Ligand Mediated Luminescence Enhancement in Cyclometalated

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Ligand Mediated Luminescence Enhancement in Cyclometalated Rhodium(III) Complexes and Their Applications in Efficient Organic Light-Emitting Devices Fangfang Wei, Shiu-Lun Lai, Shunan Zhao, Maggie Ng, MeiYee Chan, Vivian Wing-Wah Yam, and Keith Man-Chung Wong J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06308 • Publication Date (Web): 16 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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Journal of the American Chemical Society

Ligand Mediated Luminescence Enhancement in Cyclometalated Rhodium(III) Complexes and Their Applications in Efficient Organic Light-Emitting Devices Fangfang Wei,a Shiu-Lun Lai,b Shunan Zhao,a Maggie Ng,b Mei-Yee Chan,*b Vivian Wing-Wah Yam,b and Keith Man-Chung Wong*a Department of Chemistry, Southern University of Science and Technology, 1088 Xueyuan Blvd., Shenzhen, 518055, P.R. China. b Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee, Hong Kong) and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China. a

ABSTRACT: A series of luminescent cyclometalated rhodium(III) complexes have been designed and prepared. The improved luminescence property is realized by the judicious choice of a strong σ-donor cyclometalating ligand with lower-lying intraligand (IL) state that would raise the d-d excited state and introduction of lower-lying emissive IL excited state. These complexes exhibit high thermal stability and considerable luminescence quantum yields as high as up to 0.65 in thin film offering themselves as promising light-emitting materials in OLEDs. Respectable external quantum efficiencies of up to 12.2% and operational halflifetime of over 3,000 hours at 100 cd m–2 have been achieved. This work demonstrates a breakthrough as the first example of an efficient rhodium(III) emitter for OLED application and opens up a new avenue for diversifying the development of OLED materials with rhodium metal being utilized as phosphors.

INTRODUCTION Excited state properties of octahedral d6 transition metal complexes, including ruthenium(II),1,2 rhenium(I),1,3 osmium(II),1,2,4 iridium(III)1,5-7 and rhodium(III),1,8-10 have aroused tremendous interests due to their attractive photophysical and photochemical behaviors. From the last two decades, the establishment of the predominant role of luminescent cyclometalated iridium(III) system5-7 as photofunctional materials has stemmed from their overwhelming properties for the potential biological and energy related applications.6,7 Since the pioneering work of Thompson, Forrest and coworkers7a in employing cyclometalated iridium(III) complexes first reported by Watts5a,b as phosphorescent emitters in organic light-emitting devices (OLEDs), promising applications7,11 have been realized as demonstrated by their rapid adoption in smartphones and displays everywhere. Being the most important components in OLEDs, there has been a rapid surge of interest in the studies of phosphorescent emitters with heavy metal centers because of their capability to achieve 100% internal quantum efficiency from harvesting the accessible triplet excited state associated with strong spinorbit coupling (SOC).11 While most of the related works have been placed with particular emphasis on the use of iridium(III)7,11 and platinum(II)11,12 complexes, the use of metal complexes of other transition metals11,13-15 as emitters has remained a relatively niche topic in order to provide a diversity of OLED materials. Recently, Che16a,b and Li16c have independently developed different classes of palladium(II) complexes, coordinated to tetradentate ligands with Cdeprotonated donor atoms, which have also been demonstrated

to be strongly luminescent for the application in OLEDs. This strategy by using not only the strong field ligand but also the rigid scaffold with four coordination sites are anticipated to disfavor the non-radiative deactivation pathway in order to boost up the luminescence properties. Another interesting class is cyclometalated gold(III) complexes, which is isoelectronic and isostructural to the platinum(II) system. Through the choice of strong σ-donating ligand, the gold(III) complexes exhibit strong luminescence properties, as proven by the demonstration of highly efficient OLEDs based on such gold(III) complexes.11,17 Yam and co-workers have recently pioneered a unique concept of thermally stimulated delayed phosphorescence (TSDP), from which triplet excitons are upconverted from a lower-lying triplet state to a higher-lying triplet state through spin-allowed reverse internal conversion (RIC). This up-conversion process was found to significantly enhance the luminescence quantum yields (Фlum) by over 20folds.17e Similarly, high Фlum could also be obtained through the process of thermally activated delayed fluorescence (TADF) or metal assisted delayed fluorescence (MADF) arising from the reversed intersystem crossing (RISC).18 In such case, very small energy gap between the lowest singlet state (S1) and the lowest triplet excited state (T1) as well as the spatially well-separated frontier orbitals, i.e. highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), are required. There has recently been a fastgrowing interest in the use of TADF/MADF light material for the fabrication of high-efficient OLEDs.15,16c,18 Platinum group metals (PGMs) include the elements of ruthenum, rhodium, palladium, osimum, iridium and platinum, while rhodium(III) and iridium(III) are considered as very close congeners in the family of PGMs sharing similar

