Modulation of photophysical properties of copper(I) complexes containing pyridyl-imidazole (PyIm) ligands functionalized by naphthyl, phenanthryl, and anthryl groups

Article abstract of DOI:10.1016/j.ica.2018.09.059

A series of Cu(I) pyridyl-imidazole (PyIm) complexes with different aryl groups (Ar = naphthalene (P2), phenanthrene (P3), and anthracene (P4)) attached on the pyridyl ring are synthesized and characterized. The influence of these organic chromophore groups on the photophysical properties of the resulting complexes is investigated. Complexes P2–P4 show stronger light harvesting efficiencies in the visible region compared with the parent complex P1. The emitting state of complex P1 originates from the ³MLCT state with some ³LLCT character, while complexes P2 and P3 predominantly exhibit the ³LLCT character. For complex P4, the triplet emitting state is dominated by the ³(π → π) state localized on the anthryl moiety, together with a lesser contribution form the ³LLCT state. These changes in the photophysical properties were rationalized by DFT and TDDFT methods.

Full text of DOI:10.1016/j.ica.2018.09.059

Accepted Manuscript
Research paper
Modulation of photophysical properties of copper(I) complexes containing pyr-
idyl-imidazole (PyIm) ligands functionalized by naphthyl, phenanthryl, and
anthryl groups
Jinglan Wang, Caoyang Cai, Shengxian Xu, Feng Zhao, Hongying Xia, Yibo
Wang
PII:
S0020-1693(18)30848-X
https://doi.org/10.1016/j.ica.2018.09.059
ICA 18517
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
28 May 2018
28 August 2018
20 September 2018
Please cite this article as: J. Wang, C. Cai, S. Xu, F. Zhao, H. Xia, Y. Wang, Modulation of photophysical properties
of copper(I) complexes containing pyridyl-imidazole (PyIm) ligands functionalized by naphthyl, phenanthryl, and
anthryl groups, Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.09.059
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Modulation of photophysical properties of copper(I) complexes
containing pyridyl-imidazole (PyIm) ligands functionalized by
naphthyl, phenanthryl, and anthryl groups
a
a
a
a
a
b
Jinglan Wang Caoyang Cai , Shengxian Xu , , Feng Zhao *, Hongying Xia , Yibo Wang
a
School of Chemistry and Chemical Engineering, Jiangxi Science and Technology Normal
University, Fenglin Street Nanchang, Jiangxi 330013, PR China.
bKey Laboratory of Guizhou High Performance Computational Chemistry, Department of Chemistry,
Guizhou University, Guiyang 550025, PR China.
*
Corresponding author. Tel: +86 79183805183
E-mail: zhf19752003@163.com
ABSTRACT
A series of Cu(I) pyridyl-imidazole (PyIm) complexes with different aryl groups (Ar= naphthalene
(
P2), phenanthrene (P3), and anthracene (P4)) attached on the pyridyl ring are synthesized and
characterized. The influence of these organic chromophore groups on the photophysical properties of
the resulting complexes is investigated. Complexes P2-P4 show stronger light harvesting efficiencies
in the visible region compared with the parent complex P1. The emitting state of complex P1
3
3
originates from the MLCT state with some LLCT character, while complexes P2 and P3
3
predominantly exhibit the LLCT character. For complex P4, the triplet emitting state is dominated by
3
3
the ( ) state localized on the anthryl moiety, together with a lesser contribution form the LLCT state.
These changes in the photophysical properties were rationalized by DFT and TDDFT methods.
Keywords: Copper (I) complex; Pyridyl-imidazole; Molar extinction coefficient; Excited-state
lifetime; Density functional theory

