Synthesis and study on a series of phosphorescent Cu(I) complexes having sterically blocking ligands

Article abstract of DOI:10.1016/j.saa.2014.01.019

In this paper, we report six phosphorescent Cu(I) complexes with 1,10-phenanthroline-derived ligands and phosphorous ligands, 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 introduction of alkyl moieties into 2,9-positions of 1,10-phenanthroline is highly effective on restricting the geometric relaxation that occurs in excited states, which greatly enhances the photoluminescence (PL) performances, including PL quantum yield improvement, PL decay lifetime increase, and emission blue shift.

Full text of DOI:10.1016/j.saa.2014.01.019

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 341–348
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and study on a series of phosphorescent Cu(I) complexes
having sterically blocking ligands
b
Kai Zhang a, , Dong Zhang
⇑
a Correction Department of Education, The Central Institute for Correctional Police, Baoding 071000, China
b Institute of Penology, The Central Institute for Correctional Police, Baoding 071000, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
Six phosphorescent Cu(I) complexes
were synthesized.
Alkyl groups were introduced into
2,9-positions of 1,10-phenanthroline.
Crystal structures, photophysical
properties, and electronic nature
were studied.
ꢀ
The correlation between emission
performance and alkyl group was
investigated.
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 six phosphorescent Cu(I) complexes with 1,10-phenanthroline-derived ligands
and phosphorous ligands, including their synthesis, crystal structures, photophysical properties, and elec-
tronic nature. The Cu(I) center has a distorted tetrahedral geometry within the Cu(I) complexes. Theoret-
ical calculation reveals that all emissions originate from triplet metal-to-ligand-charge-transfer excited
state. It is found that the introduction of alkyl moieties into 2,9-positions of 1,10-phenanthroline is highly
effective on restricting the geometric relaxation that occurs in excited states, which greatly enhances the
photoluminescence (PL) performances, including PL quantum yield improvement, PL decay lifetime
increase, and emission blue shift.
Received 21 November 2013
Received in revised form 2 January 2014
Accepted 8 January 2014
Available online 18 January 2014
Keywords:
Cu(I) complexes
Single crystal
Steric hindrance
Luminescence
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
which initiates a continuous progress in the design and synthesis
of luminescent functional materials based on the first-row transi-
Recently, the design and synthesis of luminescent transition
metal complexes have been sparked owing to their potential appli-
cation in organoelectronic field such as chemosensors, display
materials, biosensors, and solar-energy conversion dyes [1–2].
Most of research efforts focus on expensive metal complexes of
Ir, Pt, Re, and Ru. On the other hand, there is also a strong need
to replace the expensive functional materials with cheap ones,
tion metal complexes. Luminescent Cu(I) complexes have drawn
much attention owing to their promising performance as optoelec-
trical material, as well as their advantages of low toxicity, low cost,
and redundant resource. Many luminescent Cu(I) complexes have
been reported and discussed, and the correlation between molecu-
lar structure and photophysical performance is also fully investi-
gated [3,4].
As for a typical Cu(I) complex with d10 system, the ground state
adopts a tetrahedral coordination geometry, while the excited
state tries to transfer to a tetragonally flattened one to minimize
⇑
Corresponding author.
E-mail address: zhangk826@163.com (K. Zhang).
1386-1425/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2014.01.019

342
K. Zhang, D. Zhang / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 341–348
system energy. The emission signal is usually weak because the ex-
cited state corresponds to a charge-transfer process which suffers
badly from geometric relaxation and non-radiative decay. Except
for this geometric relaxation which decreases system energy and
increases non-radiative decay, solvent attack also helps to com-
plete the non-radiative transformation from excited state to
ground state. This quenching mechanism has been firstly reported
by McMillin and coworkers, and then many following reports have
proved its correctness [3–8].
In order to suppress this exciplex quenching, emitting system
with mixed ligands have been proposed since they own long ex-
cited state lifetimes in solid state and degassed solutions [7,8].