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synthetic methodology, structural characteristics, and some physical and chemical properties.1,5-10 On the contrary, the luminescence studies of polypyridyl and cyclometalated rhodium(III) system have been much less explored,8c,9a-c,10 based on the fact that most of them are only luminescent at low temperature. The related photo-functional application of luminescent rhodium(III) system is also very rare.10c This is mainly suffered from the lack of luminescence at room temperature owing to the presence of thermally accessible non-luminescent d-d ligand field (LF) excited state. The presence of LF state at comparable energy to those of the luminescence excited states of ligand-centered (LC) and/or metal-to-ligand charge transfer (MLCT) characters, as revealed by temperature-dependent luminescence lifetime measurements,9d remains challenging to be overcome. Through the incorporation of a cyclometalating 1,3-bis(1isoquinolyl)benzene pincer ligand having the advantages of strong ligand field as well as rigid structural motif, Williams and co-workers have recently synthesized luminescent rhodium(III) complexes with the highest Фlum of up to 10% in solution state at room temperature.10e

N N Rh N N

O O

R

R

R = CH3

(1)

R = CF3

(2)

R = C6H5 (3)

Chart 1. Molecular structures of 1−3. Although tremendous efforts have been put in to tackle the shortcomings of the luminescence performance of rhodium(III) system, the reported Фlum still could not satisfactorily meet the requirement for OLED application. To the best of our knowledge, rhodium(III) system is up to now the only remaining family member of PGMs to be utilized as lightemitting material in OLEDs. In this report, we develop a series of luminescent cyclometalated rhodium(III) complexes, 1–3 (Chart 1), and demonstrate a breakthrough as the first example of an efficient rhodium(III) emitter for OLED application. Through the judicious choice of a strong σ-donor cyclometalating ligand with lower-lying intraligand (IL) state, the enhanced luminescence properties of rhodium(III) system from the integration of two strategies, i.e. raising d-d excited state and introduction of lower-lying emissive IL excited state, have been anticipated. The neutral formal charge, high thermal stability and Фlum of over 65% in solid-state thin films render these complexes possible for device fabrication by vapor deposition or solution processing technique. Notably, good device performance, including external quantum efficiencies (EQEs) up to 12.2% and fairly respectable operational halflifetime of over 3,000 hours at 100 cd m–2 in the optimized OLEDs, have been achieved from this rhodium(III) system. To the best of our knowledge, such cyclometalated rhodium(III) complexes in this work are by far the most strongly luminescent rhodium(III) system. Parallel to the recent

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development in the new class of strongly emissive 4d6 molybdenum(0) complexes with bidentate isocyanide ligand19a,b which is considered as the analogue of the long known 5d6 tungsten(0) system,19c the present study of rhodium(III) complexes with improved luminescence properties represents another viable emissive alternative of 5d6 iridium(III) system.

RESULTS AND DISCUSSION Synthesis and Characterization. For the introduction of a lower-lying IL state and the maintenance of neutral formal charge in the target complexes 1–3, the cyclometalating ligand of 2,3-diphenylquinoxaline (dpqx) and anionic acetylacetonate (acac) were chosen, respectively. The dinulcear rhodium(III) complex precursor, [Rh(dpqx)2Cl]2, was prepared by the reaction of rhodium trichloride trihydrate and dqpx in ethanol/water solution at reflux condition. Subsequent reaction of [Rh(dpqx)2Cl]2 with sodium 2,4-pentanedionate, deprotonated form of hexafluoroacetylacetone or 1,3diphenylpropane-1,3-dione in the presence of base in methanol/dichloromethane mixture at reflux temperature afforded complexes 1–3, respectively. All complexes 1–3 were characterized by 1H, 13C{1H} NMR, HR-MS and elemental analysis and their molecular structures were also determined by X-ray crystallography. As shown in Figure 1, each molecule exhibits an octahedral geometry about the rhodium(III) metal center. Similar to other related rhodium(III) and iridium(III) complexes, the nitrogen donor atoms in two cyclometalating ligands are trans to each other and all the bond lengths and bond angles (See Supporting Information) are within normal ranges.10c,e The quinoxaline and coordinated phenyl groups of dpqx ligand in 1–3 are not fully coplanar with the interplanar angels of 29.47–27.57°, 26.25–30.96°, 26.06–26.67°, respectively. Such distortion from the ideal coplanarity renders the quinoxaline plane to be pointing away from the acac ligand in order to release the steric hinderance between them. All complexes 1–3 are thermally stable with high decomposition temperatures as revealed by the thermogravimetric analysis (TGA) (Figure S1).

