Water-soluble Ir(III) complexes of deprotonated N-methylbipyridinium ligands: fluorine-free blue emitters

Abstract

New blue or blue-green emitting iridium complexes have been synthesised with cyclometalating ligands derived from the 1-methyl-3-(2′-pyridyl)pyridinium cation. Efficient luminescence is observed in MeCN or aqueous solutions, with a large range of lifetimes in the μs region and relatively high quantum yields.

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The creation of bright and stable blue emitting compounds is a major challenge in the development of light-emitting electrochemical cells and organic light-emitting diodes (OLEDs).^1,2 Iridium complexes have been investigated extensively due to their widely tunable and efficient luminescence.^2–12 A common structural type is [Ir^III(C^N)₂(N^N)]⁺, where C^N is a cyclometalating ligand like deprotonated 2-phenylpyridine, and N^N is an α-diimine. A popular strategy to blue-shift the emission is to derivatise C^N with electron-withdrawing groups (often –F or –CF₃) and/or place electron donors on N^N. Avoiding the use of fluorine is desirable to maximise the stability of the complexes in devices, and from an environmental perspective. Hence, we present an alternative approach, creating new fluorine-free Ir^III luminophores by using 1-methyl-3-(2′-pyridyl)pyridinium to generate C^N. The quaternised nitrogen opposite the cyclometalating carbon is key to blue-shifting the emission.

Ir^III complexes of C^N ligands derived from pyridinium species are extremely scarce.¹³ Notably, these known complexes are not suitable for luminescence, but were prepared in the context of catalytic studies. Complexes with quaternary N units as part of N^N have been reported, but these groups are generally not strongly coupled electronically to the Ir^III centre.^14–17 Remote ammonium groups have been attached to C^N¹⁸ or acetylacetonate¹⁹ ancillary ligands. Using Ir^III complexes in bio-sensing/imaging²⁰ is often restricted by poor water solubility,²¹ so increased positive charge is beneficial. Given this context and our general interest in photoactive complexes with quaternised pyridinium moieties,²² we targeted unusual Ir^III species combining attractive emission and solubility properties.

The new complexes 1–3 were synthesised by a standard approach, i.e. cleaving a cyclometalated chloride-bridged dimer with a N^N ligand (Scheme 1). The PF₆⁻ and Cl⁻ salts were characterised by ¹H NMR spectroscopy, electrospray mass spectrometry and elemental analyses (see ESI†). In addition, single-crystal X-ray structures have been solved for 1P·2MeCN and 3P·3Me₂CO (Tables S1 and S3, Fig. S1 and S2, ESI†). As expected, both complexes exhibit pseudooctahedral coordination at Ir, with the pyridyl rings of the C^N ligands in a trans geometry (Fig. 1). Their chemical structures bear some resemblance to the widely studied complexes of N-heterocyclic carbenes derived from imidazolium species, although such complexes are typically neutral or only +1 charged.^23–26

Scheme 1 Synthesis of the complex salts 1P–3P; their chloride counterparts 1C–3C were prepared by treating purified 1P–3P with [ⁿBu₄N]Cl in acetone.

Fig. 1 Representations of the molecular structures of the complex cations in 1P·2MeCN and 3P·3Me₂CO, with the PF₆⁻ anions, solvent molecules and H atoms removed for clarity (50% probability ellipsoids).

UV–vis absorption spectroscopic data are shown in Table 1. These spectra are almost unaffected by changing the counter-anions and solvent. They are dominated by intense bands at λ < 320 nm (Fig. S3 and S4, ESI†), assigned to π → π* and high energy metal-to-ligand charge-transfer (MLCT) transitions, involving both the C^N and N^N ligands. Weaker bands (λ_max ≈ 350–356 nm) are observed also. Cyclic voltammograms of 1P–3P in MeCN (Fig. S5, Table S2, ESI†) show an irreversible oxidation, formally assigned to a Ir^IV/III couple. The reductive region includes multiple irreversible processes, and a sharp return peak is observed for 1P and 3P, indicating adsorption onto the electrode surface.

