Pyridylpyrazole N^N ligands combined with sulfonyl-functionalised cyclometalating ligands for blue-emitting iridium(III) complexes and solution-processable PhOLEDs

Abstract

A series of blue iridium(III) complexes (12–15) comprising sulfonyl-functionalised phenylpyridyl cyclometalating ligands and pyridylpyrazole N^N ligands are reported, with an X-ray crystal structure obtained for 12. The complexes are highly emissive with photoluminescence quantum yields of 0.52–0.70 in dichloromethane solutions: two of the complexes (12 and 14) show emissions at λ^PL_max 457 nm which is considerably blue-shifted compared to the archetypal blue emitter FIrpic (λ_max 468 nm). The short excited state lifetimes (1.8–3.3 μs) and spectral profiles are consistent with phosphorescence from a mixture of ligand-centred and MLCT excited states. Density functional (DFT) and time dependent DFT (TD-DFT) calculations are in agreement with the electrochemical properties and the blue phosphorescence of the complexes. The additional mesityl substituent on the pyridylpyrazole ligand of 12 and 13 enhances the solubility of the complexes facilitating thin film formation by solution processing. Phosphorescent organic light-emitting diodes (PhOLEDs) have been fabricated using 12 or 13 in a solution-processed single-emitting layer using either poly(vinylcarbazole) (PVK) or 1,3-bis(N-carbazolyl)benzene (mCP) as host. The most blue-shifted electroluminescence (λ^EL_max 460 nm, CIE_x,y 0.15, 0.21) is obtained for an OLED containing complex 12 and mCP, with a brightness of 5400 cd m⁻² at 10 V which is high for PhOLEDs with similar blue CIE coordinates using a solution-processed emitter layer.

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Introduction

The varied applications of luminescent transition metal complexes¹ include biological labelling probes, ion sensors, water splitting, solar cells and emitters for phosphorescent organic light-emitting diodes (OLEDs)^2–8 and for solid-state lighting.^9,10 Cyclometalated iridium(III) complexes are especially prominent in the OLED field due to their ability to harvest both singlet and triplet excitons, which can result in internal quantum efficiencies approaching 100%.^11–13 Many complexes incorporate cyclometalated 2-phenylpyridine (ppy) ligands where the highest occupied molecular orbital (HOMO) is localised mostly on the iridium atom and the phenyl ring, and the lowest unoccupied MO (LUMO) is on the pyridyl ring. This spacial separation of HOMO and LUMO means that the emission wavelength of the complexes, which occurs from a mixture of triplet metal-to-ligand charge-transfer (³MLCT) states and π–π* transitions of the ligands,^14,15 can be tuned with precision by the presence of electron-withdrawing or electron-donating substituents at specific sites on the ppy framework. Additional attractive features are that the complexes generally emit with high quantum yields and relatively short excited state lifetimes on the microsecond timescale, and they possess high photochemical and chemical stability.^11–13

Blue emitting iridium complexes are important current targets.^16,17 Four main approaches are known to be effective in shifting the emission of iridium complexes towards the blue.¹⁸ These are: (1) Lowering the HOMO with respect to the LUMO by introducing electron-withdrawing groups, such as fluorine¹⁹ sulfonyl,²⁰ phosphoryl,²⁰ perfluoroalkyl,²¹ perfluoroalkyl carbonyl,²² and cyano¹³ substituents at specific positions on the phenyl ring. (2) Raising the LUMO by either adding electron-donating groups such as methoxy to the pyridyl ring,^23,24 or replacing the pyridyl ring with another heterocycle with higher LUMO energy.²⁵ (3) Using electron-withdrawing N^N ligands, e.g. pyridyl-azole, to modulate the electron density around the metal centre, thereby affecting the HOMO energy.^26–28 (4) Using strong σ-donor ligands, e.g. carbenes, to increase the crystal field splitting, thereby widening the HOMO–LUMO gap and shifting non-emissive d–d* metal-centred (MC) states to higher energies.¹⁹

The present study combines a pyridylpyrazole-based N^N ligand with sulfonyl-containing cyclometalating ligands²⁹ to obtain the new heteroleptic complexes 12–15 with emission blue-shifted compared to the archetypal blue emitter, FIrpic,¹⁶ and the analogous picolinate-complexes 1 and 2 ²⁹ (Fig. 1). Pyridylpyrazole ligands blue-shift the emission due to their high acidity, reducing the electron density around the Ir centre and hence reducing the HOMO level relative to the LUMO.³⁰ They have also been exploited as the chromophoric ligands in N-heterocyclic carbene complexes³¹ and in conjunction with other ligands.^32–34 The strongly electron-withdrawing CF₃ group on the pyrazole ring should further blue shift the emission.³⁵ The p-tolylsulfonyl and methylsulfonyl groups (complexes 1 and 2) both blue shift the emission to the same extent,²⁹ and methoxy substituents can lead to a blue shift depending on their position on the ppy ligand.^36,37 Reducing the number of aromatic fluorine substituents, e.g. by selective replacement with methoxy, can maintain the blue colour and reduce chemical degradation of a complex during device operation.³⁸ Complexes 12–15 incorporate a mesityl substituent at C4 of the ppy ligand; it is known that a mesityl group at this position enhances the quantum yields of some complexes by reducing triplet–triplet quenching without affecting blue colour purity.^39–42 We also report the new mesityl substituted pyridylpyrazole ligand 9 which achieves the desired aims of increasing the solubility of the complexes 12 and 13, in comparison with 14 and 15, thereby facilitating film formation by solution processes without affecting the emission colour.

Fig. 1 Previously reported picolinate complexes.

Synthesis and characterisation

The route to ligand 9 is shown in Scheme 1. 2-Acetyl-4-chloropyridine 6 was synthesised by reaction between 4-nitro-α-picoline-N-oxide 3 and acetyl chloride to give 4 ⁴³ in 23% yield. An intermediate oxime has been proposed in this step.⁴³ The N-oxide was reduced using PCl₃ to produce 5 ⁴³ in 73% yield, followed by reaction with methyl magnesium chloride to give 6 in 92% yield.⁴⁴ The mesityl group was then introduced via a Suzuki coupling of 6 with 2,4,6-trimethylphenylboronic acid to give 7 in 52% yield. The final steps, described in the literature for other analogues,⁴⁵ involved a crossed Claisen condensation reaction with ethyl trifluoroacetate to give a presumed diketone intermediate 8, followed by a ring closing condensation reaction with hydrazine hydrate to give 9 in 20% yield.

