Cu(I) complexes regulated by N-heterocyclic ligands: Syntheses, structures, fluorescence and electrochemical properties

Article abstract of DOI:10.1016/j.saa.2018.06.003

Three mononuclear Cu(I) complexes, namely, [Cu(2-PBO)(PPh₃)₂]·ClO₄·2CH₂Cl₂ (1), [Cu(3-PBO)(PPh₃)₂(ClO₄)]·CH₂Cl₂ (2) and [Cu(PBM)(PPh₃)₂]·ClO₄ (3) (2-PBO = 2-(2′-Pyridyl)benzoxazole, 3-PBO = 2-(3′-Pyridyl)benzoxazole, PBM = 2-(2′-Pyridyl)benzimidazole, PPh₃ = triphenylphosphine) have been synthesized and characterized by elemental analyses, IR, ¹H NMR, ¹³C NMR, X-ray single crystal diffraction and thermal analysis. Photoluminescent investigation shows that complexes 1–3 exhibit distinct tunable light green (512 nm)-to-yellow (557 nm) photoluminescence by varying the N-heterocyclic ligands. Three complexes show intense 2-PBO-based yellow, 3-PBO-based light green and intense PBM-based bright green luminescence upon irradiation with a standard UV lamp (λ_ex = 254 nm) at room temperature. Moreover, the electrochemical properties of 1–3 have been investigated by cyclic voltammetry. The results suggest the frontier molecular orbits and the HOMO-LUMO energy gaps of these cuprous complexes are effectively adjusted through the introduction of different N-heterocyclic ligands, thus achieving the selective luminescence of the cuprous complexes.

Full text of DOI:10.1016/j.saa.2018.06.003

Accepted Manuscript
Cu(I) complexes regulated by N-heterocyclic ligands: Syntheses,
structures, fluorescence and electrochemical properties
Shanshan Mao, Xintong Han, Chuang Li, Yuling Xu, Kesheng
Shen, Xinkui Shi, Huilu Wu
PII:
DOI:
Reference:
S1386-1425(18)30535-3
doi:10.1016/j.saa.2018.06.003
SAA 16162
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 29 December 2017
Revised date:
Accepted
date:
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1 June 2018
Please cite this article as: Shanshan Mao, Xintong Han, Chuang Li, Yuling Xu, Kesheng
Shen, Xinkui Shi, Huilu Wu , Cu(I) complexes regulated by N-heterocyclic ligands:
Syntheses, structures, fluorescence and electrochemical properties. Saa (2017),
doi:10.1016/j.saa.2018.06.003
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ACCEPTED MANUSCRIPT
Cu(I) complexes regulated by N-heterocyclic ligands: syntheses,
structures, fluorescence and electrochemical properties
Shanshan Mao, Xintong Han, Chuang Li, Yuling Xu, Kesheng Shen, Xinkui Shi and
Huilu Wu
School of Chemical and Biological Engineering, Lanzhou Jiaotong University,
Lanzhou, Gansu, 730070, People’s Republic of China
Abstract:
Three
mononuclear
Cu(I)
complexes,
namely,
.
[Cu(2-PBO)(PPh3)2].ClO4 2CH2Cl2 (1), [Cu(3-PBO)(PPh3)2(ClO4)].CH2Cl2 (2) and
[Cu(PBM)(PPh3)2].ClO4 (3) (2-PBO = 2-(2'-Pyridyl)benzoxazole, 3-PBO =
2-(3′-Pyridyl)benzoxazole, PBM
=
2-(2′-Pyridyl)benzimidazole, PPh3
=
triphenylphosphine) have been synthesized and characterized by elemental analyses,
1
IR, H-NMR, 13C-NMR, X-ray single crystal diffraction and thermal analysis.
Photoluminescent investigation shows that complexes 1-3 exhibit distinct tunable
light green (512nm)-to-yellow (557nm) photoluminescence by varying the
N-heterocyclic ligands. Three complexes show intense 2-PBO-based yellow,
3-PBO-based light green and intense PBM-based bright green luminescence upon
irradiation with a standard UV lamp (λex = 254 nm) at room temperature. Moreover,
the electrochemical properties of 1-3 have been investigated by cyclic voltammetry.
The results suggest the frontier molecular orbits and the HOMO-LUMO energy gaps
of these cuprous complexes are effectively adjusted through the introduction of
different N-heterocyclic ligands, thus achieving the selective luminescence of the
cuprous complexes.
Keywords:
Copper(I)
complex;
Photoluminescent;
Crystal
structures;
Electrochemical; N-heterocyclic