1. Introduction
Since the first dye-sensitized solar cells (DSSCs) employing a copper-based photosensitizer dye
was reported in 1999 by Sauvage and coworkers [1], significant progresses have been made in the
application of the Cu(I) complexes in DSSc in the recent years [2-10] due to their low cost and
generally low toxicities. However, compared with power conversion efficiencies exceeding the value
of 11% [11-14], there is still much room for improvement of the performance of the Cu(I)-based
DSSCs [15] . Clearly, it is crucial to improve the light harvesting capability and increase excited-state
lifetime of the Cu(I) complexes. In early study [16-18], Cu(I)-based dyes exhibit lower molar
3
-1
-1
extinction coefficients () on the order of 10 M cm , which means the poor light harvesting
efficiency in DSSCs. To date, the molar extinction coefficients of the Cu(I)-based dyes have been
4
-1
-1
increase to the order of 10 M cm by the judicious change of ligands [5,15], enabling greatly
enhanced Cu(I)-based DSSC performance. Nonetheless, there is very little information available on the
excited-state lifetime of such Cu(I)-based dyes in the reported literatures. It can be challenging to
possess a high light harvesting efficiency while maintaining a long-lived lifetime for a successful Cu(I)-
based photosensitizers.
Cu(I) complexes usually have the pseudo-tetrahedral coordination geometries in the ground state.
However, these Cu(I) complexes readily undergo a severe Jahn-Teller distortion (flattening) upon
excitation, which accelerates nonradiative decay of the emissive excited state, resulting in very short-
lived lifetimes[19-21]. To prolong the excited-state lifetime, the generally accepted approach is to use
the steric bulk of ligands to block geometrical distortion [22-25]. Especially, the most extensively
studied class of complexes is four-coordinate heteroleptic Cu(I) species bearing a pyridyl azolate-
based ligand together with a diphosphine ancillary ligand, exhibiting the higher quantum yields and
long-lived lifetimes [26-29]. However, this types of complexes usually possess the poor light
absorption in the visible region (), making them inferior toward DSSCs. Thus, further
functionalizations of the ligands would open the possibility to design Cu(I)-based dyes with both high

light harvesting efficiencies and long-lived lifetimes. Combining organic chromophores with the parent
complexes is an effective route to realize this goal. Herein, we present the synthesis, characterization,
and photophysical properties of a series of the pyridyl-imidazole (PyIm) Cu(I) complexes containing
naphthalene (naphthyl), phenanthrene (phenanthryl), and anthracene (anthryl) moieties (Figure 1,
complexes P2-P4). We expect enhanced light absorption abilities to be provided by the addition of
these light-harvesting groups while maintaining the long-lived excited state lifetimes. For comparison
purpose, the parent Complex P1 is also investigated as a reference. Additionally, the density functional
theory (DFT) and time-dependent density functional theory (TDDFT) were employed to explain the
photophysical properties.
Figure 1
2
2
. Experimental
.1. Materials and methods
All reagents for synthesis were purchased from Sinopharm Chemical Reagent Co. Ltd. All the
solvents for spectroscopic measurement were HPLC grade and were obtained from Vita-chemical Co.
1
Ltd. and used without further purification. H NMR spectra were performed in a Bruker AV400 MHz
spectrometer, using tetramethylsilane (TMS) as internal reference. DMSO and CDCl were used as the
3
solvents. High-resolution mass spectra (HRMS) were obtained with the Agilent 6450 Q-TOF mass
spectrometer using electrospray ionization (ESI). Elemental analyses of the complexes were carried
out on an Elementar VarioEL cube analyzer. UV-vis absorption spectra were measured using a Perkin
Elmer Lambda-900 spectrophotometer. Fluorescence spectra were determined with a Hitachi F-4600

fluorescence spectrophotometer. Photoluminescence (PL) quantum yields were determined using a
Hamamatsu system for absolute PL quantum yield measurements (type C11347). Fluorescent lifetimes
were measured with a compact fluorescent lifetime spectrometer (Hamamatsu, C11367, Japan).
2
.2. Synthetic procedure
-(naphthalen-1-yl)picolinaldehyde (2). ((5-bromopyridin-2-yl)methylene)oxonium (0.76 g,
.08 mmol), Pd(PPh3)4 (0.38g, 0.33mmol) were added to a 20ml / 20ml anhydrous toluene / THF
5
4
solution. Then, anhydrous ethanol 10 ml and 2 M K CO solution (15 ml) dissolved in H O were added
2
3
2
to the reaction mixture at 50 °C. Naphthalen-1-ylboronic acid (1.05 g, 6.12mmol) was added after half
an hour and the mixture was refluxed for 5 h under nitrogen. After the reaction had finished, the
mixture was filtered. The product was isolated using silica gel column chromatography with PE and
EtAc (5:1) as the solvent. The solvent was removed by evaporation. Recrystallization of the residue
1
from EtOH afforded a beige compound (0.94g, 4.03 mmol, 99% yield). H NMR (400 MHz, DMSO)
δ 10.12 (s, 1H), 8.96 (s, 1H), 8.19 (d, J = 2.0 Hz, 1H), 8.12 (s, 1H), 8.09 (d, J = 4.8 Hz, 2H), 7.77 (d,
J = 8.3 Hz, 1H), 7.69 – 7.61 (m, 2H), 7.60 – 7.55 (m, 2H).
5
-(phenanthren-9-yl)picolinaldehyde (3). This compound was prepared via a similar
procedure for Compound 2 from ((5-bromopyridin-2-yl)methylene)oxonium (0.75g, 4.02 mmol) and
phenanthren-9-ylboronic acid (1.33 g，6.03 mmol). The product was obtained as a pale yellow solid
1
(
1.06 g, 3.74 mmol, 93% yield). H NMR (400 MHz, DMSO) δ 10.17 (s, 1H), 9.09 – 8.99 (m, 2H),
8
7
.96 (d, J = 8.1 Hz, 1H), 8.31 (d, J = 7.8 Hz, 1H), 8.18 (d, J = 7.8 Hz, 1H), 8.11 (t, J = 10.7 Hz, 1H),
.99 (s, 1H), 7.87 – 7.77 (m, 4H), 7.73 – 7.65 (m, 1H).
5
-(anthracen-9-yl)picolinaldehyde (4). This compound was prepared via a similar procedure
for Compound 2 from ((5-bromopyridin-2-yl)methylene)oxonium (0.77 g, 4.12 mmol), and
anthracen-9-ylboronic acid (1.35g, 6.18 mmol). The product was obtained as yellow solid (0.98 g,
1
3
.46 mmol, 62% yield). H NMR (400 MHz, CDCl ) δ 10.18 (s, 1H), 8.88 (s, 1H), 8.81 (s, 1H), 8.23
3
(
s, 1H), 8.22 – 8.14 (m, 3H), 7.58 (d, J = 6.0 Hz, 2H), 7.50 (d, J = 13.2 Hz, 4H).
-(1H-imidazol-2-yl)pyridine (L1). Anhydrous ethanol (25ml), Glyoxal solution (5ml) were
2

