Synthesis and physico-chemical studies of cyclometalated heteroleptic iridium(III) complexes

Spectrochimica Acta Part A 93 (2012) 240–244

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Synthesis and physico-chemical studies of cyclometalated heteroleptic iridium(III) complexes Jayaraman Jayabharathi ∗ , Venugopal Thanikachalam, Natesan Srinivasan, Karunamoorthy Jayamoorthy Department of Chemistry, Annamalai University, Annamalainagar 608002, Tamilnadu, India

a r t i c l e

i n f o

Article history: Received 9 February 2012 Received in revised form 22 February 2012 Accepted 7 March 2012 Keywords: Energy gap law 3 MLCT–1 MLCT Iridium(III) complex Reichardt–Dimroth solvent function Marcus solvent function

a b s t r a c t Phosphorescent studies of 2-arylimidazole heteroleptic cyclometalated iridium(III) complexes with picolinic acid as the ancillary ligand were made. The observed experimental data reveals that these complexes possess dominantly 3 MLCT and 3 ␲–␲* excited states and the solvent shift of these complexes are interpreted by Reichardt–Dimroth and Marcus solvent functions. The results are consistent with prior assignments on the absorption band to a metal-to-ligand charge transfer excited state associated with chelating ligand. Emission kinetic studies exploited that the radiative transition (kr ), increases with increasing emi . © 2012 Elsevier B.V. All rights reserved.

1. Introduction The heavy transition metal complexes usually have high phosphorescence efficiencies with large population of the emitting triplet states by strong spin–orbit coupling induced by the metal center. Among these complexes, the iridium(III) complexes were particularly known to have high quantum efficiencies and relatively short excited emissive states [1–6]. Thus, the phosphorescent iridium complexes have been used as dopants for organic lightemitting devices (OLEDs) owing to utilization of both singlet and triplet excited states for emission without their significant loss by none-emissive pathways [7,8]. There are, however, some limitations in the application of phosphorescent materials to OLEDs. Compared with the short emission life-time of the fluorescent materials, the relatively long phosphorescence life-time of the iridium complexes may cause triplet–triplet (T–T) annihilation at high currents, which can be one of the factors in lowering the efficiency [9]. OLEDs based on utilizing both singlet and triplet excitons from heavy metal complexes including Ir(III), Pt(II), Ru(II), and Os(II) have been reported [10–14]. The heavy transition-metal center enhances the triplet emission resulting from promoting singlet–triplet mixing. Among these complexes, iridium complexes show the best efficiency and performance for OLEDs. For example, Okada and coworkers published an account of a highly efficient OLED based on Ir(piq)3 (iridium (III) tris(1-phenyl-isoquinolinato-N,C20)) which exhibits a maximum emission peak at 623 nm [15]. Isoquinoline

∗ Corresponding author. Tel.: +91 9443940735. E-mail address: [email protected] (J. Jayabharathi). 1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2012.03.016

derivatives are known to be one of the non-steroidal compounds that are used as anti-inflammatory and analgesic agents [16]. It is interesting to note that the isoquinoline derivatives are also potentially useful for OLED applications because they exhibit good electroluminescence properties after chelating with metal to form complexes [17]. In our laboratory, we have prepared a set of Ir complexes with imidazole ligands. 2. Experimental 2.1. Materials and methods Iridium(III)trichloride trihydrate (IrCl3 ·3H2 O, Sigma–Aldrich Ltd.), 2-ethoxyethanol (H5 C2 OC2 H4 OH, S.D. fine) and all other reagents used without further purification. 2.2. Optical measurements and compositions analysis The Ultraviolet–visible (UV–vis) spectra of the phosphorescent Ir(III)complexes were measured in an UV–vis spectrophotometer (Perkin Elmer Lambda 35) and corrected for background absorption due to solvent. Photoluminescence (PL) spectra were recorded on a (Perkin Elmer LS55) fluorescence spectrometer. The solid-state emission spectra were recorded on fluoromax2 (ISA SPEX) Xenon-Arc lamp as a source. NMR spectra were recorded on a Bruker 400 MHz NMR spectrometer. MS spectra (EI and FAB) were recorded on a Varian Saturn 2200 GCMS spectrometer. Cyclic Voltametry (CV) analyses were performed by using CHI 630A potentiostat electrochemical analyzer. Measurements of oxidation and reduction were undertaken using

