Four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes with brightly luminescence properties

Article abstract of DOI:10.1080/00958972.2016.1278075

Three four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes, [Cu(Py-Im)(POP)](PF₆) (P1), [Cu(Py-BenIm)(POP)](PF₆) (P2), and [Cu(Py-c-BenIm)(POP)](PF₆) (P3) (Py-Im?=?3-methyl-1-(pyridin-2-yl)-1H-imidazolylidene,

Full text of DOI:10.1080/00958972.2016.1278075

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Four-coordinate N-heterocyclic carbene (NHC)
copper(I) complexes with brightly luminescence
properties
Shaobo Liu, Shengxian Xu, Jinglan Wang, Feng Zhao, Hongying Xia & Yibo
Wang
To cite this article: Shaobo Liu, Shengxian Xu, Jinglan Wang, Feng Zhao, Hongying
Xia & Yibo Wang (2017): Four-coordinate N-heterocyclic carbene (NHC) copper(I)
complexes with brightly luminescence properties, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2016.1278075
To link to this article: http://dx.doi.org/10.1080/00958972.2016.1278075
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Date: 19 January 2017, At: 01:35

Journal of Coordination Chemistry, 2017
http://dx.doi.org/10.1080/00958972.2016.1278075
Four-coordinate N-heterocyclic carbene (NHC) copper(I)
complexes with brightly luminescence properties
Shaobo Liua, Shengxian Xua, Jinglan Wanga, Feng Zhaoa, Hongying Xiaa and
Yibo Wangb
aschool of Chemistry and Chemical engineering, Jiangxi science and technology normal university,
nanchang, Pr China; bKey laboratory of Guizhou high Performance Computational Chemistry, department of
Chemistry, Guizhou university, Guiyang, Pr China
ABSTRACT
ARTICLE HISTORY
received 24 July 2016
accepted 14 november 2016
Three four-coordinate N-heterocyclic carbene (NHC) copper(I)
complexes, [Cu(Py-Im)(POP)](PF6) (P1), [Cu(Py-BenIm)(POP)](PF6) (P2),
and [Cu(Py-c-BenIm)(POP)](PF6) (P3) (Py-Im = 3-methyl-1-(pyridin-
KEYWORDS
2-yl)-1H-imidazolylidene, Py-BenIm
= 3-methyl-1-(pyridin-2-yl)-
Copper(i) complex;
n-heterocyclic carbene;
four-coordinate; density
functional theory; excited-
state lifetime
1H-benzo[d]imidazolylidene, Py-c-BenIm = 3-methyl-1-(pyridin-2-
ylmethyl)-1H-benzo[d]imidazolylidene, POP = bis([2-diphenylphos-
phino]-phenyl)ether), have been synthesized without transmetalation
of the NHC–Ag(I) complex for the first time. The photophysical
properties of the resultant NHC–Cu(I) complexes have been
systematically investigated via spectroscopic methods. All complexes
exhibit good photoluminescence properties with long excited-state
lifetimes and moderate quantum yields. Density functional theory
and time dependent density functional theory calculations were
employed to rationalize the photophysical properties of the NHC–
Cu(I) complexes.
1. Introduction
N-heterocyclic carbene (NHC) copper(I) complexes have been studied because of their inex-
pensive cost and relatively low toxicity. A large number of NHC–Cu(I) complexes have been
CONTACT feng Zhao
zhaof@jxstnu.edu.cn; hongying Xia
xiahongying85@sohu.com
supplemental data for this article can be accessed at http://dx.doi.org/10.1080/00958972.2016.1278075.
© 2017 informa uK limited, trading as taylor & francis Group