Figure 1. X-Ray crystal structure of 1–3. The solvent molecules and hydrogen atoms are omitted, and only the Δ form is shown for clarity.

The photophysical data of 1–3 have been determined and the data are summarized in Table 1. Their UV-vis absorption spectra in fluid solution at 298 K (Figure 2) show intense high-energy absorption bands at 335–410 nm and less intense low-energy absorption bands at 420–530 nm. The high-energy absorption bands, which are commonly observed in the related

Table 1. Photophysical and electrochemical data of 1–3.


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Complex

Absorption In CH2Cl2 λabs, nm (ε, dm3 mol–1 cm–1)

Emission In CH2Cl2 λem, nm (Фsol, %;a τ, μs; kr, 105 s–1)

Emission In thin filmb λem, nm (Фfilm, %;c τ, μs; kr ,105 s–1)

Oxidationd Epae, V (vs. SCE)

Reductiond E1/2f, V (vs. SCE)

EHOMO,g eV

ELUMO,g eV

1

240 (45535), 285 (27280), 326 (15130), 375 (13865), 455 (5005)

612 (0.11; 0.79; 0.014)

603 (49.10; 23; 0.21)

+1.32, +1.69

–1.36, –1.62

–6.12

–3.44

2

240 (64940), 259 (51030), 283 (41085), 383 (22540), 441 (7615)

598 (0.98; 1.64; 0.060)

597 (44.80; 32; 0.14)

+1.63, +2.38

–1.28, –1.50

–6.43

–3.52

3

243 (63715), 262 (52860), 284 (41315), 370 (24355), 455 (5725)

603 (0.31; 0.81; 0.038)

602 (65.40; 25; 0.26)

+1.38, +1.67

–1.38, –1.67

–6.14

–3.42

a Luminescence quantum yield Ф , measured at room temperature using [Ru(bpy) ]Cl in degassed aqueous solution as the sol 3 2 reference (λex = 436 nm, Фlum = 0.042). b 2 wt% doped in MCP c Absolute emission quantum yields Фfilm in solid-state thin film. d In dichloromethane solution with nBu4NPF6 (0.1 M) as the supporting electrolyte at room temperature; scan rate 100 mV s-1. e Epa refers to the anodic peak potential for the irreversible oxidation waves. f E1/2=(Epa+Epc)/2; Epa and Epc are anodic peak and cathodic peak potentials, respectively. g EHOMO and ELUMO levels were calculated from electrochemical potentials, i.e., EHOMO = –e(4.8 V+Eoxpa); ELUMO = –e(4.8 V+Ered1/2).

iridium(III) analouges,7d are assignable to the spin-allowed singlet intraligand (1IL) π-π* transitions of the dpqx ligand. The low-energy absorption bands are attributed to the intraligand charge transfer (ILCT) transition from the phenyl moiety to the quinoxaline unit on the dpqx ligand, mixed with some MLCT dπ(Rh)→π*(dpqx) transition. Unlike most of the rhodium(III) complexes which are essentially non-luminescent, it is noteworthy that the present cyclometalated rhodium(III) complexes show orange-red photoluminescence (PL) with peak maxima at 598–612 nm in dichloromethane solutions at 298 K (Figure 2). This luminescence is suggested to originate from a triplet parentage, taking into consideration the large Stokes shift and the relatively long luminescence lifetimes (0.79–1.64 μs). The observation of reduced luminescence lifetimes in oxygenated solution (1, 0.08 μs; 2, 0.65 μs; 3, 0.22 μs) further supports the triplet nature of the excited state. In light of the excitation peaks that are resemble the corresponding low-energy absorption bands, the luminescence origin is reasonably assigned as the triplet excited state of intraligand charge transfer (ILCT) origin, with some mixing of MLCT dπ(Rh)→π*(dpqx) character.

moiety to the quinoxaline unit of the dpqx ligand, which is consistent with the observation of vibronic structured peaks. Such luminescence is also found to be insensitive to the solvent polarities such as toluene, dichloromethane, tetrahydrofuran and acetonitrile (Figure S2), indicating the minor contribution of MLCT character to the luminescence. Nanosecond transient absorption (TA) spectroscopy in dichloromethane solution at 298 K was investigated in order to study the nature of the excited states. From the TA difference spectra of 1–3, two positive absorption bands at 375 nm and 415 nm assignable to the radical anion absorptions of the cyclometalating ligand, are observed (Figure S3). The TA spectra also showed an additional broad absorption band ranging from 550–775 nm, with the similar lifetimes as their respective PL. These absorption bands are tentatively assigned as absorption from the triplet excited state of ILCT transition, with some mixing of MLCT character.

