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A thiocyanate-bridging dimeric cyclometalated iridium(III) complex: Synthesis, structure and phosphorescence behaviours towards metal ions
Inorganic Chemistry Communications 14 (2011) 1937–1939
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
A thiocyanate-bridging dimeric cyclometalated iridium(III) complex: Synthesis, structure and phosphorescence behaviours towards metal ions Bihai Tong a,⁎, Jia-Yan Qiang a, Ya-Qing Xu a, Qunbo Mei b, Taike Duan a, Qun Chen c, Qian-Feng Zhang a, c,⁎⁎ a
Institute of Molecular Engineering and Applied Chemistry, College of Metallurgy and Resources, Anhui University of Technology, Ma'anshan, Anhui 243002, PR China Jiangsu Key Lab of Organic Electronics & Information Displays, Institute of Advanced Materials (IAM), Nanjing University of Posts & Telecommunications (NUPT), Nanjing 210046, PR China c Department of Applied Chemistry, School of Petrochemical Engineering, Changzhou University, Jiangsu 213164, PR China b
a r t i c l e
i n f o
Article history: Received 12 July 2011 Accepted 16 August 2011 Available online 21 September 2011 Keywords: Cyclometalated iridium(III) Dimeric complex Crystal structure Photoluminescence Phosphorescence
a b s t r a c t A new neutral dimeric cyclometalated iridium complex containing bridging thiocyanate ligands, [{Ir(μ-SCN) (pqcm)2}2] (1, pqcmH = 2-phenyl-quinoline-4- carboxylic acid methyl ester), has been synthesized and structurally characterized. The photoluminescence (PL) spectrum of 1 shows emission maximum at 638 nm with a lifetime of 0.11 μs and the PL quantum yield is ca. 0.012. The phosphorescence behaviours of 1 towards different solvents and metal ions were also investigated and the strong phosphorescence quenching by acetonitrile and two equivalents of Hg 2+, Cu2+ and Ag + ions were observed. © 2011 Elsevier B.V. All rights reserved.
Luminescent cyclometalated iridium(III) complexes have been widely used in organic light-emitting diodes (OLEDs), luminescence sensitizers, and biological imaging due to their high quantum efficiency and color tenability [1]. Mononuclear iridium(III) bis-cyclometalated complexes are generally synthesized from dinuclear chloro-bridging dimers [{Ir(μ-Cl)(N^C)2}2] (N^C = cyclometalated bidentate ligand). Substitution of [{Ir(μ-Cl)(N^C)2}2] with bidentate O, O or N, O ligands L^L afforded [Ir(N^C)2(L^L)], which have been employed as dopants for OLEDs [2] and sensors for singlet oxygen [3]. Although the cyclometalated iridium(III) chloro-bridging dimers are readily prepared by the method developed by Nonoyama in 1974 [4], the multinuclear iridium(III) complexes are relatively quite rare [5−8]. It has been noted note that the conjugation SCN– or CN– anion was introduced into the cyclometalated iridium(III) complexes to increase the gap between the t2g and LUMO orbitals and sequentially to magnify the quantum yields of this class of complexes. Only two examples of thiocyanate-bridging complexes [{Ir(μ-SCN)}2(N^C)2] have been reported recently [9,10]. In this article, we set up the reaction of [{Ir(μ-Cl)(pqcm)2}2] (pqcmH= 2-phenylquinoline-4-carboxylic acid methyl ester) and KSCN, resulting in the isolation of a new dinuclear iridium(III) complex [{Ir(μ-SCN) (pqcm)2}2] which is composed of two bis-cyclometalated iridium(III) moieties bridged by two thiocyanate ligands. Herein, the initial results
⁎ Corresponding author. Tel./fax: + 86 555 2312041. ⁎⁎ Correspondence to: Q.-F. Zhang, Institute of Molecular Engineering and Applied Chemistry, College of Metallurgy and Resources, Anhui University of Technology, Ma'anshan, Anhui 243002, PR China. Tel./fax: + 86 555 2312041. E-mail addresses: [email protected] (B. Tong), [email protected] (Q.-F. Zhang). 1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2011.08.029