For example, luminescent Cu(I) complexes with a typical molecular
formula of [Cu(N–N)(POP)]+ where POP = bis(2-(diphenylphospha-
nyl)phenyl) ether have been explored as promising emitters. The
introduction of POP ligand is found to be efficient to block the sol-
vent attack and geometric relaxation that occurs in excited state
[9]. Theoretical analysis on [Cu(N–N)(POP)]+ suggests that the ste-
ric effect of POP ligand is the key factor blocking excited state close
to ground state [10]. It is also found that the highest occupied
molecular orbital (HOMO) of [Cu(N–N)(POP)]+ owns a predomi-
nant metal Cu d character, while, the lowest unoccupied orbital
(LUMO) is essentially pꢁ orbital of diamine ligand. The emission
corresponds to the radiative decay of the lowest triplet state and
is thus assigned as a character of metal-to-ligand-charge-transfer
3MLCT [d(Cu) ? pꢁ(diamine ligand)]. The photophysical character
of [Cu(N–N)(POP)]+ excited state can be modified by changing li-
gand structures.
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, 13C NMR elemental analysis, and single crystal XRD.
[Cu(Phen)(PPh3)2]BF4ꢂ1.0 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
Phen was added. The mixture was refluxed for another half 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 (CDCl3, 300 MHz): 8.67
(2H, dd, J = 1.4 Hz, J = 4.6 Hz), 8.56 (2H, dd, J = 1.0 Hz, J = 9.0 Hz),
8.03 (2H, s), 7.78 (2H, q, J = 5.3 Hz), 7.33–7.27 (6H, m), 7.14 (12H,
t, J = 8.6 Hz), 7.06–7.03 (12H, m). 13C NMR (CDCl3, 75 MHz):
149.6, 143.1, 138.1, 132.9, 131.8, 130.2, 129.7, 128.8, 127.4,
125.2. Anal. Calcd. for C48H38BCuF4N2P2: C, 67.42; H, 4.48; N,
3.28. Found: C, 67.49; H, 4.37; N, 3.17.
[Cu(DMPhen)(PPh3)2]BF4ꢂ[Cu(DMPhen)(PPh3)2]BF4 was synthe-
sized by a method similar to that of [Cu(Phen)(PPh3)2]BF4, except
that Phen was replaced by DMPhen. Yield: 80%. 1H NMR (CDCl3,
300 MHz): 8.46 (2H, d, J = 9.2 Hz), 8.01 (2H, s), 7.51 (2H, d,
J = 9.3 Hz), 7.36–7.31 (6H, m), 7.18–7.08 (24H, m), 2.14 (6H, s).
13C NMR (CDCl3, 75 MHz): d 159.1, 142.8, 138.4, 133.0, 132.1,
130.3, 128.6, 128.0, 126.3, 125.5, 27.1. Anal. Calcd. for C50H42BCuF4-
N2P2: C, 68.00; H, 4.79; N, 3.17. Found: C, 68.12; H, 4.69; N, 3.08.
[Cu(DBPhen)(PPh3)2]BF4ꢂ[Cu(DBPhen)(PPh3)2]BF4 was synthe-
sized by a method similar to that of [Cu(Phen)(PPh3)2]BF4, except
that Phen was replaced by DBPhen. Yield: 81%. 1H NMR (CDCl3,
300 MHz): 8.54 (2H, d, J = 9.4 Hz), 8.06 (2H, s), 7.54 (2H, d,
J = 9.4 Hz), 7.37–7.32 (6H, m), 7.21–7.11 (24H, m), 2.46 (4H, t,
J = 9.3 Hz), 1.02–0.93 (4H, m), 0.68–0.61 (4H, m), 0.56 (6H, t,
J = 7.5 Hz). Anal. Calcd. for C56H54BCuF4N2P2: C, 69.53; H, 5.63; N,
2.90. Found: C, 69.77; H, 5.43; N, 2.75.
In this paper, we report six phosphorescent Cu(I) complexes
with 1,10-phenanthroline-derived ligands and phosphorous li-
gands, including their synthesis, crystal structures, photophysical
properties, and electronic nature. The correlation between ligand
structure and photophysical properties of their corresponding
Cu(I) complexes is studied in detail.
[Cu(Phen)(POP)]BF4ꢂ[Cu(Phen)(POP)]BF4 was synthesized by a
method similar to that of [Cu(Phen)(PPh3)2]BF4, except that
2.0 mmol of PPh3 was replaced by 1.0 mmol of POP. Yield: 82%.
