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Article

Probing Electron Excitation Characters of Carboline-Based Bis-Tridentate Ir(III) Complexes

by
Jie Yan
1,†,
Ze-Lin Zhu
2,†,
Chun-Sing Lee
2,*,
Shih-Hung Liu
3,
Pi-Tai Chou
3,* and
Yun Chi
1,2,*
1
Department of Materials Science and Engineering, City University of Hong Kong, Hong Kong SAR, China
2
Center of Super-Diamond and Advanced Films (COSDAF) and Department of Chemistry, City University of Hong Kong, Hong Kong SAR, China
3
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Molecules 2021, 26(19), 6048; https://doi.org/10.3390/molecules26196048
Submission received: 13 September 2021 / Revised: 29 September 2021 / Accepted: 30 September 2021 / Published: 6 October 2021
(This article belongs to the Section Physical Chemistry)
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Abstract

:
In this work, we report a series of bis-tridentate Ir(III) metal complexes, comprising a dianionic pyrazole-pyridine-phenyl tridentate chelate and a monoanionic chelate bearing a peripheral carbene and carboline coordination fragment that is linked to the central phenyl group. All these Ir(III) complexes were synthesized with an efficient one-pot and two-step method, and their emission hue was fine-tuned by variation of the substituent at the central coordination entity (i.e., pyridinyl and phenyl group) of each of the tridentate chelates. Their photophysical and electrochemical properties, thermal stabilities and electroluminescence performances are examined and discussed comprehensively. The doped devices based on [Ir(cbF)(phyz1)] (Cb1) and [Ir(cbB)(phyz1)] (Cb4) give a maximum external quantum efficiency (current efficiency) of 16.6% (55.2 cd/A) and 13.9% (43.8 cd/A), respectively. The relatively high electroluminescence efficiencies indicate that bis-tridentate Ir(III) complexes are promising candidates for OLED applications.

Graphical Abstract

1. Introduction

Organic light-emitting diodes (OLEDs) have been widely employed in the fabrication of flat panel displays and solid-state lighting luminaries. In this regard, Ir(III) phosphors have received special attention for their capability in harvesting both the singlet and triplet excited states formed in the devices [1]. The triplet states account for 75% of the total excited states generated; hence, the strong spin-orbit coupling exerted by the Ir(III) metal atom can reduce the radiative lifetime of triplet excited states, resulting in a significant improvement of the overall efficiency of OLEDs. This has triggered numerous studies on the quest of chemically and photochemically stable Ir(III) metal complexes, to which the efficient phosphorescence from the coupled ligand-centered (LC) ππ* and metal-to-ligand charge transfer (MLCT) excited states tend to fulfil the criteria for higher OLED efficiency [2,3,4,5,6,7].
Traditionally, these Ir(III) emitters were constructed using bidentate cyclometalates such as 2-phenylpyridine or functional analogues (C^N) and/or monoanionic ancillary chelate, denoted as (L^X). The tris-homoleptic and heteroleptic Ir(III) complexes [Ir(C^N)3] and [Ir(C^N)2(L^X)] have been extensively designed and studied [8]. In theory, both of them are capable of affording at least two stereoisomers, which are controlled by their intrinsic kinetic and thermodynamic factors. They are named as fac- (facial) and mer- (meridional) isomers in the case of homoleptic complexes [Ir(C^N)3]. Generally, these stereoisomers possess distinctive chemical and physical properties and, hence, their interconversion should be limited during preparation. One possible method in preventing the formation of multiple stereoisomers is to employ the bis-tridentate architectures, to which the planar motif of tridentate chelates are well-known for preventing formation of conformational isomers in the octahedral coordination framework [9,10,11].
Studies on charge-neutral bis-tridentate Ir(III) complexes are limited [12,13], despite a hefty compilation of studies on analogous ionic complexes [14,15,16]. For the former, Williams and co-workers obtained these Ir(III) complexes by controlled blockage of the reactive site on the chelate [17]. Koga reported the bis-tridentate Ir(III) complex with both pyridylbiphenyl and phenylbipyridyl cyclometalate [18]. Furthermore, bis-tridentate architecture was also extended to carbazolyl-, phenoxy- and benzimidazol-2-yl-based chelates with an improved photoluminescence yield, as reported by Esteruelas et al. [19,20]. In the meantime, Chi and co-workers sought to explore efficient RGB emitters with pincer carbene ancillary (Scheme 1), aimed for potential OLED applications [21,22,23,24]. Later on, the 2-phenyl-6-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridine system (phyzn)H2 (n = 1, 2 and 3), which could serve as both the monoanionic and dianionic chelate depending on the synthetic manipulation for preparation of emissive Ir(III) complexes, attracted research attention [25,26,27,28]. Given this background, we decided to use these aforementioned (phyzn)H2 chelates, together with carbene-benzene-carboline pro-chelates (cbF)H·HF6 and (cbB)H·HF6, in building the bis-tridentate Ir(III) complexes. A carboline fragment was selected as the key component of this monoionic chelate, as it has been involved in preparation of bi-dentate chelates for Ir(III) metal complexes [29,30] that have exceptional electronic properties in various bipolar host materials [31,32,33,34,35]. Finally, within these tridentate chelates, the carboline N-donor will reside trans- to the peripheral carbene unit, avoiding the putative trans-influence that may exist in the coordinated carbene pincer chelates.

