Sensitized Near-Infrared Emission from IrIII-LnIII (Ln = Nd, Yb, Er

Article pubs.acs.org/Organometallics

Sensitized Near-Infrared Emission from IrIII-LnIII (Ln = Nd, Yb, Er) Bimetallic Complexes with a (N∧O)(N∧O) Bridging Ligand Fang-Fang Chen,†,‡,∥ Hui-Bo Wei,†,∥ Zu-Qiang Bian,*,† Zhi-Wei Liu,† En Ma,§ Zhong-Ning Chen,§ and Chun-Hui Huang† †

Beijing 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, People’s Republic of China ‡ National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China § State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China S Supporting Information *

ABSTRACT: A (N∧O)(N∧O) bridging ligand, 5-bromopyrimidine-2-carboxylic acid (bpmc), was used for connecting IrIII and LnIII centers to construct the series of d−f bimetallic complexes Ir(pdt)2(μ-bpmc)Ln(TTA)3, where pdt = 1,3-dimethyl-5phenyl-1H-[1,2,4]triazole, TTA = 4,4,4-trifluoro-1-(thiophen-2yl)-butane-1,3-dionate, and Ln = Nd, Yb, Er, Gd. Crystallographic analyses reveal that there are very short spatial distances between the d−f centers (about 6 Å), and photophysical studies demonstrate the appropriate energy level of the Ir III chromophore for sensitization of near-infrared (NIR) lanthanide ions, both of which are two important factors for efficient IrIII → LnIII energy transfer. The energy transfer rates for IrIII-NdIII, IrIIIYbIII, and IrIII-ErIII are calculated to be 3.6 × 109, 2.8 × 108, and 4.0 × 108 s−1, respectively.

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INTRODUCTION The special luminescent properties of lanthanide (LnIII) complexes have afforded them wide prospect in various applications such as organic light-emitting diodes, analytical sensors, imaging techniques, and so on.1−7 One key point in the research is seeking for proper ligands to construct the lanthanide complexes, among which transition-metal complexes are becoming a good choice due to their extended excitation band originating from the triplet metal-to-ligand charge transfer (3MLCT) absorption. Transition-metal complexes have been extensively investigated as complex ligands for the sensitization of luminescent lanthanides since 2000.8−11 Up to now, most antennas based on transition metals such as RuII/OsII,12−19 PtII,20−24 ReI,25−27 CrIII/CoIII,28−30 and IrIII 31−36 exhibit relatively low triplet states from 13000 to 18000 cm−1, which are usually used for the sensitization of lanthanides with nearinfrared (NIR) emission (NdIII, YbIII, ErIII, and PrIII). Nevertheless, some PtII 37−40 and IrIII 41−44 complexes were also successfully utilized as the chromophores for the luminescence of EuIII and TbIII, extending to visible emission. As has been proved in previous research, many factors can influence the luminescence efficiency of d−f bimetallic complexes. (i) The first is the energy level of the d-block chromophore. The energy level is considered as the most important factor, because it directly determines the extent of energy overlap, as well as the consequent energy transfer © 2014 American Chemical Society

efficiency between the d-block chromophore and the f center.42,45,46 (ii) The second is spatial distances between the d−f centers. There are two possible types of energy transfer mechanism in the 3d−4f systems, Förster and Dexter type, both showing strong dependence on the distance between the d−f centers. The former requires spectroscopic overlap between emission of the donor and absorption of the acceptor with a d−6 distance dependence (d = distance), while the latter usually demands direct donor−acceptor orbital overlap depending on e−d. However, in certain d−f systems incorporating conjugated bridging ligands, efficient indirect Dexter energy transfer can occur over substantial distances (up to 20 Å) via a superexchange process,14 which also shows an exponential decay with distance. Therefore, it can be concluded that spatial distance plays an important role in the energy transfer. However, using different principles for different systems should be taken into consideration. (iii) The third is vibrational deactivation. High-energy vibrations such as O−H, N−H, and even C−H can cause serious luminescence quenching of the lanthanide emission, especially for the NIR emission.5 According to these factors mentioned above, appropriate energy level, short spatial distance, and discouragement of Received: November 14, 2013 Published: June 25, 2014 3275

