Luminescent dinuclear copper(I) halide complexes double bridged by diphosphine ligands: Synthesis, structure characterization, properties and TD-DFT calculations

Article abstract of DOI:10.1016/j.poly.2014.07.034

Four luminescent copper(I) halide complexes of the type [CuXPh ₂P(CH₂)_nPPh₂] [X = I, n = 4 (1), 5 (2); X = Br, n = 4 (3), 5 (4)] were prepared by reacting CuX with the appropriate diphosphine in a 1:1 M ratio. All the complexes were characterized by spectroscopic analysis (IR, UV-Vis), elemental analysis and photoluminescence studies. Single-crystal X-ray diffraction revealed that 1 and 2 are both dinuclear structures which are constructed by two μ-I bridges and, especially, two diphosphine ligands as μ₂ bridges. The copper(I) iodide complexes are thermally stable, and 2 melts at 242 °C. All the complexes exhibit a strong emission in the solid state. The excited states have been assigned as a halide-to-ligand charge transfer (XLCT) state mixed with a small amount of a metal-to-ligand charge transfer (MLCT) transition, based on the TD-DFT calculations. The calculated electronic transitions have also been compared with the experimental absorption spectra to understand the reason why there is some blue-shift of the low-energy band when the halide atom changes from iodine to bromine, and an even greater blue-shift when the ligand changes from DPPP to DPPB.

Full text of DOI:10.1016/j.poly.2014.07.034

Polyhedron 81 (2014) 687–694
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Luminescent dinuclear copper(I) halide complexes double bridged
by diphosphine ligands: Synthesis, structure characterization, properties
and TD-DFT calculations
Xiayi Zhang a, Li Song b, Mingwei Hong a, Hongsheng Shi a, Kaijie Xu a, Qizhong Lin a, Yi Zhao a,
Yuan Tian a, Jiafeng Sun a, Kangying Shu a, Wenxiang Chai a,c,
⇑
a College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, PR China
b Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Education Ministry, Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018,
PR China
c State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Four luminescent copper(I) halide complexes of the type [CuXPh2P(CH2)nPPh2] [X = I, n = 4 (1), 5 (2);
X = Br, n = 4 (3), 5 (4)] were prepared by reacting CuX with the appropriate diphosphine in a 1:1 M ratio.
All the complexes were characterized by spectroscopic analysis (IR, UV–Vis), elemental analysis and pho-
toluminescence studies. Single-crystal X-ray diffraction revealed that 1 and 2 are both dinuclear struc-
Received 3 May 2014
Accepted 13 July 2014
Available online 1 August 2014
tures which are constructed by two
l-I bridges and, especially, two diphosphine ligands as l2 bridges.
Keywords:
Luminescence
Copper(I) halide complex
Diphosphine ligand
X-ray crystal structure
(TD-)DFT calculation
The copper(I) iodide complexes are thermally stable, and 2 melts at 242 °C. All the complexes exhibit a
strong emission in the solid state. The excited states have been assigned as a halide-to-ligand charge
transfer (XLCT) state mixed with a small amount of a metal-to-ligand charge transfer (MLCT) transition,
based on the TD-DFT calculations. The calculated electronic transitions have also been compared with the
experimental absorption spectra to understand the reason why there is some blue-shift of the low-energy
band when the halide atom changes from iodine to bromine, and an even greater blue-shift when the
ligand changes from DPPP to DPPB.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
disadvantages, there is considerable interest in developing
luminescent complexes of the abundant, cheap and non-toxic
Highly luminescent transition metal complexes are widely
studied due to their broad applications in organic electrolumines-
cence (OEL) [1], solar energy conversion [2], light-driven fuel pro-
duction [3,4], photochemical catalysis [5], luminescence-based
sensors [6] and probes of biological systems [7]. Each of these
applications imposes specific demands on the materials, such as
chemical or electrochemical stability, color and color purity of
the emitted light, high emission quantum yield, specific emission
decay time and so on [8]. For instance, cyclometallated d6 and d8
complexes of the third row transition series, especially iridium(III),
have been employed in OEL products because of their high phos-
phorescence quantum yield, wavelength tenability and thermal
stability [9]. However, the use of these materials inevitably
involves other inherent questions, such as high cost, limited avail-
ability and toxicity of the precious metals [10]. In view of these
metal, copper [11].