2
S. LIU ET AL.
prepared and focused on applications in transition-metal-catalyzed organic transformations
[1–7]. Despite the rapid development and widespread attention these complexes have
received, their applications in other areas remains to be explored [8, 9].
NHC ligands can form a strong bond with the metal owing to the strong σ-donation of
NHCs [10, 11] as well as the back donation from the metal d-orbital to the vacant p-orbital
on the carbene carbon, giving stable metal complexes. Thus, the photophysical properties
of these complexes are often remarkable, resulting in the highly luminescent organometallic
compounds. Many groups have turned their attention to photoluminescent properties of
transition metal complexes bearing NHC ligands, including Ir(III), Pt(II), Ag(I), Au(I), Ru(III),
and Os(II) [9, 12–20]. However, only a few articles have reported luminescence of Cu(I) com-
plexes with NHC ligands. Apart from a few reports of polynuclear emissive NHC–Cu(I) com-
plexes [21, 22], most of them are three-coordinate species [23–26]. These three-coordinate
NHC–Cu(I) complexes often exhibit interesting luminescence properties with the long life-
times and high quantum efficiency of the luminescence. However, nearly all prior studies
have employed 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene (IPr) as ligand to prepare
this class of NHC–Cu(I) complexes. It is inconvenient to tune the photophysical properties
for the corresponding NHC–Cu(I) complexes since functionalization of IPr or its derivatives
is not simple. In addition, the rotation of Cu–Ccarbene bond may have a negative influence on
the luminescence efficiency [24].
Generally, mononuclear Cu(I) complexes possess four-coordinate configuration with
pseudo-tetrahedral geometry in which the four coordination sites around Cu(I) are occupied
by two diimine ligands (N^N) or a diimine ligand and an auxiliary diphosphine ligand (P^P)
[27, 28]. A variety of different types of N^N ligands and P^P ligands can be employed to
afford a number of luminescent four-coordinate Cu(I) complexes because these ligands can
be easily functionalized and finely tuned [29–35]. Similarly, four-coordinate NHC–Cu(I) com-
plexes should gain attention, but, surprisingly, studies on luminescent properties of four-
coordinate NHC–Cu(I) complexes are scarce, with recently one example described by Wang
and coworkers [36]. In their reports, the four-coordinate NHC–Cu(I) complexes were prepared
by transmetallation of dinuclear NHC–Ag(I) complex that can be synthesized by treating
Ag2O with imidazolium salts.
In this paper, we report a direct and convenient route to prepare four-coordinate NHC–
Cu(I) complexes by direct reactions of Cu powder with the desired ligands without trans-
metallation of the NHC–Ag(I) complex. The resultant NHC–Cu(I) complexes exhibit good
photoluminescence properties with long excited-state lifetimes and moderate quantum
yields. Density functional theory (DFT) and time-dependent density functional theory
(TDDFT) were employed to rationalize the photophysical properties of those NHC–Cu(I)
complexes.
2. Experimental
2.1. Materials and methods
Imidazole, benzimidazole, 2-bromopyridine, 2-(chloromethyl)pyridine hydrochloride,
1H-benzotriazole, sodium tert-butoxide, copper(I) iodide, iodomethane, benzyl bromide,
1-(chloromethyl)naphthalene, 9-(chloromethyl)anthracene, ammonium hexafluorophos-
phate, bis-(2-diphenylphosphinophenyl)ether (POP), copper powder, anhydrous magnesium

JOURNAL OF COORDINATION CHEMISTRY
3
sulfate, anhydrous sodium sulfate, and potassium hydroxide were purchased from Sinopharm
Chemical Reagent Co. All other chemicals were analytical grade reagents.
1H, 13C and 31P NMR spectra were performed on a Bruker AV400 MHz spectrometer using
tetramethylsilane as internal reference. DMSO was the solvent. UV–vis absorption spectra
were measured using a Perkin Elmer Lambda-900 spectrophotometer. High-resolution mass
spectra (HRMS) were obtained with the Agilent 6450 Q-TOF mass spectrometer using elec-
trospray ionization (ESI). Elemental analyses of the complexes were carried out on an
Elementar VarioEL cube analyzer. 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 spec-
trometer (Hamamatsu, C11367, Japan).
2.2. Synthetic procedure
2.2.1. Synthesis of 1
A round-bottom flask containing imidazole (6.8 g, 100 mmol), 2-bromopyridine (15.8 g,
100 mmol), 1H-benzotriazole (1.19 g, 10 mmol), sodium tert-butoxide (9.81 g, 100 mmol),
CuI (0.95 g, 5 mmol), and DMSO (50 mL) was stirred at 110 °C for 12 h under N2. Cool water
(100 mL) was added, and the mixture was extracted with ethyl acetate, dried over anhydrous
magnesium sulfate and filtered. The solvent was evaporated under reduced pressure. After
purification on silica, with ethyl acetate/petroleum ether (1:1 v/v) as the eluent, the product
1
was obtained as a pale orange oily liquid. Yield: 10.6 g, 73%. H NMR (400 MHz, DMSO) δ
(ppm): 8.53 (t, J = 15.1 Hz, 2H), 8.12–7.93 (m, 2H), 7.83 (d, J = 8.2 Hz, 1H), 7.38 (t, J = 6.0 Hz,
1H), 7.17 (s, 1H). 13C NMR (101 MHz, DMSO) δ (ppm): 148.71, 148.60, 139.50, 134.94, 130.05,
122.11, 116.45, 112.63. HRMS (m/z, ESI+): Calcd for C8H7N3 ([M + Na]+) 168.0538; found
168.0537. Anal. Calcd for C8H7N3 (145.16): C, 66.19; H, 4.86; N, 28.95. Found: C, 66.15; H, 4.90;
N, 28.85.
2.2.2. Synthesis of 2
This compound was synthesized by the above-mentioned route from benzimidazole (11.8 g,
100 mmol), 2-bromopyridine (15.8 g, 100 mmol), 1H-benzotriazole (1.19 g, 10 mmol), sodium
tert-butoxide (9.81 g, 100 mmol), and CuI (0.95 g, 5 mmol). The product was obtained as a
1
pale orange oily liquid. Yield: 10.9 g, 51%. H NMR (400 MHz, DMSO) δ (ppm): 8.98 (s, 1H),
8.65 (d, J = 4.7 Hz, 1H), 8.31 (d, J = 8.0 Hz, 1H), 8.09 (t, J = 7.8 Hz, 1H), 7.96 (d, J = 8.2 Hz, 1H),
7.78 (t, J = 9.4 Hz, 1H), 7.51–7.30 (m, 3H). 13C NMR (101 MHz, DMSO) δ (ppm): 149.75, 148.85,
144.26, 142.06, 139.4, 131.90, 123.81, 122.95, 121.82, 119.85, 114.45, 113.86. HRMS (m/z, ESI+):
Calcd for C12H9N3 ([M + H]+) 196.0875; found 196.0845. Anal. Calcd for C12H9N3 (195.22): C,
73.83; H, 4.65; N, 21.52. Found: C, 73.79; H, 4.58; N, 21.48.
2.2.3. Synthesis of 3
2-(Chloromethyl)pyridine hydrochloride (16.4 g, 100 mmol) was added into a solution of
benzimidazole (11.8 g, 100 mmol), potassium hydroxide (11.5 g, 205 mmol), and anhydrous
sodium sulfate (15 g) in 50 mL of THF. The mixture was refluxed overnight under N2. After
extraction with CH2Cl2, the mixture was dried over anhydrous magnesium sulfate and filtered.
The solvent was removed by reduced pressure, and an orange solid was obtained. Yield:

4
S. LIU ET AL.
19.67 g, 94%.1H NMR (400 MHz, DMSO) δ (ppm): 8.52 (d, J = 4.6 Hz, 1H), 8.41 (s, 1H), 7.85–7.61
(m, 2H), 7.55–7.39 (m, 1H), 7.32–7.10 (m, 4H), 5.61 (s, 2H). 13C NMR (101 MHz, DMSO) δ (ppm):
155.94, 149.40, 144.58, 143.52, 137.16, 133.88, 122.90, 122.36, 121.69, 121.53, 119.45, 110.58,
49.43. HRMS (m/z, ESI+): Calcd for C13H11N3 ([M + H]+) 210.1032; found 210.1032. Anal. Calcd
for C13H11N3 (209.25): C, 74.62; H, 5.30; N, 20.08. Found: C, 74.65; H, 5.16; N, 20.12.
2.2.4. Synthesis of D1
A mixture of 1 (1.45 g, 10 mmol) and CH3I (1.42 g, 10 mmol) in toluene (20 mL) was refluxed
for 24 h under N2. The solvent was removed by filtration and the crude product was dried.
NH4PF6 (1.73 g, 10 mmol) was added into aqueous solution of the product, and the mixture
was stirred for 10 min. The precipitate was obtained and recrystallized with CH3CN and ether.
A white solid was obtained. Yield: 2.51 g, 82%. 1H NMR (400 MHz, DMSO) δ (ppm): 10.02 (s,
1H), 8.66 (d, J = 4.8 Hz, 1H), 8.50 (s, 1H), 8.22 (t, J = 7.9 Hz, 1H), 8.09–7.89 (m, 2H), 7.65 (dd,
J = 7.4, 4.9 Hz, 1H), 3.97 (s, 3H). 13C NMR (101 MHz, DMSO) δ (ppm): 149.61, 146.71, 140.90,
135.84, 125.52, 125.13, 119.35, 114.37, 36.70. HRMS (m/z, ESI+): Calcd for C9H10N3 ([M]+)
160.0875; found 160.0877. Anal. Calcd for C9H10F6N3P (305.16): C, 35.42; H, 3.30; N, 13.77.
Found: C, 35.76; H, 3.02; N, 13.83.
2.2.5. Synthesis of D2
This compound was prepared as described for D1 from 1-(2-pyridyl)benzimidazole (1.95 g,
10 mmol) and CH3I (1.42 g, 10 mmol). The product was obtained as a white solid. Yield: 3.23 g,
91%. 1H NMR (400 MHz, DMSO) δ (ppm): 10.48 (s, 1H), 8.79 (d, J = 4.0 Hz, 1H), 8.48 (d, J = 7.9 Hz,
1H), 8.30 (t, J = 7.7 Hz, 1H), 8.14 (d, J = 7.1 Hz, 1H), 8.04 (d, J = 8.1 Hz, 1H), 7.86–7.69 (m, 3H),
4.21 (s, 3H). 13C NMR (101 MHz, DMSO) δ (ppm): 149.93, 147.77, 143.12, 141.05, 132.77, 129.83,
128.15, 127.57, 125.51, 117.25, 116.17, 114.32, 34.13. HRMS (m/z, ESI+): Calcd for C13H12N3
([M]+) 210.1031; found 210.1028. Anal. Calcd for C13H12F6N3P (355.22): C, 43.96; H, 3.41; N,
11.83. Found: C, 44.15; H, 3.22; N, 11.92.
2.2.6. Synthesis of D3
This compound was prepared as described for D1 from 1-(2-picolyl)benzimidazole (2.09 g,
10 mmol) and CH3I (1.42 g, 10 mmol). The product was obtained as a white solid. Yield: 3.40 g,
92%. 1H NMR (400 MHz, DMSO) δ (ppm): 9.87 (s, 1H), 8.50 (d, J = 4.4 Hz, 1H), 8.04 (d, J = 7.9 Hz,
1H), 7.98–7.83 (m, 2H), 7.74–7.59 (m, 3H), 7.39 (dd, J = 7.1, 5.0 Hz, 1H), 5.93 (s, 2H), 4.15 (s,
3H). 13C NMR (101 MHz, DMSO) δ (ppm): 152.96, 149.58, 143.39, 137.51, 131.82, 131.12, 126.67,
126.50, 123.69, 122.71, 113.58, 113.57, 50.80, 33.28. HRMS (m/z, ESI+): Calcd for C14H14N3 ([M]+)
224.1188; found 224.1192. Anal. Calcd for C14H14F6N3P (369.25): C, 45.54; H, 3.82; N, 11.38.
Found: C, 45.87; H, 3.69; N, 11.46.
2.2.7. Synthesis of P1
D1 (0.12 g, 0.4 mmol), copper powder (0.032 g, 0.5 mmol), and POP (0.22 g, 0.4 mmol) were
reacted in CH3CN (5 mL) at 60 °C for 24 h. The resulting mixture was filtered through a plug
of Celite and concentrated to 1 mL. Addition of Et2O (10 mL) to the filtrate afforded a pale
yellow precipitate, which was collected, washed with Et2O, and recrystallized with ethanol.
Yield: 0.29 g, 80%. 1H NMR (400 MHz, DMSO) δ (ppm): 8.32 (s, 1H), 8.02 (t, J = 6.4 Hz, 3H), 7.53
(s, 1H), 7.43–7.37 (m, 5H), 7.33 (d, J = 6.9 Hz, 5H), 7.26 (d, J = 6.7 Hz, 4H), 7.17–7.05 (m, 9H),