J. Jayabharathi et al. / Spectrochimica Acta Part A 93 (2012) 240–244

241

Table 1 Absorption, emission and stokes shift values of 1 and 2. Solvent

Absorption 1

n-Hexane

1,4-Dioxan

Benzene

Chloroform

Ethyl acetate Fig. 1. The UV–vis absorption spectra of the complexes 1 and 2 in CH2 Cl2 . Dichloromethane

0.1 M tetra(n-butyl)ammoniumhexafluoro-phosphate as a supporting electrolyte, at scan rate of 0.1 V s−1 . The potentials were measured against an Ag/Ag+ (AgCl) reference electrode using ferrocene/ferrocenium (Cp2 Fe/Cp2 Fe+ ) as the internal standard. The onset potentials were determined from the intersection of two tangents drawn at the rising current and background current of the cyclic voltammogram.

1-Butanol

Ethanol

Methanol

Acetonitrile

2.3. General procedure for the synthesis of ligands The various substituted 2-arylimidazole ligands were prepared from an unusual four components assembling of butan-2,3-dione, ammonium acetate, corresponding aniline and substituted aryl aldehyde as shown in Scheme S1 [18–21].

3. Results and discussion 3.1. Photophysical properties The absorption bands of complexes (1, 2) show two kinds of bands (Fig. 1). The intense band observed around 250 nm (Table 1) in the ultraviolet part of the spectrum can be assigned to the allowed ligand centered (␲–␲*) transitions [23] and somewhat weaker bands also observed in the lower part of energy (max < 330 nm). The band position, size and extinction coefficient of the bands observed in the range 330–425 nm suggest that these are MLCT transitions (1 MLCT and 3 MLCT) [24,25]. According to our previous papers [19–21], weak bands located at longer wavelength have been assigned to the 1 MLCT ← S0 and 3 MLCT ← S0 transitions of iridium complexes. Thus the broad absorption shoulders at 382 and 425 nm observed for 1 are likely to be ascribed to the 1 MLCT ← S and 3 MLCT ← S transitions, respectively. The intensity 0 0 of the 3 MLCT ← S0 transition is close to that of 1 MLCT ← S0 transition, suggesting that 3 MLCT ← S0 transition is strongly allowed by S–T mixing of spin–orbit coupling [12] and similar observations are made for complexes 2. Absorption in the range of around 330 nm for complexes (1, 2) corresponds to the transition of the 1 MLCT state as evident from its extinction coefficient of the order 103 . The absorption like long tail toward lower energy and higher wavelength around 388 nm is assigned to 3 MLCT transition and gains intensity by mixing with the higher lying 1 MLCT transition through the spin–orbit coupling (Fig. 2) of iridium(III) [17]. Both singlet MLCT (1 MLCT) and triplet MLCT (3 MLCT) bands are typically observed for these complexes in all solvents. In order for these iridium (III) complexes to be useful as phosphors EL devices, strong spin–orbit coupling must be present to efficiently mix the singlet and triplet

a b

2 252(4.01) 330(3.76) 387(3.33) 292(3.87) 329(3.57) 386(3.26) 278(3.99) 330(3.61) 381(3.12) 270(3.98) 329(3.62) 384(3.27) 275(4.18) 330(3.71) 385(3.26) 276(4.01) 330(3.69) 385(3.20) 258(3.93) 331(3.58) 384(3.17) 267(4.04) 335(3.62) 386(3.30) 260(3.98) 333(3.41) 388(3.03) 263(4.12) 332(3.51) 384(3.07)

258(3.86) 333(3.35) 385(2.89) 286(3.98) 315(3.56) 374(3.01) 278(3.96) 332(3.59) 382(3.11) 273(4.30) 331(3.86) 385(3.21) 277(4.21) 333(3.57) 385(3.01) 275(3.98) 330(3.62) 384(3.18) 273(4.26) 332(3.68) 385(316) 274(3.91) 333(3.76) 382(3.37) 269(3.86) 334(3.38) 387(3.01) 271(4.07) 335(3.68) 386(3.32)