Previously, there have been numerous studies on copper(I)
complexes, focusing on either their structural chemistry or their
luminescence. The first important system is [Cu(NN)2]1+ complexes
which present luminescence that is markedly dependent on the
nature of the ligands. Some systematic studies have reported that
the luminescence originates from an MLCT excited state, and the
major non-radiative decay for the MLCT state is related to two
structural distortions of flattening and rocking due to the Jahn–
Teller effect of copper(II) [7,12]. In order to inhibit these distor-
tions, bulky substituents have been introduced into the phenan-
throline ligand at the 2,9-positions, and this method has been
proved to be useful in improving the luminescent quantum yield.
Similarly, the heteroleptic [Cu(NN)P2]1+ complexes with sterically
crowded ligands, such as [Cu(dbp)(POP)]1+, also show a high lumi-
nescent yield [13]. The other one important structure is tetranu-
clear cube-like copper(I) halide clusters, [CuXL]4 (X = halogen,
L = pyridine or phosphine ligand), which shows not only strong
⇑
Corresponding author at: College of Materials Science and Engineering, China
Jiliang University, Hangzhou 310018, PR China. Tel.: +86 0571 86835738.
E-mail address: wxchai@cjlu.edu.cn (W. Chai).
http://dx.doi.org/10.1016/j.poly.2014.07.034
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

688
X. Zhang et al. / Polyhedron 81 (2014) 687–694
emission in both the solid or solution states at room temperature
but also fascinating thermochromic luminescence behavior
[14,15]. Theoretic studies on two different emission bands of these
complexes revealed that the low energy phosphorescence is attrib-
uted to a cluster-centered (3CC) state and the other to a halogen to
ligand charge transfer (3XLCT) state.
2. Experimental
2.1. Materials and general methods
All the reagents were purchased commercially and employed
without further purification. Infrared spectra were measured with
a Bruker Tensor27 FT-IR spectrophotometer in the range 4000–
400 cmÀ1 in KBr lamella. The UV–Vis and fluorescence spectral
measurements were performed on Shimadzu UV3600 and Horiba
Jobin–Yvon FL3–211-P spectrometers at room temperature,
respectively. TGA-DSC curves were obtained in a static high purity
argon atmosphere with a Mettler Toledo Simultaneous Thermal
Analyzer at a heating rate of 10 °C/min.
Recently, Bräse et al. reported that dinuclear halogen-bridged
copper(I) complexes based on diphenylphosphinopyridine-type
P^N ligands show a very high luminescence quantum yield in the
solid state [16,17]. These highly emissive materials have been
regarded as an attractive singlet-harvesting emitter which benefit
from a thermally activated delayed fluorescence (TADF) approach,
based on a small energy gap between the S1 and T1 states [8]. How-
ever, there is still a scarcity of good candidates with high lumines-
cent efficiency for various applications, and rarity of the
corresponding theoretic calculations, especially on the emerging
molecules of dinuclear halogen-bridged copper(I) complexes. In
the present paper, we have studied the absorption and emission
of four dinuclear halogen-bridged copper(I) complexes constructed
from the diphosphine ligands DPPB and DPPP. Theoretical studies
based on (TD-)DFT calculations also been carried out on these com-
plexes to understand their luminescent properties.
2.2. Synthesis
2.2.1. [CuIPh2P(CH2)4PPh2] (1)
A solution of cuprous iodide (0.019 g, 0.1 mmol) in CH3CN
(5 mL) was mixed with
phino)butane (DPPB) (0.043 g, 0.1 mmol) in CH2Cl2 (5 mL) to pro-
duce a white precipitate in 82% yield. Colorless crystals for the
diffraction study were obtained from the mother solution by slow
a solution of 1,4-bis(diphenylphos-
Table 1
Crystal data and structure refinement for 1 and 2.