Correspondence author. E-mail : wuhuilu@163.com
1

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1. Introduction
The construction and characterization of copper(I) complexes containing
N-heterocyclic and triphenylphosphine ligands have drawn much attention in recent
years, due to their interesting photophysical properties, relative abundance, low cost,
electroluminescence, and solar energy conversion, [1] as well as
potentially-widespread applications in materials science, catalysis, synthetic and
medicinal chemistry. [2] The selection of N-heterocyclic ligand is paramount because,
with it, copper(I) complexes possess high stability to promote luminescence from
metal-to-ligand charge transfer (MLCT) and long fluorescence lifetime. [3] These
ligands can be easily modified by introducing different substituents, possessing a
variety of electronic, steric and conformational impacts on both the coordinated
chelate and coordination core. In addition, the phosphine auxiliary ligand play a
positive role in stabilizing the Cu(I) center, although not involved in the MLCT
transition, and exert important effects on the photophysical properties of Cu(I)
complexes. [4]
Benzoxazole and benzimidazole are typical N-heterocyclic ligand with nitrogen as
the donor atom and has proved to be a very useful intermediate for the development
of molecules of medicinal chemistry. [5] Recently the discovered benzoxazole and
benzimidazole have unique π-conjugated structure, the binding force of metal ions
with N and O atoms is different. In general, N-donor azole groups have shown strong
coordination ability to the first-row transition metal ions. [6] Accordingly, in order to
enhance the phosphorescence intensity of copper(I) complexes, the usually-used
N-heterocyclic ligands and phosphine ligands are simultaneously coordinated. [7]
N-heterocyclic ligands, due to the diversity of coordination modes and configurations,
are typically used as neutral ligands in the synthesis of complexes. [8] The
counterions are introduced to balance the charge when studying the neutral ligand,
which not only affect the coordination environment of the metal ions, but also the
overall structure of the complex. [9]
In this paper, three mononuclear copper(I) complexes with 2-PBO, 3-PBO and
2