1H NMR (CDCl3, 300 MHz): 8.72 (2H, d, J = 5.0 Hz), 8.50 (2H, d,
J = 8.7 Hz), 8.01 (2H, s), 7.71 (2H, q, J = 5.3 Hz), 7.31 (2H, dt,
J = 1.7 Hz, J = 8.6 Hz), 7.25–7.20 (4H, m), 7.10 (8H, t, J = 8.3 Hz),
7.05–6.92 (12H, m), 6.81–6.76 (2H, m). 13C NMR (CDCl3,
75 MHz): 158.3, 149.5, 143.1, 137.5, 134.2, 132.8, 132.1, 130.5,
130.0, 129.5, 128.6, 127.2, 125.1, 124.1, 120.3. Anal. Calcd. for C48-
H36BCuF4N2OP2: C, 66.33; H, 4.18; N, 3.22. Found: C, 66.20; H, 4.11;
N, 3.12.
[Cu(DMPhen)(POP)]BF4ꢂ[Cu(Phen)(POP)]BF4 was synthesized by
a method similar to that of [Cu(DMPhen)(PPh3)2]BF4, except that
2.0 mmol of PPh3 was replaced by 1.0 mmol of POP. 1H NMR
(CDCl3, 300 MHz): 8.37 (2H, d, J = 9.2 Hz), 7.85 (2H, s), 7.61 (2H,
d, J = 9.2 Hz), 7.32 (2H, dt, J = 2.2 Hz, J = 8.6 Hz), 7.24–7.15 (8H,
Experimental section
The molecular structures of the six Cu(I) complexes are shown
in Scheme 1.
1,10-phenanthroline (referred to as Phen), 2,9-dimethyl-1,10-
phenanthroline (referred to as DMPhen), Cu(BF4)2, bis(2-(diphenyl-
phosphanyl)phenyl) ether (referred to as POP), triphenylphosphane
(referred to as PPh3), polyvinylpyrrolidone (PVP), polymethyl
methacrylate (PMMA), and 2-cyanopyridine were purchased from
Aldrich Chemical Co. and used without further purifications.
The starting materials of [Cu(CH3CN)4]BF4 and 2,9-dibutyl-1,
10-phenanthroline (referred to as DBPhen) were synthesized
according to the literature procedures [7,11]. All organic solvents
were purified using standard procedures.
Scheme 1. The molecular structures of the six Cu(I) complexes.

K. Zhang, D. Zhang / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 341–348
343
Table 1
[Cu(DBPhen)(POP)]BF4ꢂ[Cu(DBPhen)(POP)]BF4 was synthesized
by a method similar to that of [Cu(DBPhen)(PPh3)2]BF4, except that
2.0 mmol of PPh3 was replaced by 1.0 mmol of POP. 1H NMR
(CDCl3, 300 MHz): 8.45 (2H, d, J = 9.4 Hz), 7.91 (2H, s), 7.64 (2H,
d, J = 9.4 Hz), 7.35–7.28 (2H, m), 7.21–7.17 (8H, m), 7.01 (8H, t,
J = 8.2 Hz), 6.96–6.90 (10H, m), 2.82 (4H, t, J = 9.2 Hz), 1.31–1.17
(4H, m), 0.76–0.65 (4H, m), 0.60 (6H, t, J = 7.5 Hz). 13C NMR (CDCl3,
75 MHz): 162.6, 158.1, 142.8, 138.1, 133.4, 132.8, 132.0, 131.4,
129.8, 128.4, 128.1, 126.1, 125.3, 125.0, 123.3, 120.1, 40.6, 30.4,
22.1, 13.7. Anal. Calcd. for C56H52BCuF4N2OP2: C, 68.54; H, 5.34;
N, 2.85. Found: C, 68.46; H, 5.22; N, 2.79.
Selected bond lengths (Å) and angles (°) of the three Cu(I) complexes.
[Cu(N–N)(POP)]BF4, N–N =
Phen
DMPhen
DBPhen
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–P(1)
Cu(1)–P(2)
Cuꢂ ꢂ ꢂO
2.071
2.104
2.097
2.109
2.271
2.279
3.257
80.51
121.44
111.58
105.45
121.74
112.91
2.063
2.084
2.231
2.261
3.205
2.269
2.273
3.151
N(1)–Cu–N(2)
N(1)–Cu–P(1)
N(2)–Cu–P(1)
N(1)–Cu–P(2)
N(2)–Cu–P(2)
P(1)–Cu–P(2)
80.83
80.89
118.38
125.75
108.12
109.08
110.81
115.22
110.00
107.76
121.43
116.44
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 RB3PW91/SBKJC level
[12], and at RB3LYP/SBKJC level for comparison (see Supporting
information for details). The initial geometries were obtained
from their single crystal structures. All computations were finished
by GAMESS software package. The graphical presentation was
m), 7.01 (8H, t, J = 8.3 Hz), 6.97–6.91 (10H, m), 2.43 (6H, s). 13C
NMR (CDCl3, 75 MHz): d 159.1, 158.3, 143.1, 138.0, 133.7, 132.7,
132.2, 131.6, 129.8, 128.4, 127.6, 126.0, 125.5, 125.3, 125.2,
120.0, 27.0. Anal. Calcd. for C50H40BCuF4N2OP2: C, 66.94; H, 4.49;
N, 3.12. Found: C, 66.73; H, 4.37; N, 3.04.