2. Experimental Section

2.1. General Information

All solvents were dried and degassed before used, and commercially available reagents were used without further purification. 2,6-Dibromo-4-methoxypyridine [36,37], 2,6-dibromo-N,N-dimethylpyridin-4-amine [38,39] and 6-(tert-butyl)-9H-pyrido[2,3-b]indole [40] were prepared using methods reported in literature. All reactions were conducted under N2 atmosphere and monitored by precoated TLC plates (0.20 nm with fluorescent indicator F254). 1H and 19F spectra were recorded with Bruker 400 MHz AVANCE III Nuclear Magnetic Resonance System. Elemental analysis was performed by an elemental carbon-hydrogen-nitrogen analyzer (Elementar). Mass spectra were obtained on 4800 Plus MALDI TOF/TOF Analyzer (ABI), where 2,5-dihydroxybenzoic acid was applied as the matrix. TGA measurements were performed on a TA Instrument TGAQ50, at a heating rate of 10 °C min−1 under N2 atmosphere. The X-ray intensity data were measured using phi and omega scan modes (APEX3) at 233 K on a Bruker D8 Venture Photon II diffractometer with microfocus X-ray sources.

2.2. Synthesis of the Bis-Tridentate Ir(III) Metal Complexes Cb1–5

Synthesis of [Ir(cbF)(phyz1)] (Cb1): A mixture of (cbF)H·HF6 (186 mg, 0.3 mmol), [Ir(COD)Cl]2 (100 mg, 0.15 mmol) and NaOAc (123 mg, 1.5 mmol) in 15 mL of degassed acetonitrile was refluxed for 12 h. After, the solvent was removed and the resulting residue was added of (phyz1)H2 (86.4 mg, 0.3 mmol), 10 mL of decalin and NaOAc (123 mg, 1.5 mmol). This mixture was refluxed for another 24 h and, after removal of decalin under reduced pressure, the residue was taken into CH2Cl2 (30 mL × 3) and the combined solution was washed with deionized water. Finally, the organic layer was dried over Na2SO4, and filtered and concentrated to dryness. The residue was purified by column chromatography (SiO2, ethyl acetate/hexane = 1:3) to yield 180 mg of yellow solid, 0.19 mmol, which is calculated to be a yield of 62%.
Other bis-tridentate Ir(III) derivatives, i.e., [Ir(cbF)(phyz2)] (Cb2), [Ir(cbF)(phyz3)] (Cb3), [Ir(cbB)(phyz1)] (Cb4) and [Ir(cbB)(phyz3)] (Cb5), were synthesized from condensation of the carboline-based chelates (cbF)H·HF6 and (cbB)H·HF6 with respective dianionic chelates (phyz1)H2, (phyz2)H2 and (phyz3)H2 under similar reaction conditions.
Spectral data of Cb1: MS (MALDI-TOF, 193Ir): m/z 956.27637 [M + H+]; 1H NMR (400 MHz, acetone-d6, 296 K) δ = 8.55 (dd, J = 7.6, 1.2 Hz, 1H), 8.35 (d, J = 9.2 Hz, 1H), 8.32 (d, J = 2.0 Hz, 1H), 8.12–8.15 (m, 3H), 8.04 (d, J = 8.0 Hz, 1H), 7.97 (d, J = 8.0 Hz, 1H), 7.87 (dd, J = 8.8, 2.0 Hz, 1H), 7.83 (s, 1H), 7.75 (d, J = 8.0 Hz, 1H), 7.42 (dd, J = 5.6, 1.2 Hz, 1H), 7.20 (d, J = 2.0 Hz, 1H), 7.06 (s, 1H), 6.98 (dd, J = 7.2, 5.6 Hz, 1H), 6.76–6.82 (m, 1H), 6.60 (td, J = 7.2, 1.2 Hz, 1H), 6.50–6.46 (m, 1H), 3.43–3.49 (m, 1H), 1.46 (s, 9H), 0.88 (d, J = 6.8 Hz, 6H); 19F NMR (376 MHz, acetone-d6, 296 K): δ = −60.39 (s, 3F), −61.46 (s, 3F). Analytical data: calculated for C43H34F6IrN7: C, 54.08; H, 3.59; F, 11.94; Ir, 20.13; N, 10.27. Found: C, 54.10; H, 3.57; N, 10.56.
