New exploration towards dinuclear iridium(II) complexes materials under chlorine-bridged precursor

Abstract

As an important precursor, the dinuclear [{Ir(C^{^}N)₂Cl}₂] (C^{^}N = ppy, dfppy) derived luminescent complex is nearly non-emissive at room temperature, but two similar chlorine-bridged dinuclear complexes [{Ir(ppy)₂}₂Cl(BTA)] (1) and [{Ir(dfppy)₂}₂Cl(BTA)] (2) (BTA = 1,2,3-benzotriazole) are proved to be bright. Herein, we report both complexes 1 and 2 from the viewpoint of experiment and theory. In order to characterize the two complexes better, single-crystal X-ray diffraction of 1 and 2 was carried out to determine their molecular structure. The emission spectra at room temperature (298 K) was measured, and the emission quantum yields for 1 and 2 in degassed CH₂Cl₂ were also given. The combined density functional theory (DFT) and time-dependent DFT (TDDFT) study was employed to gain insights into the electronic structure and radiative decay of these systems.

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Introduction

Cyclometallated iridium(III) complexes are intensively studied due to their potential use in organic light-emitting diodes (OLED),^1–10 biological labeling agents, and phosphorescent sensors.^11–17 In all these applications, the application of binuclear complexes containing cyclometallated iridium(III) centers remains scarce compared to the commonly used mononuclear complexes. However, because the bimetallic systems can exhibit an excited state substantially different from that of simple mononuclear systems, the design of supramolecular architectures containing binuclear iridium(III) complexes linked by a conjugated organic bridge is attracting more and more attention.^18–29

Moreover, the rapid growth of the chemistry of cyclometallated iridium(III) complexes stimulates fundamental investigations aimed, in particular, at elucidation of the basic synthetic pathway. To avoid a frustrating synthetic result, it is important to select a chelating ligand with appropriate frontier molecular orbitals, which can be accessed from a computational approach, and thus allow efficient electronic transitions occurring between different energy states. On the choice of all organic ligands, the most well-known compounds possess an adjacent donor group of aromatic heterocycles of C^{^}N (shown in Fig. S3†), and examples of these complexes include bis[3,5-difluoro-2-(2-pyridinyl-kN)phenyl-kC](2-pyridinecarboxylato-kN1,kO2)-Iridium (FIrpic),³⁰ bis(2,4-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate iridium(III) (FIr6),³¹ tris(2-phenylpyridine)iridium(III) (Ir(ppy)₃),³² and so on. Also, over the past three decades, to obtain in-depth mechanistic information, many researchers have been devoted to study such cyclometallation reactions of the C^{^}N ligand.^33–37 Unlike any other researchers, we now are paying more attention to the importance of a chlorine-bridged precursor. Normally, whether mononuclear or binuclear complexes containing cyclometallated iridium(III) centers, the dinuclear [{Ir(C^{^}N)₂Cl}₂] (C^{^}N ligand, such as 2-phenylpyridine (ppy), 2-(2,4-difluorophenyl)pyridine (dfppy), 2-phenylisoquinoline (piq), and so on) was an important and necessary precursor to synthesize the desired iridium complex. The precursor of dinuclear [{Ir(C^{^}N)₂Cl}₂] is nearly non-emissive at room temperature, so its importance is frequently neglected. In this regard, we want to look back on it from the view of theory and experiment research again.