Fig. 1A. Molecular structure of [Cu(Phen)(POP)]BF4, showing the interaction between phenyl ring from POP and Phen. BFꢃ and hydrogen atoms are not shown for clarity.
4
Fig. 1B. Molecular structure of [Cu(DMPhen)(POP)]BF4, showing the interaction between phenyl ring from POP and DMPhen. BFꢃ and hydrogen atoms are not shown for
4
clarity.

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K. Zhang, D. Zhang / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 341–348
Fig. 1C. Molecular structure of [Cu(DBPhen)(POP)]BF4, showing the interaction between phenyl ring from POP and DBPhen. BFꢃ and hydrogen atoms are not shown for
4
clarity.
Table 2
Summarized photophysical parameters of the six Cu(I) complexes.
Ub
sc
(ls)
Kr (ꢄ104 sꢃ1
)
Knr (ꢄ104 sꢃ1
)
a
a
[Cu(N–N)(P–P)]BF4
N–N, P–P =
kabs (nm)
kem (nm)
Phen, (PPh3)2
DMPhen, (PPh3)2
DBPhen, (PPh3)2
Phen, POP
DMPhen, POP
DBPhen, POP
368
365
363
395
385
381
541
510
504
556
527
518
0.13
0.35
0.56
0.15
0.50
0.70
8.2
18.4
33.1
4.5
13.3
20.5
1.58
1.90
1.69
3.33
3.76
3.41
10.61
3.53
1.33
18.89
3.76
1.47
a
b
c
1 nm.
10%, Measured according to the literature method of Ref. [9].
5%, I ¼ A1 expðꢃt=s1Þ þ A2 expðꢃt=s2Þ;
s
¼ ðA1s21 þ A2s22Þ=ðA1s1 þ A2s2Þ.
fluorescence spectrophotometer. Solid state photoluminescence
quantum yields were measured with the Hitachi F-4500 fluores-
cence 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 crystal data
were collected on a Siemens P4 single-crystal X-ray diffractometer
with a Smart CCD-1000 detector and graphite-monochromated
Fig. 2A. UV–Vis absorption and PL spectra of [Cu(N–N)(PPh3)2]BF4, where
N–N = Phen, DMPhen, and DBPhen.
generated by wxMacmolplt software package with contour value
of 0.025.
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 cmꢃ1, 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
Fig. 2B. UV–Vis absorption and PL spectra of [Cu(N–N)(POP)]BF4, where
N–N = Phen, DMPhen, and DBPhen.

K. Zhang, D. Zhang / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 341–348
345
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.
Results and discussion
Single crystal structures
The crystal structures of [Cu(Phen)(POP)]BF4, [Cu(DMPhen)
(POP)] BF4, and [Cu(DBPhen)(POP)]BF4 are observed, and the
selected geometric parameters are listed in Table 1. The Cu(I)
center in the three complexes has
a distorted tetrahedral
geometry. The dihedral angles between N–Cu–N and P–Cu–P
planes for the three complexes are measured to be 88.36°
for[Cu(Phen)(POP)]BF4, 82.33° for [Cu(DMPhen)(POP)]BF4, and
78.80° for [Cu(DBPhen)(POP)]BF4, respectively. The Cu–N bond
Fig. 3B. HOMO (down) and LUMO (up) of [Cu(DMPhen)(POP)]+.
length of the three Cu(I) complexes increases with the increasing
alkyl steric hindrance. Above results suggest that the introduction
of alkyl chain into 2,9-positions of 1,10-phenanthroline leads to a
crowded coordination sphere around Cu(I) center, and the Cu(I)
center tries to minimize the congestion by extending Cu–N bond
and decreasing the dihedral angle between N–Cu–N and P–Cu–P
planes.