Spectral data of Cb2: a yellow solid, with a yield of 51%. MS (MALDI-TOF, 193Ir): m/z 986.32111 [M + H+]; 1H NMR (400 MHz, acetone-d6, 296 K) δ = 8.54 (d, J = 6.4 Hz, 1H), 8.34 (d, J = 8.8 Hz, 1H), 8.31 (d, J = 2.0 Hz, 1H), 8.13 (d, J = 2.0 Hz, 1H), 8.11 (s, 1H), 7.86 (dd, J = 8.8, 2.0 Hz, 1H), 7.82 (s, 1H), 7.75 (d, J = 7.6 Hz, 1H), 7.65 (s, 2H), 7.47 (d, J = 5.2 Hz, 1H), 7.19 (d, J = 2.0 Hz, 1H), 7.08 (s, 1H), 7.00 (dd, J = 7.2, 6.0 Hz, 1H), 6.77 (t, J = 7.6 Hz, 1H), 6.58 (t, J = 7.2 Hz, 1H), 6.45 (d, J = 7.6 Hz, 1H), 4.24 (s, 3H), 3.56–3.62 (m, 1H), 1.46 (s, 9H), 0.91 (dd, J = 6.8, 4.4 Hz, 6H); 19F NMR (376 MHz, acetone-d6, 296 K): δ = −60.36 (s, 3F), −61.40 (s, 3F). Analytical data: calculated for C44H36F6IrN7O: C, 53.65; H, 3.68; F, 11.57; Ir, 19.51; N, 9.95; O, 1.62. Found: C, 53.81; H, 3.78; N, 9.85.
Spectral data of Cb3: a yellow solid with a yield of 50%. MS (MALDI-TOF, 193Ir): m/z 999.31921 [M + H+]; 1H NMR (400 MHz, acetone-d6, 296 K) δ = 8.54 (d, J = 7.2 Hz, 1H), 8.33 (d, J = 8.8 Hz, 1H), 8.31 (s, 1H), 8.08–8.13 (m, 2H), 7.85 (d, J = 8.8, 2.0 Hz, 1H), 7.79 (s, 1H), 7.72 (d, J = 7.6 Hz, 1H), 7.55 (d, J = 5.6 Hz, 1H), 7.37 (d, J = 9.6 Hz, 2H), 7.19 (d, J = 2.4 Hz, 1H), 6.96–7.03 (m, 2H), 6.72 (t, J = 7.2 Hz, 1H), 6.52 (t, J = 7.2 Hz, 1H), 6.40 (d, J = 7.2 Hz, 1H), 3.69–3.76 (m, 1H), 3.43 (s, 6H), 1.46 (s, 9H), 0.89–0.93 (m, 6H); 19F NMR (376 MHz, acetone-d6, 296 K): δ = −60.20 (s, 3F), −61.35 (s, 3F). Analytical data: calculated for C45H39F6IrN8: C, 54.15; H, 3.94; F, 11.42; Ir, 19.26; N, 11.23. Found: C, 54.43; H, 3.85; N, 10.98.
Spectral data of Cb4: a yellow solid with a yield of 54%. MS (MALDI-TOF, 193Ir): m/z 944.34766 [M + H+]; 1H NMR (400 MHz, acetone-d6, 296 K): δ = 8.50 (d, J = 7.6 Hz, 1H), 8.43 (d, J = 9.2 Hz, 1H), 8.29 (d, J = 2.0 Hz, 1H), 8.08 (t, J = 8.0 Hz, 1H), 8.02 (d, J = 2.0 Hz, 1H), 7.97–8.02 (m, 2H), 7.94 (d, J = 7.6 Hz, 1H), 7.84 (dd, J = 8.8, 2.0 Hz, 1H), 7.71 (d, J = 7.6 Hz, 1H), 7.61 (s, 1H), 7.41 (d, J = 5.6 Hz, 1H), 7.10 (d, J = 2.4 Hz, 1H), 7.04 (s, 1H), 6.91 (dd, J = 7.6, 6.0 Hz, 1H), 6.72–6.77 (m, 1H), 6.50–6.58 (m, 2H), 3.41–3.48 (m, 1H), 1.58 (s, 9H), 1.46 (s, 9H), 0.85 (d, J = 6.8 Hz, 6H); 19F NMR (376 MHz, acetone-d6, 296 K): δ = −60.26 (s, 3F). Analytical data: calculated for C46H43F3IrN7: C, 58.58; H, 4.60; F, 6.04; Ir, 20.38; N, 10.40. Found: C, 58.50; H, 4.53; N, 10.54.
Spectral data of Cb5: a greenish solid with a yield of 52%. MS (MALDI-TOF, 193Ir): m/z 987.43604 [M + H+]; 1H NMR (400 MHz, acetone-d6, 296 K): δ = 8.49 (d, J = 7.2 Hz, 1H), 8.42 (d, J = 8.8 Hz, 1H), 8.28 (d, J = 2.0 Hz, 1H), 7.99 (d, J = 2.0 Hz, 1H), 7.95 (s, 1H), 7.82 (dd, J = 8.8, 2.0 Hz, 1H), 7.67 (d, J = 8.0 Hz, 1H), 7.57 (s, 1H), 7.54 (d, J = 6.0 Hz, 1H), 7.36 (d, J = 2.0 Hz, 1H), 7.32 (s, 1H), 7.08 (d, J = 2.0 Hz, 1H), 6.98 (s, 1H), 6.92 (dd, J = 7.6, 6.0 Hz, 1H), 6.64–6.71 (m, 1H), 6.46–6.47 (m, 2H), 3.67–3.73 (m, 1H), 3.41 (s, 6H), 1.57 (s, 9H), 1.46 (s, 9H), 0.88 (m, 6H); 19F NMR (376 MHz, acetone-d6, 296 K): δ = −60.08 (s, 6F). Analytical data: calculated for C48H48F3IrN8: C, 58.46; H, 4.91; F, 5.78; Ir, 19.49; N, 11.36. Found: C, 58.37; H, 4.93; N, 11.14.
Selected crystal data of Cb1: CCDC deposition number: 2095978. C43H36F6IrN7O; M = 972.99; orthorhombic; space group = Pbca (No. 61); a = 22.6543(5) Å, b = 15.0837(3) Å, c = 27.9723(6) Å; V = 9558.4(4) Å3; Z = 8; ρCalcd = 1.352 g·cm−3; F(000) = 3856, crystal size = 0.49 × 0.05 × 0.04 mm3; λ(CuKα) = 1.54178 Å; T = 213 (2) K; µ = 5.925 mm−1; 83,799 reflections collected, 9741 independent reflections (Rint = 0.0740, Rσ = 0.0444); max. and min. transmission = 0.365 and 0.754, respectively; data/restraints/parameters = 9741/354/621; GOF = 1.041; final R1[I > 2σ(I)] = 0.0296 and wR2(all data) = 0.0764. All deposited data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: (+44) 1223-336-033; e-mail: [email protected]).

