Structural and photophysical study of copper iodide complex with P^N or P^N^P ligand

Article abstract of DOI:10.1039/c4ce00315b

Two P^N-type ligands, 8-(diphenylphosphino)quinoline (L₁) and 2-[2-(diphenylphosphino)ethyl]pyridine (L₂), and two P^N^P-type ligands, 2,6-bis((diphenylphosphino)methyl)pyridine (L₃) and 2,6-bis((di-tert-butylphosphino)methyl)pyridine (L₄), were synthesized to coordinate with copper iodide (CuI). As a result, CuI complexes with rich structures, such as discrete complexes with formulae of [Cu ₂I₂(L₁)₂] (5), [Cu₃I ₃(L₂)₂] (6), and [CuI(L₃)] (7), and polymeric complexes with repeating units of [Cu₂I₂(L ₄)] (8), and [Cu₃I₃(L₄)] (9) were synthesized and characterized by single crystal X-ray diffraction. Besides the intriguing structures, these complexes showed rich photoluminescent properties, with emission colour that varied from blue to red and a photoluminescence quantum yield (PLQY) from 1.6 to 29.9% in the solid state. Molecular orbital calculation and experimental study showed that the emissions involve halide to ligand charge transfer (XLCT), metal to ligand charge transfer (MLCT), and/or cluster-centered (CC) excited states. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ce00315b

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Structural and photophysical study of copper
iodide complex with P^N or P^N^P ligand†
Cite this: CrystEngComm, 2014, 16,
a
a
a
b
Feng Wei,ab Xiaochen Liu, Zhiwei Liu, Zuqiang Bian, Yongliang Zhao
5338
*
*
and Chunhui Huanga
Two P^N-type ligands, 8-(diphenylphosphino)quinoline (L1) and 2-[2-(diphenylphosphino)ethyl]pyridine
(L2), and two P^N^P-type ligands, 2,6-bis((diphenylphosphino)methyl)pyridine (L3) and 2,6-bis((di-tert-
butylphosphino)methyl)pyridine (L4), were synthesized to coordinate with copper iodide (CuI). As a result,
CuI complexes with rich structures, such as discrete complexes with formulae of [Cu2I2(L1)2] (5),
[Cu3I3(L2)2] (6), and [CuI(L3)] (7), and polymeric complexes with repeating units of [Cu2I2(L4)] (8), and
[Cu3I3(L4)] (9) were synthesized and characterized by single crystal X-ray diffraction. Besides the intriguing
structures, these complexes showed rich photoluminescent properties, with emission colour that varied
from blue to red and a photoluminescence quantum yield (PLQY) from 1.6 to 29.9% in the solid state.
Molecular orbital calculation and experimental study showed that the emissions involve halide to ligand
charge transfer (XLCT), metal to ligand charge transfer (MLCT), and/or cluster-centered (CC) excited
states.
Received 12th February 2014,
Accepted 14th March 2014
DOI: 10.1039/c4ce00315b
www.rsc.org/crystengcomm
example, a photoluminescence quantum yield (PLQY) close
Introduction
to
30%
for
[Cu(POP)(dmdpp)]+
(POP
=
bis[2-
Luminescent materials have been received increasing atten-
tion, particularly in view of their potential applications in
solar energy conversion,1,2 luminescence-based sensors,3,4
organic light emitting diodes (OLEDs),5–8 and biological sys-
tems.9–11 On the basis of their applications, the synthesis
and study of inexpensive, abundant luminescent Cu(I) com-
plexes has become more important. The largest class of lumi-
nescent Cu(I) complexes investigated to date is that of
(diphenylphosphino)phenyl]ether, dmdpp = 2,9-dimethyl-4,7-
diphenyl-1,10-phenanthroline) was reported, which is much
higher than the 1% for [Cu(dmp)(tbp)]+ (dmp = 2,9-dimethyl-
1,10-phenanthroline;
tbp
=
2,9-di-tert-butyl-1,10-
phenanthroline).16,17 Besides the structural rigidity, this is
also explained as the heteroleptic complex having a higher
energy MLCT excite state, which disfavors non-radiative deac-
tivations.15 Recently, a class of complexes with a formula of
[Cu(P^N)(P^P)], where P^N means an amidophosphine deriv-
ative, was found to be more promising, with a high PLQY of