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PBM ligands have been synthesized and characterized. In addition, the luminescence,
electrochemical investigations and thermal stabilities have been studied. Notably,
with the addition of different ligand, tunable light green-to-yellow emission was
realized.
2. Experimental section
Caution: Although no problems were encountered in this work, transition metal
perchlorate salts are potentially explosive and should thus be prepared in small
quantities and handled with care.
2.1. Materials and general methods
All reactions were carried out under a nitrogen atmosphere, using anhydrous
solvents or solvents treated with an appropriate drying reagent. Commercially
available reagents were used without further purification unless otherwise stated. The
C, H and N elemental analyses were determined using a Carlo Erba 1106 elemental
1
analyzer. H and 13C NMR spectra were obtained with a Mercury plus 400 MHz
NMR spectrometer with TMS as internal standard and DMSO-d6 as solvent. The IR
spectra were recorded in the 4000-400 cm−1 region with a Nicolet FT-VER-TEX 70
spectrometer using KBr pellets. Solid-state emission and excitation spectra were
obtained with a 970-CRT fluorescence spectrophotometer using pure powder samples
at room temperature. Thermogravimetric analysis (TGA) was carried out on
METTLER TOLEDO TGA1 thermal analyzer from room temperature to 900 °C with
a heating rate of 10 °C/min under N2 atmosphere. Electrochemical measurements
were taken on an LK2005A electrochemical analyser under nitrogen at room
temperature with a glassy carbon working electrode, a platinum-wire auxiliary
electrode, and an Ag/AgCl reference electrode ([Cl−] = 1.0 mol L-1), standardized by
the redox couple ferrocene/ferricinium (Fc/Fc+). The electroactive component was at
1.0×10-3 mol dm-3 concentration with tetrabutylammonium perchlorate (TBAP) (0.1
mol dm-3) as the supporting electrolyte in DMF solution.
2.2. Precursor and Ligands syntheses
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[Cu(CH3CN)2(PPh3)2][ClO4] was synthesized on the basis of literature procedures.
[10] The ligands 2-PBO, 3-PBO and PBM were prepared according to the literature
procedure with little modification. [11]
[Cu(CH3CN)2(PPh3)2][ClO4]. (81 % yield). Anal. Calcd (%) for C40H36ClCuN2O4P2:
C 62.42, H 4.71, N 3.64. Found: C 62.38, H 4.69, N 3.60. IR (KBr pellet, ν/cm−1):
3051 (w), 1479 (w), 1434 (m), 1094 (s), 746 (m), 695 (m), 513 (m).
2-PBO. A mixture of O-aminophenol (0.07 mol, 7.64 g), 2-picolinic acid (0.07 mol,
8.62 g) and 90 mL of polyphosphoric acid (PPA) were stirred at 200 °C for 5 h. (87 %
yield). M.p: 105-106 °C. 1H NMR (400 MHz, CDCl3) δ: 8.81-8.82 (d, 1H), 8.35-8.37
(d, 1H), 7.81-7.90 (m, 2H), 7.66-7.72 (m, 1H), 7.30-7.45 (m, 3H). IR (KBr pellet,
ν/cm−1): 3060 (m), 1583 (s), 1452 (s), 1243 (s), 934 (s), 811 (s), 761 (s).
3-PBO. A mixture of O-aminophenol (0.07 mol, 7.64 g), 3-picolinic acid (0.07 mol,
8.62 g) and 90 mL of polyphosphoric acid (PPA) were stirred at 200 °C for 5 h. (83 %
1
yield). M.p: 111-112 °C. H NMR (400 MHz, CDCl3) δ: 9.47 (s, 1H), 8.75-8.77 (m,
1H), 8.48-8.51 (m, 1H), 7.77-7.82 (m, 1H), 7.58-7.63 (m, 1H), 7.36-7.47 (m, 3H). IR
(KBr pellet, ν/cm−1): 3058 (w), 1574 (s), 1454 (s), 1244 (s), 925 (m), 814 (s), 761 (s).
PBM. A mixture of O-Phenylene diamine (0.1 mol, 10.89 g), 2-picolinic acid (0.1
mol, 12.3 g) and 120 mL of polyphosphoric acid (PPA) were stirred at 180 °C for 6 h.
1
(85 % yield). M.p: 224-226 °C. H NMR (400 MHz, CDCl3) δ: 13.11 (s, 1H),
8.75-8.78 (m, 1H), 8.39-8.41 (m, 1H), 8.02-8.04 (m, 1H), 7.76-7.77 (m, 1H),
7.25-7.60 (m, 4H). IR (KBr pellet, ν/cm−1): 3047 (w), 1598 (s), 1458 (s), 973 (m), 813
(s), 770 (s).
2.3. Complexes syntheses
The three complexes were prepared using a similar procedure. The ligands (0.1
mmol: 2-PBO, 19.6 mg; 3-PBO, 19.6 mg; PBM, 19.5 mg) in CH2Cl2 (4 mL) was
added to complex [Cu(CH3CN)2(PPh3)2][ClO4] (0.1 mmol, 77 mg) in CH2Cl2 (6 mL),
and the mixture was stirred at ambient temperature for 5 h to give a yellow solution.
The solvent was then removed at reduced pressure. The resultant residue was again
dissolved in 5 mL CH2Cl2, and slow diffusion of hexane into the above solution.
Yellow crystals suitable for X-ray diffraction studies were obtained after 3-5 days.
4

