Four-coordinated copper(I) complexes containing variably substituted N-heterocyclic carbenes (NHCs): Synthesis, photophysical properties and theoretical investigation

Article abstract of DOI:10.1016/j.jorganchem.2017.07.016

Three four-coordinated N-heterocyclic carbene (NHC) copper(I) complexes, [Cu(NaphIm-Py)(POP)]PF₆ (1), [Cu(AnthrIm-Py)(POP)]PF₆ (2), and [Cu(PhBenIm-c-Py)(POP)]PF₆ (3) (NaphIm-Py = 3-(naphthalen-2-ylmethyl-1-(pyridin-2-yl)-

Full text of DOI:10.1016/j.jorganchem.2017.07.016

Journal of Organometallic Chemistry 846 (2017) 351e359
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Four-coordinated copper(I) complexes containing variably substituted
N-heterocyclic carbenes (NHCs): Synthesis, photophysical properties
and theoretical investigation
Jinglan Wang , Shaobo Liu , Shengxian Xu , Feng Zhao , Hongying Xia a
Yibo Wang
a
a
a
a
a
,
*
, **
,
b
School of Chemistry and Chemical Engineering, Jiangxi Science and Technology Normal University, Fenglin Street Nanchang, Jiangxi 330013, PR China
Key Laboratory of Guizhou High Performance Computational Chemistry, Department of Chemistry, Guizhou University, Guiyang 550025, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 March 2017
Received in revised form
Three four-coordinated N-heterocyclic carbene (NHC) copper(I) complexes, [Cu(NaphIm-Py)(POP)]PF6
(
2
2
1), [Cu(AnthrIm-Py)(POP)]PF
6
(2), and [Cu(PhBenIm-c-Py)(POP)]PF
6
(3) (NaphIm-Py ¼ 3-(naphthalen-
-ylmethyl-1-(pyridin-2-yl)-1H-imidazolylidene, AnthrIm-Py ¼ 3-(anthracen-9-ylmethyl)-1-(pyridin-
2
July 2017
-yl)-1H-imidazolylidene, PhBenIm-c-Py ¼ 3-benzyl-1-(pyridin-2-ylmethyl)-1H-benzo[d]imidazolyli-
Accepted 12 July 2017
Available online 14 July 2017
dene, POP ¼ bis[2-diphenylphosphino]-phenyl)ether) have been synthesized and characterized. The
effect of varying carbene ligands with different substituents on the structural aspects and photo-
physical properties of the complexes is systematically investigated. The lowest absorption bands of 1 at
about 350 nm are assigned as metal-to-ligand charge transfer (MLCT) character with some contri-
Keywords:
Copper(I) complex
N-heterocyclic carbene
Photoluminescence property
Density functional theory
Excited-state lifetime
bution from ligand-to-ligand charge transfer (LLCT) character, while the p-p* transition of the anthryl
group are responsible for the lowest absorption band of 2. For 3, no MLCT absorption band are
observed in lower energy region. The emission wavelengths of all NHC-Cu(I) complexes are in the
range of 520e550 nm with the phosphorescent origin in the solid state. 1 and 2 with naphthyl and
anthryl groups show the lower photoluminescence efficiency due to the strong
p-p stacking in-
teractions, whereas 3 exhibit good photoluminescence properties companying with the higher
quantum yields and long excited-state lifetimes. Density functional theory (DFT) and time dependent
density functional theory (TDDFT) calculations were employed to rationalize the photophysical
properties of the NHC-Cu(I) complexes.
©
2017 Elsevier B.V. All rights reserved.
1
. Introduction
Since Tsubomura and co-workers [18] first reported the
photoluminescence properties of the NHC-Cu (I) complex in
2009, there has been a growing interest in the study of lumi-
nescent NHC-Cu(I) complexes. Thompson group in 2010 reported
the photoluminescence behavior of the two NHC-Cu(I) com-
plexes of the type [(NHC)Cu(L)] complexes (NHC¼IPr ¼ 1,3-
N-heterocyclic carbene (NHC) has been widely used as a new
class of ligands for the design of a wide variety of complexes
because of its attractive feature, such as strong electron donation
and complex stability [1]. Many NHC-based metal complexes have
been reported and utilized in a wide range field of applications
including catalysts [2e6] and medicinal fields [7e10]. Among them,
NHC copper complexes have attracted considerable attention due
to their inexpressive and relatively low toxicity [11e15]. However,
their applications in other areas remain vast room to be explored
bis(2,6-diisopropylphenyl)imidazol-2-ylidene, L1
¼
Phen
¼
1,10-phenanthroline; L2 ¼ pybim ¼ 2-(2-pyridyl)benzimidazole),
in which [(IPr)Cu(pybim)] complex exhibited high emission
quantum yields (
f
¼ 0.58) and long-lived triplet excited state
lifetimes (
t
¼ 33.1
ms) [19]. Subsequent to this work, the same
[16,17].
group in 2012 reported a series of analogous NHC-Cu(I) com-
plexes containing various pyridyl-azolate ligands [20]. These
complexes exhibited moderate to high quantum yields
*
Corresponding author.
(
m
(
f
¼ 0.23e0.62) with longer-lived excited lifetimes (
s). More recently, the [(NHC)Cu(py BMe )] complexes
py Bme
¼ di(2-pyridyl)dimethylborate) containing different
t
¼ 12e13
*
* Corresponding author.
E-mail addresses: zhf19752003@163.com (F. Zhao), xiahongying85@sohu.com
2
2
2
2
(
H. Xia).
http://dx.doi.org/10.1016/j.jorganchem.2017.07.016
022-328X/© 2017 Elsevier B.V. All rights reserved.
