Synthesis, characterization, crystal structures, and photophysical properties of a series of room-temperature phosphorescent copper(I) complexes with oxadiazole-derived diimine ligand

Article abstract of DOI:10.1016/j.ica.2010.04.041

In this paper, we report four phosphorescent Cu(I) complexes of [Cu(OP)(PPh₃J₂]BF₄, [Cu(MeOP)(PPh ₃)₂]BF₄, [Cu(OP)(POP)]BF₄, and [Cu(Me-OP)(POP)]BF₄ with oxadiazole-de

Full text of DOI:10.1016/j.ica.2010.04.041

Inorganica Chimica Acta 363 (2010) 2600–2605
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, characterization, crystal structures, and photophysical properties of a
series of room-temperature phosphorescent copper(I) complexes with
oxadiazole-derived diimine ligand
*
Fuxiang Wei , L. Fang, Y. Huang
School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221008, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, we report four phosphorescent Cu(I) complexes of [Cu(OP)(PPh3)2]BF4, [Cu(Me-
OP)(PPh3)2]BF4, [Cu(OP)(POP)]BF4, and [Cu(Me-OP)(POP)]BF4 with oxadiazole-derived diimine ligands,
where OP = 2-(5-phenyl-[1,3,4]oxadiazol-2-yl)-pyridine, Me-OP = 2-(5-p-tolyl-[1,3,4]oxadiazol-2-yl)-
pyridine, POP = bis(2-(diphenylphosphanyl)phenyl) ether, and PPh3 = triphenylphosphane, including
their synthesis, crystal structures, photophysical properties, and electronic nature. The Cu(I) center has
a distorted tetrahedral geometry within the Cu(I) complexes. Theoretical calculation reveals that all
emissions originate from triplet metal-to-ligand-charge-transfer excited state. It is found that the
inter-molecular sandwich structure triggered by inter- and intra-molecular pi-stacking within solid state
Cu(I) complexes is highly effective on restricting the geometric relaxation that occurs in excited states,
and thus greatly enhances the photoluminescence (PL) performances, including PL quantum yield
improvement, PL decay lifetime increase, and emission blue shift.
Received 2 February 2010
Received in revised form 20 April 2010
Accepted 25 April 2010
Available online 5 May 2010
Keywords:
Cu(I) complexes
Single crystal
Pi-stacking
Luminescence
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
McMillin firstly reported this type of exciplex quenching, and by
now, many other studies have confirmed this mechanism [5,6].
Recently, the development of practical components for chemi-
cal sensors, display devices, biological probes, phototherapy, and
solar-energy conversion has sparked a great interest in transition
metal complexes, especially heavy metal complexes such as Ru(II)
and Re(I) complexes [1,2]. At the same time, the strong appeal of
replacing the expensive compounds based on heavy metal ions
with cheap ones, as well as the need for a deeper understanding
on the correlation between molecular structures and photophysi-
cal properties, has pushed a continuous progress in the design of
luminescent first-row transition metal complexes. Phosphorescent
Cu(I) complexes, as a new class of optoelectrical material, have
drawn much attention due to their advantages of less toxic, low
cost, redundant resource, and environmental friendliness.
Mixed-ligand systems involving triphenylphosphane seem to
be promising because they exhibit long excited state lifetimes in
solid state and frozen solutions [7,8]. A series of new mixed-ligand
copper(I) complexes such as [Cu(N–N)(POP)]+ [POP = bis(2-
(diphenylphosphanyl)phenyl) ether] which are superior emitters
have been synthesized. It is found that the exciplex quenching is
relatively inefficient for the CT excited state in this POP system.
In addition, the introduction of sterically blocking ligands can im-
pede geometric relaxation as well as solvent attack [9]. Here, steric
effects cooperate to effectively block excited state close to ground-
state geometry, which has been proved by the theoretical studies
finished by Feng and coworkers on [Cu(N–N)(P–P)]+ system [10].