Emission

Stokes shift

1

2

1

2

432

443

2692

3401

446

448

3485

4417

430

431

2991

2976

485

493

5423

5690

483

488

5270

5482

506

510

6211

6434

510

518

6434

6669

513

521

6414

6984

518

524

6468

9756

523

542

6921

7457

UV–vis absorption measured in CH2 Cl2 solution, concentration = 1 × 10−5 M. Photoluminescence measured in CH2 Cl2 solution, concentration = 1 × 10−4 M.

excited states. Clear evidences for mixing of the singlet and triplet excited states are seen in the absorption of these complexes. 3.2. Phosphorescence spectra: mixing of excited states (LC and MLCT) Phosphorescence of mononuclear metal complexes originates from the ligand-centered excited state, metal-centered excited

Fig. 2. Spin–orbit coupling of heavy-metal facilitated triplet emission.

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J. Jayabharathi et al. / Spectrochimica Acta Part A 93 (2012) 240–244 Table 2 Photophysical properties of iridium complexes 1 and 2.

Fig. 3. The photoluminescence emission spectra of the complexes 1 and 2 in CH2 Cl2 .

state and MLCT excited state. For the cyclometalated iridium (III) complexes, the wave function of the excited triplet state T , responsible for the phosphorescence is expressed as, T = aT (␲–␲∗) + bT (MLCT)

(1)

where ‘a’ and ‘b’ are the normalized co-efficient, T (␲–␲*) and T (MLCT) are the wave function of 3 (␲–␲*) and 3 (MLCT) excited states, respectively. For these iridium complexes, the wave function of the triplet state (T ) responsible for the phosphorescence and Eq. (1) implies that the excited triplet state of these iridium complexes are mixture of T (␲–␲*) and T (MLCT) [26]. The triplet state is attributed to dominantly 3 ␲–␲* excited state when a > b and dominantly 3 MLCT excited state when b > a. The phosphorescence spectra of these complexes (1, 2) obtained at 298 K show significant broad shape and also vibronic fine structure (Fig. 3). According to our previous studies [19–21], phosphorescence spectra from the ligand centered 3 ␲–␲* state display vibronic progressions, while those from the 3 MLCT state are broad in shape. The luminescence spectra of complex 1 reveal a broad shape of the luminescence spectra (Fig. 3) whereas the vibrational sideband pattern of the photoluminescence spectra were observed for the complex 2 (Fig. 3). Complexes 1 have excited state with large contribution of 3 MLCT whereas complex 2 have excited state with large contribution of 3 ␲–␲*. All these complexes show emission at 506 and 510 nm in dichloromethane, respectively (Table 1). The emission spectra of complex 2 are red shifted when compared with complex 1 and this may be due to the electronic effect of the substituents. The emission spectrum of complex 1 is slightly blue shifted in comparison with that of complex 2, which is the result of the introduction of electron withdrawing fluorine substituent into the phenyl ring attached to C(2) carbon of the imidazole ring in complex 2. The Stokes shift (Table 1) of 1, 2 was calculated in wave numbers from the difference between the lowest energy absorption maximum and the emission maximum of each complex. The observed large Stokes shift in hydroxyl solvents may be due to the significant structural differences between the ground state and excited state of these iridium complexes [22,27]. 3.3. Marcus and Reichardt–Dimroth solvent functions – Solvatochromism Solvatochromism is a potentially important probe in accessing charge distributions in both ground and excited states and also yield information regarding charge distributions in Franck–Condon states produced by absorption. Solvent shifts in the absorption and emission of ortho-metalated iridium complexes 1, 2 were measured (Table 1) in different

Complex

Quantum yield ()

Lifetime (␮s) 298 K

kr (␮s−1 )

knr (␮s−1 )