Figure 3. Normalized PL spectra and PLQY of complexes 1–3 at different excitation wavelengths in solid-state thin film (2 wt% in MCP). Insert shows the photo of thin-film PL of 3 under UV irradiation. Figure 2. UV-Vis absorption and emission spectra of complexes 1–3 in dichloromethane solution at 298 K.

The anionic acac type ligand is anticipated to influence the dπ(Rh) orbital but not the cyclometalating ligand, and the occurrence of very similar luminescence energy of 1–3 containing the acac ligands with various substitutents, [CH3 (1); CF3 (2); C6H5 (3)], suggests that the luminescence is largely dominated by ILCT [π→π*] character from the phenyl

Intense orange luminescence of 1–3 at 597–603 nm has also been observed in the PL spectra of 1–3 in doped N,Ndicarbazolyl-3,5-benzene (MCP) thin films (Figure 3). In contrast to common square-planar metal complexes which will suffer from triplet-triplet annihilation and π-π interaction between the molecules at high doping concentration, no observable luminescence quenching as well as luminescence peak maxima shift are found in 1–3, upon increasing the



doping concentration from 2 to 10 wt% (Figures S4–S6). It is noteworthy that respectable Фlum of 0.44–0.65 has been obtained in the doped thin films (Figure 3). Nevertheless, to the best of our knowledge, these are the highest Фlum values among all reported rhodium(III) complexes, demonstrating the successful luminescence enhancement by employing a strong σ-donor cyclometalating ligand with lower-lying IL state in metal complexes with octahedral geometry. Variabletemperature PL measurement of 3 was also carried out in thin film from 298 K to 78 K. Upon cooling, the emission peaks remain unchanged except that the vibronic-structured features are becoming more apparent (Figure S7a). In addition, it is found that the emission intensity has been increased by more than two-folds with elongation of lifetimes (Figure S7b). Such behavior of increased emission intensity and elongated lifetime at low temperature has been commonly observed in most phosphorescent complexes, due to the reduced nonradiative decay processes arising from restricted molecular motions. Although some examples of luminescent transition metal systems have been reported to exhibit TADF or MADF, the large energy difference between the singlet and triplet states ΔE(S1–T1), from the computational studies (vide infra), indicates that the occurrence of such delayed fluorescence is unlikely in our system. The electrochemical properties of 1–3 were investigated by cyclic voltammetry and the potentials, together with the estimated HOMO and LUMO energy levels, are tabulated in Table 1. Upon cathodic scan, two quasi-reversible reduction couples are featured at –1.28 to –1.38 V and at –1.50 to –1.67 V (vs. SCE) (Figure S8), attributed to the successive dqpx ligand-centered reductions. Anodic shifts of the first reduction by about 0.08 V are observed in 2, relative to those in 1 and 3, resulting from the indirect influence upon coordination of the more electron-deficient hexafluoroacetylacetone (hfac) ligand with -CF3 groups. For the anodic scan, the first irreversible anodic peak at +1.32 to +1.63 V (Figure S8) is attributed to a mixed metal-/ligand-centered oxidation of the rhodium(III) metal center and ligated phenyl ring on dqpx ligand. Similarly, the more positive potential for this oxidation in 2 is due to the lower electron-richness of the rhodium(III) metal center, upon the attachment of the hfac ligand. In order to gain more insight into the electronic structures as well as the nature of the absorption and emission origins of these rhodium(III) complexes, density functional theory (DFT) and time-dependent DFT (TDDFT) calculations have been performed on 1–3. Summarized in Table S1 are the first fifteen singlet–singlet transitions of 1–3 computed by the TDDFT/CPCM (CH2Cl2) method, and some of the molecular orbitals involved in the transitions are shown in Figures S9– S11. The S0→S1 transitions of 1–3 computed at 467, 455 and 466 nm, respectively, correspond to the HOMO→LUMO excitation. The HOMO is the π orbital localized on the phenyl ring, which is ligated to the rhodium(III) metal center, of the dpqx ligand, with mixing of the dπ(Rh) orbital. The LUMO is mainly the π* orbital on the quinoxaline unit of the dpqx ligand. Therefore, the S0→S1 transition can be assigned as ILCT [π→π*] transition from the phenyl moiety to the quinoxaline unit of the dpqx ligand, with mixing of MLCT [dπ(Rh)→π*(dpqx)] transition, which is in agreement with the Table 2. Molecular orbital (MO) compositions of 1–3 at the T1 geometry optimized at the PBE0-D3 level.

