Highly luminescent copper(i) halide complexes chelated with a tetradentate ligand (PNNP): Synthesis, structure, photophysical properties and theoretical studies

Article abstract of DOI:10.1039/c8dt03452d

Two emissive copper(i) halide complexes (PNNP)Cu₂Br₂ (1) and (PNNP)Cu₂I₂ (2), which are constructed from butterfly-shaped dinuclear Cu₂X₂ cores and a new tetradentate ligand (PNNP = 1,3-bis

Full text of DOI:10.1039/c8dt03452d

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Highly luminescent copper(I) halide complexes
chelated with a tetradentate ligand (PNNP):
synthesis, structure, photophysical properties
and theoretical studies†
Cite this: DOI: 10.1039/c8dt03452d
Ji-Hui Jia,a,c Xu-Lin Chen,*a,b Jian-Zhen Liao, a Dong Liang,b
a,b
Ming-Xue Yang, a,c Rongmin Yu a and Can-Zhong Lu
*
Two emissive copper(I) halide complexes (PNNP)Cu2Br2 (1) and (PNNP)Cu2I2 (2), which are constructed
from butterfly-shaped dinuclear Cu2X2 cores and a new tetradentate ligand (PNNP = 1,3-bis(1-(2-(diphe-
nylphosphanyl)phenyl)-1H-pyrazol-3-yl)benzene), were synthesized and characterized. These chelates
exhibit bright green (λmax = 517 nm, 1) and bluish-green (λmax = 492 nm, 2) photoluminescence in the
solid state with quantum yields of 42% (1) and 58% (2), and lifetimes of 13 μs (1) and 8.8 μs (2) at room
temperature. Computational density functional theory/time-dependent density functional theory
(DFT/TDDFT) calculations were performed to elucidate the nature of their electronic transitions and to
predict their detailed photophysical properties. The results of DFT/TDDFT calculations, combined with
the temperature dependence of spectroscopic properties and emission decay behaviors, suggest that the
emission in the solid state originates from the 1,3(MLCT + XLCT + ILCT) excited states, which are in
thermal equilibrium with small energy differences of about 0.1 eV. A comparative study of the titled com-
plexes reveals that the emissive-state characteristics and photophysical properties of these complexes are
significantly affected by the ligand field strength and atomic number of the halogen atom, as well as by
the percentage of the XLCT transition involved in the lowest excited states. Compared with its bromide
counterpart (1), the iodide complex (2) shows a much higher phosphorescence quantum yield (0.94 vs.
0.50), a much shorter phosphorescence decay time (58 μs vs. 274 μs), a much larger phosphorescence
rate constant (1.6 × 104 s−1 vs. 1.8 × 103 s−1), and a larger phosphorescence contribution (25% vs. 8%) in
room-temperature emission, due to the more efficient spin–orbit coupling (SOC).
Received 24th August 2018,
Accepted 19th December 2018
DOI: 10.1039/c8dt03452d
rsc.li/dalton
have achieved great success in fabricating efficient OLEDs due
to their efficient spin–orbit coupling induced by the heavy
Introduction
Luminescent materials which can harvest both triplet and metal atoms.1 However, the rarity and high costs of these pre-
singlet excitons for electroluminescence (EL) are of great inter- cious metal-containing phosphors may greatly restrict their
est as emitters for the development of high-performance popularization. Copper(I) complexes have recently been found
organic light-emitting diodes (OLEDs). Phosphorescent to be promising inexpensive alternatives for efficient OLED
organometallic complexes based on platinum-group metals emitters because of their rich structural and photophysical pro-
perties, as well as their thermally activated delayed fluo-
rescence (TADF) emission pathway, which can harness both
aKey Laboratory of Design and Assembly of Functional Nanostructures, and Fujian
triplet and singlet excitons via efficient reverse intersystem
Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of
crossing.2–6 Copper(I) complexes generally emit from the lowest
Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China.
excited states with an extensive metal-to-ligand charge transfer
E-mail: czlu@fjirm.ac.cn, xlchem@fjirm.ac.cn
bXiamen Institute of Rare-earth Materials, Haixi Institutes,
(MLCT) character due to the low oxidation potential of the Cu+
ion.7 The spatially well-separated HOMO and LUMO orbitals of
these complexes lead to a narrow energy gap between the low-
lying MLCT emissive states (1MLCT and 3MLCT), which
ensures efficient TADF emitting at room temperature.
Chinese Academy of Sciences, Xiamen 361021, China
cUniversity of Chinese Academy of Sciences, Beijing, 100049, China
†Electronic supplementary information (ESI) available: Characterization data
and computation data. Crystallographic data have been deposited at the
Cambridge Crystallographic Data Center. CCDC 1853178 (1) and 1853179 (2).
