Synthesis, photophysical properties and DFT studies of the pyridine-imidazole (PyIm) Cu(I) complexes: Impact of the pyridine ring functionalized by different substituents

Article abstract of DOI:10.1016/j.ica.2019.01.005

A series of the pyridine-imidazole (PyIm) Cu(I) complexes with different substituents (bromide (P1), methyl (P2), and phenyl (P3)) attached on the pyridine ring are synthesized and characterized. All the complexes show the typical metal-to-ligand charge transfer (MLCT) absorption in the visible region. Complex P1 with an electron-withdrawing substituent on the pyridine ring, e.g., a bromine group, shows a red shift of the emission wavelength. Conversely, an electron-donating substituent on the pyridine ring in complex P2, e.g., a methyl group, shifts the emission to longer wavelength. Similarly, complex P3 containing the extended π-conjugation system of the ligand also exhibits a red shift of the emission wavelength. All of the complexes exhibit efficient green-yellow emission in PMMA films at room temperature with emission wavelength of 547–569 nm and quantum yields of 24.8–53.0%. Meanwhile, DFT and TDDFT methods were employed to explain the photophysical properties.

Full text of DOI:10.1016/j.ica.2019.01.005

Inorganica Chimica Acta 488 (2019) 34–40
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, photophysical properties and DFT studies of the pyridine-
imidazole (PyIm) Cu(I) complexes: Impact of the pyridine ring
functionalized by different substituents
Chaoyang Chai , Shengxian Xu , Jinglan Wang , Feng Zhao , Hongying Xia , Yibo Wangb
a
a
a
a,⁎
a,b
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
Keywords:
A series of the pyridine-imidazole (PyIm) Cu(I) complexes with different substituents (bromide (P1), methyl
(P2), and phenyl (P3)) attached on the pyridine ring are synthesized and characterized. All the complexes show
the typical metal-to-ligand charge transfer (MLCT) absorption in the visible region. Complex P1 with an elec-
tron-withdrawing substituent on the pyridine ring, e.g., a bromine group, shows a red shift of the emission
wavelength. Conversely, an electron-donating substituent on the pyridine ring in complex P2, e.g., a methyl
group, shifts the emission to longer wavelength. Similarly, complex P3 containing the extended π-conjugation
system of the ligand also exhibits a red shift of the emission wavelength. All of the complexes exhibit efficient
green-yellow emission in PMMA films at room temperature with emission wavelength of 547–569 nm and
quantum yields of 24.8–53.0%. Meanwhile, DFT and TDDFT methods were employed to explain the photo-
physical properties.
Copper (I) complex
Pyridine-imidazole
Metal-to-ligand charge transfer
Density functional theory
Molar extinction coefficient
1
. Introduction
Copper(I) complexes have drawn special attention due to their
membered imidazole ring and the facile N-alkylation of the NH group of
the imidazole ring [15–19]. It is demonstrated that the imidazole li-
gands with different functional groups have great effects on the pho-
tophysical properties of Cu(I) complexes. Recently, Wang and co-
workers reported a series of the emissive Cu(I) complexes bearing the
quinolyl-benzimidazole ligands, exhibiting bright yellow-orange lumi-
nescence in the solid states with quantum yields in the range of
0.12–0.16 and emission lifetimes in the range of 16–54 μs [20]. No-
tably, by employing the pyridine-imidazole (PyIm) as diimine ligand,
adachi and co-workers synthesized a green-emitting Cu(I) complex
displaying improved luminescence properties with 25% quantum yield
and lifetime of 15.5 μs in the solid state [21]. Most recently, our group
has prepared a series of the PyIm-based Cu(I) complexes, showing
tunable optical and electronic properties modulated by different or-
ganic chromophore groups [22].