1 2

0.33 0.40

1.1 1.0

0.30 0.40

0.60 0.60

solvents. Variation in the absorption and emission of the complexes 1, 2 was observed in different solvents. The emission peak of complexes 1, 2 are around 432 nm in n-hexane (non-polar), 513 nm in ethanol (polar protic solvent), 506 nm in CH2 Cl2 (medium polarity) and 523 nm in acetonitrile (a strongly polar aprotic solvent). The peak shift may be due to the stronger interaction between the solvents and the excited state molecules. The excited state of all iridium complexes are more stabilized in polar solvents than in non-polar solvents which lead to red shift of emission with increasing solvent polarity [24]. The photoluminescent peak of solid state (representative spectrum of complex 1 is shown in Fig. 3 of all complexes are almost similar to that of emission in non-polar solvent (n-hexane)) which shows that there is very little or no influence of molecular interaction on the excited state of iridium complexes in the solid state. Measurement of absorption solvatochromism have been interpreted with Marcus and Reichardt–Dimroth solvent functions to estimate the transition dipoles associated with low lying MLCT excited state. The linear correlation (Fig. 4) of solvent shift of absorption band positions of iridium complexes 1, 2 with Reichardt–Dimroth solvent ET parameters is indicative of the fact that the dielectric solute solvent interactions are responsible for the observed solvatochromic shift for the iridium complexes. The observed linear correlation (Fig. 5) of solvent shift of absorption band positions of iridium complexes 1 and 2 with Marcus [27] optical dielectric solvent function [(1 − Dop )/(2Dop + 1)] (Fig. 5) reveal that transition dipoles associated with absorption and the direction of excited dipole is opposite to that of the ground state-dipole (Fig. S6). Application of Marcus theory interpretation of solvent shift absorption data indicate that the absorption bands in the range 250–388 nm in these ortho-metalated Ir (III) complexes are due to MLCT transitions to the chelating ligand. Further linear correlation suggests as a result of the absorption that charge is transferred from the region around the ortho-metalated carbon atom to the region around the bridgehead of the chelating ligand and ground state dipole pointed toward the ortho-metalated carbon atom and an excited state dipole pointed toward the chelating ligand [27]. 3.4. Electronic transition theory The absolute PL quantum yields were measured by comparing fluorescence intensities (integrated areas) of a standard sample (Coumarin 46) and the unknown sample [18,21]. The main decay route of the excited state of these complexes (1, 2) and their radiative and non-radiative decay are studied in detail (Table 2). Complexes (1, 2) exhibit appreciable phosphorescence quantum yields at room temperature under nitrogen atmosphere [26]. Moreover, radiative lifetime of these complexes fall in the range of 1.0–1.1 ␮s. The radiative (kr ) and non-radiative (knr ) rate constants are calculated from the phosphorescence yield p and lifetime , p = ISC



kT kr + knr

 = (kr + knr )

−1



(2) (3)

Here, ISC is the intersystem-crossing yield. For the iridium complexes ISC is safely assumed to be 1.0 because of the strong spin–orbit interaction caused by heavy atom effects of iridium [28].

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Fig. 4. Solvatochromic absorption of 1 and 2 with Reichardt–Dimroth solvent ET parameters.

Fig. 5. Solvatochromic absorption of 1 and 2 with Marcus optical dielectric solvent function.

kr and knr are the radiative and non-radiative deactivation,  f is the life time of the S1 excited state. The low quantum yields could be explained by non-radiative path to the low-lying n–␲* state of the nitrogen atoms. Experimentally observed result indicates that the kr values of 1, 2 increase with an increase of em . According to the theory of the electronic transition, the kr value is proportional to the square of the electric dipole transition moment (MT–S ). First-order perturbation theory gives an approximate expression for MT–S [29]. MT–S = ˙ˇn < 1 n |M|1 0

(4)

where 1 n and 1 0 are the wave functions of Sn and S0 states, respectively and M is the electric dipole vector with the use of the spin–orbit coupling operator (Hso ) and the wave function of the lowest excited triplet state (3 1 ), ˇn is formulated as ˇn = 1 n |Hso |3 1 /(1 En –3 E1 )

(5)

where 1 En and 3 E1 are the energies of Sn state and the lowest excited triplet state, respectively. Now, Eq. (4) is simply expressed as MT–S = {1 n |HSO |3 1 1 1 |M|1 0 }/(1 E1 –3 E1 ) = ˛/(1 E1 − 3 E1 ) (6) (1 n |Hso |3 1 1 1 |M|1 0 )

is Eq. (6) predicts that, when ˛ = approximately constant, Since MT–S kr increases with a decrease in the energy difference (1 E1 –3 E1 ), kr also increases with a decrease in the energy difference (Table 2). In the present study, kr and knr increase with an increase in em . This fact is explained by assuming that the S1 energy does not differ significantly among the complexes 1, 2 and the energy difference (1 E1 –3 E1 ), increases with decrease in em . Actually in complexes 1, 2, the S1 ← S0 absorption bands are located at 250–290 nm. But phosphorescence S0 ← T1 exhibits large bathochromic shift ranging from 430 to 542 nm. These data provide the evidence that the energy

