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Abstract

Two new phosphorescent iridium(III) complexes, (dfbt)₂Ir(pic) and (dfbt)₂Ir(acac) (dfbt = 5,7-difluoro-2-phenylbenzothiazole, pic = picolinate, acac = acetylacetonate), have been synthesized and characterized. Upon photoexcitation, both complexes exhibit strong yellow–orange emission in CH₂Cl₂ solution at room temperature with absolute quantum yields up to 99.0% and emission lifetimes up to 1.23 μs. The spectroscopic property has been calculated by density functional theory and time-dependent density functional theory, thus suggesting that the lowest absorption and the emission originate from the metal-to-ligand charge transfer/ligand-centered transition.

Keywords

5,7-difluoro-2-phenylbenzothiazole density functional theory calculations iridium (III) complexes photoluminescence synthesis

In this paper, two new 5,7-difluoro-2-phenylbenzothiazole-based iridium(III) complexes have been synthesized and characterized. Their photophysical properties, theoretical calculations, and electrochemical behaviors have been investigated.

Introduction

Due to the favorable optoelectronic properties, cyclometalated iridium(III) complexes have been widely used to fabricate organic light-emitting diodes (OLEDs), especially white light OLEDs.^1–4 In general, the realization of white emission requires a combination of either three primary colors (red, green, and blue) or two efficient complementary colors (yellow/orange and blue). During the past decades, red-, green-, and blue-light emitters for full-color display applications have been extensively studied.^5–7 However, there is also a great demand for yellow/orange emitters in the two-color white OLEDs (WOLEDs).^8–12 Therefore, novel yellow- or orange-emitting iridium complexes with high luminescent quantum yields and excellent device performance are still need to be developed.^13–16

Since the first yellow-emitting (bt)₂Ir(acac) complex (bt = 2-phenylbenzothiazole) was reported,¹⁷ a number of bt derivatives with different substituents have been developed for OLED applications.^16,18–21 In 2004, Laskar and Chen¹⁸ modified the bt ligand by attaching –F and –CF₃ groups on the 2-phenyl ring. The resulting complexes, (4-F-bt)₂Ir(acac), (3-F-bt)₂Ir(acac), and (4-CF₃-bt)₂Ir(acac), displayed yellow emission in the range of 544–567 nm. In 2011, Wang et al.¹⁶ reported two yellow Ir(III) complexes, (CF₃-bt)₂Ir(acac) (λ_max = 564 nm) and (F-bt)₂-Ir(acac) (λ_max = 554 nm), by adding –CF₃ and –F groups to the benzothiazole ring in the bt ligand. Both complexes displayed moderate quantum efficiencies of 0.36 and 0.32 in CH₂Cl₂ solution, respectively.^16,22 In 2018, we reported a series of yellow-emitting fluorinated bt-based iridium (III) complexes with tetraphenylimidodiphosphinate (tpip) as ancillary ligand.²³ Among them, the quantum yield of 5,7-positions fluorinated bt-based iridium (III) complex [(dfbt)₂Ir(tpip)] is the highest one (48.4%). The device based on (dfbt)₂Ir(tpip) displays the best device performance with a maximum current efficiency (η_{c, max}) of up to 69.8 cd A⁻¹ and a maximum external quantum efficiency (EQEmax) of up to 24.3%. Thus, it can be seen that 5,7-difluoro-2-phenylbenzo[d]thiazole (dfbt) would be a kind of ligand framework to construct efficient yellow/orange Ir(III) complexes.

In order to further develop new dfbt-based iridium complexes, we tried to use different types of ancillary ligands, such as picolinate (pic) and acetylacetonate (acac). Hence, two corresponding iridium(III) complexes were synthesized (Scheme 1), that is, (dfbt)₂Ir(pic) and (dfbt)₂Ir(acac). Their photophysical and electrochemical properties have been studied. Especially, the luminescence properties have been investigated in solution (298 and 77 K). The lowest energy electronic transitions and the lowest triplet excited states have been calculated using density functional theory (DFT) and time-dependent DFT (TD-DFT).

Scheme 1.

Synthetic routes of complexes (dfbt)₂Ir(pic) and (dfbt)₂Ir(acac).