Scheme 1 Synthesis of ligand 9. Reagents and conditions: i, CH₃COCl, 60 °C; ii, PCl₃, CHCl₃, reflux; iii, (1) CH₃MgCl, ether, 0 °C, (2) dilute HCl; iv, Pd(OAc)₂, PPh₃, 2,4,6-trimethylphenylboronic acid, Na₂CO₃, DME/water, reflux; v, KO^tBu, F₃CCO₂Et, THF, reflux; vi, N₂H₄·H₂O, ethanol, reflux.

The desired complexes 12 and 13 were then synthesised under standard conditions;^46,47 namely, reaction of the ligand 10 with IrCl₃·3H₂O and ligand 11 ²⁹ with [Ir(COD)Cl]₂ in 2-ethoxyethanol to give a species presumed to be the bridged μ-dichloro diiridium dimer [Ir(L)₂Cl]₂. This intermediate species was then reacted in situ with ligand 9 to give the desired complexes 12 and 13 (Scheme 2). The attempted synthesis of 13 using IrCl₃·3H₂O failed to yield any product and the ligand 11 could not be recovered. Difficulties in the cyclometalation of ligands containing methoxy substituents using IrCl₃·3H₂O have been reported previously.^29,36 Complexes 14 and 15 were synthesised from 10 and 11 by analogous reactions with 3-trifluoromethyl-5-(2-pyridyl)pyrazole ligand 16.³⁰ Thermal gravimetric analysis (TGA) shows that all the complexes possess good thermal stability (Table S5 in ESI†), suggesting the complexes should be stable under device operation.

Scheme 2 Synthesis of complexes 12–15. Reagents and conditions: i, IrCl₃·3H₂O, 2-ethoxyethanol, 130 °C, followed by 9 (for 12 and 13) or 3-trifluoromethyl-5-(2-pyridyl)pyrazole 16 (for 14 and 15) DCM/ethanol, 55 °C; ii, [Ir(COD)Cl]₂, 2-ethoxyethanol, 130 °C, followed by 9 or 16, DCM/ethanol, 55 °C.

X-Ray crystal structure

The single crystal X-ray structure of the highly-functionalised complex 12 (Fig. 2) shows a distorted octahedral coordination of the Ir atom, similar to that in its analogues.³² The two Ir–C bonds are in cis positions to one another and trans to the Ir–N bonds of the N^N ligand, which are, therefore, elongated compared to the mutually trans bonds Ir–N(4) and Ir–N(5). The mesityl groups are near-perpendicular to the corresponding pyridyl rings. The crystal packing of 12 is dominated by edge-to-face contacts between arene groups, without π–π stacking.

Fig. 2 X-ray molecular structure of 12. H atoms and the disorder of CF₃ group are omitted; thermal ellipsoids are drawn at the 30% probability level. Bond distances (Å): Ir–N(1) 2.102(6), Ir–N(3) 2.142(8), Ir–N(4) 2.040(6), Ir–N(5) 2.057(6), Ir–C(10) 2.006(7) and Ir–C(22) 1.991(9). Interplanar angles (°): i/ii 7, ii/iii 90, iv/v 7, iv/vi 74, vii/viii 4, vii/ix 89.

Photophysical and electrochemical properties

The absorption and emission spectra of complexes 12–15 (Fig. 3) are consistent with pyridylpyrazole subtly lowering the energy of the HOMO. All the complexes show strong absorption bands in the 250–325 nm region, assigned to the π–π* transitions on the ligands.^16,18 Absorption bands with lower extinction coefficients in the 350–450 nm range are ascribed to singlet and triplet metal-to-ligand charge-transfer (¹MLCT and ³MLCT) states.^48,49 Here, we see differences between the picolinate complexes 1 and 2,²⁹ and complexes 12–15; the ¹MLCT bands move to higher energies on replacing the picolinate ligands with ligands 9 or 16, suggesting the pyridylpyrazole has indeed decreased the energy of the metal d-orbitals. The emission spectra of the complexes are shown in Fig. 3b. Complexes 12–15 emit in the blue region, and display a similar emission profile to that of FIrpic and the analogous picolinate complexes 1 and 2. A notable feature is that the λ_max value for 12 and 13 is blue-shifted by 7 and 8 nm relative to their analogous picolinate complexes 1 and 2, respectively. This represents a blue-shift of 11 nm and 2 nm relative to that of the benchmark sky-blue emitter FIrpic.¹⁶ The impressive 11 nm shift for complex 12 is primarily a consequence of the combined benefits of the sulfone substituents and the N^N ligand. The UV-Vis absorption and emission of complexes 14 and 15 are similar to those of complexes 12 and 13, indicating the introduction of the mesityl substituent on the N^N ligand does not impact the emission colour.

Fig. 3 (a) UV-Vis absorption spectra of the complexes in DCM [<10⁻⁵ M]. Inset shows an expansion of the 350–475 nm region. (b) Emission spectra in deaerated DCM, λ_ex = 380 nm.

The photoluminescence quantum yield (PLQY) and lifetime data are listed in Table 1. The PLQYs for complexes 12–15 are comparable to their pic analogues 1 and 2, with lifetimes and spectral profiles consistent with phosphorescence from a mixture of ligand-centred and MLCT excited states.^18,49