For ESI and crystallographic data in CIF or other electronic format, see DOI:
10.1039/c8dt03452d
The diverse coordination modes of copper(I) stimulate the
search for luminescent copper(I) complexes with various struc-
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tural features, including mononuclear, binuclear and multi- palladium (1 mmol, 1.15 g), and triethylamine (11 mmol,
nuclear species. Among them, the well-known heteroleptic 1.11 g) in 25 ml toluene were suspended. The reaction mixture
mononuclear copper(I) complexes of the type [Cu(N^N)(P^P)]+ was stirred at 80 °C for 12 h. After the completion of the reac-
that feature neutral diimine (N^N) and diphosphine (P^P) tion, the solvents were removed under reduced pressure. The
ligands have attracted particular interest due to their ready syn- reaction mixture was extracted with dichloromethane and
thesis and promising luminescence properties. Emission of these water. The combined organic layers were washed several times
copper(I) complexes tends to be quenched by the flattening dis- with brine and dried over sodium sulfate. The crude product
tortion of emissive excited states with the MLCT character and by was purified by column chromatography on silica gel (eluent:
the subsequent solvent-induced exciplex formation.8 According petroleum ether) to afford the product as a white solid (yield:
to earlier studies, the luminescence properties can be enhanced 2.3 g, 83%). The NMR data are consistent with the reported
by using rigid ligand scaffolds and by introducing sterically hin- data.36 1H NMR (400 MHz, CDCl3): δ 7.39–7.31 (m, 11H),
dered substituents on the ligand, which afford a rigid environ- 7.10–7.03 (m, 2H), 6.85–6.79 (m, 1H). 13C NMR (101 MHz,
ment around the copper center.9 Our group has initiated the CDCl3): δ 164.30 (dd, JC–F = 246.4 Hz, JC–P = 14.1 Hz), 135.40 (d,
investigation of TADF copper(I) complexes with the functiona- JC–P = 10.1 Hz), 134.24 (t, JC–P = 8.1 Hz), 133.83 (d, JC–P
lized pyridyl pyrazole chelates, which are known to effectively 20.2 Hz), 130.98 (d, JC–F = 8.1 Hz), 129.06 (s), 128.62 (d, JC–P
=
=
suppress the nonradiative processes and harness effective radia- 8.1 Hz), 124.77 (t, JC–F = 18.7 Hz, JC–P = 19.2 Hz), 124.44 (d,
tive transitions.4,10–16 Copper(I) halide complexes bearing JC–P = 2.0 Hz), 115.38–115.15 (m, JC–F = 23 Hz). 31P NMR
imine (N), phosphine (P) or imine-phosphine (N^P) ligands are (162 MHz, CDCl3): δ −18.27 (d, JP–F = 53.46 Hz). Anal. calcd for
another type of copper(I) complex which have been intensively C18H14FP: C, 77.14; H, 5.03. Found: C, 77.73; H, 5.01.
studied as potential electroluminescent materials.17–30 The struc-
Synthesis of the ligand PNNP
tural characteristics and photophysical properties of these com-
plexes are dependent on the organic ligands, halogens and the 1,3-Di(1H-pyrazol-3-yl)benzene (3 mmol, 0.63 g), (2-fluorophe-
metal–ligand–halogen ratio. In order to constitute highly emis- nyl)diphenyl-phosphine (6 mmol, 1.68 g) and Cs2CO3
sive copper(I) halide complexes, bulky or bridged ligands are (12 mmol, 3.91 g) were dissolved in 20 ml dry N,N-dimethyl-
usually preferred due to the restriction of the excited state distor- acetamide. The mixture was heated to reflux (180 °C) for 12 h.
tion by increasing the conformational rigidity.27,29 Although After cooling to room temperature, the mixture was poured
various chelating ligands such as bidentate phosphine, triden- into 300 mL of water. The resulting precipitate was collected
tate phosphine and bidentate imine-phosphine ligands have via filtration and purified by column chromatography on silica
been used to constitute efficient copper(I) halide emitters, tetra- gel (eluent: petroleum ether/ethyl acetate = 50/1) to give the
dentate-ligand-coordinated copper(I) halide complexes have not product as a white solid (yield: 1.8 g, 81%). 1H NMR (400 MHz,
been reported yet,31–33 to the best of our knowledge.