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Complex 1. (85% yield). Anal. Calcd (%) for C50H42Cl5CuN2O5P2: C 57.00, H 4.02,
N 2.66. Found: C 56.96, H 4.01, N 2.63. IR (KBr pellet, ν/cm−1): 3057 (w), 1545 (m),
1476 (w), 1434 (m), 1244 (m), 1091 (s), 745 (m), 696 (m), 511 (m). 1H NMR (400
MHz, DMSO-d6): δ 7.84 (s, 1H), 7.76 (s, 1H), 7.59 (s, 1H), 7.40 (d, 2H, J = 8.0 Hz),
7.37 (s, 1H), 7.21-7.32 (m, 20H), 7.17-7.20 (m, 12H). 13C NMR (400 MHz,
DMSO-d6): δ 163.73, 155.37, 147.60, 135.38, 134.57, 132.11, 130.92, 129.73, 129.21,
128.34, 127.94, 127.15, 123.15, 122.58, 121.35.
Complex 2. (79% yield). Anal. Calcd (%) for C49H40Cl3CuN2O5P2: C 60.75, H 4.16,
N 2.89. Found: C 60.72, H 4.13, N 2.83. IR (KBr pellet, ν/cm−1): 3054 (w), 1546 (m),
1
1478 (w), 1433 (m), 1244 (m), 1101 (s), 746 (m), 694 (m), 511(m). H NMR (400
MHz, DMSO-d6): δ 7.93-7.99 (t, 2H), 7.56-7.59 (d, 2H), 7.36-7.42 (t, 2H), 7.21-7.30
(m, 20H), 7.15-7.19 (m, 12H). 13C NMR (400 MHz, DMSO-d6): δ 165.41, 151.38,
146.99, 146.31, 143.68, 138.47, 134.44, 134.24, 130.53, 127.96, 127.64, 126.72,
126.02, 124.33, 119.91, 110.59.
Complex 3. (91% yield). Anal. Calcd (%) for C48H39ClCuN3O4P2: C 65.31, H 4.45,
N 4.76. Found: C 65.29, H 4.41, N 4.72. IR (KBr pellet, ν/cm−1): 3055 (w), 1597 (m),
1
1477 (w), 1435 (m), 1099 (s), 744 (s), 694 (s), 511 (m). H NMR (400 MHz,
DMSO-d6): δ 11.50 (s, 1H), 8.04 (d, 1H, J = 7.6 Hz), 7.58-7.62 (m, 3H), 7.23-7.41 (m,
20H), 7.18-7.20 (m, 12H), 6.96 (t, 1H, J = 8.0 Hz), 6.82 (t, 1H, J = 7.2 Hz). 13C NMR
(400 MHz, DMSO-d6): δ 149.23, 146.29, 142.40, 140.82, 139.56, 132.73, 131.47,
130.42, 128.28, 126.32, 124.96, 118.60, 115.89, 115.23.
2.4. X-ray crystallography
Crystallographic data of complexes 1-3 were collected on a Bruker Smart CCD
diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at
296 K. Data reduction and cell refinement were performed using the SMART and
SAINT programs. [12] The absorption corrections are carried out by the empirical
method. The crystal structures of 1-3 were solved by direct methods and refined by
full-matrix least-squares against F2 of data using SHELXTL program. [13] All
non-hydrogen atoms were refined by using anisotropic thermal parameters. All
hydrogen atoms were included in calculated position and refined with isotropic
5

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thermal parameters riding on the parent atoms. Crystal parameters and details of the
final refinement parameters are shown in Table 1. The selected bond lengths and bond
angles for complexes 1-3 are listed in Table 2.
[Insert Table 1]
[Insert Table 2]
3. Results and discussion
The copper(I) complexes were prepared by the reaction of
[Cu(CH3CN)2(PPh3)2][ClO4] with N-heterocyclic ligands. Synthetic routes of Cu(I)
complexes are shown in Scheme 1. The elemental analysis of complexes 1-3 is in
good agreement with the theoretical compositions. The Cu(I) complexes 1-3 are fairly
air-stable both in solution and in the solid state. (Fig. S1-S3) Complexes 1-3 are
remarkably soluble in polar aprotic solvents such as DMF, DMSO, dichloromethane
and MeCN; slightly soluble in ethanol and methanol; insoluble in water, diethyl ether,
petroleum ether and hexane.
[Insert Scheme 1]
3.1. X-ray structure characterization
Crystal structures of 1 and 3. Single-crystal X-ray diffraction analysis reveals that
complex 1 in the triclinic space group P-1 comprises an ionic structure containing
-
[Cu(2-PBO)(PPh3)2]+ and ClO4 (Fig. 1a). In asymmetric unit consists of one copper(I)
-
ion, one 2-PBO ligand, two triphenylphosphine molecules as well as one free ClO4
counterion and two guest dichloromethane molecules. The Cu(I) ion is linked to the
two nitrogen atoms (N1, N2) from 2-PBO ligands and two phosphine atoms (P1, P2)
from triphenylphosphine affording a N2P2 four-coordinated environment. The
coordination polyhedron adopted by this environment is characterized using
τ4-descriptor for four coordinated ions. [14] The τ4 values of complex 1 is 0.82 (α =
P(1)-Cu(1)-P(2) = 127.69°, β = N(2)-Cu(1)-P(1) = 116.45°). Indicate that the
6

