Sensitised near-infrared emission from lanthanides using an iridium complex as a ligand in heteronuclear Ir₂Ln arrays

Abstract

The iridium(III) complex Ir(ppy)₂(phen5f) [ppy = 2-phenylpyridinato-N,C^2′, and phen5f = 4,4,5,5,5-pentafluoro-1-(1′,10′-phenanthrolin-2′-yl)-pentane-1,3-dionate] has been synthesised and used as “complex ligands” to make heteronuclear d–f complexes by the attachment of Ln(NO₃)₃·xH₂O at the vacant coordination sites in the bridging ligand phen5f. The microanalyses and crystal structure characteristics confirmed the formation of the heteronuclear Ir₂Ln arrays. The measurement of the lowest triplet state energy level of Ir(ppy)₂(phen5f) indicates that it is suitable for the NIR (near-infrared) lanthanide ions, Nd^III, Yb^IIIand Er^III. Upon irradiation of the MLCT (metal-to-ligand charge transfer) absorption of Ir(ppy)₂(phen5f) at an excitation wavelength from 380–490 nm, the characteristic emission spectra of the three Ir₂Ln arrays (Ln = Nd, Yb, Er) in both the solid state and in CH₃CN solution were measured. According to the results, more Ir^III complexes will be designed for lanthanide NIR emission by the proper combination between the cyclometalated ligand and the tetradentate ancillary ligand.

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Introduction

Lanthanide ions, such as Nd^III, Yb^III and Er^III, are well-known near-infrared (NIR) emitters with excellent luminescence properties due to the characteristics of their f–f transitions, including lifetimes in the microsecond time domain and narrow bandwidth emissions in the 900–1600 nm region. They have become a hot topic of research because of their potential applications in many areas such as optical communication,¹ medical imaging and biological labelling.² To obtain a highly efficient NIR luminescence, many organic chromophores, as high absorbing sensitisers, were introduced to Ln^III complexes due to the low extinction coefficients of their f–f absorptions.³ On the other hand, the use of strongly absorbing d-block chromophores as sensitisers for the NIR emissions from lanthanides have attracted more and more attention since van Veggel and co-workers reported the first example of Nd^III and Yb^III luminescence sensitised by the transition metal complexes Ru(bpy)₃²⁺ and ferrocene in 2000.⁴ Subsequently, many d-block chromophores, based on related transition metals, such as Pt^II,^5,6Ru^II,⁷Re^I,⁸Os^II,⁹Pd^II,¹⁰Zn^II,¹¹Cr^III12 and Co^III13 sensitising NIR lanthanides (Nd^III, Yb^III and Er^III) were extensively studied. Because most of the energy levels of these d-block chromophores are around 13 000–18 000 cm⁻¹, energy transfers from the transition metals to the lanthanide ion centers are most efficient and the characteristic emission from these NIR lanthanides were successfully obtained.

However, the Ir^III ion, as a very important member of the transition metals, has not been used much for NIR lanthanide luminescence, though the energy transfer from the Ir^III moiety to the Eu^III complex has been studied,^14,15 and the only example, regarding the NIR emission of an Ir^III–Yb^III complex has recently been reported by De Cola and co-workers.¹⁶ In fact, as important phosphorescent materials, many Ir^III complexes with different energies of the lowest excited states have been extensively investigated by modifying cyclometalated ligands,^17,18 which give us a good strategy to seek more suitable Ir^III complexes to sensitise NIR lanthanide ions.

