Thermally Activated Delayed Fluorescence Materials Based on Copper Metal-Organic Complexes

Tingting Zhang; Xingzhi Hong; Hui Gao; Ying Ren; Jianfeng Jia; Haishun Wu

doi:10.7536/PC210211

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (2) : 411-433 DOI: 10.7536/PC210211

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Thermally Activated Delayed Fluorescence Materials Based on Copper Metal-Organic Complexes

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Abstract

The emitting materials with thermally activated delayed fluorescence (TADF) characteristics have received increasing attention in recent years. As a typical d¹⁰ metal, Cu(Ⅰ) is the most widely investigated one used to construct d¹⁰ metal-organic complexes. The copper metal-organic complexes often have certain superior luminescence properties because they can harvest both singlet and triplet excitons and exhibit much higher luminescence quantum yields close to 100%. They are cost-effectiveness and comparable with phosphorescent materials in terms of device efficiency. In addition, they have lowlying metal to ligand charge transfer(MLCT) excited states with small energy difference between the lowest singlet state and the lowest triplet state (ΔE_ST), which is a key point to facilitate the reverse intersystem crossing (RISC) process in tuning triplet excitons to singlet excitons for TADF emission. Meanwhile, the energy difference can be adjusted by different ligands or substituents. In this paper, we summarize and analyze the structure and luminescent properties of TADF copper complexes reported in recent five years, according to the types of the coordination atoms. These complexes are classified into four classes,and the coordination atoms are mainly N, P, X (halogen), C, S(O). We mainly discuss the effects of the structures on the luminescent properties. Finally, the potential applications in organic light-emitting diodes(OLEDs) are also prospected.

Contents

1 Introduction

2 TADF copper complexes with N and P coordination atoms

2.1 Mononuclear copper complexes with N and P coordination atoms

2.2 Binuclear copper complexes with N and P coordination atoms

3 TADF copper complexes with N,P,X(halogen)coordination atoms

3.1 Mononuclear copper complexes with N,P,X(halogen)coordination atoms

3.2 Binuclear copper complexes with N,P,X(halogen)coordination atoms

3.3 Polynuclear copper complexes with N,P,X(halogen)coordination atoms

4 TADF copper complexes with N,P,C coordination atoms

4.1 Copper complexes containing carbine ligands

4.2 Copper complex containing(iso-)cyanide ligand

5 TADF copper complexes with N,P,O(S)coordination atoms

5.1 Mononuclear copper complexes with N,P,O(S)coordination atoms

5.2 Binuclear copper complexes with N,P,O(S)coordination atoms

5.3 Polynuclear copper complexes with N,P,O(S)coordination atoms

6 TADF copper complexes by theoretical studying and designing

7 Summary and outlook

Graphical abstract

Keywords

thermally activated delayed fluorescence / copper / organometallic complexes / luminescent property / organic light-emitting diode

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Tingting Zhang, Xingzhi Hong, Hui Gao, Ying Ren, Jianfeng Jia, Haishun Wu. Thermally Activated Delayed Fluorescence Materials Based on Copper Metal-Organic Complexes. Progress in Chemistry, 2022, 34(2): 411-433 DOI:10.7536/PC210211

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1 Introduction

The light-emitting layer is an important component of organic light-emitting diodes (OLEDs). The external quantum efficiency (EQE) of OLEDs is directly affected by the light-emitting efficiency of the light-emitting layer material. Materials with thermally activated delayed fluorescence (TADF) are a new type of luminescent materials^[1⇓~3]. Compared with the first generation of fluorescent materials, TADF materials can convert the triplet state of molecules into singlet state by absorbing heat in the environment, thus achieving 100% theoretical internal quantum efficiency^[4,5]. This can be comparable to the performance of phosphorescent materials, and at the same time, it has the advantages of low cost, easy preparation and no precious metals, so the materials with TADF properties have a wide range of applications in OLEDs and other fields^[6⇓~8].

At present, some iconic molecules with TADF properties have attracted people's attention, and the performance and luminescence principle of similar molecules applied to OLEDs have also been reported^{[9⇓⇓⇓⇓⇓~15]}^[16]. It can be seen from the reported literatures that the molecules with TADF properties have been found to be pure organics, metal organics, fullerenes and so on, among which copper metal complexes are a class of molecules with high attention^[17⇓~19]. The energy gap ΔE_ST between the lowest excited singlet state (metal to ligand charge transfer) and triplet state (ligand to ligand charge transfer) of the copper complexes is small, which is conducive to the formation of excited States with metal to ligand charge transfer (MLCT) and ligand to ligand charge transfer (LLCT) properties, thus possibly forming the TADF phenomenon. People have been exploring to improve the performance of TADF by changing the ligand or introducing different substituents. The synthesis of copper complexes with TADF properties and the study of their performance in OLEDs devices have become a hot research topic^{[20⇓⇓⇓~24]}.

In addition, the theoretical calculation plays an important role in the study of the luminescence mechanism of the complexes^[25,26]. It is very important to understand the mechanism of TADF materials for the construction and screening of efficient TADF materials. Shuai et al. Summarized the luminescence mechanism of typical TADF molecules by analyzing the specific parameters closely related to excited state emission, such as radiation rate and intersystem crossing rate^[27]. In 2017, Zou et al. Designed several new Cu (I) complexes containing imidazole ligands with good performance^[28]. In 2018, Daniel et al. analyzed the luminescence mechanism of a class of binuclear Cu (I) complexes^[29]. In 2019, Cui et al. Studied the effect of the structure of two dinuclear copper complexes on the properties of TADF^[30].

