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Synthesis and luminescent properties of heteroleptic benzothiazolyl–naphtholates of ytterbium
Synthetic Metals 203 (2015) 117–121
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Synthesis and luminescent properties of heteroleptic benzothiazolyl– naphtholates of ytterbium Anatoly P. Pushkarev a, * , Vasily A. Ilichev a,b , Tatyana V. Balashova a,b , Artem N. Yablonskiy b,c, Boris A. Andreev b,c, Mikhail N. Bochkarev a,b, ** a b c
G.A. Razuvaev Institute of Organometallic Chemistry of Russian Academy of Sciences, Tropinina 49, 603950 Nizhny Novgorod, Russian Federation Nizhny Novgorod State University, Gagarina avenue 23/2, 603950 Nizhny Novgorod, Russian Federation Institute for Physics of Microstructures of Russian Academy of Sciences, 7 ul. Akademicheskaya, 603950 Nizhny Novgorod, Russian Federation
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 December 2014 Received in revised form 18 February 2015 Accepted 20 February 2015 Available online xxx
Photoluminescent (PL) and electroluminescent (EL) properties of heteroleptic complexes of Yb3+ containing the 3-(2-benzothiazol-2-yl)-2-naphtholate (L), 8-oxyquinolate (Q), pentafluorophenolate (OC6F5), or cyclopentadienyl (Cp) ligands were studied. All the complexes revealed NIR emission of Yb3+ ion of different intensity. The obtained data indirectly confirm the fact that the excitation of the ytterbium in naphtholate complexes proceeds due to intramolecular reduction–oxidation mechanism. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence Electroluminescence Heteroleptic complexes Ytterbium Excitation mechanism
1. Introduction The conventional simplified mechanism of excitation of lanthanide ions in the organolanthanide complexes involves excitation of coordinating ligands resulting in population of the singlet state, subsequent transfer of the excitation energy to the triplet state, from which the energy is transferred to resonant level of Ln3+ ion [1]. Such a scheme describes so called antenna effect which is manifested in an increase intensity of the metal-centered emission in complexes where the Ln3+ ion is coordinated by the sensitizing ligands. It has been found experimentally that effective energy transfer from the triplet state of ligand to respective f*-level of Ln3+ is realized when the energy gap between these levels is in the range 2500–3500 cm1 [2]. However, studying the luminescent properties of lanthanide-containing proteins Horrocks and coworkers have found that ytterbium derivatives reveal an efficient metal-centered emission although the difference between
* Corresponding author. Tel: +7 831 4627709; fax: +7 831 4627497. ** Corresponding author at: G.A. Razuvaev Institute of Organometallic Chemistry of Russian Academy of Sciences, Tropinina 49, 603950 Nizhny Novgorod, Russian Federation. Tel: +7 831 4627709; fax: +7 831 4627497. E-mail address: [email protected] (A.P. Pushkarev). http://dx.doi.org/10.1016/j.synthmet.2015.02.030 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.
the triplet level of the triptophan ligand and the sole exited level of Yb3+ (2F5/2) does not fall in the mentioned range [3]. A similar situation had taken place in the case of ytterbium complex with modified 8-oxyquinolate ligands [4]. To explain this phenomenon a new mechanism of excitation, which includes electron transfer from photoexcited anionic ligand to Ln3+ ion and formation of intermediate systems containing Ln2+ ion had been proposed [3]. Exploring the luminescent properties of lanthanide complexes with substituted phenolate and naphtholate ligands we found that efficiency of the metal-centered emission of ytterbium derivatives is much higher than that of the complexes with other lanthanides [5,6]. Especially impressive result was obtained with benzithiazolyl-naphtholate of ytterbium which demonstrated EL at 978 nm (2F5/2 ! 2F7/2 transition) with intensity of 890 mW/cm2, which is ten times greater than that of the best organolanthanide NIR electroluminophore [7]. Since in this case the conventional threesteps excitation scheme is also inapplicable for explanation of the observed hyperemission of Yb we have assumed that the redox electroexcitation mechanism that is similar to Horrocks’s mechanism of PL can be realized in the EL processes [6]. To check this assumption we have studied PL and EL properties of a series of heteroleptic ytterbium complexes containing, along with benzothiazolyl–naphtholate, 8-quinolinolate, pentafluorophenolate and cyclopentadienyl ligands each of which has different
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Fig. 1. LDI-TOF mass spectrum of complex 3.
sensitizing ability. The synthesis and structures of most of the used compounds were described previously [6,8,9] but a few of them were prepared and studied in this work.
hydroxyquinoline in DME. Complex 8 was synthesized from YbCp3 and 3-(2-benzothiazol-2-yl)-2-naphthole in a ratio of reagents 1:2 in DME solution. The complex is a dimer similar to the previously obtained yttrium and neodymium compounds [8].
2. Results and discussion 2.2. Luminescence study 2.1. Synthesis To expand the list of ytterbium compounds for investigation of the effect of ligands on the efficiency of metal-centered emission in addition to the previously obtained complexes we synthesized new heteroleptic complexes Yb(L)3(phen) (3), Yb4(L)(Q)11 (6) and Yb2(L)4(Cp)2 (8). The first of these was prepared by the reaction of Yb[N(SiMe3)2]3 with 3-(2-benzothiazol-2-yl)-2-naphthole in the presence of phenanthroline. Unlike previously studied reactions of lanthanide silylamides with substituted phenoles and naphtholes which afford binuclear complexes Ln2(L)6 [6], this process led to formation of mononuclear product 3 that was confirmed by LDITOF spectrometry (Fig. 1). The tetranuclear complex 6 was obtained in 64% yield by reaction Yb(L)(Cp)2 and 8-
PL and EL spectra of all the studied complexes contain emission bands of the ligands in the region of 350–600 nm and a characteristic bands of 2F5/2 ! 2F7/2 transition of Yb3+ ion at 978 nm accompanied by Stark’s splitting bands in the region 1000– 1100 nm (Figs. 2 and 3). The maximal intensity of NIR irradiation, power efficiency and internal quantum yield of OLEDs based on the benzothiazolyl–naphtholate complexes of Yb3+ as well as relative intensities of NIR bands in the PL spectra of these compounds are collected in the Table 1. To simplify the evaluation of relative intensities of the bands of metal-centered emission in the PL spectra the solid samples were used for the measurements. Comparing the band intensity of Yb3+ ions in the EL spectra, power efficiency and external quantum yield (EQE) of the tested
Fig. 2. PL spectra of compounds 1–9,11–13.