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experimental energy trend of the low-energy absorption bands and their spectral assignments. The higher-lying absorption band computed at ca. 380 nm is predominantly contributed by the HOMO–2→LUMO excitation in 1 and 3, and the HOMO– 1→LUMO excitation in 2. The HOMO–2 of 1 and 3 and the HOMO–1 of 2 are the π orbitals on the phenyl ring, which is ligated to the metal center, and the quinoxaline unit of the dpqx ligand. As a result, the higher-lying absorption band can be assigned as IL [π→π*] transition of the dpqx ligand, with some charge transfer from the phenyl ring to the quinoxaline moiety, supporting the experimental spectral assignment. Shown in Figure 4a is the orbital energy diagram displaying the frontier molecular orbitals and their corresponding energies. The HOMOs of 1 (–5.94 eV) and 3 (–5.95 eV) have similar energies whereas the energy of the HOMO of 2 (–6.11 eV) is significantly lower than those of 1 and 3, by ca. 0.16 eV. Similarly, the LUMOs of 1 (–2.41 eV) and 3 (–2.42 eV) are of similar energy but that of 2 (–2.53 eV) is more stable than those of 1 and 3 by ca. 0.11 eV. Compared to 1 and 3, the more electron-withdrawing –CF3 groups on the (CF3)2-acac ligand in 2 tend to stabilize both the HOMO and LUMO. This is in good agreement with the trend observed in the electrochemical study, where both the oxidation and reduction potentials are more positive in 2. The HOMO–LUMO energy gap of 2 (3.59 eV) is larger than those of 1 (3.53 eV) and 3 (3.53 eV) by 0.06 eV, resulting in the blue shift of the lowenergy absorption band from 1 and 3 to 2.

Figure 4. (a) Orbital energy diagram of complexes 1–3. (b) Plots of spin density (isovalue = 0.002) of the T1 states of 1–3. To investigate the nature of the emissive states, geometry optimization on the T1 of 1–3 has been performed with the unrestricted method (UPBE0-D3/CPCM). As shown in Figure 4b, the spin density is localized on the metal center, the quinoxaline unit and the ligated phenyl ring of the dpqx ligand. Natural bond orbital (NBO) analysis have been carried out to investigate the compositions of the HOMOs and LUMOs, which are listed in Table 2. Only the rhodium metal, the ligated phenyl ring and the quinoxaline moiety on the dpqx

1

MO

Rh

Phenyl

Quinoxaline

LUMO

2.66%

16.02%

79.56%


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2

3

Journal of the American Chemical Society HOMO

22.50%

44.66%

25.67%

LUMO

2.64%

15.57%

80.10%

HOMO

21.25%

38.32%

22.94%

LUMO

1.76%

13.49%

83.30%

HOMO

25.90%

52.79%

16.89%

ligand are included, with sums of these three compositions of less than 100%. The calculated results of 1–3 show that the HOMOs are composed of the π orbital on the ligated phenyl ring (38–53%), with some mixing of the dπ orbital on rhodium (21–26%) and π orbital on the quinoxaline moiety (17–26%). For the LUMOs, they are dominantly composed of the π* orbital on the quinoxaline moiety (80–83%). As a result, the 3ILCT [π(phenyl)→π*(quinoxaline)] character dominates over the 3MLCT [dπ(Rh)→π*(dpqx)] character in the emission of 1–3, which is contributed by the HOMO→LUMO excitation. The computed emission energies of 1–3 (Table S2) are generally over-estimated, yet the trend is well in agreement with the corresponding experimental results, i.e. 1 ≈ 3 > 2. Geometry optimizations on the S1 of 1–3 have also been performed with TDDFT to identify the emission origin. The optimized ground-state (S0), S1 and T1 geometries of 1–3, are tabulated in Tables S3–S11. The energy differences between the geometry optimized S1 and T1 states of 1–3, ΔE(S1–T1), given in Table S12 range from 0.20 to 0.38 eV, indicating a relatively low possibility for TADF to occur.

Figure 5. Characteristics of vacuum-deposited OLEDs based on 3. (a) EL spectra with different dopant concentrations. (b) EQEs with different hole-transporting layers. (c) Operational lifetime of the vacuum-deposited OLED made with 5 v/v% 3.

Solution-processed OLEDs based on 1–3 were prepared for the investigation of the electroluminescence (EL) properties of these rhodium(III) complexes. Particularly, devices with the structure of indium-tin-oxide (ITO)/poly(ethylenedioxythio-phene):poly(styrene sulfonic acid) (PEDOT:PSS; 40 nm)/emissive layer (30 nm)/tris(2,4,6trimethyl-3-(pyridine-3-yl)phenyl)borane (3TPyMB; 5 nm)/1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB; 40 nm)/LiF (1 nm)/Al (100 nm) were fabricated, in which the emissve layer was formed by spin-coating a solution of rhodium(III):MCP in dichloromethane (Figure S12). As shown in Figure S13, all devices display the vibronicstructured EL spectra and are almost identical to their PL spectra in solid-state thin films in the absence of undesired emission from adjacent carrier-transporting or host materials.