stirred in an ice bath, Then, picolinaldehyde (0.54 g, 5.02 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 EtAc as the solvent. Recrystallization of the residue from
1
EtOH afforded a brown compound (0.25 g, 1.72 mmol, 34% yield). H NMR (400 MHz, DMSO) δ
1
2.80 (s, 1H), 8.59 (d, J = 4.3 Hz, 1H), 8.04 (d, J = 7.9 Hz, 1H), 7.88 (t, J = 7.7 Hz, 1H), 7.39 – 7.31
m, 1H), 7.15 (s, 2H).
-(1H-imidazol-2-yl)-5-(naphthalen-1-yl) pyridine (L2). This compound was prepared via a
(
2
similar procedure for L1 from compound 2 (1.17g, 5.02 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 dichloromethan 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 EtAc as the solvent. Recrystallization of the residue from EtOH afforded
1
a brown compound (0.44g, 1.62 mmol, 32% yield). H NMR (400 MHz, DMSO) δ 12.95 (s, 1H), 8.71
(
s, 1H), 8.17 (t, J = 23.6 Hz, 1H), 8.05 (t, J = 7.3 Hz, 3H), 7.84 (d, J = 8.1 Hz, 1H), 7.67 – 7.55 (m,
H), 7.22 (d, J = 56.0 Hz, 2H).
-(1H-imidazol-2-yl)-5-(phenanthren-9-yl) pyridine (L3). This compound was prepared via a
4
2
similar procedure for L1 from compound 3 (1.44 g, 5.08 mmol), and Glyoxal solution (5ml). The
1
product was obtained as yellow solid (0.52g, 1.62 mmol, 32% yield). H NMR (400 MHz, DMSO) δ
1
8
7
2.96 (s, 1H), 8.99 (d, J = 8.5 Hz, 1H), 8.92 (d, J = 7.8 Hz, 1H), 8.77 (s, 1H), 8.23 (d, J = 8.0 Hz, 1H),
.13 – 8.06 (m, 2H), 7.93 (s, 1H), 7.85 (d, J = 8.2 Hz, 1H), 7.72 (ddd, J = 23.2, 13.9, 6.4 Hz, 4H),
.28 (s, 2H)
5
-(anthracen-9-yl)-2-(1H-imidazol-2-yl)pyridine (L4). This compound was prepared via a
similar procedure for L1 from compound 4 (1.17 g, 4.12 mmol), and Glyoxal solution (5ml). The
1
product was obtained as yellow solid (0.35g, 1.09 mmol, 26% yield). H NMR (400 MHz, DMSO) δ