dx.doi.org/10.1021/om401110k | Organometallics 2014, 33, 3275−3282

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of Ir(pdt)2(bpmc) and Ir-Ln (Ln = Nd, Er, Gd) are shown in Figure 1, and selected bond lengths and angles are given in Tables 1 and 2. In the crystal structure of Ir(pdt)2(bpmc), it can be found that the central IrIII ion is six-coordinated by two carbon atoms, three nitrogen atoms, and one oxygen atom. Herein, two carbon atoms and two nitrogen atoms are offered by the two

nonradiative quenching should be comprehensively considered when choosing complex ligands for d−f systems. Our group has recently reported a type of (N∧O)(N∧O) bridging ligand, 2-carboxylpyrimidine, to construct an IrIII-EuIII bimetallic complex, which gave bright red luminescence.45 In this work, in order to get a better sensitization effect for lanthanide ions in the NIR region, another IrIII complex ligand with lower triplet energy level was employed. The series of d−f bimetallic complexes Ir(pdt)2(μ-bpmc)Ln(TTA)3 (pdt = 1,3dimethyl-5-phenyl-1H-[1,2,4]triazole, bpmc = 5-bromopyrimidine-2-carboxylic acid, TTA = 4,4,4-trifluoro-1-(thiophen-2yl)butane-1,3-dionate, and Ln = Nd, Yb, Er, Gd) were synthesized and investigated. Herein, the modified (N∧O)(N∧O) bridging ligand bpmc was chosen because it can not only produce a lower energy level but also introduce the “heavy atom effect” by the bromo substituent. In addition, Ln(TTA)3 compounds were adopted in the synthesis of bimetallic complexes to achieve saturated coordination and minimize the vibration quenching from solvent molecules. Results show that NIR emission can be obtained with rapid energy transfer rates of 3.6 × 109, 2.8 × 108, and 4.0 × 108 s−1 for NdIII, YbIII, and ErIII, respectively.

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RESULTS AND DISCUSSION Synthesis and Structure. Synthetic routes for the IrIII-LnIII bimetallic complexes Ir(pdt)2(μ-bpmc)Ln(TTA)3 (Ln = Nd, Yb, Er, Gd) are outlined in Scheme 1 (for clarity in expression, Scheme 1. Synthetic Route of Ir(pdt)2(μ-bpmc)Ln(TTA)3a

a

Ln = Nd, Yb, Er, Gd, abbreviated as Ir-Nd, Ir-Yb, Ir-Er, and Ir-Gd, respectively

the Ir-Ln bimetallic complexes are abbreviated Ir-Nd, Ir-Yb, IrEr, and Ir-Gd, respectively). The bridging ligand bpmc is used to connect the transition-metal center and the lanthanide center. Ir(pdt)2(bpmc) was prepared from (pdt)2Ir(μ-Cl)2Ir(pdt)2 and bmpc according to ref 47, except that the reaction was controlled at a milder temperature of 343 K. (The Br atom on the pyrimidine ring will be substituted by a 2-ethoxyethoxy group if the reaction mixture is refluxed in 2-ethoxyethanol as usual at 408 K, resulting in an unwanted structure with higher energy level which is unfavorable for the energy transfer.) A single crystal of Ir(pdt)2(bpmc) was obtained by slow evaporation of its CH2Cl2/n-hexane/methanol solution. A mixture of Ir(pdt)2(bpmc) and Ln(TTA)3·2H2O (Ln = Nd, Yb, Er, Gd) in the molar ratio 1:1 was dissolved in CH2Cl2, and then an appropriate amount of n-hexane was added. After evaporation for several days, dark orange single crystals of IrNd, Ir-Er, and Ir-Gd were obtained. The X-ray crystal structures

Figure 1. ORTEP (Oak Ridge thermal ellipsoid plot) diagrams of Ir(pdt)2(bpmc) (a) and Ir-Ln (Ln = Nd (b), Er (c), Gd (d)). Thermal ellipsoids are shown at the 20% probability level. The hydrogen atoms and solvent molecules are omitted for clarity. 3276


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Table 1. Selected Bond Lengths and Angles in the Complexes Ir(pdt)2(bpmc)