For a typical phosphorescent [Cu(N–N)(POP)]+ complex, the high-
est occupied molecular orbital (HOMO) has predominant metal
Cu d character, while, the lowest unoccupied orbital (LUMO) is
Generally, emission signals from charge-transfer (CT) excited
states of copper(I) complexes are typically weak and short lived be-
cause the lowest energy CT state of a d10 system involves excitation
essentially p* orbital localized on the diimine ligand. The photolu-
from a metal–ligand d
r
* orbital [3,4]. An important consequence is
minescence (PL) corresponds to the lowest triplet T1 and is thus
that the excited state typically prefers a tetragonally flattened
geometry whereas the ground state usually adopts a tetrahedral-
like coordination geometry appropriate for a closed-shell ion. Aside
from reducing energy content, the geometric relaxation that occurs
in excited states facilitates relaxation back to ground state.
assigned as a character of metal-to-ligand-charge-transfer 3MLCT
[d(Cu) ? p*(diimine ligand)]. The photophysical properties of
[Cu(N–N)(POP)]+ complexes are usually sensitive towards both
ligands structures and environment surroundings.
In this paper, we report four phosphorescent Cu(I) complexes
with oxadiazole-derived diimine ligands and phosphorous ligands,
including their synthesis, crystal structures, photophysical
properties, and electronic nature. The Cu(I) center has a distorted
* Corresponding author. Tel./fax: +86 516 83880131.
E-mail address: weifuxiang2001@163.com (F. Wei).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.04.041

F. Wei et al. / Inorganica Chimica Acta 363 (2010) 2600–2605
2601
tetrahedral geometry within the Cu(I) complexes. Theoretical cal-
culation reveals that all emissions originate from triplet metal-
to-ligand-charge-transfer excited state. It is found that the inter-
molecular sandwich structure triggered by inter- and intra-molec-
for 3 days. The mixture was then poured into cold water and fil-
tered. The obtained crude product was purified by column chroma-
tography. Yield: 62%. 1H NMR (CDCl3): d 7.48 (1H, m), 7.54 (3H, m),
7.90 (1H, m), 8.22 (1H, t), 8.24 (1H, t), 8.33 (1H, d, J = 6.0), 8.83 (1H,
d, J = 3.6). Anal. Calc. for C13H9N3O: C, 69.95; H, 4.06; N, 18.82.
Found: C, 69.89; H, 4.12; N, 18.71%. IR (KBr pellet): 3051, 1618,
ular
p-stacking within solid state Cu(I) complexes is highly effec-
tive on restricting the geometric relaxation that occurs in excited
states, and thus greatly enhances the PL performances, including
PL quantum yield improvement, PL decay lifetime increase, and
emission blue shift.
1543, 1484, 1446, 1382, 1139, 1071, 1029, 791, 710 cmꢀ1
.
Me-OP. Me-OP was synthesized by a method similar to that
of OP, except that benzoyl chloride was replaced by 4-methyl-
benzoyl chloride. Yield: 51%. 1H NMR (CDCl3): d 2.45 (3H, s), 7.31
(2H, d, J = 6.0), 7.47 (1H, t), 7.90 (1H, t), 8.10 (2H, d, J = 6.0), 8.31
(1H, d, J = 6.0), 8.81 (1H, d, J = 3.6). Anal. Calc. for C14H11N3O: C,
70.87; H, 4.67; N, 17.71. Found: C, 70.95; H, 4.77; N, 17.58%. IR
(KBr pellet): 3051, 2927, 2859, 1603, 1548, 1481, 1454 1381,
2. Experimental
A synthetic procedure for the two oxadiazole-derived diimine
ligands of 2-(5-phenyl-[1,3,4]oxadiazol-2-yl)-pyridine (referred
as OP) and 2-(5-p-tolyl-[1,3,4]oxadiazol-2-yl)-pyridine (referred
as Me-OP), as well as their corresponding Cu(I) complexes, is
shown in Scheme 1.
1142, 1076, 1037, 818, 784, 719 cmꢀ1
.