up to 70% and a tunable emission colour from red to
green.18,19
[Cu(N^N)2]+, where N^N indicates
a chelating bisimine
ligand, typically a substituted 1,10-phenanthroline. Studies
on these complexes show that luminescence originates from
a metal to ligand charge transfer (MLCT) excited state, which
is critical to the rigidity of the ligand.12–15 Thereafter, atten-
tion has been paid to heteroleptic [Cu(N^N)(P^P)]+ com-
plexes, where P^P denotes a bisphosphine ligand, because
they exhibit a greatly enhanced emission performance. For
In addition to monomeric complexes, several multimeric
Cu(I) complexes, especially with halide as the chelator, have
been reported with a strong emission.20–24 Among them, the
formulated [Cu4I4L4] (where L is pyridine or substituted pyri-
dine) clusters are known to be highly luminescent, and usu-
ally show two emission bands termed as high energy (HE)
and low energy (LE), which sharply change their relative
intensities upon temperature variation, leading to lumines-
cent thermochromism.24 Based on density function theory
(DFT) calculation and experimental investigation, the excited
states have been assigned to halide to ligand charge transfer
(XLCT) for the HE band, mixed halide to metal charge trans-
fer (XMCT) and cluster-centered (CC) for the LE band, respec-
tively.14,24,25 It is also notable that a number of Cu(I) halide
complexes with planar diamond {Cu2(μ-X)2} (X = Br, I) units
a Beijing National Laboratory for Molecular Sciences (BNLMS), State Key
Laboratory of Rare Earth Materials Chemistry and Applications, College of
Chemistry and Molecular Engineering, Peking University, Beijing 100871,
P.R. China. E-mail: zwliu@pku.edu.cn
b College of Chemistry and Chemical Engineering, Inner Mongolia University,
Hohhot 01002, P.R. China. E-mail: nmgzyl100@163.com
† Electronic supplementary information (ESI) available: X-ray crystallographic
files in CIF format are all available. CIFs have also been deposited at the
Cambridge Crystallographic Database Centre. CCDC 981184–981188 for com-
pounds 5–9, respectively. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4ce00315b
5338 | CrystEngComm, 2014, 16, 5338–5344
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have been reported. By varying the N-heteroaromatic ligands,
the emissions tuned from red to blue, and were assigned to
the MLCT excited state with some mixing of the XLCT excited
state.26
To date, a number of Cu(I) halide complexes have been
reported with isolated P- and N-ligands,25–27 N^N ligands,28
P^P ligands,8,29 S ligands,30 and so forth, while there have
been fewer investigations into the Cu(I) halide complex with
P^N or P^N^P ligands, especially in view of their
photophysical properties.31–34 Herein, we report on the struc-
tural and photophysical properties of five CuI complexes
[Cu2I2(L1)2] (5), [Cu3I3(L2)2] (6), [CuI(L3)] (7), [Cu2I2(L4)] (8),
and [Cu3I3(L4)] (9) which are derived from two P^N ligands
APEX diffractometer using graphite-monochromated Mo Kα
radiation, and structures were determined using direct
methods with standard Fourier techniques using the Bruker
AXS software package. In some cases, Patterson maps
were used in place of the direct methods procedure. Detailed
crystal parameters and structure refinements of complexes
5–9 are given in Table 1.
Computational methods
All of the molecular orbital calculations were performed with
the Titan (version 1.0.7) program at the B3LYP level using
an LACVP** basis set. The geometry for single-point DFT
calculations was taken from the crystal structure of each
complex.
8-(diphenylphosphino)quinoline
(L1)
and
2-[2-(diphenylphosphino)ethyl]pyridine (L2), and two P^N^P
ligands 2,6-bis((diphenylphosphino)methyl)pyridine (L3) and
2,6-bis((di-tert-butylphosphino)methyl)pyridine (L4) (Scheme 1),
respectively.
