Fang-Fang
Chen
a,
Zu-Qiang
Bian
*a,
Bin Lou
a,
En
Ma
b,
Zhi-Wei
Liu
a,
Dao-Bo
Nie
a,
Zhu-Qi
Chen
a,
Jiang
Bian
a,
Zhong-Ning
Chen
b and
Chun-Hui
Huang
*a
aBeijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. E-mail: chhuang@pku.edu.cn; bianzq@pku.edu.cn; Tel: +86-10-6275-7156
bState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China
First published on 9th September 2008
The iridium(III) complex Ir(ppy)2(phen5f) [ppy = 2-phenylpyridinato-N,C2′, 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, NdIII, YbIIIand ErIII. 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 IrIII complexes will be designed for lanthanide NIR emission by the proper combination between the cyclometalated ligand and the tetradentate ancillary ligand.
However, the IrIII 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 IrIII moiety to the EuIII complex has been studied,14,15 and the only example, regarding the NIR emission of an IrIII–YbIII complex has recently been reported by De Cola and co-workers.16 In fact, as important phosphorescent materials, many IrIII 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 IrIII complexes to sensitise NIR lanthanide ions.
Recently, we reported an IrIII complex Ir(dfppy)2(phen5f) as a complex ligand, which can efficiently sensitise EuIII.15 In this work, we have changed the cyclometalated ligand from dfppy [2-(4′,6′-difluorophenyl)-pyridinato-N,C2′] to ppy (2-phenylpyridinato-N,C2′) in order to decrease the triplet energy level of the whole d-block complex ligand and make it more suitable for the NIR LnIII (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.
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Scheme 1 Synthetic route of the Ir2Ln arrays. |
The crystals of Ir(ppy)2(phen5f) were obtained from a slowly evaporating solution of acetone (Fig. 1a). The central ion IrIII 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 IrIII ion and the ancillary ligand phen5f is longer than those between the IrIII ion and the cyclometalated ligand ppy, which showed that the combination of the IrIII ion and the C⁁N ligand anion was tighter than that of the IrIII ion and the N⁁N ancillary ligand.
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 NdIII 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 IrIII complex ligands in [(ppy)2Ir(μ-phen5f)]2Nd(NO3)3 are similar to the individual IrIII 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
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) |
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.
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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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. |
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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.
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) |
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.
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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. |
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Fig. 5 Crystal packing for Ir(ppy)2(phen5f). |
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Fig. 6 Crystal packing for [(ppy)2Ir(μ-phen5f)]2Nd(NO3)3. |
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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 LnIII ion was changed from GdIII to other LnIII (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 IrIII moieties to the LnIII (Ln = Nd, Yb, Er) centers. The weighted-average lifetimes of the IrIII-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 IrIII→LnIII energy transfer rates kET from the equation6kET = 1/τq − 1/τu, where τq is the residual lifetime of the IrIII-based emission undergoing quenching by a lanthanide ion, and τu is the “unquenched” lifetime in the reference complex Ir2Gd array, so the IrIII→LnIII 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 IrIII centered emission.
Upon irradiation of the MLCT absorption of the Ir complex at λex 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 4F3/2→4I9/2, 4I11/2 and 4I13/2 transitions of NdIII, respectively. For the YbIII complex, there is only one emission band around 978 nm and it is affected by crystal-field splitting, which is assigned to the 2F5/2→2F7/2 transition. An observed emission band around 1530 nm is attributed to the 4I13/2→4I15/2 transition for the ErIII complex. Moreover, the quantum yield of the Ir2Yb complex was determined by the equation ΦLn = τobs/τ0, in which τobs is the observed emission lifetime and τ0 is the radiative or ‘natural’ lifetime, 2 ms for YbIII. The luminescence decay of YbIII 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.
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Fig. 8 Solid state NIR emission spectra (powdered samples) of Ir2Nd (red), Ir2Yb (black) and Ir2Er (blue) arrays. |
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.
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).
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/Å3 | 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![]() |
65![]() ![]() |
Data/restraints/parameters | 7008/313/523 | 15![]() |
Goodness-of-fit on F2 | 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 |
Footnote |
† CCDC reference numbers 631669 and 648231. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b810016k |
This journal is © The Royal Society of Chemistry 2008 |