Recently, we reported an Ir^III complex Ir(dfppy)₂(phen5f) as a complex ligand, which can efficiently sensitise Eu^III.¹⁵ In this work, we have changed the cyclometalated ligand from dfppy [2-(4′,6′-difluorophenyl)-pyridinato-N,C^2′] to ppy (2-phenylpyridinato-N,C^2′) in order to decrease the triplet energy level of the whole d-block complex ligand and make it more suitable for the NIR Ln^III (Ln = Nd, Yb, Er) ions.¹⁹ As shown in Scheme 1, the d-block “complex ligand” used is Ir(ppy)₂(phen5f), in which phen5f = 4,4,5,5,5-pentafluoro-1-(1′,10′-phenanthrolin-2′-yl)-pentane-1,3-dionate.¹⁵ The hetero-trinuclear Ir₂Ln arrays (Ln = Nd, Yb, Er, Gd, Eu) have been obtained by the reaction of Ir(ppy)₂(phen5f) and Ln(NO₃)₃·xH₂O. We will describe the structural and photophysical properties of Ir(ppy)₂(phen5f) and the Ir₂Ln arrays in this paper.

Scheme 1 Synthetic route of the Ir₂Ln arrays.

Results and discussion

Syntheses and structures

The complex Ir(ppy)₂(phen5f) was prepared from a reaction of the chloride-bridged Ir^III dimer (ppy)₂Ir(μ-Cl)₂Ir(ppy)₂ and the ligand Hphen5f under similar conditions to those for the synthesis of Ir(C⁁N)₂(O⁁O),¹⁸ which was also very similar to the synthesis of Ir(dfppy)₂(phen5f).¹⁵

The crystals of Ir(ppy)₂(phen5f) were obtained from a slowly evaporating solution of acetone (Fig. 1a). The central ion Ir^III is six coordinated by two carbon atoms and four nitrogen atoms, very similar to the complex Ir(dfppy)₂(phen5f).¹⁵ Herein, the C⁁N sites arise from two cyclometalated ppy ligands and the N⁁N sites are contributed by the ancillary ligand phen5f. The Ir–C and Ir–N bonds in Ir(ppy)₂(phen5f) (Table 1) are also similar to those in Ir(dfppy)₂(phen5f). Furthermore, the Ir–N bond lengths between the Ir^III ion and the ancillary ligand phen5f is longer than those between the Ir^III ion and the cyclometalated ligand ppy, which showed that the combination of the Ir^III ion and the C⁁N ligand anion was tighter than that of the Ir^III ion and the N⁁N ancillary ligand.

Table 1 Selected bond lengths and angles for Ir(ppy)₂(phen5f)

Selected bond lengths/Å		Selected angles/°
Ir(1)–C(22)	1.998(11)	C(22)–Ir(1)–N(2)	80.8(4)
Ir(1)–N(2)	2.052(7)
Ir(1)–C(11)	2.028(8)	C(11)–Ir(1)–N(1)	80.6(3)
Ir(1)–N(1)	2.063(7)
Ir(1)–N(3)	2.166(7)	N(3)–Ir(1)–N(4)	76.1(3)
Ir(1)–N(4)	2.239(7)

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Fig. 1 ORTEP diagrams of Ir(ppy)₂(phen5f) (a) and [(ppy)₂Ir(μ-phen5f)]₂Nd(NO₃)₃ (b). The hydrogen atoms and solvent molecules are omitted for clarity.

The bimetallic complexes [(ppy)₂Ir(μ-phen5f)]₂Ln(NO₃)₃ (Ln = Nd, Yb, Er, Gd, Eu) were simply obtained by mixing a 2 : 1 molar ratio of Ir(ppy)₂(phen5f) and Ln(NO₃)₃·xH₂O in a MeOH solution and allowing the MeCN–MeOH solution by slow diffusion of Et₂O. X-Ray quality crystals were obtained for Ln = Nd. The structure and metric parameters of [(ppy)₂Ir(μ-phen5f)]₂Nd(NO₃)₃ are shown in Fig. 1b and Table 2. The Nd^III ion is ten coordinated by ten oxygen atoms, in which four oxygen atoms come from two β-diketate fragments in Ir(ppy)₂(phen5f) while the other six oxygen atoms come from the three nitrates. The Ir^III complex ligands in [(ppy)₂Ir(μ-phen5f)]₂Nd(NO₃)₃ are similar to the individual Ir^III complex according to the comparison between the corresponding bond lengths and angles (Table 1 and Table 2). It is important that the configuration of the two carbonyl groups in the β-diketate has been changed from the trans form to the cis form upon coordination. The non-bonding distances for Nd⋯Ir are 6.170 and 6.216 Å, which may efficiently improve the d→f energy transfer in the hetero-bimetallic complex.²⁰