3. Results and Discussion

3.1. Syntheses and Characterizations

Synthesis of dianionic chelate (phyzn)H2 (n = 1, 2 and 3) followed the literature precedents [37,41]. Commercially available 2,6-dibromopyridine and self-synthesized 2,6-dibromo-4-methoxypyridine [36] and 2,6-dibromo-N,N-dimethylpyridin-4-amine [38,39] were employed as the respective starting materials. For preparation of the carbene-benzene-carboline pro-chelates (cbF)H·HF6 and (cbB)H·HF6, 1,3-dibromo-5-(trifluoromethyl)benzene and 1,3-dibromo-5-(tert-butyl)benzene were first coupled with functional α-carboline using the multi-step protocol described in Scheme S2 of electronic supporting information (ESI). The isolated intermediates were next reacted with imidazole in presence of both CuO and K2CO3, followed by methylation of peripheral imidazole in giving the N-methyl imidazolium entity. Finally, their iodide anion was metathesized with PF6‒ anion with addition of excessive, aqueous KPF6, giving an immediate precipitation of a white solid of (cbF)H·HF6 and (cbB)H·HF6 as the intended tridentate chelates.
After that, the preparation of the bis-tridentate Ir(III) complexes Cb1‒5 was conducted using a one-pot and two-step method. As a generalized protocol, the carboline chelate (cbF)H·HF6 (or (cbB)H·HF6) was first heated with [Ir(COD)Cl]2 and sodium acetate in degassed acetonitrile. The intermediate was next reacted with a series of second chelate (phyzn)H2 (n = 1, 2 and 3) in decalin to afford the desired Ir(III) complexes in moderate yields. The mass spectrometry and 1H and 19F NMR spectroscopies, together with a single crystal X-ray diffraction study on Cb1, were examined to offer the needed characterizations. Their structural drawings are depicted in Scheme 2 for scrutiny.
Figure 1 depicts the molecular drawing of Cb1, with thermal ellipsoids drawn at a level of 30% probability. The crystal of Cb1 for X-ray diffraction was obtained via the slow diffusion of hexane into a saturated CH2Cl2 solution of Cb1 at RT. The Ir(III) metal atom constituted a slightly distorted octahedral coordination arrangement with two mutually orthogonal tridentate chelates. The phyz1 chelate is essentially planar, while that of the tridentate chelate cbF underwent a slight distortion at the outer hexagonal ring of the carboline unit, which can be attributed to the unfavourable steric interaction between carboline and central benzene fragments. In agreement with the prediction of trans-influence [42], the carbene Ir-C distance (Ir-C(39) = 2.004(3) Å) is relatively shorter than the typical Ir-C distances observed in other bis-tridentate Ir(III) complexes bearing symmetrically arranged carbene pincer chelates (2.043 − 2.062 Å) [43,44]. Concomitantly, the Ir-C distance of central benzene group (Ir-C(31) = 2.011(3) Å) elongated slightly in comparison to that of the corresponding carbene pincer chelates (1.950–1.960 Å).

3.2. Photophysical and Electrochemical Properties

Figure 2 reveals both the absorption and emission spectra of Cb1‒5 recorded in the degassed CH2Cl2 solution, to which the corresponding photophysical data are summarized in Table 1. All Ir(III) complexes give similar absorption patterns, and the higher energy bands above 380 nm are attributed to the spin-allowed ππ* transition, while those occurring at the longer wavelength regions of 380‒450 nm are assigned to the singlet metal-to-ligand charge transfer (1MLCT). The next lower absorption bands spanning the region from 450 nm up to the onset are ascribed to the mixed spin-forbidden ligand-centered ππ* transition and MLCT transition processes.
Upon photoexcitation, an intense green emission was observed among Cb1, Cb2 and Cb3 in the degassed CH2Cl2 solution with the peak wavelength at 525, 521 and 529 nm, respectively. The slight shifting of peak indicates the substituent effects of the pyridinyl coordination unit. It is worth noting that the shoulder at the right of emission profile gradually vanished in accordance with the sequence of hydrogen, methoxy, dimethylamino presented, manifesting an increased MLCT contribution for a structureless profile. In addition, the radiative rate constant (kr) for Cb1 to Cb3 (2.0, 3.2 and 3.4 × 105 s−1), which was calculated from quantum yield (Φ) divided by the observed lifetime (τobs), revealed an acending trend to the increased MLCT contribution, as it fostered stronger spin-orbital coupling and faster phosphorescence. This tendency was also observed by comparing the second set of the Ir(III) complexes Cb4 and Cb5, with the radiative rate constant being 2.9 × 105 s−1 and 3.3 × 105 s−1, respectively. Furthermore, for Cb3 and Cb5, the bathochromic shift can also be rationalized with the electron-donating effect of NMe2 substituent at the 4-position of pyridinyl group, giving a higher-lying HOMO level and hence a narrower energy gap.
Figure 3 shows the electrochemical properties of bis-tridentate Ir(III) complexes Cb1−5, with numerical data listed in Table 2. All complexes present reversible oxidation and irreversible reduction waves. Replacing CF3 with the tert-butyl substituent in the monoanionic carbene pincer chelate induces a cathodic shift on the oxidation potential, e.g., Cb1 (0.56 V) to Cb4 (0.35 V). For Cb1, Cb2 and Cb3, the oxidation potentials experience a decrease from 0.56 V and 0.53 V to 0.45 V, with changing 4-hydrogen atom on the pyridinyl fragment to methoxy and dimethylamino substituents. A similar trend is also observed between Cb4 and Cb5, which varied from 0.35 V to 0.25 V, after the introduction of the dimethylamino group. Meanwhile, the reduction potentials are also influenced by the substituent effect as mentioned earlier. Among Ir(III) complexes Cb1‒3, Cb3 exhibits the most destabilized LUMO by giving the most negative potential at −2.48 V, which can be explained by the strongest electron-donating ability of the dimethylamino group. Moreover, both the Ir(III) complexes Cb4 and Cb5 (−2.50 V and −2.56 V, respectively) with the tert-butyl substituent on the monoanionic tridentate chelate display more negative reduction potentials than that of the CF3 substituted counterparts Cb1, Cb2 and Cb3 (−2.42 V, −2.45 V and −2.48 V, respectively), showing that the LUMO is not associated with this pyridinyl coordination unit.