4-Methyl-benzoyl chloride, benzoyl chloride, sodium azide, zinc
bromide, Cu(BF4)2, bis(2-(diphenylphosphanyl)phenyl) ether (re-
ferred as POP), triphenylphosphane (referred as PPh3), polyvinyl-
pyrrolidone (PVP), and 2-cyanopyridine were purchased from
Aldrich Chemical Co., and used without further purifications. The
starting materials of [Cu(CH3CN)4]BF4 and 2-(2H-tetrazol-5-yl)-
pyridine (referred as TP) were synthesized according to the litera-
ture procedures [7,11]. All organic solvents were purified using
standard procedures.
2.2. Synthesis of Cu(I) complexes
All the Cu(I) complexes were synthesized according to the clas-
sic literature procedure [7]. Their identity was confirmed by 1H
NMR, elemental analysis, IR spectra, and single crystal XRD.
[Cu(OP)(PPh3)2]BF4. About 1.0 mmol of [Cu(CH3CN)4]BF4 and
2.0 mmol of PPh3 were dissolved in 10 mL of CH2Cl2. The mixture
was refluxed for 30 min at room temperature. Then 1.0 mmol of
OP was added. The mixture was refluxed for another half an hour.
The solvent was removed by rotary evaporation. The crude product
was further purified by recrystallization from the mixed solvent of
tetrahydrofuran/ether. Yield: 85%. 1H NMR: d 7.21 (24H, m), 7.36
(6H, t), 7.586, (4H, m), 8.13 (2H, d, J = 5.4), 8.26, (3H, m). Anal. Calc.
for C49H39BCuF4N3OP2: C, 65.53; H, 4.38; N, 4.68. Found: C, 65.48;
2.1. Synthesis of diimine ligands
OP. The mixture of 20 mmol of TP, 22 mmol of benzoyl chloride,
and 30 mL of pyridine was brought to reflux under N2 atmosphere
Scheme 1. A synthetic procedure for OP, Me-OP, and their corresponding Cu(I) complexes.

2602
F. Wei et al. / Inorganica Chimica Acta 363 (2010) 2600–2605
H, 4.41; N, 4.77%. IR (KBr pellet): 3060, 1612, 1557, 1474, 1431,
1158, 1091, 1057, 740 cmꢀ1
.
[Cu(Me-OP)(PPh3)2]BF4. [Cu(Me-OP)(PPh3)2]BF4 was synthe-
sized by a method similar to that of [Cu(OP)(PPh3)2]BF4, except
that OP was replaced by Me-OP. Yield: 80%. 1H NMR: d 2.48 (3H,
s), 7.21 (19H, m), 7.25 (5H, m), 7.37 (8H, m), 7.67, (1H, t), 8.04
(2H, d, J = 6.0), 8.24 (2H, t), 8.33 (1H, t). Anal. Calc. for C50H41BCuF4-
N3OP2: C, 65.84; H, 4.53; N, 4.61. Found: C, 65.87; H, 4.44; N, 4.52%.
IR (KBr pellet): 3077, 2922, 2863, 1617, 1564, 1434, 1191, 1164,
1086, 1050, 747 cmꢀ1
[Cu(OP)(POP)]BF4. [Cu(OP)(POP)]BF4 was synthesized by
.
a
method similar to that of [Cu(OP)(PPh3)2]BF4, except that 2.0 mmol
of PPh3 was replaced by 1.0 mmol of POP. Yield: 82%. 1H NMR: d
6.83 (2H, m), 7.00 (6H, m), 7.22 (9H, m), 7.28 (6H, m), 7.30 (5H,
m), 7.60 (4H, m), 8.14 (3H, t), 8.38 (2H, m). Anal. Calc. for
C
49H37BCuF4N3O2P2: C, 65.52; H, 4.09; N, 4.61. Found: C, 65.56;
H, 4.13; N, 4.67%. IR (KBr pellet): 3048, 1601, 1549, 1474, 1428,
1389, 1263 (m), 1221, 1180, 1152, 1058, 741 cmꢀ1
.