Table 2 Selected bond lengths and angles for [(ppy)₂Ir(μ-phen5f)]₂Nd(NO₃)₃

Selected bond lengths/Å		Selected angles/°
Nd(1)–O(1)	2.436(7)	O(1)–Nd(1)–O(2)	69.1(3)
Nd(1)–O(2)	2.458(8)
Nd(1)–O(3)	2.447(8)	O(3)–Nd(1)–O(4)	69.4(3)
Nd(1)–O(4)	2.479(7)
Nd(1)–O(6)	2.556(8)	O(6)–Nd(1)–O(5)	49.8(2)
Nd(1)–O(5)	2.576(8)
Nd(1)–O(9)	2.589(9)	O(9)–Nd(1)–O(8)	49.6(3)
Nd(1)–O(8)	2.603(9)
Nd(1)–O(12)	2.522(9)	O(12)–Nd(1)–O(11)	49.9(3)
Nd(1)–O(11)	2.575(9)
Ir(1)–C(35)	1.996(11)	C(35)–Ir(1)–N(4)	80.8(4)
Ir(1)–N(4)	2.054(9)
Ir(1)–C(24)	2.024(12)	C(24)–Ir(1)–N(3)	79.7(4)
Ir(1)–N(3)	2.065(9)
Ir(1)–N(1)	2.168(10)	N(1)–Ir(1)–N(2)	76.6(4)
Ir(1)–N(2)	2.204(11)
Ir(2)–C(63)	2.004(14)	C(63)–Ir(2)–N(7)	80.0(5)
Ir(2)–N(7)	2.058(10)
Ir(2)–C(74)	2.016(11)	C(74)–Ir(2)–N(8)	79.4(4)
Ir(2)–N(8)	2.060(9)
Ir(2)–N(5)	2.127(9)	N(5)–Ir(2)–N(6)	77.8(4)
Ir(2)–N(6)	2.195(10)

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The ESI-MS spectra of the Ir₂Ln complexes also support the formula of a relative heterometallic molecule (Table 3). For the Ir₂Nd complex, the ESI-MS spectrum exhibits a peak at m/z 970.00, corresponding to the major species {[(ppy)₂Ir(μ-phen5f)]₂Nd(NO₃)}²⁺, indicating that this discrete molecule exists in solution. Similarly, the peaks at m/z 985.08, 982.05 and 978.00 in the ESI-MS spectra, also suggest the formation of {[(ppy)₂Ir(μ-phen5f)]₂Ln(NO₃)}²⁺ (Ln = Yb, Er, Gd) in solution, respectively.

Table 3 Molecular peaks of the Ir₂Ln arrays in DMSO

Ir2Ln	Cation	m/z
Ir2Nd	{[(ppy)2Ir(μ-phen5f)]2Nd(NO3)}^2+	970.00
Ir2Yb	{[(ppy)2Ir(μ-phen5f)]2Yb(NO3)}^2+	985.08
Ir2Er	{[(ppy)2Ir(μ-phen5f)]2Er(NO3)}^2+	982.05
Ir2Gd	{[(ppy)2Ir(μ-phen5f)]2Gd(NO3)}^2+	978.00