including structural characterization and the phosphorescence behaviour towards different solvents and metal ions are reported in this paper. The neutral thiocyanate-bridging dimeric cyclometalated iridium(III) complex [{Ir(μ-SCN)(pqcm)2}2]·C6H14 (1) was obtained from the reaction of the corresponding chloro-bridging cyclometalated iridium(III) dimer and an excess KSCN in CH2Cl2/CH3OH (1:1) at reflux. 1 is stable in air and soluble in common organic solvents such as CH2Cl2, THF, and toluene. The formulation of 1 was confirmed by NMR, ESI mass spectroscopy, and infrared spectroscopy. The detail descriptions of these characterizations for 1 are available as supplementary materials. The strong absorptions at 2121 and 1729 cm –1 in the IR spectrum of 1 may be attributed to the SCN and C=O stretching vibrations of the coordination thiocyanate ligands and the carboxyl groups in the pqcm ligands, respectively. Complex 1 crystallizes in the monoclinic space group P21/c with four molecules per unit cell. The unit cell of 1 comprises neutral dinuclear complex and disordered hexane solvents molecules in the crystal lattice. The perspective view of 1 is depicted in Fig. 1. Each of two iridium(III) centers in 1 adopts a distorted octahedral coordination geometry and is ligated by two chelating pqcm and two bridging thiocyanate ligands. The metalated carbon atoms of the two pqcm ligands are in a mutually cis arrangement, and the two bridging thiocyanate ligands bind the two iridium centers in a μ-S,N fashion. The sulfur and nitrogen atoms of two different thiocyanate ligands lie in the equatorial plane trans to the metalated carbon atoms of the pqcm moieties. The average Ir–C and Ir–N(pqcm) bond lengths in 1 are 1.976(18) and 2.084(13) Å, respectively, which are in the ranges
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B. Tong et al. / Inorganic Chemistry Communications 14 (2011) 1937–1939
4.0 3.5
294 nm 352 nm
Absorbance
3.0 2.5 2.0 1.5 469 nm
1.0 0.5 300
400
500
600
Wavelength (nm) Fig. 2. UV–vis absorption spectrum of 1 in CH2Cl2.
of the corresponding bond lengths in the cyclometalated iridium(III) complexes [11,12]. The average Ir−S distance of 2.566(5) Å in 1 is longer than those of 2.496(1) Å in [{Ir(Buppy)2}2(μ-SCN)2] (Buppy = 2-(4′tert-butylphenyl)-pyridine) [10]. The Ir–S–C≡N units are approximately linear [C(1)−N(1)−Ir(2) =172.6(13) and C(2)−N(2)−Ir(1) = 162.5(13)°] and the S−Ir−N(SCN) angles are close to 90° [S(1)− Ir(1)−N(2) = 89.9(4)° and S(2)−Ir(2)−N(1) = 86.1(3)°]. Thus, the eight-membered Ir2(SCN)2 core in 1 can be roughly described as a rectangle. In the absorption spectrum of complex 1 (Fig. 2), the band at ca. 294 nm is assigned to a typical spin-allowed intraligand 1π−π* transitions of pqcm in the complex since the free pqcm ligand also absorbs bands at a similar energy. The strong band at 352 nm corresponds likely to spin-allowed metal-to-ligand charge transfer ( 1MLCT) and the weak band at 469 nm probably arises from the formally spin-forbidden 3MLCT transition. The room-temperature photoluminescence spectrum of 1 in degassed CH2Cl2 solution shows that the excitation maximum is dominated by a broad band centered at 585 nm which is an absorption obviously arising from the 3MLCT states. The emission maximum for 1 is an intense luminescence at 638 nm, corresponding to the red light emitting. The excited state lifetime of 1 was determined to be 0.11 μs which is obviously shorter than those of [Ir(ppy)3] (ppy = phenylpyridinato) (2.0 μs) [13] and the other related neutral cyclometalated iridium(III) complexes [14]. This would be advantageous for complex 1 to be a candidate for the design of highly efficient OLED devices. Actually, the phosphorescence quantum efficiency in CH2Cl2 solution is ca. 0.012 used an aqueous solution of [Ru(bpy)3]Cl2 (Φ = 0.042) as the standard solution [15].
Et2O Toluene Dioxane EA DCM THF DMF EtOH ACN NMP
1000 800
Intensity (a.u.)