Parameter
Compound
1
2
Empirical formula
Color and habit
Crystal size (mm)
Crystal system
Space group
a (Å)
C
56H56Cu2I2P4
C58H60Cu2I2P4
colorless block
0.20 Â 0.13 Â 0.12
triclinic
P1
9.9143(7)
11.7428(6)
12.2391(6)
104.439(4)
97.588(5)
colorless block
0.20 Â 0.13 Â 0.11
triclinic
ꢀ
ꢀ
P1
10.2805(6)
11.2895(6)
12.1393(6)
70.073(5)
b (Å)
c (Å)
a
(°)
b (°)
86.801(4)
c
(°)
78.032(5)
1295.59(12)
1
94.680(5)
1358.04(14)
1
V (Å3)
Z
Formula weight
1233.79
1.581
2.172
616
1261.82
1.543
2.074
632
Dcal. (Mg/m3)
Absorption coefficient (mmÀ1
F(000)
)
Parameter/restraints/data (obs.)
Final R indices (obs.)
R indices (all)
289/0/3794
R1 = 0.0372, wR2 = 0.0568
R1 = 0.0564, wR2 = 0.0641
1.002
0.001, 0.000
0.510, À0.505
298/0/4024
R1 = 0.0352, wR2 = 0.0585
R1 = 0.0521, wR2 = 0.0643
0.980
0.000, 0.000
0.478, À0.504
Goodness-of-fit (GOF)
Largest and mean delta/sigma
Largest difference peak (e ÅÀ3
)
Table 2
Selected bond lengths (Å) and angles (°) for 1 and 2 from X-ray experiments and DFT calculations.
Exp.
Cal.
Exp.
Cal.
Compound 1
Cu(1)–P(1)
2.2711(12)
2.7438(5)
2.7307(5)
124.50(4)
98.46(3)
2.3900
2.8310
2.8029
124.609
98.440
108.318
Cu(1)–P(2)
Cu(1)–I(1)i
2.2798(10)
2.7307(5)
2.3981
2.8029
Cu(1)–I(1)
I(1)–Cu(1)i
P(1)–Cu(1)–P(2)
P(2)–Cu(1)–I(1)i
P(2)–Cu(1)–I(1)
Compound 2
Cu(1)–P(1)
P(1)–Cu(1)–I(1)i
P(1)–Cu(1)–I(1)
I(1)–Cu(1)–I(1)i
116.51(3)
99.77(3)
109.215(19)
113.872
101.150
110.303
107.93(3)
2.2623(10)
2.7486(5)
2.7078(5)
121.01(4)
104.47(3)
114.04(3)
2.3945
2.8124
2.8049
121.150
103.759
113.351
Cu(1)–P(2)
Cu(1)–I(1)ii
2.2589(11)
2.7078(5)
2.3931
2.8049
Cu(1)–I(1)
I(1)–Cu(1)ii
P(2)–Cu(1)–P(1)
P(1)–Cu(1)–I(1)ii
P(1)–Cu(1)–I(1)
P(2)–Cu(1)–I(1)ii
P(2)–Cu(1)–I(1)
I(1)ii–Cu(1)–I(1)
115.42(3)
103.63(3)
95.632(15)
114.808
102.749
99.119
Symmetry code: (i) Àx + 1, Ày, Àz + 1; (ii) Àx, Ày + 1, Àz.

X. Zhang et al. / Polyhedron 81 (2014) 687–694
689
Table 3
Some strong low-energy singlet electronic transitions for 1–4 calculated by TD-DFT.