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Photophysical properties

When compared with the bimetallic complex {[(dfppy)₂Ir(μ-phen5f)]₃EuCl}Cl₂,¹⁵ which exhibited highly efficient red emission, the complex [(ppy)₂Ir(μ-phen5f)]₂Eu(NO₃)₃ showed hardly any observable emission from Eu^III (Fig. 2). It is implied that the ligand change from dfppy to ppy should decrease the lowest triplet energy level of the whole Ir^III complex to a great extent. Therefore, we recorded the low temperature emission of [(ppy)₂Ir(μ-phen5f)]₂Gd(NO₃)₃ in an EtOH rigid-matrix under the same experimental conditions as for the measurement of {[(dfppy)₂Ir(μ-phen5f)]₃GdCl}Cl₂.¹⁵ From the phosphorescence spectra of [(ppy)₂Ir(μ-phen5f)]₂Gd(NO₃)₃ in solution at 77 K (Fig. 2), the triplet energy level of Ir(ppy)₂(phen5f) was obtained as 2.19 eV (1.76 × 10⁴ cm⁻¹),²¹ which was much lower than the triplet energy level of Ir(dfppy)₂(phen5f) (2.64 eV, 2.12 × 10⁴ cm⁻¹). It was obvious that Ir(ppy)₂(phen5f) should be more suitable for sensitising the NIR lanthanide ions Nd^III, Yb^III and Er^III instead of Eu^III.

Fig. 2 Emission spectra of {[(dfppy)₂Ir(μ-phen5f)]₃GdCl}Cl₂ (red dash), and [(ppy)₂Ir(μ-phen5f)]₂Gd(NO₃)₃ (black dash) in an EtOH rigid-matrix (1 × 10⁻⁵ M) at 77 K, and {[(dfppy)₂Ir(μ-phen5f)]₃EuCl}Cl₂ (red solid), and [(ppy)₂Ir(μ-phen5f)]₂Eu(NO₃)₃ (black solid) in CH₃CN (1 × 10⁻⁵ M) at room temperature.

Fig. 3 UV-vis absorption spectra of Ir(ppy)₂(phen5f) (black line) and heteronuclear Ir₂Ln arrays (Ln = Gd (red), Nd (blue), Yb (green) and Er (orange)) in CH₃CN (1 × 10⁻⁵ M).

The UV-vis absorption spectrum of Ir(ppy)₂(phen5f) in CH₃CN (Fig. 3) shows a high energy band (π–π*) from 220–300 nm, which may originate from the cyclometalated ligand ppy and the ancillary ligand phen5f, while the weaker absorption tail from 440–480 nm (centered at ca. 466 nm) should be assigned to the ³MLCT transition in the Ir(ppy)₂⁺ moiety²² and Ir to phen5f (N⁁N) ligand.¹⁵ Moreover, the absorption spectra of the bimetallic complexes Ir₂Ln (Ln = Nd, Yb, Er, Gd) are obviously similar in shape but different in absorbance to that of Ir(ppy)₂(phen5f) at room temperature due to the weak absorption from Ln(NO₃)₃. The extinction coefficients of the bimetallic complexes Ir₂Ln are about two times higher than those of Ir(ppy)₂(phen5f), as expected (Table 4), which also shows the adduct formation of Ir(ppy)₂(phen5f) and Ln(NO₃)₃ in a ratio of 2 : 1.

Table 4 The absorbance peaks and extinction coefficients for the Ir complex and bimetallic complexes

Complex	Absorbance λ/nm (logε)
Ir	229 (4.7), 266 (4.8), 377 (3.8), 415 (3.5), 464 (2.9)
Ir2Nd	228 (5.0), 252 (5.0), 267 (5.0), 380 (4.2), 416 (3.8), 467 (3.2)
Ir2Yb	230 (5.0), 252 (5.0), 268 (5.0), 379 (4.2), 417 (3.8), 465 (3.2)
Ir2Er	228 (5.0), 251 (5.0), 268 (5.0), 380 (4.2), 418 (3.8), 467 (3.2)
Ir2Gd	229 (5.0), 253 (5.0), 268 (5.0), 378 (4.2), 415 (3.8), 465 (3.2)

