Four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes bearing functionalized 3-benzyl-1-(pyridyl)-1H-imidazolylidene ligands: Synthesis, photophysical properties and computational study

Article abstract of DOI:10.1016/j.poly.2019.114240

Seven luminescent copper(I) complexes bearing four-coordinate N-heterocyclic carbene (NHC) ligands with varying electron-withdrawing substituents including –CF₃, –CN, –COCH₃, and –CHO groups at the pyridine ring part of the carbene are reported in this study. P1-P4 without the methyl group show better light absorption in low-energy region compared with P5-P7 with the methyl group at the α-position of the pyridine ring. The emitting state of all the complexes is regarded as the ³MLCT/³LLCT character. P1-P4 exhibit the emission maximum in the range of 537–547 nm with the photoluminescence quantum yields (PLQY) of 12.7–21.9% in PMMA films and excited state lifetimes (τ) of 14.7–22.6 μs. The emission wavelengths of P5-P7 are blue-shifted to 505–513 nm compared with those of P1-P4, showing the higher PLQY of 38.0–57.3% and longer τ of 36.2–84.8 μs. Theoretical calculations were used to rationalize the photophysical properties.

Full text of DOI:10.1016/j.poly.2019.114240

Polyhedron xxx (xxxx) xxx
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes
bearing functionalized 3-benzyl-1-(pyridyl)-1H-imidazolylidene
ligands: Synthesis, photophysical properties and computational study
b
Bingbing Yang a, Jinglan Wang a, Shengxian Xu a, Hongyun Chen a, Feng Zhao a, , Yibo Wang
⇑
a School of Chemistry and Chemical Engineering, Jiangxi Science and Technology Normal University, Fenglin Street Nanchang, Jiangxi 330013, PR China
b Key Laboratory of Guizhou High Performance Computational Chemistry, Department of Chemistry, Guizhou University, Guiyang 550025, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Seven luminescent copper(I) complexes bearing four-coordinate N-heterocyclic carbene (NHC) ligands
with varying electron-withdrawing substituents including –CF3, –CN, –COCH3, and –CHO groups at the
pyridine ring part of the carbene are reported in this study. P1-P4 without the methyl group show better
Received 22 August 2019
Accepted 6 November 2019
Available online xxxx
light absorption in low-energy region compared with P5-P7 with the methyl group at the a-position of
the pyridine ring. The emitting state of all the complexes is regarded as the 3MLCT/3LLCT character. P1-P4
Keywords:
Carbene ligand
Copper(I) complex
Photoluminescence efficiency
Excited-state lifetime
Density functional theory
exhibit the emission maximum in the range of 537–547 nm with the photoluminescence quantum yields
(PLQY) of 12.7–21.9% in PMMA films and excited state lifetimes (
s) of 14.7–22.6 ls. The emission
wavelengths of P5-P7 are blue-shifted to 505–513 nm compared with those of P1-P4, showing the higher
PLQY of 38.0–57.3% and longer
photophysical properties.
s
of 36.2–84.8
ls. Theoretical calculations were used to rationalize the
Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction
emissive NHC-Cu(I) complexes because it possesses strong r dona-
tion ability and the stability of the corresponding complex [23]. So
far, most of these complexes reported have a three-coordinate
geometry around the Cu center with the formula [Cu(NHC)
(N^N)]0/+1, where the NHC ligand acts as a monodentate ligand,
while the N^N ligand is a chelating diimine. Many of these com-
plexes show the high photoluminescence quantum efficiencies
and long excited state lifetimes. However, nearly all of the reported
complexes were synthesized using a very bulky monodentate
ligand, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr).
Thus, selection scope of the demanding NHC ligand is rather lim-
ited. On the other hand, NHC-Cu(I) complexes with the four-coor-
dinate geometry should be a potent choice in this hot topic. To our
surprise, studies on the synthesis and emission properties related
to the four-coordinate NHC-Cu(I) complexes are relatively rare.
Recently, our group and Wang et al. [24–28] synthesized this type
of complexes. NHC-Cu(I) complexes can be easily prepared from
the commercially available Cu powders as the starting materials
[24]. These types of complexes all exhibit high emissive quantum
yields due to the high rigidity of the framework. Through this
route, we reported the synthesis of a series of the similar structural
complexes, and the corresponding photophysical properties can be
tailored by varying different aryl substituents bound to the NHC
ligands [25,26]. Notably, our results showed that the NHC-Cu(I)
complexes containing the benzyl-substituted imidazolylidene ring
Copper(I) complexes have important application as the emitting
component layer in lighting field because of their lower cost and
toxicities [1–5]. However, their low photoluminescence efficiency
makes them less popular compared with other noble metal com-
plexes in OLED fields because of their flexible geometry framework
and smaller spin-orbit coupling [6,7]. The structure distortion of
the complexes, which seriously increase the nonradiative decay,
must be suppressed to improve the photoluminescence efficiency.
Many advances have been achieved in designing strongly lumines-
cent Cu(I) complexes in recent years [8–12]. A particularly impor-
tant class of these complexes is the [Cu(N^N)(P^P)]+ complexes
with the mixed-ligand, where N^N is a diimine ligand (including
2,20-bipyridine, 1.10-phenanthroline, or their derivatives) and the
ancillary ligand, P^P, denotes diphosphine ligands (such as bis[2-
(diphenylphosphino)phenyl]ether (POP)) [13–17]. Although these
complexes are known to exhibit remarkable photophysical proper-
ties, it is still highly desirable to develop a novel family of lumines-
cence Cu(I) complexes by the structure modification of the ligands.
Very recently, many research groups [18–22] have been inter-
ested in employing N-heterocyclic carbene (NHC) ligand to prepare
⇑
Corresponding author.