difference (1 E1 –3 E1 ), increases with the decrease in em and therefore, kr increases with an increase in em [29]. 3.5. Effect of substituent on tuning wavelength Substituent effect of the d orbital (t2g ) stabilization of iridium is through carbon atom-iridium bonding and the stabilization is due to inductive effect of the substituents. Therefore, the HOMO stability and the emission energy gap are controlled by the nature and number of substituents and its inductive influence on the aromatic ring. The photophysical study of these complexes demonstrates that the electron withdrawing fluorine substituent increases the absorption and emission energies of complexes by stabilizing the HOMO level. Besides increasing the emission energy, the lower HOMO energies decrease the energy separation between the 1 MLCT and 3 LC states, which in turn modified the excited state properties of the iridium complexes. The redox potentials of the cyclometalated iridium complexes were measured relative to an internal ferrocene reference (Cp2 Fe/Cp2 Fe+ = 0.45 V versus SCE in CH2 Cl2 solvent) [30–33]. The reduction occurs primarily on the more electron accepting heterocyclic portion of the cyclometalated 2-arylimidazole ligands (LUMO contribution) whereas the oxidation process is to largely involve in the Ir-phenyl center (HOMO contribution). The calculated energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are given in Table 3. Table 3 Cyclic voltammetry data of the complexes 1 and 2. Complex

E(onset) (V)

HOMO (eV)

LUMOa (eV)

Eg (eV)

1 2

0.32 0.25

−5.12 −5.05

−2.30 −2.75

2.82 2.30

a

Measurement was carried out in CH2 Cl2 solution, concentration = 1 × 10−3 M.

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The iridium complexes show reversible oxidation behavior and HOMO and the LUMO energies were calculated based on the HOMO energies and the lowest-energy absorption edges of the UV–vis absorption spectra [31]. These results reveal that the complex 1 show emission with the shorter wavelength (506 nm) whereas 2 exhibits emission with maximum wavelength around 510 nm in dichloromethane. From the energy gap values it was concluded that all the reported dopant (1, 2) are green emitters. 4. Conclusions In summary we succeeded in the preparation of green emitting Ir(III) complex dopants using various substituted imidazole ligands. Complexes (1, 2) discussed here showed 3 MLCT predominant mixing states for their lowest excited triplet states, but the degree of mixing between 3 MLCT and 3 ␲–␲* states of the excited states varied. The solvent shifts are interpreted in terms of Reichardt–Dimroth and Marcus solvent functions. The results are consistent with prior assignments on the absorption band to a metal-to-ligand charge transfer excited state associated with chelating ligand. Acknowledgments One of the authors Dr. J. Jayabharathi is thankful to Department of Science and Technology [No. SR/S1/IC-73/2010], University Grants commission (F. No. 36-21/2008 (SR)) and Defence Research and Development Organisation (DRDO) (NRB-213/MAT/10-11) for providing funds to this research study. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.saa.2012.03.016. References [1] B.A. Baldo, D.F. O’Brian, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151–154. [2] B.A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson, S.R. Forrest, Appl. Phys. Lett. 75 (1999) 4–6. [3] M.A. Baldo, M.E. Thompson, S.R. Forrest, Nature 403 (2000) 750–753. [4] M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, Y. Taga, Appl. Phys. Lett. 79 (2001) 156–158.