JOURNAL OF COORDINATION CHEMISTRY
5
6.92 (s, 4H), 6.66 (s, 2H), 3.44 (s, 3H); 13C NMR (101 MHz, DMSO) δ (ppm): 157.93, 148.14,
140.51, 133.66, 132.91, 132.38, 132.10, 131.80, 131.61, 131.42, 129.82, 128.73, 125.00, 124.72,
123.83, 122.44, 120.47, 116.92, 112.23, 37.52; 31P NMR (162 MHz, DMSO) δ (ppm): −8.80 (s),
−143.57 (quint). HRMS (m/z, ESI+): Calcd for C45H37CuN3OP2 ([M]+) 760.1708; found 760.1723.
Anal. Calcd for C45H37CuF6N3OP3 (906.25): C, 59.64; H, 4.12; N, 4.64. Found: C, 59.97; H, 4.47;
N, 4.52.
2.2.8. Synthesis of P2
This compound was prepared with the same method as for P1 from D2 (0.14 g, 0.4 mmol),
copper powder (0.032 g, 0.5 mmol), and POP (0.22 g, 0.4 mmol). The product was a pale
yellow powder. Yield: 0.25 g, 77%. 1H NMR (400 MHz, DMSO) δ (ppm): 8.33 (d, J = 6.5 Hz, 2H),
8.19 (s, 1H), 8.11 (d, J = 7.5 Hz, 1H), 7.68 (d, J = 7.1 Hz, 1H), 7.53 (s, 2H), 7.44–7.35 (m, 5H), 7.28
(s, 6H), 7.12 (dd, J = 32.8, 14.6 Hz, 12H), 6.95 (s, 4H), 6.70 (s, 2H), 3.60 (s, 3H); 13C NMR (101 MHz,
DMSO) δ 157.76, 150.20, 149.43, 147.43, 136.13, 134.66, 133.71, 133.11, 132.20, 131.45, 130.55,
129.48, 127.85, 125.88, 125.36, 124.34, 123.60, 121.21, 119.53, 112.06, 38.63; 31P NMR
(162 MHz, DMSO) δ(ppm): −8.87 (s), −143.49 (quint). HRMS (m/z, ESI+): Calcd for C49H39CuN3OP2
([M]+) 810.1859; found 810.1843. Anal. Calcd for C49H39CuF6N3OP3 (956.31): C, 61.54; H, 4.11;
N, 4.39. Found: C, 61.55; H, 3.86; N, 4.06.
2.2.9. Synthesis of P3
This compound was prepared with the same method as for P1 from D3 (0.15 g, 0.4 mmol),
copper powder (0.032 g, 0.5 mmol), and POP (0.22 g, 0.4 mmol). The product was a pale
yellow powder. Yield: 0.34 g, 88%. 1H NMR (400 MHz, DMSO) δ (ppm):8.08 (d, J = 4.8 Hz, 1H),
8.01 (d, J = 8.0 Hz, 1H), 7.81 (d, J = 3.8 Hz, 2H), 7.50 (d, J = 7.9 Hz, 2H), 7.47–7.40 (m, 3H),
7.38–7.30 (m, 7H), 7.27–7.19 (m, 9H), 7.13 (dd, J = 18.1, 9.9 Hz, 7H), 7.04–6.95 (m, 2H), 6.75
(d, J = 3.1 Hz, 2H), 5.50 (s, 2H), 3.21 (s, 3H); 13C NMR (101 MHz, DMSO) δ (ppm): 156.57, 149.51,
145.70, 140.86, 137.69, 133.75, 132.80, 132.31, 131.83, 131.20, 130.80, 129.57, 129.01, 128.00,
125.94, 124.87, 124.30, 123.73, 122.01, 120.80, 119.29, 111.87, 110.01, 50.11, 48.51; 31P NMR
(162 MHz, DMSO) δ(ppm): −8.81 (s), −143.58 (quint). HRMS (m/z, ESI+): Calcd for C50H41CuN3OP2
([M]+) 824.2015; found 824.2014. Anal. Calcd for C50H41CuF6N3OP3 (970.34): C, 61.89; H, 4.26;
N, 4.33. Found: C, 61.86; H, 4.70; N, 4.50.
2.3. DFT calculations
All calculations were performed using the Gaussian 09 [37] program package. The B3LYP
exchange-correlation function [38, 39] was used to optimize the ground state (S0 state)
geometries of P1–P3 using the polarized continuum model (PCM) [40] in CH2Cl2 media. The
6-31G* basis set [41, 42] was used for C, H, N, O, and P atoms. The LANL2DZ basis set [43]
was adopted for Cu. On the basis of the optimized ground geometries, TDDFT method [44,
45] associated with PCM in CH2Cl2 media were used to simulate absorption spectra of the
complexes. The first 200 singlet vertical excitations were obtained from 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 [46] and VMD
programs [47].