Table 1 Photophysical data for the iridium complexes

Complex	λ ^absmax (ε)^a/nm (×10^3 M^−1 cm^−1)	λ ^emmax ^b/nm	PLQY/ΦPL ^a^,^c	τ P ^a^,^d/μs	k r/10^5 s^−1	k nr/10^5 s^−1
a Data obtained in DCM solution at 20 °C. b Data obtained in degassed DCM solution with λex = 380 nm. c Measured relative to Ir(ppy)3ΦPL = 0.46 in degassed DCM at 20 °C; estimated error ±5%. d Estimated error ±5%.
FIrpic	277 (50.1), 301 (34.2), 304 (32.6), 337 (13.8, sh), 357 (8.9, sh), 400 (6.2), 454 (0.8)	468, 496, 531 (sh)	0.67	1.72	3.90	1.92
1 ^29	258 (s sh, 108.5), 271 (128.6), 297 (123.4), 307 (sh, 112.5), 339 (s sh, 29.2), 379 (15.7), 450 (1.0)	464, 490, 524 (sh)	0.67	1.78	3.76	1.85
2 ^29	275 (104.4), 291 (101.9), 324 (sh, 47.8), 381 (14.5), 405 (sh, 7.9), 456 (0.9)	474, 497, 536 (sh)	0.58	2.13	2.72	1.97
12	274 (144.4), 293 (136.7), 370 (19.4), 419 (sh, 2.7), 447 (0.7)	457, 486, 521 (sh)	0.62	2.30	2.70	1.65
13	255 (sh, 100.4), 277 (125.8), 292 (133.5), 321 (59.6) (sh), 375 (16.3), 406 (sh, 5.1), 454 (0.8)	466, 494, 531 (sh)	0.52	3.13	1.66	1.53
14	274 (171.0), 291 (161), 374 (17.9), 418 (2.1), 450 (0.5)	457, 486, 522 (sh)	0.70	2.74	2.55	1.09
15	276 (129.3), 295 (122.75), 325 (50.8), 371 (18.2), 411 (3.2), 454 (0.7)	467, 494, 531 (sh)	0.64	3.30	1.94	1.09

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The electrochemical behaviour of complexes 12–15 was investigated by cyclic voltammetry (CV) in a 0.1 M NⁿBu₄PF₆ acetonitrile solution, and compared to FIrpic and picolinate analogues 1 and 2.²⁹ The voltammograms of FIrpic, 12–15 are shown in Fig. 4 and S13,† and the key parameters are listed in Table 2. Whereas the picolinate complexes (FIrpic, 1 and 2) show a single quasi-reversible oxidation wave, complexes 12–15 show two oxidation waves. In the case of 12 and 13 the two waves are irreversible, whereas for 14 and 15 the first oxidation wave is quasi-reversible.

Fig. 4 Cyclic voltammograms of the iridium complexes.

Table 2 Cyclic voltammetric data of the iridium complexes

Complex	E ^ox11/2 ^a/V	E ^ox21/2 ^a/V	Obs E(HOMO)^c/V	Complex	Calc. HOMO/eV	Calc. LUMO/eV	Calc. HLG^d/eV
a Redox data were obtained in 0.1 M N^nBu4PF6 acetonitrile solutions and are reported vs. FcH/FcH^+.^50 b Not reversible. c E(HOMO) = −4.8 − E^ox11/2 with respect to the FcH/FcH^+ couple at 4.8 V. d HLG = HOMO–LUMO energy gap.
FIrpic^29	0.89	—	−5.69	FIrpic′	−5.49	−1.87	3.62
1 ^29	1.01	—	−5.81	1′	−5.64	−1.94	3.70
2 ^29	0.89	—	−5.69	2′	−5.53	−1.89	3.64
12	1.02^b	1.28^b	−5.82	12′	−5.74	−1.83	3.91
13	0.91^b	1.17^b	−5.71	13′	−5.62	−1.77	3.85
14	1.07	1.41^b	−5.87	14′	−5.77	−1.89	3.88
15	0.97	1.28^b	−5.77	15′	−5.64	−1.84	3.80

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As complexes 12–15 contain a pyridylpyrazole-based ligand it is assumed the second oxidation occurs either on this ligand or is due to an interaction of this ligand with the metal. Exchanging the picolinate ancillary ligand for pyridylpyrazole results in an increase in the first oxidation potential by only 10–20 mV (1vs. 12, 2vs. 13), consistent with pyridylpyrazole subtly lowering the energy of the HOMO. The presence of the mesityl group on the N^N ligand (complexes 12 and 13) decreases the oxidation potentials by ca. 50 mV compared to the unsubstituted complexes 14 and 15. No reduction waves were observed within the solvent window to −2.0 V. This is consistent with the calculated large HOMO–LUMO gap (Table 2).

Computations

Electronic structure calculations were carried out on the four new iridium complexes 12–15 to explore the frontier orbitals (Fig. 5 and Tables S1–S4†) in order to support the electrochemical and photophysical observations. The full geometries were optimised at B3LYP/LANL2DZ:3-21G* to compare directly with computed data of FIrpic, 1 and 2 reported earlier.²⁹ The optimised geometries are denoted as 12′–15′ to identify them as computed models and distinguish the predicted data from experimental data. Comparison between optimised (12′) and X-ray determined (12) geometry reveals good agreement with differences in bond lengths below 0.04 Å.

Fig. 5 Frontier molecular orbitals for 12′ and 13′. All contours are plotted at ±0.04 (e per bohr³)^1/2. Ir : phenyl : pyrazolyl : pyridyl(pypz) : pyridyl(ppy) % orbital contribution ratio are listed for each orbital.

Complexes 12′–15′ have HOMOs similar to FIrpic with the HOMO on iridium and phenyl (ppy) (Fig. 5). The lower HOMO energies on the complexes 12′–15′, compared to FIrpic, reflect the electron-withdrawing properties of the sulfone groups. Frontier orbital energies for all complexes are listed in Table 2 for direct comparison with the observed CV data. The trends in observed and computed HOMO energies are in very good agreement. The iridium contributions (32–33%, Table 3) in the HOMOs of 12′–15′ are lower than the corresponding contribution in FIrpic′ (44%) suggesting increased ligand contributions in the MLCT transitions involved in the emissions of 12′–15′ with respect to MLCT transitions for FIrpic′.

Table 3 Iridium atom contributions in HOMOs and predicted (TD-DFT) emission wavelengths of iridium complexes

Complex	% Ir HOMO	S0 ← T1^a/nm	Complex	Observed λ^emmax ^b/nm
a Values from TD-DFT data on S0 optimised geometries with scaling energy factor of 0.945 based on DCM at 298 K. b Observed highest energy band from emission spectra (Table 1).
FIrpic′^29	44	471	FIrpic	471
1′ ^29	39	467	1	464
2′ ^29	40	473	2	474
12′	33	455	12	457
13′	33	459	13	466
14′	32	454	14	457
15′	32	459	15	467

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The LUMOs in 12′–15′ have substantial contributions from the pyridyl groups of the ppy and the pyridylpyrazole ligands which raise their orbital energies compared to FIrpic′ where the LUMO is entirely located on the pyridyl group of the ppy ligands. The pyridylpyrazole ligands are, therefore, not ancillary. Consequently, the HOMO–LUMO energy gaps (HLGs) in complexes 12′–15′ are larger than the HLG in FIrpic′ (Table 2) which is consistent with the observed blue-shifted emissions of 12–15 compared to FIrpic. The LUMOs in 13′ and 15′ contain more pyridylpyrazole character than ppy character (Fig. 5) which is explained by the increased number of electron-donating methoxy groups present in the ppy ligands of these complexes.