structural diversities and rich photophysical properties as well as pro-
mising applications in numerous areas such as catalysis, organic light-
emitting devices (OLEDs), dye-sensitized solar cells (DSSCs) [1–7]. The
combination of electron rich metal centers, conjugated organic ligands
and high degree of inherent covalence in soft acid-base bonding can
produce low energy electronic interactions between the metal center
and the ligands and the resulting compound possesses interesting op-
tical and electronic properties. However, the traditional tetrahedral
copper(I) complexes display weak emission, low quantum yields and
short excited state lifetimes because of the excited-state distortion,
which accelerates nonradiative decay of the emissive excited state
[
8–10]. To improve the photophysical properterties of the Cu(I) com-
+
plexes, the heteroleptic [Cu(N^N)(P^P)] complexes, where N^N de-
notes diimine ligands and P^P represents diphosphine ligands, have
been designed and synthesized with rich luminescence properties
Excellent photophysical properties possessed with these imidazole-
based Cu(I) complexes encouraged us to explore the luminescent
properties of analogous Cu(I) complexes of this type. As an extension of
our works, we report herein the synthesis and characterization of a few
PyIm-based Cu(I) complexes with different substituents such as bro-
mide, methyl, and phenyl groups attached onto the position A in the
pyridine ring of the PyIm ligand (see Fig. 1). The photophysical
[
11–14].
Among the various N^N ligand systems, the imidazole ligand and its
derivatives have been frequently employed in the syntheses of the lu-
minescence Cu(I) complexes due to the electron-rich nature of the five-
⁎
Corresponding author.
E-mail address: zhf19752003@163.com (F. Zhao).
https://doi.org/10.1016/j.ica.2019.01.005
Received 3 November 2018; Received in revised form 26 December 2018; Accepted 4 January 2019
Available online 05 January 2019
0020-1693/ © 2019 Elsevier B.V. All rights reserved.

C. Chai et al.
Inorganica Chimica Acta 488 (2019) 34–40
Fig. 1. Structures of the Cu(I) complexes in this study.
1
properties of the resulting complexes were systematically investigated
both experimentally and theoretically. We believed that these findings
are useful for the development of highly efficient luminescent materials
of the PyIm-based Cu(I) complexes.
(5 mL). The product was obtained as white solid. Yield: 68%. H NMR
(400 MHz, DMSO) δ 12.73 (s, 1H), 8.45 (s, 1H), 7.94 (s, 1H), 7.71 (s,
1H), 7.13 (d, J = 60.4 Hz, 2H), 2.36 (s, 3H).
2
.2.3. 5-phenylpicolinaldehyde (L3′)
2
. Experimental section
5-bromopicolinaldehyde (0.87 g, 4.66 mmol), Pd(PPh
3
)
4
(0.65 g,
0
.59 mmol) were added to a 20 mL/20 mL anhydrous toluene/THF so-
2.1. Materials and method
lution. Then, anhydrous ethanol 10 mL and 2 M K
2
CO solution (15 mL)
3
dissolved in H O were added to the reaction mixture at 50 °C.
2
All reagents were obtained from commercial sources, unless other-
Phenylboronic acid (0.68 g, 5.6 mmol) was added after half an hour and
the mixture was refluxed for 8 h under nitrogen. After the reaction had
finished, the mixture was filtered. The product was isolated using silica
gel column chromatography with PE and EtOAc (5:1) as the solvent.
The solvent was removed by evaporation. Recrystallization of the re-
wise stated. Complex P0 was prepared according to our previous report
[
22]. NMR spectra were recorded on a Bruker AV400 MHz spectro-
meter. UV-vis absorption spectra were measured with a Perkin Elmer
Lambda-900 spectrophotometer. Elemental analyses of the complexes
were carried out on an Elementar VarioEL cube analyzer. Fluorescence
spectra were determined with a Hitachi F-4600 fluorescence spectro-
photometer. Photoluminescence (PL) quantum yields were determined
using a Hamamatsu system for absolute PL quantum yield measure-
ments (type C11347). Fluorescent lifetimes were measured with a
compact fluorescent lifetime spectrometer (Hamamatsu, C11367,
Japan).
1
sidue from EtOH afforded a beige compound. Yield:76%. H NMR
(400 MHz, DMSO) δ 10.05 (s, 1H), 9.17 (s, 1H), 8.35 (d, J = 8.0 Hz,
1H), 8.02 (d, J = 8.1 Hz, 1H), 7.86 (d, J = 7.3 Hz, 2H), 7.54 (dt,
J = 22.9, 6.8 Hz, 3H).