DMSO-d6): δ 8.12 (d, J = 2.3 Hz, 2H), 7.90 (s, 1H), 7.67 (dd, J =
In this study, a new tetradentate (PNNP) chelating ligand 7.7, 3.9 Hz, 2H), 7.57 (t, J = 7.7 Hz, 2H), 7.38 (dd, J = 14.4,
was used to construct emissive dinuclear copper(I) halide com- 5.3 Hz, 14H), 7.29 (dd, J = 7.7, 1.4 Hz, 2H), 7.21 (td, J = 7.2,
plexes, of which the crystal structures, the photophysical pro- 3.1 Hz, 8H), 7.12 (t, J = 7.7 Hz, 1H), 6.98 (dd, J = 7.2, 3.6 Hz,
perties, and the theoretical calculations of molecular orbitals 2H), 6.84 (d, J = 2.4 Hz, 2H). 13C NMR (101 MHz, DMSO-d6):
and excited states have been systematically studied.
δ 151.45 (s), 144.56 (d, JC–P = 21.2 Hz), 138.09 (d, JC–P = 12.1 Hz),
135.60 (s), 133.84 (d, JC–P = 20.2 Hz), 133.47 (s), 132.57 (s), 132.06
(d, JC–P = 23.2 Hz), 130.50 (s), 129.15–128.98 (m), 128.34 (s),
125.16 (s), 124.76 (d, JC–P = 2.0 Hz), 122.62 (s), 105.18 (s). 31P NMR
(162 MHz, DMSO-d6): δ −13.86 (s). Anal. calcd for C48H36N4P2: C,
78.89; H, 4.97; N, 7.67. Found: C, 78.71; H, 5.37; N, 7.49.
Experimental section
General procedures
All reactions were performed under a nitrogen atmosphere
unless specified otherwise. The starting chemicals were pur-
chased from commercial sources and used as received.
Solvents were dried over molecular sieves. 1,3-Di(1H-pyrazol-3-
yl)benzene was synthesized by following the literature route.14
1H NMR and 31P NMR spectra were recorded on a Bruker
Avance III 400 MHz NMR spectrometer. 31P chemical shifts
were referenced to an external standard, 85% phosphoric acid
(δ = 0 ppm). Elemental analyses (C, H, N) were carried out with
an Elementar Vario EL III elemental analyzer.
Synthesis of copper(I) halide complexes
The two copper(I) halide complexes were prepared according
to the following general procedure: a mixture of CuI or CuBr
(0.2 mol) and the PNNP ligand (0.1 mmol) in CH2Cl2 (10 ml)
was stirred for 1 h at room temperature. The resulting products
were purified and isolated through crystallization by slow
diffusion of ether into the CH2Cl2 solution. Single crystals of
these copper(I) halide complexes suitable for X-ray diffraction
measurement were obtained via the same method.
(PNNP)Cu2Br2 (1). Yellow powders, 66 mg, yield: 65%. 1H NMR
(400 MHz, DMSO-d6): δ 9.32 (s, 1H), 7.98 (s, 2H), 7.77–7.31 (m,
Synthesis of (2-fluorophenyl)diphenylphosphine34–36
In a Schlenk tube equipped with a reflux condenser and a 29H), 6.85 (dd, J = 18.2, 10.7 Hz, 4H). 31P NMR (162 MHz, DMSO-
magnetic bar, diphenylphosphine (10 mmol, 1.86 g), 1-fluoro- d6): δ −25.49 (s). Anal. calcd for C48H36Br2Cu2N4P2·(C2H5)2O:
2-iodobenzene (10 mmol, 2.22 g), tetrakis(triphenylphosphine) C, 56.65; H, 3.57; N, 5.51. Found: C, 56.98; H, 4.19; N, 5.21.
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(PNNP)Cu2I2 (2). Yellow powders, 69 mg, yield: 62%.
1H NMR (400 MHz, DMSO-d6): δ 9.25 (s, 1H), 7.99 (d, J =
2.1 Hz, 2H), 7.71–7.35 (m, 29H), 6.86 (t, J = 5.2 Hz, 4H).
Results and discussion
Syntheses and crystal structures
31P NMR (162 MHz, DMSO-d6): δ −28.03 (s). Anal. calcd for The tetradentate ligand PNNP was easily synthesized in three
C48H36I2Cu2N4P2·1/2(C2H5)2O: C, 51.86; H, 3.26; N, 5.04. steps with an overall yield of 80%, and the graphical synthesis
Found: C, 51.93; H, 3.53; N, 4.83.