A.P. Pushkarev et al. / Synthetic Metals 203 (2015) 117–121
Fig. 3. EL spectra of compounds 3–5.
compounds showed that the previously studied homoleptic dimeric complex 1 has the highest characteristics. Other complexes with L ligands 2–6 also exhibited emission at 978 nm efficiency of which is much higher than that of L free compounds 7, 13 and complex 6, containing one L group per four Yb atoms, but three–four times lower than that of 1. Note that high power efficiency and EQE of OLED based on 13 is explained by low charge transporting properties of this compound causing low operating current. The difference in the EL characteristics of 2–5 is insignificant and is explained apparently by some differences in the charge transport properties of these materials. Since the energies of triplet levels of L 16,010 cm1 and Q 16,942 cm1 [12] ligands are considerably higher than the energy of resonant level of ytterbium 2F5.2 10,248 cm1 [13] both these ligands, according to the conventional excitation scheme, can not be good sensitizers for Yb3+ ions. However, as it can be seen from Table 1, the effect of L and Q ligands on the EL efficiency of ytterbium is dramatically different. While homoleptic 8-oxyquinolate complex 7 shows very low efficiency of ytterbium, which agrees well with the above rule of optimal 3T1 – f * energy gap, the naphtholate 1 exhibits a record high intensity of NIR luminescence, which contradicts to this rule. The heteroleptic complexes 2–6 occupy an intermediate position. These data suggest: (i) the sensitization of Yb3+ by Q ligands proceeds according to the conventional three-level scheme but sensitization of ytterbium by
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L ligands is realized by another way: probably according to redox mechanism which allows to achieve high population of 2F5/2 excited level of Yb3+; (ii) in the binuclear complexes only one of two Yb–L groupings is involved into luminescence process; (iii) in this grouping the L is a terminal ligands rather than bridging. Comparison of data for the complex Yb4(L)(Q)11 containing monodentate L ligand and the data for the other naphtholate– quinolate complexes containing chelate L ligands (Table 1) indicates that the sensitizing ability of the benzothiasolyl– naphtholate ligands in the studied complexes depends on the type of metal–ligand bonding. Instability in air prevented the measurement of EL characteristics of cyclopentadienyl and pentafluorophenolate derivatives but they were used in the PL study which showed that the luminescence upon the photo- and electroexcitation obeys the same regularities, i.e., the metal-centered emission of the complexes containing L ligands is much higher than that of L free compounds. Mononuclear and binuclear complexes containing at least one Yb–L fragment exhibit the same or close intensity of the band at 978 nm (complexes 1–4, 11, 12). The cyclopentadienyl compounds 8 and 9 revealed reduced NIR efficiency (probably due to quenching effect of Cp ligands) but it is also equal for both complexes. The found regularity is broken in the case of complexes 5 and 6: the compounds have Yb–L groupings but display moderate (for 5) or weak (for 6) metal-centered emission. In the case of 6 the reason of weak Yb luminescence apparently is h1 coordination mode of L. The reduced NIR luminescence of 5 probably is due to specific crystal packing but confirmation of this could not be obtained because of the low quality of the crystals. Thus, the data of PL study suggest that the sensitization of luminescence of Yb3+ ions upon photoexcitation by the ligands L and Q (or C6F5O) also has a different nature. In favor of an electron transfer excitation mechanism in the ytterbium–naphtholate complexes says comparison of PL spectra of 6 and its yttrium analog Y4(L)(Q)11 in visible region (Fig. 4). The latter contains two bands at 420 and 528 nm the first of which is assigned to p– p* transitions in the aromatic ligands and the second one is typical for ligand-metal charge transfer (LMCT) whereas the spectrum of the ytterbium complex comprises only one band at 420 nm. It is reasonable to assume that in the ytterbium complex the complete charge transfer, i.e., electron
Table 1 Efficiency of NIR emission of OLEDs based on ytterbium complexes and relative intensity of the Yb+3 band in the PL spectra of the same complexes. Complex
Yb2(L)6d Yb(L)4Lie Yb(L)3(phen) Yb2(L)4(Q)2 Yb2(L)2(Q)4 Yb4(L)(Q)11 Yb3(Q)9f Yb(L)2(Cp) Yb(L)(Cp)2 Yb(Cp)3 Yb2(L)4(OC6F5)2 Yb2(L)2(OC6F5)4 Yb(OC6F5)3(phen)g a b c d e f g
No.
1 2 3 4 5 6 7 8 9 10 11 12 13
ELa
PL intensity arb.un.
Max. irradiance mW/cm2 (V)b
Power efficiency mW/W
EQE (%)c
889 (19.2) 220 (21) 149 (27.5) 251 (27) 314 (30) – 47 (16) – – – – – 94 (20)
0.915 0.36 0.1 0.2 0.09 – 0.013 – – – – – 0.823
1.6 0.4 0.17 0.32 0.16 – 0.023 – – – – – 1.4
Obtained with OLED devices of configuration ITO/TPD/complex/BATH/Yb. In the brackets voltage at which the maximum irradiance was observed. At driving voltage 12 V. EL data from [6]. EL data from [9]. El data from [10]. EL data from [11].
60 60 60 60 26 11 20 18 18 0 60 54 12
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luminescence decay curves for 1–4, 7, 11 and 12 are described by simple exponential characteristic. At first sight this fact manifests interligand energy transfer from triplet excited state of Q (4) or OC6F5 (11, 12) to 3T1 state of L ligand and further to 2F5/2 resonant level of Yb3+ because otherwise the exponential fitting of decay curves would be more complicated. However, the data obtained do not contradict the proposed redox mechanism of Yb3+ excitation since the observed different decay times may be related to the different rates of the redox processes which occur in the 3-(2benzothiazol-2-yl)-2-naphtholate containing complexes. 3. Conclusions Fig. 4. Normalized PL spectra of Ln4(L)(Q)11, (Ln = Y, Yb) in THF solution, lex 360 nm.
