LA EMPRESA

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Introduction

As outlined in the previous chapters, Iridium(III) complexes are attractive phosphors because of their excellent photophysical properties and the facile and wide emission colour tunability as a function of ligand identity.5,207 _{In electroluminescent devices such as organic}

light emitting diodes (OLEDs)170,262 and light-emitting electrochemical cells (LEECs),21,23,172 blue emissive materials are critical components for full-colour displays and for the generation of white light in the context of solid-state lighting.238 _{Charged complexes are particularly}

germane for LEECs. Typically, heteroleptic cationic Ir(III) complexes of the form [Ir(C^N)2(N^N)]+ consist of two cyclometalating C^N ligands typically based on a 2-

phenylpyridinato (ppy) scaffold and one five-membered chelating diimine N^N ancillary ligand such as 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen) or their derivatives.238 The emission energy is normally tuned through substituent decoration about these ligand moieties, with electron-withdrawing groups attached to the C^N ligands and electron-donating groups incorporated onto the N^N ligand used in concert to increase the HOMO-LUMO gap, and by extension the energy of the emissive triplet state. As discussed in Chapters 1, 4 and 5, much less attention has been devoted to the effect of changing the chelate ring size on the emission energy, particularly in the context of the incorporation of a nonconjugating methylene space group between the coordinating rings.

The use of six-membered chelate ancillary ligands on cationic iridium(III) complexes is more common, though there are only a handful of reports here as well (Figure 87). Examples include the use of a di(pyridin-2-yl)methane (dpm)263 that incorporates a methylene spacer to interrupt the -conjugation of the ligand such as [Ir(ppy)2(dpm)]PF6, 49; no photophysics was

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Figure 87: Structural representation of iridium(III) complexes bearing conjugated and nonconjugated ancillary ligands reported in the literature.

Other studies have focused on the functionalisation of this methylene bridge. For instance, the complexes [Ir(ppy)2(dpyOH-R)]Cl [with R = H, 50 and CH2CN, 51 and dpyOH-

R is di(pyridin-2-yl)methanol and 3-hydroxy-3,3-di(pyridine-2-yl)propanenitrile, respectively] have been investigated.264 The effect of successfully interrupting the direct electronic crosstalk between the coordination moieties can be demonstrated by comparing the photophysical properties of 50 (with PL = 477, 507 and 547 nm, PL = 10% in MeCN) and the reference

complex 12, [Ir(ppy)2(bpy)]PF6, (PL = 602 nm, PL = 9% in MeCN),187 where a large blue-

shift of 125 nm (4353 cm-1) is observed. The complex 51264 (with PL = 535 nm, PL = 49% in

MeCN) is a bright emitter exhibiting predominantly MLCT emission. The two complexes [Ir(ppy)2(dpy-R)]Cl [with R = O, 52 and N-NH2, 53 and dpy-R is di-2-pyridylketone and 2,2’-

(hydrazonomethylene)dipyridine, respectively] are poorly emissive in acetonitrile, with PL <

0.5%. The former exhibits an unstructured emission centred at 678 nm, whereas the latter displays a blue-shifted, structured emission profile (PL = 480, 510 nm).264

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A more widely studied six-membered chelate N^N ligand is di(pyridin-2-yl)amine (dpa).263,265–268 With [Ir(ppy)2(dpa)]PF6, 54 (PL = 483, 514 (sh) nm, PL = 43% in CH2Cl2,

Figure 87) a significant blue-shift and increase in PL can be observed with respect to 12, which

in this case is due to the presence of the electron-releasing central amine.265

Through sulfur-bridged six-membered chelate N^N ligands (di(pyridine-2-yl)sulfane and its oxidised derivatives), the emission energy could be tuned as a function of the oxidation state of the central sulfur atom.269 Ligand-centred (3LC) emission was observed when the sulfur was in the +2 (55, with PL = 478, 510, 548 (sh) nm, PL = 4% in CH2Cl2), and +4 oxidation

states (56, with PL = 478, 510, 548 (sh) nm, PL = 1% in CH2Cl2). Through oxidation of the

sulfur atom to the +6 oxidation state (57, with PL = 552 nm, PL = 3% in CH2Cl2) a red-shift

and emission with 3MLCT character was observed.

Examples of nonconjugated six-membered chelate ancillary rings employing coordinating heterocycles other than pyridine include those using bis(tetrazolate)270,271 (e.g., NBu4[Ir(ppy)2(b-trz)], 58 where b-trzH2 isdi(1H-tetrazol-5-yl)methane, with PL = 498, 520

(sh) nm, PL = 75% in MeCN), and bis(pyrazole)272,273 (e.g., [Ir(ppy)2(bpm)]PF6, 59 where bpm

is bis(pyrazol-1-yl)methane, with PL = 477 nm, PL = 21% in MeCN, Figure 87) or bis-

NHC274–279 (e.g., [Ir(ppy)2(dmdiim)]PF6, 60 where (dmdiimH2)I2 is 1,1’-dimethyl-3,3’-

methylenediimidazolium diiodide, with PL = 475, 503 nm, PL = 38% in MeCN) complexes.