route and related NMR spectra are shown in the ESI
(Scheme S1 and Fig. S6–S19†). The copper(I) halide complexes
(PNNP)Cu2Br2 (1) and (PNNP)Cu2I2 (2) were prepared by reac-
X-ray crystallographic analysis
Diffraction data for 1 and 2 were collected on a SuperNova, tions of PNNP and the corresponding cuprous halide with a
Dual, Cu at zero, Atlas diffractometer with graphite-monochro- molar ratio of 1 : 2. After purification by recrystallization, the
mated Cu Kα radiation (λ = 1.54184 Å). The structures were resulting products were characterized by 1H NMR, 31P NMR
solved by direct methods and refined by full-matrix least- and elemental analysis. The tetradentate chelating ligand was
squares methods using the SHELXL-97 program package.37 designed according to the supposition that its great steric hin-
Hydrogen atoms were added in idealized positions. All non- drance and rigidity around the copper(I) core could inhibit the
hydrogen atoms were refined anisotropically. These two copper- excited-state distortion and solvent-induced exciplex for-
halide crystals contain badly disordered solvent molecules, and mation. As expected, these complexes are robust both in solu-
SQUEEZE routine implemented on PLATON was used to tion and in the solid state, retaining both their color and
remove electron densities corresponding to disordered solvent luminescence after several weeks under an air atmosphere,
molecules.38 The details of the crystal and structure refine- presumably due to the bulky and rigid coordination environ-
ments are listed in Table S1.† Selected bond lengths and bond ment of the copper(I) ions. They also display high thermal
angles are listed in Table S2 in the ESI.† CCDC 1853178 and stability without degradation until 329 °C for 1 and 328 °C for 2,
1853179† contain the supplementary crystallographic data for respectively, as shown by thermal gravimetric analysis (Fig. S1,
complexes 1 and 2, respectively.
ESI†).
The crystal structures of these complexes were determined
by single-crystal X-ray diffraction analysis. Selected bond dis-
Photophysical measurements
UV-vis absorption spectra were recorded with a PerkinElmer tances and angles are summarized in Table S1.† Their mole-
Lambda 45 UV-vis spectrophotometer. Photoluminescence cular structures and ORTEP plots are shown in Fig. 1. Each
spectra were recorded on a HORIBA Jobin–Yvon FluoroMax-4 complex contains a dimeric Cu2(µ-X)2 (X = Br or I) unit which
spectrometer. Lifetime measurements were performed by is symmetrically chelated with a single tetradentate ligand.
multichannel scaling (MCS) mode on the same fluorimeter Due to the narrow coordination space provided by the ligand,
equipped with a spectraLED pulsed source (373 nm). Signals the Cu2(µ-X)2 units of complex 1 and complex 2 adopt similar
were collected using a FluoroHub module and analyzed by the butterfly-shaped configurations with dihedral angles of 9.78°
DAS6 Decay Analysis software (HORIBA Jobin–Yvon). and 14.14° between the X1–Cu1–X2 plane and the X1–Cu2–X2
Photoluminescence quantum yields were defined as the (X = Br, I) plane, respectively, which are different from the
number of photons emitted per photon absorbed by the reported planar Cu2X2 geometry of halogen-bridged copper(I)
system and measured using the FluoroMax-4 spectrometer complexes.44 The coordination geometry around the Cu atoms
equipped with an integrating sphere. The variable-temperature is nearly tetrahedral, considering that the dihedral angles
measurements were carried out on corresponding instruments between the N1–Cu–P1 plane and the X1–Cu–X2 (X = Br, I)
by using an additional LINKAM THMS600 system.
plane are in the range of 83.7° and 86.7°, respectively. As listed
in Table S2,† the Cu–X distances of complexes 1 and 2
elongate with an increase in the van der Waals radius of X,
Theoretical calculations
The density functional theory (DFT) and time-dependent while the intramolecular Cu⋯Cu distance of complex 2
density functional theory (TDDFT) calculations were per- (2.96 Å) is slightly shorter than that of complex 1 (3.04 Å)
formed with the Gaussian 09 program package,39 using the which features a more planar Cu2(µ-X)2 core. The Cu⋯Cu dis-
hybrid Becke three-parameter Lee–Yang–Parr (B3LYP) func- tances of both complexes are longer than the van der Waals
tional level.40,41 The input structures were extracted from the radius of copper (2.8 Å), indicating no interaction between two
X-ray crystallographic data. In all calculations, the relativistic copper atoms. Interestingly, the distances between one of the
effective core potential (RECP) and the associated basis set halogen ligands and the nearest aromatic hydrogen atoms of
Lanl08(f) and Lanl08(d),42 which are the revised version of the the “bridgehead” phenyl ring are 3.054 Å and 3.259 Å for
original Hay–Wadt basis set, were employed for the Cu(I), Br, complex 1 and complex 2, respectively, indicating the presence
and I atoms. An all-electron basis set of 6-31G* was used for P, of intramolecular hydrogen bonding interactions between
C, and H. SOC is not taken into account in the calculations. them (see ESI, Fig. S20).‡
Visualization of the optimized structures and frontier mole-
cular orbitals was performed with GaussView. The partition
orbital compositions were analyzed using the Multiwfn 2.4
program.43
‡These intramolecular hydrogen bonding interactions may lead to the downfield
shift (to ∼9.2 ppm) of the aromatic C–H hydrogen resonance.