[Cu(Me-OP)(POP)]BF4. [Cu(Me-OP)(POP)]BF4 was synthesized
by a method similar to that of [Cu(OP)(POP)]BF4, expect that OP
was replaced by Me-OP. Yield: 83%. 1H NMR: d 2.45 (3H, s), 6.81
(2H, m), 7.00 (5H, m), 7.21 (11H, m), 7.28, (3H, m), 7.31 (5H, m),
7.37 (3H, d, J = 6.3), 7.55 (2H, m), 8.01 (2H, m), 8.14 (1H, d,
J = 3.6), 8.34 (2H, m). Anal. Calc. for C50H39BCuF4N3O2P2: C, 64.84;
H, 4.24; N, 4.54. Found: C, 65.89; H, 4.28; N, 4.41%. IR (KBr pellet):
3051, 2931, 2858, 1617, 1552, 1430, 1261, 1210, 1186, 1157, 1097,
ꢀ
Fig. 1a. Crystal structure of [Cu(OP)(PPh3)2]BF4. BF4 and hydrogen atoms are not
shown for clarity.
1053, 743 cmꢀ1
.
2.3. Methods and measurements
Density functional theory (DFT) and singlet excitation calcula-
tions using time-dependent density functional theory (TD-DFT)
were performed on the Cu(I) complexes at RB3LYP/SBKJC level.
The initial geometries were obtained from their single crystal
structures. All computations were finished by GAMESS software
package.
Excited state lifetimes were obtained with a 355 nm light gen-
erated from the third-harmonic-generator pump, using pulsed
Nd:YAG laser as the excitation source. The Nd:YAG laser possesses
a line width of 1.0 cmꢀ1, pulse duration of 10 ns, and repetition fre-
quency of 10 Hz. A Rhodamine 6G dye pumped by the same
Nd:YAG laser was used as the frequency-selective excitation
source. All PL spectra were measured with a Hitachi F-4500 fluo-
rescence spectrophotometer. Solid state photoluminescence quan-
tum yields were measured with the Hitachi F-4500 fluorescence
spectrophotometer equipped with an integrating sphere [9]. UV–
Vis absorption spectra were recorded using a Shimadzu UV-
3101PC spectrophotometer. 1H NMR spectra were obtained with
a Varian INOVA 300 spectrometer. Elemental analysis was per-
formed on a Carlo Erba 1106 elemental analyzer. Single crystals
data were collected on a Siemens P4 single-crystal X-ray diffrac-
tometer with a Smart CCD-1000 detector and graphite-monochro-
ꢀ
Fig. 1b. Crystal structure of [Cu(Me-OP)(PPh3)2]BF4. BF4 and hydrogen atoms are
not shown for clarity.
geometry. The dihedral angles between N–Cu–N and P–Cu–P
planes for the three complexes are measured to be 83.85°,
87.47°, and 87.51°, respectively. Cu–N bond lengths of the three
Cu(I) complexes localize in a region of 2.1–2.2 Å which are compa-
rable to literature values [6–9]. The Cu–N(pyridine) bond length is
longer than the Cu–N(oxadiazole) bond length in PPh3 system. In
POP system, however, the Cu–N(pyridine) bond length is shorter
than the Cu–N(oxadiazole) bond length, indicating that the Cu–N
bond is affected by phosphorous ligands. In addition, it is observed
that Cu–N bond lengths in POP system are usually shorter than
those in PPh3 system, indicating a stronger Cu–N bond in POP sys-
tem. On the other hand, Cu–P bond lengths of the three Cu(I) com-
plexes are similar to each other. The oxygen atom of POP ligand in
[Cu(OP)(POP)]BF4 localizes at a distance of ꢁ3 Å away from the
Cu(I) center and opposite to the coordinated N atoms, indicating
mated Mo Ka radiation, operating at 50 kV and 30 A at 298 K. All
hydrogen atoms were calculated. All data were recorded using sin-
gle crystal samples in the air at room temperature without being
specified.
3. Results and discussion
3.1. Single crystal structures
The crystal structures of [Cu(OP)(PPh3)2]BF4, [Cu(Me-
OP)(PPh3)2]BF4, and [Cu(OP)(POP)]BF4 are depicted in Figs. 1a–d.
The selected geometric parameters shown in Table 1 suggest that
the Cu(I) center in the three complexes has a distorted tetrahedral

F. Wei et al. / Inorganica Chimica Acta 363 (2010) 2600–2605
2603
Table 1
Selected bond lengths (Å) and angles (°) of the three Cu(I) complexes.