1
3.02 (s, 1H), 8.77 (s, 1H), 8.62 (s, 1H), 8.31 (d, J = 8.0 Hz, 1H), 8.21 (d, J = 8.4 Hz, 2H), 7.99 (dd,
J = 8.0, 1.8 Hz, 1H), 7.67 – 7.46 (m, 6H), 7.25 (s, 2H).
General procedure for preparation of PyIm-Cu(I) complexes. ImPy-Cu(I) complexes P1-P4
were synthesized by the following route: [Cu(CH CN) ](PF ) (0.124 g, 0.4mmol) and POP (0.216g,
3
4
6
0
.4 mmol) reacted in dichloromethane (15 mL) at 25C for 2 h. Then, the corresponding PyIm 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 plug of Celite and concentrated to ca. 1
mL. Addition of Et O (10ml) to the filtrate afforded a pale yellow precipitate, which was collected and
2
washed with Et O. And the product was recrystallized with ethanol.
2
[
Cu(PyIm)(POP)](PF ), P1. The product was a yellow powder (0.17g, 0.191 mmol, 48% yield).
6
1H NMR (400 MHz, DMSO) δ 13.76 (s, 1H), 8.13 (s, 2H), 8.05 (s, 1H), 7.53 (s, 1H), 7.37 (d, J = 6.7
Hz, 8H), 7.28 (s, 11H), 7.12 (s, 2H), 7.08 (s, 2H), 7.06 (s, 1H), 7.04 (s, 1H), 6.96 – 6.78 (m, 3H), 6.66
1
3
(
s, 2H). C NMR (400 MHz, DMSO) δ 158.4 (s), 149.6 (s), 145.2 (s), 139.2 (s), 134.2 (s), 133.2 (s),
1
1
7
4
32.5 (s), 131.7 (s), 130.4 (s), 129.2 (s), 125.4 (s), 124.8 (s), 124.3 (s), 124.2 (s), 124.0 (s), 122.0 (s),
+
+
20.9 (s), 120.4 (s). HRMS (m/z, ESI ): calcd. For C H CuN OP ([M] ) 746.1551; found
4
4
35
3
2
46.1530. Anal. Calcd. For C H CuF N OP (892.23): C 59.23, H 3.95, N 4.71; found: C 59.61, H
4
4
35
6
3
3
.02, N 4.53.
[
Cu(Naph-PyIm)(POP)](PF ), P2. The product was a brown powder (0.17g, 0.167 mmol, 42%
6
1
yield). H NMR (400 MHz, DMSO) δ 13.92 (s, 1H), 8.36 (s, 1H), 8.29 (s, 1H), 8.16 (s, 1H), 8.02 (s,
2
2
1
,
H), 7.65 (s, 2H), 7.58 (s, 3H), 7.50 (s, 3H), 7.33 (s, 18H), 7.04 (s, 2H), 6.93 (s, 3H), 6.86 – 6.76 (m,
1
3
H), 6.67 (s, 2H). C NMR (100 MHz, DMSO) δ 157.7, 134.0 – 133.9, 133.6, 133.4 , 132.9 – 132.8,
31.9 , 130.1, 128.9, 128.7 , 128.5, 128.4, 128.2 – 128.0, 126.9 , 126.2 , 125.4 , 124.9 , 123.9 , 123.5
123.4– 123.3 , 120.2, 119.7. HRMS (m/z, ESI+): calcd. For C H CuN OP ([M]+) 872.2021;
5
4
41
3
2
found 872.2012. Anal. Calcd. For C H CuF N OP (1018.38): C 63.69, H 4.06, N 4.13; found: C
5
4
41
6
3
3
6
3.38, H 4.27, N 4.43.