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geometries45–48 (Table 1). The T1 and S1 energies were calcu-
lated, at the optimized structures, to be 1.86 eV (666.40 nm)
and 2.01 eV (616.10 nm) for complex
1 and 1.94 eV
(638.56 nm) and 2.09 eV (593.05 nm) for complex 2, respect-
ively. These energies are smaller than the experimental values
mentioned below (Table 2); this may be due to the underesti-
mation of excited-state energies by using B3LYP when spin–
orbit coupling is not taken into account. The major contri-
butions of the low energy transitions involve charge transfer
transitions from the HOMO and HOMO−1, to the PNNP
ligand based LUMO and LUMO+1 (Fig. 2). The two highest
energy occupied molecular orbitals essentially localize on
copper atoms, halogen atoms, and P atoms of PNNP ligands
(Table S3†). The low-lying transitions are therefore character-
ized as metal-to-ligand charge transfer (MLCT) transitions
mixed with halogen-to-ligand charge transfer (XLCT) tran-
sitions and intra-ligand charge-transfer (ILCT) transitions. The
spatially well-separated frontier orbitals involved in these intra-
molecular charge transfer (ICT) transitions lead to small
energy gaps (ΔEST) between S1 and T1 states, which is pre-
dicted to be 0.15 eV for both complex 1 and complex 2. Such
small ΔEST values have proved to be essential for efficient
TADF emission at ambient temperature for both organic mole-
cules and metal complexes, including copper(I) complexes. As
for the metal complexes, spin–orbit coupling (SOC) which can
serve to induce room temperature phosphorescence, should
also be taken into account (Fig. 3). The radiative rate constant
krT of T1 → S0 phosphorescence can be expressed as:49
, ψS jHSOjψ >2 μ2S
T1
n
kTr ꢀ γ
ð1Þ
n
2
ΔEðSn ꢁ T1Þ
16π3103n3EP3
3hε0
γ ¼
Fig. 1 (a) The molecular structure drawing of the copper(I) complexes,
and ORTEP diagrams of (b) complex 1 and (c) complex 2 with thermal
ellipsoids at 50% probability level. Solvent molecules and H atoms are
omitted for clarity.
where HSO is the Hamiltonian for SOC and ΔE(Sn − T1) is the
energy difference between Sn and T1 states. μS is the S0 → Sn
n
transition dipole moment, EP represents the emission energy
(T1 → S0) in cm−1, and n, h, and ε0 are the refractive index,
Planck’s constant, and the permittivity in vacuum, respectively.
Accordingly, the phosphorescence rate constant is governed by
Theoretical calculations and analysis
DFT and TDDFT calculations were carried out to investigate the spin–orbit coupling matrix, S0 → Sn transition moment,
the molecular orbitals and electronic states of the presented the energy gap between Sn and T1, and the phosphorescence
copper(I) complexes. The emission properties of these com- emission energy gap. Provided that the orbital angular
plexes were investigated by calculations performed in the momentum changes, which may couple with the flip of elec-
triplet state (T1) and the singlet state (S1) optimized tron spin, the spin–orbit coupling matrix is determined by the
Table 1 Vertical emissions computed at the B3LYP/LANL2DZ levels for complexes 1 and 2
States
λcal (nm)
f
Assignments
MLCT
XLCT
ILCT
1
2
S1
S2
S3
T1
S1
S2
S3
T1
616.10
480.52
478.42
666.40
593.05
490.29
470.52
638.56
0.005
0.0021
0.0016
0.0
0.006
0.0027
0.0014
0.0
HOMO → LUMO (98%)
HOMO → LUMO+1 (82%)
HOMO−1 → LUMO (81%)
HOMO → LUMO (97%)
HOMO → LUMO (97%)
HOMO−1 → LUMO (97%)
HOMO → LUMO+1 (96%)
HOMO → LUMO (95%)
47.10%
45.79%
54.21%
43.09%
42.87%
43.64%
40.52%
38.81%
10.84%
15.14%
28.35%
9.47%
16.30%
37.56%
23.85%
13.20%
14.97%
14.04%
3.61%
13.14%
14.57%
4.53%
12.16%
13.06%
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Table 2 Photophysical parameters of the copper(I) complexes 1 and 2 in the solid state
298 K
77 K
Complex
λema (nm)
Φb (%)
τc (μs)
krd (S−1
)
λema (nm)
Φb (%)
τc (μs)
krd (S−1
)
ΔESTe (eV)
1
2
517
494
42
58
13
8.8
3.2 × 104
6.6 × 104
533
505
50
94
274
58
1.8 × 103
1.6 × 104
0.09
0.10
a Emission maximum. b Photoluminescence quantum yield (relative error 5%). c Emission decay time. d Radiative decay rate constant. e Energy
gap between S1 and T1 determined from the emission spectra at 77 K and 293 K.