Similar to the corresponding PL studies, only small changes of ±0.01 in the CIE x and y values for all the devices are observed with increasing dopant concentration from 2 to 10 wt%. Remarkably, good performance with high maximum current efficiency of 9.4 cd A–1 and EQE of 6.4 % is achieved for the optimized device made with 8 wt% 2 (Figure S14). Table S13 summarizes the key parameters for solutionprocessed devices based on 1–3. Table 3. Key parameters of vacuum-deposited OLEDs the structure of ITO/HTM/5 v/v% 3:mCBP (20 nm)/Tm3PyP26PyB (50 nm)/LiF (1 nm)/Al (150 nm) based on 3 with different HTMs. HTL

Max. CE / cd A–1

Max. PE / lm W–1

Max. EQE / %

λmax / % (FWHM / nm)a

CIE x,yb

Ic

16.7

10.5

11.7

600 (84)

(0.63,0.37)

IId

17.2

10.8

12.1

600 (84)

(0.63,0.37)

IIIe

17.5

11.0

12.2

600 (84)

(0.63,0.37)

IVf

16.0

10.0

11.3

600 (86)

(0.63,0.37)

Vg

17.1

7.7

11.9

600 (86)

(0.63,0.37)

a Measured at 100 cd m–2. b CIE coordinates measured at 100 cd m–2. c MoOx (2 nm)/α-NPD (40 nm)/TCTA (5 nm). d α-NPD (40 nm)/TCTA (5 nm). e TAPC (40 nm)/TCTA (5 nm). f MoOx (2 nm)/TCTA (40 nm). g TCTA (40 nm).

Using 3 with the highest Фlum in solid-state thin film and the highest decomposition temperature, vacuum-deposited OLEDs with the structure of ITO/molybdenum oxide (MoOx; 2 nm)/di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane (TAPC; 40 nm)/4,4ˈ,4ˈˈ-tris(carbazol-9-yl)triphenylamine (TCTA; 5 nm)/x% rhodium(III):MCP (20 nm)/1,3,5-tris(6-(3(pyridin-3-yl)phenyl)pyridine-2-yl)benzene (Tm3PyP26PyB; 50 nm)/LiF (1 nm)/Al (150 nm) were also fabricated (Figure S15), in which 3 was doped into MCP at different concentrations (i.e. x = 2, 5, 8, 11, and 14 v/v%). Almost identical EL spectra were featured (Figure 5a) as in the corresponding solution-processed OLEDs. Maximum current efficiency of 9.9 cd A–1 and EQE of 7.0% were achieved for the 5 v/v% doped device (Figure S16 and Table S14). In order to improve the efficiencies, various host materials, including TCTA, 3,3ˈ-di(9H-carbazol-9-yl)biphenyl (mCBP) and beryllium bisbenzo[h]quinolin-10-olate (Bebq2), were employed. Remarkably, device efficiencies could be improved to 11.9 cd A–1 and 8.1% when mCBP was used as the host (Figure S17 and Table S15). Further enhancement could be done by either removing the hole-injecting MoOx or using a hole-transporting material (HTM) with lower hole mobility, i.e. N,Nˈ-bis(naphthalene-1-yl)-N,Nˈ-bis(phenyl)-2,2ˈ-dimethylbenzidine (α-NPD) or TCTA. Apparently, the current efficiencies and EQEs could be significantly boosted up to ~17.5 cd A–1 and ~12.2%, respectively (Figure 5b and Figure S18). While TCTA is an excellent electron-blocking material, the insertion of a thin TCTA layer (i.e. 5 nm) at the HTM/emissive interface can effectively accumulate electrons within the emissive layer for exciton formation and light emission. Table 3 summarizes the key parameters for these vacuum-deposited devices based on 3. The reduced holetransport can result in a better balance in the hole and electron currents in the emissive layer and thus improved device



efficiency. The operational stability for the vacuum-deposited device based on 3 was also explored. Particularly, the vacuumdeposited device was measured by accelerated testing at a constant driving current density of 20 mA cm–2. Impressively, the device exhibits an operational half-lifetime (i.e. the time required for the luminance to drop to 50% of its initial value) of ~52.7 hours at an initial brightness of 1,084 cd m–2 (Figure 5c). This corresponds to ~946 hours at 1,000 cd m–2 and over 3,000 hours at 100 cd m–2. The respectable EQE values and satisfactory operational stability clearly demonstrate the capability of such cyclometalated rhodium(III) complexes serving as promising phosphorescent dopants, and more importantly, this work represents the first successful demonstration of application studies of rhodium(III) complexes in OLEDs.