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coordination geometry around Cu(I) is best described as a distorted trigonal pyramidal,
which are comparable to those reported in analogous complexes containing
N-heterocyclic chelating and triphenylphosphine, i.e. [Cu(bptzH)(PPh3)2][ClO4] (τ4 =
0.83) [1] and [Cu(pbb)(PPh3)2][BF4] ( τ4 = 0.82). [4] In complex 1, adjacent
[CuPBO(PPh3)2]+ motif are connected together by C-H…π (3.1-3.57 Å) interactions to
generating a 2D layer supramolecular framework (Fig. 2a). The 2D sheets are
interlinked to produce a 3D supramolecular framework through C-H…π (3.64 Å)
stacking interactions and two dichloromethane molecules filled inside each unit (Fig.
2b).
[Insert Fig. 1]
[Insert Fig. 2]
Compared to the 1, complex 3 present the similar arrangement with some small
discrepancy (Fig. 1b). Firstly, in asymmetric unit of complex 3 consists of one
copper(I) ion, one PBM ligand, two triphenylphosphine molecules as well as one free
-
ClO4 counterion. Secondly, the τ4 values of 3 is 0.86 (α = P(2)-Cu(1)-P(1) = 126.93°,
β = N(1)-Cu(1)-P(2) = 111.78°), which are comparable to those reported in analogous
complexes containing N-heterocyclic chelating and triphenylphosphine, i.e.
[CuL2(PPh3)2][BF4] ( τ4 = 0.86). [15] Indicate that the coordination geometry around
Cu(I) is best described as a distorted trigonal pyramidal but the 1 is more distorted,
which ascribes to the presence of dichloromethane molecules in the complex 1 unit.
Furthermore, adjacent [CuPBM(PPh3)2]+ motif are connected together by C-H…π
(2.84-3.13 Å) interactions to generating a 2D layer supramolecular framework (Fig.
3a). The 2D sheets are interlinked to produce a 3D supramolecular framework
-
through C-H…π (3.0-3.58 Å) stacking interactions and ClO4 counterions filled inside
each unit (Fig. 3b).
[Insert Fig. 3]
Crystal structure of 2. Complex 2 crystallizes in the triclinic space group P-1 and
7