Fig. 1. The perspective view of 1. Displacement ellipsoids have been drawn at the 35% probability level. Hydrogen atoms are omitted for the clarity. Selected bond lengths (Å) and angles (°): Ir(1)-C(3) 1.98(2), Ir(1)-C(20) 1.975(17), Ir(1)-N(2) 2.085(14), Ir(1)-N(3) 2.101(12), Ir(1)-N(4) 2.062(14), Ir(1)-S(1) 2.566(5), Ir(2)-C(37) 1.951(16), Ir(2)-C(54) 1.997(17), Ir(2)-N(1) 2.147(14), Ir(2)-N(5) 2.085(12), Ir(2)-N(6) 2.089(14), Ir(2)-S(2) 2.567(5); C(20)-Ir(1)-C(3) 88.5(7), C(20)-Ir(1)-N(4) 80.5(6), C(3)-Ir(1)-N(4) 94.7(6), C(20)-Ir(1)-N(2) 173.8(6), C(3)-Ir(1)-N(2) 97.6(6), N(4)-Ir(1)-N(2) 100.2(5), C(20)-Ir(1)-N(3) 93.2(6), C(3)-Ir(1)-N(3) 78.2(6), N(4)-Ir(1)-N(3) 170.6(5), N(2)Ir(1)-N(3) 86.9(4), C(20)-Ir(1)-S(1) 84.1(5), C(3)-Ir(1)-S(1) 171.9(5), N(4)-Ir(1)S(1) 80.8(4), N(2)-Ir(1)-S(1) 89.9(4), N(3)-Ir(1)-S(1) 105.6(4), C(37)-Ir(2)-C(54) 92.5(6), C(37)-Ir(2)-N(5) 79.9(6), C(54)-Ir(2)-N(5) 94.1(5), C(37)-Ir(2)-N(6) 94.4(5), C(54)-Ir(2)-N(6) 79.4(6), N(5)-Ir(2)-N(6) 171.2(5), C(37)-Ir(2)-N(1) 173.5(5), C(54)Ir(2)-N(1) 93.0(6), N(5)-Ir(2)-N(1) 103.1(5), N(6)-Ir(2)-N(1) 83.3(4), C(37)-Ir(2)S(2) 88.8(4), C(54)-Ir(2)-S(2) 173.6(4), N(5)-Ir(2)-S(2) 80.0(4), N(6)-Ir(2)-S(2) 106.7(4), N(1)-Ir(2)-S(2) 86.1(3), C(1)-S(1)-Ir(1) 102.8(6), C(2)-S(2)-Ir(2) 103.3(6), C(1)-N(1)-Ir(2) 172.6(13), C(2)-N(2)-Ir(1) 162.5(13).
The photoluminescence spectra of complex 1 in different solutions and in solid state were tested at room temperature, as shown in Fig. 3. It may be found that complex 1 emits the strongest luminescence in the non-polar Et2O solution and the weakest in the polar NMP solution. The emission maximum wavelengths are obviously red-shifted from 638 nm in Et2O to 673 nm in NMP. This red-shift phenomenon has been found in some related cyclometalated iridium(III) complexes [16]. It is interesting to note that the emission maximum in CH3CN is an exception to the rule, indicated by the emission intense reduced along with the wavelength undergoing a blue-shift, which may infer that the dimeric structure of 1 may be unstable in the CH3CN solution, probably decomposed to be a monomeric complex due to the substitution of the thiocyanate ligands by the stronger acetonitrile molecule. The emission intensity of 1 is rather weak in solid state along with the wavelength red-shifting to be 673 nm. The redshift of the emission spectrum is related to the strong intermolecular interactions, a possible indicative of the strong excited-state quenching of complex 1 in solid state. The effects of different metal ions Hg 2+, Ag +, Cu 2+, Fe 2+, Co 2+, Ni 2+, Zn 2+, Mg 2+, Cr 3+, Pb 2+ and Cd 2+ on the fluorescent spectral properties of 1 were also investigated (see Fig. 4). When two equiv. of Hg 2+ were added to the CH2Cl2 solution of 1, the luminescence intensity of 1 decreased to be ca. 8.8%. The similar changes were observed when the Ag + and Cu 2+ ions were added to the CH2Cl2 solution of 1, accordingly, the luminescence intensity of 1 in CH2Cl2 decreased to be ca. 11.2% and 12.7%, respectively. Obviously, the strong complexation interactions between 1 and Hg 2+ or Ag +/Cu 2+ ion occurred. No obvious spectral variations were observed upon addition of other metal ions, as displayed in Fig. 4.