transfer from a ligand to Yb3+ occurs resulting in disappearance of the LMCT band and implementation of redox mechanism of excitation of Yb3+ ions. In order to get additional evidence of the existence of different excitation mechanisms in the synthesized complexes timeresolved spectroscopic measurements were carried out. The measurements of Yb3+ metal-centered emission decay were performed at lmon = 978 nm using laser pulse irradiation with lex = 355 nm (decay curves are depicted in Fig. 5). It was found that the decay times for the compounds are within the range of 2– 10 ms, which is typical for organolanthanide derivatives of ytterbium: 6.98 (1), 9.92 (2), 8.36 (3), 8.91 (4), 3.89 (5), 2.02 (6), 9.91 (7), 1.69 (8), 0.98 (9), 9.45 (11), 7.06 (12) and 2.29 ms (13). A luminescence lifetime of Ln3+ ion surrounded by organic ligands may give some useful information for the rational molecular design of efficient emissive materials as it is sensitive to electrostatic ligand field symmetry at the ion site, considerably depends on the presence of luminescence quenching groups as well as luminescence concentration quenching and also reflects sophisticated energy transfer processes which may occur in the luminescent lanthanide containing systems. In terms of ligand field symmetry at the Yb3+ site one can notice that the metal ion in the derivatives 1 [6], 2 [9], 4 [8] and 7 [10] is eight coordinated and has a distorted antiprismatic environment. On the contrary, the Yb3+ ions in the compounds 8 and 9 [8] are ten and twelve coordinated, respectively and have unsymmetrical environment. Since the ligand field symmetry reduction at the ion site leads to increasing of radiative probability of electric dipole transitions the luminescence lifetime in the case of 8 and 9 should be shorter than that of 1, 2, 4 and 7. On the other hand, the organolanthanides 6, 8, 9 and 13 can demonstrate fast luminescence decay due to the presence of non-chelating L ligand, Cp groups or neutral phenanthroline ligand having a lot of C—H groups which effectively quench NIR luminescence. It should be noted also that
Fig. 5. Metal-centered luminescence decay curves for the complexes 1–9 and 11– 13.
To get evidence of the existence of different excitation mechanisms of Yb3+ ions in organic complexes the data of EL and PL of a series of mononuclear, dinuclear and tetranuclear homoleptic and heteroleptic complexes of ytterbium with 3-(2benzothiazol-2-yl)-2-naphtholate, 8-oxyquinolate, pentafluorophenolate and cyclopentadienyl ligands were collected and analyzed. The data obtained indicate that the benzothiazolyl– naphthyl ligands sensitize the metal-centered luminescence of Yb3 + much more efficiently than others used organic ligands. The conventional three-level energy diagram of the luminescence of organolanthanide emitters can not explain the observed phenomenon since the energy gap between the resonant 2F5/2 level of ytterbium and triplet level of the ligand L (as well as the ligands Q and C6F5O) about 6000 cm1 is much more the optimal value 2500–3500 cm1. On the contrary, the results are in good agreement with the redox mechanism of luminescence, which explains the high population of the excited state of ytterbium when some types of ligands are used. Taking into account that enhanced luminescence of Yb3+ was observed as well in the benzoxasolyl– naphtholates [6], benzoxasolyl–phenolates, benzothiasolyl–phenolates [5] complexes one may assume that S^N- and O^N-chelate aromatic ligands are suitable for the intramolecular ligand to metal electron transfer and realization of specific chemiluminescence of ytterbium. It should be noted that such an approach opens the way for the search of new highly luminescent complexes of not only ytterbium but as well samarium, europium and probably thulium because these lanthanides like ytterbium have relatively low Ln2 + /Ln3+ potentials. 4. Experimental 4.1. General procedures All experiments were performed in evacuated tubes using standard Schlenk techniques, thus excluding traces of air and water. Solvents (DME and THF) were purified by distillation from sodium/benzophenone ketyl. THF for spectral studies was purified by treatment NdI2, as described previously [14]. Silylamide complexes Ln[N(SiMe3)2]3, cyclopentadienyl complexes Ln(Cp)3 and methylcyclopentadienyl complexes Ln(CpMe)3 (Ln = Y, Nd, Tm, Yb) were prepared according to the published procedure [15,16]. 8Hydroxyquinoline and pentafluorophenol were obtained from commercial sources, (2-benzothiazol-2-yl)naphthol were synthesized as described earlier [17]. The C, H, N elemental analyses were performed by the Microanalytical laboratory of IOMC on Euro EA 3000 elemental analyser. The lanthanides content was analyzed by complexometric titration. IR spectra were obtained on a PerkinElmer 577 spectrometer and recorded from 4000 to 450 cm1 as a Nujol mull on KBr plates. The mass spectra were recorded on a Bruker Microflex LT mass spectrometer. The samples were excited at a wavelength of 337 nm at a maximum pulsed laser beam intensity of 150 mJ/pulse at 60 Hz. Absorption spectra were recorded on a UV–vis instrument PerkinElmer Lambda-25 from
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200 to 800 nm. Emission spectra were registered from 300 to 700 nm on a fluorescent spectrometer PerkinElmer LS-55.
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spectrometer Acton-2300 and detected with a cooled InP/InGaAsbased PMT Hamamatsu H10330A-75 in the IR range (900– 1500 nm).