3.3. Theoretical Calculation

We then conducted the density functional theory (DFT) calculations at PBE0/LANL2DZ (Ir) and PBE0/6-31g(d,p) (H, C, N, F, O) levels using CH2Cl2 as the solvent to optimize the ground-state (S0) geometries of all molecules. In addition, time-dependent (TD) DFT calcualtions at the same levels were performed to optimize the geometries of the excited states and to probe the transition characteristics of the studied Ir(III) complexes. The calculated transition energies and major assignments of Ir(III) complexes Cb1–5 in CH2Cl2 solution are summarized in Table 3 and Tables S1–S5, respectively. The frontier molecular orbitals involved in the major transitions were also depicted in Figure 4 and Figures S1–S5. The calculated S0 → S1 transition in terms of wavelength was estimated to be Cb1: 402.7 nm, Cb2: 391.2 nm, Cb3: 394.8 nm, Cb4: 417.8 nm and Cb5: 413.4 nm, which are close to the onset of the absorption spectra in Figure 2. After structural optimization of the excited states S1 and T1, the computed wavelengths for S1 → S0 and T1 → S0 vertical transition were Cb1: 488.2 and 588.4 nm, Cb2: 484.2 and 573 nm, Cb3: 490 and 557.4 nm, Cb4: 511 and 587.7 nm and Cb5: 520.2 and 598.9 nm, respectively. For Cb1–5, the calculated S1 → S0 wavelengths were all close to the onset of the emission spectra while the T1 → S0 wavelengths were akin to the experimental emissive peaks as recorded in Figure 2. The trends of S0 → S1 absorption and T1 → S0 emission were in good agreement with their corresponding absorption and phosphorescence spectra, respectively.
Moreover, the S0 → S1 absorption was derived mainly from HOMO → LUMO+1 for Cb1 and Cb4 and HOMO → LUMO for Cb2, Cb3 and Cb5, respectively (Table 3). The S1 → S0 and T1 → S0 emission were all assigned to LUMO → HOMO for Cb1–5. For the ground state S0 of Cb1–5, the electron density distribution of the HOMO was mainly localized at the central Ir(III) metal atom (31%‒34%) and delocalized over the chromophoric chelate 2-phenyl-6-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridine (phyz) and carbene-benzene-carboline (cb), while the electron density distribution of the LUMO and LUMO+1 was mainly localized at the cb or phyz chelate, respectively, accompanying a little contribution at the Ir(III) atom (1–3%) (Figure 4 and Figures S1‒S5). For the excited states S1 and T1 of Cb1–5, the electron density distribution of the HOMO was mainly localized at the central Ir(III) metal atom (29–36%) and delocalized over the phyz and cb fragment, while the electron density distribution of the LUMO was mainly localized at the cb or phyz chelate, together with a few contribution at the Ir(III) atom (2–4%). Moreover, it is notable that LUMO is partially shifted to carboline moiety in Cb3, while completely moved to carboline moiety as observed in Cb5. We attributed this to the introducing of the dimethylamino substituent at the pyridinyl unit of the dianionic chelate that greatly increased the associated π* orbital energy, such that the LUMO is now dominated by the relatively unaffected carboline π* orbital. Overall, the S0 → S1, S1 → S0 and T1 → S0 transitions were all mainly ascribed to the metal-to-ligand charge transfer (MLCT) process (19–31%), accompanied by minor ligand-to-ligand charge transfer (LLCT) or intraligand charge transfer (ILCT). These high MLCT characters were in nice relevance to the moderate emission quantum yield (41–69%) of the emissive complexes Cb1–5 in Table 1 and Table 3. Furthermore, with regard to the calculated HOMO energy levels of S0, S1 and T1, Cb3 was higher than Cb1 and Cb2 due to the electron-donating effect of NMe2 substituent at the 4-position of pyridinyl group in Cb3. Additionally, Cb5 is higher than that of Cb4 (Table 2 and Figures S1–S5). The trend of calculated HOMO energy levels is in good agreement with the experimental results (vide supra).