2.2.4. 2-(1H-imidazol-2-yl)-5-phenylpyridine (L3)
Anhydrous ethanol (25 mL), Glyoxal solution (5 mL) were stirred in
an ice bath, Then, L3′ (0.94 g, 5.15 mmol) and concentrated ammonia
were added to the mixed solution under nitrogen for 1 h. The mixed
solution was cooled to room temperature and allowed to stand for 12 h,
extracted with dichloromethane and washed by water until neutral, and
evaporated under reduced pressure to remove the solvent. The product
was isolated using silica gel column chromatography with EtOAc as the
solvent. Recrystallization of the residue from EtOH afforded a brown
2.2. Synthetic procedure
2
.2.1. 5-bromo-2-(1H-imidazol-2-yl)pyridine(L1)
Anhydrous ethanol (25 mL) , glyoxal solution (5 mL) were stirred in
an ice bath, Then, 5-bromopicolinaldehyde (0.76 g, 4.12 mmol) and
concentrated ammonia were added to the mixed solution under ni-
trogen for 1 h. The mixed solution was cooled to room temperature and
allowed to stand for 12 h, extracted with dichloromethane and washed
by water until neutral, and evaporated under reduced pressure to re-
move the solvent. The product was isolated using silica gel column
chromatography with EtOAc as the solvent. Recrystallization of the
1
compound. Yield: 56%. H NMR (400 MHz, DMSO) δ 12.87 (s, 1H),
8.91 (s, 1H), 8.19 (dd, J = 8.2, 2.2 Hz, 1H), 8.11 (d, J = 8.3 Hz, 1H),
7.80 (d, J = 7.4 Hz, 2H), 7.53 (t, J = 7.5 Hz, 2H), 7.44 (t, J = 7.3 Hz,
1H), 7.22 (s, 2H).
1
residue from EtOH afforded a brown compound. Yield: 81%. H NMR
2.2.5. General procedure for preparation of Im-Py-Cu(I) complexes
Cu(I) complexes were synthesized by the following route: [Cu
(CH CN) ](PF ) (0.124 g, 0.4 mmol) and bis(2-(diphenylphosphanyl)
(
400 MHz, DMSO) δ 13.02 (s, 1H), 8.54 (d, J = 5.3 Hz, 1H), 8.24 (s,
1
H), 7.78–7.63 (m, 1H), 7.26 (s, 2H).
3
4
6
phenyl ether (POP) (0.216 g, 0.4 mmol) reacted in dichloromethane
(15 mL) at 25 °C for 2 h. Then, the corresponding ligand (0.4 mmol) was
dissolved in the degassed dichloromethane solution and injected into
the mixed solution for 2 h. The resulting mixture was filtered through a
2
.2.2. 2-(1H-imidazol-2-yl)-5-methylpyridine(L2)
This compound was prepared via a similar procedure for L0 from 5-
methylpicolinaldehyde (0.81 g, 5.08 mmol), and glyoxal solution
35

C. Chai et al.
Inorganica Chimica Acta 488 (2019) 34–40
Scheme 1. The synthetic pathways of the PyIm derivatives and the corresponding Cu(I) complexes.
plug of Celite and concentrated to ca. 1 mL. Addition of Et
2
O (10 mL) to
3. Results and discussion
the filtrate afforded a pale yellow precipitate, which was collected and
washed with Et
2
O. And the product was recrystallized with ethanol.
3.1. Synthesis
1
P1. The product was a yellow powder. Yield: 82%. H NMR
400 MHz, DMSO) δ 13.76 (s, 1H), 8.43 (s, 1H), 7.95 (s, 1H), 7.62 (d,
(
The synthetic pathways of the ligands and the corresponding Cu(I)
complexes are outlined in Scheme 1. The ligands L0–L2 were readily
synthesized from commercially available starting materials according
to the literature procedure [35]. L3 was prepared in two steps involving
the Suzuki coupling reaction [36] of phenylboronic acid with 5-bro-
mopicolinaldehyde, followed by the cyclocondensation reaction of 5-
J = 5.6 Hz, 2H), 7.38 (s, 8H), 7.29 (s, 11H), 7.15–7.02 (m, 5H), 6.87 (s,
1
3
3
1
1
H), 6.68 (s, 2H), C NMR (100 MHz, DMSO) δ 158.4, 150.1, 144.7,
41.8, 134.3, 134.1, 133.9, 133.4, 132.8, 131.7, 131.5, 131.4, 130.70
29.4, 125.7, 124.1, 124.0, 123.8, 121.9, 120.9. 31P NMR (162 MHz,
DMSO)
δ
−13.36 (s), −144.50 (quint). Anal. Calcd. For
C
44
H
34CuBrF
6
N
3
OP
3
(971.12): C 54.42, H 3.53, N 4.33; found: C 54.61,
phenylpicolinaldehyde (L3′), oxalaldehyde, and aqueous NH
3
in
H 3.12, N 4.53.