In recent years, a series of novel copper complexes with TADF properties have been reported. However, the effect of different coordination atoms on the properties of TADF copper complexes has not been specifically reviewed. In this paper, the copper complexes containing different coordination atoms with TADF properties reported in the past five years were investigated, and their structural characteristics and luminescent properties were classified and discussed according to the types of coordination atoms, which will be helpful for the future study of copper organic complexes and TADF photofunctional compounds.

2 Copper Complexes with N and P as Coordinated Atoms

These TADF copper complexes with coordinating atoms of N and P are both neutral and cationic, and are divided into mononuclear and binuclear complexes to summarize their structures and properties.

2.1 Mononuclear complex

When the coordinating atoms are N and P, the complexes are mainly four-coordinated. According to the type of ligand, copper complexes can be roughly divided into three categories. Table 1 summarizes the ligand and photophysical properties of these complexes, and figs. 1A and 1b show the structure of complexes 1-29.

The first type is the neutral or ionic complexes of Cu (I) with two bidentate ligands, and the complexes with the general formula Cu (P ^ P) (N ^ N) (P ^ P is called the main ligand and N ^ N is called the auxiliary ligand) are the main type. The main ligand of complexes 1 ~ 18 is bis (2- (diphenylphosphino) phenyl) ether (abbreviated as POP) or its derivatives, and the photophysical properties of these complexes are different. In 2015, Lu et al. Synthesized copper complexes 1 ~ 5 with pyridine-pyrazolediimine as auxiliary ligands. These complexes all have excellent blue luminescence properties. The photoluminescence quantum yields (PLQYs) of 2 are nearly 100%, which has application potential in the field of OLEDs^[31]. Subsequently, they synthesized blue-green luminescent complexes 6 and 7 with TADF properties by introducing strong electron donor 9,9-dimethylacridine to the diimine ligand^[32]. The results show that the EQE of the device doped with 7 is 7.42%, the current efficiency (CE) is 20.24 cd/A, and the maximum brightness is 5579 cd/m². The luminescent color of complexes 8 ~ 13 is (yellow) green, and the auxiliary ligand in 8 ~ 11 is carbazole, 12 is triazolopyridine ligand, and 13 is tetrazolopyridine ligand^{[22,33⇓ ~35]}. 8 ~ 10 can be sublimated and applied in OLEDs, 11 with up to 98% PLQYs. The device based on 8 exhibits better luminescence performance than that of complex 7, with an EQE of 14.81% and a CE of 47.03 cd/A, and a maximum brightness of 11 010 cd/m². Wolf et al. Synthesized complexes 14 and 15. The sulfur atoms with different oxidation States introduced into the auxiliary ligand N ^ N can change the electron density on the nitrogen atom, thus affecting the emission wavelength^[36]. In 2020, de Souza et al. Synthesized TADF copper complexes 16 ~ 18 with auxiliary ligand phen and its derivatives, and pointed out a new method to reduce the overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied mole cular orbital (LUMO), thereby reducing the luminescence lifetime, that is, introducing S and Se elements into the auxiliary ligand phen^[37]. After the introduction of Se element on the phen ligand in complex 18, the luminescence lifetime is reduced to about 0.8 μs, which is the lowest lifetime reported for such [Cu(N^N)(P^P)]⁺ complexes.

It is worth mentioning that when the auxiliary ligand in complex 19 is pyridine-imidazole, the luminescence of the complex does not have TADF properties either in the solid phase or in the device, and the luminescence changes from blue in the solid phase to yellow in the device, which is a rare phenomenon that the luminescence position shifts by about 100 nm after being applied to the device^[38]. In 2017, Che et al. Synthesized new TADF copper complexes 20 ~ 22 with green, yellow and red colors. P ^ P is a nested borate, which can achieve different colors by adjusting the substituents on phen ligands^[39]. When the complex 20 is used in green light emitting devices, the EQE can reach 15.2% and the CE is 48.15 cd/A. When the complex 22 is used in red light emitting devices, the EQE can reach 10.2%.

The second type is the complexes of Cu (I) with one bidentate ligand and two monodentate ligands. The general formula is Cu(N^N)(L)₂,L usually triphenylphosphine (abbreviated as PPh₃). Comparing the structures of complexes 23 and 11 reported by Yang et al., only the ligand with the coordination atom P is different, that is, after replacing the bidentate ligand POP with two monodentate PPh₃, the gap ΔE_ST of singlet S₁ and triplet T₁ increases from 0.13 eV to 0.18 eV, and the PLQYs decreases to 45%^[34]. Complex 24 is also a different ligand containing a P coordinating atom than complex 12^[35]. The results show that although the PLQYs of 24 is higher than that of 12 (89.87% vs. 27.82%), the CE of 24 is lower than that of 12 (2.1 cd/A vs. 2.1 CD/A), which is related to the better conjugated structure of 24. Similarly, when complexes 25 ~ 27 reported by de Souza et al. In 2020 were compared with complexes 16 ~ 18, respectively, the PLQYs were slightly lower after the conjugation of the molecular structure was reduced^[37].