E-mail address: zhf19752003@163.com (F. Zhao).
https://doi.org/10.1016/j.poly.2019.114240
0277-5387/Ó 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: B. Yang, J. Wang, S. Xu et al., Four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes bearing functionalized 3-benzyl-
1-(pyridyl)-1H-imidazolylidene ligands: Synthesis, photophysical properties and computational study, Polyhedron, https://doi.org/10.1016/j.
poly.2019.114240

2
B. Yang et al. / Polyhedron xxx (xxxx) xxx
1-(6-(1H-imidazol-1-yl)pyridin-3-yl)ethanone (4). Yellow
solid. Yield: 0.59 g, 40%. 1H NMR (400 MHz, DMSO) d 9.03 (s, 1H),
8.65 (s, 1H), 8.46 (d, J = 8.6 Hz, 1H), 8.05 (s, 1H), 7.96 (d,
J = 8.6 Hz, 1H), 7.17 (s, 1H), 2.64 (s, 3H).
3-fluoro-6-(1H-imidazol-1-yl)-2-methylpyridine (5). White
solid. Yield: 0.72 g, 51%. 1H NMR (400 MHz, DMSO) d 8.62 (d,
J = 69.9 Hz, 1H), 8.14–7.86 (m, 2H), 7.85–7.72 (m, 1H), 7.38–7.09
(m, 1H), 2.72–2.44 (m, 3H).
3-bromo-6-(1H-imidazol-1-yl)-2-methylpyridine (6). This
compound was prepared via a similar procedure for 1 from
2,5-Dibromo-6-methylpyridine (1.99 g, 8 mmol). Yield: 1.02 g,
54%. 1H NMR (400 MHz, DMSO) d 8.52 (s, 1H), 8.19 (d, J = 8.4 Hz,
1H), 7.93 (s, 1H), 7.61 (d, J = 8.5 Hz, 1H), 7.12 (s, 1H), 2.59 (s, 3H).
6-(1H-imidazol-1-yl)-2-methylnicotinonitrile
(7).
Yield:
0.79 g, 52%. 1H NMR (400 MHz, DMSO) d 8.63 (s, 1H), 8.45 (d,
J = 8.2 Hz, 1H), 8.02 (s, 1H), 7.86 (d, J = 8.4 Hz, 1H), 7.17 (s, 1H),
2.70 (s, 3H).
Fig. 1. Chemical structure of the NHC-Cu(I) complexes P1-P7.
have higher photoluminescence efficiencies than the analogous
ones containing naphthyl and anthryl substituents.
With the aim of expanding our work and motivated by the rich
photophysical properties of the type of complexes, we present the
preparation of a series of the NHC-Cu(I) complexes containing dif-
(Ph-Im-triflumePy)(PF6) (L1). Yield: 0.34 g, 30%. m.p. 122 °C.
1H NMR (400 MHz, DMSO) d 10.38 (s, 1H), 9.11 (s, 1H), 8.84–8.52
(m, 2H), 8.27 (d, J = 8.6 Hz, 1H), 8.08 (s, 1H), 7.76–7.27 (m, 5H),
5.66 (d, J = 73.6 Hz, 2H).
(Ph-Im-cyPy)(PF6) (L2). Yield: 0.86 g, 85%. m.p. 210 °C. 1H NMR
(400 MHz, DMSO) d 10.38 (s, 1H), 9.18 (s, 1H), 8.77 (d, J = 8.5 Hz,
1H), 8.61 (s, 1H), 8.24 (d, J = 8.6 Hz, 1H), 8.08 (s, 1H), 7.47 (dt,
J = 23.3, 8.0 Hz, 5H), 5.55 (s, 2H).
ferent electron-withdrawing groups attached at the
a-position of
pyridine ring, remaining the benzyl-substituted imidazolylidene
ring unchanged (Fig. 1). The excellent photophysical properties of
these complexes make them suitable for future application as
OLED emitters. The photophysical properties were comprehen-
sively investigated using experimental and theoretical methods.
(Ph-Im-alPy)(PF6) (L3). Yield: 0.74 g, 72%. m.p. 203 °C. 1H NMR
(400 MHz, DMSO) d 10.40 (s, 1H), 10.18 (s, 1H), 9.16 (s, 1H), 8.65 (d,
J = 8.6 Hz, 2H), 8.22 (d, J = 8.5 Hz, 1H), 8.08 (s, 1H), 7.47 (dt, J = 16.4,
8.0 Hz, 5H), 5.55 (s, 2H).
2. Experimental
(Ph-Im-AcPy)(PF6) (L4). Yield: 0.67 g, 63%. m.p. 144 °C.
1H NMR (400 MHz, DMSO) d 10.37 (s, 1H), 9.15 (s, 1H), 8.78–8.53
(m, 2H), 8.24–7.95 (m, 2H), 7.66–7.28 (m, 5H), 5.55 (s, 2H), 2.70
(s, 3H).
2.1. Materials and methods
Chemical reagents were obtained from Sinopharm Chemical
Reagent Co Ltd. and used without further purification. A Bruker
AV400 MHz spectrometer was utilized to obtain the 1H NMR spec-
tra. An Agilent 6450 Q-TOF mass spectrometer was used to obtain
high-resolution mass spectra (HRMS). An Elementar VarioEL cube
analyzer was employed to obtain elemental analysis data (C, H,
and N) of the complexes. Crystallographic data of P7 were analyzed
on a Bruker SMART APEX II CCD diffractometer using a multiscant-
echnique. The structure has been deposited as supplemental mate-
rial at the Cambridge Crystallographic Data Center CCDC 1906684.
A Perkin Elmer Lambda-900 spectrophotometer and a Hitachi
F-4600 spectrophotometer were used to record UV–Vis absorption
and emission spectra, respectively. A Hamamatsu-C11347 system
equipped with an integrating sphere was employed to obtain abso-
lute photoluminescence quantum yields. The excited state life-
times were measured using a Hamamatsu C11367 spectrometer.