0

352
J. Wang et al. / Journal of Organometallic Chemistry 846 (2017) 351e359
NHC ligands were described. The color of these complexes was
observed to turn from blue to red while retaining high emission
efficiencies, depending on the type of the NHC ligands [21]. In
2. Experimental
2.1. Materials and methods
2
014, Marion and Gaillard employed the dipyridylamine ligands
to synthesize a series of [(NHC)Cu(N^N)] complexes, where N^N
denotes a dipyridylamine ligand. These NHC-Cu(I) complexes
display strong blue emission with the highest quantum yield of
The known intermediates, 2-(1H-imidazol-1-yl)pyridine (Im-
Py) and 1-(yridine-2-ylmethyl)-1H-benzo[d]imidazole (BenIm-c-
Py), were prepared according to procedures previously reported
[24]. Other chemicals with analytical grade regent were purchased
from Sinopharm Chemical Reagent Co and used as received without
further purification.
0
.88 and the excited state lifetime of 51 ms [22]. These pioneering
works have inspired us to turn our attention to the photophysical
properties of NHC-Cu(I) complexes. However, a perusal of the
literature for luminescent NHC-Cu(I) complexes shows that the
three-coordinated species are widely studied and nearly all prior
works have employed compound IPr as ligand to prepare this
class of three-coordinated NHC-Cu(I) complexes. It is inconve-
nient to tune the photophysical properties for the corresponding
NHC-Cu(I) complexes since the functionalization of IPr ligand or
its derivatives is not simple.
On the other hand, from a conceptual point of view, the four-
coordinated NHC-Cu(I) complexes seem also interesting since
they can allow a broader range of ligand selection to conve-
niently tune the photophysical properties. Surprisingly, studies
on the luminescent properties of the four-coordinated NHC-Cu(I)
complexes are very scarce. Wang and coworkers in 2016 reported
the first study on the luminescence behavior of the four-
coordinated NHC-Cu(I) complexes, in which the photo-
luminescence band maximum was found to be blue-shifted from
1
H NMR, C NMR, and 31P NMR spectra in DMSO-d
13
6
were per-
1
13
formed in a Bruker AV400 MHz spectrometer. For H and C NMR,
the chemical shifts were referenced to the residual signals of the
31
deuterated solvent. P NMR signals were referenced using 85%
PO as an external standard. High-resolution mass spectra
H
3
4
(HRMS) were obtained with the Agilent 6450 Q-TOF mass spec-
trometer using electrospray ionization (ESI). Elemental analyses of
the complexes were carried out on an Elementar VarioEL cube
analyzer. UV-vis absorption spectra were measured using a Perkin
Elmer Lambda-900 spectrophotometer. Fluorescence spectra were
determined with a Hitachi F-4600 fluorescence spectrophotometer.
Photoluminescence (PL) quantum yields were determined using a
Hamamatsu system for absolute PL quantum yield measurements
(type C11347). Fluorescent lifetimes were measured with
a
compact fluorescent lifetime spectrometer (Hamamatsu, C11367,
Japan). The X-ray experiment of the single-crystal was done on a
Bruker SMART APEX II CCD diffractometer using a MULTI scan
570 to 520 nm with high emission intensity [23]. Most recently,
our groups developed a efficient preparation method for this
kind of complexes without transmetalation of Ag(I) complexes
and the resulting complexes all exhibit good photoluminescence
properties [24].
technique and Mo Ka radiation. The structure was solved using the
SHELXTL-97 program. Crystallographic data for the structure of 1
have been deposited as supplemental material at the Cambridge
Crystallographic Data Center CCDC 1523539.
In this paper, with the aim of further tuning the luminescent
properties of this type of the four-coordinated NHC-Cu(I) com-
plexes, we report the synthesis, structural characterization, and
photophysical properties of the three four-coordinated NHC-Cu(I)
complexes bearing variably substituted carbene ligands with
various aryl groups (Fig. 1). The influence of the different structural
carbene ligands on the luminescent behaviors of the NHC-Cu(I)
complexes have been investigated. In addition, density functional
theory (DFT) and time-dependent density functional theory
2.2. Synthetic procedure
(NaphIm-Py)(PF ). A mixture of compound ImPy (1.45 g,
6
10 mmol) and 1-(chloromethyl)naphthalene (1.77 g, 10 mmol) in
toluene (20 mL) was refluxed for 24 h under N . The solvent was
removed by filtration, and the crude product was dried. NH PF
6
(1.73 g, 10 mmol) was added into the aqueous solutions of the
product. And the mixture was stirred for 10min. The precipitate
2
4
(
TDDFT) were employed to rationalize the photophysical properties
was obtained, and recrystallized with CH CN and ether. The prod-
uct was obtained as a white solid. Yield: 2.76 g, 64%. H NMR
3
1
of those NHC-Cu(I) complexes.
(
400 MHz, DMSO-d
6
):
d
10.31 (d, J ¼ 12.4 Hz, 1H), 8.68 (s, 1H), 8.57
Fig. 1. Molecular structures of the four-coordinated NHC-Cu(I) complexes containing different groups and their abbreviations in this study.

J. Wang et al. / Journal of Organometallic Chemistry 846 (2017) 351e359
353
þ
þ
(
7
s, 1H), 8.28 (d, J ¼ 8.1 Hz, 1H), 8.26e8.19 (m, 1H), 8.15e7.99 (m, 4H),
ESI ): calcd. For C59
H
45CuF
45CuN
3
OP
2
([M] ) 936.2334; found 936.2359.
13
.78e7.57 (m, 5H), 6.14 (d, J ¼ 37.8 Hz, 2H). C NMR (101 MHz,
Anal. Calcd. For C59
found: C 65.11, H 4.21, N 3.83.