ethanol. The corresponding Cu(I) complexes were prepared by reacting
1
P2. The product was a pale yellow powder. Yield: 68%. H NMR
400 MHz, DMSO) δ 13.48(s, 1H), 8.05 (s, 1H), 7.88 (s, 2H), 7.51 (s,
[Cu(CH
3
CN)
4
]PF with the corresponding ligands at the rate of 1:1.
6
(
These Cu(I) complexes were confirmed with H NMR, C NMR, and ele-
mental analysis.
2
7
H), 7.41(s, 1H), 7.38 (d, J = 8.3 Hz, 6H), 7.28 (s, 10H), 7.15 (s, 3H)
13
.07 (t, J = 7.4 Hz, 3H), 6.87 (s, 3H), 6.66 (s, 2H), 2.07 (s, 3H).
C
NMR (100 MHz, DMSO) δ 162.2, 157.8, 149.1, 138.9, 133.6, 132.9,
3.2. Photophysical properties
1
1
31.9, 131.3, 131.2, 130.9, 129.9, 128.6, 124.9, 123.7, 123.6, 123.4,
31
20.9, 120.2, 119.3, 17.6. P NMR (162 MHz, DMSO) δ −13.64 (s),
Fig. 2 shows the UV–Vis spectra of P0–P3 in CH
2
Cl solution at
2
−
144.49 (quint). Anal. Calcd. For C45
6
H37CuF N
3
OP
3
(906.25): C
room temperature. The corresponding absorbance data are presented in
Table 1. All the Cu(I) complexes display intense absorption bands in the
region of 250–340 nm, which are assigned to spin-allowed π − π* li-
gand centered (LC) transitions of the diimine and POP ligands. On the
other hand, the lower energy bands at the range of 350–425 nm result
from spin-allowed metal Cu to ligand charge transfer (MLCT) for all the
complexes. It is notable that the MLCT band for P1 bearing an electron-
5
9.64, H 4.12, N 4.64; found: C 59.41, H 4.02, N 4.54.
1
P3. The product was a pale yellow powder. Yield: 51%. H NMR
(
400 MHz, DMSO) δ 13.78 (s, 1H), 8.36 (d, J = 23.6 Hz, 1H), 8.27 (s,
1
7
H), 8.18 (s, 1H), 7.60 (s, 2H), 7.40 (d, J = 27.1 Hz, 19H), 7.16 (s, 6H),
13
.07 (d, J = 6.7 Hz, 3H), 6.81 (s, 3H), 6.66 (s, 2H). C NMR (100 MHz,
DMSO) δ 157.2, 145.7, 145.2, 135.0, 134.4, 132.8, 132.0, 131.4–130.8,
1
1
30.0, 129.2, 128.9, 128.7, 126.3, 125.0, 124.3–123.3, 120.4,
31
19.9–119.7. P NMR (162 MHz, DMSO) δ −13.42 (s), −144.50
(
quint). Anal. Calcd. For C50
6
H39CuF N
3
OP (968.32): C 62.02, H 4.06,
3
N 4.34; found: C 61.84, H 4.02, N 4.54.
2.3. DFT calculations
The B3LYP exchange-correlation function [23,24] was used to op-
timize the ground state geometries of all the complexes using the po-
larized continuum model (PCM) [25] in CH
2
Cl
2
media under the
Gaussian 09 [26] program package. The 6-31G* basis set [27,28] was
used for the C, H, N, O, and P atoms. The LANL2DZ basis set [29] was
adopted for the Cu and Br atoms. On the basis of the optimized ground
geometries, TDDFT method [30,31] associated with PCM in CH
2
Cl
2
media were used to simulate the absorption spectra of the complexes
studied. The first 200 singlet vertical excitations were obtained using
the TDDFT/B3LYP method to construct the calculated absorption
spectra. The calculated emission energies of all the complexes were
obtained by the TDDFT method with the M06-2X functional [32] at
B3LYP optimized ground state geometry. The calculated electronic
density plots for the frontier molecular orbitals were prepared using
Multiwfn analyzer soft [33] and VMD program [34].