Fig. 3 Energy level diagram with emission processes for the investi-
gated complexes in which ISC, RISC, and SOC represent intersystem
crossing, reverse intersystem crossing, and spin–orbit coupling,
respectively.
matrix of complex 2 is larger than that of complex 1. Moreover,
the energy gap (0.59 eV) between S2 and T1 of complex 2 is
smaller than the energy gap (0.73 eV) between S3 and T1 of
complex 1, and the oscillator length (ƒ) value of S2 of complex
2 is calculated to be 0.0027 which is larger than the oscillator
Fig. 2 Selected frontier orbitals of complexes 1 and 2 according to DFT
length (ƒ) value (0.0016) of S3 of complex 1. As a result, the
phosphorescence rate constant of complex 2 is expected to be
larger than that of complex 1 according to eqn (1), which is in
accordance with the experimental results (Table 2).
calculations.
atomic number and contribution of the heavy atoms (includ-
ing copper and halogen atoms) which directly participate in
the transitions.50 For complex 1, both S1 and S2, with the
Photophysical properties
dominant HOMO → LUMO transition, carry the same type of The UV/Vis absorption spectra of the ligand PNNP and com-
metal 4d character and halogen outermost p orbital character plexes 1 and 2 in CH2Cl2 are shown in Fig. 4. Both the com-
as T1 (see Table 1). Similarly, for complex 2, S1 carries the plexes and the ligand display similar broad and intense
same type of orbital character as T1. According to the El-Sayed absorption bands (ε > 2 × 103 M−1 cm−1) in the wavelength
rule, the spin–orbit admixtures of these states with the same range from 200 to 300 nm, which are assigned to the spin-
orbital character are expected to be small.2,51–53 The nearest allowed ligand-centered transitions (π–π* and n–π*) of PNNP.
singlet state with a heavy atom contribution different from The complexes exhibit weaker absorption bands in the
that in the T1 state is S3 and S2 (mainly stemming from the 325–375 nm region, which are not present for the free ligand.
HOMO−1 → LUMO transition) for complex 1 and complex 2, These additional bands can be assigned to transitions invol-
respectively. As listed in Table 1, both the T1 and S2 states of ving the Cu(I) atoms and halide atoms according to the results
complex 2 show transitions with more contributions from the of the density functional theory (DFT) and time-dependent
halogen atoms than those of the T1 and S3 states of complex 1. density functional theory (TDDFT) calculations. As shown in
Also considering the larger atomic number of the iodine atom Table S6,† the S0 → S1 absorption bands of complex 1 and
relative to that of the bromine atom, the spin–orbit coupling complex 2 mainly involve the transitions of HOMO → LUMO
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these complexes which are significantly affected by the nature
of the halogen ions directly bound to copper atoms. According
to orbital transition analysis (Table 1), the lowest excited states
S1 and T1 of complex 1 consist of MLCT (47.10%) + XLCT
(10.84%) + ILCT (14.97%) transitions and MLCT (43.10%) +
XLCT (9.47%) + ILCT (13.14%) transitions, respectively, while
the S1 and T1 states of complex 2 consist of MLCT (42.87%) +
XLCT (16.30%)
+ ILCT (14.57%) transitions and MLCT
(38.81%) + XLCT (13.20%) + ILCT (13.06%) transitions,
respectively. On the one hand, both complexes show appreci-
able MLCT contributions in the lowest excited states. The
iodine ion has a relatively weaker ligand field strength as com-
pared to the bromide ion. With a decrease in the ligand field
strength, the splitting of the d-orbitals decreases, leading to a
larger MLCT energy and, therefore, to shorter λmax. On the
other hand, the lowest excited states of the iodide complex
have much greater XLCT contributions than those of its
bromide counterpart. Due to the larger atomic number of
iodine and the more significant participation of the iodine 5p
orbital in the low-lying transitions, the certain low-lying
excited states of the iodide complex may invoke stronger spin–
orbit coupling and, hence the rate of intersystem crossing
(ISC) would be enhanced relative to that of the bromide
complex (see the Theoretical calculations section). The
enhanced rate of ISC processes, such as T1 → S0, S1 → T1 and
T1 → S1, is expected to shorten the decay time and improve the
quantum yield of either phosphorescence or TADF (vide infra).