3.4. Fabrication of OLED Devices

All these new Ir(III) complexes showed a high decomposition temperature (>283 °C, Figure S6), which is suitable for conducting device fabrication via thermal deposition. In view of their better photophysical properties, Cb1 and Cb4 were selected as the dopant emitter in fabrication of OLED devices with architecture: ITO/TAPC (40 nm)/TCTA (10 nm)/mCP (10 nm)/8 wt.% dopant in mCP (20 nm)/TmPyPB (45 nm)/LiF (1 nm)/Al. Figure 5 presents the chemical structures of the employed materials and device configuration. The obtained device characteristics and key parameters are summarized in Figure 6 and Table 4 for scrutiny. Here, 1,1-bis((di-4-tolylamino)phenyl)cyclohexane (TAPC) and tris(4-carbazoyl-9-ylphenyl)amine (TCTA) are taken as the hole-transporting and electron-blocking layer. 1,3-Bis(N-carbazolyl)benzene (mCP) serves as both the hole-blocking layer and host in the emissive layer. 1,3,5-Tri(3-pyridyl-3-phenyl)benzene (TmPyPB), LiF and Al are acting as the electron-transporting layer, electron injection layer and cathode, respectively.
As showed in Figure 6, their normalized EL spectra resemble the PL spectra recorded in the degassed CH2Cl2 solution, confirming that the emission is solely generated from the emitters, from which EL of Cb4 is also red-shifted compared to that of Cb1. Moreover, the Cb4-based device shows a relatively lower current density at the same voltage compared to that of the Cb1-based device, which can be ascribed to the carrier trapping effect of Cb4 with a narrower energy gap than that of Cb1 [45,46]. In contrast, the Cb1-based device exhibited a bright green emission with EL peak at 530 nm and a maximum luminance of 12420 cd/m2 at 11.5 V, while the Cb4-based device delivered a yellow EL peak centered at 559 nm with a maximum luminance of 21480 cd/m2 at 13.0 V. A maximum external quantum efficiency (current efficiency) of 16.6% (55.2 cd/A) and 13.9% (43.8 cd/A) was also observed for Cb1- and Cb4-based devices, respectively. More importantly, both OLED devices present a small efficiency roll-off at 1000 cd/m2 (15.4% and 12.1% for Cb1 and Cb4-based devices, respectively), evidencing good carrier balance during device operation.

4. Conclusions

In summary, by introducing varied substituents at the 4-position of central pyridinyl fragment of dianionic chelate or on the central phenyl coordination unit of carboline-based monoanionic pincer chelate, a series of five bis-tridentate Ir(III) complexes were successfully designed and synthesized, with an isolation yield higher than 50% and absence of any isomeric product. This result is consistent with those documented in literature [37,43]. The addition of methoxy and dimethylamino substituents at the 4-position of central pyridinyl fragment of dianionic chelate effectively increased the electron density at the Ir(III) metal center, which increased the MLCT contribution at the excited states, and gave a structureless emission profile. As for Ir(III) complexes Cb4 and Cb5, the tert-butyl substituent on the 4-position of the phenyl ring also red-shifted the emission and exhibited slightly reduced emission quantum yields. Next, Cb1 and Cb4 were doped into the emission layer for fabrication of OLEDs, achieving a maximum external quantum efficiency (current efficiency) of 16.6% (55.2 cd/A) and 13.9% (43.8 cd/A), respectively. The well-performed electroluminescence efficiencies indicate that the studied bis-tridentate Ir(III) complexes and their future derivations are promising candidates for OLED applications.

Supplementary Materials

The following are available online. General experimental procedures of all measurements and calculations, synthetic protocol of chelates, original electrochemical data and detailed TD-DFT results of studied Ir(III) metal complexes. Scheme S1. Synthetic protocol given the employed dianionic chelates (phyz)H2; Scheme S2. Synthetic protocol given the employed carboline chelates (cbF)H·HF6 and (cbB)H·HF6; Figure S1. Frontier molecular orbitals pertinent to the optical transitions for the ground state S0, excited state T1 and S1 of Ir(III) complex Cb1. The electron density distributions of Ir atoms in each molecular orbital are shown; Figure S2. Frontier molecular orbitals pertinent to the optical transitions for the ground state S0, excited state T1 and S1 of Ir(III) complex Cb2. The electron density distributions of Ir atoms in each molecular orbital are shown; Figure S3. Frontier molecular orbitals pertinent to the optical transitions for the ground state S0, excited state T1 and S1 of Ir(III) complex Cb3. The electron density distributions of Ir atoms in each molecular orbital are shown; Figure S4. Frontier molecular orbitals pertinent to the optical transitions for the ground state S0, excited state T1 and S1 of Ir(III) complex Cb4. The electron density distributions of Ir atoms in each molecular orbital are shown; Figure S5. Frontier molecular orbitals pertinent to the optical transitions for the ground state S0, excited state T1 and S1 of Ir(III) complex Cb5. The electron density distributions of Ir atoms in each molecular orbital are shown; Figure S6. Thermal gravimetric analysis of studied Ir(III) complexes Cb 1–5 with a decomposition temperature (Td) showing a loss of 5% in weight; Table S1. The calculated wavelengths, transition probabilities and charge transfer character of the optical transitions for Ir(III) complex Cb1 in CH2Cl2; Table S2. The calculated wavelengths, transition probabilities and charge transfer character of the optical transitions for Ir(III) complex Cb2 in CH2Cl2; Table S3. The calculated wavelengths, transition probabilities and charge transfer character of the optical transitions for Ir(III) complex Cb3 in CH2Cl2; Table S4. The calculated wavelengths, transition probabilities and charge transfer character of the optical transitions for Ir(III) complex Cb4 in CH2Cl2; Table S5. The calculated wavelengths, transition probabilities and charge transfer character of the optical transitions for Ir(III) complex Cb5 in CH2Cl2.

Author Contributions

Conceptualization, J.Y. and Y.C.; methodology, J.Y., Z.-L.Z., S.-H.L., P.-T.C. and Y.C.; software, J.Y., Z.-L.Z., S.-H.L., P.-T.C. and Y.C.; validation, C.-S.L., P.-T.C. and Y.C.; writing—original draft preparation, J.Y., S.-H.L. and Y.C.; writing—review and editing, Z.-L.Z., P.-T.C. and Y.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was mainly funded by NSFC–NSFC/RGC Joint Research Scheme, grant number N_CityU102/19.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available in the Supplementary Materials.