TD-DFT computations were carried out on the S₀ optimised geometries of 12′–15′ to predict emission wavelengths of these complexes. The initial excitation is assumed to give the lowest energy singlet excited state S₁ which in the presence of the iridium centre results in intersystem crossing (ISC) to form the triplet excited state T₁ and phosphorescence is observed from the S₀ ← T₁ process. The small Stokes shifts observed for these complexes suggest that the T₁ geometry is similar to the corresponding S₀ geometry. The reverse process S₀ ← T₁ is thus considered to have the same nature as the computed S₀ → T₁ process with the predicted emission wavelength adjusted to take into account the Stokes shift. Table 3 shows there is close agreement between the predicted S₀ ← T₁ wavelengths and the observed emission wavelengths for all the complexes.

Phosphorescent organic light-emitting devices

PhOLEDs containing complex 12, 13 or 2 as the dopant complex were fabricated to compare directly with our reference devices containing FIrpic and 1. The initial standard device architecture comprised of a simple single-emissive-layer which was a blend of poly(vinylcarbazole) (PVK) as the host material, OXD-7 (an electron-transporting material) and the Ir complex to give the architecture: ITO/PEDOT:PSS (45 nm)/PVK:OXD-7 (30 wt%):Ir complex (15 wt%) (60 nm)/TPBi (30 nm)/LiF (1 nm)/Al (100 nm). The emissive layer was spin-coated from chlorobenzene solution to prevent possible degradation of the complexes which is known to occur during thermal evaporation of complexes which have fluorinated ligands.^38,51–54 TPBi was used as an additional electron transporting layer adjacent to the cathode to improve charge balance and to ensure that excitons are confined in the emitter layer.⁵⁵ The efficiency and luminance data of the devices are summarised in Table 4 and the electroluminescence (EL) spectra are shown in Fig. 6a. The EL and PL spectra are similar for all the complexes due to efficient exciton confinement on the emissive molecules. The λ^EL_max of complex 12 is significantly bluer (by 6 nm) than the corresponding pic complex 1. However, as a trade-off, 12 has lower brightness (panel e), lower EQE (panel c), and lower current and power efficiencies (panels d and f). Based on the electrochemical data, the HOMO of 12 is lower in energy than 1, which could be partially responsible for the reduced performance. Complex 13, however, performed better than 2, with comparable brightness and turn-on voltage (panel e), and with superior EQE, current and power efficiencies. The λ^EL_max of 13 is also blue-shifted relative to 2; however, this is offset by a broader emission resulting in similar CIE coordinates for both complexes. The additional mesityl substituent on the N^N ligand of 12 and 13 enhances the solubility of the complexes facilitating thin film formation by solution processing.

Fig. 6 Electroluminescent device data for complexes FIrpic, 1, 2, 12 and 13 in PVK host and for 12 in mCP host.

Table 4 Summary of device data

Complex	λ ^ELmax/nm	Brightness (max, and at 10 V)/cd m^−2	Turn-on voltage^c/V	EQEmax/%	Current efficiency/cd A^−1	Power efficiency/lm W^−1	CIE coordinates/(x,y)^d
a In PVK host. b In mCP host. c Measured at a brightness of 10 cd m^−2. d Measured at 12 V.
FIrpic^a	475	7340, 2380	6.2	5.4	12.2	5.1	(0.19, 0.38)
1	470	2072, 175	8.5	3.5	6.1	1.9	(0.16, 0.30)
2	478	516, 160	8.5	1.2	2.6	0.9	(0.22, 0.40)
12	464	1066, 55	9.5	2.3	4.1	1.2	(0.17, 0.28)
13	473	456, 195	7.9	1.7	3.9	1.4	(0.23, 0.40)
12	460	5428, 5400	6.2	6.3	6.9	3.5	(0.15, 0.21)

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We further explored the bluest complex 12 using mCP as a wide energy gap host which is more appropriate for a deep blue emitter in the architecture: ITO/TAPC (45 nm)/mCP:Ir complex 12 (15 wt%) (45 nm)/Tm3PyBP (5 nm)/TPBi (30 nm)/LiF (1 nm)/Al (100 nm). TAPC served as a hole-transporting layer and Tm3PyBP serves as an additional hole/exciton-blocking layer to optimise the device structure.⁵⁶ Compared to the 12:PVK device, the 12:mCP device displayed a significantly lower turn-on voltage and further blue shifted electroluminescence to λ^EL_max 460 nm, CIE_x,y (0.15, 0.21) with an increased maximum EQE of 6.3% and increased brightness at 10 V of 5400 cd m⁻² which is high compared to reported PhOLEDs with similar blue CIE coordinates using a solution-processed emitter layer.⁵⁷

Conclusions

To summarise, replacing the picolinate ancillary ligand of the heteroleptic iridium complexes 1 and 2 with a pyridylpyrazole-based ligand affords the new complexes 12–15 with blue shifted emission. This is ascribed to a reduction of electron density around the metal centre which results in a lowering of the energy of the HOMO of the complexes, as observed in the solution electrochemical data. (TD-)DFT calculations are in excellent agreement with the observed photophysical and electrochemical properties of the complexes. Blue-emitting PhOLEDs have been fabricated with a solution-processed emissive layer structure. Notably, devices of complex 12 possess λ^EL_max 460 nm and CIE_x,y (0.15, 0.21) with brightness at 10 V of 5400 cd m⁻². This study demonstrates that the rational design of new complexes with specific functionalities on the cyclometalating and the N^N ligands is an effective route to new efficient blue emitters and PhOLEDs using relatively simple device architectures.