Ir-Nd

Ir-Er Selected Bond Lengths (Å) 2.473(5) Er(1)−O(2) 2.745(6) Er(1)−N(8) 2.375(5) Er(1)−O(3) 2.407(5) Er(1)−O(4) 2.383(6) Er(1)−O(5) 2.409(5) Er(1)−O(6) 2.407(8) Er(1)−O(7) 2.410(7) Er(1)−O(8) Selected Angles (deg) 62.65(17) O(2)−Er(1)−N(8) 70.40(18) O(3)−Er(1)−O(4) 71.5(2) O(6)−Er(1)−O(5) 71.8(4) O(7)−Er(1)−O(8)

Ir(1)−C(1) Ir(1)−N(3) Ir(1)−C(11) Ir(1)−N(6) Ir(1)−N(7) Ir(1)−O(1)

1.988(5) 2.020(4) 2.019(5) 2.042(4) 2.121(4) 2.172(3)

Nd(1)−O(2) Nd(1)−N(8) Nd(1)−O(3) Nd(1)−O(4) Nd(1)−O(5) Nd(1)−O(6) Nd(1)−O(7) Nd(1)−O(8)

C(1)−Ir(1)−N(3) C(11)−Ir(1)−N(6) N(7)−Ir(1)−O(1)

80.09(18) 79.47(18) 76.63(14)

O(2)−Nd(1)−N(8) O(3)−Nd(1)−O(4) O(6)−Nd(1)−O(5) O(7)−Nd(1)−O(8)

Ir-Gd

2.3640(18) 2.615(2) 2.2714(19) 2.306(2) 2.2698(18) 2.311(2) 2.307(2) 2.289(2)

Gd(1)−O(2) Gd(1)−N(8) Gd(1)−O(3) Gd(1)−O(4) Gd(1)−O(5) Gd(1)−O(6) Gd(1)−O(7) Gd(1)−O(8)

2.412(5) 2.664(7) 2.364(5) 2.373(5) 2.386(5) 2.321(5) 2.314(6) 2.379(5)

64.86(6) 73.25(7) 74.12(7) 74.28(8)

O(2)−Gd(1)−N(8) O(3)−Gd(1)−O(4) O(6)−Gd(1)−O(5) O(7)−Gd(1)−O(8)

63.83(18) 74.91(19) 72.6(2) 71.5(2)

Table 2. Selected Bond Lengths (Å) for Bridging Ligands in Different Complexes O(1)−C(21) O(2)−C(21) C(21)−C(22) N(7)−C(22) N(8)−C(22)

Ir(pdt)2(bpmc)

Ir-Nd

Ir-Gd

Ir-Er

1.245(6) 1.233(6) 1.527(6) 1.344(6) 1.321(6)

1.258(9) 1.253(8) 1.520(10) 1.346(9) 1.314(9)

1.254(9) 1.235(9) 1.528(10) 1.318(10) 1.356(10)

1.254(3) 1.243(3) 1.522(3) 1.345(3) 1.317(3)

Figure 2. Emission spectra of (a) Ir(pdt)2(bpmc) at room temperature (solid line for 4 × 10−5 M in CH2Cl2 solution excited at 380 nm and dashed line for powder solid excited at 470 nm) and (b) Ir(pdt)2(bpmc) in 2-Me-THF at 77 K (3 × 10−5 M).

difference in atomic radii between O and N, which indicates that the coordination between NdIII and O is tighter than that between NdIII and N. Similar coordination properties could be also found in Ir-Er and Ir-Gd (Figure 1 and Table 1). The Ln− O and Ln−N bond lengths in the bimetallic complexes exhibit a tendency to decrease in the order NdIII > GdIII > ErIII, which is in agreement with the lanthanide contraction, gradual decreasing of the ionic radius of 1.11 Å > 1.05 Å > 1.00 Å.48 Since LnIII ions have relatively large ionic radii, a high coordination number of 8 or 9 is usually preferred. Achieving saturated coordination by using proper organic ligands can benefit the NIR luminescence from LnIII (Ln = Nd, Yb, Er) ions because the serious vibrational quenching caused by the coordinated solvent molecules can be eliminated. This is the reason Ln(TTA)3 was used as the rare-earth source instead of LnCl3 or Ln(NO3)3 in this work. Interestingly, it is found, according to our previous research, that using different rareearth sources can result in different ratios between IrIII and LnIII. For example, when LnCl3·6H2O was used as the reaction reagent, the IrIII/LnIII ratio was 3/1 (another Cl− coordi-