Results and discussion

The UV-Vis absorption and photoluminescence (PL) emission spectra of (dfbt)₂Ir(pic) and (dfbt)₂Ir(acac) at room temperature (RT) and the PL spectra at 77 K are depicted in Figure 1. The photophysical data are summarized in Table 1. The strong absorption bands around 260–340 nm can be assigned to the spin-allowed intraligand ¹LC (¹π→π*) charge-transfer transition. The weak and broad bands in the 350–420 and 420–550 nm regions can be attributed to spin-allowed ¹MLCT (metal-to-ligand charge transfer) transition and the spin-forbidden ³MLCT transition, respectively.²⁴ Obviously, the maximal absorption wavelength of (dfbt)₂Ir(acac) is red-shifted compared to that of (dfbt)₂Ir(pic), depending on the impact of different ancillary ligands.

Figure 1.

(a) UV-Vis absorption and PL emission spectra of (dfbt)₂Ir(pic) and (dfbt)₂Ir(acac) in CH₂Cl₂ solution at room temperature. (b) The PL emission spectra in CH₂Cl₂ solution at 77 K.

Table 1.

Photophysical data of complexes (dfbt)₂Ir(pic) and (dfbt)₂Ir(acac).

Complex	absλ_abs^a (nm)ɛ (10⁴ cm⁻¹ M⁻¹)	Emission at RT					Emission at 77 K
Complex	absλ_abs^a (nm)ɛ (10⁴ cm⁻¹ M⁻¹)	λ_em^a (nm)	τ^a (μs)	Φ_PL^b (%)	k_r^c (10⁵ s⁻¹)	k_nr^c (10⁵ s⁻¹)	λ_em^d (nm)	τ^d (μs)
(dfbt)₂Ir(pic)	283 (7.4), 318 (6.7), 330 (6.8), 357 (2.5), 394 (1.6), 440 (1.2), 473 (1.1)	572	1.23	99.0	8.05	0.081	558, 591(sh)	3.62
(dfbt)₂Ir(acac)	273 (7.0), 318 (6.5), 332 (6.2), 363 (1.7), 413 (1.2), 450 (1.1), 493 (0.93)	591	0.98	54.9	5.60	4.60	566, 613(sh)	3.04

RT: room temperature.

Data were collected from degassed CH₂Cl₂ solutions at RT.

Absolute emission quantum yields measured in degassed CH₂Cl₂ solution.

Radiative decay rate (k_r) and nonradiative decay rate (k_nr) were estimated from the measured quantum yields and lifetimes.

Data were collected from degassed CH₂Cl₂ solutions at 77 K.

The emission spectra of the two Ir(III) complexes at RT (Figure 1(a)) show a broad band covering a range from 572 (full width at half maximum (FWHM) = 84 nm) to 591 nm (FWHM = 83 nm). Their emission colors are tuned from yellow [CIE (0.58, 0.42)] to orange [CIE (0.52, 0.48)] through the change of ancillary ligands. Moreover, their RT emission profiles are featureless, indicating that a ³MLCT excited-state emission is dominant. Upon cooling to 77 K, their low-temperature PL spectra in CH₂Cl₂ solution undergo a pronounced blue shift and display a well-resolved vibronic structure (Figure 1(b)), originating from the mixing between the ³MLCT and ³LC emission. These results suggest that the ligands should have significant contributions to the lowest lying excited stated. This speculation will be confirmed by DFT calculations below.

To investigate the effect of the ancillary ligand, the absolute photoluminescent quantum yields (Φ_PL) of both complexes in CH₂Cl₂ solution at RT were measured by integrating sphere method. As shown in Table 1, the Φ_PL of (dfbt)₂Ir(pic) (99.0%) is much higher than that of (dfbt)₂Ir(acac) (54.9%), while the luminescent lifetime (τ) is longer both at RT (1.23 vs 0.98 μs) and at 77 K (3.62 vs 3.64 μs). Based on the Φ_PL and τ data, the radiative and nonradiative rate constants (k_r and k_nr) can be calculated through the equations, k_r = Φ_PL / τ and k_nr = (1 − Φ_PL) / τ²⁵ (Table 1). From the results, we found that complex (dfbt)₂Ir(pic) possess relatively larger radiative rate constant and smaller nonradiative rate constant than those of (dfbt)₂Ir(acac), which indicates the quantum yields follow the order (dfbt)₂Ir(pic) > (dfbt)₂Ir(acac).