H
6
N
3
OP
3
(1082.47): C 65.46, H 4.19, N 3.88;
DMSO-d
6
):
d
149.62, 140.98, 135.78, 133.88, 130.86, 130.18, 129.38,
1
5
28.17, 127.75, 126.94, 126.16, 125.72, 124.23, 123.37, 120.19, 114.83,
0.95.
AnthrIm-Py)(PF
procedure for (NaphIm-Py)(PF
chloromethyl)anthracene (2.27 g, 10 mmol). The product was ob-
[Cu(PhBenIm-c-Py)(POP)](PF
powder. Yield: 0.34 g, 88%. H NMR (400 MHz, DMSO-d
6
), 3. The product was a pale yellow
1
6
):
d
8.77 (s,
(
6
). This compound was prepared via a similar
) from ImPy (1.45 g,10 mmol) and 9-
1H), 8.32 (d, J ¼ 4.6 Hz, 1H), 8.21 (s, 1H), 8.16 (d, J ¼ 8.5 Hz, 2H), 8.06
(d, J ¼ 13.7 Hz, 2H), 7.56 (d, J ¼ 8.9 Hz, 2H), 7.53e7.47 (m, 5H), 7.45
(s, 1H), 7.44e7.34 (m, 9H), 7.33 (s, 1H), 7.32e7.23 (m, 5H), 7.18 (d,
J ¼ 8.0 Hz, 8H), 7.08 (t, J ¼ 7.5 Hz, 2H), 6.69 (d, J ¼ 2.8 Hz, 2H), 6.41 (s,
6
(
1
tained as a pale yellow solid. Yield: 2.89 g, 60%. H NMR (400 MHz,
DMSO-d ):
13
6
d
10.11 (s, 1H), 8.88 (s, 1H), 8.64 (d, J ¼ 4.5 Hz, 1H), 8.58
6
1H), 5.57 (s, 2H). C NMR (101 MHz, DMSO-d ) d 156.93, 154.29,
(
8
d, J ¼ 4.7 Hz, 1H), 8.56 (s, 1H), 8.45 (s, 1H), 8.27 (s, 1H), 8.24 (s, 1H),
148.59, 145.88, 141.36, 137.93, 135.48, 134.50, 133.77, 133.02, 132.66,
132.34, 131.29, 130.90, 130.55, 129.56, 129.17, 127.68, 125.74, 123.87,
122.57, 121.00, 119.35, 116.77, 112.69, 111.96, 50.24, 48.25. P NMR
.17 (t, J ¼ 7.8 Hz, 1H), 8.01 (d, J ¼ 8.2 Hz, 1H), 7.78e7.71 (m, 2H),
13
31
7
.64 (dd, J ¼ 14.4, 7.7 Hz, 4H), 6.63 (s, 2H). C NMR (101 MHz,
DMSO-d ): 148.01, 145.13, 139.34, 133.36, 129.95, 129.67, 129.23,
28.31, 126.76, 124.48, 124.12, 122.25, 122.02, 121.70, 118.49, 113.31,
4.37.
PhBenIm-c-Py)(PF
similar procedure for (NaphIm-Py)(PF
6
d
(162 MHz, DMSO-d
6
):
43CuN
43CuF
d
ꢁ8.99 (s), ꢁ143.69 (quint). HRMS (m/z,
þ
þ
1
4
ESI ): calcd. For C52
H
3
OP
2
([M] ) 850.2177; found 850.2163.
Anal. Calcd. For C58
H
6
N
3
3
OP (996.38): C 62.68, H 4.35, N 4.22;
(
6
). This compound was prepared via a
) from 1-(2-picolyl)benz-
found: C 62.51, H 4.21, N 4.13.
6
imidazole (2.09 g, 10 mmol) and benzyl bromide (1.71 g, 10 mmol).
The product was obtained as a white solid. Yield: 3.61 g, 81%. H
2.3. DFT calculations
1
NMR (400 MHz, DMSO-d
8
2
6
):
.18 (t, J ¼ 13.6 Hz, 1H), 8.06 (t, J ¼ 6.8 Hz, 3H), 7.99 (dd, J ¼ 6.2,
.8 Hz, 1H), 7.90 (t, J ¼ 7.5 Hz, 1H), 7.72e7.62 (m, 5H), 7.58 (t,
d
9.95 (s, 1H), 8.46 (d, J ¼ 4.4 Hz, 1H),
All calculations were performed using the Gaussian 09 [25]
program package. The B3LYP exchange-correlation function
26,27] was used to optimize the ground state (S state) geometries
of complexes 1e4 using the polarized continuum model (PCM) [28]
in CH Cl media. The 6-31G* basis set [29,30] was used for the C, H,
N, O, and P atoms. The LANL2DZ basis set [31] was adopted for the
Cu atom. On the basis of the optimized ground geometries, TDDFT
2 2
method [32,33] associated with PCM in CH Cl media were used to
simulate the absorption spectra of complexes studied. The first 200
singlet vertical excitations were obtained form the TDDFT output
file to construct the calculated absorption spectra. Calculated
electronic density plots for the frontier molecular orbitals were
prepared using Multiwfn analyzer soft [34] and VMD program [35].
[
0
J ¼ 7.6 Hz, 1H), 7.50 (d, J ¼ 7.0 Hz, 1H), 7.42e7.32 (m, 1H), 6.39 (s,
H), 5.96 (s, 2H). 13C NMR (101 MHz, DMSO-d
): 152.83, 149.59,
43.94, 143.45, 137.54, 133.87, 131.55, 131.29, 130.86, 129.04, 128.76,
28.26, 128.14, 126.85, 126.73, 123.71, 122.72, 113.88, 50.96, 50.02.