cyclometalated ligands pdt and the third nitrogen atom and one oxygen atom are contributed by the ancillary ligand bpmc. The C−O bond lengths in Ir(pdt)2(bpmc) are 1.245(6) Å (O(1)− C(21)) and 1.233(6) Å (O(2)−C(21)), respectively (Figure 1 and Table 2). The former O is coordinated to IrIII, while the latter O is free. Both of the C−O bonds are shorter than an ordinary single C−O bond (∼1.36 Å) but longer than an ordinary double CO bond (∼1.21 Å),48 indicating that it is highly conjugated in the bpmc part, which will benefit its coordination with the lanthanide ion. Ir-Nd is taken as an example to elucidate the coordination structure of the bimetallic complexes. The lanthanide ion is eight-coordinated, six by oxygen atoms from three TTA ligands and two by one oxygen and one nitrogen atom from bpmc. The six NdIII−O bonds between NdIII and O of TTA have similar lengths (Table 1). Nervertheless, for the bpmc part, the NdIII− O and NdIII−N bonds are obviously different. The former has a bond length of 2.473(5) Å (Nd(1)−O(2)), while the latter length is 2.745(6) Å (Nd(1)−N(8)). The NdIII−O bond is much shorter than the NdIII−N, bond even considering the 3277


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Figure 3. UV/vis absorption spectra (a) and luminescence spectra (b) of solutions of Ir(pdt)2(bpmc) in CH2Cl2 (4 × 10−5 M) on gradual addition of small portions of Nd(TTA)3·2H2O at room temperature. Luminescence spectra (b) were measured with an excitation of 470 nm.

nated),42,45 while if Ln(NO3)3·3H2O was chosen, the IrIII/LnIII ratio changed into 2/1 (three other NO3− coordinated).32 In this work, Ln(TTA)3(H2O)2 (Ln = Nd, Yb, Er, Gd) was employed and the Ir III /Ln III ratio became 1/1. This phenomenon can be explained by the different coordination abilities and steric hindrances of the corresponding anions (Cl−, NO3−, and TTA−). A diagrammatic sketch showing the structure of these heteronuclear complexes with different IrIII/LnIII ratios is given in the Supporting Information (Figure S1). Photophysical Properties. The emission spectra of Ir(pdt)2(bpmc) are shown in Figure 2. In CH2Cl2 solution (4 × 10−5 M), an emission peak around 625 nm was observed, while the solid powder spectrum exhibited a peak close to 578 nm. Obviously, there is a red spectral shift from solution to the solid state. A probable explanation is the existence of an ILET (interligand energy transfer) process,49 which may result in a severe solvent effect in solution that does not happen in the solid state. Therefore, we suppose that the 3MLCT energy level of Ir(pdt)2+ is higher than the 3LX (ligand centered) energy level of bpmc, and the emission of Ir(pdt)2(bpmc) can be ascribed to an ILET process from Ir(pdt)2+ to bpmc followed by the 3LX emission of bpmc. This speculation can be further supported by theoretical calculations (see Computational Studies). In addition, the low-temperature emission spectrum of Ir(pdt)2(bpmc) was recorded in 2-Me-THF at 77 K, which displayed a peak at about 562 nm. Thus, the calculated triplet energy level of Ir(pdt)2(bpmc), acting as a whole complex ligand, is around 1.78 × 104 cm−1 (2.22 eV). This energy level is not high enough to sensitize the low-lying excited state of EuIII (5D0, 1.75 × 104 cm−1), because an energy gap of >2000 cm−1 is usually needed. However, it is suitable to transfer energy to NIR-emitting lanthanide ions. In order to test the sensitization ability, spectroscopic titration were recorded by addition of Nd(TTA)3·2H2O to Ir(pdt)2(bpmc) in CH2Cl2 solution at room temperature (Figure 3). The UV/vis absorption spectra exhibit increasing intensities at 250 and 350 nm which originate from the absorption of TTA, while the luminescence spectra show decreasing tendency of the IrIII emission. When the Nd/Ir concentration ratio reached 1/1, the emission from Ir(pdt)2(bpmc) was almost completely quenched. Therefore, it can be concluded that effective energy transfer occurs between Ir(pdt)2(bpmc) and {Nd(TTA)3}. Figure 4 shows the UV/vis absorption spectra of Ir(pdt)2(bpmc) and Ir-Ln (Ln = Nd, Yb, Gd). It can be clearly