Complex
Transition
S1
Energy (103 cmÀ1
30.9
)
Wavelength (nm)
323.3
Oscillator strength
0.0221
Major contributions
1
HOMO ? LUMO (93%)
HÀ1 ? LUMO (2%)
HOMO ? L+3 (+83%)
HÀ1 ? LUMO (9%)
HÀ3 ? L+2 (+4%)
S4
S5
31.6
31.7
316.0
315.7
0.0135
0.0054
HÀ1 ? LUMO (+82%)
HOMO ? L+3 (+9%)
HOMO ? LUMO (+3%)
H-4 ? L+1 (+3%)
S8
S9
S1
S3
33.1
33.1
30.7
30.9
302.4
302.2
326.0
323.6
0.0322
0.0521
0.0032
0.0447
HOMO ? L+4 (+60%)
H-1 ? L+3 (31%)
HOMO ? L+6 (3%)
HÀ1 ? L+3 (+56%)
HOMO ? L+4 (+35%)
HOMO?L+3 (2%)
HOMO ? LUMO (+91%)
H-4 ? L+1 (+4%)
H-1 ? L+3 (+2%)
2
HÀ1 ? LUMO (+58%)
HOMO ? L+3 (+36%)
H-4 ? L + 2 (+3%)
S5
S8
S9
S1
31.2
31.8
32.1
31.5
320.2
314.4
312.0
317.9
0.0030
0.0095
0.0073
0.0322
HOMO ? L+3 (+57%)
HÀ1 ? LUMO (38%)
HÀ2 ? LUMO (+91%)
HÀ1 ? L+3 (+4%)
HÀ1 ? L+3 (+89%)
HÀ2 ? LUMO(5%)HOMOÀ > LUMO(2%)
HOMO ? LUMO(+88%)
HÀ1 ? LUMO(+3%)
HOMO ? L+3 (2%)
HÀ4 ? L+1 (2%)
3
S4
S5
S8
31.9
32.4
33.2
313.3
308.7
301.5
0.0044
0.0107
0.0063
HOMO ? L + 3 (+44%)
HÀ1 ? LUMO (+44%)
HÀ2 ? L+2 (4%)
HÀ4 ? L+1 (3%)
HÀ2 ? L+1 (2%)
HÀ1 ? LUMO (+42%)
HOMO ? L + 3 (41%)
HOMO ? LUMO (7%)
H-2 ? L+1(3%)
HÀ1 ? L+3(+2%)
HÀ1 ? L+3 (+82%)
HOMO ? L+3 (+6%)
H-2 ? L+2 (+6%)
S9
S1
33.6
30.8
297.7
324.7
0.0570
0.0157
HOMO ? L + 4 (+95%)
HOMO ? LUMO (+90%)
HÀ3 ? L+1 (7%)
4
S4
31.3
319.8
0.0102
HOMO ? L+3 (+85%)
H-3 ? L+2 (+10%)
H-1 ? LUMO (+4%)
HÀ1 ? LUMO (+91%)
HOMO ? L+3 (4%)
HÀ2 ? LUMO (+92%)
HOMO ? L+4 (+2%)
HÀ1 ? L+3 (+74%)
HOMO ? L+4(+15%)
HÀ2 ? LUMO (3%)
S5
S8
S9
32.6
32.9
33.0
307.0
303.7
302.7
0.0199
0.0072
0.0027
evaporation of the solvent over about a week. IR (cmÀ1): 3050(w),
2902(w), 2844(w), 1586(w), 1486(m), 1437(s), 1097(m), 740(s),
692(vs), 513(s), 482(m). Anal. Calc. for C56H56Cu2I2P4: C, 54.51; H,
4.57. Found: C, 54.11; H, 4.04%.
week. M.p. 242 °C. IR (cmÀ1): 3047(w), 2922(w), 2858(w),
1584(w), 1480(m), 1432(s), 1098(m), 744(s), 697(vs), 506(s). Anal.
Calc. for C58H60Cu2I2P4: C, 55.20; H, 4.79. Found: C, 55.76; H, 4.21%.
2.2.3. [CuBrPh2P(CH2)4PPh2] (3)
Complex 3 was prepared following a similar procedure to that
for 1, only using cuprous bromide instead of cuprous iodide. Yield:
79%. IR (cmÀ1): 3051(w), 2897(w), 2851(w), 1584(w), 1484(m),
1437(s), 1101(m), 735(s), 691(vs), 516(s), 481(m). Anal. Calc. for
2.2.2. [CuIPh2P(CH2)5PPh2] (2)
A solution of cuprous iodide (0.019 g, 0.1 mmol) in CH3CN
(5 mL) was mixed with
a solution of 1,5-bis(diphenylphos-
phino)pentane (DPPP) (0.044 g, 0.1 mmol) in CH2Cl2 (5 mL) to pro-
duce a colorless solution. After stirring for an hour, the solvents
were removed under reduced pressure to give a white residue
which was then washed with CH3CN and EtO, and dried in air.