4.2. Synthesis 4.4. Device fabrication 4.2.1. Synthesis of Yb(L)3(phen) (3) A solution of 3-(2-benzothiazol-2-yl)-2-naphthole (107 mg, 0.39 mmol) and 1,10-phenanthroline (23 mg, 0.13 mmol) in DME (10 ml) was added to a solution of Yb[N(SiMe3)2]3 (84 mg, 0.13 mmol) in 5 ml DME. The reaction mixture was stirred for 30 min at the room temperature. DME and volatile products were removed in vacuum and remaining oily substance was extracted with mixture of DME and hexane. The solution was separated from the precipitate by decantation. The solid product dried under vacuum to give 3 as the yellow powder. Yield 112 mg (74%). Anal. Calcd. (%) for C63H38N5O3S3Yb (1182.24): C, 64.00; H, 3.24; N, 5.92; S, 8.14; Yb, 14.64. Found (%): C, 63.98; H, 3.21; N, 5.97; S, 8.11; Yb, 14.69. IR (n, cm1): 1623 (s), 1587 (s), 1516 (w), 1424 (m), 1376 (m), 1342 (s), 1317 (w), 1289 (m), 1274 (m), 1186 (m), 1142 (m), 1124 (m), 1100 (w), 1014 (w), 973 (w), 949 (m), 922 (w), 864 (s), 842 (m), 810 (w), 783 (w), 758 (s), 691 (w), 630 (m), 566 (w), 525 (w), 503 (w). 4.2.2. Synthesis of Yb4(L)(Q)11 3DME (6) A solution of 8-hydroxyquinoline (25 mg, 0.17 mmol) in DME (5 ml) was added to a solution of Yb(L)(Cp)2 (50 mg, 0.086 mmol) in 5 ml DME. The reaction mixture was stirred for 30 min at 40 E. Slow cooling of the mixture to room temperature afforded yellow crystals of 6 which were separated by decantation, washed with cold DME and dried in vacuum. Yield 39 mg (64%). Anal. Calcd. (%) for C128H139N12O18SYb4 (2857.77): C, 53.80; H, 4.90; N, 5.88; S, 1.12; Yb, 24.22. Found (%): C, 53.82; H, 4.87; N, 5.91; S, 1.15; Yb, 24.25. IR (n, cm1): 1625 (w), 1600 (w), 1587 (w), 1574 (m), 1498 (m), 1377 (s), 1347 (w), 1327 (w), 1315 (m), 1272 (m), 1232 (w), 1189 (w), 1175 (w), 1142 (w), 1104 (m), 1015 (m), 949 (w), 863 (w), 823 (w), 803 (w), 786 (m), 752 (m), 649 (w), 627 (w), 609 (w), 593 (w), 569 (w), 492 (w). 4.2.3. Synthesis of Yb2(L)4(Cp)2 (8) A solution of 3-(2-benzothiazol-2-yl)-2-naphthole (90 mg, 0.32 mmol) in DME (5 ml) was added to a solution of YbCp3 (60 mg, 0.16 mmol) in 5 ml DME. The reaction mixture was stirred for 30 min at 40 E. The precipitated orange–yellow crystals of 8 were separated by decantation, washed with cold DME and dried in vacuum. Yield 90 mg (70%). Anal. Calcd. (%) for C78H48N4O4S4Yb2 (1579.58): C, 59.31; H, 3.06; N, 3.55; S, 8.12; Yb, 21.91. Found (%): C, 59.38; H, 3.03; N, 3.57; S, 8.15; Yb, 21.92. IR (n, cm1): 1626 (s), 1587 (s), 1490 (s), 1450 (s), 1343 (s), 1275 (s), 1192 (m), 1170 (m), 1145 (m), 1103 (s), 1010 (m), 947 (s), 920 (m), 883 (m), 863 (m), 806 (m), 777 (s), 752 (s), 627 (m), 566 (m), 497 (m), 465 (s). 4.3. Time-resolved spectroscopy The PL decay times were measured under pulsed excitation with a third harmonic of a Spectra-Physics Nd:YAG laser at 355 nm (pulse duration – 10 ns). The PL signal was dispersed with a grating
The simple non-doped three-layer devices ITO/TPD (20 nm)/Lncomplex (50 nm)/BATH (20 nm)/Yb (150 nm), consisting of triphenyldiamine derivative (TPD) as a hole transport layer, 4,7-diphenyl1,10-phenanthroline (BATH) as a hole-blocking layer and a lanthanide chelated heterocyclic complex as an emissive layer, were fabricated in a vacuum chamber (106 Torr) with different resistive heaters for organic and metal layers. A commercial ITO on a glass substrate with 10 V/& was used as the anode material (Luminescence Technology Corp.) and commercial Yb, 99.9% trace metals basis (Sigma–Aldrich) as the cathode material. The deposition rate for the organic compounds and metallocomplexes was 1 nm s1.The active area of the devices was 5 mm 5 mm. The EL spectra and current–voltage–luminance characteristics were measured using an Ocean Optics USB-2000 fluorimeter, the computer controlled GW Instek PPE 3323 power supply and GW Instek GDM 8246 digital multimeter under ambient conditions. Acknowledgment This work was supported by the Russian Scientific Foundation (project 14-13-01158). References [1] S.I. Weissman, J. Chem. Phys. 10 (1942) 214–217. [2] S.V. Eliseeva, J.-C.G. Bunzli, Chem. Soc. Rev. 39 (2010) 189–227. [3] W.D. Horrocks Jr., J.P. Bolender, W.D. Smith, R.M. Supkowski, J. Am. Chem. Soc. 119 (1997) 5972–5973. [4] Y. Zhong, L. Si, H. He, A.G. Sykes, Dalton Trans. 40 (2011) 11389–11395. [5] M.A. Katkova, A.P. Pushkarev, T.V. Balashova, A.N. Konev, G.K. Fukin, S.Y. Ketkov, M.N. Bochkarev, J. Mater. Chem. 21 (2011) 16611–16620. [6] A.P. Pushkarev, V.A. Ilichev, T.V. Balashova, D.L. Vorozhtsov, M.E. Burin, D.M. Kuzyaev, G.K. Fukin, B.A. Andreev, D.I. Kryzhkov, A.N. Yablonskiy, M.N. Bochkarev, Russ. Chem. Bull. 62 (2013) 392–397. [7] H. Wei, G. Yu, Z. Zhao, Z. Liu, Z. Bian, C. Huang, Dalton Trans. 42 (2013) 8951–8960. [8] T.V. Balashova, A.P. Pushkarev, R.V. Rumyantcev, G.K. Fukin, I.D. Grishin, M.N. Bochkarev, J. Organomet. Chem. 777 (2015) 42–49. [9] M.E. Burin, T.V. Balashova, D.L. Vorozhtsov, A.P. Pushkarev, M.A. Samsonov, G.K. Fukin, M.N. Bochkarev, Russ. J. Coord. Chem. 39 (2013) 667–679. [10] E.V. Baranov, G.K. Fukin, T.V. Balashova, A.P. Pushkarev, I.D. Grishin, M.N. Bochkarev, Dalton Trans. 42 (2013) 15699–15705. [11] A.P. Pushkarev, V.A. Ilichev, A.A. Maleev, A.A. Fagin, A.N. Konev, A.F. Shestakov, R.V. Rumyantzev, G.K. Fukin, M.N. Bochkarev, J. Mater. Chem. C 2 (2014) 1532–1538. [12] H.D. Burrows, M. Fernandes, J.S. de Melo, A.P. Monkman, S. Navaratnam, J. Am. Chem. Soc. 125 (2003) 15310–15311. [13] C.-K. Duan, P.A. Tanner, J. Phys. Chem. A 114 (2010) 6055–6062. [14] M.N. Bochkarev, A.A. Fagin, N.O. Druzhkov, J. Organomet. Chem. 695 (2010) 2774–2780. [15] G. Wilkinson, J.M. Birmingham, J. Am. Chem. Soc. 76 (1954) 6210. [16] J.M. Birmingham, G. Wilkinson, J. Am. Chem. Soc. 78 (1956) 42–44. [17] M.E. Burin, D.M. Kuzyaev, M.A. Lopatin, A.P. Pushkarev, V.A. Ilichev, D.L. Vorozhtsov, A.V. Dmitriev, D.A. Lypenko, E.I. Maltsev, M.N. Bochkarev, Synth. Met. 164 (2013) 55–59.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Synthesis and luminescent properties of heteroleptic benzothiazolyl– naphtholates of ytterbium Anatoly P. Pushkarev a, * , Vasily A. Ilichev a,b , Tatyana V. Balashova a,b , Artem N. Yablonskiy b,c, Boris A. Andreev b,c, Mikhail N. Bochkarev a,b, ** a b c