6
S. LIU ET AL.
3. Results and discussion
3.1. Synthesis and characterization
The synthetic pathways for the NHC precursors and the four-coordinate NHC–Cu(I) com-
plexes are shown in scheme 1. The imidazolium salts D1–D3 were prepared according to
the literature procedure [48, 49]. According to literature [36], four-coordinate NHC–Cu(I)
complexes can be synthesized via two steps. First, the intermediate NHC–Ag(I) complex was
Scheme 1. the synthetic pathways of the nhC precursors and the corresponding nhC–Cu(i) complexes.

JOURNAL OF COORDINATION CHEMISTRY
7
prepared by reaction of Ag2O with the imidazolium salt. Second, the target complexes were
obtained by mixing the NHC–Ag(I) complex with Cu powder [50] and an additional ligand.
This method employs both expensive metal as transmetallation agent and harsh reaction
conditions (under degassed anhydrous acetonitrile). In our experiment, the four-coordinate
NHC–Cu(I) complexes were unexpectedly synthesized through a one-pot procedure, where
all components (the imidazolium salt, Cu powder, and the auxiliary ligand) were mixed in
acetonitrile under mild reaction conditions (60 °C, 24 h).
The resultant NHC–Cu(I) complexes were identified by 1H, 13C, and 31P NMR spectra. The
1H NMR spectra of D1–D3 exhibit pro-carbenic proton resonance signals at 10.02, 10.48, and
9.87 ppm, respectively. As expected, upon coordination of the NHC ligands, the signal of
pro-carbenic proton resonances was absent from the 1H NMR spectra illustrating that Cu–
Ccarbene bonds are formed. The 13C NMR spectra of P1–P3 exhibit resonances at 157.93, 157.76,
and 156.57 ppm ascribed, respectively, to the carbenic carbon. Usually 13C chemical shifts
of carbenic carbon of NHC–Cu(I) complexes appear at 150–204 ppm depending on the ancil-
lary ligands [23–26, 36, 51–54]. Additionally, 31P NMR spectra are consistent with the presence
of POP, showing a doublet at −8.80, 8.87, and 8.81 ppm for P1–P3, respectively. The presence
of the [PF6]− was confirmed by the characteristic septet at −143.57, −143.49, and −143.58 ppm
for P1–P3, respectively [55]. The present work represents the first route for synthesis of the
four-coordinate NHC–Cu(I) complexes without the use of Ag(I) complexes as transmetalation
agents. We believe that our approach can be applied for various NHC ligands and the exten-
sion of a family of these NHC–Cu(I) complexes are in progress.
3.2. Photophysical properties
Figure 1 shows the UV–vis spectra of P1–P3 in CH2Cl2 solution (1 × 10−4 M) at room temper-
ature. The corresponding absorbance values of the relevant bands are shown in table 1. At
high energy all of the complexes show intense absorption bands at 220–250 nm with shoul-
ders at 260–300 nm, which are quite similar to those of free ligands (figure 1S, Supporting
Figure 1. absorption spectra of P1–P3 in dichloromethane (1 × 10−4 m) at 298 K.