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All the complexes, Ir(ppy)₂(phen5f) and Ir₂Ln (Ln = Nd, Yb, Er, Gd) in CH₃CN, showed hardly any observable emission in the visible region at room temperature. However, they exhibited quite different solid state luminescence behaviour (Fig. 4). Under excitation at 450 nm, the powdered Ir(ppy)₂(phen5f) luminesced yellow-green with a maximum at ca. 553 nm, which might originate from face-to-face π–π interactions between the aromatic molecules in the solid state and be ascribed to an excimer formation.²² The crystal packing for Ir(ppy)₂(phen5f) is shown in Fig. 5. The crystal packing (a) shows that the π–π spacing between the phen5f ligands of adjacent molecules is ca. 3.55 Å, and the crystal packing (b) shows that the middle parts of the two phenanthroline rings are almost face-to-face with some extent of π–π overlap. Moreover, the angle between the two phenanthroline rings is about 1.6° (Table 5), which confirms that the two planes are near parallel. The solid state luminescence of the Ir₂Gd complex undergoing excitation with an absorption range from 400–490 nm exhibited a broad and structureless emission at 638 nm, which should also arise from the intermolecular interactions including not only the π–π interactions between the phen5f ligands but also the π–π interactions between the ppy ligands (Fig. 6 and Table 5). Because the crystal structure of [(ppy)₂Ir(μ-phen5f)]₂Gd(NO₃)₃ has not been obtained, we will take [(ppy)₂Ir(μ-phen5f)]₂Nd(NO₃)₃ as a reference to discuss the packing of this class of complex in the solid state. The crystal packing (a) shows that the π–π spacing between the phen5f ligands of adjacent molecules is ca. 3.58 Å with an angle between them of 1.4°, and the crystal packing (b) shows that the π–π spacing between the parallel ppy ligands of adjacent molecules is ca. 3.45 Å. The latter caused the red shift in the emission maximum for the bimetallic Ir₂Gd arrays compared with that for the monomeric Ir(ppy)₂(phen5f). Moreover, the emission spectra of Ir(ppy)₂(phen5f) and Ir₂Gd complexes in solution at 77 K are fairly overlapping (Fig. 7), which also supports the hypothesis that their solid state luminescence originates from the π–π interactions and the formation of the excimer.

Table 5 Crystal packing data for Ir(ppy)₂(phen5f) and [(ppy)₂Ir(μ-phen5f)]₂Nd(NO₃)₃

Complex	λ em/nm	π–π spacing	Distance/Å	Angle^a/°
a It presents the angle between two phen5f planes or two ppy planes.
Ir	553	phen5f	3.55	1.6
Ir2Nd	638	phen5f	3.58	1.4
		ppy	3.45	0.0

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Fig. 4 Solid state emission spectra (powdered samples) in the visible region of Ir(ppy)₂(phen5f) (black) under excitation at 450 nm and heteronuclear Ir₂Ln arrays (Ln = Gd (red), Nd (blue), Yb (green) and Er (orange)) at room temperature under excitation at 490 nm.

Fig. 5 Crystal packing for Ir(ppy)₂(phen5f).

Fig. 6 Crystal packing for [(ppy)₂Ir(μ-phen5f)]₂Nd(NO₃)₃.

Fig. 7 Emission spectra of Ir(ppy)₂(phen5f) (black) and [(ppy)₂Ir(μ-phen5f)]₂Gd(NO₃)₃ (red) in an EtOH rigid-matrix (1 × 10⁻⁵ M) at 77 K.