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its asymmetric unit consists of a mononuclear [Cu(3-PBO)(PPh3)2(ClO4)] motif and
one guest dichloromethane molecule (Fig. 4). The coordination environment around
the copper(I) center is best portrayed as a NOP2 distorted trigonal pyramidal
environment (τ4 = 0.84 where α = P(2)-Cu(1)-P(1) = 127.35° and β = P(1)-Cu(1)-O(1)
= 113.49°), ligated by one nitrogen atom (N1) from 3-PBO, one oxygen atom (O1)
from the perchlorate anion and two phosphine atoms (P1, P2) from two distinct PPh3.
The bond lengths are Cu1-N1 = 2.088, Cu1-P1 = 2.2565 and Cu1-P2 = 2.2535 Ǻ,
respectively, which, to the best of our knowledge, are similar to the corresponding
values for reported Cu(I) complexes. [16] The Cu1-O1 (2.295 Å) distances in 2 is
compatible to those observed in other related perchlorate-coordinated Cu(I) complex
[Cu(OClO3)(CO)(H2CPz’2)]
(Cu(I)-O(ClO4)
=
2.24
Å),
[17]
and
[{Cu(L1’’)(CO)}2(μ-ClO4)](ClO4) (Cu1-O(ClO4)= 2.252 Å and Cu2-O(ClO4) = 2.311
Å). [18] It is notable that complex 3 unit extends to a 2D supramolecular layered
framework through C-H…π (2.83-3.37 Å) interactions as shown in Fig. 5a. The 2D
sheets are interlinked to produce a 3D supramolecular framework through C-H…π
(2.77 Å) and C-H…Cl (3.44 Å) stacking interactions (Fig. 5b).
[Insert Fig. 4]
[Insert Fig. 5]
3.2. IR spectra
The IR spectrum (Fig. S4) exhibits the main characteristic bands of the 2-PBO,
3-PBO and PBM fragments for complexes 1-3. The weaker peaks at 1244 cm-1 for
complex 1 and 2 are attributed to the ν(C-O) of the oxazole rings, [19] characteristic
bands at 1545 cm-1 for complex 1, 1546 cm-1 for complex 2 and 1597 cm-1 for
complex 3 are assigned to the ν(C=N) stretching vibration of the oxazole and
imidazole rings, [20] respectively. The middle peaks at 511 cm-1 for complexes 1-3
may be stretching vibrations of Cu-P bonds. The perchlorate complex 1 and 3 exhibit
strong bands at 1091-1099 cm-1, suggest the stretching vibration of non-coordinated
-
ClO4 anion in the complexes, while the intense band for complex 2 at 1101 cm-1 is
8

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-
attributed to the stretching vibration of coordinated ClO4 anion. [10, 21] This fact
agrees with the result determined by X-ray diffraction.
3.3. Thermal properties
Thermogravimetric analysis was carried out between 30 and 900 °C in a static
atmosphere of nitrogen to investigate the thermal stabilities of complexes 1-3 as
depicted in Fig. 6. The decomposition of the three complexes starts at 207-248 °C and
is completed at ca 813-825 °C, and the weight of the last residue is in agreement with
that required for CuO. The TGA illustrates that the complexes 1-3 possesses good
thermal stability and thus render it as a potential candidate in display devices and
solar-energy conversion.
[Insert Fig. 6]
3.4. Photophysical properties
Copper(I) complexes are promising candidates for photoactive materials with
potential applications as luminescent materials. [22] Thus, the solid-state
photoluminescence properties of complexes 1-3 and the corresponding free ligands
have been investigated using pure powder samples at room temperature under the
same experimental conditions. As shown in Fig. 7, The complexes and ligands exhibit
broad emission and excitation band, all the excitation band with a maximum at
355-414 nm, which is attributed to the π-π* electron transition of the ligands. [23] The
free 2-PBO and PBM ligands display violet emissions at 402 nm and 3-PBO display
blue emissions at 472 nm, respectively, upon excitation at 355 nm, which may be
attributed to π*-π transition. [24] For complexes 1 and 3, the maximal emission peaks
occur at 557 nm (λex = 414 nm, yellow luminescence) and 520 nm (λex = 398 nm,
green luminescence) respectively. Compared with the free 2-PBO and PBM ligands, a
red-shift of 149 nm was observed for complex 1 and 117 nm for complex 3, which
may be attributed to metal-to-ligand charge-transfer [MLCT]. [3] However, the
fluorescence spectra of 2 show two bands upon excitation at λex = 360 nm. The
9