600 400 200 0 580
600
620
640
660
680
700
720
Wavelength (nm) Fig. 3. The emission spectra of 1 in different solvents.
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B. Tong et al. / Inorganic Chemistry Communications 14 (2011) 1937–1939
200
900
+
Intensity (a.u.)
160 140 120 100 80 60 +
Ag
40
Hg
2+ 2+
Cu
0 600
620
640
660
700 600
400 300
680
700
720
740
0 575
In order to explore the complexation abilities of 1 with metal ions, the changes in UV–vis absorption spectra (see Fig. 5) were investigated by the titration experiments. Upon addition of Hg2+, the absorption bands of 1 at 294 and 352 nm gradually increased firstly then turned to decrease, resulting in formation of a new band centered at 475 nm with a distinct isosbestic point at 445 nm, which may indicate the strong ground state interactions between 1 and the Hg2+ ion. The emission spectra titration of 1 with Hg2+ ion was also tested. It can be observed from Fig. 6 that the emission maximum is almost entirely quenched upon addition of two equiv. of Hg2+ ion which likely leads to an entire dissociation of the thiocyanate-bridging cyclometalated iridium(III) dimer. In summary, we have synthesized a thiocyanate-bridging dimeric cyclometalated iridium(III) complex in a high yield. The emission spectra of the dimer are sensitive to strong polar solvents and heavy metal ions. The present dimeric cyclometalated iridium(III) complex is found to be a candidate for the effective red emitter with a neutral charge which is desirable for an example in the OLED applications, and more related multinuclear cyclometalated iridium(III) complex with new designed ligands are underway in our laboratory.
Acknowledgments This project was supported by the National Natural Science Foundations of China (grant nos. 50903001 and 50803027) and the Program for New Century Excellent Talents in University of China (NCET-08-0618). Bihai Tong thanks the Fund of Anhui University of Technology for Youth (No. QZ200907) for the assistance.
4.0 3.5
Absorbance
3.0 2.5 2.0 eq.
1.5 2+
0 Hg
0.5 0.0 300
400
500
600
625
650
675
700
725
750
Wavelength (nm)
Fig. 4. Fluorescence spectra of 1 in the presence of 2 equiv. of different metal ions.
1.0
2.0 eq.
200
Wavelength (nm)
2.0
2+
0 Hg
500
100
20 580
800
Intensity (a.u.)
Ag 2+ Cd 2+ Co 3+ Cr 2+ Cu 2+ Mg 2+ Pb 2+ Fe 2+ Ni 2+ Zn 2+ Hg
180
1939
600
Wavelength (nm) Fig. 5. Changes in the UV absorption spectra of 1 on addition of Hg2+.
Fig. 6. Changes in the emission spectra of 1 on addition of Hg2+.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi: 10.1016/j.inoche.2011.xx.xxxx (CCDC820784). References [1] Z.Q. Chen, Z.Q. Bian, C.H. Huang, Functional Ir III complexes and their applications, Advanced Materials 22 (2010) 1534–1539. [2] L.X. Xiao, Z.J. Chen, Q. Bo, J.X. Luo, Recent progresses on materials for electrophosphorescent organic light-emitting devices, Advanced Materials 23 (2011) 926–952. [3] P.I. Djurovich, D. Murphy, Cyclometalated iridium and platinum complexes as singlet oxygen photosensitizers: quantum yields, quenching rates and correlation with electronic structures, Dalton Transactions (2007) 3763–3770. [4] M. Nonoyama, Benzo[h]quinolin-10-yl-N iridium (III) complexes, Bulletin of the Chemical Society of Japan 47 (1974) 767–768. [5] V.L. Whittle, J.A. Gareth Williams, Cyclometallated, bis-terdentate iridium complexes as linearly expandable cores for the construction of multimetallic assemblies, Dalton Transactions (2009) 3929–3940. [6] A. Auffrant, A. Barbieri, F. Barigelletti, Bimetallic iridium (III) complexes consisting of Ir(ppy)2 Units (ppy= 2-Phenylpyridine) and two laterally connected N^N chelates as bridge: synthesis, separation, and photophysical properties, Inorganic Chemistry 46 (2007) 6911–6919. [7] A. Auffrant, A. Barbieri, F. Barigelletti, Dinuclear iridium (III) complexes consisting of back-to-back