[Cu(OP)(PPh3)2]BF4 [Cu(Me-OP)(PPh3)2]BF4 [Cu(OP)(POP)]BF4
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–P(1)
Cu(1)–P(2)
Cuꢂ ꢂ ꢂO
2.154
2.095
2.278
2.237
N/Aa
2.175
2.129
2.274
2.229
N/Aa
78.58
99.47
98.60
112.67
123.40
130.40
2.098
2.130
2.246
2.247
3.132
79.24
109.43
116.90
113.98
115.92
115.73
N(1)–Cu–N(2) 78.47
N(1)–Cu–P(1) 100.37
N(2)–Cu–P(1) 109.33
N(1)–Cu–P(2) 121.63
N(2)–Cu–P(2) 117.69
P(1)–Cu–P(2) 120.96
a
Not available.
to the
p–p attraction between Me-OP rings. The dihedral angle
between Me-OP rings is measured to be as small as 6.95° with a
distance of ꢁ3.391 Å, which confirms that there is face-to-face
p–p stacking between the two Me-OP rings. In addition to this kind
of inter-molecular
intra-molecular
(PPh3)2]BF4 molecules, which actually increases
and allows [Cu(Me-OP)(PPh3)2]BF4 molecules to take a more orga-
nized geometry. The large P–Cu–P bond angle of [Cu(Me-
OP)(PPh3)2]BF4 allows one of PPh3’s phenyl ring to align almost
parallel to Me-OP ring so that their mean planes intersect with
an angle of 6.63°, and the approximate distance between PPh3 phe-
p
-stacking interaction, it appears that there is
-stacking interaction between [Cu(Me-OP)-
attraction
p
p–p
ꢀ
Fig. 1c. Crystal structure of [Cu(OP)(POP)]BF4. BF4 and hydrogen atoms are not
shown for clarity.
nyl ring and Me-OP ring is only 3.375 Å. No similar
p-stacking is
observed in POP system, which may be caused by the P–Cu–P bond
angle limitation of 120° as mentioned.
3.2. Photophysical properties
The photophysical parameters of the four Cu(I) complexes are
summarized in Table 2. Fig. 2 shows the UV–Vis absorption spectra
of [Cu(OP)(PPh3)2]BF4, [Cu(Me-OP)(PPh3)2]BF4, [Cu(OP)(POP)]BF4,
and [Cu(Me-OP)(POP)]BF4 in CH2Cl2 solutions with a concentration
of 1 ꢃ 10ꢀ4 mol/L. It can be observed that the four absorption spec-
tra are quite similar to each other due to their similar diimine and
phosphorous ligands. Each absorption spectrum is typically com-
posed of a high-energy absorption band ranging from 350 to
220 nm and a low-energy absorption band ranging from 350 to
500 nm. The former one corresponds to ligands
p ? p* transitions
according to literature reports [9,10,12]. While, the latter one is
experimentally assigned as MLCT transition absorption. Compared
with the absorption spectrum of [Cu(Phen)(POP)]BF4 ending at
ꢁ500 nm, it can be seen that the oxadiazole moiety in OP and
Me-OP ligand exerts no obvious effect on the absorption edges
(kedg) of their corresponding Cu(I) complexes [9].
As shown in Fig. 3, the four Cu(I) complexes exhibit broad emis-
sion spectra (kem) at room temperature, peaking at 553 nm for
[Cu(OP)(PPh3)2]BF4, 516 nm for [Cu(Me-OP)(PPh3)2]BF4, 569 nm
Fig. 1d. Inter- and intra-molecular p-stacking of [Cu(Me-OP)(PPh3)2]BF4.
a weak interaction between oxygen atom and Cu(I) center. Similar
Cuꢂ ꢂ ꢂO separations have also been reported in POP-based Cu(I)
complexes that contain 1,10-phenanthroline (Phen) and Phen-
derived ligands [10,12].