The third type is Cu (I) complexes with a tridentate ligand and a monodentate ligand, which are rarely reported. In 2019, Koshevoy et al. reported a series of TADF molecules containing tridentate-coordinated phosphine ligands and monodentate-coordinated thiocyanate (SCN) ligands, in which the luminescence colors of complexes 28 and 29 are green and yellow, respectively, which are caused by mixed charge transitions from ligand SCN to ligand phosphine and metal to ligand^[40]. Complexes in which the other coordinating atoms are S-coordinated are described in Section 5.

2.2 Binuclear complex

Binuclear TADF copper complexes with N and P as coordinating atoms usually have two kinds of ligands, and sometimes the substituents introduced on the ligands affect the luminescence wavelength or luminescence properties of the complexes. The ligand and photophysical properties of these complexes are summarized in Table 2. Fig. 2 shows the structure of complexes 30-40.

In 2016, Yu et al. Synthesized a series of dinuclear copper complexes 30-32 bridged by tetraphosphine ligands, in which the Cu atoms are four-coordinated^[41]. The solid state material exhibits orange photoluminescence at room temperature, with a lifetime of 7.4 ~ 20.5 μs and PLQYs of 7% ~ 45%. The calculated and experimental results show that the ΔE_ST of these complexes are very small and exhibit TADF properties. Subsequently, Yu et al. Synthesized three dinuclear Cu (I) complexes 33 ~ 35 bridged by tetraimine molecules. The other ligand is POP, and both copper are four-coordinated^[42]. These solid complexes emit green light at room temperature, the luminescence property is TADF, and the quantum efficiency can reach 79%. Although their emission spectra are similar, the effect of substituents on photoluminescence (PL) and electroluminescence (EL) efficiency is obvious (Table 2). Complex 35 with carbazole substituent showed the highest efficiency in PL and EL with EQE of 8.3%, CE of 27.1 cd/A in solution-processed OLEDs, and the highest brightness of 2525 cd/m², while complex 34 with trifluoromethyl substituent showed lower quantum efficiency than the other complexes.

In a series of complexes synthesized by Wolf et al. In 2019, sulfur atoms of different oxidation States were introduced into the ligand N ^ N, of which 36 formed a copper three-coordinated binuclear complex with coordinated atoms of N and P^[36]. The results show that TADF complexes with different spatial configurations can be obtained with the change of spatial confinement and sulfur oxidation state. Complex 36 is the first binuclear Cu (I) complex bridged by a POP (DPEPhos) ligand. In 2020, Artem 'ev et al. Synthesized a series of paddle-wheel-like binuclear copper complexes 37 ~ 40 showing efficient TADF properties^[43]. In these complexes, Cu (Ⅰ) is bridged to P and N by three diphenyl (2-pyrimidinyl) phosphine (PymPPh₂) ligands in a "head-to-tail" manner. Complex 39 has no other ancillary ligands, and the copper and the coordinating atom are three-coordinated. In complexes 37, 38, and 40, one or both of the coppers are also coordinated to the ligands MeCN and PhCN, so the copper is four-coordinated to the ligands. At room temperature, these complexes exhibit TADF properties, their PLQYs can reach 65%, and their lifetimes are 9.6 ~ 27 μs.

3 Copper Complexes with N, P, X (Halogen) as Coordination Atom

3.1 Mononuclear complex

Copper complexes with coordination atoms of N, P, X (halogen) can be divided into the following three categories according to the number of teeth of the ligand. The ligand and photophysical properties of these complexes are summarized in Table 3. Fig. 3 shows the structure of complexes 41-79.

The first type is Cu (I) four-coordinated complexes with a bidentate ligand, a monodentate ligand and a halogen X, the general formula is Cu (P ^ P) (L) X. In 2017, Huang et al. Synthesized four mononuclear copper complexes with bidentate phosphine, pyridine derivatives and halogen ligands, among which complexes 41 and 42 have bidentate ligand POP^[44]. Although these complexes contain similar pyridine derivative ligands, it is shown that the torsion angle between the N-heterocyclic torus and the N-Cu-X plane of complex 41 is much larger than that of other complexes, resulting in a red shift of luminescence. In 2018, Liu et al. Synthesized a new class of bidentate phosphine ligand complexes 43 ~ 45, which also emit strong blue light at room temperature with a peak emission wavelength of 464 ~ 479 nm^[45].

In 2019, they synthesized new complexes 46 ~ 48 containing thiophene bidentate phosphine ligands. Due to different halogens, the emission color of the three complexes can be from dark blue to green at 293 K, the peak wavelength is 447 ~ 460 nm, and the luminescence lifetime is 114 ~ 752 μs. This is the first reported copper complex with ultra-long fluorescence lifetime^[46]. This is due to the very small MLCT and halide to ligand charge transfer (XLCT) components in the excited state transition properties, and the emission is mainly from intraligand charge transfer (ILCT) and charge transfer between different ligands. In 2020, Liu et al. Synthesized a series of TADF copper complexes 49 ~ 54 containing thiophene bidentate ligands^[47]. The introduction of trimethylsilyl into the diphosphine ligand to form 49 ~ 51 not only greatly improves the solubility of the complex, but also improves the PLQYs, and can also fine-tune the emission color. These complexes emit strong blue-green to yellowish green light with peak emission wavelength of 485 ~ 535 nm, luminescence lifetime of 4.2 ~ 48.9 μs, and PLQYs of 3% ~ 52%. The emission properties of the six complexes are mainly derived from MLCT, XLCT and ILCT. In addition, complex 50 exhibits a yellow-green emission with an EQE of 7.74%, which is higher than that of complex 48 when applied to the device.