Results and discussion

Herein the typical precursor of dinuclear [{Ir(C^{^}N)₂Cl}₂] (C^{^}N = ppy, dfppy) was selected as a breakthrough for us. As shown in Fig. 1 and Table S2 (in ESI†), the HOMO for [{Ir(ppy)₂Cl}₂] and [{Ir(dfppy)₂Cl}₂] is similar, the HOMO resides on ppy (dfppy) (44%), iridium (50%), chlorine (6%); the LUMO for [{Ir(ppy)₂Cl}₂] and [{Ir(dfppy)₂Cl}₂] is slightly different, and the LUMO of [{Ir(ppy)₂Cl}₂] resides on ppy (92%) and iridium (5%), while the LUMO of [{Ir(dfppy)₂Cl}₂] resides on dfppy (82%) and iridium (14%). Overall, if there is an imaginary axis of symmetry through the two chlorine atoms, we can see that both HOMO and LUMO for [{Ir(ppy)₂Cl}₂] and [{Ir(dfppy)₂Cl}₂] are symmetrical in strict sense. And then, because of the symmetrical fragments, the different of orbitals between HOMO and LUMO, which is so slightly (7–18%), cause the charge transfer transitions are very weak, such as HOMO → LUMO intraligand charge transfer (ILCT) transition, metal-to-ligand charge transfer (MLCT) and ligand-to-ligands charge transfer (LLCT) transitions. Thus, on the basis of the above conclusion, we can have good understanding of the phenomenon that [{Ir(ppy)₂Cl}₂] and [{Ir(dfppy)₂Cl}₂] are nearly non-emissive at room temperature; beyond that, to further explore the two precursors, it is also easy to discover that the symmetry is the barricade of light emission. So if we view the issue from a different perspective, to devastate the symmetry of [{Ir(ppy)₂Cl}₂] and [{Ir(dfppy)₂Cl}₂], even slightly, what will the result bring forth? Whether the frontier molecular orbitals will be different? And whether the new complexes which are in coincidence with our purposes will be bright at room temperature?

Fig. 1 A schematic illustration of the reaction and the electron density plots of HOMO and LUMO of 1 and 2.

Fortunately, two new complexes [{Ir(ppy)₂}₂Cl(BTA)] (1) and [{Ir(dfppy)₂}₂Cl(BTA)] (2) (BTA = 1,2,3-benzotriazole) which have good agreement with above thesis were obtained by us. Treatment of a mixture of [{Ir(C^{^}N)₂Cl}₂] (C^{^}N = ppy, dfppy) and benzotriazole (BTA) resulted in the crude product of complex 1 and 2, and further purification was subjected to column chromatography on silica gel. Single-crystal X-ray diffraction of 1 and 2 was carried out to determine their molecular structure. As shown in Fig. S1 and Table S1 (in ESI†), complex 1 and 2 retain the primary molecular fragment of [{Ir(ppy)₂Cl}₂] and [{Ir(dppy)₂Cl}₂] on the whole except one chlorine atom was substituted by BTA. In both complexes, each Ir³⁺ cation also adopts a six-coordinate approach, and the iridium atom exhibits the octahedral coordination geometry with cis-C,C and trans-N,N dispositions. For complex 1, the average bond lengths of Ir–C, Ir–N and Ir–Cl are 1.986(2), 2.092(8) and 2.467(5) Å, respectively. For 2, the corresponding values are 2.002(2), 2.085(6) and 2.460(1) Å, respectively. All other bond lengths and angles are within the normal values for cyclometalated ppy, dfppy and the BTA ligand bound to iridium(III). Taking complex 1 as an example to compare before-and-after frontier molecular orbitals, the HOMO and LUMO of [{Ir(ppy)₂Cl}₂] reside on one chlorine atom 0.1% and 3%, while the HOMO and LUMO of [{Ir(ppy)₂}₂Cl(BTA)] (Table S2†) reside on BTA 0.1% and 1%. The BTA devastates the good symmetry of [{Ir(ppy)₂Cl}₂], but it doesn't obtain obviously more ratio of HOMO and LUMO. Similar result is shown in complex 2. Compared to the precursor of complex 2, the BTA also devastates the symmetry, but it doesn't obtain obviously more ratio of HOMO and LUMO in the fragment of complex 2.