To deeply understand the photophysical properties of (dfbt)₂Ir(pic) and (dfbt)₂Ir(acac), DFT and TD-DFT calculations were performed using the Gaussian 09 software package.²⁶ A similar approach has been shown to be effective at reproducing the ground- and excited-state properties of related cyclometalated Ir(III) complexes.^27,28 For comparison, the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) molecular frontier orbital distributions and the energy levels of (bt)₂Ir(pic), (dfbt)₂Ir(pic), (bt)₂Ir(acac), and (dfbt)₂Ir(acac) are presented in Figure 2. The calculated singlet and triplet transitions are provided in Table 2. The electron density distributions are summarized in Table 3.

Figure 2.

Some frontier molecular orbitals of complexes (bt)₂Ir(pic), (dfbt)₂Ir(pic), (bt)₂Ir(acac), and (dfbt)₂Ir(acac).

Table 2.

Contributions, oscillator strengths and transition characters of singlet and triplet transitions for complexes (dfbt)₂Ir(pic) and (dfbt)₂Ir(acac).

Complex		Major assignments	Transition	Character	Oscillation strength
(dfbt)₂Ir(pic)	S₁	HOMO→LUMO	MLCT/LC	dπ_Ir/π_dfbt → π^*_dfbt	0.1136
	T₁	HOMO→LUMO	MLCT/LC	dπ_Ir/π_dfbt → π^*_dfbt	0
(dfbt)₂Ir(acac)	S₁	HOMO→LUMO	MLCT/LC	dπ_Ir/π_dfbt → π^*_dfbt	0.1162
	T₁	HOMO→LUMO	MLCT/LC	dπ_Ir/π_dfbt → π^*_dfbt	0

HOMO: highest occupied molecular orbital; LUMO: lowest unoccupied molecular orbital; MLCT: metal-to-ligand charge transfer; LC: ligand-centered.

Table 3.

Frontier orbital energy and electron density distribution for complexes (dfbt)₂Ir(pic) and (dfbt)₂Ir(acac).

Complex	Orbital	Energy (eV)	Ir	C^N ligand	Ancillary ligand
(dfbt)₂Ir(pic)	HOMO	−5.558	48.86	45.89	5.25
	LUMO	−2.122	3.11	95.33	1.56
(dfbt)₂Ir(acac)	HOMO	−5.419	50.82	43.75	5.44
	LUMO	−2.024	3.32	95.59	1.09

HOMO: highest occupied molecular orbital; LUMO: lowest unoccupied molecular orbital.

The HOMOs are principally distributed on the iridium atom and phenyl moiety of cyclometalated ligands, whereas the LUMOs are primarily located on the whole cyclometalated ligands (Table 3). Therefore, the calculated lowest lying singlet and triplet transitions of (dfbt)₂Ir(pic) and (dfbt)₂Ir(acac) are both mainly assigned to MLCT/LC characters from HOMO→LUMO transitions (Table 2). From Figure 2, it can be seen that the energy gap of (dfbt)₂Ir(pic)/(dfbt)₂Ir(acac) (3.436 eV/3.395 eV) is lower than that of (bt)₂Ir(pic)/(bt)₂Ir(acac) (3.520 eV/3.475 eV), respectively. The results indicate that the introduction of two fluorine atoms on cyclometalated ligand can lead to a red-shifted emission (λ_{max,(bt)2Ir(pic)} = 541 nm,²⁹ λ_{max,(dfbt)2Ir(pic)} = 572 nm; λ_{max,(bt)2Ir(acac)} = 557 nm,²⁹ λ_{max,(dfbt)2Ir(acac)} = 591 nm). Furthermore, by the exchange of pic to acac, the energy gap of the corresponding complex is also altered; thus, the actual emission spectra are changed accordingly. Although the ancillary ligands have little contributions to the HOMO/LUMO level, they also cause a slight difference in the electron density distribution by interacting with the Ir d-orbitals, which would be confirmed by electrochemical analysis.

Through the DFT calculation, we found that the triplet energy of ancillary ligand (pic = 3.399 eV, acac = 3.548 eV) is higher than those of the cyclometalated ligand (dfbt = 2.758 eV) and the complexes ((dfbt)₂Ir(pic) = 2.419 eV, (dfbt)₂Ir(acac) = 2.387 eV). Thus, both complexes give emission from an excited state on the “dfbt₂Ir” fragment, supporting the assignment of the lowest energy excited state. Meanwhile, we also calculated the triplet energies of bt ligand (2.731 eV) and the complexes ((bt)₂Ir(pic) = 2.456 eV, (bt)₂Ir(acac) = 2.425 eV). Likewise, the two complexes emit from states within the “C^N₂Ir” fragment as discussed above.