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high-energy band is centred at about 481 nm in the spectra, while the low-energy
band is at 512 nm (light green luminescence). The high-energy emission is due to the
fluorescence emission from a ligand-centerd π*-π transition of 3-PBO, while the latter
red-shift of 39 nm emission bands are assigned to the MLCT excited state. The λmax
values of copper(I) complexes 1-3 are in the order 1 > 3 > 2, which is suggested that
the arylation of the N-heterocyclic group might has some important impacts on the
luminescence properties of cuprous complexes. We found that the increase in
electron-donating capacity of the nitrogen ligands induces an obvious red-shift on the
emission spectra.
[Insert Fig. 7]
3.5. Electrochemical properties
The electrochemical properties of 1-3 were studied by cyclic voltammetry in DMF
solution. Experimental results show that complexes 1-3 all has a redox couple. The
data are collected in Table 3 and a voltammogram is shown in Figure S5. The
separation between the cathodic and anodic peak potentials ΔEp (Epa - Epc) is greater
than 60 mV, and the current ipa/ipc > 1, indicating an electrochemically irreversible
process which can be assigned to the anodic waves were associated with a
Cu(I)-based oxidation process (CuI/CuII), and the cathodic waves with the
ligand-based reduction process ([Cu(L)]/[Cu(L•)]-). [25] The data of HOMO/LUMO
energy levels and the energy gap (Eg) of the complexes 1-3 are collected in Table 3.
The HOMO/LUMO levels of the complexes 1-3 were calculated with formulas:
EHOMO = - (4.8 ev + EOX - E(Fc/Fc+)) and ELUMO = EHOMO + Eg, [26] the energy gap
(Eg) was calculated as Eg = 1240/λonset (eV) [27]. These calculations give the HOMO
_
_
_
energy levels to be 4.428 for 1, 4.495 for 2 and 4.467 for 3. The energy gaps
between the LUMO and the HOMO levels of 1-3 were thus derived to be 2.43, 2.84
and 2.59 eV, respectively. As can be seen from Table 3, with the order of complexes 2,
3 and 1, the HOMO level of the complexes increases accordingly, and the energy gaps
is decreased accordingly. This indicates that the complex 2 is most stable, followed by
the complexes 3 and 1. Moreover, due to energy gaps narrowing, the fluorescence
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should be redshift. This result is consistent with the experimental results of the
fluorescence properties of the complexes. To sum up, the frontier molecular orbits and
the HOMO-LUMO energy gaps of these cuprous complexes are effectively adjusted
through the introduction of different N-heterocyclic ligands, thus achieving the
selective luminescence of the cuprous complexes.
[Insert Table 3]
4. Conclusion
In summary, three mononuclear complexes of copper(I) with N-heterocyclic
ligands were synthesized and characterized. The structural analysis revealed that in
complexes 1-3, the all Cu(I) ions are four-coordinated and the geometric structure
around the central Cu(I) atom possessing a slightly distorted trigonal pyramidal
geometry. In addition, these Cu(I) complexes exhibit distinct tunable light
green-to-yellow photoluminescence, attributed to the MLCT excited state. Moreover,
the selective luminescence of the cuprous complex is achieved by the introduction of
different N-heterocyclic ligands. The investigation indicates these copper(I)
complexes are the new promising luminescence materials in the future.
Acknowledgments
The present research was supported by the National Natural Science Foundation of
China (Grant No. 21367017), Foundation of A Hundred Youth Talents Training
Program of Lanzhou Jiaotong University (Grant No. 152022) and the Science and
Technology Program of Gansu Province (Grant No. 17JR5RA090).
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Table 1 Crystal and structure refinement data for complexes 1-3.