2
1
1
6
d
2
2
General procedure for preparation of NHC-Cu(I) complexes.
NHC-Cu(I) complexes 1e3 were synthesized by the following route:
a solution of imidazolium salt (0.4 mmol), copper powder (0.032 g,
0
6
.5 mmol) and POP (0.22 g, 0.4 mmol) reacted in CH
0 C for 24 h. The resulting mixture was filtered through a plug of
3
CN (5 mL) at
ꢀ
Celite and concentrated to ca. 1 mL. Addition of Et
filtrate afforded a pale yellow precipitate, which was collected and
washed with Et O. And the product was recrystallized with ethanol.
Cu(NaphIm-Py)(POP)](PF ), 1. The product was a pale yellow
2
O (10 ml) to the
2
3. Results and discussion
[
6
1
powder. Yield: 0.34 g, 88%. H NMR (400 MHz, DMSO-d
J ¼ 15.2 Hz, 1H), 8.19 (d, J ¼ 8.2 Hz, 1H), 8.13 (t, J ¼ 7.5 Hz, 1H), 7.98
d, J ¼ 8.2 Hz, 2H), 7.69 (d, J ¼ 8.2 Hz, 2H), 7.65e7.60 (m, 1H),
6
): d 8.54 (d,
3.1. Synthesis and characterization
(
The synthetic pathways for the NHC precursors and the corre-
7
.59e7.49 (m, 2H), 7.41 (t, J ¼ 7.2 Hz, 2H), 7.29 (dd, J ¼ 14.1, 6.9 Hz,
sponding four-coordinate NHC-Cu(I) complexes are shown in
Scheme 1. The imidazolium salt (NaphIm-Py)(PF ), (AnthrIm-
Py)(PF ) and (PhBenIm-c-Py)(PF ) were prepared according to the
8
6
H), 7.23e7.18 (m, 1H), 7.08 (d, J ¼ 8.0 Hz, 2H), 7.02 (dd, J ¼ 13.5,
2
.0 Hz, 5H), 6.97 (d, J ¼ 7.5 Hz, 1H), 6.86 (d, J ¼ 3.9 Hz, 4H), 6.74 (dd,
2
2
J ¼ 18.7, 6.1 Hz, 5H), 6.48 (d, J ¼ 2.8 Hz, 2H), 6.34 (d, J ¼ 7.0 Hz, 1H),
.48 (s, 2H). 13C NMR (101 MHz, DMSO-d
48.50, 141.30, 134.34, 133.53, 133.13, 132.84, 132.51, 132.34, 132.19,
32.12, 130.60, 130.33, 130.11, 129.28, 129.02, 128.26, 127.04, 126.68,
25.67,125.46,124.93,123.71,123.62,123.49,123.28,122.69,120.76,
literature procedure [36]. The four-coordinate NHC-Cu(I) com-
plexes was prepared by the established synthetic method [24].
5
1
1
1
6
) d 158.34, 158.28, 150.30,
1
13
31
Spectroscopic data of H NMR, C NMR, P NMR, HRMS and MS
confirmed the formation of the desired NHC-Cu(I) complexes (see
1
31
Experimental Section). The H NMR spectra of the imidazolium salt
118.11, 113.04, 52.43. P NMR (162 MHz, DMSO-d
6
H
):
43CuN
6
43CuF N
d
ꢁ9.39
þ
(Naph-ImPy)(PF
exhibit pro-carbenic proton resonance signals at 10.31, 10.11 and
.95 ppm, respectively. As expected, upon coordination of the NHC
2 2 2
), (Anth-ImPy)(PF ) and (PhBenIm-c-Py)(PF )
(
(
(
s), ꢁ143.61 (quint). HRMS (m/z, ESI ): calcd. For C55
3
OP
OP
2
þ
[M] ) 886.2177; found 886.2142. Anal. Calcd. For C55
H
3
3
9
1032.40): C 63.99, H 4.20, N 4.07; found: C 63.81, H 4.09, N 4.32.
Cu(AnthrIm-Py)(POP)](PF ), 2. The product was a pale yellow
ligands, the signal of pro-carbenic proton resonances was absent
[
6
1
1
from the H NMR spectra illustrating that Cu-Ccarbene bonds are
6
powder. Yield: 0.34 g, 88%. H NMR (400 MHz, DMSO-d ): d 8.77 (s,
13
formed. In the C NMR spectra, the signals for the carbene carbon
1
H), 8.32 (d, J ¼ 4.6 Hz, 1H), 8.21 (s, 1H), 8.16 (d, J ¼ 8.5 Hz, 2H), 8.06
atoms of 1e3 appear at 158.34,157.94 and 156.93 ppm, respectively.
(
d, J ¼ 13.7 Hz, 2H), 7.56 (d, J ¼ 8.9 Hz, 2H), 7.53e7.47 (m, 5H), 7.45
13
Usually the C chemical shifts of known carbonic carbon atom of
(
s, 1H), 7.44e7.34 (m, 9H), 7.33 (s, 1H), 7.32e7.23 (m, 5H), 7.18 (d,
NHC-Cu(I) complexes appear in the range 150e204 ppm depending
J ¼ 8.0 Hz, 8H), 7.08 (t, J ¼ 7.5 Hz, 2H), 6.69 (d, J ¼ 2.8 Hz, 2H), 6.41 (s,
31
H), 5.57 (s, 2H). 1 C NMR (101 MHz, DMSO-d
49.77, 148.51, 140.65, 133.77, 132.97, 132.18, 132.00, 131.77, 131.61,
30.88, 130.28, 130.17, 129.54, 129.35, 128.96, 127.25, 125.25, 123.86,
3
the ancillary ligands [19e23,37e39]. Additionally, the P NMR
1
6
) d 157.94, 157.89,
spectra are consistent with the presence of POP ligand, a singlet
1
1
at ꢁ9.39, ꢁ8.88 and ꢁ8.99 ppm for 1e3, respectively. The presence
-
31
of the [PF
6
] counterion was confirmed by the observation of a
123.77, 122.80, 122.37, 121.60, 120.55, 117.59, 112.66, 46.59. P NMR
characteristic quintet at ꢁ143.61, ꢁ143.52 and ꢁ143.69 ppm for
(
162 MHz, DMSO-d ):
6
d
ꢁ8.88 (s), ꢁ143.52 (quint). HRMS (m/z,
1e3, respectively [40].