(Ph-Im-flumePy)(PF6) (L5). Yield: 0.69 g, 67%. m.p. 229 °C. 1H
NMR (400 MHz, DMSO) d10.17 (s, 1H), 8.48 (s, 1H), 8.09 (t,
J = 8.7 Hz, 1H), 8.02 (s, 1H), 7.93 (d, J = 8.6 Hz, 1H), 7.52 (s, 1H),
7.51 (s, 1H), 7.45 (d, J = 6.3 Hz, 1H), 7.41 (d, J = 8.7 Hz, 1H), 5.53
(s, 2H), 4.39 (t, J = 4.8 Hz, 2H), 2.53 (d, J = 8.8 Hz, 3H).
(Ph-Im-bromePy)(PF6) (L6). Yield: 0.64 g, 54%. m.p. 179 °C. 1H
NMR (400 MHz, DMSO) d 10.22 (s, 1H), 8.52 (s, 1H), 8.45 (d,
J = 8.6 Hz, 1H), 8.03 (s, 1H), 7.82 (d, J = 8.6 Hz, 1H), 7.51 (d,
J = 6.3 Hz, 2H), 7.43 (d, J = 7.5 Hz, 3H), 5.53 (s, 2H), 2.67 (s, 3H).
(Ph-Im-cymePy)(PF6) (L7). Yield: 0.39 g, 37%. m.p. 114 °C. 1H
NMR (400 MHz, DMSO) d 10.32 (s, 1H), 8.69 (d, J = 8.5 Hz, 1H),
8.59 (s, 1H), 8.06 (d, J = 8.2 Hz, 2H), 7.52 (d, J = 7.2 Hz, 2H), 7.48–
7.36 (m, 3H), 5.55 (s, 2H), 2.78 (s, 3H).
P1. Yellow powder. Yield: 0.2 g, 48%. m.p. 306 °C. 1H NMR
(400 MHz, DMSO) d 8.51 (d, J = 10.9 Hz, 2H), 8.31 (d, J = 8.7 Hz, 1H),
7.91 (s, 1H), 7.52 (s, 1H), 7.47–7.28 (m, 11H), 7.26–7.13 (m, 10H),
7.02 (dt, J = 26.6, 7.4 Hz, 4H), 6.90–6.70 (m, 6H), 6.53 (s, 2H), 5.27
(s, 2H). Anal. Calcd. For C52H40CuF9N3OP3 (1050.35): C 59.46, H
3.84, N 4.00; found: C 59.62, H 3.58, N 4.33. HRMS (m/z, ESI+): calcd.
For C52H40CuF3N3OP2 ([M]+) 904.1895; found 904.1847.
P2. Yellow powder. Yield: 0.21 g, 51%. m.p. 307 °C. 1H NMR
(400 MHz, DMSO) d 10.38 (s, 1H), 9.18 (s, 1H), 8.77 (d, J = 8.5 Hz,
1H), 8.65–8.45 (m, 2H), 8.41–8.14 (m, 2H), 8.06 (d, J = 15.0 Hz,
2H), 7.53 (d, J = 7.5 Hz, 3H), 7.49–6.96 (m, 21H), 6.89 (s, 3H), 6.77
2.2. Synthetic procedure
Compounds 1–7, ligands L1-L7 and the corresponding com-
plexes P1-P7 were synthesized according to the similar method
in our previously paper [24]. The structural data were listed in
the following:
2-(1H-imidazol-1-yl)-5-(trifluoromethyl) pyridine (1). White
solid. Yield: 0.78 g, 46%. 1H NMR (400 MHz, DMSO) d 8.87 (s, 1H),
8.65 (s, 1H), 8.42 (d, J = 8.6 Hz, 1H), 8.05 (d, J = 9.9 Hz, 2H), 7.17
(s, 1H).
6-(1H-imidazol-1-yl)nicotinonitrile (2). Pale white solid.
Yield: 0.89 g, 66%. 1H NMR (400 MHz, DMSO) d 8.98 (s, 1H), 8.65
(s, 1H), 8.54 (d, J = 8.6 Hz, 1H), 8.05 (d, J = 7.9 Hz, 2H), 7.18 (s, 1H).
6-(1H-imidazol-1-yl)nicotinaldehyde (3). White solid. Yield:
0.68 g, 49%. 1H NMR (400 MHz, DMSO) d 10.08 (s, 1H), 9.00 (s,
1H), 8.67 (s, 1H), 8.42 (d, J = 8.5 Hz, 1H), 8.18–7.90 (m, 2H), 7.18
(s, 1H).
(d, J = 7.4 Hz, 1H), 6.59 (s, 1H), 5.55 (s, 2H). Anal. Calcd. For C52H40
-
CuF6N4OP3 (1007.36): C 62.00, H 4.00, N 5.56; found: C 61.62, H
3.88, N 5.72. HRMS (m/z, ESI+): calcd. For C52H40CuN4OP2 ([M]+)
861.1973; found 861.1969.
P3. Yellow powder. Yield: 0.18 g, 45%. m.p. 311 °C. 1H NMR
(400 MHz, DMSO) d 9.71 (s, 1H), 8.48 (d, J = 12.7 Hz, 2H), 8.26 (d,
J = 8.5 Hz, 2H), 7.55 (s, 1H), 7.32 (ddt, J = 26.9, 14.0, 7.6 Hz, 17H),
7.18–6.95 (m, 10H), 6.86–6.76 (m, 5H), 6.59 (s, 2H), 5.14 (s, 2H).
Anal. Calcd. For C53H43CuF6N3O2P3 (1024.39): C 62.14, H 4.23, N
Please cite this article as: B. Yang, J. Wang, S. Xu et al., Four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes bearing functionalized 3-benzyl-
1-(pyridyl)-1H-imidazolylidene ligands: Synthesis, photophysical properties and computational study, Polyhedron, https://doi.org/10.1016/j.
poly.2019.114240

B. Yang et al. / Polyhedron xxx (xxxx) xxx
3
4.10; found: C 62.03, H 4.12, N 4.02. HRMS (m/z, ESI+): calcd. For
geometries. Multiwfn [38] and VMD software [39] were used for
visualizing the frontier molecular orbitals.