Acknowledgments

This work was also partially supported by fundings from Research Grant Council and City University of Hong Kong, Hong Kong SAR. Works conducted in Taiwan were supported by Ministry of Science and Technology (MOST) and National Taiwan University. We were also grateful to the National Center for High-Performance Computing (NCHC) of Taiwan for the valuable computer time and research facilities.

Conflicts of Interest

The authors declare no conflict of interest.

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Molecules 26 06048 sch001 550
Scheme 1. Structural drawings of both the carbene pincer chelate and chelates employed in the present study.
Scheme 1. Structural drawings of both the carbene pincer chelate and chelates employed in the present study.
Molecules 26 06048 sch001
Molecules 26 06048 sch002 550
Scheme 2. Structural drawings of the bis-tridentate Ir(III) complexes Cb1–5.
Scheme 2. Structural drawings of the bis-tridentate Ir(III) complexes Cb1–5.
Molecules 26 06048 sch002
Molecules 26 06048 g001 550
Figure 1. Structural drawing of Cb1 with ellipsoids shown at 30% probability. Nitrogen atoms in blue, carbon atoms in black, iridium atom in red, fluorine atoms in green. All hydrogen atoms are omitted for clarity. Selected bond distances: Ir-N(1) = 2.149(3), Ir-N(3) = 2.055(3), Ir-N(4) = 2.120(3), Ir-C(14) = 2.019(4), Ir-C(31) = 2.011(3) and Ir-C(39) = 2.004(3) Å; Selected bond angles: C(14)-Ir-N(1) = 156.82(14), C(39)-Ir-N(4) = 169.77(14), C(31)-Ir-N(4) = 89.79(13) and C(31)-Ir-N(3) = 177.19(13)°.
Figure 1. Structural drawing of Cb1 with ellipsoids shown at 30% probability. Nitrogen atoms in blue, carbon atoms in black, iridium atom in red, fluorine atoms in green. All hydrogen atoms are omitted for clarity. Selected bond distances: Ir-N(1) = 2.149(3), Ir-N(3) = 2.055(3), Ir-N(4) = 2.120(3), Ir-C(14) = 2.019(4), Ir-C(31) = 2.011(3) and Ir-C(39) = 2.004(3) Å; Selected bond angles: C(14)-Ir-N(1) = 156.82(14), C(39)-Ir-N(4) = 169.77(14), C(31)-Ir-N(4) = 89.79(13) and C(31)-Ir-N(3) = 177.19(13)°.
Molecules 26 06048 g001
Molecules 26 06048 g002 550
Figure 2. UV−vis absorption and normalized emission spectra of the studied Ir(III) complexes Cb1−5 in degassed CH2Cl2 solution at RT.
Figure 2. UV−vis absorption and normalized emission spectra of the studied Ir(III) complexes Cb1−5 in degassed CH2Cl2 solution at RT.
Molecules 26 06048 g002
Molecules 26 06048 g003 550
Figure 3. Cyclic voltammograms of Ir(III) metal complexes Cb1–5.
Figure 3. Cyclic voltammograms of Ir(III) metal complexes Cb1–5.
Molecules 26 06048 g003
Molecules 26 06048 g004 550
Figure 4. Frontier molecular orbitals HOMO and LUMO in the T1 excited state for the Ir(III) complexes Cb1–5 in CH2Cl2 solution. “Ir” indicates the relative electron density distribution at the Ir atoms.
Figure 4. Frontier molecular orbitals HOMO and LUMO in the T1 excited state for the Ir(III) complexes Cb1–5 in CH2Cl2 solution. “Ir” indicates the relative electron density distribution at the Ir atoms.
Molecules 26 06048 g004
Molecules 26 06048 g005 550
Figure 5. Schematic device configuration, energy level diagram and chemical structures of the materials used in devices.
Figure 5. Schematic device configuration, energy level diagram and chemical structures of the materials used in devices.
Molecules 26 06048 g005
Molecules 26 06048 g006 550
Figure 6. Summary of electroluminescence data: (a) normalized EL spectra of Cb1 and Cb4, (b) current density-voltage-luminance (J-V-L) characteristics and (c) plot of current efficiency-luminance-EQE.
Figure 6. Summary of electroluminescence data: (a) normalized EL spectra of Cb1 and Cb4, (b) current density-voltage-luminance (J-V-L) characteristics and (c) plot of current efficiency-luminance-EQE.
Molecules 26 06048 g006
Table
Table 1. The corresponding photophysical properties of Ir(III) complexes Cb1–5 in degassed CH2Cl2 solution at RT.
Table 1. The corresponding photophysical properties of Ir(III) complexes Cb1–5 in degassed CH2Cl2 solution at RT.
λabs (nm); (ε × 104 M−1 cm−1) [a]λem (nm) [b]Φ (%) [b,c]τobs (μs) [b]kr (s−1) [d]knr (s−1) [d]