Synthesis
L1. 8-(Diphenylphosphino)quinoline was synthesized in a
way similar to the reported procedure, the NMR data of
which are in agreement with the literature value.35 To a
solution of 8-chloroquinoline (1.63 g, 10 mmol) and
18-crown-6 (3.43 g, 13 mmol) in THF was added slowly a 0.5 M
solution of potassium diphenylphosphine (24 mL, 12 mmol) in
THF at −78 °C. The mixture was allowed to stir at room tem-
perature for 12 h and then carefully poured into ice water. A
yellow precipitate was obtained by filtration and
chromatographed on a silica gel column with CH2Cl2 as eluent
followed by recrystallization in CH2Cl2/CH3CN to get 1.35 g
white powder, yield 43%.
Experimental
Instruments and measurements
1H-NMR spectra were recorded using a Varian-400 MHz NMR
spectrometer. Chemical shift data for each signal were
reported in ppm units with CDCl3 as reference, where δ
(CDCl3) = 7.26. Elemental analyses were performed using a
Model CE 440 CHN Analyser. The photoluminescence spec-
tra were measured using
a PTI Quanta Master model
C-60SE spectrophotometer, equipped with a 928 PMT detector
and corrected for detector response. Phosphorescent
lifetimes at room temperature were measured by time-
correlated single-photon counting using an IBH Fluorocube
instrument equipped with a 405 nm LED excitation source,
whereas the lifetimes at 77 K with longer range were mea-
sured on the PTI fluorimeter equipped with a microsecond
Xe flash lamp. Quantum yield measurements of both solid
and solution samples were carried out using a Hamamatsu
C9920 system equipped with a xenon lamp, calibrated inte-
grating sphere and model C10027 photonic multichannel
analyser.
L2. 2-[2-(Diphenylphosphino)ethyl]pyridine was synthesized
according to the reported procedure and the NMR data are in
agreement with the literature value.36
L3 and L4. 2,6-Bis((diphenylphosphino)methyl)pyridine
and 2,6-bis((di-tert-butylphosphino)methyl)pyridine were
synthesized according to the procedure described below, and
used without purification by considering their stability in
air.37
Cu2I2(L1)2 (5). To a CH3CN/CH2Cl2 (20/10 mL) solution of
L1 (313 mg, 1.0 mmol) was added CuI (190 mg, 1.0 mmol) in
CH3CN (15 mL). Red crystals were obtained after 3 days in
1
93% yield (470 mg). H-NMR (400 MHz, CDCl3): δ 9.38 (d, J =
4.4 Hz, 1H), 8.25 (d, J = 9.2 Hz, 1H), 7.92 (t, J = 8.4 Hz, 1H),
7.80 (t, J1 = 7.2 Hz, J2 = 6.4 Hz, 1H), 3.34 (d, J = 9.2 Hz, 4H),
1.35 (d, J = 13.6 Hz, 36H). 31P-NMR (400 MHz, CDCl3): δ
−29.8. Anal. calcd. for C42H32Cu2I2N2P2: C, 50.07; H, 3.20; N,
2.78. Found: C, 49.86; H, 3.17; N, 2.80%.
X-ray crystallography
Suitable single crystals for X-ray measurement of the complexes
5–9 were obtained as described in the preparation section.
The crystals were mounted on a glass fiber with Paratone-N
oil. X-ray diffraction data were collected using a Bruker SMART
Cu3I3(L2)2 (6). To a CH3CN (20 mL) solution of L2 (325 mg,
1.1 mmol) was added CuI (210 mg, 1.1 mmol) in CH3CN (20 mL)
quickly without stirring, white crystals were obtained after
1
1 h in 35% yield (150 mg). H-NMR (400 MHz, CDCl3): δ 9.26
(s, 1H), 7.68 (t, J = 8.6 Hz, 1H), 7.42 (s, 4H), 7.34 (t, J =
7.4 Hz, 2H), 7.23 (m, 6H), 3.45 (s, 2H), 2.89 (s, 2H). 31P-NMR
(400 MHz, CDCl3): δ −16.2. Anal. calcd. for C38H36Cu3I3N2P2: C,
39.55; H, 3.14; N, 2.43. Found: C, 39.57; H, 3.10; N, 2.50%.