Fig. 2. Absorption spectra in dichloromethane (C = 1 × 10−5 mol/L) at 298 K
of complexes P0–P3.
36

C. Chai et al.
Inorganica Chimica Acta 488 (2019) 34–40
Table 1
Photophysical properties of complexes P0–P3.
Absorption
Emission
em (nm)/E (eV)
λ
abs/nm (ε × 104 M−1 cm−1
)
λ
τ/μs
Φ/%
k
r
/104 s−1
k
nr/104 s−1
P0
P1
P2
P3
260 (2.5), 291 (2.2), 375 (0.23)
271 (3.0), 305 (2.1), 375 (0.35)
261 (2.2), 298 (1.8), 375 (0.20)
274 (3.0), 317 (2.3), 375 (0.43)
550/2.25
559/2.22
547/2.27
569/2.18
13.9
13.7
26.4
23.5
50.4
46.3
53.0
24.8
3.63
3.38
2.01
1.05
3.57
3.92
1.78
3.20
introduction of electron-donating substituents will increase the energy
gap of the complexes, which agrees with the theoretical results (see
below, in Section 3.3, P0:3.96 eV; P1:3.79 eV; P2:4.02 eV; P3:3.83 eV).
In addition, complexes P1 and P3 exhibit the enhanced light absorption
−
1
−1
abilities (ε(375 nm) = 3460 and 4320 M cm , respectively) compared
−
1
−1
with those of P0 and P2 (ε(375 nm) = 2340 and 2060 M
cm , re-
spectively).
The emission spectra of all the complexes in PMMA film (10 wt%) at
room temperature are shown in Fig. 3, exhibiting the emission maxima
in the green-yellow region at 550, 559, 547, and 569 nm, respectively.
The shapes of the emission spectra are broad and unstructured, in-
dicating the excited state is assigned as the triplet charge transfer
3
character ( CT) in nature. It is clearly seen that the emissive wave-
lengths can be modulated by an introduction of different substituents
on the pyridine ring. For P1, a H atom is replaced with the electron-
withdrawing bromide substituent, which resulted in a red shift of the
emission maximum of 9 nm in comparison with P0. In contrast, the
emission is blue-shifted by 3 nm in P2 where an electron-donating
methyl group is attached. Additionally, the emission maximum of P3 is
more red-shifted by 19 nm compare to that of P0 due to the efficient π-
extension caused by the addition of phenyl group in the ligand. The
blue or red shift observed in the emission spectra could be explained by
the change of the energy level of the lowest unoccupied molecular or-
bitals (LUMO) of the complex. The addition of donor groups such as
methyl group on the pyridine ring destabilizes the LUMO of the com-
plex and ultimately increases the energy gap of the complex, leading to
a blue shift of emission. Conversely, the addition of acceptor groups
such as bromide group or the extension of π-conjugation on the pyr-
idine ring stabilizes the LUMO of the complex and consequently de-
creases the energy gap of the complex, thus shifting the emission to
longer wavelengths. These results are in good agreement with the
theoretical calculations (see below in Section 3.3)
Fig. 3. Normalized emission spectra of complexes P0–P3 in PMMA films (10 wt
).
%
The emission lifetimes (τ) and quantum yields (Φ) of P0-P3 in
PMMA films (10 wt%) are also measured and the corresponding data
are provided in Table 1. the τ values of all the complexes are in range of
1
3.7–26.4 μs, indicating that the emissive behavior is originated from
the triplet excited state. The reference complex P0 has a higher
quantum yield (Φ = 50.4%). P1 with lower emission energy displays
slightly lower quantum yield (Φ = 46.3%), whereas P2 with higher
emission energy shows higher quantum yield (Φ = 53.0%), compared
to that of P0. This is consistent with the energy gap law [37], i.e., a
decrease in emission quantum efficiency with decreasing emission en-
ergy. Furthermore, the increase in the nonradiative rate constants (knr
)
(
see Table 1) across P2 to P0 to P1 supports the present observations. It
4
Fig. 4. The energy levels and atomic orbital compositions calculated for the
HOMO and LUMO of complexes P0–P3.