tpy-(ph)n-tpy bridging ligands (n = 0, 1, or 2) and terminal cyclometallating tridentate N-C-N ligands, Inorganic Chemistry 45 (2006) 10990–10997. [8] A.R. McDonald, D. Mores, C.M. Donega, Supramolecular dendriphores: anionic organometallic phosphors embedded in polycationic dendritic species, Organometallics 28 (2009) 1082–1092. [9] M.K. Nazeeruddin, R. Humphry-Baker, D. Berner, S. River, L. Zuppiroli, M. Graetzel, Highly phosphorescence iridium complexes and their application in organic lightemitting devices, Journal of the American Chemical Society 125 (2003) 8790–8797. [10] M.K. Lau, K.M. Cheung, Q.F. Zhang, Y. Song, W.T. Wong, I.D. Williams, W.H. Leung, Iridium (III) and rhodium(III) cyclometalated complexes containing sulfur and selenium donor ligands, Journal of Organometallic Chemistry 689 (2004) 2401–2410. [11] S.J. Lee, K.M. Park, K. Yang, Y. Kang, Blue phosphorescent Ir(III) complex with high color purity: fac–Tris(2′,6′-difluoro-2,3′-bipyridinato-N, C4′)iridium(III), Inorganic Chemistry 48 (2009) 1030–1037. [12] L.Q. Chen, C.L. Yang, J.G. Qin, J. Gao, D.G. Ma, Tuning of emission: synthesis, structure and photophysical properties of imidazole, oxazole and thiazole-based iridium (III) complexes, Inorganica Chimica Acta 359 (2006) 4207–4214. [13] A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide, Homoleptic cyclometalated iridium complexes with highly efficient red phosphorescence and application to organic light-emitting diode, Journal of the American Chemical Society 125 (2003) 12971–12979. [14] W. Wong, G. Zhou, X. Yu, H. Kwok, B. Tang, Amorphous diphenylaminofluorenefunctionalized iridium complexes for high-efficiency electrophosphorescent light-emitting diodes, Advanced Functional Materials 16 (2006) 838–846. [15] J. van Houten, R.J. Watts, Temperature dependence of the photophysical and photochemical properties of the Tris(2,2′-bipyridyl)ruthenium(II) ion in aqueous solution, Journal of the American Chemical Society 98 (1976) 4853–4858. [16] Q. Zhao, S.J. Liu, M. Shi, Tuning photophysical and electrochemical properties of cationic iridium (III) complex salts with imidazolyl substituents by proton and anions, Organometallics 26 (2007) 5922–5930.
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
A thiocyanate-bridging dimeric cyclometalated iridium(III) complex: Synthesis, structure and phosphorescence behaviours towards metal ions Bihai Tong a,⁎, Jia-Yan Qiang a, Ya-Qing Xu a, Qunbo Mei b, Taike Duan a, Qun Chen c, Qian-Feng Zhang a, c,⁎⁎ a
Institute of Molecular Engineering and Applied Chemistry, College of Metallurgy and Resources, Anhui University of Technology, Ma'anshan, Anhui 243002, PR China Jiangsu Key Lab of Organic Electronics & Information Displays, Institute of Advanced Materials (IAM), Nanjing University of Posts & Telecommunications (NUPT), Nanjing 210046, PR China c Department of Applied Chemistry, School of Petrochemical Engineering, Changzhou University, Jiangsu 213164, PR China b
a r t i c l e
i n f o
Article history: Received 12 July 2011 Accepted 16 August 2011 Available online 21 September 2011 Keywords: Cyclometalated iridium(III) Dimeric complex Crystal structure Photoluminescence Phosphorescence
a b s t r a c t A new neutral dimeric cyclometalated iridium complex containing bridging thiocyanate ligands, [{Ir(μ-SCN) (pqcm)2}2] (1, pqcmH = 2-phenyl-quinoline-4- carboxylic acid methyl ester), has been synthesized and structurally characterized. The photoluminescence (PL) spectrum of 1 shows emission maximum at 638 nm with a lifetime of 0.11 μs and the PL quantum yield is ca. 0.012. The phosphorescence behaviours of 1 towards different solvents and metal ions were also investigated and the strong phosphorescence quenching by acetonitrile and two equivalents of Hg 2+, Cu2+ and Ag + ions were observed. © 2011 Elsevier B.V. All rights reserved.