In addition, Cu (I) forms a three-coordinate complex with a bidentate ligand and a halogen X. In 2015, Osawa et al. synthesized a series of three-coordinate copper complexes 55 ~ 59^[48]. The luminescence properties of these complexes are similar, that is, the alkyl substituent at the ortho position of the phenyl group has little effect on the excited state properties. At room temperature, the solid complexes all exhibit highly efficient blue-green emission with peak wavelength in the range of 473 ~ 517 nm and PLQYs of 38% ~ 95%. Organic light-emitting devices containing these five complexes exhibit bright green luminescence with CE of 55.6 ~ 69.4 cd/A and EQE of 18.6% ~ 22.5%.

The second type is the Cu (I) complexes with monodentate ligands whose three coordinating atoms are N or P and halogen X. In 2015, Kato et al. Synthesized 60 ~ 65 tetradentate copper complexes with ligands of triphenylphosphine, pyridine or naphthyridine, and halogen^[49]. All complexes show high quantum efficiency (38% ~ 95%), and the emission color can be changed from red to blue by changing the nitrogen-containing ligands and halogen ligands. The results show that the transition properties of excited States are different at room temperature. The luminescence properties of 60 ~ 62 are delayed fluorescence and phosphorescence, and the luminescence of 64 ~ 65 is caused by singlet radiation, showing obvious TADF properties. When the auxiliary ligand is isoquinoline, the radiation mainly comes from the triplet state, and there is no TADF property. In 2016, Huang et al. replaced the P ^ P ligand in the synthesized complexes 41 and 42 with PPh₃ to form complexes 66 and 67. The blue emission properties at room temperature are the same as those of 41 and 42, which are caused by the mixed transition of singlet metal to pyridine ligand and halogen to pyridine ligand^[44].

The third type is Cu (I) complexes with a tridentate ligand and a halogen X. In 2015, Yersin et al. Synthesized a class of neutral copper complexes 68 ~ 76 containing tripodal ligands^[50]. At room temperature, the emission color of the complexes is from yellow to red, and the PLQYs is about 5% ~ 35%. It is shown that the emission spectrum is a mixture of TADF and short-lived phosphorescence, which is the result of the large degree of spin-orbit coupling. In 2016, Xu et al. Synthesized copper complexes 77 ~ 79 with TADF and phosphorescence dual-emission properties, which contain three-coordinated triphosphine ligands, and copper and coordination atoms form a tetrahedral configuration^[51]. Complex 79 essentially achieves a balance between the two emission properties, with phosphorescence accounting for 39%, PLQYs exceeding 80%, and EQE exceeding 15% when applied to devices.

3.2 Binuclear complex

There are many binuclear copper complexes with coordination atoms of N, P and X (halogen). The ligand and photophysical properties of complexes 80 ~ 130 are summarized in Table 4, and the structural diagrams of complexes 80 ~ 130 are shown in Figures 4A and 4B. These complexes can be divided into three categories according to the bridging mode between the two coppers.

The characteristic of the first kind of complexes is that copper interacts with N and P in turn to form a ring, and most of them are four-coordinated copper complexes, and a few are three-coordinated molecules, such as complexes 80-82. Yersin et al. Showed that the PLQYs of dinuclear complex 80 could reach 92% at room temperature, and its emission properties were 20% phosphorescence and 80% TADF^[52]. The spin-coupling interaction makes the T₁state have a large zero-field splitting parameter (ΔE_ZFS is 15 cm^-1), which is rare in copper complexes. Some binuclear complexes, such as complexes 83 ~ 88, are copper tetradentate complexes with P ^ N ^ N tridentate ligands and halogens^[53,54]. Complexes 83 ~ 85 emit bright green light in solid state at room temperature with PLQYs of 23% ~ 74%^[53]. In 2020, Artem 'ev et al. Synthesized TADF binuclear complexes 86 ~ 88^[54]. It also emits bright green light at room temperature with a lifetime of 14.5 ~ 20.0 μs and PLQYs of 51% ~ 55%. It is found that the change of halogen atoms from chlorine to iodine has a significant effect on the luminescence properties. With the decrease of the ligand field intensity, the maximum emission wavelength was blue-shifted from 550 nm to 520 nm. In addition, the luminescence of complex 86 is contributed by 73% phosphorescence and 27% TADF, and the luminescence of complex 87 is mainly contributed by 65% phosphorescence and 35% TADF. However, the TADF luminescence property of 88 is dominant (61%) and the contribution of phosphorescence drops to 39%. Related theoretical studies confirmed that the decrease of ΔE_ST was the main reason for the change of luminescence composition, which promoted the effective luminescence of TADF while suppressing the phosphorescence properties.

The second type is TADF copper tetradentate binuclear complexes with halogen X as the bridging atom. The binuclear center generally has a typical butterfly-like structure [Cu(μ-X)₂Cu]. This type of binuclear copper complexes is the most reported so far.