Yield: 97%. Crystals for the diffraction study were obtained from
same solution by slow evaporation of the solvent over about a
C56H56Cu2Br2P4: C, 59.01; H, 4.95. Found: C, 59.55; H, 4.63%.
2.2.4. [CuBrPh2P(CH2)5PPh2] (4)
Complex 4 was prepared following a similar procedure to that
for 2, only using cuprous bromide instead of cuprous iodide. Yield:

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X. Zhang et al. / Polyhedron 81 (2014) 687–694
92%. IR (cmÀ1): 3049(w), 2897(w), 2858(w), 1585(w), 1484(m),
1434(s), 1100(m), 740(s), 697(vs), 510(s). Anal. Calc. for C58H60Cu2
Br2P4: C, 59.65; H, 5.18. Found: C, 59.06; H, 5.71%.
2.3. Crystal structure determination
Diffraction intensities for complexes 1 and 2 were collected at
20 °C on an Oxford Xcalibur (Atlas Gemini ultra) diffractometer
with the
x-scan technique (see Table 1). The structures were
solved with the direct method and refined with full-matrix least-
squares using the SHELX package [18,19]. Anisotropic displacement
parameters were applied to all non-hydrogen atoms, and the
organic hydrogen atoms were generated geometrically (C–H
0.96 Å). Selected bond lengths and angle parameters are collected
in Table 2 for the two complexes.
2.4. Theoretical calculations
Fig. 1. The isolated dinuclear molecular structure of complex 1.
The calculations described here were carried out using the
Gaussian 09 suite of programs [20]. The structures of the four
Cu(I) iodide/bromine complexes were fully optimized in the
ground state (S0) using the DFT method with the three-parameters
hybrid functional B3LYP and the LACVP⁄ basis set [21,22], which is
a combination basis set including the relativistic effective core
potential standard basis set LANL2DZ for Cu, P and I/Br and the
standard 6-31G⁄ basis set for C and H. Polarization functions were
also added for P and I/Br, i.e. P (n(d) = 0.34) and I (n(d) = 0.266)/
Br(n(d) = 0.389) [23]. The optimized molecular structure parame-
ters in the S0 states are summarized in Table 2 for 1 and 2. To
obtain estimates of the vertical electron excitation energies, which
include some account of the electron correlation, time-dependent
density functional theory (TD-DFT) was performed on the calcu-
lated S0 geometries using same B3LYP method and LACVP⁄ basis
set. The excitation energies, oscillator strength parameters and cor-
responding major orbital contributions for the five main electronic
transitions are gathered in Table 3 for the four complexes.
3. Results and discussion
Fig. 2. The packing diagram of complex 1.
3.1. Structural characterization
The X-ray diffraction data revealed that complex 1 has an iso-
lated dinuclear structure with a four-membered {Cu2(
in which each Cu(I) atom exhibits a tetrahedral configuration con-
structed by two 2-I and two P atoms from two 2-bridged DPPB
l2-I)2} ring,
l
l
ligands. The {Cu2I2} unit is planar, as is usual for halogen-bridged
copper complexes, and its midpoint coincides with the crystallo-
graphic inversion center of the molecule. On the two sides of the
{Cu2I2} plane, as shown in Fig. 1, two DPPB ligands bridge the
two Cu atoms, so that the dinuclear structure is supported further.
To date, many similar double bridged structures supported by
DPPM ligand, due to its suitable linkage distance, have been
reported [24–27], however, these double bridged structures in
cuprous coordination chemistry are very rare when the DPPM
ligand is replaced by the other diphosphine ligands [28–30]. The
two Cu–I bond lengths of 2.7307(5) and 2.7438(5) Å are similar
to those previously reported [31]. The two Cu–P bond lengths of
2.2711(12) and 2.2798(10) Å and the P–Cu–P angle of 124.50(4)°
are all comparable to those of reported copper-phosphine com-
Fig. 3. The isolated dinuclear molecular structure of complex 2.