In a word, the trend of the above photophysical properties can
be rationalized qualitatively by the ligand field strength and
the atomic number of the halogen atom, as well as the percen-
tage of XLCT transition involved in the lowest excited states.
Temperature dependence of the luminescence properties
was further investigated to understand the nature of the emis-
sive states. The solid complexes display red-shifted emissions,
higher PLQYs, and considerably longer decay times at 77 K rela-
tive to those recorded at 298 K. Correspondingly, the radiative
rate constants of 1 and 2 decrease by nearly 94% and 76%,
respectively, as the temperature decreases from 77 K to 298 K.
These temperature-dependent behaviors indicate that the emis-
sions of these complexes may arise from two thermally equili-
brated excited states with different radiative decay rates, i.e., S1
and T1. To prove the existence of these thermally equilibrated
emitting states, more observed decay times (τobs) of these com-
plexes were taken at varied temperatures from 77 to 298 K, and
the temperature-dependent results are shown intuitively in
Fig. 6a and b. When the exciton population is assumed to be in
thermal equilibrium among the excited states, the observed
decay time can be expressed as a function of the temperature as
shown in eqn (2) that has been used generally in the literature.54
Fig. 4 Absorption spectra of the complexes and the free ligand in
CH2Cl2 (c ≈ 2 × 10−5).
and HOMO → LUMO+1, respectively. Orbital component ana-
lysis shows that the electron density in the HOMOs is mainly
distributed over the Cu atoms, halide atoms and P atoms,
while that in the LUMO of complex 1 and LUMO+1 of complex
2 is dominantly localized on the ligand PNNP. Accordingly, the
origin of the low-energy absorption bands involves MLCT tran-
sitions, XLCT transitions and ILCT transitions.
The two complexes exhibit bright photoluminescence at
various temperatures. Fig. 5 shows the emission spectra of 1
and 2 powders recorded at 293 and 77 K. The spectra are
broad and structureless, even at liquid-nitrogen temperature,
indicating that the emissive excited states have substantial
charge-transfer characteristics. At 293 K, 1 and 2 display blue-
green emission with peak maxima of 517 and 492 nm, photo-
luminescence quantum yields (PLQYs) of 42% and 58%, and
lifetimes of 13 and 8.8 μs, respectively. The radiative decay rate
constants (kr = ϕPL × τ−1) are then calculated to be 3.2 × 104
and 6.6 × 104 for 1 and 2, respectively. Obviously, complex 2
shows a blue-shifted spectrum, a higher photoluminescence
quantum yield (PLQY), a shorter decay time, and a larger
radiative decay rate constant than complex 1. These behaviors
can be ascribed to the different excited-state characteristics of
ꢀ
ꢁ
1
3
ΔEST
1 þ exp ꢁ
KBT
ꢀ
ꢁ
τobs
¼
ð2Þ
1
1
ΔEST
þ
exp ꢁ
τðT1Þ 3τðS1Þ
KBT
Herein, kB and T denote the Boltzmann constant and the
absolute temperature, respectively. τ(S1) and τ(T1) are the indi-
Fig. 5 Emission spectra of complexes 1 and 2 in the powder state at
77 K and 298 K.
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Fig. 6 Temperature dependence of decay time for (a) complex 1 and (b) complex 2 (the insets show the fitting parameters), and the contributions
of TADF and phosphorescence to the luminescence of (c) complex 1 and (d) complex 2 in dependence of temperature according to eqn (S3)
and (S4).†
vidual decay times of S1 and T1, and ΔEST is the energy gap emission lifetimes of 13 μs and 8.8 μs, respectively. Obviously,
between these two states. By fitting eqn (2) to the experimental the room-temperature emission of these copper(I) complexes
data of overall decay times observed at different temperatures, consists of both TADF and phosphorescence. We evaluated the
the parameters were obtained with values of (S1) = 196 ns, temperature-dependent relative contributions of TADF and
(T1) = 271 μs, and ΔEST = 0.09 eV for 1 and (S1) = 53 ns, (T1) = phosphorescence in the overall emission.55 As depicted in
57 μs, and EST = 0.11 eV for 2, respectively. The fitted (T1) values Fig. 6c and d, the low-temperature (below 100 K) emission
approximate to the decay times (274 μs for 1 and 58 μs for 2) of these complexes originates absolutely from T1, namely
observed at 77 K, and the obtained energy differences ΔEST are pure phosphorescence. The phosphorescence contribution
exactly equal to those estimated from the emission spectra decreases as the temperature increases further, accompanied by
(Table 2). As depicted in Fig. 6a and b, the solid fitting curves an increasing contribution of emission from the S1 state
match well with the experimental data. At low temperatures (TADF). At room temperature (293 K), the phosphorescence and
(below 100 K for 1 and below 120 K for 2), nearly constant TADF account for 92% and 75% of the total emission for
values of ca. 271 μs and 57 μs are observed for complex 1 and complex 1 and complex 2, respectively. The larger phosphor-
complex 2, respectively, indicating that the exciton population escence contribution (25%) in the room-temperature emission
is predominantly frozen in the T1 state, and thus each copper of complex 2, as compared with that of complex 1, is due to the
(I) complex emits almost pure phosphorescence at such low higher phosphorescence rate constant induced mainly by the
temperatures. This corresponds to the mono-exponential more efficient SOC.