8
S. LIU ET AL.
Table 1. Photophysical properties of P1–P3.
kr (104 s−1
)
knr (104 s−1
)
Complex
λabs (nm)
λem (nm)
τ (μs)
Φ (%)
P1
P2
P3
333
335
/
558
560
528
37.5
44.5
29.0
31.3
30.7
40.0
0.84
0.69
1.38
1.83
1.56
2.07
Figure 2. normalized emission spectra of P1–P3 in Pmma films (20% wt).
Information) and thus attributed to the π → π* transitions of both NHC precursors and POP
ligands. In addition, at lower energy the weak absorption band at 333 nm of P1, not observed
in absorption spectra of the free ligands, can be tentatively assigned to metal-(Cu)-to-ligand
charge-transfer (MLCT) transition. P2 exhibits a slightly red-shifted MLCT absorption band
at 335 nm as compared to P1, indicating negligible effect of the extended π-conjunction of
imidazole ring. For P3, however, no MLCT absorption band was observed from 320 to 380 nm,
which was apparently overlapped by the π → π* absorption bands of the ligands, because
this absorption wavelength shifted to the high energy region due to broken conjugation
between the pyridine ring and carbene ring.
The solid-state photoluminescence spectra of P1–P3 in PMMA film are shown in
figure 2 and the relevant emission data are also provided in table 1. All complexes exhibit
unstructured broad bands, which are typical for triplet 3MLCT origin. P1 and P2 emit yellow
light with similar emission maxima around 560 nm, indicating that the fused phenyl group
on the imidazole ring did not have great influence on the excited-state energies of the
complex. However, P3 gives green emission at 528 nm. The stronger blue-shifts in the emis-
sion wavelengths for P3 indicate that the broken conjugation between the imidazolylidene
ring and adjacent pyridine ring is effective strategy to achieve high-energy emission.
The photoluminescence quantum yields (PLQY) of all complexes are moderate, reaching
values between 0.31 and 0.40 at room temperature in PMMA film. These PLQYs are compa-
rable to the values reported for three-coordinate Cu(I) complexes [23]. P1–P3 show PLQYs
of 31.3, 30.7, and 40.0%, respectively. A slight decrease of PLQYs with the extended phenyl
ring fused on the imidazolylidene ring in P2 compared to P1 was tentatively ascribed to

JOURNAL OF COORDINATION CHEMISTRY
9
self-quenching of the excited states induced by enhancement of the intermolecular π–π
stacking. P3 exhibited much higher PLQY than P1 and P2, indicating that the presence of
non-coplanarity within the NHC ligands may restrict intermolecular π–π stacking. The excit-
ed-state lifetimes of all complexes are 29.0–44.5 μs, which is somewhat longer compared to
the reported three-coordinate NHC–Cu(I) complexes [24, 25]. To summarize, the newly syn-
thesized four-coordinate NHC–Cu(I) complexes exhibit fairly good photophysical properties
with higher PLQYs and long-lived lifetimes. By independently modifying the NHC ligands
and auxiliary ligands, it is possible to endow a series of similar complexes with specific pho-
tophysical features to be used as potential candidates for the application in optoelectronics
and luminescence-based technologies.
3.3. Theoretical calculations
3.3.1. Ground-state geometries and molecular orbital properties
The optimized ground-state structures in CH2Cl2 for P1–P3 are shown in figure 3 and selected
bond lengths and angles are summarized in table 2. The optimized structural parameters
are in agreement with the experimental values of similar structural analogs [36]. The NHC
ligands in P1 are nearly coplanar and the dihedral angle between the imidazolylidene ring
and pyridine ring is lower than 2.20°. However, for P2, the degree of coplanarity diminishes
to a certain extent and the dihedral angle between the benzo[d]imidazolylidene ring and
pyridine ring is 19.1° larger than that of P1. The observed structural differences between
them can be rationalized by steric hindrance that results from repulsion between two hydro-
gens bound to C2 and C3. Compared to P1 and P2, the benzo[d]imidazolylidene ring and
pyridine ring in the NHC ligands of P3 are linked by a methylene bridge and the coplanarity
between them is broken. These variations in the twisting angles are probably responsible
for the difference in their photophysical properties.
Figure 4 shows the energy levels and atomic orbital compositions calculated for the HOMO
and LUMO of P1–P3. The HOMOs of all complexes are mainly localized on Cu and the lone
pair electrons on P in POP ligands. Therefore, the energy levels of the HOMOs remain almost
unchanged (−5.94, −6.00, and −5.93 eV for P1, P2, and P3, respectively) since the NHC ligands
have little contribution on the composition of the HOMO. The LUMOs of all complexes are
predominately localized on the NHC ligands, with little contribution from the metal, as shown
in figure 4. As mentioned above, the twist distortion between the imidazolylidene ring and
pyridine ring diminishes the degree of coplanarity in NHC ligands for P2, which should
Figure 3. the optimized ground-state geometric structures in Ch2Cl2 for P1–P3.