C
53H43CuN3O2P2 ([M]+) 878.2127; found 878.2133.
P4. Yellow powder. Yield: 0.176 g, 43%. m.p. 297 °C. 1H NMR
(400 MHz, DMSO) d 8.56–8.42 (m, 2H), 8.21 (d, J = 8.5 Hz, 2H),
7.52 (s, 1H), 7.47–7.11 (m, 21H), 7.03 (dt, J = 26.4, 7.4 Hz, 4H),
6.91–6.64 (m, 6H), 6.55 (s, 2H), 5.26 (s, 2H), 2.22 (s, 3H). Anal.
Calcd. For C52H41CuF6N3O2P3 (1010.36): C 61.82, H 4.09, N 4.16;
found: C 62.04, H 4.15, N 4.02. HRMS (m/z, ESI+): calcd. For
3. Results and discussion
3.1. Synthesis and characterization
C
52H41CuN3O2P2 ([M]+) 864.1970; found 864.1975.
The pyridyl-imidazole derivatives 1–7 can be prepared through
the CAN coupling reaction [40]. The pyridyl-imidazolium salts
L1-L7 were synthesized as previously described (Scheme 1) [41].
P1-P7 were synthesized by the reported synthetic method [24].
The corresponding analysis data, including 1H NMR, HRMS and
elemental analyses, support the structures of these complexes
(see Experimental Section).
complex P7 (CCDC 1906684) was crystallized from CH2Cl2/CH3-
CH2OH solution of complexes with slow evaporation speed. The
crystal structure is shown in Fig. 2. The X-ray analysis reveals that
P5. White powder. Yield: 0.23 g, 57%. m.p. 291 °C. 1H NMR
(400 MHz, DMSO) d 8.36 (s, 1H), 8.10 (t, J = 8.1 Hz, 1H), 8.00 (d,
J = 7.1 Hz, 1H), 7.47–7.36 (m, 8H), 7.28 (t, J = 6.5 Hz, 8H),
7.22–7.04 (m, 11H), 6.91 (s, 4H), 6.78 (s, 2H), 6.70 (d, J = 7.2 Hz,
2H), 5.24 (d, J = 28.0 Hz, 2H), 1.72 (s, 3H). Anal. Calcd. For
C
53H42CuF6N4OP3 (1021.38): C 62.32, H 4.14, N 5.49; found: C
62.14, H 4.08, N 5.36. HRMS (m/z, ESI+): calcd. For C53H42CuN4OP2
([M]+) 875.2130; found 875.2145.
P6. White powder. Yield: 0.21 g, 52%. m.p. 271 °C. 1H NMR
(400 MHz, DMSO) d 8.36 (d, J = 8.7 Hz, 2H), 7.87 (d, J = 8.6 Hz,
1H), 7.38 (dd, J = 22.7, 15.1 Hz, 8H), 7.26 (t, J = 7.3 Hz, 7H), 7.20–
6.99 (m, 11H), 6.87 (s, 4H), 6.75 (s, 2H), 6.68 (d, J = 7.9 Hz, 2H),
5.21 (s, 2H), 1.83 (s, 3H). Anal. Calcd. For C52H42BrCuF6N3OP3
(1075.27): C 58.08, H 3.94, N 3.91; found: C 58.14, H 4.02, N
3.36. HRMS (m/z, ESI+): calcd. For C52H42BrCuN3OP2 ([M]+)
928.1282; found 928.1277.
P7. Yellow powder. Yield: 0.27 g, 67%. m.p. 263 °C. 1H NMR
(400 MHz, DMSO) d 8.60 (d, J = 8.5 Hz, 1H), 8.42 (s, 1H), 8.09 (d,
J = 8.6 Hz, 1H), 7.48 (s, 1H), 7.38 (dd, J = 14.8, 7.3 Hz, 6H), 7.26 (s,
8H), 7.22–7.00 (m, 11H), 6.89 (s, 4H), 6.78–6.65 (m, 4H), 5.25 (s,
2H), 1.89 (s, 3H). Anal. Calcd. For C52H42CuF7N3OP3 (1014.37): C
61.57, H 4.17, N 4.14; found: C 61.35, H 4.07, N 4.08. HRMS (m/z,
ESI+): calcd. For C52H42CuFN3OP2 ([M]+) 868.2083; found 868.2088.
2.3. DFT calculations
The singlet geometries (S0 state) of all complexes were
optimized using the B3LYP functional [29,30] associated with
the PCM [31] model in CH2Cl2 media under Gaussian 09 [32]. The
6-31G* basis set [33,34] was used for the C, H, N, O, and P atoms,
whereas the LANL2DZ basis set [35] was employed for the Cu atom.
The simulated UV–Vis spectra were calculated using the TDDFT
method [36,37] in CH2Cl2 media on the basis of the optimized S0
Fig. 2. X-ray crystal structure of P7 (50% probability thermal ellipsoids).
Scheme 1. Synthetic route for complexes P1-P7.
Please cite this article as: B. Yang, J. Wang, S. Xu et al., Four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes bearing functionalized 3-benzyl-
1-(pyridyl)-1H-imidazolylidene ligands: Synthesis, photophysical properties and computational study, Polyhedron, https://doi.org/10.1016/j.
poly.2019.114240

4
B. Yang et al. / Polyhedron xxx (xxxx) xxx
two phosphorus atoms (P1, P2) of the POP ligand and one nitrogen
atom (N1) of the pyridine moiety bind to the metal Cu ion center,
while the fourth coordination site is occupied by a carbene carbon
atom (C1) of imidazolylidene ring, thus leading to a four-coordi-
nated tetrahedral geometry. The Cuꢀ ꢀ ꢀO separation of 3.001 Å is
shorter than 3.20 Å, indicating minimum interaction between O
atom and Cu(I) center. The dihedral angle between the N1-Cu1-
C1 and P1-Cu1-P2 planes is a measurement of the degree of the
flattening distortion of the Cu(I) complexes, which is 90° for the
idealized tetrahedral geometry. Here, this angle in complex 7 is
80.5°, indicating the presence of a significant flattening distortion.