Luminescent cyclometalated iridium(III) complexes have been widely used in organic light-emitting diodes (OLEDs), luminescence sensitizers, and biological imaging due to their high quantum efficiency and color tenability [1]. Mononuclear iridium(III) bis-cyclometalated complexes are generally synthesized from dinuclear chloro-bridging dimers [{Ir(μ-Cl)(N^C)2}2] (N^C = cyclometalated bidentate ligand). Substitution of [{Ir(μ-Cl)(N^C)2}2] with bidentate O, O or N, O ligands L^L afforded [Ir(N^C)2(L^L)], which have been employed as dopants for OLEDs [2] and sensors for singlet oxygen [3]. Although the cyclometalated iridium(III) chloro-bridging dimers are readily prepared by the method developed by Nonoyama in 1974 [4], the multinuclear iridium(III) complexes are relatively quite rare [5−8]. It has been noted note that the conjugation SCN– or CN– anion was introduced into the cyclometalated iridium(III) complexes to increase the gap between the t2g and LUMO orbitals and sequentially to magnify the quantum yields of this class of complexes. Only two examples of thiocyanate-bridging complexes [{Ir(μ-SCN)}2(N^C)2] have been reported recently [9,10]. In this article, we set up the reaction of [{Ir(μ-Cl)(pqcm)2}2] (pqcmH= 2-phenylquinoline-4-carboxylic acid methyl ester) and KSCN, resulting in the isolation of a new dinuclear iridium(III) complex [{Ir(μ-SCN) (pqcm)2}2] which is composed of two bis-cyclometalated iridium(III) moieties bridged by two thiocyanate ligands. Herein, the initial results
⁎ Corresponding author. Tel./fax: + 86 555 2312041. ⁎⁎ Correspondence to: Q.-F. Zhang, Institute of Molecular Engineering and Applied Chemistry, College of Metallurgy and Resources, Anhui University of Technology, Ma'anshan, Anhui 243002, PR China. Tel./fax: + 86 555 2312041. E-mail addresses: [email protected] (B. Tong), [email protected] (Q.-F. Zhang). 1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2011.08.029
including structural characterization and the phosphorescence behaviour towards different solvents and metal ions are reported in this paper. The neutral thiocyanate-bridging dimeric cyclometalated iridium(III) complex [{Ir(μ-SCN)(pqcm)2}2]·C6H14 (1) was obtained from the reaction of the corresponding chloro-bridging cyclometalated iridium(III) dimer and an excess KSCN in CH2Cl2/CH3OH (1:1) at reflux. 1 is stable in air and soluble in common organic solvents such as CH2Cl2, THF, and toluene. The formulation of 1 was confirmed by NMR, ESI mass spectroscopy, and infrared spectroscopy. The detail descriptions of these characterizations for 1 are available as supplementary materials. The strong absorptions at 2121 and 1729 cm –1 in the IR spectrum of 1 may be attributed to the SCN and C=O stretching vibrations of the coordination thiocyanate ligands and the carboxyl groups in the pqcm ligands, respectively. Complex 1 crystallizes in the monoclinic space group P21/c with four molecules per unit cell. The unit cell of 1 comprises neutral dinuclear complex and disordered hexane solvents molecules in the crystal lattice. The perspective view of 1 is depicted in Fig. 1. Each of two iridium(III) centers in 1 adopts a distorted octahedral coordination geometry and is ligated by two chelating pqcm and two bridging thiocyanate ligands. The metalated carbon atoms of the two pqcm ligands are in a mutually cis arrangement, and the two bridging thiocyanate ligands bind the two iridium centers in a μ-S,N fashion. The sulfur and nitrogen atoms of two different thiocyanate ligands lie in the equatorial plane trans to the metalated carbon atoms of the pqcm moieties. The average Ir–C and Ir–N(pqcm) bond lengths in 1 are 1.976(18) and 2.084(13) Å, respectively, which are in the ranges
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B. Tong et al. / Inorganic Chemistry Communications 14 (2011) 1937–1939
4.0 3.5
294 nm 352 nm
Absorbance
3.0 2.5 2.0 1.5 469 nm
1.0 0.5 300
400
500
600
Wavelength (nm) Fig. 2. UV–vis absorption spectrum of 1 in CH2Cl2.