Cb1278 (4.31), 318 (2.19), 358 (0.96), 420 (0.3)506(sh), 525693.42.0 × 1050.91 × 105
Cb2280 (4.02), 352 (0.96), 416 (0.3)495(sh), 521481.53.2 × 1053.4 × 105
Cb3282 (4.9), 316 (2.63), 358 (1.09), 418 (0.28)529411.23.4 × 1054.9 × 105
Cb4284 (3.62), 354 (1.02), 398 (0.37), 446 (0.2)544582.02.9 × 1052.1 × 105
Cb5284 (3.96), 316 (2.32), 356(0.99), 418 (0.27)555461.43.3 × 1053.8 × 105
[a] UV-vis spectra were recorded in CH2Cl2 at a concentration 10−5 M at RT; [b] PL spectra, lifetime, and quantum yields were recorded in degassed CH2Cl2 at RT; [c] Coumarin (C153) in EtOH (Q.Y. = 58% and λmax = 530 nm) was employed as standard; [d] kr = radiative decay rate constant and knr = nonradiative decay rate constant.
Table
Table 2. Electrochemical data of the Ir(III) metal complexes Cb1–5 in acetonitrile at RT.
Table 2. Electrochemical data of the Ir(III) metal complexes Cb1–5 in acetonitrile at RT.
Eox½ (V) (ΔEp) [a]Erepc (V) [b]HOMO (eV) [c]Energy Gap (eV) [d]LUMO (eV) [e]
Cb10.56 (0.07)−2.42−5.362.64−2.72
Cb20.53 (0.07)−2.45−5.332.66−2.67
Cb30.45 (0.07)−2.48−5.252.65−2.60
Cb40.35 (0.08)−2.50−5.152.52−2.63
Cb50.25 (0.07)−2.56−5.052.53−2.52
[a] All electrochemical potentials were measured in a 0.1 M acetonitrile solution of TBAPF6 and reported in volts using Fc+/Fc as the reference. Eox½ (V) refers to [(Epa + Epc)/2], where Epa and Epc are anodic and cathodic waves, respectively. ΔEp = Epa ‒ Epc; [b] Erepc is the cathodic wave potential for the irreversible reduction wave; [c] HOMO = ‒(Eox½ + 4.8); [d] energy gap = 1240/[PL onset (nm)]; [e] LUMO = HOMO + Energy gap.
Table
Table 3. The main transition characters, calculated wavelengths and contributing percentages of the lowest energy absorption and emission bands of Ir(III) complexes Cb1–5 in CH2Cl2 solution.
Table 3. The main transition characters, calculated wavelengths and contributing percentages of the lowest energy absorption and emission bands of Ir(III) complexes Cb1–5 in CH2Cl2 solution.
ComplexStateλ (nm)fMain AssignmentsMLCT
Cb1S0 → T1459.10HOMO → LUMO + 1 (19%)13.52%
S0 → S1402.70.0105HOMO → LUMO + 1 (87%)24.82%
T1 → S0588.40LUMO → HOMO (72%)18.97%
S1 → S0488.20.0083LUMO → HOMO (96%)30.30%
Cb2S0 → T1449.20HOMO → LUMO (29%)18.62%
S0 → S1391.20.0425HOMO → LUMO (94%)29.18%
T1 → S05730LUMO → HOMO (73%)20.40%
S1 → S0484.20.053LUMO → HOMO (98%)30.01%
Cb3S0 → T1437.40HOMO → LUMO (26%)19.13%
S0 → S1394.80.0509HOMO → LUMO (94%)30.36%
T1 → S0557.40LUMO → HOMO (72%)19.92%
S1 → S04900.0553LUMO → HOMO (98%)31.27%
Cb4S0 → T1462.10HOMO → LUMO + 1 (87%)23.64%
S0 → S1417.80.0074HOMO → LUMO + 1 (85%)23.09%
T1 → S0587.70LUMO → HOMO (72%)19.15%
S1 → S05110.0062LUMO → HOMO (96%)27.39%
Cb5S0 → T1449.50HOMO → LUMO (83%)25.75%
S0 → S1413.40.0432HOMO → LUMO (96%)29.78%
T1 → S0598.90LUMO → HOMO (76%)23.85%
S1 → S0520.20.0423LUMO → HOMO (98%)28.86%
Table
Table 4. Device performances of OLEDs based on bis-tridentate Ir(III) complexes Cb1 and Cb4.
Table 4. Device performances of OLEDs based on bis-tridentate Ir(III) complexes Cb1 and Cb4.
λEL [nm]Von [V] [a]Max. Luminescence [cd/m2] (@voltage [V])EQE [%] [b]CE [cd/A] [b]
Cb1503(sh), 5303.512420 (11.5)16.6, 16.5, 15.455.2, 52.9, 51.1
Cb45593.521480 (13)13.9, 13.1, 12.143.8, 43.0, 38.8
[a] Voltage at 1 cd/m2; [b] data recorded at maximum efficiency, 100 and 1000 cd/m2, respectively.
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Yan, J.; Zhu, Z.-L.; Lee, C.-S.; Liu, S.-H.; Chou, P.-T.; Chi, Y. Probing Electron Excitation Characters of Carboline-Based Bis-Tridentate Ir(III) Complexes. Molecules 2021, 26, 6048. https://doi.org/10.3390/molecules26196048

AMA Style

Yan J, Zhu Z-L, Lee C-S, Liu S-H, Chou P-T, Chi Y. Probing Electron Excitation Characters of Carboline-Based Bis-Tridentate Ir(III) Complexes. Molecules. 2021; 26(19):6048. https://doi.org/10.3390/molecules26196048

Chicago/Turabian Style

Yan, Jie, Ze-Lin Zhu, Chun-Sing Lee, Shih-Hung Liu, Pi-Tai Chou, and Yun Chi. 2021. "Probing Electron Excitation Characters of Carboline-Based Bis-Tridentate Ir(III) Complexes" Molecules 26, no. 19: 6048. https://doi.org/10.3390/molecules26196048

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