This flattening distortion is caused by intramolecular steric inter-
for all these complexes correspond to carbene- and POP-based
p
? p* intraligand transitions. For the series P1-P4, the weaker
shoulder in 350–450 nm (
e
< 1.64 ꢁ 104 Mꢂ1 cmꢂ1) can be confi-
dently regarded as metal Cu to ligand charge transfer (1MLCT).
The onset of this 1MLCT absorption band slightly shifts to lower
energy from P1 to P4, which is consistent with the energy gap
energy in the order P4 < P3 < P2 < P1. Support for this notion is
seen in the theoretical results discussed below. Unlike P1-P4, P5-
P7 exhibit very weak 1MLCT bands between 300 and 400 nm
(e
< 0.78 ꢁ 104 Mꢂ1 cmꢂ1). This significant decrease in absorption
ability is confidently attributed to the presence of the methyl
group at the a-position of the pyridine ring for P5-P7. This condi-
action between the methyl groups at
a
-position of the pyridine
tion causes the steric hindrance with the POP ligand, thereby leav-
ing the pyridine ring away from the Cu atom. Thus, the probability
of S0 ? S1 transition (1MLCT band) from Cu to the pyridine ring
decreases and consequently exhibits the weak absorption feature.
The emission spectra of P1-P7 in the solid state are provided in
Fig. 4. The broad and unstructured emissions can be observed for
all of the complexes in the green spectral region from 505 to
547 nm, which can be assigned as the triplet 3MLCT character. As
shown in Fig. 4, the blue-shift of the emission maximum for P5-
P7 can be observed compared with that of P1-P4. In addition,
P5-P7 also show much higher photoluminescence quantum yields
(PLQY) (38%-57.3%) than P1-P4 (12.7%-21.9%, Table 1). The
moiety and phenyl ring of POP ligand, which severely influences
the photophysical properties of the complexes (see below). The
lengths of CuAP bond range from 2.259 to 2.295 Å and these data
are similar to those of the Cu(I) complexes reported. The length of
the CuACcarbene bond is 1.976 Å, which is much shorter than that
of the CuAN1 distance (2.333 Å) due to the stronger bond between
the carbene carbon and Cu atoms.
3.2. Photophysical properties
The UV–Vis spectra of P1-P7 in CH2Cl2 are provided in Fig. 3 and
presence of the methyl substituent at the
plays a crucial role for the enhancement of the PLQY as it restricts
a-position of pyridine
the important photophysical parameters are collected in Table 1.
Intense bands (
e
> 2.19 ꢁ 104 Mꢂ1 cmꢂ1) in the 250–300 nm region
Fig. 3. Absorption spectra in dichloromethane at 298 K of complexes P1-P7.
Table 1
Photophysical properties of complexes P1-P7.
Absorption
Emission
kabs/nm (
e
ꢁ 104 Mꢂ1 cmꢂ1
)
kem/nm
s
/
ls
PLQY/%
kr/104 sꢂ1
knr/104 sꢂ1
P1
P2
P3
P4
P5
P6
P7
272 (7.93), 355 (1.64)
276 (7.10), 376 (0.63)
282 (10.1), 369 (1.45)
282 (8.74), 376 (1.01)
274 (2.20), 367 (0.38)
282 (2.19), 334 (0.53)
272 (3.04), 331 (0.78)
537
547
547
547
531
515
505
22.6
21.5
18.5
14.7
36.2
84.8
40.7
17.3
16.9
21.9
12.7
52.3
57.3
38.0
0.77
0.79
1.18
0.86
1.44
0.68
0.93
3.66
3.87
4.22
5.94
1.32
0.50
1.52
Please cite this article as: B. Yang, J. Wang, S. Xu et al., Four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes bearing functionalized 3-benzyl-
1-(pyridyl)-1H-imidazolylidene ligands: Synthesis, photophysical properties and computational study, Polyhedron, https://doi.org/10.1016/j.
poly.2019.114240

B. Yang et al. / Polyhedron xxx (xxxx) xxx
5
structural relaxation in the excited state, which is corroborated by
the results obtained from DFT calculations (see below). The com-
plexes showing the higher PLQY also have longer excited state life-
times (
= 14.7–22.6
PLQY and , the radiative rate (kr)and the nonradiative rate con-
s
= 36.2–84.8
ls for P5-P7) compared with other complexes
(s
ls for P1-P4). On the basis of the known values of
s
stants (knr) can be obtained (Table 1). Although the kr values of
P1-P4 (kr = (0.77–1.18) ꢁ 104 sꢂ1) are comparable to those of
P5-P7 (kr = (0.68–1.44) ꢁ 104 sꢂ1), the knr values of P1-P4
(knr = (3.66–5.94) ꢁ 104 sꢂ1
) are greater than those of P5-P7
(knr = (0.50–1.52) ꢁ 104 sꢂ1). Thus, it is concluded that the dramat-
ically improved photophysical properties of the series P5-P7 is
mainly due to the additional methyl groups at the
a-position of
pyridine ring by the effective suppression of the non-radiative
decay process. Additionally, the electron-donating nature of the
methyl substituent also has important influences for the improve-
ment of the photophysical properties of the complexes given that
the incorporation of the methyl substituent at the
pyridine ring upshifts the * orbital energies of the complexes
and therefore results in the blue-shift of the emission wavelength.
a-position of
p
Fig. 4. Normalized emission spectra of P1-P7 in PMMA films (10 wt%).
Fig. 5. Energies and electron density distributions for the HOMO and LUMO of P1-P7.
Fig. 6. Molecular structures of P2 and P5 in the S0 states.