Experimental section

Materials, synthesis and characterisation

All commercially available chemicals were used without further purification. Reactions requiring an inert atmosphere were performed under a blanket of argon gas, which was dried over a phosphorus pentoxide column. Anhydrous solvents were dried through an HPLC column on an Innovative Technology Inc. solvent purification system. Column chromatography was performed using 40–60 μm mesh silica gel. Analytical TLC was performed on plates pre-coated with silica gel (Merck, silica gel 60F254) and visualised using UV light (254, 315, 365 nm). NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer. Chemical shifts are referenced to tetramethylsilane [TMS, Si(CH₃)₄] at 0.00 ppm. Melting points were determined in open-ended capillaries using a Stuart Scientific SMP3 melting point apparatus at a ramping rate of 1 °C min⁻¹ and were recorded to the nearest 0.1 °C. ESI and MALDI mass spectra were recorded on a Thermo-Finnigan LTQ FT (7.0 T magnet) spectrometer. ASAP mass spectra were recorded on a Waters Xevo QTOF spectrometer. GCMS spectra were recorded on a Thermo-Finnigan Trace GCMS (EI and CI ion sources). Elemental analyses were obtained on an Exeter Analytical Inc. CE-440 elemental analyzer.

Solution electrochemistry and photophysics

Cyclic voltammetry experiments were recorded using a BAS CV50 W electrochemical analyzer fitted with a three-electrode system consisting of a Pt disk (∅ = 1.8 mm) as the working electrode, a Pt wire as an auxiliary electrode and an Ag/AgNO₃ (0.1 M [NEt₄][ClO₄] in CH₃CN) system as the reference electrode. Experiments were conducted in dry acetonitrile solution with NⁿBu₄PF₆ (0.1 M) as the supporting electrolyte at a scan rate of 100 mV s⁻¹. The reference electrode was assumed to be stable and was referenced externally to ferrocene (Cp₂Fe) and decamethylferrocene (Cp*₂Fe) which displayed potentials (E_1/2) of −0.41 V and +0.10 V, respectively, versus Ag/AgNO₃ under these conditions. It was not possible to use Cp₂Fe or Cp*₂Fe as an internal reference because their addition to the solutions of the complexes resulted in a distortion of the redox waves of the complexes.

Solution state photophysical data were obtained using freshly prepared solutions of the complexes in DCM. Emission and lifetime measurements used thoroughly degassed solutions achieved by three freeze–pump–thaw cycles, and quartz cuvettes with a path length of 1 cm. The solutions had absorbance below 0.10 to minimise inner filter effects. UV-vis absorption measurements were recorded using a Unicam UV2-100 spectrometer operated with Unicam Vision (ver. 3.50) software. Baseline correction was achieved by reference to pure solvent in the same cuvette. Absorption measurements were obtained using quartz cuvettes with a path length of 2 cm. Excitation and emission spectra were recorded on a Jobin–Yvon–Horiba SpexFluoromax 3 Spectrometer. Solutions of the complexes in degassed DCM [<10⁻⁵ M] were used for decay measurements. Samples were excited with a pulsed nitrogen laser emitting at 337.1 nm. Emission was focused onto a spectrograph and detected on a sensitive gated iCCD camera (Stanford Computer Optics) with sub-nanosecond resolution. Solution PLQYs were recorded in degassed solvent, and determined using the relative method, with Ir(ppy)₃ (Φ_PL = 0.46 in degassed dichloromethane, determined in-house vs. Φ_PL = 0.546 in 0.5 M H₂SO₄) as the reference. The PLQYs were calculated according to the following equation:

where subscripts ‘x’ and ‘ref’ denote the material being measured and the reference, respectively. Φ represents the PLQY, Grad is the gradient from the plot of integrated fluorescence intensity vs. absorbance, and η is the refractive index of the solvent.

Thermal analyses

Thermogravimetric analysis (TGA) was performed in a nitrogen atmosphere using a PerkinElmer Pyris 1 TGA instrument. The complexes were heated at a rate of 10 °C min⁻¹ from room temperature up to 600 °C.

Computational studies

All calculations were carried out with the Gaussian 09 package.⁵⁸ All optimised S₀ geometries for 12′–15′ were carried out using B3LYP⁵⁹ with the pseudopotential (LANL2DZ)⁶⁰ for iridium and 3-21G* basis set for all other atoms.⁶¹ This model chemistry was selected for direct comparison with computed data on FIrpic′, 1′ and 2′.²⁹ Electronic structure and TD-DFT calculations were also from the optimised geometries at B3LYP/LANL2DZ:3-21G*. The MO diagrams and orbital contributions were generated with the aid of Gabedit⁶² and GaussSum⁶³ packages, respectively.

Phosphorescent organic light-emitting devices (PhOLEDs)

PhOLEDs with poly(vinylcarbazole) (PVK) host were fabricated on indium tin oxide (ITO)-coated glass substrates of thickness 125 nm and possessing a sheet resistance of 15 Ω per square. Poly(3,4-ethylenedioxythiophene) doped with high work function hole injection layer poly(styrenesulfonic acid) (PEDOT:PSS) (HIL1.3) from CLEVIOS was spin-coated at 2500 rpm for 60 s to produce a ∼50 nm thick hole-injecting/transporting layer (HTL). The PEDOT:PSS layer was annealed at ca. 200 °C for 5 min to remove any residual water. A chlorobenzene solution of 25 mg ml⁻¹ of poly(vinylcarbazole) (PVK) (M_w = 90 000) was doped with 30% w/w of (1,3-phenylene)bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole] (OXD-7). Blended devices were made by mixing 15% w/w of the Ir complexes. The prepared mixtures were filtered with a 0.45 μm pore filter and spin-coated at 2500 rpm for 1 min on the top of the PEDOT:PSS layer and baked for 10 min at 120 °C. Each sample was shadow masked to produce four identical devices of area 4 × 5 mm; the samples were then introduced into a nitrogen glove box, where 30 nm of 2,2′,2′′-(1,3,5-benzenetriyl)tris-[1-phenyl-1H-benzimidazole] (TPBi) was evaporated as an electron injection/hole blocking layer at a rate of ∼1 A s⁻¹ under vacuum at a pressure of ca. 1 × 10⁻⁶ Torr, followed by 0.8 nm LiF and a 100 nm capping layer of aluminium under the same evaporation conditions. Therefore, the device configuration for all complexes was: ITO/PEDOT:PSS (50 nm)/PVK:OXD-7 (35%):Ir-complex (15%)/TPBi (30 nm)/LiF (0.8 nm)/Al (100 nm). All samples were encapsulated inside a glove box using DELO UV cured epoxy (KATIOBOND) and capped with a 1.2 × 1.2 cm microscope glass slide then exposed to UV light for 3 min. Current–voltage data, device efficiency, brightness and electroluminescence spectra were measured in a calibrated Labsphere LMS-100 integrating sphere. A home written NI LabVIEW programme was used to control an Agilent 6632B DC power supply, and the emission properties of the device were measured using an Ocean Optics USB4000 CCD fibre optic spectrometer. The thicknesses of the various layers in the device were measured with a J A Woolam VASE Ellipsometer using thin films which had been spin-coated on Si/SiO₂ substrates under the same conditions as the device films.