CuI(L3) (7). To
a
stirred solution of potassium
Scheme 1 Chemical structure of ligands L1–L4 used to coordinate
with CuI in this study.
diphenylphosphine (6.7 mL, 3.4 mmol) was added slowly a
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Table 1 Summary of X-ray data collection and refinement of the complexes 5–9
5
6
7
8
9
Formula
Mw
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
C42H32Cu2I2N2P2
1007.52
Monoclinic
C12/c1
38.973(8)
11.992(2)
17.704(4)
90.00
C38H36Cu3I3N2P2
1153.95
Orthorhombic
P212121
10.185(1)
13.224(3)
28.904(6)
90.00
C31H27CuINP2
665.92
Monoclinic
P21/n
15.983(3)
11.391(2)
16.357(3)
90.00
C23H43Cu2I2NP2
776.40
Monoclinic
C2/c
23.986(5)
13.948(3)
21.765(4)
90.00
C23H43Cu3I3NP2
966.84
Monoclinic
C2/c
15.373(1)
16.444(1)
14.240(1)
90.00
β (°)
105.94(1)
90.00
7956(3)
90.00
90.00
3893.2(12)
4
108.82(3)
90.00
2818.7(10)
4
109.18(3)
90.00
6877(2)
8
122.25(1)
90.00
3044.3(4)
4
γ (°)
Volume (Å3)
Z
8
T (K)
153(2)
1.682
3936
148(2)
1.969
2216
183(2)
1.569
1328
133(2)
1.500
3056
120(2)
2.109
1856
Dcalcld (g cm−3
F(000)
)
θ range (°)
Index ranges
1.78–27.52
−50 ≤ h ≤ 50
−15 ≤ k ≤ 15
−18 ≤ l ≤ 22
1.358
1.69–27.38
−12 ≤ h ≤ 12
−16 ≤ k ≤ 15
−36 ≤ l ≤ 21
1.238
2.16–27.51
−20 ≤ h ≤ 17
−14 ≤ k ≤ 14
−14 ≤ l ≤ 20
1.377
1.71–27.53
−28 ≤ h ≤ 30
−9 ≤ k ≤ 17
−25 ≤ l ≤ 27
1.692
2.00–27.50
−19 ≤ h ≤ 17
−15 ≤ k ≤ 21
−16 ≤ l ≤ 18
1.248
GOF on F2
R [I > 2σ(I)]
R (all data)
0.0498
0.0382
0.0537
0.0431
0.0424
0.0543
0.0399
0.0501
0.0464
0.0339
solution of 2,6-bis(chloromethyl)pyridine (0.30 g, 1.7 mmol)
in THF at −78 °C. The mixture was stirred at room
temperature for 2 h and transferred into a drop funnel under
nitrogen atmosphere. The slurry was added dropwise into a
CH3CN solution of CuI (324 mg, 1.7 mmol) and the mixture
stirred for 2 h. The complex 7 was then obtained as light
yellow crystal by flash column chromatography using
CH2Cl2 : CH3COOEt (v/v, 10 : 1) as eluent, followed by
recrystallization from CH2Cl2/CH3CN. Yield 41% (450 mg) for
the two steps. 1H-NMR (400 MHz, CDCl3): δ 7.57 (t, J = 7.6 Hz,
1H), 7.48 (m, 8H), 7.32 (t, J = 7.2 Hz, 4H), 7.21 (t, J = 7.2 Hz,
8H), 7.12 (d, J = 8.0 Hz, 2H), 3.89 (s, 4H). 31P-NMR (400 MHz,
CDCl3): δ −3.3. Anal. calcd. for C31H27CuINP2: C, 55.91; H,
4.09; N, 2.10. Found: C, 55.34; H, 4.07; N, 2.15%.
Cu2I2(L4) (8). To a stirred solution of 2,6-dimethylpyridine
(300 mg, 2.8 mmol) in THF at −78 °C was added slowly
n-BuLi (1.6 M, 3.5 mL). The mixture was stirred at 45 °C for
1 h and followed by the dropwise addition of di-tert-
butylchlorophosphine (1.0 g, 5.6 mmol) at −78 °C. The
resulting solution was stirred overnight and then added
dropwise into a solution of CuI (535 mg, 2.8 mmol) in
CH3CN. The mixture was stirred for two days, the precipitate
was filtered and the complex 8 was obtained as grey yellow
crystals by recrystallization in CH2Cl2/CH3CN. Yield 64% for
the two steps. 1H-NMR (400 MHz, CDCl3): δ 7.54 (t, J = 8.0 Hz,
1H), 7.16 (d, J = 7.6 Hz, 2H), 3.34 (d, J = 9.2 Hz, 4H), 1.35 (d, J =
13.6 Hz, 36H). 31P-NMR (400 MHz, CDCl3): δ 60.6, 34.0. Anal.
calcd. for C23H43Cu2I2NP2: C, 35.58; H, 5.58; N, 1.80. Found: C,
36.03; H, 5.64; N, 1.88%.