[
Cu(Phen-PyIm)(POP)](PF ), P3. The product was a pale yellow powder (0.18g, 0.168 mmol,
6
1
4
8
5
2% yield). H NMR (400 MHz, DMSO) δ 13.91 (s, 1H), 8.74 (s, 1H), 8.43 (s, 1H), 8.17 (s, 2H),
.15 (s, 2H), 7.63 (s, 1H), 7.55 (t, J = 7.2 Hz, 2H), 7.39 (s, 3H), 7.34 (s, 6H), 7.26 (s, 10H), 7.10 (s,
1
3
H), 7.02 – 6.95 (m, 3H), 6.92 (s, 2H), 6.78 (d, J = 6.9 Hz, 2H), 6.68 (s, 2H). C NMR (100 MHz,
DMSO) δ 157.6 , 149.1 , 144.3, 144.1, 139.8, 136.1, 133.6, 132.7, 131.9, 130.4 , 130.0 , 129.7 ,
1
1
9
4
29.1, 128.9 , 128.7, 128.6, 128.5 , 128.0, 127.6, 127.4 , 127.2, 127.1, 125.1, 124.9, 123.5, 122.9 ,
21.60, 120.17 , 119.7. HRMS (m/z, ESI+): calcd. For C H CuN OP ([M]+) 922.2177; found
5
8
43
3
2
22.2184. Anal. Calcd. For C H CuF N OP (1068.44): C 65.20, H 4.06, N 3.93; found: C 65.71, H
5
8
43
6
3
3
.42, N 3.50.
[
Cu(Anthr-PyIm)(POP)](PF ), P4. The product was a yellow powder (0.15 g, 0.140 mmol, 35%
6
1
yield) . H NMR (400 MHz, DMSO) δ 13.83 (s, 1H), 8.86 (d, J = 8.3 Hz, 1H), 8.81 (d, J = 8.2 Hz,
1
1
5
1
1
H), 8.27 (d, J = 9.1 Hz, 2H), 8.10 (s, 1H), 7.87 (d, J = 6.7 Hz, 1H), 7.68 (t, J = 8.1 Hz, 4H), 7.57 (s,
H), 7.41 (s, 3H), 7.25 (d, J = 7.2 Hz, 12H), 7.17 (s, 6H), 6.96 (t, J = 7.1 Hz, 3H), 6.81 (d, J = 6.7 Hz,
1
3
H), 6.59 (s, 2H). C NMR (100 MHz, DMSO) δ 157.7-157.5 , 150.3, 141.4-141.2 , 133.5, 133.2,
31.7, 130.6, 130.4 ,130.3, 129.9, 129.7,129.5, 128.9, 128.7 ,128.4, 128.2,128.0, 127.8, 127.4, 127.1,
26.5, 125.4 , 124.8, 123.6-123.42, 120.0, 119.6. HRMS (m/z, ESI+): calcd. For C H CuN OP
2
5
8
43
3
(
[M]+) 922.2177; found 922.2165. Anal. Calcd. For C H CuF N OP (1068.44): C 65.20, H 4.06,
58 43 6 3 3
N 3.93; found: C 65.33, H 4.32, N 3.51.
2
.3. DFT calculations
The B3LYP exchange-correlation function [30,31] was used to optimize the ground state (S state)
0
geometries of all the complexes using the polarized continuum model (PCM) [32] in CH Cl media
2
2
under the Gaussian 09 [33] program package. The 6-31G* basis set [34,35] was used for the C, H, N,
O, and P atoms. The LANL2DZ basis set [36] was adopted for the Cu atom. On the basis of the
optimized ground geometries, TDDFT method [37, 38] associated with PCM in CH Cl media were
2
2
used to simulate the absorption spectra of complexes studied. The first 200 singlet vertical excitations
were obtained form the TDDFT output file to construct the calculated absorption spectra. Calculated

electronic density plots for the frontier molecular orbitals were prepared using Multiwfn analyzer soft
39] and VMD program [40].
[
3
3
. Results and discussion
.1. Synthesis
The synthetic pathways for the ligands and the corresponding Cu(I) complexes are shown in
Scheme 1. Compounds 2-4 were synthesized by the straightforward cross-coupling reaction under
standard Suzuki coupling condition [41]. The ligands L1-L4 were prepared according to the literature
procedure [42]. Complexes P1-P4 were obtained by the conventional reactions of [Cu(CH CN) ]PF
3
4
6
with one equivalent of the POP ligand and one equivalent of the corresponding ligands. The structures
1
of these complexes were confirmed by H NMR, HRMS and elemental analyses (see experimental
section).
Scheme 1
3
.2. Photophysical properties

Figure 2
Table 1
Absorption
/nm ( 1 0 M cm )
Emission
em /nm / s / %
4
-1
-1
4 -1
4 -1

k
r
/10 s
knr/10 s
abs
P1 260 (2.5), 291 (2.2), 370 (0.24)
P2 284 (1.8), 319 (1.6), 370 (0.40)
P3 255 (5.7), 322 (2.1), 370 (0.67)
P4 285 (3.4), 350 (1.10), 370 (1.4), 388 (1.3)
550
578
581
548
13.9 50.4
58.6 10.5
63.9 10.0
12.8 10.0
3.6
3.6
1.5
1.4
7.0
0.18
0.16
0.78
UV-Vis spectra of the complexes P1-P4 in CH Cl solution at room temperature are shown in
2
2
Figure 2 and the corresponding photophysical data are listed in Table 1. All these complexes exhibit
4
-1
intense bands (  > 1.6 1 0 M-1 cm ) in the region 250-340 nm assigned to π→π* transitions
involving both the PyIm and POP ligands. For the parent complex P1, the weaker and broader band
3
-1
between 340 and 420 nm ((370 nm) 2.4 1 0 M-1 cm ) is observed. This can be typically attributed to
metal Cu to ligand charge transfer (MLCT) transition. The similar absorption transition (MLCT) can
be observed for complexes P2 and P3 in the region of 360-420 nm. In case of complex P4, the MLCT
transition can not be distinguished in low energy region, which is overlapped by the characteristic
absorption band of anthryl group with three vibronic bands at 350, 370, and 388 nm. It is particularly
noteworthy that complexes P2-P4 with naphthyl, phenanthryl, and anthryl groups on the PyIm ligands,