Devices with 1,3-bis(N-carbazolyl)benzene (mCP) host were fabricated as follows. TAPC (1,1-bis{4-[N,N-di(p-tolyl)amino]phenyl}cyclohexane), TPBi, and Tm3PyPB (1,3,5-tris(3-pyridyl-3-phenyl)benzene) were obtained from commercial sources. Indium tin oxide (ITO)-coated glass with a sheet resistance of 10 Ω per square was used as the substrate. Before device fabrication, the ITO-coated glass substrate was precleaned and exposed to UV-ozone for 15 min. PEDOT:PSS was then spin-coated onto the clean ITO substrate as a hole-injection layer. Next a mixture of complex 12 and mCP in chlorobenzene was spin-coated (10 mg mL⁻¹; 3000 rpm) to form a ca. 45 nm thick emissive layer and annealed at 80 °C for 30 min to remove the residual solvent. Finally, a 5 nm-thick hole/exciton blocking layer of Tm3PyPB was vacuum deposited followed by a 30 nm-thick electron-transporting layer (ETL) of TPBi, and a cathode composed of a 1 nm-thick layer of LiF and aluminum (100 nm) was sequentially deposited through shadow masking at a pressure of 10⁻⁶ Torr. The OLED devices had a pixel sise of 4 mm × 4 mm. All measurements were performed at room temperature under ambient conditions.

Synthesis of ligands and complexes

1-(4-Mesitylpyridin-2-yl)ethanone 7. 2-Acetyl-4-chloropyridine 6 ⁴⁴ (1.5 g, 9.68 mmol), 2,4,6-trimethylphenylboronic acid (1.75 g, 10.67 mmol) and PPh₃ (657 mg, 2.50 mmol) were dissolved in DME (70 mL) and the solution was degassed for 20 min by bubbling with argon. Separately, an aqueous solution of Na₂CO₃ (4.18 g, 39.43 mmol in 17 mL) was degassed. The solutions were combined and Pd(OAc)₂ (130 mg, 0.58 mmol) was added. The mixture was heated to reflux overnight under an atmosphere of argon. The solution was then cooled to RT, and the majority of the solvent was removed in vacuo. DCM and water were added, and the organic layer separated. The aqueous layer was extracted with additional DCM, and the extracts combined and the solvent removed in vacuo. The crude residue was purified by column chromatography (DCM), followed by recrystallisation from hexane to give 1-(4-mesitylpyridin-2-yl)ethanone 7 (1.2 g, 52%); m.pt. 118.9–119.9 °C; δ_H (400 MHz; CDCl₃) 8.74 (1H, dd, J 4.9, 0.8), 7.88 (1H, dd, J 1.7, 0.8), 7.31 (1H, dd, J 4.9, 1.7), 6.95–6.94 (2H, m), 2.79 (3H, s), 2.33 (3H, s), 1.97 (6H, s); δ_C (101 MHz; CDCl₃) 200.07, 153.73, 151.14, 149.18, 138.05, 135.64, 135.08, 128.61, 128.41, 123.06, 26.19, 21.19, 20.72; HRMS (FTMS + ESI): calcd for [C₁₆H₁₇NO + H]⁺: 240.1388. Found: 240.1388.

4-Mesityl-2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridine 9. A solution of 4-mesityl-2-acetylpyridine 7 (1.20 g, 5.01 mmol) in THF (15 mL) was added dropwise to a stirred suspension of KO^tBu (0.68 g, 6.06 mmol) in dry THF solution (30 mL) which was cooled in an ice bath. After 5 min ethyl trifluoroacetate (0.65 mL, 5.51 mmol) was added slowly. The solution was allowed to stir at 0 °C for 15 min before being removed from the ice bath. The mixture was heated to reflux overnight before being cooled, and the solvent was removed under reduced pressure. Water (30 mL) was added to the resulting solid to form a suspension which was neutralised with conc. HCl. The solution was then extracted with DCM. The organic phase was dried over MgSO₄ and the solvent removed to give the presumed 1,3-dione intermediate 8 (not characterised). This was dissolved in ethanol (30 mL) and hydrazine hydrate (0.34 mL, 95%) was added. The solution was heated to reflux overnight before the solvent was removed in vacuo. The residue was dissolved in DCM and washed with water to remove unreacted hydrazine hydrate. The solvent was removed, and the oily residue was redissolved in ethanol and conc. HCl (0.5 mL) was added (to promote complete dehydration). The solution was heated to reflux for 5 h, before being cooled and the solvent removed. The crude solid was dissolved in DCM and washed with water, before being dried over MgSO₄ and concentrated in vacuo. The crude solid was purified by column chromatography (1 : 3 EtOAc : DCM v/v) followed by recrystallisation from hexane to give an off-white solid, 4-mesityl-2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridine 9 (277 mg, 20%); m.pt. 171.1–172.9 °C; δ_H (400 MHz; CDCl₃) 11.95 (1H, br s), 8.70 (1H, dd, J 5.0, 0.9), 7.47 (1H, dd, J 1.5, 0.9), 7.15 (1H, dd, J 5.0, 1.5), 6.96–7.00 (2H, m), 6.91 (1H, s), 2.35 (3H, s), 2.02 (6H, s); δ_F (376 MHz; CDCl₃) −62.34 (3F, s); δ_C (101 MHz; CDCl₃) 151.57, 149.86, 146.91, 144.42 (q, J 38.4), 143.10, 138.08, 135.43, 134.97, 128.52, 125.07, 121.40, 121.14 (q, J 267.9), 101.34, 21.06, 20.57; HRMS (FTMS + ESI): calcd for [C₁₈H₁₆N₃F₃ + H]⁺: 332.1375. Found: 332.1379.