On the other hand, Cu–P bond lengths of the three Cu(I) com-
plexes are similar to each other, showing no response towards
the introduction of alkyl chain. The oxygen atom of POP ligand in
the three Cu(I) complexes localizes at a distance of ꢅ3.2 Å away
from the Cu(I) center and opposite to the coordinated N atoms,
indicating 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 [10,13].
Fig. 3A. HOMO (down) and LUMO (up) of [Cu(Phen)(POP)]+.

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K. Zhang, D. Zhang / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 341–348
3.021 Å. As for [Cu(DBPhen)(POP)]BF4 which owns the most in-
tense steric hindrance, large P–Cu–P bond angle of
[Cu(DBPhen)(POP)]BF4 allows one of POP’s phenyl ring to align al-
most parallel to DBPhen ring so that their mean planes intersect
with an angle of 16.41°, and the minimum distance between phe-
nyl ring from POP and diamine ligand is only 3.018 Å. As above
mentioned, the excited state Cu(I) center tends to waste its energy
by switching its coordination sphere from a tetragonally flattened
one to a tetrahedral-like one, this interaction between phenyl ring
from POP and diamine ligand may effectively block the geometry
transition, leading to improved photophysical performances,
which will be discussed below.
Photophysical properties
The photophysical parameters of the six Cu(I) complexes dis-
persed in PMMA films (10 wt%) are summarized in Table 2. Fig. 2
shows the UV–Vis absorption spectra of the six Cu(I) complexes
in CH2Cl2 solutions with a concentration of 1 ꢄ 10ꢃ5 mol/L. It can
be observed that the spectral band shapes are quite similar to each
other due to their similar diimine and phosphorous ligands. Each
absorption spectrum is typically composed of a high-energy
absorption band ranging from 350 nm to 220 nm and a low-energy
absorption band ranging from 350 nm to 500 nm. The former one
corresponds to ligands
p ?
pꢁ transitions according to literature
reports [9,10,13]. While, the latter one is experimentally assigned
as MLCT transition absorption. Compared with the absorption
spectrum of [Cu(Phen)(POP)]BF4, it can be seen that the introduc-
tion of alkyl chain into 2,9-positions of 1,10-phenanthroline leads
to a blue shift to the absorption spectra of corresponding [Cu(N–
N)(POP)]BF4 complexes, where N–N = DMPhen and DBPhen [9].
In addition, [Cu(N–N)(PPh3)2]BF4 need more energy for onset elec-
tronic transition compared with [Cu(N–N)(POP)]BF4, where N–
N = Phen, DMPhen, and DBPhen.
As shown in Fig. 2, the six Cu(I) complexes exhibit broad emis-
sion spectra (kem) at room temperature, and their emission peaks
are listed in Table 2. Those emissions render no vibronic progres-
sions, indicating that the excited states own a CT character. Large
Stokes shifts between absorption peaks (kabs) and emission peaks
(kem) are observed for the Cu(I) complexes (173 nm for
[Cu(Phen)(PPh3)2]BF4, 145 nm for [Cu(DMPhen)(PPh3)2]BF4, and
141 nm for [Cu(DBPhen)(PPh3)2]BF4, while 161 nm for
[Cu(Phen)(POP)]BF4, 142 nm for [Cu(Phen)(POP)]BF4, and 137 nm
for [Cu(DBPhen)(POP)]BF4). Those large Stokes shifts reveal that
the six Cu(I) complexes experience intense geometric relaxation
in excited state, and this geometric relaxation tends to become
weak with the increasing increasing alkyl steric hindrance from
Phen to DBPhen.
The PL quantum yields (
sured, along with their PL decay lifetimes (
U
) for the six Cu(I) complexes are mea-
), as shown in Table 2.
Fig. 3C. HOMO (down) and LUMO (up) of [Cu(DBPhen)(POP)]+.
s
Their phosphorescent nature can be confirmed by the long-lived
excited state. It is notable that both PL quantum yield and PL decay
lifetime tend to increase with the increasing alkyl steric hindrance
from Phen to DBPhen, which means that luminescence is greatly
enhanced by the introduction of alkyl chain into 2,9-positions of
1,10-phenanthroline. Based on the quantum yields and lifetime
data on hand, the radiative probability and nonradiative probabil-
ity values, Kr and Knr, are calculated with (1) and (2) as follows:
N–Cu–N bond angles of the three Cu(I) complexes are quite
similar to each other (ꢅ80°) with small variations, which means
that the bite angles in Phen-derived ligands are nearly invariable.