Complex
1
C50H42Cl5CuN2O5P2
1053.59
Triclinic
P-1
2
C49H40Cl3CuN2O5P2
968.66
3
C48H39ClCuN3O4P2
882.75
Empirical formula
Molecular weight
Crystal system
Space group
a (Å)
Triclinic
P-1
Triclinic
P-1
10.559(15)
12.683(19)
18.90(3)
75.33(3)
88.62(3)
80.71(2)
2416(6)
2
11.7103(15)
12.6368(17)
16.629(2)
87.499(2)
78.906(2)
76.064(2)
2343.6(5)
2
11.506(5)
12.772(6)
16.401(8)
82.244(7)
75.018(7)
66.439(6)
2132.8(17)
2
b (Å)
c (Å)
α (°)
β (°)
γ (°)
V (Å3)
Z
Dcalcd (g/cm3)
μ (mm−1)
1.448
1.373
1.375
0.844
0.753
0.698
F(000)
1080
996
912
Total reflections
Unique reflections
Rint
GOF on F2
R1 (I > 2σ(I))
wR2 (I > 2σ(I))
Δρmax, eÅ–3
Δρmin, eÅ–3
11447
11951
11378
8042
8239
7943
0.0339
0.959
0.0230
0.0192
1.045
1.029
0.0663
0.1922
0.961
0.0651
0.0462
0.1878
0.1342
0.760
0.812
-0.752
-1.602
-0.659
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Table 2 Selected bond distances (Å) and angles (°) for complexes 1-3.
Parameter
Value
Parameter
Value
1
Cu(1)-N(2)
Cu(1)-N(1)
Cu(1)-P(1)
Cu(1)-P(2)
P(1)-Cu(1)-P(2)
2
2.100(4)
2.121(4)
2.207(3)
2.255(3)
127.69(7)
N(2)-Cu(1)-N(1)
N(2)-Cu(1)-P(1)
N(1)-Cu(1)-P(1)
N(2)-Cu(1)-P(2)
N(1)-Cu(1)-P(2)
79.15(17)
116.45(14)
115.05(15)
106.61(13)
100.52(13)
Cu(1)-N(1)
Cu(1)-P(2)
Cu(1)-P(1)
Cu(1)-O(1)
N(1)-Cu(1)-P(2)
3
2.088(4)
N(1)-Cu(1)-P(1)
P(2)-Cu(1)-P(1)
N(1)-Cu(1)-O(1)
P(2)-Cu(1)-O(1)
P(1)-Cu(1)-O(1)
109.29(14)
127.35(5)
88.0(2)
2.2535(15)
2.2565(14)
2.295(6)
102.64(18)
113.49(18)
108.95(14)
Cu(1)-N(2)
Cu(1)-N(1)
Cu(1)-P(2)
Cu(1)-P(1)
N(2)-Cu(1)-N(1)
2.092(3)
N(2)-Cu(1)-P(2)
N(1)-Cu(1)-P(2)
N(2)-Cu(1)-P(1)
N(1)-Cu(1)-P(1)
P(2)-Cu(1)-P(1)
107.95(8)
111.78(7)
110.45(8)
109.64(7)
126.93(4)
2.102(2)
2.2610(11)
2.2650(11)
80.28(10)
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Table 3 The HOMO and LUMO energy levels of the complexes 1-3.
ipa/ipc
λonset (nm)
EOX (v)
EHOMO (ev)
Eg (ev)
ELUMO (ev)
Complex
ΔEp(v)
1
2
3
0.212
0.315
0.174
1.191
1.166
1.094
511
436
479
0.230
0.297
0.269
-4.428
-4.495
-4.467
2.43
2.84
2.59
-1.998
-1.655
-1.877
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Figure captain
Scheme 1 Syntheses of complexes 1-3.
Fig. 1 ORTEP diagram of 1 and 3 with ellipsoids shown at the 30% probability level
(The hydrogen atoms, counterion and solvent molecules are omitted for clarity).
Fig. 2 (a) 2D framework built by C-H…π interactions in complex 1; (b) A 3D
supramolecular lattice of 1 directed by C-H…π stacking interactions. (The red dotted
sticks are represented C-H…π interactions).
Fig. 3 (a) 2D framework built by C-H…π interactions in complex 3; (b) A 3D
supramolecular lattice of 3 directed by C-H…π stacking interactions. (The red dotted
sticks are represented C-H…π interactions).
Fig. 4 ORTEP diagram of 2 with ellipsoids shown at the 30% probability level (The
hydrogen atoms and solvent molecule are omitted for clarity).
Fig. 5 (a) 2D framework built by C-H…π interactions in complex 2; (b) A 3D
supramolecular lattice of 2 directed by C-H…π and C-H…Cl stacking interactions.
(The red dotted sticks are represented C-H…π and C-H…Cl interactions).
Fig. 6 TGA curves of complexes 1-3 in N2 atmosphere.
Fig. 7 (a) Solid-state excitation and emission spectra of 2-PBO and 1; (b) Solid-state
excitation and emission spectra of 3-PBO and 2; (c) Solid-state excitation and
emission spectra of PBM and 3. (insert) Photoluminescent images of ligands and
complexes 1-3 under UV radiation (254 nm).
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Highlights
 Three mononuclear Cu(I) complexes have been synthesized and characterized.
 In complexes 1-3, the all Cu(I) are four-coordinated and possessing a slightly
distorted trigonal pyramidal geometry.
 Complexes 1-3 exhibit distinct tunable photoluminescence by varying the
N-heterocyclic ligands.
20

Graphics Abstract

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7