3
respectively, significantly exhibit the enhanced light absorption abilities ((370 nm) (4.0-6.7) 1 0 M-1
-1
cm ) in the lower energy region compared to complex P1, which means that these chromophoric
groups are potentially useful for improving light harvesting capacity of complexes.
Figure 3
The emission spectra of P1-P4 with the excitation wavelengths at 350, 340, 345 and 340 nm,
respectively, at 298 K in PMMA films (10 wt %) are shown in Figure 3 and the corresponding
parameters are also listed in Table 1. Comparing with emission of the free ligands (see Figure 1S,
Supporting Information), the redshifts of the emission wavelength of the corresponding complexes are
observed, suggesting that the emitting states of the complexes are triplet in nature. The parent complex
P1 displays a broad and structureless emission band centered at 550 nm, indicating that the emitting
3
state originates from the lowest excited triplet state (T ) with metal-to-ligand charge transfer ( MLCT)
1
character. The emission peaks of the complexes P2 and P3 centered at 578 and 581 nm, respectively,
are red-shifted by 28-31 nm compared to that of the complex P1 due to the increased conjugation
systems on the PyIm ligands. Despite the similarity in the emission spectral shapes of the complexes P2
and P3 with respect to complex P1, the luminescence lifetimes of the complexes P2 and P3 (=58.6

and 63.9 s , respectively, see Figure 4) are much longer than that of the complex P1 (=13.9 s ), thus
suggesting that the emitting states of the complexes P2 and P3 have predominantly triplet ligand-to-
3
3
ligand charge transfer ( LLCT) character other than MLCT character, as the long-lived emissions
3
from the LLCT state are usually observed [43]. In general, for the typical transition metal complexes,
3
two types of the excited states contribute to the emission, namely, the MLCT state and the ligand-
3
3
centered ( IL) state. Emission from a MLCT state is commonly observed because this state typically
3
3
lies lower in energy than the IL state. However, the IL state may become the lowest energy triplet
state if the ligands are functionalized by organic groups with low triplet energy. Thus, the emission
3
would be predominantly from the LC state. Herein, the functionalization of the PyIm ligands with
3
3
naphthyl and phenanthryl groups stabilizes the LC state in a larger degree. As a consequence, the LC
state becomes the lowest emitting state. Based on the structural similarity compared to complexes P2
3
and P3, it is to be expected that the emitting state of the complex P4 has a LLCT character. However,
3
3
the slightly short lifetime (=12.8 s ) indicates that a mixed LLCT and ( - * ) character is more
appropriate for complex P4. Further support for the assignments of these excited states is
corroborated by the spin density distribution calculations (see below). Despite the presence of the
stronger - stacking interaction caused by the larger - conjugation system in the ligands seems to
somewhat compromise the emission quantum yields of the complexes P2-P4 compared with complex
P1 (see Table 1), the introduction of organic chromophores into the parent Cu(I) complexes is still an
efficient route for designing highly efficient Cu(I)-based dyes having both higher light absorption
abilities and long-lived lifetimes.
Based on the known and values, the radiative rate constant (k ) and nonradiative rate constant
r
(
k ) are calculated and summarized in Table 1. One can see that the values of k and k are the same
nr r nr
order of magnitude as those obtained in the literature [26, 27]. The k for complex P1 is larger than that
r
for complexes P2-P4 by about 4-23-fold, leading to much higher value for complex P1. The values of
k and k for complexes P2 and P3 are almost identical, whereas the values of k and k of the complex
r
nr
r
nr

P4 are larger by 4-5 times than those of P2 and P3. The change in k and k values suggests a change
r
nr
of the character of the emitting transition on going from complex P1 to P4, which are in line with the
assignments of the emitting states as mentioned above.
Figure 4
3
.3. Theoretical calculations
Based the optimized ground geometries of the complexes P1-P4 in CH Cl solution at the
2
2
B3LYP/6-31G(d)/LANL2DZ levels of theory, the energy levels, atomic orbital compositions, and
energy gaps of the HOMO and LUMO of the complexes are shown Figure 5. For the parent complex
P1, the highest occupied molecular orbital (HOMO) is localized on the metal Cu atom and POP ligand,
while the lowest unoccupied molecular orbital (LUMO) mainly distributes on the pyridyl ring.
Therefore, the functionalization of the pyridyl ring would have a significantly influence on the LUMO
level. Here, the induction of naphthyl and phenanthryl groups into the pyridyl ring extends the whole -
conjugation system of the pyridyl-imizole ligand, giving a decrease of the LUMO energy level. As
expected, the LUMO energy levels of the complexes P2 and P3 are calculated to be -2.15 and -2.17 eV
lower than complex P1 (-2.02 eV), respectively, while the HOMO energy levels of the complexes P1-