plexes [11]. No obvious intra-molecular p–p stacking interactions
could be found between any two aromatic rings. In the solid state,
As shown in Fig. 3, similar to 1, complex 2 is still an isolated
the molecules of 1 are only stabilized by weak Van der Waals’
dinuclear structure, except that the
instead of DPPB. The Cu(I) atom exhibits a tetrahedral configura-
tion, coordinated by two 2-I and two atoms from two
l2-bridging ligand is DPPP
forces, without any obvious hydrogen bonding or
interactions (Fig. 2).
p–p stacking
l
P

X. Zhang et al. / Polyhedron 81 (2014) 687–694
691
l2-bridged DPPP ligands. In its core {Cu2I2} unit, the two Cu–I bond
3.2. TG–DSC analysis
lengths of 2.7078(5) and 2.7486(5) are still within the range of
reported lengths [31], but they are more different from each other
than observed in 1. Moreover, the Cu–P bond lengths of 2.2589(11)
and 2.2623(10) Å and the P–Cu–P angle of 121.01(4)° are all smal-
ler than that in 1. All of these changes should be the result of the
ligand swapping from DPPB to DPPP; in detail, the longer alkyl lin-
ker of DPPP not only shortens the Cu–P bond length due to its
stronger electron donating ability, but it also compresses the two
Cu–P bonds, leading to a smaller P–Cu–P angle due to the bigger
size of the –(CH2)5– linker. This bigger linker also lengthens the
The coinstantaneous TG and DSC curves obtained on the poly-
crystalline samples of 1 and 2 are shown in Fig. 5, where the puri-
ties of the samples have first been checked by PXRD
measurements. The TG and DSC curves show that 1 is stable rang-
ing from room temperature to 285 °C. The complex then has a
great weight loss of 72.65% from 285 to 450 °C and a slow weight
loss after 450 °C. Finally, there is almost no residuum, probably due
to the fact that the decomposition product is volatile. As shown in
Fig. 5b, the DSC curve shows that there is an endothermic peak at
about 240 °C for 2 in its TG plot, however there is no obvious sign
of decomposition. Further measurement of the melting point indi-
cated that 2 begins to melt from 242 °C. After melting, the TG curve
proves that 2 is stable up to 310 °C, then the complex has a great
weight loss of 72.09% from 310 to 460 °C, and a slow weight loss
after 460 °C. The residuum of 2 should be inorganic compound
with 5.43% at about 800 °C. In general, the thermal stability of this
material is good; its melting temperature of 242 °C is especially
very suitable for using non-solution processes to fabricate devices,
e.g. OLEDs [33].
Cu—Cu distance (3.371 Å in 1
versus 3.664 Å in 2) so that the
{Cu2I2} unit is more loose in 2 and the Cu–I bond lengths are more
different from each other in 2. In the solid state, the molecules of 2
are also only stabilized by weak Van der Waals’ forces, without any
obvious intra- or inter-molecular hydrogen bonding or
ing interactions (Fig. 4).
p-p stack-
Based on the crystallographic data, the calculated molecular
structures of 1 and 2 in the gas phase have also been studied at
the B3LYP level, without any geometric restrains. Obviously, as
reported in the literature [32], all of the bond lengths herein are
overestimated compare to the experimental data (Table 2), but
all the differentiations of the two molecules are still visible from
the calculated data and are in reasonable agreement with the crys-
tallographic data, including the previously mentioned bond
lengths and angles. Accordingly, the aforementioned linker effect
on the molecular structure could also be supported by the DFT cal-
culations. In addition, the calculated IR spectra are both coincident
with the experimental spectra (see Fig. S1).
3.3. UV absorption spectra
The electronic absorption spectra of complexes 1–4, recorded
on the as-synthesized powder samples, are shown in Fig. 6. There
are two different bands in their absorption spectra in the range
200–400 nm: one broad band observed at 200–300 nm and the
other stronger band observed at 300–400 nm. In the high energy
Fig. 4. The packing diagram of complex 2.
Fig. 5. The TG and DSC curves of: (a) complex 1 and (b) complex 2.