Iridium complex 12. IrCl₃·3H₂O (66 mg, 0.19 mmol) was added to a stirred solution of 2-(2-fluoro-4-methoxy-3-tosylphenyl)-4-mesitylpyridine 10 ²⁹ (169 mg, 0.38 mmol) in 2-ethoxyethanol (5 mL). The solution was heated to 130 °C under an argon atmosphere overnight, during which time a bright yellow precipitate formed, presumed to be the dichloro bridged dimer. The solvent was removed in vacuo and the precipitate was redissolved in DCM/ethanol (20 mL, 3 : 1 v/v). 4-Mesityl-2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridine 9 (77 mg, 0.28 mmol) was added and the solution was heated at 60 °C overnight under an argon atmosphere. The solution was cooled and the solvent was removed in vacuo. The residue was purified by column chromatography (EtOAc in DCM, 2.5% increased to 4%) to give a yellow solid, iridium complex 12 (190 mg, 70%); Anal. calc. for C₇₄H₆₅F₅IrN₅O₆S₂: C, 60.39; H, 4.45; N, 4.76. Found: C, 60.53; H, 4.71; N, 4.50; δ_H (400 MHz; CDCl₃) 8.12–8.17 (1H, m), 8.03–8.08 (1H, m), 7.89 (2H, d, J 8.2), 7.88 (2H, d, J 8.2), 7.67 (1H, d, J 5.9), 7.64 (1H, d, J 5.9), 7.59 (1H, dd, J 1.7, 0.8), 7.57 (1H, d, J 5.9), 7.27 (4H, t, J 8.7), 7.02–6.89 (8H, s), 6.78 (2H, td, J 6.1, 1.8), 5.79 (1H, s), 5.74 (1H, s), 3.47 (3H, s), 3.44 (3H, s) 2.39 (3H, s), 2.38 (3H, s), 2.34 (3H, s), 2.33 (6H, s), 2.09 (3H, s), 2.01 (3H, s), 1.98 (3H, s), 1.97 (3H, s), 1.86 (6H, s); δ_F (376 MHz; CDCl₃) −59.87 (3F, s), −110.50 (1F, s), −111.36 (1F, s); HRMS (FTMS + ESI): calcd for [C₇₄H₆₅N₅F₅O₆S₂¹⁹¹Ir + H]⁺: 1470.3981. Found: 1470.3995. Single crystals of 12·3CH₂Cl₂ for X-ray study were grown by layering of hexane over a DCM solution of 12; they instantly decomposed when taken out of the solvent. The X-ray experiment was carried out at T = 120 K on a D8 Venture (Bruker AXS) 3-circle diffractometer equipped with a IμS Mo microsource (Mo-Kα radiation, focussing mirrors, [small lambda, Greek, macron] = 0.71073 Å) and a PHOTON 100 CMOS area detector. The structure was solved by direct methods (SHELXS v. 2013/1 program⁶⁴) and refined by full-matrix least squares using SHELXL v. 2014/7 (ref. 65) and OLEX2⁶⁶ software. Crystal data: C₇₄H₆₇F₅IrN₅O₆S₂·3CH₂Cl₂, M = 1726.40, triclinic, space group P1̄ (no. 2), a = 14.209(1), b = 18.215(1), c = 18.780(1) Å, α = 67.984(2), β = 86.202(2), γ = 69.653(2)°, V = 4213.6(5) ų, Z = 2, D_c = 1.361 g cm⁻³, μ = 1.89 mm⁻¹, 57 359 reflections with 2θ ≤ 50°, 14 940 unique, R_int = 0.048, R = 0.069 [11 521 data with I ≥ 2σ(I)], wR(F²) = 0.201 (all data). CCDC 1495066.† The structure contains DCM of crystallisation, of which 1.5 molecules per asymmetric unit were located and the rest (presumably disordered along infinite channels parallel to the x axis) was masked using PLATON SQUEEZE program.^67,68 The large atomic displacement parameters, together with practical absence of Bragg diffraction at 2θ > 50°, indicate severe internal strain, probably caused by partial solvent loss.

Iridium complex 13. [Ir(COD)Cl]₂ (90 mg, 0.13 mmol) was added to a stirred solution of 2-(2,4-dimethoxy-3-(methylsulfonyl)phenyl)-4-mesitylpyridine 11 ²⁹ (226 mg, 0.55 mmol) in 2-ethoxyethanol (5 mL). The solution was heated to 130 °C under an argon atmosphere overnight before being cooled to RT. Water was added and a yellow precipitate formed which was filtered, washed with more water and dried. The precipitate, presumed to be the intermediate dichloro bridged dimer, was dissolved in DCM/ethanol (40 mL, 3 : 1 v/v) and 4-mesityl-2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridine 9 (90 mg, 0.27 mmol) was added and the solution was heated at 55 °C overnight under an argon atmosphere. The solution was cooled and the solvent was removed in vacuo. The residue was purified by column chromatography (EtOAc in DCM: gradient elution from 0% to 15%) to give a yellow solid, iridium complex 13 (98 mg, 27%); Anal. calc. for C₆₄H₆₃F₃IrN₅O₈S₂: C, 57.21; H, 4.73; N, 5.21. Found: C, 57.17; H, 5.07; N, 4.68; δ_H (400 MHz; CD₂Cl₂) 8.47 (1H, d, J 1.4), 8.38 (1H, d, J 1.4), 7.80–7.74 (3H, m), 7.65 (1H, dd, J 1.8, 0.8), 6.98 (5H, s), 6.95 (2H, s), 6.91 (1H, dd, J 5.7, 1.8), 6.84 (1H, dd, J 5.9, 1.9), 6.81 (1H, dd, J 5.9, 1.9), 6.03 (1H, s), 6.01 (1H, s), 3.88 (3H, s), 3.85 (3H, s), 3.61 (3H, s), 3.60 (3H, s), 3.22 (3H, s), 3.19 (3H, s), 2.33 (3H, s), 2.32 (3H, s), 2.31 (3H, s), 2.09 (3H, s), 2.06 (3H, s), 2.05 (3H, s), 2.01 (3H, s), 1.88 (3H, s), 1.85 (3H, s); δ_F (376 MHz; CDCl₃) −58.48 (s); HRMS (FTMS + ESI): calcd for [C₆₄H₆₃F₃N₅O₈S₂¹⁹¹Ir + H]⁺: 1342.3754. Found: 1342.3802.