Figure 4. UV/vis absorption spectra of the solutions of Ir(pdt)2(bpmc) and Ir-Ln (Ln = Nd, Yb, Gd) in CH2Cl2 (4 × 10−5 M).

seen that there is no obvious difference among the spectra of Ir(pdt)2(bpmc) and Ir-Ln (Ln = Nd, Yb, Gd) in the longwavelength region from 400 to 520 nm, which implies that the absorbance in this region can be mainly attributed to the 3 MLCT absorption of Ir(pdt)2(bpmc). In other words, the absorption range of TTA is shorter than 400 nm. Therefore, an excitation wavelength longer than 400 nm was chosen to investigate the IrIII → LnIII energy transfer properties of the bimetallic systems through selective excitation of Ir(pdt)2(bpmc) using visible light.50 The solid-state emission spectra of Ir-Ln (Ln = Nd, Yb, Gd) were recorded and are shown in Figure 5. On excitation at 470

Figure 5. Solid-state emission spectra (powdered samples) of Ir-Ln (Ln = Gd, Nd, Yb, Er) in the visible region on excitation at 470 nm at room temperature. 3278


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the π−π stacking data are summarized in Table 3. For Ir-Gd, one can clearly find the π−π stacking not only between the pdt

nm, the powdered Ir-Gd emitted red light with a maximum at ca. 630 nm. In comparison with the solid-state emission of Ir(pdt)2(bpmc), which peaked at 578 nm (Figure 2), an obvious red shift of about 50 nm was observed. To get a further understanding of this phenomenon, crystal-packing views of Ir(pdt)2(bpmc), Ir-Gd and Ir-Nd are illustrated in Figure 6 and

Table 3. Crystal-Packing Data for Ir(pdt)2(bpmc), Ir-Gd, and Ir-Nd complex

λem/nm

π−π spacing

distance/Å

anglea/deg

Ir(pdt)2(bpmc) Ir-Gd

578 630

Ir-Nd

630

pdt pdt TTA pdt TTA

3.49 3.45 4.19 3.41 4.05

1.5 1.3 1.4 1.2 1.0

a

It presents the angle between two pdt planes or two thiophene rings in TTA.

ligands but also between thiophene rings of the TTA ligands. However, in the packing view of Ir(pdt)2(bpmc), only stacking between the pdt ligands was observed. This may be the probable reason for the red spectral shift of Ir-Gd in comparison to Ir(pdt)2(bpmc). Ir-Nd, Ir-Yb, and Ir-Er exhibit crystal-packing conditions and emission wavelengths similar to those of Ir-Gd. By comparison of the emission intensities in Figure 5, it can be found that Ir-Nd, Ir-Yb, and Ir-Er demonstrate much weaker emission than Ir-Gd. Apparent differences can also be found from the experimental results of the emission lifetimes. The weighted-average lifetime of Ir-Gd at 630 nm is 13 ns, while those of other Ir-Ln species are much shorter: 0.27, 2.8, and 2.1 ns for Nd, Yb, and Er, respectively. Both of these different behaviors indicate that efficient energy transfer from the IrIII moieties to LnIII (Ln = Nd, Yb, Er) centers occurs, which effectively relaxes the excitation state energy of IrIII. Nevertheless, such energy transfer cannot take place in Ir-Gd due to the extremely high energy level of the GdIII ion. The IrIII → LnIII energy transfer rates kET can be estimated from the equation kET = 1/τq −1/τu,51 where τq is the residual lifetime of the IrIII-based emission undergoing quenching by a lanthanide ion and τu is the “unquenched” lifetime of the reference complex Ir-Gd. Therefore, the IrIII → LnIII energy transfer rates for Ir-Nd, Ir-Yb, and Ir-Er could be calculated to be 3.6 × 109, 2.8 × 108, and 4.0 × 108 s−1, respectively. X-ray diffraction data show that the nonbonding distances between the IrIII center and LnIII centers are very short, 6.172, 6.140, and 6.058 Å for IrIII- - -NdIII, IrIII- - -GdIII and IrIII- - -ErIII, respectively, which implies that the bridging ligand bpmc is a good choice to obtain a rather short spatial distance and also rapid energy transfer in d−f bimetallic systems. Upon irradiation at 470 nm (absorption region of the Ir moiety), NIR emission of the bimetallic complexes Ir-Ln (Ln = Nd, Yb, Er) were measured both in the solid state and in CH2Cl2 solution (only the solid emissions are shown in Figure 7 for clarity). As expected, three strong emission bands were obtained at ca. 873, 1058, and 1332 nm for Ir-Nd, corresponding to the 4F3/2 → 4I9/2, 4I11/2, and 4I13/2 transitions of NdIII, respectively. For Ir-Yb, only one emission band was detected around 978 nm, which is affected by crystal-field splitting and can be assigned to the 2F5/2 → 2F7/2 transition. The emission band around 1526 nm for Ir-Er can be attributed to the 4I13/2 → 4I15/2 transition. Moreover, the quantum yields of Ir-Yb and Ir-Nd were calculated according to the equation ΦLn = τobs/τ0, in which τobs is the observed emission lifetime and τ0 is the radiative or “natural” lifetime (1.2 ms for YbIII and 0.25 ms for NdIII).5,23 Under the present experimental