In complexes 89 ~ 100, the atoms coordinated to copper are P and X. Yu et al. reported light yellow complexes 89 ~ 91 containing diphosphine ligands^[55]. The copper atoms in the complex are four-coordinated, forming a tetrahedral geometry. The two copper atoms are connected by two halogens, while each copper is further chelated with a bisphosphine ligand. The distance between copper atoms in each complex is more than 2.9494 Å, and there is no interaction. These complexes can emit intense blue-green light with a lifetime of 4.9 ~ 5.9 μs and a quantum efficiency of 42% ~ 95% at room temperature. In addition, the coordination ability (I^-<Br^-<Cl^-) of the halogen ligand has no obvious effect on the maximum emission position of the series of complexes. In 2016, Kato et al. studied three binuclear copper complexes 92 ~ 94, which are stable and luminescent in solution^[56]. When the pyridine N atom is introduced into the phenyl group of the bridged diphenylphosphine group, the broad emission light adjustment from 497 nm to 638 nm can be realized, and the emission color is from green to red-orange. It has not been reported in luminescent copper complexes before that the emission wavelength varies in a wide range without changing the structure and luminescent properties. In the same year, Liu et al. Also synthesized a series of halogen-bridged complexes 95 ~ 97^[57]. The structure features that two electron-donating methyl groups are introduced to the ortho position of the diphosphine. When these complexes are used as emitters in organic light-emitting devices, they show bright green luminescence. The current efficiency of complex 95 can reach 32.9 cd/A, and the maximum external quantum efficiency is 10.1%. In 2019, Liu et al. Prepared dimers 98 and 99 formed by diphosphine ligands with thiophene rings, and studied their photoluminescence and electroluminescence properties^[58]. Both of them have excellent photoluminescence (PL) properties, with blue-green emission color (487 nm and 483 nm), short decay time (9. 46 μs and 7. 62 μs), and high PL quantum efficiency (69% and 86%). The CE of 99 is as high as 32.2 cd/A, the peak brightness is 3.67×10³cd/m², and the maximum EQE is 14.5%. In 2020, Balducci et al. Synthesized a dinuclear complex 100 of a diphosphine ligand with a stilbene group^[59]. Due to the rigid backbone of the phosphonium group, there is a stable π-π interaction between the two phenyl rings, resulting in a very short distance between the two phosphorus atoms, well below twice the van der Waals radius.

In complexes 101-105, the atoms coordinated to copper are N and X. In 2018, Artem 'ev et al. Synthesized a class of yellow binuclear complexes 101-105 containing phosphine oxide ligands, with iodine as the bridging atom^[60]. The strong emission wavelength range of these solid compounds is 536 ~ 576 nm, and the PLQYs of complex 101 is as high as 63%, and the luminescence lifetime is 10. 0 μs. Combining the photophysical properties with the theoretical calculation results, the TADF properties of the complexes at room temperature can be attributed to the mixed transitions of MLCT and XLCT.

In the complexes 106 ~ 114, the atoms coordinated to copper are N, P and X. In 2018, Kato et al. Synthesized 106 ~ 108 TADF complexes containing pyrrolidinopyridine ligands^[61]. The three complexes exhibit blue emission at about 450 nm. Subsequently, they synthesized two luminescent copper polymers 109 ~ 110, and structural analysis showed the formation of rhombic binuclear { Cu₂I₂(PPh₃)₂ } and one-dimensional coordination chains bridged by tripyridine substituted benzene ligands^[62]. Both show blue and yellow thermally activated delayed fluorescence, respectively, with moderate luminescent quantum efficiency. In addition, mechanochromic luminescence was observed. It is shown that grinding induces the generation of triplet cluster excited States, and the emission properties change from TADF to phosphorescence. In 2020, Kobayashi et al. Quantitatively synthesized strong emission copper polymers 111 ~ 113 by a new method of melting ligand thermal reaction under solvent-free conditions^[63]. The results show that the photophysical properties of the complexes obtained by thermal reaction are almost the same as those obtained by solution reaction, which may be due to the good thermal stability of the polymers, which can prevent the structure from being destroyed by heating. The thermal reaction of fused ligands provides a new method for the synthesis of novel TADF copper complexes. In 2021, Mauro et al. Synthesized a series of binuclear complexes, only 114 of which had TADF properties, indicating that the substituents on N and P ligands had obvious effects on the spectral properties^[64].

In the third type of complexes, the two copper atoms are connected by a halogen bridge and a bidentate ligand, which is generally a N-heterocyclic phospine (NHetPHOS) derivative containing nitrogen heterocycles and a bidentate ligand containing N and P coordination atoms. In addition, there is a monodentate ligand atom P in the complex, and complexes 115 ~ 128 have such coordination characteristics. In 2016, Heske et al. Introduced a new binuclear copper complex 115 containing pyridine diphenylphosphine ligands, and studied its performance as an organic light-emitting device in printing^[65]. The results show that the structure of the complex is stable in various printing solvents, and the complex can be particularly used for ink-jet printing, the EQE of the device is more than 11%, and the brightness can reach 3.4×104 cd/m². In 2017, Gerhards et al. Studied the ground state and excited state structures of complexes 116 and 117, with excited state lifetimes of about 3 μs and 4 μs, respectively^[66]. In 2018, Yersin et al. Synthesized a complex 118 bridged by iodine and triazole phosphine ligands, which emits bright sky-blue light at room temperature, with PLQYs of 85% and emission lifetime of 7 μs, including 13% phosphorescence and 87% TADF luminescence, which is rare^[67]. In 2019, Klopper et al. introduced a new class of binuclear copper complexes 119 ~ 128^[68]. These complexes also have butterfly-like structures and have good solubility in many solvents, especially the trifluoromethyl-containing 120 and 123 complexes. The results show that TADF has TADF properties in both solid and solution, and the medium such as thin film and solution has a significant effect on the emission wavelength.