N–Cu–N bond angles of the three Cu(I) complexes are quite
similar to each other (ꢁ79°) with small variations. In contrast,
P–Cu–P bond angle varies dramatically from 115.73° for [Cu(OP)-
(POP)]BF4 to 130.40° for [Cu(Me-OP)(PPh3)2]BF4 as shown in Table
1. Considering POP’s natural bite angle of 102.2°, with a flexibility
range from 86° to 120°, the large P–Cu–P bond angle in [Cu(OP)-
(POP)]BF4 suggests a crowded coordination environment around
Cu(I) center [13].
Table 2
Summarized photophysical parameters of the four Cu(I) complexes.
a
a
[Cu(N–N)(P–
P)]BF4 N–N, P–P =
kedg
kem
Ub
sc
Kr
Knr
(nm) (nm)
(l
s)
(ꢃ103 sꢀ1
)
(ꢃ103 sꢀ1
)
OP, (PPh3)2
Me-OP, (PPh3)2
OP, POP
485
485
500
500
553
516
569
560
0.02 3.23
0.95 60.31 15.7
0.01 1.28
0.01 3.15
6.2
303.4
0.88
773.5
314.3
It is also observed that there is inter- and intra-molecular
p-
7.8
3.2
Me-OP, POP
stacking within solid state [Cu(Me-OP)(PPh3)2]BF4 due to Me-
OP’s large coplanar conjugation plane, as shown by Fig. 1d. As for
solid [Cu(Me-OP)(PPh3)2]BF4, every two molecules are bonded
head-to-head, where Me-OP moiety is defined as the ‘‘head”, due
a
b
c
1 nm.
10%, measured according to the literature method of Ref. [9].
5%, I = A1exp(ꢀt/ 1) + A2exp(ꢀt/ 2), 1 + A2s2).
s
s
s
= (A1s21 + A2 22)/(A1
s
s

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F. Wei et al. / Inorganica Chimica Acta 363 (2010) 2600–2605
(POP)]BF4), except for [Cu(Me-OP)(PPh3)2]BF4 (1239 cmꢀ1). The
small Stokes shift reveals that the geometric relaxation in
[Cu(Me-OP)(PPh3)2]BF4 excited state is largely and effectively
suppressed.
The PL quantum yields (U) are measured to be 0.02 for
[Cu(OP)(PPh3)2]BF4, 0.95 for [Cu(Me-OP)(PPh3)2]BF4, 0.01 for
[Cu(OP)(POP)]BF4, and 0.01 for [Cu(Me-OP)(POP)]BF4, respectively.
Correspondingly, the PL decay lifetimes (
3.23 for [Cu(OP)(PPh3)2]BF4, 60.31
(PPh3)2]BF4, 1.28 for [Cu(OP)(POP)]BF4, and 3.15 ls
s
ls
) are measured to be
for [Cu(Me-OP)-
for
l
s
l
s
[Cu(Me-OP)(POP)]BF4, respectively. It is notable that [Cu(Me-OP)-
(PPh3)2]BF4 gives a largely improved PL quantum yield and a
largely increased lifetime than the others, which means that its
luminescence is greatly enhanced. Based on the quantum yields
and lifetime data on hand, the radiative probability and nonradia-
tive probability values, Kr and Knr, are calculated with Formula (1)
and Formula (2) as follows.
Kr
U
¼
ð1Þ
ð2Þ
Fig. 2. UV–Vis absorption spectra of the four Cu(I) complexes in CH2Cl2 solutions
with a concentration of 1 ꢃ 10ꢀ4 mol/L. Inset: a magnified view of their absorption
edges.
Kr þ Knr
¼ Kr þ Knr
1
s
As shown in Table 2, the radiative probability value of [Cu(Me-
OP)(PPh3)2]BF4 is typically two times bigger than the correspond-
ing values of the other three Cu(I) complexes. On the other hand,
the nonradiative probability value of [Cu(Me-OP)(PPh3)2]BF4 is
three orders of magnitude smaller than the corresponding values
of the other three Cu(I) complexes. Thus, we come to a conclusion
that the luminescence enhancement of [Cu(Me-OP)(PPh3)2]BF4 is
mainly caused by the effective suppression of Knr decay process.