Figure 6. Crystal-packing views for Ir(pdt)2(bpmc) (a), Ir-Gd (b), and Ir-Nd (c).

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energy of 2.21 eV (560 nm), which is thought to be more appropriate for the sensitization of NIR lanthanide ions. The triplet emissive transition is LUMO+1 → HOMO, which can be ascribed to an emission from bpmc (see the electron cloud distribution in Table 5). Moreover, one can observe that LUMO and LUMO+1 of Ir(pdt)2(bpmc) are located on the bridging ligand bpmc, while higher molecular orbitals from HOMO+2 to HOMO+5 mainly lie on the Ir(pdt)2+ part, which proves the aforementioned supposition that the triplet energy level of bpmc (3LX) is lower than the 3MLCT level of Ir(pdt)2+ and that an interligand energy transfer step exists during the emission process. This result also indicates the possible energy transfer pathway from the iridium complex to the lanthanide center is through the triplet excited state of the bridging ligand (3LX) in heteronuclear IrIII-LnIII complexes. These theoretical calculations together with the experimental results are perfectly supported by each other.

Figure 7. Solid-state NIR emission spectra (powdered samples) of IrLn (Ln = Nd, Yb, Er) under excitation at 470 nm at room temperature.

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CONCLUSION Three bimetallic complexes Ir(pdt)2(μ-bpmc)Ln(TTA)3 (Ln = Nd, Yb, Er) were synthesized using the (N∧O)(N∧O) bridging ligand bpmc. Their crystal structures and photophysical properties were determined. X-ray diffraction data reveal that the IrIII−LnIII distances are all around 6 Å, which benefits efficient energy transfer. A photophysical study shows that energy transfer rates are 3.6 × 109, 2.8 × 108, and 4.0 × 108 s−1 for Ir-Nd, Ir-Yb, and Ir-Er, respectively, which lie in high rank among similar kinds of works.14,34,52 In addition, the complex ligand Ir(pdt)2(bpmc) successfully moved the excitation wavelength from the UV to the visible region (up to 520 nm).

conditions, an accurate lifetime value of ErIII could not be detected due to its weak intensity; therefore, the estimated quantum yield could not be obtained either. The luminescence decay of Ir-Yb was measured at 980 nm in the solid state and in solution, and the lifetimes were 17 and 18 μs, respectively (Table 4). Similarly, the luminescence decay of Ir-Nd measured Table 4. Luminescence Properties of Ir-Ln in the Solid State at Room Temperature in Air complex