[5] F.C. Chen, Y. Yang, M.E. Thompson, J. Kido, Appl. Phys. Lett. 80 (2002) 2308–2310. [6] Y. You, K.S. Kim, T.K. Ahn, D. Kim, S.Y. Park, J. Phys. Chem. C 111 (2007) 4052–4060. [7] A. Kohler, J.S. Wilson, R.H. Friend, Adv. Mater. 14 (2002) 701–707. [8] A.B. Tamayo, B.D. Alleyne, P.I. Djurovich, S. Lamansky, I. Tsyba, N.N. Ho, R. Bau, M.E. Thompson, J. Am. Chem. Soc. 125 (2003) 7377–7387. [9] S.J. Yeh, M.F. Wu, C.T. Chen, Y.H. Song, Y. Chi, M.H. Ho, S.F. Hsu, C.H. Chen, Adv. Mater. 17 (2005) 285–289. [10] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H. Lee, C. Adachi, P.E. Burrows, S.R. Forrest, M.E. Thompson, J. Am. Chem. Soc. 123 (2001) 4304–4312. [11] F. Chen, Y. Yang, M.E. Thompson, J. Kido, Appl. Phys. Lett. 80 (2002) 2308–2316. [12] M.A. Baldo, D.F. OBrien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature (London) 395 (1998) 151–154. [13] H. Rudmann, S. Shimada, M.F. Rubner, J. Am. Chem. Soc. 121 (2002) 4918–4921. [14] J.H. Kim, M.S. Liu, A.K.Y. Jen, B. Carlson, L.R. Dalton, C.F. Shu, R. Dodda, Appl. Phys. Lett. 83 (2003) 776–779. [15] S. Okada, H. Iwawaki, M. Furugori, J. Kamatani, S. Igawa, T. Moriyama, S. Tsuboyama, A. Miura, T. Takiguchi, H. Mizutani, SID 02 DIGEST (2002) 1360–1363. [16] Q. Chao, L. Deng, H. Shih, L.M. Leoni, D. Genini, D.A. Carson, H.B. Cottam, J. Med. Chem. 42 (1999) 3860–3873. [17] Y.J. Su, H.L. Huang, C.L. Li, C.H. Chien, Y.T. Tao, P.T. Chou, S. Datta, R.S. Lin, Adv. Mater. 15 (2003) 884–888. [18] J. Jayabharathi, V. Thanikachalam, N. Srinivasan, K. Saravanan, J. Fluoresc. 21 (2011) 595–606. [19] J. Jayabharathi, V. Thanikachalam, K. Jayamoorthy, M. Venkatesh Perumal, Spectrochim. Acta A 79 (2011) 6–16. [20] J. Jayabharathi, V. Thanikachalam, K. Saravanan, N. Srinivasan, M. Venkatesh Perumal, Spectrochim. Acta A 78 (2011) 794–802. [21] J. Jayabharathi, V. Thanikachalam, N. Srinivasan, M. Venkatesh Perumal, K. Jayamoorthy, Spectrochim. Acta A 79 (2011) 137–147. [22] M. Nonoyama, Bull. Chem. Soc. Jpn. 47 (1974) 767–768. [23] J. Jayabharathi, V. Thanikachalam, M. Venkatesh Perumal, N. Srinivasan, Spectrochim. Acta A 79 (2011) 236–244. [24] J. Jayabharathi, V. Thanikachalam, N. Srinivasan, M. Venkatesh Perumal, Spectrochim. Acta A 79 (2011) 338–347. [25] K. Saravanan, N. Srinivasan, V. Thanikachalam, J. Jayabharathi, J. Fluoresc. 21 (2011) 65–80. [26] J. Jayabharathi, V. Thanikachalam, K. Saravanan, J. Photochem. Photobiol. A 208 (2009) 13–20. [27] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, R. Kwong, I. Tsyba, M. Bortz, B. Mmui, R. Bau, M.E. Thompson, Inorg. Chem. 40 (2001) 1704–1711. [28] B.X. Mi, P.F. Wang, M.W. Liu, H.L. Kwong, N.B. Wong, C.S. Lee, S.T. Lee, Chem. Mater. 15 (2003) 3148–3151. [29] J. DePriest, G.Y. Zheng, N. Goswami, D.M. Eichhorn, C. Woods, D.P. Rillema, Inorg. Chem. 39 (2000) 1955–1963. [30] S. Okada, K. Okinaka, H. Iwawaki, M. Furugori, M. Hashimoto, T. Mukaide, J. Kamatani, S. Igawa, A. Tsuboyama, T. Takiguchi, K. Ueno, Dalton Trans. 9 (2005) 1583–1590. [31] J. Lundin, A.G. Blackman, K.C. Gordon, D.L. Officer, Angew. Chem., Int. Ed. 45 (2006) 2582–2584. [32] A.P. Wilde, R.J. Watts, J. Phys. Chem. 95 (1991) 622–629. [33] S.D. Cummings, R. Eisenberg, J. Am. Chem. Soc. 118 (1996) 1949–1960.