3.3. Theoretical calculations
As mentioned, the emissive state of phosphorescent Cu(I) com-
plexes is usually affected by both ligand electronic nature and
coordination geometry. In order to get a further understanding
on the electronic nature of the Cu(I) complexes, we perform a
DFT/TD-DFT calculation, which has been proved to be a powerful
tool to investigate electronic properties of transition metal com-
plexes, on [Cu(OP)(PPh3)2]+ and [Cu(Me-OP)(PPh3)2]+ at RB3LYP/
SBKJC level [10].
As shown in Table 3, the highest occupied molecular orbital
(HOMO) of [Cu(OP)(PPh3)2]+ has an evident metal Cu character, ad-
mixed with large contributions from the phosphorous ligand,
while, the lowest unoccupied molecular orbital (LUMO) of
Fig. 3. PL spectra of the four Cu(I) complexes. Inset:
[Cu(OP)(PPh3)2]BF4, [Cu(OP)(POP)]BF4, and [Cu(Me-OP)(POP)]BF4.
a magnified view for
[Cu(OP)(PPh3)2]+ is essentially diimine ligand
p* orbital of OP.
for [Cu(OP)(POP)]BF4, and 560 nm for [Cu(Me-OP)(POP)]BF4,
respectively. Those emissions render no vibronic progressions,
indicating that the excited states own a CT character. Large Stokes
shifts between absorption edges and emission peaks are observed
for the Cu(I) complexes (2535 cmꢀ1 for [Cu(OP)(PPh3)2]BF4,
2425 cmꢀ1 for [Cu(OP)(POP)]BF4, and 2143 cmꢀ1 for [Cu(Me-OP)-
Similar case is observed in [Cu(Me-OP)(PPh3)2]+. The calculated
onset transition of [Cu(OP)(PPh3)2]+ and [Cu(Me-OP)(PPh3)2]+
corresponds to an electronic transition from HOMO to LUMO,
and the onset excitation energy value of [Cu(OP)(PPh3)2]+ listed
in Table 3 correlates quite well with the experimentally recorded
Table 3
Calculated percentage composition of frontier 1 molecular orbitals and the first four singlet excitations of [Cu(OP)(PPh3)2]+ and [Cu(Me-OP)(PPh3)2]+ calculated at RB3LYP/SBKJC
level.
Orbital/transition
[Cu(OP)(PPh3)2]+ composition/energy
[Cu(Me-OP)(PPh3)2]+ composition/energy
LUMO+1
LUMO
HOMO
Cu(1.6%)OP(82.8%)PPh3(15.7%) ꢀ3.97 eV
Cu(2.7%)OP(88.4%)PPh3(8.9%) ꢀ4.92 eV
Cu(26.7%)OP(5.4%)PPh3(67.9%) ꢀ8.04 eV
Cu(36.9%)OP(17.0%)PPh3(46.0%) ꢀ8.48 eV
Cu(45.6%)OP(34.9%)PPh3(19.6) ꢀ8.72 eV
Cu(1.1%)Me-OP(88.9%)PPh3(10.1%) ꢀ3.90 eV
Cu(2.2%)Me-OP(87.7%)PPh3(10.1%) ꢀ4.81 eV
Cu(26.9%)Me-OP(6.9%)PPh3(66.2%) ꢀ8.16 eV
Cu(34.6%)Me-OP(18.3%)PPh3(47.0%) ꢀ8.42 eV
Cu(49.6%)Me-OP(36.0%)PPh3(14.4%) ꢀ8.68 eV
HOMOꢀ1
HOMOꢀ2
S0 ? S1
S0 ? S2
S0 ? S3
S0 ? S4
HOMO ? LUMO(99.6%) 2.550 eV
HOMOꢀ1 ? LUMO(88.9%) 2.769 eV
HOMOꢀ2 ? LUMO(89.2%) 2.824 eV
HOMO ? LUMO+1(86.4%) 3.470 eV
HOMO ? LUMO(74.7%) 2.690 eV
HOMOꢀ1 ? LUMO(72.6%) 2.809 eV
HOMOꢀ2 ? LUMO(89.0%) 2.924 eV
HOMO ? LUMO+1(85.7%) 3.582 eV

F. Wei et al. / Inorganica Chimica Acta 363 (2010) 2600–2605
2605
(3.48 ls) is nearly twenty times shorter than that of [Cu(Me-
OP)(PPh3)2]BF4 crystal. For comparison, we also disperse
[Cu(OP)(PPh3)2]BF4 crystal into PVP film, and its emission spec-
trum is also shown in Fig. 4. The [Cu(OP)(PPh3)2]BF4/PVP film
exhibits a blue shifted emission peaking at 533 nm compared with
[Cu(OP)(PPh3)2]BF4 crystal with a luminescence decay lifetime of
3.27 ls. This slightly enhanced luminescence should be caused
by the rigid surrounding environment of PVP [15,16].