Table 1 Absorption and emission data at 298 K in solutions ca. 1.0 × 10⁻⁵–2.0 × 10⁻⁴ M. Luminescence data measured in the presence (ox) or absence (deox) of oxygen

Complex salt	Absorption, λmax/nm (ε/10^3 M^−1 cm^−1)	Emission, λ/nm	τ ^c/μs		ϕ (%)
			deox	ox	deox	ox
a In MeCN. b In water. c Estimated experimental errors ±10%.
1P	237 (54.3), 255sh (48.5), 302 (25.9), 313 (24.5), 352sh (5.3)	444, 474max, 504, 548	3.5	1.2	24	4.7
1C	237 (48.3), 255sh (42.4), 302 (22.8), 312 (21.8), 353sh (4.8)	442, 470max, 504, 547	12.1	3.9	27	9.7
2P	236 (47.4), 257 (43.2), 308 (21.6), 316 (20.9), 350sh (5.4)	466, 494max, 525, 574	3.8	1.5	43	16
2C	237 (52.7), 259 (52.1), 308 (25.2), 317 (24.3), 350sh (5.8)	462, 494max, 529, 575	4.3	2.6	42	24
3P	236 (51.8), 259sh (44.9), 299 (23.9), 311 (23.2), 356sh (4.5)	440, 470max, 502, 546	3.8	1.2	43	11
3C	237 (58.0), 260 (49.9), 300 (25.4), 312 (25.9), 355sh (5.5)	440, 468max, 500, 547	9.5	2.9	45	14

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Excitation at 315–400 nm in deoxygenated and oxygenated MeCN or aqueous solutions leads to bright blue (1 and 3) or blue-green (2) luminescence (Table 1). The spectra show significant fine structure, indicating primarily ligand-centred emission. As for the absorption spectra, changing the counter-anion and solvent has only slight effects, and the excitation profiles remain constant in all cases while monitoring at all the emission maxima. The spectra are very similar for R = H or ^tBu (λ_em = 468–474 nm), but shifted significantly to lower energy when R = CF₃ (λ_em = 494). The fact that replacing H with ^tBu has little effect while –CF₃ groups give a red-shift suggests that the character of the emitting state varies. The almost identical spectra of 1 and 3 (Fig. 2) indicate mainly ³LC emission involving C^N with little ³MLCT contribution. However, the red-shift for 2 suggests that the emission is associated with N^N. The quantum yields ϕ are not affected significantly by the counter-anions under deoxygenated conditions, but are substantially enhanced when R = CF₃ or ^tBu (ϕ ≈ 42–45%) as opposed to H (ϕ ≈ 24–27%). In oxygenated conditions, 2 shows the largest ϕ values. All complexes have emission lifetimes τ in the μs region, covering a large range of values (ca. 1–12 μs), with monoexponential decay kinetics. 1C and 3C show relatively long τ values in water, considerably longer than for 2C (Fig. 3).

Fig. 2 Emission spectra of 1P (blue), 2P (green) and 3P (red) with excitation at 350 nm in MeCN.

Fig. 3 Emission decay traces in water of 1C (blue), 2C (green) and 3C (red) following 375 nm excitation with a ps pulsed diode laser.

The observed blue emissions from the fluorine-free complexes 1 and 3 are remarkable since, as mentioned above, decorating the C^N ligands with F or fluorinated groups is a common strategy to blue-shift the emission of this type of complex. The influence of the pyridinium fragment located para to the cyclometalating carbon is clearly shown by comparing the emission properties of 3P (λ_max = 470 nm, ϕ_deox = 43%, τ_deox = 3.8 ms) with other reported complexes [Ir^III(C^N)₂{4,4′-(^tBu)₂bpy}]⁺. When using the heavily fluorinated cyclometalating ligand derived from 2-(2,4-difluorophenyl)-5-trifluoromethylpyridine, the emission in MeCN (λ_max = 470 nm, ϕ_deox = 68%, τ_deox = 2.3 ms) is similar to that of 3P.²⁷ On the other hand, when C^N is deprotonated 2-phenylpyridine, the emission in MeCN is red-shifted strongly (λ_max = 581 nm) with a lower ϕ_deox (24%) and shorter τ_deox (0.56 ms).²⁸