The electrochemical properties of both Ir(III) complexes have been investigated by cyclic voltammetry (CV) using ferrocene as the internal standard (Fc⁺/Fc = +0.38 V vs SCE (saturated calomel electrode) in acetonitrile)³⁰ (Figure 3) and their data are listed in Table 4. For both the complexes, the first irreversible oxidation waves (0.79 V for (dfbt)₂Ir(pic); 0.68 V for (dfbt)₂Ir(acac)) can be associated with the bis-cyclometalated phenyl-Ir moiety, as evidenced by DFT calculations. According to the formula of E_HOMO = −[E_onset^ox + 4.8],³¹ their HOMO energy levels are determined to be −5.59 and −5.48 V, respectively. We note that the introduction of pic ancillary ligand in (dfbt)₂Ir(pic) can cause great stabilization of the HOMO level. In other word, pic ancillary ligand can increase the oxidation potential of (dfbt)₂Ir(pic), making the phenyl-Ir moiety harder to oxidize. That is because that pic ligand decrease more electron density of Ir center (48.86% for (dfbt)₂Ir(pic)) than acac ligand (50.82% for (dfbt)₂Ir(acac)). Clearly, the ancillary ligands can influence the HOMO energy by modifying the electron density at the metal center, which are in good agreement with the above DFT calculations. Unfortunately, for both Ir(III) complexes, no reduction processes were observed in the acetonitrile.¹⁹ Thus, their LUMO energy levels were deduced from the HOMO and E_opt,_g values. From Table 4, it can be seen that the HOMO and LUMO energy levels derived from the experimental data are comparable to those obtained by the theoretical calculation.

Figure 3.

Cyclic voltammograms for complexes (dfbt)₂Ir(pic) and (dfbt)₂Ir(acac) in acetonitrile. Voltammograms were recorded at a scan rate of 100 mV s⁻¹ and referenced to internal Fc⁺/Fc.

Table 4.

Electrochemical and theoretical data of complexes (dfbt)₂Ir(pic) and (dfbt)₂Ir(acac).

Complex	E_onset^ox (V)^a	E_HOMO/LUMO (eV)^b	E_HOMO/LUMO (eV)^c	E_opt,g (eV)^b
(dfbt)₂Ir(pic)	0.79	−5.59/−2.97	−5.56/−2.12	2.62
(dfbt)₂Ir(acac)	0.68	−5.48/−2.96	−5.42/−2.02	2.52

HOMO: HOMO: highest occupied molecular orbital; LUMO: lowest unoccupied molecular orbital.

Redox measurements were carried out in CH₃CN solution, and values are reported to Fc⁺/Fc.

Deduced from the equation E_HOMO = −[E_onset^ox + 4.8] eV and LUMO = HOMO + E_opt,g, respectively.

Obtained from theoretical calculations.

In conclusion, we report herein on the synthesis and photophysical properties of two new dfbt-based iridium(III) complexes, (dfbt)₂Ir(pic) and (dfbt)₂Ir(acac). Both complexes exhibit yellow and orange emission in dichloromethane (DCM) solution at RT through changing ancillary ligands. The low-temperature PL spectra reveal the significant impact of the ancillary ligands on the emission energy. The theoretical calculations have also been performed to rationalize the photophysical and electrochemical properties. The calculated results suggest that the emission properties are mainly ³MLCT/³LC nature, involving iridium d-orbitals and π→π* orbitals of the cyclometalated units.

Experimental

All reactions were performed under nitrogen atmosphere. All of the reagents were purchased from commercial sources and used without further purification, unless stated otherwise. 5,7-Difluoro-2-phenylbenzo[d]thiazole (dfbt) and the corresponding dichloro-bridged [Ir(dfbt)₂Cl]₂ were synthesized according to our previous literature.²³

¹H NMR spectra were recorded on a Bruker AM 400 MHz instrument, and chemical shifts were reported in ppm relative to Me₄Si as internal standard. MALDI-TOF-MS spectra were recorded on a Bruker Autoflex ^IITM TOF/TOF instrument. The elemental analyses were measured on a Vario EL Cube Analyzer system. UV-Vis spectra were recorded using a Hitachi U3900/3900H spectrophotometer. Fluorescence spectra were carried out on a Hitachi F-7000 spectrophotometer in degassed CH₂Cl₂ solutions at RT and 77 K. Luminescence lifetime curves were measured on a HORIBA FL-3 fluorescence spectrometer under 375 nm excitation of a Delta-Diode laser. The absolute PL quantum yields (Φ_PL) was measured with HORIBA FL-3 fluorescence spectrometer equipped with an integrating sphere.