354
J. Wang et al. / Journal of Organometallic Chemistry 846 (2017) 351e359
Scheme 1. The synthetic pathways of the NHC precursors and the corresponding NHC-Cu(I) complexes.
Single crystal of 1 suitable for X-ray diffraction analysis was
Table 1
Photophysical properties of 1e3.
obtained by slow evaporation of dichloromethane/ethanol solution
of the complexes. The crystal structures and the detailed crystal-
lographic data are provided in Fig.1S and Table 1S in the Supporting
Information. The Cu-N bond lengths (2.176 Å) and Cu-P bond
lengths (2.240e2.256 Å) are normal for the four-coordinate Cu(I)
complexes. The dihedral angles defined by P-Cu-P and C-Cu-N
4
ꢁ1
knr/104sꢁ1
Complex
labs/nm
lem/nm
t
/m
s
F
/%
k /10 s
r
1
2
3
350
352/370/391
/
540
550
526
75.3
20.9
33.42
6.10
3.30
46.7
0.08
0.16
1.40
1.25
4.62
1.59
ꢀ
planes are 80.96 for 1. It is clearly seen that the short C-H,,,F
contacts are in the range of 2.701e2.691 Å, probably indicating the
3.2. Photophysical properties
UV-Vis spectra of complexes 1e3 in CH Cl solution were shown
present of intramolecular hydrogen-bonding interactions between
-
the two atoms. However, the present of a [PF6] counterion has
2
2
little effect on the electrical structure and absorption spectra of the
complexes, as demonstrated theoretically by Lu et al. [41,42].
Additionally, it is worthwhile noticing that the bond lengths of Cu-
in Fig. 2 at room temperature and the corresponding absorption
maxima of the relevant bands were summarized in Table 1. Ab-
sorption spectra for all NHC-Cu(I) complexes showed intense ab-
sorption bands in the region of 230e280 nm, which was assigned to
C
carbene of 1 is 1.970 Å, which is obviously shorter than those of Cu-
N bond of the similar structural tetrahedral Cu(I) complexes
2.070e2.141 Å) [43], suggesting that the carbon atom of the NHC
1
the ligand-centered pp* transitions of both NHC and POP ligands.
(
In the lower-energy region, the less intense absorption shoulders at
around 350 nm for complex 1, not present in the free NHC and POP
ligands, may be tentatively assigned to metal Cu to ligand charge
transfer (MLCT) characters. Complex 2 in the lower-energy region
showed the vibrationally structured bands at 352, 370, 391 nm.
ligand forms stronger bond to the Cu center. These important bond
lengths and angles have been anticipated by DFT calculations (see
below).
1
These bands were assigned as the characteristic pp* transition of
the anthryl moiety [44]. Additionally, it is worth noting that the
typical weak MLCT bands for complex 2 were overlapped under the
1
considerably intense pp* transition of the anthryl moiety, which
precludes the visualization of this bands in the spectrum. For
complex 3, the conjugation between the pyridine ring and imida-
zolylidene ring is broken owing to the present of a single methyl
group, giving an increase of the energy gap. Therefore, the lower
energy side of the absorption spectrum is blue-shifted compared
with that of complex 1 and 2.
Emission spectra of the NHC-Cu(I) complexes 1e3 in PMMA film
as 20% dopant are shown in Fig. 3 and the relevant emission data
are also provided in Table 1. All complexes show broad and un-
structured shapes, which is a characteristic feature of triplet
3
charge-transfer ( CT) nature of the emitting state. The maximum
emission wavelengths for 1 and 2 are located at 540 and 550 nm,
indicating that the substituent from naphthyl to anthryl group at
N3 position of the imidazolylidene ring has the slight influences on
the emissive properties of the complexes. In contrast, the emission
band of 3, located at 526 nm, is blue-shifted compared with the
maxima of 1 and 2. The blue shifted emission of 3 is because of the
broken of
p-conjugation system in the carbene ligands. Here, the
Fig. 2. Absorption spectra in dichloromethane at 298 K of 1e3.

J. Wang et al. / Journal of Organometallic Chemistry 846 (2017) 351e359
355
Table 2
Selected parameters of the optimized geometric structures of 1e3 in CH
Cl
2 2
solvent
in the ground and lowest-lying triplet excited states at the (B3LYP)/6-31g*/LANL2DZ
level.