G.A. Razuvaev Institute of Organometallic Chemistry of Russian Academy of Sciences, Tropinina 49, 603950 Nizhny Novgorod, Russian Federation Nizhny Novgorod State University, Gagarina avenue 23/2, 603950 Nizhny Novgorod, Russian Federation Institute for Physics of Microstructures of Russian Academy of Sciences, 7 ul. Akademicheskaya, 603950 Nizhny Novgorod, Russian Federation
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 December 2014 Received in revised form 18 February 2015 Accepted 20 February 2015 Available online xxx
Photoluminescent (PL) and electroluminescent (EL) properties of heteroleptic complexes of Yb3+ containing the 3-(2-benzothiazol-2-yl)-2-naphtholate (L), 8-oxyquinolate (Q), pentafluorophenolate (OC6F5), or cyclopentadienyl (Cp) ligands were studied. All the complexes revealed NIR emission of Yb3+ ion of different intensity. The obtained data indirectly confirm the fact that the excitation of the ytterbium in naphtholate complexes proceeds due to intramolecular reduction–oxidation mechanism. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence Electroluminescence Heteroleptic complexes Ytterbium Excitation mechanism
1. Introduction The conventional simplified mechanism of excitation of lanthanide ions in the organolanthanide complexes involves excitation of coordinating ligands resulting in population of the singlet state, subsequent transfer of the excitation energy to the triplet state, from which the energy is transferred to resonant level of Ln3+ ion [1]. Such a scheme describes so called antenna effect which is manifested in an increase intensity of the metal-centered emission in complexes where the Ln3+ ion is coordinated by the sensitizing ligands. It has been found experimentally that effective energy transfer from the triplet state of ligand to respective f*-level of Ln3+ is realized when the energy gap between these levels is in the range 2500–3500 cm1 [2]. However, studying the luminescent properties of lanthanide-containing proteins Horrocks and coworkers have found that ytterbium derivatives reveal an efficient metal-centered emission although the difference between
* Corresponding author. Tel: +7 831 4627709; fax: +7 831 4627497. ** Corresponding author at: G.A. Razuvaev Institute of Organometallic Chemistry of Russian Academy of Sciences, Tropinina 49, 603950 Nizhny Novgorod, Russian Federation. Tel: +7 831 4627709; fax: +7 831 4627497. E-mail address: [email protected] (A.P. Pushkarev). http://dx.doi.org/10.1016/j.synthmet.2015.02.030 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.
the triplet level of the triptophan ligand and the sole exited level of Yb3+ (2F5/2) does not fall in the mentioned range [3]. A similar situation had taken place in the case of ytterbium complex with modified 8-oxyquinolate ligands [4]. To explain this phenomenon a new mechanism of excitation, which includes electron transfer from photoexcited anionic ligand to Ln3+ ion and formation of intermediate systems containing Ln2+ ion had been proposed [3]. Exploring the luminescent properties of lanthanide complexes with substituted phenolate and naphtholate ligands we found that efficiency of the metal-centered emission of ytterbium derivatives is much higher than that of the complexes with other lanthanides [5,6]. Especially impressive result was obtained with benzithiazolyl-naphtholate of ytterbium which demonstrated EL at 978 nm (2F5/2 ! 2F7/2 transition) with intensity of 890 mW/cm2, which is ten times greater than that of the best organolanthanide NIR electroluminophore [7]. Since in this case the conventional threesteps excitation scheme is also inapplicable for explanation of the observed hyperemission of Yb we have assumed that the redox electroexcitation mechanism that is similar to Horrocks’s mechanism of PL can be realized in the EL processes [6]. To check this assumption we have studied PL and EL properties of a series of heteroleptic ytterbium complexes containing, along with benzothiazolyl–naphtholate, 8-quinolinolate, pentafluorophenolate and cyclopentadienyl ligands each of which has different
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Fig. 1. LDI-TOF mass spectrum of complex 3.
sensitizing ability. The synthesis and structures of most of the used compounds were described previously [6,8,9] but a few of them were prepared and studied in this work.
hydroxyquinoline in DME. Complex 8 was synthesized from YbCp3 and 3-(2-benzothiazol-2-yl)-2-naphthole in a ratio of reagents 1:2 in DME solution. The complex is a dimer similar to the previously obtained yttrium and neodymium compounds [8].
2. Results and discussion 2.2. Luminescence study 2.1. Synthesis To expand the list of ytterbium compounds for investigation of the effect of ligands on the efficiency of metal-centered emission in addition to the previously obtained complexes we synthesized new heteroleptic complexes Yb(L)3(phen) (3), Yb4(L)(Q)11 (6) and Yb2(L)4(Cp)2 (8). The first of these was prepared by the reaction of Yb[N(SiMe3)2]3 with 3-(2-benzothiazol-2-yl)-2-naphthole in the presence of phenanthroline. Unlike previously studied reactions of lanthanide silylamides with substituted phenoles and naphtholes which afford binuclear complexes Ln2(L)6 [6], this process led to formation of mononuclear product 3 that was confirmed by LDITOF spectrometry (Fig. 1). The tetranuclear complex 6 was obtained in 64% yield by reaction Yb(L)(Cp)2 and 8-
PL and EL spectra of all the studied complexes contain emission bands of the ligands in the region of 350–600 nm and a characteristic bands of 2F5/2 ! 2F7/2 transition of Yb3+ ion at 978 nm accompanied by Stark’s splitting bands in the region 1000– 1100 nm (Figs. 2 and 3). The maximal intensity of NIR irradiation, power efficiency and internal quantum yield of OLEDs based on the benzothiazolyl–naphtholate complexes of Yb3+ as well as relative intensities of NIR bands in the PL spectra of these compounds are collected in the Table 1. To simplify the evaluation of relative intensities of the bands of metal-centered emission in the PL spectra the solid samples were used for the measurements. Comparing the band intensity of Yb3+ ions in the EL spectra, power efficiency and external quantum yield (EQE) of the tested
Fig. 2. PL spectra of compounds 1–9,11–13.