CV was performed on a CHI 1210B electrochemical workstation, with a glassy carbon electrode as the working electrode, a platinum wire as the counter electrode, an Ag/AgCl electrode as the reference electrode, and 0.1 M n-Bu₄NClO₄ as the supporting electrolyte. The redox potentials are reported relative to a ferrocenium/ferrocene (Fc⁺/Fc) redox couple used as an internal reference (0.38 V vs SCE in acetonitrile).

The geometry of complexes (dfbt)₂Ir(pic) and (dfbt)₂Ir(acac) was optimized by the DFT method with B3LYP (Becke three-parameter Lee-Yang-Parr) hybrid DFT and the 6-31G* basis set. The LANL2DZ basis set was used for the iridium(III), whereas the 6-31G* basis set was adopted for the ligands. All calculations were carried out with Gaussian 09 software package.

General procedure for the preparation of the Ir(III) complexes (dfbt)₂Ir(pic) and (dfbt)₂Ir(acac)

A mixture of the above dichloro-bridged dimer complex [Ir(dfbt)₂Cl]₂, the corresponding ancillary ligand (picolinic acid or pentane-2,4-dione, 2.5 equiv.), and Na₂CO₃ (5.0 equiv.) was dissolved in ethoxyethanol (6 mL) and was refluxed for 12 h under nitrogen. After cooled to RT, the mixture was diluted with water and extracted multiple times with DCM. The combined organic phases were washed with brine, dried over Na₂SO₄. After concentrated, the residue was purified by column chromatography with petroleum ether/DCM (5:1) eluent to afford pure Ir(III) complex.

(dfbt)₂Ir(pic) (43.8 mg): Orange solid; yield 48.2%; ¹H NMR (400 MHz, CDCl₃): δ 8.26 (d, J = 7.6 Hz, 1H), 8.12 (d, J = 7.6 Hz, 1H), 7.95 (t, J = 7.6 Hz, 1H), 7.85 (d, J = 7.6 Hz, 1H), 7.78 (d, J = 7.6 Hz, 1H), 7.70 (d, J = 7.6 Hz, 1H), 7.45–7.48 (m, 1H), 7.03 (t, J = 7.6 Hz, 1H), 6.93–6.97 (m, 2H), 6.76–6.84 (m, 4H), 6.61 (d, J = 7.6 Hz, 1H), 6.27 (d, J = 8.0 Hz, 1H). MS (MALDI-TOF) calcd for C₃₂H₁₆F₄IrN₃O₂S₂, 807.02 ([M+H]⁺); found, 807.96; Anal. calcd for C₃₂H₁₆F₄IrN₃O₂S₂: C, 47.64; H, 2.00; N, 5.21; found: C, 47.62; H, 2.01; N, 5.20.

(dfbt)₂Ir(acac) (53.5 mg): Orange solid; yield 51.7%; ¹H NMR (400 MHz, CDCl₃): δ 7.63–7.69 (m, 4H), 6.98 (td, J = 9.1, 2.2 Hz, 2H), 6.90 (t, J = 7.6 Hz, 2H), 6.70 (t, J = 7.6 Hz, 2H), 6.40 (d, J = 7.6 Hz, 2H), 5.20 (s, 1H), 1.79 (s, 6H). MS (MALDI-TOF) calcd for C₃₁H₁₉F₄IrN₂O₂S₂, 784.05 ([M+H]⁺); found, 785.06. 685.53 (M-acac); Anal. calcd for C₃₁H₁₉F₄IrN₂O₂S₂: C, 47.50; H, 2.44; N, 3.57; found: C, 47.48; H, 2.43; N, 3.58.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was partially supported by the Natural Science Foundation of Hainan Province (Nos. 219MS043 and 218QN236).

ORCID iD

Zhi-Gang Niu

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New yellow/orange-emitting heteroleptic iridium(III) complexes with 5,7-difluoro-2-phenylbenzothiazole ligand

Abstract

Keywords

Introduction

Results and discussion

Experimental

General procedure for the preparation of the Ir(III) complexes (dfbt)2Ir(pic) and (dfbt)2Ir(acac)

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References

General procedure for the preparation of the Ir(III) complexes (dfbt)₂Ir(pic) and (dfbt)₂Ir(acac)