3.3 Polynuclear complex

Baumann et al. Summarized the trinuclear and tetranuclear copper complexes formed by bridging ligand NHetPHOS reported before 2014, for example, 129 ~ 130 are trinuclear complexes containing triazophos ligand^[69]. It is found that the color and solubility of the complexes can be adjusted by different NHetPHOS ligands, which provides an idea for the wide application of high-performance OLEDs devices. In 2019, the TADF behavior of the hexanuclear copper complex synthesized by Sinyashin et al. Is shown by the dinuclear complex after dissociation in solution, because its luminescence spectrum in solution is similar to that of the dissociation product^[70].

4 Copper complexes with coordinating atoms of N, P, C

Table 5 summarizes the ligand and photophysical properties of copper complexes with coordinating atoms of N, P, and C. Fig. 5 shows the structure of complexes 131-161.

4.1 Copper Complexes Containing Carbene Ligand

Carbene is an important group in the ligands coordinated by carbon atoms. It forms mononuclear and binuclear TADF complexes with copper, and has two coordination, three coordination and four coordination.

In 2019, Thompson et al. Reported six luminescent two-coordinated copper complexes 131 ~ 136 formed by N-heterocyclic carbene ligands and carbazole derivatives^[71]. The emission color of the complex can be changed from purple to red by introducing different substituents on the carbene acceptor and the carbazole donor. It has been shown that 132 exhibits photoluminescence quantum efficiency of up to 100% in solution and in polystyrene films. 133 has a small energy difference ΔE_ST(0.06 eV) and an unusually large zero-field splitting energy ΔE_ZFS(85 cm^-1). The organic light-emitting diode fabricated with 133 as the green emitting dopant has a high EQE (19.4%) and a brightness of 5.4×10⁴cd/m².

In the complexes 137 ~ 142, besides the monodentate carbene, there is a bidentate ligand with a nitrogen coordinating atom, which forms a three-coordinated substance with copper. Complexes 137 and 138 reported by Thompson et al. Have similar chemical structures with more than 70% PLQYs, but only 137 shows thermally activated delayed blue fluorescence, while 138 shows pure yellow phosphorescence^[72]. This different luminescence property is related to the twist angle between the two ligands. Changing this angle will have a great impact on the energy difference between S₁ and T₁, thus affecting whether the complex has the nature of TADF. In 2017, Linares et al. studied the structures and related properties of complexes 139 ~ 142^[73]. The results show that the nature of the bridging group between pyridine rings in bidentate ligands plays an important role in the luminescent properties of these complexes. When the two pyridine rings changed from planar structure (complexes 139 and 140) to boat-like structure (complexes 141 and 142) with different bridging groups, the luminescence quantum efficiency increased significantly. In addition, devices based on 141 and 142 exhibit brightness and current efficiency that exceed the performance of devices based on 139 and 140. This provides useful information on how to enhance the optoelectronic properties of such complexes.

Among the complexes 143 ~ 150, bidentate carbene and bidentate phosphine POP are the ligands, forming four-coordinated copper complexes. In 2016, Wang et al. Synthesized copper complexes 143 and 144, both of which showed high stability in the solid state, but the emission wavelength could be adjusted by different carbene ligand structures^[74].

Although they all have TADF properties, the transition properties are different. The excited state properties of 143 are dominated by MLCT transition, while 144 includes MLCT and ligand-centered transition (LC). In 2018, they synthesized carbenes with different substituents and mononuclear cationic four-coordinated copper complexes 145-148 formed by POP ligands^[75]. These complexes have a typical distorted tetrahedral configuration, and the Cu — N bond length is shorter when the substituents on the carbene ligand are electron-donating methyl and methoxy than when the substituents are electron-withdrawing fluorine and chlorine. At room temperature, the emission wavelength can be adjusted from 489 nm to 539 nm by the group on the carbene ligand, and the PLQYs is 42% -69%. In 2020, Zhao et al. Synthesized and characterized two new nitrogen-containing heterocyclic carbene complexes 149 and 150. Whether there is a conjugated phenyl group on the imidazole ring will adjust the emission color from blue to green, and at the same time, it has a high PLQYs of nearly 100% at room temperature, which is rare in four-coordinated nitrogen heterocyclic carbene complexes^[76].

In 2016, Cisnetti et al. studied the binuclear copper complexes 151 ~ 153 formed by carbene and chlorine ligands^[77]. The distance between the two coppers is less than 2.88 Å, indicating that there is an interaction between them, which can enhance the spin-orbit coupling, so that both the forward and reverse intersystem crossing processes can be realized T₁↔S₁, thus showing the TADF performance. However, in complex 154, when the substituent on the heterocyclic carbene is methoxy, the unusually strong spin-orbit coupling makes it only show phosphorescence properties.

4.2 Copper complexes with (ISO) cyano ligands

Ligands with a coordinating atom of C also have an (ISO) cyano group. In 2017, Ko et al. Synthesized six neutral copper complexes 155 ~ 160 containing isocyanoborate ligands^[78]. The results show that the cationic emitters can be converted into neutral emitters with similar emission properties by the introduction of anionic isocyanoborate ligands. Like the related cationic copper complexes, the TADF of these solid-state complexes is caused by MLCT from the [d(Cu)→π^*(N^N)] excited state. In 2020, Yam et al. Formed a multinuclear copper complex 161 by tridentate phosphine ligand and cyano ligand, which was formed by three [Cu₃(μ-dpmp)₂] fragments connected by cyano. By studying its solid-state photophysical behavior, it was shown that it had TADF characteristics^[79].