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X. Zhang et al. / Polyhedron 81 (2014) 687–694
Fig. 6. The UV absorption spectra of complex 1–4 in the solid state.
region, the ligand-centered (p–
p⁄) transition of the diphosphine
ligands should be dominant [34]. The low-energy transition should
be derived from a charge transfer mechanism, which should be
either assigned as a metal to ligand charge transfer (MLCT) or as
a halide to ligand charge transfer (XLCT), according to previous lit-
erature [11]. These assignments were further investigated and
identified by theoretical calculations on complexes 1–4 using
time-dependent density functional theory (TD-DFT). The frontier
orbitals for these compounds are shown in Fig. 7, and other molec-
ular orbitals related to the lowest energy transition states (Table 3)
are also shown in Figs. S2–S5. In each complex, the HOMO is lar-
gely localized on the Cu2X2 core, with minor participation of the
phosphorus atom of the diphosphine ligand. Furthermore, it is
comprised primarily of l2-X non-bond p orbitals mixed with vary-
ing degrees of Cu–d character. On the other hand, the LUMO is
entirely located on the diphosphine ligand, and is mainly delocal-
ized over the p⁄ orbitals of the phenyl groups. Actually, as shown
in Table 3 and Figs. S2–S5, five strong low-energy absorptions in
each complex were all found to be transitions from the Cu2X2 core
to the diphosphine ligand, with the characters of each excited state
being similar to the HOMO ? LUMO excited state (S1 state) in
complex 1. Therefore, it is certain that the low-energy band
observed in experimental absorption spectra should be mainly
assigned as XLCT mixed with a little MLCT transition. The calcu-
lated electronic absorption spectra that focus on 10 low-energy
transition singlet states for complexes 1–4 are also shown in
Fig. 8. In accordance with the experimental spectra, there is some
blue-shift of the low-energy band when the halide atom changes
from iodine to bromine, and an even greater blue-shift when the
ligand changes from DPPP to DPPB. The effect of altering the halide
should result from the decrease in polarizability on going from
iodine to bromine, or the decrease of their delocalization [35].
The later effect of the ligand should be attributed to the stronger
donation of the –(CH2)5– linker in DPPP because the donating
group could decrease the HOMO–LUMO gap, leading to a smaller
excited energy [36].
Fig. 7. HOMO and LUMO frontier orbital plots of complexes 1–4 based on TD-DFT
calculations.
3.4. Luminescence properties
Fig. 8. The calculated electronic absorption spectra of complexes 1–4 focused on
the 10 lowest energy transitions.
Previously, the luminescent properties of copper(I) halide com-
plexes have been studied not only focusing on the synthetic chem-
istry but also the structural chemistry [11,34]. In 1999, Ford P.C.
reviewed the photoluminescence (PL) properties of many multinu-
clear copper(I) compounds and summarized five possible excited
states for luminescent mechanisms, typically MLCT, XLCT, halide
to metal charge transfer (XMCT), metal-centered (MC) and cluster
centered (CC) excited states [14]. In this case, the light emitting
from the powder sample is strong for each complex at room tem-
perature. As shown in Fig. 9, although the band shapes in the PL
excitation spectra of complexes 1–4 do not agreed with the corre-
sponding UV absorption spectra, similar shifts are also observed

X. Zhang et al. / Polyhedron 81 (2014) 687–694
693
Fig. 9. The PL excitation and emission spectra of complexes 1–4 as a powder samples.
herein. The PL excitations could then also be assigned as
Appendix A. Supplementary data
(XLCT + MLCT), just like the absorption spectra. Different from
the excitation spectra, the PL emission spectra appear to be more
complicated, which could be the result of many photophysical pro-
cesses or structural relaxations of these complexes. For example,
the PL emission band of complex 2 is broader than the others, pre-
sumably the reason for this is that the molecular structure of 2 is
loosest and the DPPP ligand is more flexible, so that its structural
relaxation is more severe. Consequently, the PL emission intensity
of complex 2 is also weaker than the others. Thus for a high effi-
ciency of the PL emission, it is not good to employ diphosphine
ligands with a long flexible linker, although it could reduce the
excited energy.
CCDC 990122 and 990123 contains the supplementary crystal-
lographic data for 1 and 2. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.poly.2014.07.034.
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