10
S. LIU ET AL.
Table 2. selected parameters of the optimized geometric structures of P1–P3 in Ch2Cl2 in the ground
state.
P1
P2
P3
Exptla
Bond lengths (Å)
Cu–C1
Cu–n1
Cu–P1
Cu–P2
2.028
2.324
2.387
2.406
2.027
2.326
2.385
2.415
2.042
2.345
2.414
2.450
1.967
2.181
2.261
2.247
Bond angles (°)
P1–Cu–P2
C1–Cu–n1
112.46
76.71
112.52
76.74
112.05
90.51
110.19
80.02
athe experimental data extracted from ref. [36] is for an analog of P1.
Figure 4. the energy levels and atomic orbital compositions calculated for the homo and lumo of P1–P3.
destabilize the LUMO energy of P2. However, the LUMO energy level of P2 (−1.71 eV) is
slightly stabilized by 0.03 eV with respect to P1 (−1.68 eV). This is a clear indication that the
effect of the extended π-conjugation due to the introduction of the fused phenyl ring on
the imidazolylidene ring is dominant. With respect to P3, it is clearly shown in figure 4 that
the LUMO resides on the pyridine unit not on the whole NHC ligand, which reduces conju-
gation of the NHC ligands with a consequent destabilization of the LUMO energy level.
Therefore, the LUMO energy of P3 (−1.53 eV) is destabilized by 0.15 eV compared to that of
P1. Additionally, the calculated HOMO–LUMO energy gap decreases in the order P3 > P2 > P1,
in reasonably good agreement with the blue-shift of the onset of lowest-lying absorption
maximum observed in the experiments.

JOURNAL OF COORDINATION CHEMISTRY
11
3.3.2. Theoretical absorption spectra
To gain insight into the transition character of absorption spectra of the complexes, TDDFT
calculations were undertaken to simulate the absorption spectra. To discuss the electronic
transitions, the relative frontier molecular orbital compositions of 1–3, which are expressed
in terms of the four molecular fragments, in CH2Cl2 are shown in tables 1S–3S (see Supporting
Information). The dominant energy singlet-singlet vertical excitations, their oscillator
strengths, assignment configurations, and excitations with maximum coefficients for each
of the three complexes are listed in table 4S in Supporting Information. A comparison of
calculated and experimental absorption spectra for 1–3 is presented in figure 5. The exper-
imental spectra are well reproduced by the calculations.
For P1, the lowest-lying absorption band calculated at 341 nm, in good agreement with
the 333 nm experimental values, can be assigned as the HOMO → LUMO transition, which
originates from the second excited state, the S2 state. A composition analysis of the HOMO
and LUMO (see table 1S) clearly shows that this excitation can be ascribed as [d(Cu) + π(POP)]
→ [π*(Py-im)] with the metal-to-ligand charge-transfer (MLCT)/ligand-to-ligand charge-trans-
fer (LLCT) character. For lower wavelengths of 275–325 nm, three intense bands at 300 nm
(S6, f = 0.0626), 292 nm (S9, f = 0.0622), and 286 nm (S10, f = 0.0799) are calculated, which are
not observed in experimental spectrum for P1. These excitations arise from HOMO to
LUMO + 3/LUMO + 4/LUMO + 5 with MLCT and intraligand charge-transfer (LLCT) transition
within POP ligands. In the 220–275 nm range, an intense absorption band centered at about
230 nm, followed by a shoulder at 256 nm, are exactly calculated, which correspond well to
the experimental values of 233 and 267 nm, respectively, However, the transition characters
of those absorption bands seem to be more complicated. At ~230 nm, the band is mainly
composed of the S47, S54, S55, S66, S67, S82, and S85 states, as seen from the inset of figure 5(a),
showing the mixed characters of MLCT/LLCT/ILCT. At ~256 nm, the S25, S28, S29, and S30 states
are responsible for the shoulder band with major MLCT and minor ILCT characters.
For P2, the calculated lowest-lying absorption band localized at 336 nm, corresponding
to the weaker absorption band at 335 nm observed in experiments, should be assigned to
the HOMO[d(Cu) + π(POP)] → LUMO[π*(Py-BenIm)] transition with MLCT/LLCT character. The
highest-lying absorption band calculated at 237 nm, which corresponds well to the exper-
imental value of 236 nm, appears to mainly originate from the S58 state, having the mixed
transitions of HOMO-4[π(Py-BenIm)] → L + 3[π*(POP)] (20.3%) and HOMO-4[π(Py-BenIm)] →
L + 4[π*(Py-BenIm)+π*(POP)] (18.6%) with LLCT/ILCT transition. Additionally, the S13 state
originated from H-4[π(Py-BenIm)] → L[π*(Py-BenIm)] (42.7%) and H-1[d(Cu)] → L + 3[π*(POP)]
(30.0%) with ILCT/MLCT character responsible for the absorption band at 277 nm, which
agrees well with the experimental value of 284 nm.
For P3 (figure 5(c) and table 4S), the calculated lowest-lying absorption band at 333 nm
from the S1 state, not observed in experiments, is dominated by the H[d(Cu) + π(POP)] → L
[π*(Py-c-BenIm)] transition (75.5%) and H-1[d(Cu)] → L [π*(Py-c-BenIm)] transition (19.2%)
with MLCT/LLCT character. The broad shoulder at 249–293 nm, corresponding to the exper-
imental absorption shoulder at 278–291 nm, originated mainly from the contributions of
the S8, S11, S18, and S38 states with MLCT/LLCT transition, accompanied by some contribution
from the S24, S29, and S32 states with ILCT transition. The highest-lying absorption band cal-
culated at 213 nm, which corresponds to the experimental value of 231 nm, is mainly com-
posed of the S137, S151, and S180 states, showing ILCT/LLCT/MLCT transition. In addition,
Rydberg state may be involved in this band (see the detailed assignments in table 4S).