decay behaviors observed at 77 K. Notably, complex 1 shows a
much longer phosphorescence decay time than that of
complex 2, indicating the stricter spin forbiddenness of the
T1 → S0 transition for complex 1, as is discussed in detail in the
Conclusions
above section. As the temperature decreases further, the fitting In summary, two binuclear copper(I) halide complexes (PNNP)
curves drop sharply, implying that a gradual thermal popu- Cu2Br2 (1) and (PNNP)Cu2I2 (2) exhibiting intense green and
lation of the upper lying S1 state occurs, which can be expected bluish-green photoluminescence have been designed and syn-
due to the small energy gap between S1 and T1. At room tem- thesized by using a bulky tetradentate ligand. The photo-
perature, the downward trend of the fitting curves of both com- physical and theoretical studies of these complexes indicate
plexes 1 and 2 becomes much less noticeable with the observed that their room-temperature luminescence in the solid state
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consists of TADF and phosphorescence which originate from 13 Q. Zhang, X. L. Chen, J. Chen, X. Y. Wu, R. M. Yu and
two thermally equilibrated 1,3(MLCT + XLCT + ILCT) excited
C. Z. Lu, RSC Adv., 2015, 5, 34424–34431.
states. The iodide complex (2) with the heavier halogen ligand 14 D. Liang, X. L. Chen, J. Z. Liao, J. Y. Hu, J. H. Jia and
shows larger spin–orbit admixtures of up-lying singlet states in C. Z. Lu, Inorg. Chem., 2016, 55, 7467–7475.
the T1 state. The more efficient spin–orbit coupling (SOC) of 15 L. Lin, D.-H. Chen, R. Yu, X.-L. Chen, W.-J. Zhu, D. Liang,
complex 2, as compared with that of complex 1, apparently
increases the photoluminescence quantum yield and shortens
J.-F. Chang, Q. Zhang and C.-Z. Lu, J. Mater. Chem. C, 2017,
5, 4495–4504.
the emission decay time. These results may provide a reference 16 F. Zhang, Y. Guan, X. Chen, S. Wang, D. Liang, Y. Feng,
for designing efficient copper(I) halide emitters with short
decay times which are required in electroluminescence
applications.
S. Chen, S. Li, Z. Li, F. Zhang, C. Lu, G. Cao and B. Zhai,
Inorg. Chem., 2017, 56, 3742–3753.
17 M. Hashimoto, S. Igawa, M. Yashima, I. Kawata,
M. Hoshino, M. Osawa and J. Am, Chem. Soc., 2011, 133,
10348–10351.
18 Z. W. Liu, M. F. Qayyum, C. Wu, M. T. Whited,
P. I. Djurovich, K. O. Hodgson, B. Hedman, E. I. Solomon
and M. E. Thompson, J. Am. Chem. Soc., 2011, 133, 3700–
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Conflicts of interest
The authors declare no competing financial interest.
19 D. Volz, D. M. Zink, T. Bocksrocker, J. Friedrichs,
M. Nieger, T. Baumann, U. Lemmer and S. Bräse, Chem.
Mater., 2013, 25, 3414–3426.
Acknowledgements
This work was supported by the Strategic Priority Research
Program of the Chinese Academy of Sciences (XDB20000000),
the Key Program of Frontier Science, CAS (QYZDJ-SSW-SLH033),
the National Natural Science Foundation of China (21521061,
51672271, 21671190, 21403236 and 21805281) and the Natural
Science Foundation of Fujian Province (2006L2005).
20 D. M. Zink, M. Bachle, T. Baumann, M. Nieger, M. Kuhn,
C. Wang, W. Klopper, U. Monkowius, T. Hofbeck, H. Yersin
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21 D. M. Zink, D. Volz, T. Baumann, M. Mydlak, H. Flügge,
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