P3 remain almost unchanged. Unlike complexes P1-P3, the HOMO and LUMO of the complex P4 are
mainly distributed on the anthryl group. The HOMO energy is calculated to be -5.59 eV, which is less
stable than those of the complexes P1-P3. The LUMO energy (-2.21 eV) is calculated to be more
stable compared to P1 and P2. The HOMO-LUMO energy gap can be obtained in the order P1 (3.96
eV) > P2 (3.84 eV) > P3 (3.82 eV) > P4 (3.38 eV), which is consistent with the absorption onset in
low energy region observed in the experiment spectra.
Figure 5
Table 2 (see the attached files)
The calculated spectra of the complexes P1-P4 employing TDDFT method in CH Cl solution are
2
2
shown in Figure 6, together with the experimental spectra for comparison purpose. Table 2 shows the
relative molecular orbital and transition characters (see the detailed information in Figure 2S, Table
1
S-4S in the Supporting Information). The calculated spectra reproduced nicely the shapes and
positions of the experimental spectra. For complex P1, the first lowest excited state S , calculated at
1
3
74 nm, mainly composed of HOMO L UMO transition, is in good agreement with the experimental

shoulder centered at about 370 nm. A composition analysis of the HOMO and LUMO (see Table 1S,
Supporting Information) clearly shows that this excitation can be ascribed as MLCT/LLCT character,
supporting the original assignment of the absorption bands in the experiment. The experimental
absorption bands in the region of 250-320 nm are mainly originated from the combination transitions
of the S , S , S , S , S and S states with the ( * ) transition character within both POP and PyIm
6
7
11
15
19
28
ligands but mixed with little contribution of MLCT/LLCT transition (see Table 2). For complex P2,
the experimental shoulder at > 340 nm appears to be composed of the S , S , and S with the
1
2
3
calculated peaks at 353, 365, and 384 nm, respectively, showing primary LLCT character from the S2
state since the oscillator strength of this transition is largest (f=0.1560), with the mixing of some
MLCT character originated from the S and S states. The calculated absorption bands in the range
1
3
2
60-340 nm are dominated by the mixed transitions of the S , S , S , and S states with the transition
6 8 10 13
character of ( * )/LLCT. For complex P3, the S , S , and S states, calculated at 355, 368, and 385
1
2
3
nm, respectively, are responsible for the experimental shoulder at > 360 nm with LLCT/MLCT
character. The two intense transitions at 313 and 305 nm, originating from the S and S , respectively,
7
9
appear to give rise to the 310-360 nm shoulder of the experimental spectrum with ( * )/LLCT
character. An intense band calculated at 248 nm, originating from the S state, can be related to the
6
7
absorption peak experimentally found at 260 nm with ( * )/LLCT character. With respect to complex
P4, the characteristic absorption bands of the anthryl moiety in the range of 340 -400 nm in the
experimental spectrum are reflected by the calculated two absorption bands at 394 and 428 nm from
the S and S states, respectively, which are originated from the starting orbitals of the HOMO
1
2
[
π(Anth)] to the arriving orbitals of LUMO/LUMO+1[π*(Anth)] with ( * ) character. It is noteworthy
that the S state calculated at 384 nm with MLCT/LLCT character has a comparative contribution to
3
this experimental shoulder, which is not well distinguished in the experimental spectrum. The weak
shoulder experimentally found in the range 270-320 nm is composed of a series of lower intensity
transitions, dominated by the S , S , S , S , S , S and S (see inset of Figure 6d). These
1
1
12
13
17
19
20
25

transitions have ( * )/LLCT/MLCT character, originating from the starting orbitals of the HOMO-
/HOMO-2/HOMO-4 combinations to the arriving orbitals of
1
LUMO/LUMO+1/LUMO+3/LUMO+4/LUMO+5/LUMO+6. A very intense band calculated at 248
nm originated from the S state is assigned as ( * )/LLCT character, which is related to the 256 nm
8
0
band of the experimental spectrum. Notably, this 256 nm absorption band exhibits the oscillator
-1
-1
strength as high as 1.3140, which is also reflected by a very high value of 142 500 M cm in the
experiment.