As the Ln^III ion was changed from Gd^III to other Ln^III (Ln = Nd, Yb, Er), the emission intensity largely diminished in the Ir₂Ln arrays under the same experimental conditions (Fig. 4). It is apparent that an efficient energy transfer occurs from the Ir^III moieties to the Ln^III (Ln = Nd, Yb, Er) centers. The weighted-average lifetimes of the Ir^III-based luminescence decay at 638 nm are 129 ns (Ln = Gd) and <2 ns (Ln = Nd, Yb, Er, almost undetectable). Using the emission lifetimes, we can estimate the Ir^III→Ln^III energy transfer rates k_ET from the equation⁶k_ET = 1/τ_q − 1/τ_u, where τ_q is the residual lifetime of the Ir^III-based emission undergoing quenching by a lanthanide ion, and τ_u is the “unquenched” lifetime in the reference complex Ir₂Gd array, so the Ir^III→Ln^III energy transfer rates for Nd, Yb, and Er may all be estimated to be above 4 × 10⁸ s⁻¹, which could well imply that all of the Nd, Yb, and Er ions are good quenchers of the Ir^III centered emission.

Upon irradiation of the MLCT absorption of the Ir complex at λ_ex from 380–490 nm, the emission spectra of the three Ir₂Ln arrays (Ln = Nd, Yb, Er) were measured in both the solid state and the CH₃CN solution (Fig. 8, the emission of the solution is not shown due to the analogue of solid). As expected, three strong emission bands were observed at ca. 872 nm, 1057 nm and 1330 nm, corresponding to the ⁴F_3/2→⁴I_9/2, ⁴I_11/2 and ⁴I_13/2 transitions of Nd^III, respectively. For the Yb^III complex, there is only one emission band around 978 nm and it is affected by crystal-field splitting, which is assigned to the ²F_5/2→²F_7/2 transition. An observed emission band around 1530 nm is attributed to the ⁴I_13/2→⁴I_15/2 transition for the Er^III complex. Moreover, the quantum yield of the Ir₂Yb complex was determined by the equation Φ_Ln = τ_obs/τ₀, in which τ_obs is the observed emission lifetime and τ₀ is the radiative or ‘natural’ lifetime, 2 ms for Yb^III. The luminescence decay of Yb^III was measured at 1012 nm in the solid state and at 980 nm in solution, and the lifetimes were 17.9 μs and 22.1 μs, respectively and the corresponding quantum yields were about 1 × 10⁻².

Fig. 8 Solid state NIR emission spectra (powdered samples) of Ir₂Nd (red), Ir₂Yb (black) and Ir₂Er (blue) arrays.

Conclusions

The proper combination between the cyclometalated ligand (ppy) and the tetradentate ancillary ligand (phen5f) sufficiently decreased the triplet energy level of the whole Ir^III complex as a ligand for lanthanide ions. The heteronuclear [(ppy)₂Ir(μ-phen5f)]₂Ln(NO₃)₃ (Ln = Nd, Yb, Er) arrays exhibited good quenching properties when compared with the analogous [(ppy)₂Ir(μ-phen5f)]₂Gd(NO₃)₃ array. Furthermore, the bridging ligand, phen5f, played an important role in linking the Ir^III center to the Ln^III center, which made the space separation very close and the energy transfer efficient. The near-infrared emission upon photoexcitation of the Ir^III centered antenna chromophore was successfully obtained. According to the results, more Ir^III complexes will be designed for lanthanide NIR emission by proper combination between cyclometalated ligands and tetradentate ancillary ligands.

Experimental

Physical measurements

The ¹H NMR spectra were recorded using an ARX-400 NMR spectrometer. The chemical shift data for each signal were reported in ppm units with tetramethylsilane (TMS) as the internal reference. Elemental analyses (C, H, N) were performed on a VARIO EL instrument. Mass spectra were recorded on an Ultraflex tandem TOF mass spectrometer and an HPLC-Q-TOF micro mass spectrometer. IR spectra were recorded using a Bio-Rad FTS-65A/896 FTIR system. UV-vis absorption spectra were recorded on a Shimadzu UV-3100 spectrometer. The photoluminescence (PL) spectra were recorded on an Edinburgh Analytical Instruments FLS920 spectrometer.