1
2
3
S0
T1
Exptla
S0
T1
S0
T1
Bond length(Å)
Cu-P1
Cu-P2
Cu-N1
Cu-C1
2.392
2.416
2.331
2.027
2.497
2.489
2.035
1.983
2.256
2.240
2.176
1.970
2.390
2.418
2.332
2.026
2.390
2.419
2.323
2.027
2.411
2.460
2.353
2.045
2.537
2.556
2.020
2.028
Bond angle (deg)
P1-Cu-P2 112.57 104.19 111.59 111.66 111.65 111.82 101.78
P1-Cu-N1 110.74 132.94 118.15 105.85 106.16 112.15 137.61
P1-Cu-C1 115.56 101.42 110.50 126.93 127.22 117.13 96.85
P2-Cu-N1 111.32 100.89 109.85 105.00 104.60 104.23 103.74
P2-Cu-C1 123.75 141.82 123.85 118.76 118.44 118.16 134.30
N1-Cu-C1 76.66
81.14
79.90
76.79
76.96
90.29
88.90
Distortion Parameter (deg)
q
q
q
x
y
z
78.6
87.1
87.5
0.909
83.8
87.7
58.7
0.787
66.6
87.0
88.4
0.848
66.6
86.6
88.7
0.847
79.1
87.2
85.9
0.904
85.2
88.0
60.1
0.803
x
CD
Fig. 3. Normalized emission spectra of 1e3 in PMMA films (10%).
a
In this work.
emission bands of 1 and 3 are assigned to a dp(Cu)/p*(ligand)
3
excited state ( MLCT), whereas that of 3 can be regard as triplet
n
Table 1. A comparison of the k and f shows a simple trend that the
photoluminescence quantum yield is mainly determined by the
3
ligand-localized (intraligand, IL) excited state inside the anthryl
moiety, which are corroborated by the spin density distributions
radiative decay. It may be noted that the values of knr for 1 and 3 are
4
4
(
see below).
roughly the same (1.25 ꢂ 10 for 1 and 1.59 ꢂ 10 for 3), while that
4
To gain a deeper understanding of the emitting states of com-
of 2 (4.62 ꢂ 10 ) is much larger than those of the other two com-
plexes 1e3, the photoluminescence quantum yields (
state lifetimes ( ) were also measured. As shown in Table 1, the
values of 1 and 2 are 6.1 and 3.3%, respectively. These values are
f
) and excited
plexes. This is additional evidence that the nature of the emitting
t
f
state of 2 is difference from that of 1 and 3.
f
significantly smaller to the values reported for the similar NHC-
3.3. Theoretical calculations
Cu(I) complexes [19], which may be rationalized by the strong
intermolecular
structures of naphthyl group in 1 and anthryl groups in 2. The
values decrease in the order 1 > 2, in accordance with the increase
of the stacking ability of the substituents (naphthyl < anthryl).
In contrast, an increase in for 3 (
compared to 1 and 2, presumably because of the presence of a
nonplanar form of the carbene ligand with the smaller planar
structure of phenyl group in 3.
p
-
p
stacking interactions caused by the large planar
3.3.1. Ground state geometries, molecular orbital properties, and
singlet-singlet excited states
f
The optimized ground state geometric structures in CH Cl
2
2
p-
p
solvent for complexes 1e3 are shown in Fig. 4 and selected bond
lengths and angles are summarized in Table 2. The optimized
structural parameters of the complexes 1 are in general agreement
with the corresponding experimental values. The geometry for all
the complexes is best described as a distorted tetrahedral geometry
around the Cu(I) center. In general, the degree of distortion in the
Cu coordination often be assessed as the flattening distortion
f
f
¼ 46.7%) is observed as
The
confirming their phosphorescence origin in nature. Based on the
known and values, the radiative rate constant (k ) and non-
radiative rate constant (knr) are calculated and summarized in
t values of all complexes are in the range of 20e76 ms,
f
t
r
expressed by the dihedral angles (
ligand planes (qLL ¼ 90.0 for the ideal tetrahedral geometry).
q
LL) between the two chelate
ꢀ
2 2
Fig. 4. The optimized ground state geometric structures in CH Cl solvent for 1e3.

356
J. Wang et al. / Journal of Organometallic Chemistry 846 (2017) 351e359
To gain a comprehensive description of the distortion of the
Cu(I) center from the ideal tetrahedral geometry, the overall dis-
tortions around Cu(I) center in the three-dimensional space are
highly desired. In 1984, White and coworkers firstly reported the
effective approach to describe the distortions in terms of the an-
gles
q
x
,
q
y
, and
q
z
[45]. The
LL) usually reported in the literature, corre-
and are regar-
z
q value is generally similar to the
dihedral angle (
sponding the flattening distortion; while the
q
q
x
q
y
ded as the ‘rocking’ and ‘waggling’ distortions, respectively [46].
ꢀ
For the ideal tetrahedral geometry,
q
x
¼
q
y
¼
q
z
¼ 90.0 . Based on
the six angles around the Cu(I) center (see Table 2), the calculation
of , and was done and the corresponding data are detailed
in Table 2. As shown in Table 2, the rocking distortion with the
values in the range of 66.6 e79.1 are much greater than the
waggling and flattening distortions with the and values in the
range of 85.9 e89.0 for all the complexes. The longer Cu-N1
bonds compared to the Cu-Ccarbene bonds provide further evi-
dences for this distortion, as the rocking moves one bond toward
q
x
,
q
y
q
z
q
x
ꢀ
ꢀ
q
y
q
z
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
the axial position. The
respectively) is approximately the same with the
mentioned above. In 2004, Coppens and coworkers employed a
single parameter to conveniently quantify the degree of overall
distortion as follows [47]:
q
z
values (
q
z
¼ 87.5 , 88.4 , 85.9 for 1e3,
q
LL values
x
À
Á
Fig. 5. The energy levels and atomic orbital compositions calculated for the HOMO and
LUMO of 1e3.