CONCLUSION In summary, we have developed a new class of luminescent rhodium(III) complexes in which the luminescence quenching problem from the lowest-lying d–d state is overcome by the incorporation of a strong σ-donor cyclometalating ligand with lower-lying IL state. These complexes exhibit high thermal stability and Фlum as high as up to 0.65 in thin film offering themselves as promising lightemitting materials in OLEDs. Notably, efficient solutionprocessed and vacuum-deposited OLEDs based on these rhodium(III) complexes with respectable EQEs of 6.4% and 12.2%, respectively, and fairly respectable operational halflifetime of over 3,000 hours have been realized. This work represents for the first time the application studies of rhodium(III) complexes in OLEDs and opens up a new avenue for diversifying the development of OLED materials. Apart from the main application of rhodium in catalysis for nitrogen oxides reduction in exhaust gases in catalytic converters for cars, the breakthrough of another potential application of rhodium in OLEDs is demonstrated. Modification of the cyclometalating ligand as well as the ancillary ligand is in progress in order to tune the luminescence color and further improve the EL performance.

EXPERIMENTAL SECTION Synthesis. All the solvents for synthesis were all of analytical grade. Rhodium(III) chloride hydrate was purchased from Innochem. 2,3-Diphenylquinoxaline and sodium 2,4pentanedionate hydrate were purchased from Aladdin Chemical Company. Diphenyl-1,3-propanedione and hexafluoroacetylacetone were purchased from Energy Chemical Company. [Rh(dpqx)2Cl]2. This was synthesized by following a literature procedure.9,10 Rhodium trichloride trihydrate (100 mg, 0.38 mmol) was mixed with 2,3-diphenylquinoxaline (268 mg, 0.95 mmol) and dissolved in ethanol/water solution (20 mL, ethanol/water = 3:1, v/v). The reaction mixture was heated to reflux in the dark for 22 h, and was then cooled to room temperature. Deionized water (20 mL) was added, and the reaction mixture was refrigerated for 3 h. The precipitate was collected on a glass filter frit and washed with methanol (3×5 mL). The remaining precipitate was dissolved in dichloromethane (20 mL) and filtered. The filtrate was evaporated, and the orange product was collected to yield [Rh(dpqx)2Cl]2 . Yield 200 mg (75 %).

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[Rh(dpqx)2(acac)] (1). A mixture of [Rh(dpqx)2Cl]2 (100 mg, 0.07 mmol) and sodium 2,4-pentanedionate hydrate (17.4 mg, 0.14 mmol) in 10 mL of methanol/dichloromethane (1:1 v/v) was refluxed under an inert atmosphere of nitrogen for 8 h. The solution was then cooled to room temperature; water was added to the reaction mixture and extracted with CH2Cl2. The combined organic layer was dried over anhydrous Mg2SO4 and filtered to obtain the filtrate, to which diethyl ether and dichloromethane were added to obtain [Rh(dpqx)2(acac)] as red crystals. Yield 88 mg (81 %). 1H NMR (400 MHz, CDCl3) δ (ppm): 8.48 (d, J = 8.5 Hz, 2H), 8.11 (d, J = 8.4 Hz, 2H), 7.99 (m, 4H), 7.68 (t, J = 7.5 Hz, 2H), 7.60 (m, 6H), 7.55 (t, J = 7.8 Hz, 2H), 7.08 (dd, J = 7.9 Hz and 1.2 Hz, 2H), 6.66 (td, J = 7.2 Hz and 1.2 Hz, 2H), 6.60 (td, J = 7.2 Hz and 1.2 Hz, 2H), 6.52 (d, J = 7.6 Hz, 2H), 4.64 (s, 1H), 1.61 (s, 6H). HRMS (ESI). Calcd for C45H33O2N4Rh+H: m/z 765.1692. Found: m/z 765.1731. Elemental analysis (%) calcd for C45H33O2N4Rh•(CH2Cl2)0.50 (found): C 67.71 (67.86), H 4.25 (4.33), N 6.94 (6.88). [Rh(dpqx)2(hfac)] (2). A mixture of [Rh(dpqx)2Cl]2 (100 mg, 0.07 mmol), hexafluoroacetylacetone (hfac) (29.6 mg, 0.14 mmol) and K2CO3 (97 mg, 0.7 mmol) in 20 mL of methanol/dichloromethane (1:1 v/v) was refluxed under an inert atmosphere of nitrogen for 8 h. The solution was then cooled to room temperature; water was added to the reaction mixture and extracted with CH2Cl2. The combined organic layer was dried over Mg2SO4 and filtered to obtain the filtrate, to which diethyl ether and dichloromethane were added to obtain 2 as red crystal. Yield: 85 mg (70 %). 1H NMR (400 MHz, CDCl3) δ (ppm): 8.16 (d, J = 8.4 Hz, 2H), 8.11 (d, J = 8.6 Hz, 2H), 7.94 (m, 4H), 7.74 (t, J = 7.5 Hz, 2H), 7.62 (m, 6H), 7.56 (t, J = 7.7 Hz, 2H), 7.07 (dd, J = 7.9 Hz and 1.2 Hz, 2H), 6.72 (td, J = 7.6 Hz and 1.2 Hz, 2H), 6.65 (td, J = 7.6 Hz and 1.2 Hz, 2H), 6.49 (d, J = 7.6 Hz, 2H), 5.43 (s, 1H). HRMS (ESI). Calcd for C45H27O2N4F6Rh+H: m/z 873.1127. Found: m/z 873.1155. Elemental analysis (%) calcd for C45H27O2N4F6Rh•(CH2Cl2)0.65 (found): C 59.09 (59.23), H 3.07 (3.38), N 6.04 (5.88). [Rh(dpqx)2(Ph2-acac)] (3). The synthetic procedure was similar to that of complex 2 except that 1,3-diphenylpropane1,3-dione (Ph2-acac) was used instead of hfac. Red crystals were obtained after re-crystallization from slow vaporization of diethyl ether into dichloromethane solution of 3. Yield 87 mg (69 %). 1H NMR (400 MHz, CDCl3) (ppm): 8.58 (d, J = 8.8 Hz, 2H), 8.04 (d, J = 8.4 Hz, 2H), 7.93 (m, 4H), 7.65 (d, J = 7.5 Hz, 4H), 7.58 (m, 8H), 7.32 (m, 8H), 7.13 (dd, J = 8.0 Hz and1.2 Hz, 2H), 6.71 (td, J = 7.2 Hz and 1.2 Hz, 2H), 6.64 (td, J = 7.2 Hz and 1.2 Hz, 2H), 6.58 (d, J = 7.2 Hz, 2H), 5.98 (s, 1H). HRMS (ESI). Calcd for C55H37O2N4Rh+H: m/z 889.2005. Found: m/z 889.2014. Elemental analysis (%) calcd for C55H37O2N4Rh•(CH2Cl2)1.50 (found): C 66.78 (66.97), H 3.97 (3.94), N 5.51 (5.48). X-ray Crystallography. Single crystals of 1‒3 suitable for X-ray diffraction studies were grown by slow vapour diffusion of diethyl ether into dichloromethane solution of 1‒3, respectively. Single-crystal X-ray diffraction analysis were performed on a Bruker APEX-II CCD diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 293K, 150K, 170K, respectively, for 1‒3. All absorption corrections were performed using multiscan. The structure was solved by direct methods and refined by full-matrix leastsquares techniques on F2 with the SHELXL-97 or SHELXL-