Please cite this article as: B. Yang, J. Wang, S. Xu et al., Four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes bearing functionalized 3-benzyl-
1-(pyridyl)-1H-imidazolylidene ligands: Synthesis, photophysical properties and computational study, Polyhedron, https://doi.org/10.1016/j.
poly.2019.114240

6
B. Yang et al. / Polyhedron xxx (xxxx) xxx
3.3. Theoretical calculations
relationship between the properties and structure of the investi-
gated complexes, the highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) of complexes
were theoretically calculated at B3LYP/6-31G(d)/LANL2DZ levels
in CH2Cl2 solution (Fig. 5). As shown in the figure, the HOMO is
centered on the metal Cu atom and POP ligand for P1-P7. The
HOMO energies of P1-P7 are similar (ꢂ5.99 eV to ꢂ6.06 eV). On
The molecular geometries of all complexes were optimized, and
the structural parameters of the representative complex P7 are in
good agreement with X-ray crystal data (Fig. 1S and Table 1S,
Supporting Information); this ensures a proper description of the
electronic structures for all complexes. To extensively explore the
Fig. 7. Superimposition of the S0 (red) and T1 (blue) structures of studied complexes. (For interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
Please cite this article as: B. Yang, J. Wang, S. Xu et al., Four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes bearing functionalized 3-benzyl-
1-(pyridyl)-1H-imidazolylidene ligands: Synthesis, photophysical properties and computational study, Polyhedron, https://doi.org/10.1016/j.
poly.2019.114240

B. Yang et al. / Polyhedron xxx (xxxx) xxx
7
the other hand, the LUMO mainly distributes on the pyridine ring,
and the LUMO energies exhibit larg variation (ꢂ1.66 eV to
ꢂ2.56 eV) for P1-P7. The addition of the substituents to the pyri-
dine ring significantly affects LUMO than HOMO. The HOMO-
LUMO gap gradually decreases following the order P7 (4.33 eV)
> P6 (4.24 eV) > P1 (3.96 eV) > P5 (3.73 eV) > P2 (3.63 eV) > P3
(3.59 eV) > P4 (3.46 eV). These data are concordant with the onset
of the lowest-lying absorption bands in the experiment.
ure of the UV–Vis spectra. The corresponding transition charac-
ters are shown in Table 2. The simulated spectra of P1-P7 are
provided in Fig. 2S in the Supporting Information, together with
the experimental spectrum for the purpose of comparison. For
all complexes, the red-shift of the calculated lowest-lying sin-
glet-singlet excited state S1 can be observed in the following
order: P7 (335 nm) ? P6 (342 nm) ? P1 (372 nm) ? P5
(392 nm) ? P2 (406 nm) ? P3 (409 nm) ? P4 (428 nm), in line
with the trend of HOMO-LUMO gaps mentioned above. The S1
state mainly originates from the HOMO ? LUMO transition and
The low-energy transitions are calculated for P1-P7 employing
TDDFT method in CH2Cl2 solution to further investigate the nat-
Table 2
Electronic absorptions of complexes P1-P7 in CH2Cl2 based on TDDFT calculations at the B3LYP/6-31g*/LANL2DZ level, together with the experimental values.
Excited state
Transition
Coeff
E(eV)/(nm)
Oscillator
Assign
Exptl/nm
P1
P2
S1
S7
S10
S12
S13
H ? L
0.62900(79.1%)
0.69104(95.5%)
0.69372(96.2%)
0.66212(87.6%)
0.43024(37.0%)
ꢂ0.39071(13.5%)
0.68769(94.6%)
0.44404(39.4%)
0.35822(25.6%)
0.25078(12.6%)
0.68596(94.1%)
0.28221(15.9%)
0.40275(32.4%)
3.32/372
4.18/296
4.29/288
4.37/284
4.39/282
0.0642
0.0782
0.0673
0.0467
0.0543
MLCT(Cu?PyIm)/LLCT(POP?PyIm)
355
272
272
272
272
H ? L + 4
H ? L + 5
H ? L + 5
H-5 ? L
H-4 ? L
(
(
(
p?*
p?*
p?*
p
p
p
)(POP)/MLCT(Cu?POP)
)(POP)/MLCT(Cu?POP)
)(POP)/MLCT(Cu?POP)
LLCT(POP?PyIm)/MLCT(Cu?PyIm
LLCT(POP?PyIm)/MLCT(Cu?PyIm
S1
S9
H ? L
H-5 ? L
H-4 ? L
H-2 ? L + 1
H ? L + 3
H-20 ? L
H-9 ? L + 1
3.05/406
4.15/298
0.0581
0.0641
LLCT(POP?PyIm)/MLCT(Cu?PyIm
376
276
LLCT(POP?PyIm)/(
LLCT(Benzyl?PyIm)/(
MLCT(Cu?PyIm)/LLCT(POP?PyIm)
MLCT(Cu?POP)/( ?*
?* (PyIm)/MLCT(Cu?PyIm)
p?*
p)
(PyIm)
p
?*p)
(PyIm)
S10
S43
4.20/295
4.87/254
0.0831
0.0994
p
p
)
276
248
(POP)
(
p
p)
LLCT(POP?PyIm)
P3
S1
S9
H ? L
H-5 ? L
H-4 ? L
0.69531(96.7%)
ꢂ0.35591(25.3%)
0.54374(59.1%)
0.68055(92.6%)
0.67949(92.3%)