12
S. LIU ET AL.
Figure 5. Comparison of the calculated (red line) and experimental (black line) absorption spectra in
Ch2Cl2 for (a) P1, (b) P2, and (c) P3. red vertical lines correspond to oscillator strength of calculated
singlet-singlet transitions.

JOURNAL OF COORDINATION CHEMISTRY
13
Figure 6. spin-density distribution contours (isovalue = 0.003) for the lowest triplet-state t1 of P1–P3.
the values of the unpaired-electron spin-density population are depicted together with the electronic
nature of the states.
3.3.3. Triplet excited states and emission properties
To gain insight into the nature of the emission properties of P1–P3, their lowest-lying triplet
excited-state geometries (T1) were optimized at the spin-unrestricted UB3LYP/6-31G*/
LANL2DZ level with a spin multiplicity of three. Where stated, toluene was chosen as the
solvent because it is less polar and thus acts similar to compare with the doped PMMA films.
The nature of the emitting excited state of each complex can be identified by the
unpaired-electron spin-density distribution, as shown in figure 6. The unpaired electron
spin-density distributions in the T1 state are very similar for the three NHC–Cu(I) complexes
(Cu, 0.61e; POP, 0.28e; Py-Im, 1.10e for P1, Cu, 0.62e; POP, 0.27e; Py-BenIm, 1.20e for P2, Cu,
0.57e; POP, 0.26e; Py-c-BenIm, 1.16e for P3) and confirm the emission behavior of 3MLCT/3LLCT
transitions and illustrate the higher contribution of the d(Cu) to the NHC ligand with some
contributions of POP ligand to the NHC ligand. Based on the energy difference between T1
and S0 at the geometry of T1, the phosphorescence emission energy was further calculated
by performing a single-point calculation. The calculated emission energies of 2.01 eV for P1,
1.98 eV for P2, and 2.18 eV for P3 are in reasonably good agreement with the experimental
values (2.22 eV for P1, 2.21 eV for P2, and 2.35 eV for P3). The theoretical values exactly
reproduce the slight red-shift in the experimental emission energy on going from P1 to P2
and a large blue-shift for P3 compared to that of P1 and P2.
4. Conclusion
In this paper, three four-coordinate NHC–Cu(I) complexes, [Cu(Py-Im)(POP)](PF6) (P1), [Cu(Py-
BenIm)(POP)](PF6) (P2), and [Cu(Py-c-BenIm)(POP)](PF6) (P3) (Py-Im = 3-methyl-1-(pyridin-2-
yl)-1H-imidazolylidene, Py-BenIm = 3-methyl-1-(pyridin-2-yl)-1Hbenzo[d]imidazolylidene,
Py-c-BenIm = 3-methyl-1-(pyridin-2-ylmethyl)-1Hbenzo[d]imidazolylidene and POP = bis([2-di-
phenylphosphino]phenyl)ether), have been synthesized and characterized. These complexes
are obtained by addition of the NHC precursors to the mixture solution of Cu powders and
POP ligands. The structure features a typical tetrahedral copper(I) center with two phosphorus
atoms from POP, a carbon and a nitrogen of NHC. Here, the type of NHC ligand is an important
factor influencing the structure of the corresponding NHC–Cu(I) complexes. Recently, a series
of heterometallic NHC–Cu(I) complexes have been synthesized through employing the anal-
ogous bipyridine-functionalized imidazolium salts as the NHC precursors and exhibit similar
emission wavelengths compared to the synthesized complexes in this study [22]. The

14
S. LIU ET AL.
photophysical properties of P1–P3 were investigated by spectroscopic methods and theoret-
ical calculations. Electronic absorption spectra show three bands at 220–250, 260–300, and
320–380 nm with mixed π–π* and charge-transfer characters. The assignments of the absorp-
tion bands are interpreted with the aid of TDDFT calculations. The four-coordinate NHC–Cu(I)
complexes show strong 3MLCT phosphorescence at room temperature in the solid state with
emission wavelengths at 560 nm for P1 and P2 and 528 nm for P3, with quantum yields of
0.30–0.40 and lifetimes of 29–45 μs. The nature of the emitting states is corroborated by the
unpaired-electron spin-density calculations. The present study is an important addition to the
understanding of the relationship between the structural aspects and the photophysical prop-
erties of four-coordinate NHC–Cu(I) complexes because there are only a few reports on such
complexes.
Acknowledgement
The authors thank the Guizhou University High Performance Computation Chemistry Laboratory
(GHPCC) for help with computational studies.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
The work was supported by the National Natural Science Foundation of China [grant number 21462020],
[grant number 21443010], [grant number 21563013]; Jiangxi Science andTechnology Normal University
Key Laboratory of Organic-inorganic Composite Materials (Key training base); and the Natural Science
Foundation of Jiangxi Province [grant number 20151BAB203006].
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