In contrast, P–Cu–P bond angle varies dramatically from 110.81°
for [Cu(Phen)(POP)]BF4 to 116.44° for [Cu(DM)(POPh)]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 an-
gles suggest a crowded coordination environment around Cu(I)
center, which is consistent with above analysis [14].
It is also observed from their molecular structures that the
interaction between phenyl ring from POP and Phen-derived
ligands increases with the increasing alkyl steric hindrance. As
shown in Fig. 1, in [Cu(Phen)(POP)]BF4, the minimum distance
between phenyl ring from POP and diamine ligand is 3.543 Å, the
corresponding value for [Cu(DMPhen)(POP)]BF4 decreases to
Kr
U
¼
ð1Þ
ð2Þ
Kr þ Knr
¼ Kr þ Knr
1
s
As shown in Table 2, the radiative probability values of [Cu(N–
N)(POP)]BF4 are typically two times bigger than [Cu(N–N)(PPh3)2]-
BF4, where N–N = Phen, DMPhen, and DBPhen. On the other hand,

K. Zhang, D. Zhang / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 341–348
347
Table 3
Calculated percentage composition of the first five singlet excitations of [Cu(N–N)(POP)]+ calculated at RB3PW91/SBKJC level.
[Cu(N–N)(POP)]+, N–N =
Phen
DMPhen
DBPhen
S0 ? S1
S0 ? S2
S0 ? S3
S0 ? S4
S0 ? S5
HOMO ? LUMO(91.7%)
460.04 nm
HOMO ? LUMO+1(97.3%)
451.40 nm
HOMO ? LUMO(75.5%)
435.79 nm
HOMO ? LUMO(89.1%)
449.18 nm
HOMO ? LUMO+1(93.3%)
406.82 nm
HOMOꢃ1 ? LUMO(72.5%)
407.21 nm
HOMOꢃ1 ? LUMO(90.4%)
HOMO ? LUMO+1(93.1%)
402.48 nm
HOMOꢃ1 ? LUMO(85.6%)
447.81 nm
399.79 nm
HOMOꢃ1 ? LUMO+1(88.3%)
410.11 nm
HOMOꢃ2 ? LUMO(94.4%)
HOMOꢃ2 ? LUMO(95.8%)
375.49 nm
385.21 nm
HOMOꢃ2 ? LUMO(83.9%)
HOMOꢃ1 ? LUMO+1(93.9%)
371.49 nm
HOMOꢃ1 ? LUMO+1(95.8%)
404.66 nm
359.17 nm
3MLCT excited states, showing no luminescence mechanism differ-
ence. Consequently, the luminescence enhancement with increas-
ing alkyl steric hindrance is not caused by their electronic nature
difference.
Further confirmation on luminescence enhancement
In order to further confirm that the luminescence enhancement
is triggered by the increasing alkyl steric hindrance, we compare
the photophysical properties of [Cu(Phen)(PPh3)2]BF4 in crystal
state and in PMMA film. As shown in Fig. 4, the PL spectrum of
crystal [Cu(Phen)(PPh3)2]BF4 peaks at 522 nm, compared with
541 nm from [Cu(Phen)(PPh3)2]BF4 doped PMMA film. In addition,
the crystal sample exhibits a much higher PL quantum yield of
0.75, compared with 0.13 from [Cu(Phen)(PPh3)2]BF4 doped PMMA
film. It is observed that the PL performances of [Cu(Phen)(PPh3)2]-
BF4 in crystal state is much superior to those in dispersed film,
which can be explained as follows.
McMillin and coworker’s reports confirm that the dominant
nonradiative decay process is the geometric relaxation that occurs
at excited state from a tetragonally flattened geometry to a
tetrahedral-like one [3,5–7]. According to Zhang and coworkers,
there exists face-to-face pi–pi stacking in crystal state
[Cu(Phen)(PPh3)2]BF4 which is believed to be a partially rigid struc-
ture. Thus, it is expected that this geometric relaxation is somehow
suppressed by the rigid environment, resulting in the enhanced
luminescence of [Cu(Phen)(PPh3)2]BF4 in crystal state [15]. After
being dispersed in PMMA film, the rigid environment around
[Cu(Phen)(PPh3)2]BF4 molecules is expected to be destroyed. The
alkyl moieties at 2,9-positions of 1,10-phenanthroline should be
the only reason for improved PL performance. In other words, their
steric effect effectively limits the excited state geometric relaxa-
tion, showing better PL performance.