kET (s−1)a

τobs (μs)b

ΦLn (%)c

Ir-Nd Ir-Yb Ir-Er

3.6 × 10 2.8 × 108 4.0 × 108

0.85 17 d

0.34 1.4 d

9

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EXPERIMENTAL SECTION

The 1H NMR spectra were recorded on an ARX-400 NMR spectrometer. Chemical shift data are 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. UV− vis absorption spectra were measured 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 TM 300 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 were obtained by using an Edinburgh Xe900 450 W pulse xenon lamp as the excitation light source. Solvents for reactions were distilled prior to use. Solvents used in photophysical experiments were of spectroscopic grade. Synthesis. pdt. The new cyclometalated ligand 1,3-dimethyl-5phenyl-1H-[1,2,4]triazole (pdt) was synthesized by using ethyl acetimidate hydrochloride instead of ethyl butyrimidate hydrochloride as the initial material in accord with the requirement of the target molecule.52,53 (pdt)2Ir(μ-Cl)2Ir(pdt)2. The IrIII chloro-bridged dimer was synthesized following the literature method.47,53 The crude product was purified via chromatography by using CH2Cl2 as eluent, and the IrIII chloro-bridged dimer (pdt)2Ir(μ-Cl)2Ir(pdt)2 was obtained as a yellow powder. Ir(pdt)2(bpmc). (pdt)2Ir(μ-Cl)2Ir(pdt)2 (0.50 mmol), bpmc (1.1 mmol), and anhydrous sodium carbonate (5.0 mmol) were mixed and heated to 343 K in 2-ethoxyethanol under an N2 atmosphere for 24 h. After it was cooled to room temperature, the mixture was poured into water. The crude product was obtained by filtration and then dissolved in CH2Cl2. The solution was dried in anhydrous sodium sulfate overnight. After evaporation of the solvent, the residue was purified via

The IrIII → LnIII energy transfer rates kET, estimated from the equation kET = 1/τq −1/τu,52 where τq is the residual lifetime of the IrIII-based emission (∼600 nm) undergoing quenching by a lanthanide ion (0.27, 2.8, and 2.1 ns for Nd, Yb, and Er, respectively) and τu is the “unquenched” lifetime of the reference complex Ir-Gd (13 ns); relative error ±5%. bObserved emission lifetimes of the lanthanide ions for IrLn on excitation at 470 nm; relative error ±5%. cEstimated intrinsic quantum yields from the equation ΦLn = τobs/τ0, in which τobs is the observed emission lifetime and τ0 is the radiative or “natural” lifetime (1.2 ms for YbIII, 0.25 ms for NdIII);5,23 relative error not given considering the values are roughly estimated. dNot observed. a

at 1062 nm gave lifetimes of 0.85 and 1.1 μs in the solid state and in CH2Cl2 solution, respectively. The difference between the lifetime values in the solid state and in CH2Cl2 solution is not obvious. Therefore, the corresponding quantum yields for YbIII and NdIII are ca. 1 × 10−2 and 4 × 10−3 both in the solid state and in CH2Cl2 solution. Computational Studies. Time-dependent density functional theory (TD-DFT) calculations were carried out to study the triplet excited states of Ir(pdt)2(bpmc) at optimized triplet geometry. The calculated emission wavelengths, energy levels, and molecular orbitals involved in the emissive transition are presented in Table S2 (Supporting Information), and the electron cloud distributions of some related molecular orbitals around the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are shown in Table 5. In comparison with our previously reported iridium complex ligand,45 Ir(pdt)2(bpmc) exhibits a relatively low triplet state 3280


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Table 5. HOMO and LUMO Distributions of Ir(pdt)2(bpmc) Calculated using TD-DFT