0.67214(90.3%)
ꢂ0.27774(15.4%)
ꢂ0.23922(11.4%)
0.31396(19.7%)
0.22540(10.1%)
ꢂ0.26830(14.4%)
3.03/409
4.12/300
0.0475
0.1012
LLCT(POP?PyIm)/MLCT(Cu?COCH3)
LLCT(Benzyl?PyIm)/LLCT(POP?COCH3)
369
282
LLCT(Benzyl?PyIm)/(p?*p)
(PyIm)
S10
S14
S18
S47
H ? L + 3
H ? L + 4
H ? L + 5
H-21 ? L
H-5 ? L + 1
H-4 ? L + 1
H ? L + 1
H ? L + 2
4.17/297
4.29/289
4.36/284
4.89/253
0.0672
0.0752
0.0673
0.1494
(
(
(
p?*
p?*
p?*
p
p
p
)(POP)/MLCT(Cu?POP)
)(POP)/MLCT(Cu?POP)
)(POP)/MLCT(Cu?POP)
282
282
282
251
MLCT(Cu?COCH3)/LLCT(PyIm?COCH3)
LLCT(Benzyl?PyIm)/LLCT(POP?PyIm)/(
?* (PyIm)/LLCT(Benzyl?PyIm)
MLCT(Cu?PyIm)/LLCT(POP?PyIm)
MLCT(Cu?POP)/( ?*
p
?*p)
(PyIm)
(
p
p)
p
p)
(POP)
P4
S1
S8
H ? L
H-5 ? L
H-4 ? L
0.69524(96.6%)
0.46033(42.3%)
0.44165(39.0%)
0.60043(72.1%)
0.69380(96.2%)
0.57057(65.1%)
0.29990(17.9%)
0.24839(12.3%)
0.50466(50.9%)
0.24171(11.6%)
0.30875(19.0%)
0.27314(14.9%)
2.89/428
4.01/308
0.0350
0.0717
MLCT(Cu?CHO)/LLCT(POP?PyIm)
LLCT(POP?PyIm)/LLCT(POP?PyIm)/(
LLCT(PyIm?CHO)
376
282
p?*p)
(PyIm)
S14
S19
S34
H ? L + 3
H ? L + 4
H-2 ? L + 2
H ? L + 7
H-5 ? L + 1
H-2 ? L + 3
H-21 ? L
H-2 ? L + 3
H-1 ? L + 6
4.19/295
4.29/288
4.63/267
0.0701
0.0631
0.1153
(
(
(
(
p?*
p?*
p?*
p?*
p
p
p
p
)(POP)/MLCT(Cu?POP)
)(POP)/MLCT(Cu?POP)
)(POP)/MLCT(Cu?POP)
)(POP)/MLCT(Cu?POP)
282
282
253
S42
S43
4.79/258
4.80/258
0.1047
0.1145
LLCT(POP?PyIm)/(
?* (POP)/MLCT(Cu?POP)
LLCT(POP?PyIm)/LLCT(POP?COH)
?* (POP)/MLCT(Cu?POP)
p?*
p)(PyIm)/LLCT(Benzyl?PyIm)
253
253
(
p
p)
(
p
p)
MLCT(Cu?POP)
P5
S1
S8
S9
H ? L
H ? L + 3
H-5 ? L
0.69966(97.9%)
0.68495(93.8%)
ꢂ0.32598(21.2%)
0.53207(56.6%)
0.68606(94.1%)
0.63880(81.6%)
0.60753(73.8%)
0.25011(12.5%)
0.29233(17.1%)
0.26651(14.2%)
3.16/392
4.15/298
4.21/294
0.0673
0.0628
0.0980
LLCT(POP?PyIm)/MLCT(Cu?PyIm)
367
274
274
(
p
?*
LLCT(Benzyl?PyIm)/LLCT(POP?PyIm)
LLCT(Benzyl?PyIm)/( ?*
?* (POP)/MLCT(Cu?POP)
MLCT(Cu?POP)/( ?* (POP)/LLCT(PyIm?POP)
?* (POP)/MLCT(Cu?POP)
LLCT(POP?PyIm)/MLCT(Cu?PyIm)
LLCT(Benzyl?PyIm)/( ?*
?* (POP)/MLCT(Cu?POP)
p)(POP)/MLCT(Cu?POP)
H-4 ? L
p
p)
(PyIm)
S10
S11
S14
S43
H ? L + 4
H-1 ? L + 2
H ? L + 5
H-18 ? L
H-4 ? L + 1
H-3 ? L + 3
4.31/287
4.36/284
4.41/280
4.93/251
0.0805
0.0694
0.0769
0.1500
(
p
p
)
274
274
274
248
p
p)
(
p
p)
p
p)
(PyIm)
(
p
p)
P6
P7
S1
S8
S10
S12
H ? L
0.68756(94.5%)
0.68498(93.8%)
0.67396(90.8%)
0.46746(43.7%)
0.36825(27.1%)
3.62/342
4.29/288
4.39/282
4.51/275
0.0900
0.0749
0.0620
0.0724
LLCT(POP?PyIm)/MLCT(Cu?PyIm)
334
282
282
282
H ? L + 4
H ? L + 5
H-4 ? L
H-3 ? L
(
(
(
p?*
p?*
p?*
p
p
p
)
)
)
(POP)/MLCT(Cu?POP)
(POP)/MLCT(Cu?POP)
(PyIm)
LLCT(POP?PyIm)
S1
S4
H ? L
0.65737(86.4%)
0.53040(56.2%)
0.43110(37.1%)
0.65813(86.6%)
0.67319(90.6%)
3.69/335
4.11/302
0.0786
0.0603
LLCT(POP?PyIm)/MLCT(Cu?PyIm)
LLCT(POP?PyIm)/MLCT(Cu?PyIm)
331
272
H ? L + 2
H ? L + 3
H ? L + 4
H ? L + 5
(
(
(
p?*
p?*
p?*
p
p
p
)(POP)/MLCT(Cu?POP)
)(POP)/MLCT(Cu?POP)
)(POP)/MLCT(Cu?POP)
S8
S10
4.28/289
4.37/283
0.0465
0.0688
272
272
Please cite this article as: B. Yang, J. Wang, S. Xu et al., Four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes bearing functionalized 3-benzyl-
1-(pyridyl)-1H-imidazolylidene ligands: Synthesis, photophysical properties and computational study, Polyhedron, https://doi.org/10.1016/j.
poly.2019.114240

8
B. Yang et al. / Polyhedron xxx (xxxx) xxx
can be regarded as 1MLCT/1LLCT character for all complexes given
that the HOMOs of P1–P7 are located on the Cu (28.7%ꢂ32.7%)
and POP (59.1%ꢂ62.7%), whereas their LUMOs dominantly reside
on the PyIm ligand (63.2%ꢂ91.7%) (Fig. 3S, Table 2S-8S, Support-
ing Information). The complete assignments for the absorption
transitions are collected in Table 2.
imposed with the pyridine ring in the S0 state, meaning that the
degree of the flattening distortion is very small. In order to qualita-
tively evaluate the structural deviation from the S0 to the T1 states,
the root-mean-square deviation (RMSD) between the S0 and the T1
states was calculated [42] and the calculated results are presented
in Fig. 7. The RMSD values are in the range of 0.9–0.717 for P1-P4,
which are significantly larger than those of P5-P7 (0.653–0.686).