Fig. 4. PL spectrum of [Cu(Phen)(PPh3)2]BF4 and in crystal state and in PMMA film.
the nonradiative probability values of [Cu(N–N)(POP)]BF4 and
[Cu(N–N)(PPh3)2]BF4 decrease dramatically with the increasing al-
kyl steric hindrance from Phen to DBPhen. Thus, we come to a con-
clusion that the introduction of alkyl chain into 2,9-positions of
1,10-phenanthroline effectively suppresses Knr decay process, lead-
ing to the luminescence enhancement.
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 confirm that the luminescence
enhancement is caused by the steric hindrance effect, instead of
electronic effect, we perform a DFT/TD-DFT calculation, which
has been proved to be a powerful tool to investigate electronic
properties of transition metal complexes, on [Cu(N–N)(POP)]+ at
RB3PW91/SBKJC level, where N–N = Phen, DMPhen, and DBPhen
[10].
Conclusion
As shown in Fig. 3, the highest occupied molecular orbitals
(HOMOs) of [Cu(N–N)(POP)]+ have an evident metal Cu character,
admixed with large contributions from the phosphorous ligand of
POP, while, the lowest unoccupied molecular orbitals (LUMOs) of
[Cu(N–N)(POP)]+ are essentially diimine ligand pꢁ orbital of Phen.
The calculated onset transitions correspond to an electronic transi-
tion from HOMO to LUMO, and the onset excitation energy values
listed in Table 3 correlates quite well with the experimentally re-
corded absorption edge shown in Fig. 2. It is thus confirmed that
the onset electronic transition of [Cu(N–N)(POP)]+ is a MLCT one.
Correspondingly, the emissive state of [Cu(N–N)(POP)]+ which de-
rives from onset electronic excitation is also a MLCT one. Combined
with the long excited state lifetimes as mentioned, we come to a
conclusion that the emissions of [Cu(N–N)(POP)]BF4 originate from
In this paper, we report six phosphorescent Cu(I) complexes
with 1,10-phenanthroline-derived ligands and phosphorous li-
gands, 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. The correlation
between ligand structure and photophysical properties of their
corresponding Cu(I) complexes is studied in detail. It is found that
the introduction of alkyl moieties into 2,9-positions of 1,10-
phenanthroline is highly effective on restricting the geometric
relaxation that occurs in excited states, which greatly enhances
the PL performances, including PL quantum yield improvement,
PL decay lifetime increase, and emission blue shift. This finding

348
K. Zhang, D. Zhang / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 341–348
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[4] N. Armaroli, Chem. Soc. Rev. 30 (2001) 113–124.
[5] L. Zhang, S. Yue, B. Li, D. Fan, Inorg. Chim. Acta 384 (2012) 225–232.
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[7] D.G. Cuttell, S.M. Kuang, P.E. Fanwick, D.R. McMillin, R.A. Walton, J. Am. Chem.
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[8] R.A. Rader, D. McMillin, M.T. Buckner, T.G. Matthews, D.J. Casadonte, R.K.
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may be useful when designing high-performance phosphorescent
Cu(I) complexes.
Acknowledgments
The authors gratefully thank the support and help from The
Central Institute for Correctional Police, Academic innovation team
of young teachers 2012.
[9] Q. Zhang, Q. Zhou, Y. Cheng, L. Wang, D. Ma, X. Jing, F. Wang, Adv. Mater. 16
(2004) 432–436.
[10] L. Yang, J.K. Feng, A.M. Ren, M. Zhang, Y.G. Ma, X.D. Liu, Eur. J. Inorg. Chem. 10
(2005) 1867–1879.
Appendix A. Supplementary material
[11] L. Shi, B. Li, S. Yue, D. Fan, Sensor Actuat. B Chem. 137 (2009) 386–392.
[12] K. Su, J. Wei, X. Hu, H. Yue, L. Lv, Y. Wang, Z. Wen, Acta Phys. – Chim. Sin. 16
(2000) 643–651.
[13] S.M. Kuang, D.G. Cuttell, D.R. McMillin, P.E. Fanwick, R.A. Walton, Inorg. Chem.
41 (2002) 3313–3322.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.01.019.
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