The steady state near-infrared (NIR) emission spectra were measured on an Edinburgh FLS920 fluorescence spectrometer equipped with a Hamamatsu R5509–72 supercooled photomultiplier tube at 193 K and a TM300 emission monochromator with NIR grating blazed at 1000 nm. The corrected spectra were obtained via a calibration curve supplied with the instrument. The emission lifetimes above 10 μs were obtained by using an Edinburgh Xe900 450 W pulse xenon lamp as the excitation light source. Because the emission lifetimes of the Ir₂Nd and Ir₂Er arrays are both below 10 μs, accurate lifetime values could not be obtained under the experimental conditions.

Syntheses

[Ir(ppy)₂(phen5f)] . (ppy)₄Ir₂Cl₂ (0.5 mmol),²³ Hphen5f (1.1 mmol) and anhydrous sodium carbonate (5 mmol) were refluxed in 2-ethoxyethanol under a N₂ atmosphere for 12 h. After cooling to room temperature, the mixture was poured into water. The crude product was obtained by filtration and then dissolved in CHCl₃. The solution was dried in anhydrous sodium sulfate overnight. After evaporation of the solvent, the residue was purified viachromatography, using an acetone–petroleum ether mixture as the eluent. The pure product was obtained as a yellow powder. Yield: 45%. ¹H NMR (400 MHz, DMSO-d₆): δ/ppm 8.84–8.70 (m, 2H), 8.53–8.18 (m, 3H), 8.16–8.14 (m, 1H), 8.08–7.73 (m, 6H), 7.68–7.66 (m, 1H), 7.56–7.38 (m, 2H), 7.08–6.96 (m, 1H), 6.88–6.84 (m, 2H), 6.84–6.73 (m, 2H), 6.64–6.44 (m, 1H), 6.13–5.92 (m, 2H), 4.27–4.11 (m, 1H). Anal. calcd for C₃₉H₂₄F₅IrN₄O₂·H₂O: C 52.88, H 2.96, N 6.32. Found: C 52.74, H 3.14, N 6.07. MALDI-TOF-MS: m/z 869 [M + H]⁺. FT-IR: ν/cm⁻¹ 3600–3400 (w), 3100–3000 (w), 1609 (m), 1535 (vs), 1481 (s), 1441 (m), 1424 (m), 1323 (m).

[(ppy)₂Ir(μ-phen5f)]₂Ln(NO₃)₃ (Ln = Nd, Yb, Er, Gd, Eu). Ir(ppy)₂(phen5f) and 0.5 equiv. of Ln(NO₃)₃·xH₂O were stirred in methanol for 6 h to give an orange-red solution. Crystallization from a MeCN–MeOH solution by slow diffusion of Et₂O led to microcrystalline [(ppy)₂Ir(μ-phen5f)]₂Ln(NO₃)₃.

Anal. calcd for C₇₈H₄₈F₁₀Ir₂N₁₁NdO₁₃·H₂O: C 44.95, H 2.42, N 7.39. Found: C 45.18, H 2.77, N 7.10. FT-IR: ν/cm⁻¹ 3600–3500 (w), 3100–3000 (w), 1622 (s), 1610 (s), 1525 (m), 1480 (vs), 1443 (vs), 1321 (s).

Anal. calcd for C₇₈H₄₈F₁₀Ir₂N₁₁O₁₃Yb·H₂O: C 44.34, H 2.39, N 7.29. Found: C 44.34, H 2.48, N 7.24. FT-IR: ν/cm⁻¹ 3600–3500 (w), 3100–3000 (w), 1610 (s), 1527 (s), 1481 (vs), 1443 (s), 1322 (s).

Anal. calcd for C₇₈H₄₈ErF₁₀Ir₂N₁₁O₁₃·3H₂O: C 43.72, H 2.54, N 7.19. Found: C 43.43, H 2.66, N 7.23. FT-IR: ν/cm⁻¹ 3600–3500 (w), 3100–3000 (w), 1622 (s), 1610 (s), 1480 (vs), 1443 (s), 1322 (s).

