the polynuclear Cu(I)
complexes [25] containing 2-(2-pyridyl)benzimidazolyl group
and the PPh3 ancillary ligand. The presence of the extra copper
(I) centers in polynuclear complexes shortens the decay lifetime
considerably. Complexes 1 and 2 exhibit remarkably long decay
lifetimes which confirmed that the observed MLCT emission is
from the triplet state. These long decay lifetimes of 1 and 2 make
them highly susceptible to solvent-induced thermal quenching
and cause the lack of emission of 1 and 2 in solutions at ambient
temperature. Compared to complex 1, the complex 2 has a shorter
decay lifetime, and the DPEphos ligand is known to substantially
increase the emission quantum efficiency compared to the PPh3 li-
gand, thus as emitters for OLEDs complex 2 should be better than
complex 1.
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In summary, two mononuclear Cu (I) complexes based on 1-[4-
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a hole-transporting carbazole moiety have been synthesized and
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