5 Copper Complexes with N, P, O (S) as Coordinated Atoms

Copper complexes with N, P, O (S) are mononuclear and binuclear. The ligand and photophysical properties of these complexes are summarized in Table 6. Fig. 6 shows the structure of complexes 162-178.

5.1 Mononuclear complex

In the three-coordinate copper complexes 162 and 163 reported by Osawa in 2014, the coordinating atoms are S and P^[80]. Both complexes emit strong blue-green delayed fluorescence with solid PLQYs up to 95%, which is related to the spatial orientation of the aryl ring SPh. In 2017, Vogler studied the photophysical properties of complex 164 with coordination atom O^[81]. It is found that the complex has a single broad emission band with strong fluorescence and weak phosphorescence in argon saturated solution. If the solution is one that is saturated with air, the emission band will weaken. This is rare because in general the fluorescence of oxygen-containing complexes is not quenched by oxygen.

In the TADF copper complexes synthesized by Koshevoy et al. In 2019, complexes 165 and 166 have weakly coordinated isothiocyanato boronic acid ligands -SCN:B(C₆F₅)₃, which are less studied in transition metal coordination chemistry, and S in SCN coordinates with copper^[40]. These two complexes can be efficiently converted and synthesized by complexes 28 and 29 in which the SCN ligand is terminally coordinated. The boronation of SCN group changes the nature of S₁ and T₁ States, increases the transition from metal to ligand phosphine, and changes the nature of excited state transition into the mixed transition from metal to phosphine ligand and between phosphine ligands [d,p(M,P)→π(P^3/4)], which is obviously different from the excited state nature of ligand SCN in 28 and 29 in which only N participates in coordination.

In the series of TADF copper complexes synthesized by Wolf et al in the same year, the copper in complex 167 is four-coordinated with two P atoms of POP and the N and O atoms of 6,6 '-sulfonylbis (2-phenylpyridine) (Ph-DPSO₂), which is the first mononuclear copper complex with sulfone groups^[36]. Compared with the mononuclear complex 14 in which only N and P are coordinated, the emission spectrum of the complex 14 shows a red shift. Coordination of emitting electrochemical cells has a promising application in light-emitting electrochemical cells (LEECs), and the coordination mode of oxygen provides a new idea for the development of interesting luminescent complexes.

5.2 Binuclear complex

In 2017, Kato et al. Synthesized binuclear copper complexes 168 and 169 bridged by pyrazine^[82]. In the two molecules, the coordination environment of copper is different. 168 has an asymmetric structure in which one copper atom coordinates to the N atom of acetonitrile to form a P₂N₂ coordination environment, and the other copper atom coordinates to the O atom of water to form a P₂N₁O₁ coordination structure. While in 169, both copper formed nearly the same P₂N₁O₁ coordination structure. 168 emits a very bright yellow-green light with PLQYs of 70%, while 169 emits a very weak yellow light with PLQYs of only 5%. This shows that the ligand (water or acetonitrile) around copper is one of the key factors affecting the quantum efficiency of luminescence. The results show that when the ligand changes from acetonitrile to water, the ligand around copper changes from tetrahedral configuration to quasi-square planar structure, which greatly reduces the luminescence performance, that is, the rigid structure around copper is very important to the luminescence performance.

In 2018, Artem 'ev et al. Synthesized binuclear complexes 170 ~ 172, in which the bridging ligand is SCN, and the N and S atoms participate in coordination at the same time. Unlike the complexes 101 ~ 105 described above, the binuclear center does not have a butterfly or rhombic structure, but forms an almost planar eight-membered ring^[60]. These substances containing thiocyanate ligands also emit yellow-green light in the solid state. Similarly, in 2019, Koshevoy et al. Constructed dinuclear complexes 173 and 174 using thiocyanate and phosphine ligands with bidentate coordination properties, and both N and S in the ligand SCN participated in the coordination^[40]. The bridging of SCN in the excited state reduces its contribution to the highest occupied molecular orbital, so the excited state properties of 173 and 174 increase the transition from metal to ligand phosphine [d,p(M,P)→π(P^3/4)] compared with those of complexes 28,29 introduced earlier. Subsequently, Wolf et al. Synthesized a binuclear complex 175 in which both the N and O atoms of the ligand 6,6 '-sulfonylbis (2-methylpyridine) (Me-DPSO₂) were coordinated, and the emission spectrum was red-shifted compared with that of the binuclear complex 36 in which only N and P were coordinated^[36]. This is due to the strong electron-withdrawing ability of the sulfone group, which affects the electron-donating strength of the pyridine ring of the diimine ligand. This is the first binuclear copper complex involving a sulfone group. In a series of paddle-wheel-like binuclear copper complexes synthesized by Artem'ev et al., which showed efficient TADF, the auxiliary ligands of 176 and 177 were MeOH and Artem 'ev, respectively, and the atom coordinated to copper was oxygen. The PLQYs of 176 and 177 were higher than those of 37 ~ 40, both up to 85%, and the lifetime was about 20 μs^[43].