The photophysical properties comparison between pure and
dispersed samples indicates that the absence of inter-molecular
p–p stacking breaks down the rigid sandwich structure, and conse-
quently causes the obvious red-shifted emission, reduced emission
intensity, together with the greatly shortened excited state life-
time, which can be served as a powerful evidence for our hypoth-
esis demonstrated above.
4. Conclusion
In this paper, we report four phosphorescent Cu(I) complexes
with oxadiazole-derived diimine ligands and phosphorous ligands,
including their syntheses, crystal structures, photophysical proper-
ties, and their electronic nature. The Cu(I) center has a distorted
tetrahedral geometry within the Cu(I) complexes. Theoretical cal-
culation reveals that all emissions originate from 3MLCT excited
state. It is found that the inter-molecular sandwich structure trig-
Fig. 4. PL spectra of [Cu(OP)(PPh3)2]BF4/PVP (10 wt.%) and [Cu(Me-OP)(PPh3)2]BF4/
PVP (10 wt.%) films. Inset: PL decay curves of the two films.
absorption edge shown in Fig. 2. As for [Cu(Me-OP)(PPh3)2]+, the
calculated onset excitation energy value is slightly bigger than
the one calculated from its absorption edge, which may be caused
by the distorted geometry of [Cu(Me-OP)(PPh3)2]+ triggered by
p-
gered by inter- and intra-molecular
p-stacking within solid state
stacking. It is thus confirmed that the onset electronic transition
of [Cu(OP)(PPh3)2]+ and [Cu(Me-OP)(PPh3)2]+ is a MLCT one. Corre-
spondingly, the emissive state of [Cu(OP)(PPh3)2]+ and [Cu(Me-
OP)(PPh3)2]+ is also a MLCT one. Combined with the long excited
state lifetimes as mentioned, we come to a conclusion that the
emissions of both [Cu(OP)(PPh3)2]BF4 and [Cu(Me-OP)(PPh3)2]BF4
originate from 3MLCT excited states, showing no luminescence
mechanism difference. Consequently, the luminescence enhance-
ment of [Cu(Me-OP)(PPh3)2]BF4 is not caused by the electronic nat-
ure difference between [Cu(OP)(PPh3)2]BF4 and [Cu(Me-
OP)(PPh3)2]BF4.
Cu(I) complexes is highly effective on restricting the geometric
relaxation that occurs in excited states, and thus greatly enhances
the PL performances, including PL quantum yield improvement, PL
decay lifetime increase, and emission blue shift. This finding may
be useful when designing high-performance phosphorescent
Cu(I) complexes.
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China [No. 90202034] and Youth Foundation of China
University of Mining and Technology (2007A051).
3.4. Analysis on luminescence enhancement
Appendix A. Supplementary material
As above mentioned, the luminescence enhancement is mainly
caused by the suppression of Knr decay process. McMillin and cow-
orker’s reports confirm that the dominant nonradiative decay pro-
cess is the geometric relaxation that occurs at excited state from a
tetragonally flattened geometry to a tetrahedral-like one [3,5–7].
Since the crystal data suggest that [Cu(Me-OP)(PPh3)2]BF4 mole-
cules adopt the ‘‘head-to-head” dual-molecular structure due to
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.04.041.
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