of the corresponding bond lengths in the cyclometalated iridium(III) complexes [11,12]. The average Ir−S distance of 2.566(5) Å in 1 is longer than those of 2.496(1) Å in [{Ir(Buppy)2}2(μ-SCN)2] (Buppy = 2-(4′tert-butylphenyl)-pyridine) [10]. The Ir–S–C≡N units are approximately linear [C(1)−N(1)−Ir(2) =172.6(13) and C(2)−N(2)−Ir(1) = 162.5(13)°] and the S−Ir−N(SCN) angles are close to 90° [S(1)− Ir(1)−N(2) = 89.9(4)° and S(2)−Ir(2)−N(1) = 86.1(3)°]. Thus, the eight-membered Ir2(SCN)2 core in 1 can be roughly described as a rectangle. In the absorption spectrum of complex 1 (Fig. 2), the band at ca. 294 nm is assigned to a typical spin-allowed intraligand 1π−π* transitions of pqcm in the complex since the free pqcm ligand also absorbs bands at a similar energy. The strong band at 352 nm corresponds likely to spin-allowed metal-to-ligand charge transfer ( 1MLCT) and the weak band at 469 nm probably arises from the formally spin-forbidden 3MLCT transition. The room-temperature photoluminescence spectrum of 1 in degassed CH2Cl2 solution shows that the excitation maximum is dominated by a broad band centered at 585 nm which is an absorption obviously arising from the 3MLCT states. The emission maximum for 1 is an intense luminescence at 638 nm, corresponding to the red light emitting. The excited state lifetime of 1 was determined to be 0.11 μs which is obviously shorter than those of [Ir(ppy)3] (ppy = phenylpyridinato) (2.0 μs) [13] and the other related neutral cyclometalated iridium(III) complexes [14]. This would be advantageous for complex 1 to be a candidate for the design of highly efficient OLED devices. Actually, the phosphorescence quantum efficiency in CH2Cl2 solution is ca. 0.012 used an aqueous solution of [Ru(bpy)3]Cl2 (Φ = 0.042) as the standard solution [15].
Et2O Toluene Dioxane EA DCM THF DMF EtOH ACN NMP
1000 800
Intensity (a.u.)
Fig. 1. The perspective view of 1. Displacement ellipsoids have been drawn at the 35% probability level. Hydrogen atoms are omitted for the clarity. Selected bond lengths (Å) and angles (°): Ir(1)-C(3) 1.98(2), Ir(1)-C(20) 1.975(17), Ir(1)-N(2) 2.085(14), Ir(1)-N(3) 2.101(12), Ir(1)-N(4) 2.062(14), Ir(1)-S(1) 2.566(5), Ir(2)-C(37) 1.951(16), Ir(2)-C(54) 1.997(17), Ir(2)-N(1) 2.147(14), Ir(2)-N(5) 2.085(12), Ir(2)-N(6) 2.089(14), Ir(2)-S(2) 2.567(5); C(20)-Ir(1)-C(3) 88.5(7), C(20)-Ir(1)-N(4) 80.5(6), C(3)-Ir(1)-N(4) 94.7(6), C(20)-Ir(1)-N(2) 173.8(6), C(3)-Ir(1)-N(2) 97.6(6), N(4)-Ir(1)-N(2) 100.2(5), C(20)-Ir(1)-N(3) 93.2(6), C(3)-Ir(1)-N(3) 78.2(6), N(4)-Ir(1)-N(3) 170.6(5), N(2)Ir(1)-N(3) 86.9(4), C(20)-Ir(1)-S(1) 84.1(5), C(3)-Ir(1)-S(1) 171.9(5), N(4)-Ir(1)S(1) 80.8(4), N(2)-Ir(1)-S(1) 89.9(4), N(3)-Ir(1)-S(1) 105.6(4), C(37)-Ir(2)-C(54) 92.5(6), C(37)-Ir(2)-N(5) 79.9(6), C(54)-Ir(2)-N(5) 94.1(5), C(37)-Ir(2)-N(6) 94.4(5), C(54)-Ir(2)-N(6) 79.4(6), N(5)-Ir(2)-N(6) 171.2(5), C(37)-Ir(2)-N(1) 173.5(5), C(54)Ir(2)-N(1) 93.0(6), N(5)-Ir(2)-N(1) 103.1(5), N(6)-Ir(2)-N(1) 83.3(4), C(37)-Ir(2)S(2) 88.8(4), C(54)-Ir(2)-S(2) 173.6(4), N(5)-Ir(2)-S(2) 80.0(4), N(6)-Ir(2)-S(2) 106.7(4), N(1)-Ir(2)-S(2) 86.1(3), C(1)-S(1)-Ir(1) 102.8(6), C(2)-S(2)-Ir(2) 103.3(6), C(1)-N(1)-Ir(2) 172.6(13), C(2)-N(2)-Ir(1) 162.5(13).