chromatography, using a CH2Cl2/methanol (2/1) mixture for the elution. The pure product was obtained as a yellow powder. 1H NMR (400 MHz, CDCl3; δ, ppm): 9.02 (d, 1H, J = 2.7 Hz), 7.99 (d, 1H, J = 2.7 Hz), 7.54 (d, 1H, J = 7.7 Hz), 7.48 (d, 1H, J = 7.7 Hz), 7.02−6.98 (m, 1H), 6.95−6.84 (m, 3H), 6.60 (d, 1H, J = 7.3 Hz), 6.26 (d, 1H, J = 7.0 Hz), 4.29 (s, 3H), 4.22 (s, 3H), 2.41 (s, 3H), 1.62 (s, 3H). Highresolution ESI-MS: m/z observed [M + H]+ 739.0736, calculated 739.0756; observed [M + Na]+ 761.0557, calculated 761.0576. Anal. Found: C, 41.02; H, 3.36; N, 14.88. Calcd for C25H22N8O2BrIr· 0.2CH3OH: C, 40.63; H, 3.08; N, 15.04. Ln(TTA)3·2H2O (Ln = Nd, Yb, Er, Gd). Three equivalents of 4,4,4trifluoro-1-(thiophen-2-yl)butane-1,3-dione (HTTA) was dissolved in ethanol, and 3 equiv of NaOH was added to the solution. After it was refluxed, the mixture was added dropwise to 1 equiv of LnCl3·6H2O (Ln = Nd, Yb, Er, Gd) in ethanol. The mixture was then refluxed for a few hours. After removal of the solvent, the crude product was recrystallized in EtOH/H2O, and the corresponding products Ln(TTA)3·2H2O were obtained. Ir(pdt)2(μ-bpmc)Ln(TTA)3 (Ln = Nd, Yb, Er, Gd). Equal amounts of Ir(pdt)2(bpmc) and Ln(TTA)3·2H2O (Ln = Nd, Yb, Er, Gd) were dissolved in CH2Cl2, and then some n-hexane was added to the mixture. The product crystals were obtained from the solution through slow evaporation. Ir(pdt)2(μ-bpmc)Nd(TTA)3. Anal. Found: C, 37.87; H, 2.45; N, 7.09. Calcd for C49H34BrF9IrN8NdO8S3: C, 38.06; H, 2.22; N, 7.25. Highresolution ESI-MS: m/z observed [M + Na]+ 1565.9314, calculated 1565.9304. Ir(pdt)2(μ-bpmc)Yb(TTA)3. Anal. Found: C, 37.54; H, 2.44; N, 6.98. Calcd for C49H34BrF9IrN8YbO8S3: C, 37.36; H, 2.18; N, 7.11. Highresolution ESI-MS: m/z observed [M + Na]+ 1597.9535, calculated 1597.9615. Ir(pdt)2(μ-bpmc)Er(TTA)3. Anal. Found: C, 37.35; H, 2.46; N, 6.98. Calcd for C49H34BrF9IrN8ErO8S3: C, 37.50; H, 2.18; N, 7.14. Highresolution ESI-MS: m/z observed [M + Na]+ 1589.9657, calculated 1589.9530. Ir(pdt)2(μ-bpmc)Gd(TTA)3. Anal. Found: C, 37.58; H, 2.42; N, 7.05. Calcd for C49H34BrF9IrN8GdO8S3: C, 37.74; H, 2.20; N, 7.19. Highresolution ESI-MS: m/z observed [M + Na]+ 1581.9651, calculated 1581.9468. X-ray Crystallography. The X-ray diffraction data were collected on a Rigaku MicroMaxTM-007 CCD diffractometer by the ω scan technique at 131 K using graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation. An absorption correction by multiscan was applied to the intensity data. The structures were solved by direct methods, and the heavy atoms were located from an E map. The remaining non-hydrogen atoms were determined from 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 package. A summary of the refinement details and resulting factors is given in the Supporting Information. CCDC-730202 for Ir(pdt)2(bpmc), 730198 for Ir(pdt)2(μ-bpmc)Nd(TTA)3, 730203 for Ir(pdt)2(μ-bpmc)Er(TTA)3, 730197 for Ir(pdt)2(μ-bpmc)Gd(TTA)3 contain the supplementary crystallographic data for this paper. Full crystallographic data are provided in CIF files in the Supporting Information and are also available from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Calculation Methods. Calculations on the triplet excited states of the iridium complexes were carried out using B3LYP TD-DFT at optimized triplet geometries. The LANL2DZ relativistic effective core potential (ECP) was applied to describe the core electrons of the Ir atoms, and full electron calculations were carried out for the other atoms with the 6-31g* basis set. All of the calculations were carried out using the Gaussian 09 program.54

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ASSOCIATED CONTENT

* Supporting Information S

Table S1 and CIF files giving crystallographic data for Ir(pdt)2(bpmc) and Ir-Ln (Ln = Nd, Er, Gd) and Table S2 and text giving an explanation of computational studies on the triplet state energy of related iridium complexes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for Z.-Q.B.: [email protected]. Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National High Technology Research and D e vel o p me n t Pr o g r a m o f C h i na (8 6 3 p r o g r a m , 2011AA03A407), the National Natural Science Foundation of China (NNSFC, Nos. 21371012 and 21321001), the Major State Basic Research Development Program of China (973 program, 2014CB643800), and The Specialized Research Fund 3281


Organometallics

Article

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for the Doctoral Program of Higher Education (20120001120116).

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Sensitized Near-Infrared Emission from IrIII-LnIII (Ln = Nd, Yb, Er

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