Results and discussion
Synthesis and structures
Since the d10 electronic configuration of Cu(I) enforces no
stereochemical demand, the coordination sphere is there-
fore largely determined by electronic and molecular
mechanical factors. For example, mononuclear CuI(3-Mepy)3
to polymer [CuI(py)]∞ (3-Mepy = 3-methylpyridine; py = pyridine)
have been prepared from the simple combination of
CuI and pyridine-type ligands in different molar ratios.14,25
In our case, the complexes 5 and 6 were obtained at cer-
tain ratios of CuI to ligand, whereas the complexes 7 and
8 were obtained with unclear ratios. Although we have
found that complex 7 was obtained whether the input
CuI : L2 ratio is 2 : 1 or 1 : 1, it should be noted that the
complex structure with a certain ligand may vary if forced
by mass action law.14
Fig. 1 shows the perspective view of the dimeric complex
5. Two CuI ions are bridged by two μ2-I ions, and coordinated
by two bidentate P^N ligands L1 additionally, each CuI ion is
Cu3I3(L4) (9). The complex 9 was obtained as green yellow
crystals by gradient temperature sublimation of the complex
8 at high vacuum (10−6 Torr), yield: ~1%. Anal. calcd. for
C23H43Cu3I3NP2: C, 28.57; H, 4.48; N, 1.45. Found: C, 28.58;
H, 4.35; N, 1.46%.
Fig. 1 ORTEP drawing of the complex 5 with ellipsoids at the 30%
probability level. Hydrogen atoms are omitted for clarity.
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in a distorted tetrahedral geometry. The Cu(1)–Cu(2) distance
is 2.631(37) Å, which is much shorter than those found
in dimeric complexes with isolated
P and N ligands
(2.872–3.303 Å),26 while comparable with those in complexes
bridged by 1,8-nap (2.51–2.63 Å).28,38 With such a short Cu–Cu
distance, the dihedral angle between the Cu(1)–I(1)–Cu(2) and
Cu(1)–I(2)–Cu(2) faces is 137.7°, thus the {Cu2(μ2-I)2} core
observed here is one of the most distorted cores.39
The reaction of CuI with L2 gives a trinuclear complex 6. A
perspective view of the structure is depicted in Fig. 2. The
three CuI ions are in a trigonal planar array with two μ3-I ions
[I(1) and I(2)] above and below the plane, respectively, and
one μ2-I [I(3)) ion as a bridge of one side of the triangle, while
each of the other two sides is connected by an L2 ligand. The
μ3-I binds to the Cu with bond lengths of 2.7205–2.7996 Å,
which is longer than those of μ2-I (2.6980–2.7095 Å). Further-
more, for the CuI ions, those bridged by μ2-I ions have a sig-
nificantly shorter Cu–Cu separation of 2.5236 Å, suggesting a
metal–metal bonding character.
Fig. 3 ORTEP drawing of the complex 7 with ellipsoids at the 30%
probability level. Hydrogen atoms are omitted for clarity.
The complex 7 was found to be a mononuclear Cu(I) com-
pound and has a similar structure to its bromide counter-
part.31 As shown in Fig. 3, the CuI ion adopts four-coordinate
geometry, three of which are from the ligand L3. However,
considering that the angle sum Σ{[P(2)–Cu(1)–P(1)]
+
Fig. 4 ORTEP drawing of the repeating units of the complex 8 with
ellipsoids at the 30% probability level. Hydrogen atoms are omitted for
clarity.