The photoluminescence spectra of complex 1 in different solutions and in solid state were tested at room temperature, as shown in Fig. 3. It may be found that complex 1 emits the strongest luminescence in the non-polar Et2O solution and the weakest in the polar NMP solution. The emission maximum wavelengths are obviously red-shifted from 638 nm in Et2O to 673 nm in NMP. This red-shift phenomenon has been found in some related cyclometalated iridium(III) complexes [16]. It is interesting to note that the emission maximum in CH3CN is an exception to the rule, indicated by the emission intense reduced along with the wavelength undergoing a blue-shift, which may infer that the dimeric structure of 1 may be unstable in the CH3CN solution, probably decomposed to be a monomeric complex due to the substitution of the thiocyanate ligands by the stronger acetonitrile molecule. The emission intensity of 1 is rather weak in solid state along with the wavelength red-shifting to be 673 nm. The redshift of the emission spectrum is related to the strong intermolecular interactions, a possible indicative of the strong excited-state quenching of complex 1 in solid state. The effects of different metal ions Hg 2+, Ag +, Cu 2+, Fe 2+, Co 2+, Ni 2+, Zn 2+, Mg 2+, Cr 3+, Pb 2+ and Cd 2+ on the fluorescent spectral properties of 1 were also investigated (see Fig. 4). When two equiv. of Hg 2+ were added to the CH2Cl2 solution of 1, the luminescence intensity of 1 decreased to be ca. 8.8%. The similar changes were observed when the Ag + and Cu 2+ ions were added to the CH2Cl2 solution of 1, accordingly, the luminescence intensity of 1 in CH2Cl2 decreased to be ca. 11.2% and 12.7%, respectively. Obviously, the strong complexation interactions between 1 and Hg 2+ or Ag +/Cu 2+ ion occurred. No obvious spectral variations were observed upon addition of other metal ions, as displayed in Fig. 4.
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Wavelength (nm) Fig. 3. The emission spectra of 1 in different solvents.
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B. Tong et al. / Inorganic Chemistry Communications 14 (2011) 1937–1939
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In order to explore the complexation abilities of 1 with metal ions, the changes in UV–vis absorption spectra (see Fig. 5) were investigated by the titration experiments. Upon addition of Hg2+, the absorption bands of 1 at 294 and 352 nm gradually increased firstly then turned to decrease, resulting in formation of a new band centered at 475 nm with a distinct isosbestic point at 445 nm, which may indicate the strong ground state interactions between 1 and the Hg2+ ion. The emission spectra titration of 1 with Hg2+ ion was also tested. It can be observed from Fig. 6 that the emission maximum is almost entirely quenched upon addition of two equiv. of Hg2+ ion which likely leads to an entire dissociation of the thiocyanate-bridging cyclometalated iridium(III) dimer. In summary, we have synthesized a thiocyanate-bridging dimeric cyclometalated iridium(III) complex in a high yield. The emission spectra of the dimer are sensitive to strong polar solvents and heavy metal ions. The present dimeric cyclometalated iridium(III) complex is found to be a candidate for the effective red emitter with a neutral charge which is desirable for an example in the OLED applications, and more related multinuclear cyclometalated iridium(III) complex with new designed ligands are underway in our laboratory.
Acknowledgments This project was supported by the National Natural Science Foundations of China (grant nos. 50903001 and 50803027) and the Program for New Century Excellent Talents in University of China (NCET-08-0618). Bihai Tong thanks the Fund of Anhui University of Technology for Youth (No. QZ200907) for the assistance.
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Fig. 4. Fluorescence spectra of 1 in the presence of 2 equiv. of different metal ions.
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Wavelength (nm) Fig. 5. Changes in the UV absorption spectra of 1 on addition of Hg2+.
Fig. 6. Changes in the emission spectra of 1 on addition of Hg2+.
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