Recently, Chi and co-worker reported a nonplanar tetradentate N^N^N^N chelate bearing a pyrazole unit and a nonconjugated tripodal arranged terpyridine, which can coordinate to iridium forming a six-membered ring.280 They obtained sky blue efficient OLEDs using this complex as the dopant emitter. In each of these literature examples the spacer disrupts the conjugation across the coordinating moieties, enabling a blue-shifted emission. The strongly donating character of the coordinating heterocycle contributed to the blue-to-sky-blue emission of these complexes.

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The work presented in this chapter deals about the development of charged blue- emitting iridium(III) complexes for solution-processed OLEDs and LEECs. Here we investigate the coordination of the nonconjugated diimine 2,2'-(phenylmethylene)dipyridine (pmdp, Figure 88) to iridium as the N^N ligand, in combination with either 2-(2,4- difluorophenyl)-4-mesitylpyridinato (dFmesppy) or 4-mesityl-2-phenylpyridine (mesppy) as C^N ligands to form complexes [Ir(dFmesppy)2(pmdp)]PF6, 61and [Ir(mesppy)2(pmdp)]PF6, 62,respectively (Figure 88).

Figure 88: Structural representations of 2,2'-(phenylmethylene)dipyridine (pmdp), and complexes 61 and 62 and their reference complexes 63 and 21 respectively.

The mesityl group was incorporated onto the C^N ligands to increase the solubility of the resultant complexes in organic solvents (e.g. MeCN and CH2Cl2), without affecting their

emission energy due to the mutually orthogonal conformation between the mesityl substituent and the pyridine of the C^N ligands, thereby disrupting any formal conjugation.174 _{The impact}

of the use of the pmdp ligand is studied through comparison with two reference complexes 63

and 21 bearing the same C^N ligands and a conjugated N^N ancillary ligand [4,4'-di-tert-butyl- 2,2'-bipyridine (dtBubpy)]. The photophysical properties of these complexes are corroborated by density functional theory (DFT) and time-dependent DFT (TD-DFT) investigations.

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Results and Discussion

Synthesis

The ancillary ligand (pmdp) was obtained in 40% yield as a beige solid following a modified procedure281 wherein 2-benzylpyridine was treated with n-BuLi at -78 °C and subsequently reacted with 2-fluoropyridine under SNAr conditions (Figure 89).

Figure 89: Synthesis route for 2,2'-(phenylmethylene)dipyridine; (a) i) THF, n-BuLi, N2, -78

°C, 1 h; ii) 2-fluoropyridine, THF, -78 °C to r.t., 18 h. (3) reflux, 3 h.

Complexes 61 and 62 were obtained as their hexafluorophosphate salts in a two-step synthesis following our previously reported protocol (Figure 90).174 In the first step, the bis(µ- Cl) dimer was obtained in high yields (91% and 94% for R = H and R = F, respectively) as a yellow solid by treatment of the corresponding C^N ligand with IrCl3.6H2O in a 3:1 mixture of

2-ethoxyethanol/H2O (125 °C, 24 h). This dimer was then cleaved with 2,2'-

(phenylmethylene)dipyridine in a 1:1 mixture of CH2Cl2/MeOH (40 °C, 18 h) to afford the

cationic Ir(III) complexes as their chloride salts. After column chromatography on silica (eluent: 8% MeOH in CH2Cl2) followed by an ion exchange with aqueous NH4PF6 and

recrystallisation, complexes 61 and 62 were isolated as yellow solids in excellent yield (81% and 89%, respectively) as their hexafluorophosphate salts.

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Figure 90: Procedure for the synthesis of target complexes (61 and 62) in this study. a) IrCl3.6H2O, corresponding C^N ligand, 2-ethoxyethanol/water (3:1), 125 °C, 24 h. b) 1. 2,2'-

(phenylmethylene)dipyridine, MeOH/CH2Cl2 (1:1), 40 °C, 18h. 2. aq. NH4PF6.

Complexes 61 and 62 were characterised by 1H, 13C and 31P NMR spectroscopy and, for 61, 19_{F NMR spectroscopy in CDCl}

3; ESI-HR mass spectrometry, elemental analysis, and

melting point determination. Due to the phenyl group on the N^N ligand, the two C^N ligands are no longer symmetrical as in the above examples of Chapter 2 – 4. This translates into the double amount of resonances for the C^N ligand. A characteristic signal in the 1H NMR spectra is the singlet at around 5.90 – 5.30 ppm arising from the C-H on the bridge of the N^N ligand. As expected the 13_{C NMR also shows a higher number of resonances compared to the examples}

from Chapter 2 – 4. For 61, a splitting of resonances for the C-F carbons is detected leading to not well resolved doublets. With respect to the complexes in Chapter 2 these complexes behave in a comparable way in HR-ESI mass spectrometry and elemental analysis. The structure of complex 61 was determined by single crystal X-ray diffraction.

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