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Fig. 3. EL spectra of compounds 3–5.
compounds showed that the previously studied homoleptic dimeric complex 1 has the highest characteristics. Other complexes with L ligands 2–6 also exhibited emission at 978 nm efficiency of which is much higher than that of L free compounds 7, 13 and complex 6, containing one L group per four Yb atoms, but three–four times lower than that of 1. Note that high power efficiency and EQE of OLED based on 13 is explained by low charge transporting properties of this compound causing low operating current. The difference in the EL characteristics of 2–5 is insignificant and is explained apparently by some differences in the charge transport properties of these materials. Since the energies of triplet levels of L 16,010 cm1 and Q 16,942 cm1 [12] ligands are considerably higher than the energy of resonant level of ytterbium 2F5.2 10,248 cm1 [13] both these ligands, according to the conventional excitation scheme, can not be good sensitizers for Yb3+ ions. However, as it can be seen from Table 1, the effect of L and Q ligands on the EL efficiency of ytterbium is dramatically different. While homoleptic 8-oxyquinolate complex 7 shows very low efficiency of ytterbium, which agrees well with the above rule of optimal 3T1 – f * energy gap, the naphtholate 1 exhibits a record high intensity of NIR luminescence, which contradicts to this rule. The heteroleptic complexes 2–6 occupy an intermediate position. These data suggest: (i) the sensitization of Yb3+ by Q ligands proceeds according to the conventional three-level scheme but sensitization of ytterbium by
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L ligands is realized by another way: probably according to redox mechanism which allows to achieve high population of 2F5/2 excited level of Yb3+; (ii) in the binuclear complexes only one of two Yb–L groupings is involved into luminescence process; (iii) in this grouping the L is a terminal ligands rather than bridging. Comparison of data for the complex Yb4(L)(Q)11 containing monodentate L ligand and the data for the other naphtholate– quinolate complexes containing chelate L ligands (Table 1) indicates that the sensitizing ability of the benzothiasolyl– naphtholate ligands in the studied complexes depends on the type of metal–ligand bonding. Instability in air prevented the measurement of EL characteristics of cyclopentadienyl and pentafluorophenolate derivatives but they were used in the PL study which showed that the luminescence upon the photo- and electroexcitation obeys the same regularities, i.e., the metal-centered emission of the complexes containing L ligands is much higher than that of L free compounds. Mononuclear and binuclear complexes containing at least one Yb–L fragment exhibit the same or close intensity of the band at 978 nm (complexes 1–4, 11, 12). The cyclopentadienyl compounds 8 and 9 revealed reduced NIR efficiency (probably due to quenching effect of Cp ligands) but it is also equal for both complexes. The found regularity is broken in the case of complexes 5 and 6: the compounds have Yb–L groupings but display moderate (for 5) or weak (for 6) metal-centered emission. In the case of 6 the reason of weak Yb luminescence apparently is h1 coordination mode of L. The reduced NIR luminescence of 5 probably is due to specific crystal packing but confirmation of this could not be obtained because of the low quality of the crystals. Thus, the data of PL study suggest that the sensitization of luminescence of Yb3+ ions upon photoexcitation by the ligands L and Q (or C6F5O) also has a different nature. In favor of an electron transfer excitation mechanism in the ytterbium–naphtholate complexes says comparison of PL spectra of 6 and its yttrium analog Y4(L)(Q)11 in visible region (Fig. 4). The latter contains two bands at 420 and 528 nm the first of which is assigned to p– p* transitions in the aromatic ligands and the second one is typical for ligand-metal charge transfer (LMCT) whereas the spectrum of the ytterbium complex comprises only one band at 420 nm. It is reasonable to assume that in the ytterbium complex the complete charge transfer, i.e., electron
Table 1 Efficiency of NIR emission of OLEDs based on ytterbium complexes and relative intensity of the Yb+3 band in the PL spectra of the same complexes. Complex
Yb2(L)6d Yb(L)4Lie Yb(L)3(phen) Yb2(L)4(Q)2 Yb2(L)2(Q)4 Yb4(L)(Q)11 Yb3(Q)9f Yb(L)2(Cp) Yb(L)(Cp)2 Yb(Cp)3 Yb2(L)4(OC6F5)2 Yb2(L)2(OC6F5)4 Yb(OC6F5)3(phen)g a b c d e f g
No.
1 2 3 4 5 6 7 8 9 10 11 12 13
ELa
PL intensity arb.un.
Max. irradiance mW/cm2 (V)b
Power efficiency mW/W
EQE (%)c
889 (19.2) 220 (21) 149 (27.5) 251 (27) 314 (30) – 47 (16) – – – – – 94 (20)
0.915 0.36 0.1 0.2 0.09 – 0.013 – – – – – 0.823
1.6 0.4 0.17 0.32 0.16 – 0.023 – – – – – 1.4
Obtained with OLED devices of configuration ITO/TPD/complex/BATH/Yb. In the brackets voltage at which the maximum irradiance was observed. At driving voltage 12 V. EL data from [6]. EL data from [9]. El data from [10]. EL data from [11].
60 60 60 60 26 11 20 18 18 0 60 54 12
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luminescence decay curves for 1–4, 7, 11 and 12 are described by simple exponential characteristic. At first sight this fact manifests interligand energy transfer from triplet excited state of Q (4) or OC6F5 (11, 12) to 3T1 state of L ligand and further to 2F5/2 resonant level of Yb3+ because otherwise the exponential fitting of decay curves would be more complicated. However, the data obtained do not contradict the proposed redox mechanism of Yb3+ excitation since the observed different decay times may be related to the different rates of the redox processes which occur in the 3-(2benzothiazol-2-yl)-2-naphtholate containing complexes. 3. Conclusions Fig. 4. Normalized PL spectra of Ln4(L)(Q)11, (Ln = Y, Yb) in THF solution, lex 360 nm.