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2014 program package.20 CCDC-1910469-1910471 contain the supplementary crystallographic data for 1‒3, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

ASSOCIATED CONTENT This material is available free of charge via the Internet at http://pubs.acs.org. Experimental details, TGA data, nanosecond transient absorption spectra, other PL results in MCP thin film and cyclic voltammograms of 1−3, computational and electroluminescence data. (PDF) X-ray crystallographic data for compound 1 (CIF) X-ray crystallographic data for compound 2 (CIF) X-ray crystallographic data for compound 3 (CIF)

AUTHOR INFORMATION Corresponding Author [email protected] [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT K.M.C.W. acknowledges the “Young Thousand Talents Program” award and the start-up fund administered by the Southern University of Science and Technology. This project is also supported by National Natural Science Foundation of China (grant no. 21771099) and Shenzhen Technology and Innovation Committee (grant no. JCYJ20170307110203786 and JCYJ20170817110721105).

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Figure 1. X-Ray crystal structure of 1–3. The solvent molecules and hydro-gen atoms are omitted, and only the Δ form is shown for clarity. 404x231mm (300 x 300 DPI)



Figure 2. UV-Vis absorption and emission spectra of complexes 1–3 in dichloromethane solution at 298 K. 119x84mm (300 x 300 DPI)


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Figure 3. Normalized PL spectra and PLQY of complexes 1–3 at different excitation wavelengths in solid-state thin film (2 wt% in MCP). Insert shows the photo of thin-film PL of 3 under UV irradiation. 119x80mm (300 x 300 DPI)



Figure 4. (a) Orbital energy diagram of complexes 1–3. (b) Plots of spin density (isovalue = 0.002) of the T1 states of 1–3. 134x131mm (300 x 300 DPI)


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Figure 5. Characteristics of vacuum-deposited OLEDs based on 3. (a) EL spectra with different dopant concentrations. (b) EQEs with different hole-transporting layers. (c) Operational lifetime of the vacuumdeposited OLED made with 5 v/v% 3. 106x61mm (300 x 300 DPI)



Table of Content 138x83mm (300 x 300 DPI)


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Ligand Mediated Luminescence Enhancement in Cyclometalated

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