ð90 þ
q
x
Þ 90 þ
q
3
y
ð90 þ
q
z
Þ
x
¼
1
80
As seen in Table 2, the
which means that the distortion of molecular structure of 1 is
very small. For 2, the degree of overall distortion is slightly
x
value for 1 can reach as high as 0.909,
Herein, these dihedral angles are defined by P1-Cu-P2 and C1-Cu-
ꢀ
ꢀ
ꢀ
N3 planes and are 88.2 for 1, 88.5 for 2, and 85.9 for 3, respec-
tively. Obviously, the distortion of the geometry for 3 is slightly
larger than that of 1 and 2 owing to the present of the flexible
methylene spacer between the pyridine moiety and the benzo[d]
imidazolylidene ring. The bond lengths of Cu-Ccarbene of 1e3 are
calculated to be 2.027, 2.026, and 2.045 Å, respectively, which are in
good agreement with the shorter Cu-Ccarbene bond lengths in
experiments.
increased (
x
¼ 0.848), owing to the larger anthryl group inserted
into the N3 position of the benzo[d]imidazolylidene ring. With
respect to 3, the degree of overall distortion is smaller compared
to 2 since the small size of the phenyl group is introduced into
the N3 position. Thus, the overall distortion of the complex
studied away from an ideal tetrahedral geometry is in the order:
2
> 3>1.
Table 3
2 2
Electronic absorptions of 1e3 in CH Cl based on TDDFT calculations at the (B3LYP)/6-31g*/LANL2DZ level, together with the experimental values.
Excited state
Transition
Coeff
E(eV)/(nm)
3.58/346
Oscillator
0.0401
Assign
Exptl/nm
350
1
S
S
S
2
H-2/L
H/L
0.47842(45.8%)
0.44380(39.4%)
0.58494(68.4%)
0.34409(23.7%)
0.25266(12.8%)
ꢁ0.24507(12.0%)
0.19754(7.80%)
ꢁ0.18898(7.14%)
0.37322(27.9%)
0.70148(98.4%)
0.52195(54.5%)
0.39830(31.7%)
0.66500(88.4%)
0.34141(23.3%)
ꢁ0.33764(22.8%)
ꢁ0.30612(18.7%)
0.63568(80.8%)
0.64506(83.2%)
0.44058(38.8%)
ꢁ0.36868(27.2%)
0.34364(23.6%)
0.28763(16.5%)
ꢁ0.21317(9.09%)
0.44445(39.5%)
MLCT(Cu/NHC)/ILCT(NHC)
MLCT(Cu/NHC)/LLCT(POP/NHC)
ILCT(Naph)
MLCT(Cu/Naph)/LLCT(NHC/Naph)
LLCT(Naph/NHC, POP)
ILCT(Naph)
ILCT(Naph)
MLCT(Cu/POP)/LLCT(NHC/POP)
LLCT(Naph/NHC, POP)
ILCT(Anth)
13
161
H-1/Lþ1
H-2/Lþ1
H-1/Lþ16
H-1/Lþ10
H-5/Lþ1
H-3/Lþ15
H-1/Lþ16
H/L
4.33/286
5.74/216
0.1660
0.2284
272
237
S
S
S
171
1
5.79/214
3.15/393
3.66/338
0.2496
0.1521
0.0725
232
391/370/352
335
2
3
6
H-1/Lþ1
H-2/Lþ1
H-1/Lþ6
H/Lþ8
MLCT(Cu/NHC)/LLCT(POP/NHC)
MLCT(Cu/NHC)/ILCT(NHC)
MLCT(Cu/POP)/ILCT(POP)
ILCT(Anth)
S
S
21
71
4.34/286
4.99/248
0.0744
0.9646
274
235e262
H-5/L
ILCT(Anth)
H-16/L
H/L
MLCT(Cu/Anth)/LLCT(POP/Anth)
MLCT(Cu/NHC)/LLCT(POP/NHC)
MLCT(Cu/POP)/ILCT(POP)
MLCT(Cu/NHC)/LLCT(POP/NHC)/ILCT(POP)
MLCT(Cu/POP)/LLCT(NHC/POP)/ILCT(POP)
MLCT(Cu/POP)/LLCT(NHC/POP)/ILCT(POP)
MLCT(Cu/NHC)/ILCT(POP)/LLCT(POP/NHC)
MLCT(Cu/POP)/ILCT(POP)
MLCT(Cu/NHC)/ILCT(POP)/ILCT(NHC)
S
S
S
1
7
8
3.73/332
4.23/293
4.25/292
0.0496
0.0745
0.0835
/
H/Lþ4
290
279
H/Lþ3
H-1/Lþ1
H-1/Lþ2
H/Lþ18
H-13/Lþ4
H-1/Lþ18
S
S
130
166
5.62/221
5.79/214
0.0526
0.0496
234
234

J. Wang et al. / Journal of Organometallic Chemistry 846 (2017) 351e359
357
Fig. 5 shows the energy levels and atomic orbital compositions
calculated for the HOMO and LUMO of 1e3. The HOMO of 1 and 3 is
mainly localized on the Cu atom and the lone pair electrons on the P
atoms in POP ligands. Therefore, the HOMO energy remains almost
unchanged (ꢁ5.96 eV for 1; ꢁ5.94 eV for 3). On the other hand, the
LUMO of 1 and 3 is predominately localized on the carbene ligands.
However, the LUMO energy level of 3 is destabilized by 0.14 eV
compared with 1, due to the broken of
p-conjuction between the
pyridine moiety and the benzo[d]imidazolylidene ring. Unlike 1
and 3, the HOMO and LUMO of 2 are mainly distributed on the
anthryl moiety. The HOMO energy is calculated to be ꢁ5.68 eV,
which is less stable than those of 1 and 3. The LUMO energy
(
ꢁ2.19 eV) is calculated to be more stable compared to 1 and 3. The
HOMO-LUMO energy gap can be obtained in the order
4.40 eV) > 1 (4.28 eV) > 2 (3.49 eV), in reasonably good agreement
3
(
with the blue-shifted lowest-lying absorption onset observed in the
experiments.