transfer from a ligand to Yb3+ occurs resulting in disappearance of the LMCT band and implementation of redox mechanism of excitation of Yb3+ ions. In order to get additional evidence of the existence of different excitation mechanisms in the synthesized complexes timeresolved spectroscopic measurements were carried out. The measurements of Yb3+ metal-centered emission decay were performed at lmon = 978 nm using laser pulse irradiation with lex = 355 nm (decay curves are depicted in Fig. 5). It was found that the decay times for the compounds are within the range of 2– 10 ms, which is typical for organolanthanide derivatives of ytterbium: 6.98 (1), 9.92 (2), 8.36 (3), 8.91 (4), 3.89 (5), 2.02 (6), 9.91 (7), 1.69 (8), 0.98 (9), 9.45 (11), 7.06 (12) and 2.29 ms (13). A luminescence lifetime of Ln3+ ion surrounded by organic ligands may give some useful information for the rational molecular design of efficient emissive materials as it is sensitive to electrostatic ligand field symmetry at the ion site, considerably depends on the presence of luminescence quenching groups as well as luminescence concentration quenching and also reflects sophisticated energy transfer processes which may occur in the luminescent lanthanide containing systems. In terms of ligand field symmetry at the Yb3+ site one can notice that the metal ion in the derivatives 1 [6], 2 [9], 4 [8] and 7 [10] is eight coordinated and has a distorted antiprismatic environment. On the contrary, the Yb3+ ions in the compounds 8 and 9 [8] are ten and twelve coordinated, respectively and have unsymmetrical environment. Since the ligand field symmetry reduction at the ion site leads to increasing of radiative probability of electric dipole transitions the luminescence lifetime in the case of 8 and 9 should be shorter than that of 1, 2, 4 and 7. On the other hand, the organolanthanides 6, 8, 9 and 13 can demonstrate fast luminescence decay due to the presence of non-chelating L ligand, Cp groups or neutral phenanthroline ligand having a lot of C—H groups which effectively quench NIR luminescence. It should be noted also that
Fig. 5. Metal-centered luminescence decay curves for the complexes 1–9 and 11– 13.
To get evidence of the existence of different excitation mechanisms of Yb3+ ions in organic complexes the data of EL and PL of a series of mononuclear, dinuclear and tetranuclear homoleptic and heteroleptic complexes of ytterbium with 3-(2benzothiazol-2-yl)-2-naphtholate, 8-oxyquinolate, pentafluorophenolate and cyclopentadienyl ligands were collected and analyzed. The data obtained indicate that the benzothiazolyl– naphthyl ligands sensitize the metal-centered luminescence of Yb3 + much more efficiently than others used organic ligands. The conventional three-level energy diagram of the luminescence of organolanthanide emitters can not explain the observed phenomenon since the energy gap between the resonant 2F5/2 level of ytterbium and triplet level of the ligand L (as well as the ligands Q and C6F5O) about 6000 cm1 is much more the optimal value 2500–3500 cm1. On the contrary, the results are in good agreement with the redox mechanism of luminescence, which explains the high population of the excited state of ytterbium when some types of ligands are used. Taking into account that enhanced luminescence of Yb3+ was observed as well in the benzoxasolyl– naphtholates [6], benzoxasolyl–phenolates, benzothiasolyl–phenolates [5] complexes one may assume that S^N- and O^N-chelate aromatic ligands are suitable for the intramolecular ligand to metal electron transfer and realization of specific chemiluminescence of ytterbium. It should be noted that such an approach opens the way for the search of new highly luminescent complexes of not only ytterbium but as well samarium, europium and probably thulium because these lanthanides like ytterbium have relatively low Ln2 + /Ln3+ potentials. 4. Experimental 4.1. General procedures All experiments were performed in evacuated tubes using standard Schlenk techniques, thus excluding traces of air and water. Solvents (DME and THF) were purified by distillation from sodium/benzophenone ketyl. THF for spectral studies was purified by treatment NdI2, as described previously [14]. Silylamide complexes Ln[N(SiMe3)2]3, cyclopentadienyl complexes Ln(Cp)3 and methylcyclopentadienyl complexes Ln(CpMe)3 (Ln = Y, Nd, Tm, Yb) were prepared according to the published procedure [15,16]. 8Hydroxyquinoline and pentafluorophenol were obtained from commercial sources, (2-benzothiazol-2-yl)naphthol were synthesized as described earlier [17]. The C, H, N elemental analyses were performed by the Microanalytical laboratory of IOMC on Euro EA 3000 elemental analyser. The lanthanides content was analyzed by complexometric titration. IR spectra were obtained on a PerkinElmer 577 spectrometer and recorded from 4000 to 450 cm1 as a Nujol mull on KBr plates. The mass spectra were recorded on a Bruker Microflex LT mass spectrometer. The samples were excited at a wavelength of 337 nm at a maximum pulsed laser beam intensity of 150 mJ/pulse at 60 Hz. Absorption spectra were recorded on a UV–vis instrument PerkinElmer Lambda-25 from
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200 to 800 nm. Emission spectra were registered from 300 to 700 nm on a fluorescent spectrometer PerkinElmer LS-55.
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spectrometer Acton-2300 and detected with a cooled InP/InGaAsbased PMT Hamamatsu H10330A-75 in the IR range (900– 1500 nm).