[(P(1)–Cu(1)–I(1)] + [I(1)–Cu(1)–P(2)]} is 359.95°, the four atoms
[P(1), P(2), I(1), and Cu(1)] are in the same plane. Herein the
CuI ion has a much different coordination environment to
the quasi-tetrahedron for most of the reported Cu(I) com-
plexes. The coordination of the CuI ion and the pyridine N
atom is evident from the Cu–N length of 2.185(2) Å, which
results in a dihedral angle of 121.0° between the pyridine
ring and the plane in which Cu is seated.
ions and one P atom. In the complex, the ligand L4 provides
one bidentate P^N coordinate for one {Cu2(μ2-I)2} unit and
the rest of P for another {Cu2(μ2-I)2} unit to form an extended
one-dimensional chain along the b-axis.
The complex 8 (Fig. 4) has a {Cu2(μ2-I)2} unit similar to
that observed in complex 5, where the two CuI ions are
bridged by two μ2-I ions with a Cu–Cu distance of 2.7913(8) Å
and a dihedral angle between the Cu(1B)–I(1B)–Cu(2B) and
Cu(1B)–I(2B)–Cu(2B) faces of 142.3°. However, two kinds of
CuI ion geometries found here: one CuI ion (Cu1B) adopts a
distorted tetrahedral mode with two μ2-I ions and one
bidentate P^N fragment, while the other CuI ion (Cu2B)
adopts a Y-shaped trigonal configuration with the two μ2-I
The complex 9 (Fig. 5) is found to have an unprecedented
polymeric chain that involves alternating {Cu2(μ2-I)2} and CuI
units coordinated by P atoms and pyridine N atoms along
the ac-axis, respectively. The {Cu2(μ2-I)2} unit has a parallelo-
gram configuration with a long Cu–Cu distance of 3.435(2) Å,
which is different from those observed in complexes 5 and 8,
but similar to those reported ones with isolated P- and N-
ligands. The {Cu2(μ2-I)2} and CuI units are connected by a
Fig. 5 ORTEP drawing of the repeating units of complex 9 with
ellipsoids at the 30% probability level. Hydrogen atoms are omitted for
clarity.
Fig. 2 ORTEP drawing of the complex 6 with ellipsoids at the 30%
probability level. Hydrogen atoms are omitted for clarity.
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Fig. 6 Emission spectra of complexes 5–9 in the solid state (λex
=
380 nm).
Fig.
7
Molecular orbitals of complexes 5–7 calculated by Titan
(version 1.0.7) using geometry obtained from single crystal X-ray
diffraction.
short Cu–Cu interaction of 2.570(2) Å and a weak Cu–I inter-
action of 2.916(1) Å with μ2-I bridges.
room temperature and less than 20 nm blue-shifted maxi-
mum excitation wavelengths, except for the complex 7. Com-
plex 7 has maximum excitation and emission wavelengths
around 404 and 598 nm at 77 K, which is 46 nm blue-shifted
and 34 nm red-shifted compared to its excitation and emis-
Photophysical properties
At room temperature, the solid samples of 5, 6, 7, 8, and 9
display photoluminescence with a maximum wavelength at
664, 468, 564, 534, and 480 nm, and a PLQY of 1.6, 22.2,
16.2, 15.4, and 29.9%, respectively (Fig. 6 and Table 2). The
emissions have lifetimes on the microsecond scale, implying
that these emissions are arising from triplet excited states.
To gain insight into the emission properties, we performed
single-point DFT calculations on the ground states of these
complexes since such investigations have been proven to be
very helpful in understanding the photophysical properties,
which will be instructive to examine the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) of the complex. Considering that calculations
on polymeric complexes are costly we only examined the
HOMOs and LUMOs of complexes 5, 6, and 7. As shown in
Fig. 7, the calculated energy levels around the frontier
orbitals are similar to the reported CuI complexes, where the
HOMOs are mainly focused on iodide and copper, while the
LUMOs are on the N-heteroaromatic rings. With reference to
the calculation results and previous work on the CuI com-
plexes, these emissions could be ascribed to transitions from
mixed XLCT and MLCT excited states.
sion at room temperature, respectively. With such
a
temperature-dependent photoluminescent property, the
change in sample colour and emission colour can be recog-
nized by the naked eye. Based on literature reports,40,41 the
luminescence of Cu(I) complexes is very sensitive to rigid-
ity, due to the divalent copper characteristics in the excited
state that lead to structural reorganization from a tetrahedral
to a square-planar geometry.