2-(3-(Trifluoromethyl)-1H-pyrazol-5-yl)pyridine 16. 2-Acetylpyridine (1.00 g, 8.25 mmol) and ethyltrifluoroacetate (2.35 g, 16.51 mmol) were added dropwise simultaneously to a stirred suspension of NaH (396 mg, 16.51 mmol) in THF (20 mL, dry) with caution (gas evolution). The mixture was stirred at RT for 30 min before being heated to reflux overnight. The reaction was cooled and quenched with water. The reaction mixture was acidified to pH 6 with dilute HCl (2 M) and then extracted with EtOAc (3 × 60 mL). The organic phases were combined, dried over MgSO₄, filtered and the solvent was removed in vacuo to give the crude 1,3-dione. The dione was dissolved in ethanol (20 mL) and N₂H₄·H₂O (4 mL) was added. The solution was heated to reflux overnight before being cooled to RT. The solvent was removed in vacuo and the residue was purified by column chromatography (hexane : EtOAc 3 : 2 v/v) to give an off-white solid, 2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridine 16 (1.31 g, 75%); δ_H (400 MHz; CDCl₃) 11.74 (1H, s), 8.65 (1H, ddd, J 4.9, 1.8, 1.0), 7.82 (1H, td, J 7.8, 1.7), 7.65 (1H, dt, J 7.9, 1.1), 7.33 (1H, ddd, J 7.6, 4.9, 1.1), 6.96 (1H, s); δ_F (376 MHz; CDCl₃) −62.35 (s).

Iridium complex 14. 2-(2-Fluoro-4-methoxy-3-tosylphenyl)-4-mesitylpyridine 10 ²⁹ (110 mg, 0.23 mmol) and IrCl₃·3H₂O (37 mg, 0.10 mmol) were combined in 2-ethoxyethanol (5 mL), and the mixture was heated to reflux overnight under a nitrogen atomosphere. The reaction was the cooled to RT, and the solvent removed in vacuo. The residue was redissolved in DCM/EtOH (3 : 1 v/v, 20 mL) and 2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridine 16 (28 mg, 0.13 mmol) and Na₂CO₃ (11 mg, 0.10 mmol) were added. The solution was heated to 55 °C for 6 h, before being cooled to RT and the solvent removed in vacuo. The residue was purified by column chromatography (0–3% EtOAc in DCM) to give the product as a yellow solid, iridium complex 14 (109 mg, 77%); Anal. calc. for C₆₅H₅₅F₅IrN₅O₆S₂: C, 57.68; H, 4.10; N, 5.17. Found: C, 57.84; H, 4.33; N, 4.92; δ_H (400 MHz, CDCl₃) 8.11 (1H, s), 8.05 (1H, s), 7.95–7.78 (6H, m), 7.70–7.64 (1H, m), 7.56 (1H, d, J 6.0), 7.50–7.43 (2H, m), 7.29 (1H, d, J 0.8), 7.26–7.22 (2H, m), 7.18–7.11 (1H, m), 7.01 (1H, s), 6.93 (4H, d, J 6.4), 6.77 (1H, dd, J 5.9, 1.8), 6.71 (1H, dd, J 6.0, 1.8), 5.72 (1H, s), 5.70 (1H, s), 3.44 (3H, s), 3.41 (3H, s), 2.39 (3H, s), 2.38 (3H, s), 2.33 (6H, s), 1.99 (3H, s), 1.97 (6H, s), 1.93 (3H, s); δ_F (376 MHz; CDCl₃) −59.85 (3F, s), −110.73 (1F, s); −111.48 (1F, s); HRMS (FTMS + ESI): calcd for [C₆₅H₅₅N₅O₆F₅S₂¹⁹¹Ir + H]⁺: 1352.3198. Found: 1352.3193.

Iridium complex 15. 2-(2,4-Dimethoxy-3-(methylsulfonyl)phenyl)-4-mesitylpyridine 11 ²⁹ (198 mg, 0.48 mmol) and [Ir(COD)Cl]₂ (79 mg, 0.12 mmol) were combined in 2-ethoxyethanol (5 mL), and the mixture was heated to reflux overnight under a nitrogen atmosphere. The reaction was the cooled to RT, and the solvent removed in vacuo. The residue was redissolved in DCM/EtOH (3 : 1 v/v, 20 mL) and 2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridine 16 (63 mg, 0.30 mmol) and Na₂CO₃ (50 mg, 0.47 mmol) were added. The solution was heated to 55 °C for 5 h, before being cooled to RT and the solvent removed in vacuo. The residue was purified by column chromatography (0–10% EtOAc in DCM) to give the product as a yellow solid, iridium complex 15 (28 mg, 56%); Anal. calc. for C₅₅H₅₃F₃IrN₅O₈S₂: C, 53.91; H, 4.36; N, 5.72. Found: C, 53.48; H, 4.43; N, 5.55; δ_H (400 MHz, CDCl₃) 8.46 (1H, d, J 1.8), 8.40 (1H, d, J 1.8), 7.88 (2H, m), 7.82 (1H, d, J 5.9), 7.75 (1H, d, J 5.6), 7.52 (1H, d, J 5.9), 7.16 (1H, td, J 6.0, 2.6), 7.04 (1H, s), 6.98 (2H, s), 6.96 (2H, s), 6.85 (1H, dd, J 5.9, 1.9), 6.72 (1H, dd, J 5.9, 1.9), 5.96 (1H, s), 5.89 (1H, s), 3.91 (3H, s), 3.91 (3H, s), 3.60 (3H, s), 3.58 (3H, s), 3.30 (3H, s), 3.27 (3H, s), 2.35 (6H, s), 2.03 (6H, s), 2.01 (3H, s), 1.97 (3H, s); δ_F (376 MHz; CDCl₃) −60.00 (s); HRMS (FTMS + ESI): calcd for [C₅₅H₅₃N₅O₈F₃S₂¹⁹¹Ir + H]⁺: 1224.2972. Found: 1224.2987.

Acknowledgements

We thank EPSRC grant EL/L02621X/1 for funding the work in Durham.

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Footnote

† Electronic supplementary information (ESI) available: Thermogravimetric data; NMR spectra; additional computational and CV data. CCDC 1495066. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt02289a

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