is worth noting that P1 and P3 have similar knr values (3.92 × 10 and
4
−1
3.20 × 10 s
respectively) but the Φ value of P3 (Φ = 24.8%) is
nearly two times lower than that of P1, which is not enough to be ra-
tionalized via the energy gap law alone. The additional decrease in Φ
presumably results from the intermolecular π-π stacking interactions, as
can be inferred from the extended π-conjugation system in P3
withdrawing Br substituent is red-shifted compared to that of the parent
P0. In contrast, the MLCT band for P2 with an electron-donating methyl
substituent is observed at the blue-shifted region. Additionally, a red
shift of the MLCT band for P3 with the extended π-conjugation between
the phenyl ring and pyridine-imidazole ligand can be observed com-
pared to that of the parent P0. These results indicate that increasing the
electron-withdrawing character or extension of the conjugation on the
pyridine ring will reduce the energy gap of the complexes, while
3.3. Theoretical calculations
To provide further insight on the structural and optical properties of
the investigated Cu(I) complexes, DFT/TDDFT calculations were
37

C. Chai et al.
Inorganica Chimica Acta 488 (2019) 34–40
Fig. 5. Comparison of the calculated (red line) and experimental (black line)
absorption spectra in CH
2
Cl solution for P0 (a), P1 (b), P2 (c), and P3 (d). Red
2
vertical lines correspond to 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.)
performed for these Cu(I) complexes. The energy levels, atomic orbital
compositions, and energy gaps of the HOMO and LUMO of the com-
plexes in the optimized ground singlet geometries are shown Fig. 4. The
highest occupied molecular orbital (HOMO) of P0–P3 is mainly loca-
lized on both the Cu d orbitals (37.2–38.5%) and the π orbitals
(
57.9–59.4%) on the phenyl ring of POP ligand, while the LUMO is
located on the π* orbitals (82.7–93.1%) of pyridine-imidazole ligand.
Therefore, the introduction of substituents onto the pyridine ring would
have a significantly influence on the LUMO level. For P1, the LUMO is
stabilized by 0.22 eV with respect to the reference complex P0 due to
the electron-withdrawing effect of the Br group, while the LUMO of P2
is slightly destabilized by 0.06 eV due to the electron-donating effect of
the methyl group. For P3, the electron density of LUMO is delocalized
over the whole ligand framework constituted by the pyridyl, carbene
and phenyl segments. Thus, the LUMO is stabilized by 0.14 eV com-
pared with that of P0. As expected, the HOMO energies for all the
complexes remain almost unaltered. The HOMO-LUMO energy gap
increases in the order P2 (4.02 eV) > P0 (3.96 eV) > P3
(
3.83 eV) > P1 (3.79 eV). This trend is in good agreement with the
absorption onset in low energy region observed experimentally.
TDDFT method is employed to simulate the absorption spectra of all
the complexes using PCM model. As shown in Fig. 5, one can see that
the calculated spectra are overall well reproduced the experimental
spectra. For all the complexes, the calculated first singlet excited state
(S ) is mainly originated from the HOMO → LUMO excitation with
1
MLCT/LLCT character, and this transition is responsible for the lowest
lying absorption band in the range 350–425 nm in experiment. The
calculated absorption bands in the 250–320 nm range are much pro-
nounced, which are composed of a mixture of several of the excitation
transitions. For example, for P0, the mixed transitions of the S
6
and S
7
dominantly contribute to the calculated absorption bands centered at
2
93 nm with (π → π*)/MLCT character (see Table 2), correlating well
with the experimental shoulders centered at 291 nm. The highest en-
ergy absorption bands centered at 263 nm calculated mainly originate
from the mixed transitions of the S28 and S31 with MLCT/LLCT/ (π →
π*) character, which are coincident with the absorption band of 260 nm
observed experimentally. The detailed assignments of the transitions for
the other complexes are provided in Table 2.