The singlet ground (S₀) and lowest triplet excited (T₁) states of 1–3 were optimised by using density functional theory (DFT) (Fig. S6–S9, Tables S3–S8, ESI†). The calculated ground-state structures for 1 and 3 reproduce well the X-ray crystallographic ones. The LUMO is located on the C^N (69–90%) and N^N (6–27%) ligands. The HOMO is located at the Ir atom (50–55%) and the C^N ligands (40–46%), and is essentially invariant. Such relatively high C^N contributions are consistent with the irreversible oxidations observed by cyclic voltammetry (see above). The spectra simulated by time-dependent DFT agree relatively well with those measured. The weak low energy bands are due to a single transition for 1 and 3, a mixture of HOMO → LUMO and HOMO → LUMO+1 (347 nm, 1) or HOMO → LUMO (348 nm, 3). For 2, transitions occur at 342 nm (HOMO → LUMO+1) and 336 nm (HOMO−2 → LUMO), with the latter being ca. 5-fold more intense. These results indicate ¹MLCT character with ¹ML′CT and also ¹LL′CT for 1 and 2 (L = C^N; L′ = N^N).

The T₁ geometries resemble the S₀ ones, except that 1 and 3 now have unequal Ir–C and (chemically equivalent) Ir–N distances, while in 2, these pairs of distances are equal. The calculated emission energies (Table 2) follow the experimental trend (1 ≈ 3 > 2). The spin densities for the T₁ state (Fig. 4) show mainly ³LC involving one C^N ligand with some ³MLCT contribution for 1 and 3. In contrast, for 2, the spin density is located on N^N largely, indicating that the emission has ³L′C character with some ³ML′CT. Therefore, on excitation from the C^N/Ir-centred HOMO−2 to the C^N-centred LUMO (336 nm transition) in 2, there is efficient inter-ligand energy transfer to the emitting ³L′C excited state of N^N. Such energy transfer is expected if the ³L′C state lies below the ³MLCT.²⁹ In 2, the presence of the electron-withdrawing –CF₃ groups stabilises the π* orbitals of the N^N ligand, lowering the energy of the ³L′C state.

Fig. 4 M06/Def2-QZVP/SVP-calculated spin density plots for the T₁ state of complexes 1–3.

Table 2 Predicted and measured emission data in MeCN

Complex	E 0,0/eV (λ/nm)	E AE/eV (λ/nm)	λ max/nm (exp)
E 0,0 calculated by using the DFT-optimised geometries for T1 and S0. EAE calculated by using the DFT-optimised T1 geometry for both states (adiabatic electronic emission).
1	2.91 (426)	2.46 (504)	474
2	2.76 (450)	2.28 (544)	494
3	2.91 (426)	2.47 (503)	470

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To conclude, using 1-methyl-3-(2′-pyridyl)pyridinium to generate C^N affords new water-soluble Ir^III complexes. Their excited-state and emissive behaviour can be switched between two types by modifying N^N. The bright blue emission in MeCN and water of the fluorine-free complexes 1 and 3 suggests potential uses in highly efficient OLEDs or bioimaging. The tunability of the emission properties is shown, not only by the emission maxima, but also in the range of quantum yields (ca. 5–45%) and lifetimes, from quite short (ca. 1 μs) to relatively long (ca. 12 μs). Much further scope exists for modifying properties, for example by using groups other than methyl on the quaternised N atom.

We thank the EPSRC for support (grant EP/J018635/1), and the BBSRC for a PhD studentship (M.K.P.). N.S.S. is supported by an EPSRC Established Career Fellowship (grant EP/J020192/1) and was funded by a Royal Society Wolfson Merit Award. We thank Dr Louise S. Natrajan and Michael B. Andrews for assistance with the luminescence measurements.

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Footnote

† Electronic supplementary information (ESI) available: Details of synthetic, characterisation and theoretical studies. CCDC 1407686 and 1407687. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt02591e

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