These theoretical data reveal that the presence of the methyl sub-
It is interesting to compare the ground geometry (S0) of P2 with
that of P5 since their structures are similar, except for groups at the
a-position of the pyridine ring. Fig. 6 presents the optimized S0
stituent at the a-position of the pyridine ring in P5-P7 effectively
structures of P2 and P5 in CH2Cl2 solution. The larger difference
observed is that the CuAN1 bond length of P5 (2.547 Å) is signifi-
cantly longer compared with that of P2 (2.353 Å), which is due to
suppresses the structural distortion of the excited states. Thus,
the structural relaxation that occurred from the T1 to the S0 states
is weakened, leading to the higher PLQY, which are consistent with
the experimental values.
steric repulsion of the POP ligand with the methyl group at the a-
position of the pyridine ring. Thus, the interaction between the Cu
(I) center and the pyridine moiety is weak and consequently
decreases the probability of charge transfer from metal to the pyr-
idine ring. This decrease is reflected by P5 exhibiting a very low
absorption ability observed experimentally in the MLCT region in
contrast with P2.
It is well known that the structural flattening of the T1 states of
the Cu(I) complexes significantly influence on the luminescence
efficiency. Here, the spin-unrestricted U-DFT approach was
employed to optimize the T1 state geometries of all the investi-
gated complexes. A comparison between T1 and S0 geometries is
provided in Fig. 7. For P1-P4, a marked flattening distortion in
the T1 state was observed compared with the molecular structure
in the S0 state. The pyridine ring in the T1 state was titled out of
the plane of the pyridine part in the S0 state. However, for P5-P7,
one can see that the pyridine ring in the T1 state was almost super-
On the basis of the optimized T1 geometry, the TDDFT method
was performed to calculate the triplet emission wavelengths of
P1-P7. Here, the B3LYP functional underestimates the emission
energy of the investigated Cu(I) complexes (Table 9S, Supporting
Information). Thus, the emission energies were calculated at TD-
CAM-B3LYP/6-31G(d)/LANL2DZ level. The results are presented
in Table 3. The calculated emission wavelengths are red-shifted
in the following order P7 (501 nm) ? P6 (508 nm) ? P5
(531 nm) ? P1 (573 nm) ? P2 (578 nm), reproducing the experi-
mental trend, except for P3 and P4. The emitting states of all com-
plexes originate mainly from the LUMO ? HOMO transitions. On
the basis of the HOMO and LUMO compositions in the T1 state
(Tables 10S, Supporting Information), the nature of the excited
states can be assigned as the 3MLCT/3LLCT characters for all com-
plexes. The representative spin density plots of P1, P2 and P7 are
shown in Fig. 8, further support the assignment of the emission
state with the 3MLCT/3LLCT character. The spin density distribu-
tions of other complexes are provided in Fig. 4S in the Supporting
Information.
Table 3
Calculated emission wavelengths and
T1 ? S0 transition of P1-P7 using TDDFT
method with CAM-B3LYP functional. The experimental values are also provided.
4. Conclusion
kCemalc/nm
Configuration
Assignment
kEemxpl/nm
P1
P2
P3
P4
P5
P6
P7
573
578
564
574
531
508
501
L ? H (77.6%)
L ? H (75.2%)
L ? H (71.2%)
L ? H (70.6%)
L ? H (69.6%)
L ? H (71.3%)
L ? H (71.0%)
3MLCT/3LLCT
3MLCT/3LLCT
3MLCT/3LLCT
3MLCT/3LLCT
3MLCT/3LLCT
3MLCT/3LLCT
3MLCT/3LLCT
537
547
547
547
531
515
505
In summary, seven Cu(I) complexes bearing the four-coordinate
N-heterocyclic carbene ligands with different electron-withdraw-
ing substituents are reported. P1-P4 show the stronger MLCT
absorption band compared with P5-P6 with the methyl groups at
the
a-position of the pyridine ring in the visible region. P5-P7 with
the emission maximum in the range of 505–531 nm show excel-
Fig. 8. Spin density distribution contours of P1, P2 and P7 (isovalue = 0.004) in the T1 state.
Please cite this article as: B. Yang, J. Wang, S. Xu et al., Four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes bearing functionalized 3-benzyl-
1-(pyridyl)-1H-imidazolylidene ligands: Synthesis, photophysical properties and computational study, Polyhedron, https://doi.org/10.1016/j.
poly.2019.114240

B. Yang et al. / Polyhedron xxx (xxxx) xxx
9
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Declaration of Competing Interest
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The authors declare that they have no known competing finan-
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to influence the work reported in this paper.
Acknowledgements
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characterization of phosphorescent three-coordinate Cu(I)–NHC complexes,
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We thank the National Natural Science Foundation of China
(No. 21563013 and 201961012) and the Natural Science Founda-
tion of Jiangxi Province (No. 2010GZH0035) for their financial sup-
port. We are also grateful to the Guizhou University High
Performance Computation Chemistry Laboratory (GHPCC) for their
help with theoretical analysis.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at
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1-(pyridyl)-1H-imidazolylidene ligands: Synthesis, photophysical properties and computational study, Polyhedron, https://doi.org/10.1016/j.
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