Figure 6
To gain further understanding of the nature of the triplet emitting states, the triplet excited state
geometries of the complexes P1-P4 were optimized using the spin-unrestricted U-DFT approach,
imposing a spin multiplicity of 3. The calculated emission energy based on S CF method [44] was
calculated to be 1.96, 1.82 and 1.83 eV, for complexes 1-3, respectively, which is in reasonably good
agreement with the corresponding experimental values (2.25, 2.15 and 2.13 eV) with exception of the
complex 4 that its calculated emission energy (1.41 eV) was considerably lower than the experimental
value (2.26 eV). The large discrepancy between calculated and experimental values for complex 4
might be partly explained by the large the structural relaxation of the excited state. Figure 7 displays
the spin density distributions of all the complexes based on the optimized lowest triplet state geometry.
For the parent complex P1, the unpaired electron spin-density distributions in the T state (Cu, 0.54e;
1
POP, 0.32e; PyIm, 1.15e) clearly illustrates the higher contribution of the d(Cu) to the PyIm ligand
with some contribution of POP ligand to the PyIm ligand and confirm the emission behavior of
3
3
MLCT/ LLCT character. The emitting state of the complexes P2 and P3 undoubtedly can be assigned
3
as the LLCT character with PyIm N aph transition for P2, PyIm P hen transition for P3, which is also

supported by the spin-density distributions in T state (Cu, 0.005e; POP, 0.003e; PyIm, 0.79e; Naph,
1
1
.30e for P2; Cu, 0.006e; POP, 0.003e; PyIm, 0.72e; Phen, 1.26e for P3). With respect to complex P4,
the unpaired electrons in T state are dominantly localized on anthryl group with a small contribution
1
from the PyIm ligand (Cu, 0.002e; POP, 0.002e; PyIm, 0.28e; Anth, 1.82e), indicating that the
3
*
3
emitting state of the complex P4 has a LC  character with a LLCT contribution from the PyIm
ligand to anthryl group. These theoretical results further support the assignments of the nature of the
emitting excited state.
Figure 7
4
. Conclusion
In summary, Cu(I) complexes containing the pyridyl-imidazole (PyIm) ligand functionalized by
the different organic chromophores have been synthesized and characterized. Complexes P2-P4
exhibit the enhanced light absorption ability in the visible region compared with the parent complex P1
with the long-lived excited state lifetimes, which are potentially useful for the harvesting of solar
3
3
energy. The emitting state of the complexes can be altered from the MLCT/ LLCT state (complex
3
3
* 3
P1) to the LLCT state (complexes P2 and P3) to the (  )/ LLCT state (complex P4) by modulating
the orbital energy levels of the ligands. These structure-property relationships are further rationalized
by DFT and TDDFT calculations.
Acknowledgements. The work was supported by the National Natural Science Foundation of China
(
No. 21563013) and the Natural Science Foundation of Jiangxi Province (No. 20151BAB203006).

The authors thank the Guizhou University High Performance Computation Chemistry Laboratory
GHPCC) for help with computational studies.
(
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Figure Captions
Figure 1. Molecular structures of Cu(I) complexes containing different aryl groups in this study.
Scheme 1. The synthetic pathways of the PyIm derivatives and the resulting Cu(I) complexes. [a]
Purchased from market.
-5
Figure 2. Absorption spectra in dichloromethane (C=1 1 0 mol/L) at 298K of complexes P1-P4.
Figure 3. Normalized emission spectra of complexes P1-P4 in PMMA films (10wt%).
Figure 4. Emission decay curves in PMMA films (10wt%) at 298 K for P1 (a), P2 (b), P3 (c), P4 (d).
Red solid lines drawn through the data points correspond to the best exponential fitting. Excitation
wavelength =370 nm, emission monitored at 550, 580, 582, and 545 nm for P1-P4, respectively.

Figure 5. The energy levels and atomic orbital compositions calculated for the HOMO and LUMO of
complexes P1-P4.
Figure 6. Comparison of the calculated (red line) and experimental (black line) absorption spectra in
CH Cl solution for P1 (a), P2 (b), P3 (c), and P4 (d). Red vertical lines correspond to oscillator
2
2
strength of calculated singlet-singlet transitions.
Figure 7. Spin density distribution contours (isovalue = 0.004) for the lowest triplet state T of
1
complexes P1-P4. The values of the unpaired-electron spin-density population are depicted together
with the electronic nature of the states.
Table captions
Table 1. Photophysical properties of complexes P1-P4.
Table 2. Electronic absorptions of complexes P1-P4 in CH Cl based on TDDFT calculations at the
2
2
(
B3LYP)/6-31g*/LANL2DZ level, together with the experimental values.