5.3 Polynuclear complex

Few polynuclear complexes containing O (S) coordination atoms have been reported. In 2019, Artem 'ev et al. Synthesized a long copper complex chain 178 linked by N, S and I by a simple grinding method. The monomer is a trinuclear copper coordination unit, and there is no interaction between copper and copper^[83]. At room temperature, the complex emits blue-green light with TADF property, the PLQYs can reach 70%, and the luminescence lifetime is 4. 0 μs.

6 Theoretical Study and Design of TADF Copper Complex

With the continuous progress of theoretical calculation methods, it has unique advantages in the study of the excited state properties of TADF copper complexes. By analyzing the luminescence mechanism of the known complexes, we can provide a direction for the experimental design and synthesis of more superior complexes. Fig. 7 shows the structures of some complexes analyzed and designed by relevant theoretical calculations.

Lv et al. Studied the structure and luminescence process of TADF complex 179 in depth^[84]^[85]. It is shown that the reverse intersystem crossing rate from T₁ to S₁ can compete with the radiative decay rate of the T₁state. This means that the T₁state can return to the S₁ state, that is, TADF can occur. In addition, the calculation shows that the free rotation of the phenyl ring in the pop ligand is an important reason why the T₁ can return to the S₁. Subsequently, they further studied the luminescence properties of related complexes 180-182 by theoretical calculation methods^[86]. The calculation shows that the vibrational spin-orbit coupling can significantly enhance the intersystem crossing rate, which will affect the decay path of the excited state.

Cui et al. Studied the TADF properties of two copper complexes 183 and 184 containing tetrazolopyridine and POP ligands by QM/MM method in gas phase, solution and crystal environment^[87]. The calculated results show that the molecular vibrational ratio is significantly suppressed in the gas phase and in solution due to the rigidity of the crystal environment, and thus the intersystem crossing rate in the crystal is slightly reduced accordingly. This indicates that the environmental effect is very important for the regulation of the photophysical properties of copper complexes. In 2018, Sundermeyer et al. Found that the performance of the synthesized complex 185 needed to be improved when applied to OLEDs^[88]^[89]. Therefore, they first studied the effect of the degree of distortion between ligands on the luminescent properties by theoretical calculation, and then proposed the structures of 186 and 187, that is, introducing o-methyl and n-butyl on the aryl group of phosphine ligands to increase the steric hindrance and reduce the degree of distortion, and then carried out experimental synthesis. The experimental results show that the luminescent quantum efficiency of the designed complexes is obviously improved. In 2020, Zou et al. First theoretically studied the relationship between the structure of two experimentally synthesized TADF molecules 188 and 189 and the luminescence mechanism. There are nested boronic acid, pyridine or phenanthroline ligands in their structures^[90]^[91]. The calculation results show that the benzene ring between the two pyridyl units in 189 enhances the rigid structure of the molecule, resulting in the easy occurrence of S₁→S₀ radiation process, the more effective reverse intersystem crossing process, and the better relative stability of the T₁ state, which is inferred to be the reason for its excellent TADF performance. Therefore, analogous complexes 190 and 191 were designed to extend the conjugated structure in the N ^ N ligand in order to further improve the quantum efficiency. In addition, the introduction of electron-withdrawing pyrazinyl will increase the crossing speed of the antisystem by two orders of magnitude compared with 189, and will have better TADF performance.

Theoretical studies on the luminescence mechanism of binuclear complexes and the design of new complexes have also been reported. In 2019, Yu et al. applied a design strategy based on a symmetric structure that would rapidly reduce the luminescence lifetime of the TADF to the binuclear copper compounds 192-195^[92]. Theoretical calculation and experimental research prove that this design concept is very successful. The luminescence lifetime of 192 is only 1.2 μs, which is about three times less than that of other copper mononuclear or binuclear complexes found so far.

7 Conclusion and prospect

In recent years, the coordination atoms of copper complexes with TADF properties are mainly N, P, X (halogen), C, S (O) and so on, and the ligands are mainly phosphine derivatives, nitrogen heterocyclic derivatives, nitrogen heterocyclic carbene and (ISO) cyano. From the reported literatures, it can be seen that the strength of the ligand field, the conjugation of the ligand itself, the angle or spatial orientation between the ligands and the environment will affect the TADF properties and luminescence efficiency of copper complexes. Because these factors will change the energy difference between the singlet and triplet States, the energy difference is generally around 0.1 eV for TADF performance. At the same time, it will also affect the degree of spin coupling, thus changing the proportion of TADF in the luminescence component.

Some of the copper complexes with TADF properties reported in the experiment have excellent luminescent properties and have potential applications in the field of OLEDs, and some of them have shown good photoelectric properties as devices. At the same time, it is necessary to study the luminescence mechanism of these complexes by means of theoretical calculation. Complexes with good luminescent properties can be designed by using the principles or laws of theoretical calculations, which provides useful information for experimental synthesis and has important guiding significance for the design of new luminescent copper complexes. Of course, the designed structure is ultimately subject to further synthesis and development in experiments.

Although some progress has been made in the experimental and theoretical studies of TADF copper complexes, further studies are still needed for practical applications. It is necessary to make rational use of the adjustability of luminescence properties, explore complexes with simple synthesis methods, stable properties and efficient luminescence, and expand their applications in the field of optoelectronic materials and devices. Because of the unique luminescent properties of TADF copper complexes, and with the continuous integration and mutual penetration of theoretical research and experimental research, more and more researchers will pay attention to their research, and more and better results will emerge.

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