Since the nature of the lowest absorption bands for all the com-
1
1
plexes has a MLCT/ LLCT character, we reasoned that the nature of
the emitting state from the lowest lying triplet excited state would also
3
3
have MLCT/ LLCT character. In order to investigate the origin of the
emitting state for these complexes, the calculated emission energy is
obtained using the TDDFT method. The B3LYP functional seems to
underestimate the emission energy of the investigated Cu(I) complexes,
while the M06-2X functional gives more satisfactory results (see
Table 5S in the Supporting Information). Therefore, we have employed
the M06-2X functional using 6-31G(d)/LANL2DZ level for further
emission energy calculations. The calculated emission energy, wave-
length, and transition nature are complied in Table 3, along with the
experimental values. The involved molecular orbital compositions are
provided in Table 5S (Supporting Information). As can be noticed, the
calculated emission energies are in excellent agreement with the ex-
perimental values and decrease in the order P2 > P0 > P1 > P3
along with the same order of the experimental results. For all the
complexes, the lowest energy emissions are mainly from the transitions
of LUMO → HOMO. For P0, from an analysis of the electron density
distribution of HOMO and LUMO, the emitting state of P0 can be as-
signed as 3MLCT/ LLCT character. The emitting states of P1 and P2
3
38

C. Chai et al.
Inorganica Chimica Acta 488 (2019) 34–40
Table 2
Electronic absorptions of complexes P0–P3 in CH
2
Cl
2
based on TDDFT calculations at the (B3LYP)/6-31 g*/LANL2DZ level, together with the experimental values.
Excited state
Transition
Coeff
E(eV)/(nm)
Oscillator
Assign
Exptl/nm
P0
S
S
1
6
H → L (188–189)
H → L + 3
H-3 → L
0.69998(98.0%)
0.47974(46.0%)
−0.46027(42.4%)
0.49661(49.3%)
−0.46027(42.4%)
0.47326(44.8%)
0.43145(37.2%)
0.35091(24.6%)
−0.34494(23.8%)
−0.25967(13.5%)
3.32/374
4.16/298
0.0752
0.1299
MLCT(Cu→PyIm)/LLCT(POP→PyIm)
(π → *π)(POP)/MLCT(Cu→POP)
(π → *π)(PyIm)
370
291
S
S
S
7
H → L + 3
H-3 → L
4.17/298
4.76/260
4.78/259
0.1451
0.0756
0.0386
(π → *π)(POP)/MLCT(Cu→POP)
(π → *π)(PyIm)
291
260
260
28
31
H-2 → L + 4
H-3 → L + 1
H-2 → L + 4
H-3 → L + 1
H-10 → L
MLCT(Cu→POP)/LLCT(PyIm→POP)
(π → *π)(PyIm)
MLCT(Cu→POP)/LLCT(PyIm→POP)
(π → *π)(PyIm)
MLCT(Cu→POP)/LLCT(POP→PyIm)
P1
P2
S
S
S
S
S
1
5
6
8
H → L (191–192)
0.70189(98.5%)
0.65028(84.6%)
0.67343(90.7%)
0.67711(91.7%)
0.44244(39.2%)
−0.38064(29.0%)
0.60143(72.3%)
0.70113(97.4%)
0.63880(81.6%)
0.59681(71.2%)
0.69391(96.3%)
0.68682(94.3%)
0.47572(45.3%)
−0.28425(16.2%)
−0.26250(13.8%)
0.50053(50.1%)
0.23497(11.0%)
3.14/395
3.96/331
4.00/310
4.10/303
4.49/276
0.0747
0.2281
0.0476
0.0936
0.2100
MLCT(Cu→PyIm)/LLCT(POP→PyIm)
380
305
305
305
271
H-3 → L
(π → *π)(PyIm)
H → L + 2
H → L + 3
H-3 → L + 1
H → L + 6
H → L + 8
(π → *π)(POP)/MLCT(Cu→PyIm)
(π → *π)(POP)/MLCT(Cu→PyIm)
LLCT(PyIm→POP)
18
MLCT(Cu→Br)/LLCT(POP→PyIm)
(π → *π)(POP)/MLCT(Cu→POP)
S
23
4.55/273
0.0781
271
S
S
S
S
S
S
1
6
7
8
9
24
H → L (192–193)
H → L + 3
H-3 → L
3.34/371
4.07/304
4.11/302
4.17/298
4.24/292
4.67/265
0.0787
0.0910
0.2050
0.0666
0.0923
0.0578
MLCT(Cu→PyIm)/LLCT(POP→PyIm)
(π → *π)(POP)/MLCT(Cu→POP)
(π → *π)(PyIm)
360
298
298
298
298
261
H → L + 4
H → L + 5
H-5 → L