Anal. calcd for C₇₈H₄₈F₁₀Ir₂N₁₁O₁₃Gd·2H₂O: C 44.30, H 2.48, N 7.28. Found: C 44.07, H 2.87, N 7.15. FT-IR: ν/cm⁻¹ 3600–3500 (w), 3100–3000 (w), 1610 (s), 1526 (m), 1480 (vs), 1444 (s), 1321 (s).

Anal. calcd for C₇₈H₄₈EuF₁₀Ir₂N₁₁O₁₃·H₂O: C 44.79, H 2.41, N 7.37. Found: C 44.62, H 2.67, N 7.64. FT-IR: ν/cm⁻¹ 3600–3500 (w), 3100–3000 (w), 1610 (s), 1526 (m), 1480 (vs), 1443 (s), 1321 (s).

X-Ray crystallography

The X-ray diffraction data were collected on a Rigaku MicroMax-007 CCD diffractometer by the ω scan technique using graphite-monochromated Mo/Kα radiation. An absorption correction by multi-scan was applied to the intensity data. The structures were solved by direct methods and the heavy atoms were located from E-map. The remaining non-hydrogen atoms were determined from the successive difference Fourier syntheses. The non-hydrogen atoms were refined anisotropically, whereas the hydrogen atoms were generated geometrically with isotropic thermal parameters. The structures were refined on F² by full-matrix least-squares methods using the SHELXTL-97 program.²⁴ Because the crystal structure of [(ppy)₂Ir(μ-phen5f)]₂Nd(NO₃)₃ contained some solvent molecules that were highly disordered, PLATON/SQUEEZE²⁵ was used to correct the data for the presence of the disordered solvent. A summary of the refinement details and resulting factors are given in Table 6.

Table 6 Crystallographic data for the crystal structures

Complex	Ir(ppy)2(phen5f)	[(ppy)2Ir(μ-phen5f)]2Nd(NO3)3·2.5CH3CN·CH3CH2OCH2CH3
Empirical formula	C39H24F5IrN4O2	C87H65.5F10Ir2N13.5NdO14
Formula weight/g mol^−1	867.82	2242.67
Temperature/K	293(2)	113(2)
λ/Å	0.71073	0.71070
Crystal system	Monoclinic	Monoclinic
Space group	P21/c	P21/c
a/Å	12.612(2)	15.4979(18)
b/Å	17.190(3)	17.028(2)
c/Å	18.641(3)	34.616(4)
β/°	98.554(3)	90.743(7)
V/Å^3	3996.4(13)	9134.3(19)
Z	4	4
D calcd /g cm^−3	1.442	1.631
Absorption coefficient/mm^−1	3.400	3.555
F(000)	1696	4392
Crystal size/mm	0.14 × 0.10 × 0.06	0.22 × 0.18 × 0.16
Theta range for data collection/°	2.02–25.02	1.68–25.00
Reflections collected/unique	19 499/7008 [Rint = 0.0790]	65 277/15 938 [Rint = 0.0856]
Data/restraints/parameters	7008/313/523	15 938/114/1231
Goodness-of-fit on F^2	1.098	1.107
Final R indices [I>2σ (I)]	R1 = 0.0515, wR2 = 0.1064	R1 = 0.0761, wR2 = 0.1865
R indices (all data)	R1 = 0.1168, wR2 = 0.1343	R1 = 0.0925, wR2 = 0.1974

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Acknowledgements

The authors are grateful to the National Basic Research Program (2006CB601103) and the NNSFC (20471004, 20221101, 20423005, 20671006) for financial support. The authors also thank Prof. Chun-Ying Duan and Dr Cheng He at Dalian University of Technology for their kind help with the EIS-MS measurements.

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Footnote

† CCDC reference numbers 631669 and 648231. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b810016k

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