The low-lying singlet-singlet transitions of all the complexes
were calculated by using the TDDFT approach to provide insight
into the nature of the electronic transitions. Table 3 shows the
transition energies to the low-lying singlet excited states for 1e3
associated with the dominant singlet-singlet vertical excitations,
their oscillator strengths, assignment configurations, along with
the corresponding experimental values. To conveniently discuss the
electronic transition characters, the relative frontier molecular
orbital compositions of complexes 1e3, which are expressed in
terms of the molecular fragments, in CH
and Table 2S (see Supporting Information).
For 1, the second low-lying singlet-singlet excited state S
calculated at 346 nm, is mainly composed of HOMO-2/LUMO
45.8%) and HOMO/LUMO (39.4%) transitions, in good agreement
2 2
Cl are shown in Fig. 2S
2
,
(
with the experimental values of 350 nm. An composition analysis of
the HOMO, HOMO-1, and LUMO (see Table 2S, Supporting Infor-
mation) clearly shows that this excitation can be ascribed as metal-
to-ligand charge transfer (MLCT)/ligand-to-ligand charge transfer
(
LLCT)/intraligand (ILCT) charge transfer character. For 2, the
lowest-lying singlet-singlet excited state S , calculated at 393 nm, is
dominantly originated from HOMO[ (anthryl)]/LUMO
*(anthryl)] transition with pp*character, corresponding to the
1
p
[p
experimental absorption bands in the range of 350e400 nm even
though the calculated spectra do not display vibronic fine structure
of anthryl group. For 3, the calculated lowest-lying absorption band
1
at 332 nm from the S state, not observed in experiments, is
dominated by the HOMO/LUMO transition with MLCT/LLCT
character. In addition, other transitions with the larger oscillator
strengths are also presented in Table 2. A comparison of the shapes
of calculated and experimental absorption spectra for 1e3 is pre-
sented in Fig. 6. It is found that the experimental spectra are overall
well reproduced by the calculations.
3.3.2. Triplet state geometries and emission properties
To gain insight into the nature of the emission properties of
1
1
e3, their lowest-lying triplet excited-state geometries (T ) were
optimized at the spin-unrestricted UB3LYP/6-31G*/LANL2DZ level
with a spin multiplicity of 3 and selected bond lengths and angles
are also presented in Table 2. Where stated, toluene was chosen as
the solvent, because it is the less polar solvent and thus acts as the
similar case to compare with the doped PMMA films. The calcu-
lated results show that the triplet geometrical parameters around
Cu(I) center of 1 and 3 have large differences from the corre-
sponding ground state structures, meaning that the metal char-
acter is involved in the excited state. The Cu-P1 and Cu-P2 bond
lengths are relatively longer than that in the corresponding
ground states, while the Cu-N1 and Cu-C1 bond lengths are
shorted, indicating electron transfer from the POP ligand to NHC
ligand in the excited states. The similar variation trend in the
excited states suggests that 1 and 3 are emitting from the same
excited state. For 2, the main geometric parameters around Cu(I)
center did not change significantly in the lowest-lying triplet state
compared to the ground state, which provides the additional
Fig. 6. Comparison of the calculated (red line) and experimental (black line) absorp-
tion spectra in CH Cl solution for 1 (a), 2 (b), and 3 (c). Red vertical lines correspond to
2 2
oscillator strength of calculated singlet-singlet transitions. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)

358
J. Wang et al. / Journal of Organometallic Chemistry 846 (2017) 351e359
Fig. 7. Spin density distribution contours (isovalue ¼ 0.004) for the lowest triplet state T
1
of 1e3. The values of the unpaired-electron spin-density population are depicted together
with the electronic nature of the states.
evidence of non-metal character involved in the excited state. It is
worth noting that the dihedral angles between P1-Cu-P2 and C1-
Acknowledgements
ꢀ
ꢀ
Cu-N3 planes are seriously flatten for 1 and 3 (
q
z
¼ 58.7 and 60.1 ,
The work was supported by the National Natural Science
Foundation of China (No.21462020 and 21563013), Jiangxi Science
and Technology Normal University Key Laboratory of Organic-
inorganic Composite Materials (Key training base), and the Natu-
ral Science Foundation of Jiangxi Province(No. 20151BAB203006).
The authors thank the Guizhou University High Performance
Computation Chemistry Laboratory (GHPCC) for help with
computational studies.
respectively). The values gradually increase in the following
x
order: 1 (0.787) < 3 (0.803). This order is correlated with the
emission energy: the smaller the distortion in the excited state,
the higher the emission energy. For 2, although the distortion
(
x
¼ 0.847) in the excited state is smaller compared with 1 and 3,
its emission energy is red-shifted. This is due to the fact that the
nature of the emitting excited state of 2 is different from that of 1
and 3.
The nature of the emitting excited state of each complex can be
quantitatively identified by the unpaired-electron spin-density
distribution, as shown in Fig. 7. The unpaired electron spin-density
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2017.07.016.
distributions in the T
1
state are very similar for 1 and 3 (Cu, 0.61e;
POP, 0.27e; NHC, 1.11e for 1; Cu, 0.57e; POP, 0.26e; NHC, 1.17e for 3)
3
3
and confirm the emission behavior of MLCT/ LLCT characters and
illustrates the higher contribution of the d(Cu) to the NHC ligand
with some contribution of POP ligand to the NHC ligand. The sig-
nificant difference for 2 compared with 1 and 3 is that for the
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