The complex 9 is not stable in solution, as CuI precipitates
when it is dissolved, thus we measured only complexes 5–8 in
solution. The four complexes display intense emissions
with long lifetimes in 2-MeTHF glassy solution at 77 K. Com-
pared to the spectra of the solid sample at 77 K, all com-
plexes have a blue-shifted maximum emission wavelength,
i.e., 26, 20, 50, and 30 nm for complexes 5–8, respectively.
At room temperature, no visible emission was found for com-
plexes 5–8 in 2-MeTHF.
Due to a divalent copper characteristically having a
square-planar geometry in the excited state, Cu(I) complexes
usually show weak emission in solution at room tempera-
ture, as the excited Cu(II) center is exposed to and could be
attacked by solvent molecules, leading to non-radiative
As the temperature is decreased to 77 K, all of the com-
plexes have a similar emission spectrum to that observed at
Table 2 Photophysical properties of the complexes 5–9
Solid sample
Solution sample
λmax (nm)
Lifetime (μs)
λmax (nm)
Lifetime (μs)
PLQY (%)
PLQY (%)
RT
77 K
RT
77 K
RT
77 K
RT
77 K
5
6
7
8
9
1.6
22.2
16.2
15.4
29.9
664
468
564
534
480
678
462
598
534
484
0.5
16.1/4.1/0.9
8.4/3.5
6.8/3.2/1.2
2.8/0.4
51.6/23.8
53.2/27.4
103.2
211.5/78.3
91.0/36.9
—
—
652
442
548
504
—
—
100.7/23.1
47.4/34.4
208.3/66.9
189.6/65.8
—
19.5
1.3
1.3
—
505/650
363/537
363/609
—
0.8
0.4
0.5
—
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the solid state. Room temperature and low temperature
(77 K) emissions and lifetimes, as well as single-point DFT
calculations have been examined on these CuI complexes,
indicating that the transitions mainly originate from triplet
XLCT, MLCT, and/or CC excited states.
Acknowledgements
We greatly acknowledge the financial support by The
National Basic Research Program of China (no.
2006CB601103), The National Natural Science Foundation of
China (NNSFC, no. 20971006, 90922004, 21201011), and The
Specialized Research Fund for the Doctoral Program of
Higher Education (20120001120116).
Fig. 8 Excitation (for emissions at 500 and 650 nm) and emission
(excitation wavelength of 300 nm) spectra of complex
dichloromethane solution at room temperature.
6 in
Notes and references
transition. In our case, no visible emission was observed for
complex 5 in CH2Cl2 solution. Excitation of 7 or 8 at 300 nm
gave a weak green or red emission with a peak around 537 or
609 nm, as well as an additional emission peak around 363 nm,
respectively. It is interesting to note that complex 8 has a
low-energy emission that usually observed in CuI complexes
with a short Cu–Cu interaction. However, the Cu–Cu distance
observed here is 2.7913(8) Å. With the Cu–Cu distance and
emission peak at 363 nm that may come from the ligand in
mind, it is tempting to assign the weak red emission as
decomposed species. The photophysical properties of com-
plex 6 in CH2Cl2 solution also deserve attention. In addition
to the emission around 500 nm that is similar to that
observed in the solid state at room temperature, an emission
shoulder around 650 nm was examined, which has a similar
excitation spectrum to the emission at 500 nm (Fig. 8). Since
the shortest Cu–Cu distance in complex 6 was examined as
2.5236(11) Å, it may be reasonable to assign this emission to
the 3CC excited state.
It should be noted that we have checked the possibility of
whether these compounds could be used in chemical vapour
deposition (CVD)-based OLEDs, for example, the quantum
yield of compound 5 is too low (1.6% in solid), compounds 6
and 7 decomposed during the thermal gradient sublimation,
and compound 8 converted to the emissive compound 9
while with an extremely low yield (~1%). Unfortunately, these
compounds may not be good candidates for CVD-based
OLEDs. Searching for new CuI-based compounds for OLEDs
is in progress.
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5344 | CrystEngComm, 2014, 16, 5338–5344
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