4.2. Synthesis 4.4. Device fabrication 4.2.1. Synthesis of Yb(L)3(phen) (3) A solution of 3-(2-benzothiazol-2-yl)-2-naphthole (107 mg, 0.39 mmol) and 1,10-phenanthroline (23 mg, 0.13 mmol) in DME (10 ml) was added to a solution of Yb[N(SiMe3)2]3 (84 mg, 0.13 mmol) in 5 ml DME. The reaction mixture was stirred for 30 min at the room temperature. DME and volatile products were removed in vacuum and remaining oily substance was extracted with mixture of DME and hexane. The solution was separated from the precipitate by decantation. The solid product dried under vacuum to give 3 as the yellow powder. Yield 112 mg (74%). Anal. Calcd. (%) for C63H38N5O3S3Yb (1182.24): C, 64.00; H, 3.24; N, 5.92; S, 8.14; Yb, 14.64. Found (%): C, 63.98; H, 3.21; N, 5.97; S, 8.11; Yb, 14.69. IR (n, cm1): 1623 (s), 1587 (s), 1516 (w), 1424 (m), 1376 (m), 1342 (s), 1317 (w), 1289 (m), 1274 (m), 1186 (m), 1142 (m), 1124 (m), 1100 (w), 1014 (w), 973 (w), 949 (m), 922 (w), 864 (s), 842 (m), 810 (w), 783 (w), 758 (s), 691 (w), 630 (m), 566 (w), 525 (w), 503 (w). 4.2.2. Synthesis of Yb4(L)(Q)11 3DME (6) A solution of 8-hydroxyquinoline (25 mg, 0.17 mmol) in DME (5 ml) was added to a solution of Yb(L)(Cp)2 (50 mg, 0.086 mmol) in 5 ml DME. The reaction mixture was stirred for 30 min at 40 E. Slow cooling of the mixture to room temperature afforded yellow crystals of 6 which were separated by decantation, washed with cold DME and dried in vacuum. Yield 39 mg (64%). Anal. Calcd. (%) for C128H139N12O18SYb4 (2857.77): C, 53.80; H, 4.90; N, 5.88; S, 1.12; Yb, 24.22. Found (%): C, 53.82; H, 4.87; N, 5.91; S, 1.15; Yb, 24.25. IR (n, cm1): 1625 (w), 1600 (w), 1587 (w), 1574 (m), 1498 (m), 1377 (s), 1347 (w), 1327 (w), 1315 (m), 1272 (m), 1232 (w), 1189 (w), 1175 (w), 1142 (w), 1104 (m), 1015 (m), 949 (w), 863 (w), 823 (w), 803 (w), 786 (m), 752 (m), 649 (w), 627 (w), 609 (w), 593 (w), 569 (w), 492 (w). 4.2.3. Synthesis of Yb2(L)4(Cp)2 (8) A solution of 3-(2-benzothiazol-2-yl)-2-naphthole (90 mg, 0.32 mmol) in DME (5 ml) was added to a solution of YbCp3 (60 mg, 0.16 mmol) in 5 ml DME. The reaction mixture was stirred for 30 min at 40 E. The precipitated orange–yellow crystals of 8 were separated by decantation, washed with cold DME and dried in vacuum. Yield 90 mg (70%). Anal. Calcd. (%) for C78H48N4O4S4Yb2 (1579.58): C, 59.31; H, 3.06; N, 3.55; S, 8.12; Yb, 21.91. Found (%): C, 59.38; H, 3.03; N, 3.57; S, 8.15; Yb, 21.92. IR (n, cm1): 1626 (s), 1587 (s), 1490 (s), 1450 (s), 1343 (s), 1275 (s), 1192 (m), 1170 (m), 1145 (m), 1103 (s), 1010 (m), 947 (s), 920 (m), 883 (m), 863 (m), 806 (m), 777 (s), 752 (s), 627 (m), 566 (m), 497 (m), 465 (s). 4.3. Time-resolved spectroscopy The PL decay times were measured under pulsed excitation with a third harmonic of a Spectra-Physics Nd:YAG laser at 355 nm (pulse duration – 10 ns). The PL signal was dispersed with a grating
The simple non-doped three-layer devices ITO/TPD (20 nm)/Lncomplex (50 nm)/BATH (20 nm)/Yb (150 nm), consisting of triphenyldiamine derivative (TPD) as a hole transport layer, 4,7-diphenyl1,10-phenanthroline (BATH) as a hole-blocking layer and a lanthanide chelated heterocyclic complex as an emissive layer, were fabricated in a vacuum chamber (106 Torr) with different resistive heaters for organic and metal layers. A commercial ITO on a glass substrate with 10 V/& was used as the anode material (Luminescence Technology Corp.) and commercial Yb, 99.9% trace metals basis (Sigma–Aldrich) as the cathode material. The deposition rate for the organic compounds and metallocomplexes was 1 nm s1.The active area of the devices was 5 mm 5 mm. The EL spectra and current–voltage–luminance characteristics were measured using an Ocean Optics USB-2000 fluorimeter, the computer controlled GW Instek PPE 3323 power supply and GW Instek GDM 8246 digital multimeter under ambient conditions. Acknowledgment This work was supported by the Russian Scientific Foundation (project 14-13-01158). References [1] S.I. Weissman, J. Chem. Phys. 10 (1942) 214–217. [2] S.V. Eliseeva, J.-C.G. Bunzli, Chem. Soc. Rev. 39 (2010) 189–227. [3] W.D. Horrocks Jr., J.P. Bolender, W.D. Smith, R.M. Supkowski, J. Am. Chem. Soc. 119 (1997) 5972–5973. [4] Y. Zhong, L. Si, H. He, A.G. Sykes, Dalton Trans. 40 (2011) 11389–11395. [5] M.A. Katkova, A.P. Pushkarev, T.V. Balashova, A.N. Konev, G.K. Fukin, S.Y. Ketkov, M.N. Bochkarev, J. Mater. Chem. 21 (2011) 16611–16620. [6] A.P. Pushkarev, V.A. Ilichev, T.V. Balashova, D.L. Vorozhtsov, M.E. Burin, D.M. Kuzyaev, G.K. Fukin, B.A. Andreev, D.I. Kryzhkov, A.N. Yablonskiy, M.N. Bochkarev, Russ. Chem. Bull. 62 (2013) 392–397. [7] H. Wei, G. Yu, Z. Zhao, Z. Liu, Z. Bian, C. Huang, Dalton Trans. 42 (2013) 8951–8960. [8] T.V. Balashova, A.P. Pushkarev, R.V. Rumyantcev, G.K. Fukin, I.D. Grishin, M.N. Bochkarev, J. Organomet. Chem. 777 (2015) 42–49. [9] M.E. Burin, T.V. Balashova, D.L. Vorozhtsov, A.P. Pushkarev, M.A. Samsonov, G.K. Fukin, M.N. Bochkarev, Russ. J. Coord. Chem. 39 (2013) 667–679. [10] E.V. Baranov, G.K. Fukin, T.V. Balashova, A.P. Pushkarev, I.D. Grishin, M.N. Bochkarev, Dalton Trans. 42 (2013) 15699–15705. [11] A.P. Pushkarev, V.A. Ilichev, A.A. Maleev, A.A. Fagin, A.N. Konev, A.F. Shestakov, R.V. Rumyantzev, G.K. Fukin, M.N. Bochkarev, J. Mater. Chem. C 2 (2014) 1532–1538. [12] H.D. Burrows, M. Fernandes, J.S. de Melo, A.P. Monkman, S. Navaratnam, J. Am. Chem. Soc. 125 (2003) 15310–15311. [13] C.-K. Duan, P.A. Tanner, J. Phys. Chem. A 114 (2010) 6055–6062. [14] M.N. Bochkarev, A.A. Fagin, N.O. Druzhkov, J. Organomet. Chem. 695 (2010) 2774–2780. [15] G. Wilkinson, J.M. Birmingham, J. Am. Chem. Soc. 76 (1954) 6210. [16] J.M. Birmingham, G. Wilkinson, J. Am. Chem. Soc. 78 (1956) 42–44. [17] M.E. Burin, D.M. Kuzyaev, M.A. Lopatin, A.P. Pushkarev, V.A. Ilichev, D.L. Vorozhtsov, A.V. Dmitriev, D.A. Lypenko, E.I. Maltsev, M.N. Bochkarev, Synth. Met. 164 (2013) 55–59.