(π → *π)(POP)/MLCT(Cu→POP)
(π → *π)(POP)/MLCT(Cu→POP)
LLCT(POP→PyIm)
H-3 → L + 2
H-11 → L
(π → *π)(PyIm)/LLCT(PyIm→POP)
MLCT(Cu→PyIm)/LLCT(POP→PyIm)
(π → *π)(PyIm)/LLCT(PyIm→POP)
LLCT(POP→PyIm)
S
25
H-3 → L + 2
4.71/263
0.1140
261
H-5 → L
P3
S
S
S
1
4
15
H → L (208–209)
H-1 → L
0.70115(98.3%)
0.56539(63.9%)
0.40967(33.6%)
−0.26447(14.0%)
−0.23228(10.8%)
3.23/383
3.85/322
4.43/280
0.0778
0.5463
0.1816
MLCT(Cu→PyIm)/LLCT(POP→PyIm)
375
317
274
LLCT(Phenyl→PyIm)
H-1 → L + 1
H-2 → L + 1
H-9 → L
(π → *π)(PyIm)/LLCT(Phenyl→PyIm)
MLCT(Cu→PyIm)/LLCT(POP→PyIm)
LLCT(Phenyl→PyIm)/LLCT(POP→PyIm))
moiety (82.2%) and the phenyl ring (15.1%), thus, the transition
Table 3
3
Emission energies and transition natures of P0–P3 calculated by TDDFT method
characters of the emission can be described as LLCT [π*(PyIm) →
using M062X/6-31G(d)/LANL2DZ level, as well as the experimental values.
π(Phenyl)] character.
Calc
Calc
Expl
Expl
To support our assignments of the emitting state, a spin density
analysis has been calculated and the corresponding spin density dis-
tributions are shown in Fig. 6. As shown in Fig. 6, the unpaired electron
spin-density distributions in the lowest lying triplet excited state for
P0–P2 are similar (Cu, 0.54e; POP, 0.32e; PyIm, 1.15e for P0; Cu,
0.53e; POP, 0.33e; PyIm, 1.14e for P1; Cu, 0.54e; POP, 0.31e; PyIm,
λ
em
(nm)/Eem
Configuration
Assignment
λ
em
(nm)/Eem
(
eV)
(eV)
3
3
3
3
3
3
3
P0 511/2.43
P1 530/2.34
P2 500/2.48
P3 581/2.14
L → H (81.1%)
L → H (82.1%)
L → H (80.7%)
L → H (92.5%)
MLCT/ LLCT 550/2.25
MLCT/ LLCT 559/2.22
MLCT/ LLCT 547/2.27
LLCT
569/2.18
1
.14e for P2) and clearly demonstrate the higher contribution of the d
(
Cu) to the PyIm ligand with some contribution of POP ligand to the
3 3
have similar character to that calculated for P0, see Table 3. With re-
spect to P3, the HOMO is localized on the PyIm moiety (73.7%) and the
phenyl ring (30.9%) whereas the LUMO is also localized on the PyIm
PyIm ligand, confirming the emission behavior of MLCT/ LLCT char-
acter. With respect to complex P3, the unpaired electrons in the emis-
sion state are localized on both PyIm ligand and the Phenyl ring (Cu,
Fig. 6. Spin density distribution contours (isovalue = 0.004) for the lowest triplet state T of complexes P0–P3. The values of the unpaired-electron spin-density
1
population are depicted together with the electronic nature of the states.
39

C. Chai et al.
Inorganica Chimica Acta 488 (2019) 34–40
0
.027e; POP, 0.007e; PyIm, 1.57e; Phenyl, 0.42e), indicating that the
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47–569 nm and quantum yields of 24.8–53.0%. The addition of sub-
[
stituents onto the pyridine moiety has a direct influence on the energy
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TDDFT studies reproduce well the experimental data and perfectly ex-
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[
Acknowledgements
(
(
2019) 237–244.
The work was supported by the National Natural Science
Foundation of China (No. 21563013) and the Natural Science
Foundation of Jiangxi Province (No. 2010GZH0035 and No.
[
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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ica.2019.01.005.
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