In order to better characterize these two complexes, computed singlet to singlet excitations together with the experimental absorption spectra were also studied by us. In experiment (Fig. 2), absorption spectra show intense features below 340 nm and less intense features in the range of 340–400 nm. Obviously, the calculated absorption spectra (Fig. S2 in ESI†) can reproduce well the experimental features in terms of band positions, intensities, and separations. This spectral region presents three dominant transitions computed at 341, 310 and 289 nm and at 334, 302 and 284 nm for complex 1 and 2, respectively. Compare to complex 1, the complex 2 shown a little blue shift about 5–8 nm, this because the electrondrawing group –F on the Ph moiety results the spectrum shift weekly. Few other transitions of lower intensity are also computed in the same region, and most probably they contribute to yield the complex broad band structure experimentally detected. The main computed transitions agree well with the maxima of absorption experimentally defined at 281 nm and at 275 nm for complex 1 and 2, respectively. In particular, the blue shift of these bands from complex 1 and 2, also expected by simple inspection of the HOMO–LUMO gap for the two compounds, is qualitatively and quantitatively reproduced by calculations. An admixture of metal-to-ligand charge transfer (MLCT) states appears at lower energies after 430 nm with lower extinction coefficients. Excitation at either the π → π* absorption band or the MLCT absorption band in CH₂Cl₂ solutions leads to the same MLCT emission maxima at 484 nm (bluish-green color) and 478 nm (bluish-green color) for 1 and 2, respectively, except for the concurrent change in the emission intensities. As shown in Fig. 2, the fluorinated complex 2 shows a significantly larger value compared with the nonfluorinated analogue complex 1, suggesting an important substituent effect on the emission properties. Besides, the hypsochromic shift was observable from emission spectra because of fluorinated impact on phenylpyridine. As for its reasons, it is probable that the fluoride has the effect of stabilizing the HOMO level (both from the metal and ligand orbitals) while only slightly affecting the ligand LUMO level, as a consequence, both the MLCT and ligand centered (LC) transitions move to higher energy.^38,39 On the basis of the singlet (or triplet) excited-state geometries, we also obtained the computed emission spectra of complex 1 and 2 in CH₂Cl₂ solution, and the results are listed in Table S3 in ESI,† associated with the emissive assignments. Table S3 (in ESI†) shows that the calculated lowest energy emissions of the two complexes are localized at 477.93 and 474.9 nm, respectively. The predicted emission wavelengths slightly deviate from the experimental data by 6 and 3 nm for complex 1 and 2, respectively, which is comparable with experimental values. The emission quantum yields for 1 and 2 in CH₂Cl₂ are given in Table S4 (in ESI†). Their emission quantum yields are 1% and 2.3%, respectively, by reference to an aerated aqueous solution of quinine sulfate in 0.1 M H₂SO₄ (quantum yield 54% at 360 nm in the literature⁴⁰) as a standard. Compared to other dinuclear complexes, the efficiency of our materials in CH₂Cl₂ solution for the complexes is relatively higher.^18,26–29

Fig. 2 The UV-vis absorption and photoluminescence spectra of complex 1 (green) and complex 2 (purplish red) in CH₂Cl₂.

To find the origin of the luminescence of two peculiar dinuclear complexes, DFT and TD-DFT calculations were employed to gain insights into the electronic structure of these systems. From a structural point of view, in order to assess the quality of our computational approach, the computed ground-state structure of complex 1, fully optimized at the DFT level, was compared with the available X-ray diffraction data. Generally, excellent agreement between the computed and observed bond lengths and angles is found. When we try to analyze frontier molecular orbitals of complex 1 and 2, a distinctive long-range electron transfer (LRET) process was raised. As shown in Fig. 1 and Table S2,† the HOMO and LUMO of complex 1 and 2 are greatly different from those of their precursors. The HOMO and LUMO of [{Ir(ppy)₂Cl}₂] and [{Ir(dfppy)₂Cl}₂] are systematically arranged, while the HOMO and LUMO of [{Ir(ppy)₂}₂Cl(BTA)] and [{Ir(dfppy)₂}₂Cl(BTA)] are largely tipped to one sides of the molecular structure. In these fragments, HOMO → LUMO MLCT and LRET process are both met. The emission of complex 1 and 2 can be assigned to have MLCT and LRET process, especially to LRET, it is crucial to the process of luminescence. As far as we know, the literature which is concerned with LRET process abounds in reports of the proteins,^41–51 porphyrin–quinone molecules,^52,53 phenothiazines,⁵⁴ and even mononuclear cyclometallated complexes,⁵⁵ however, binuclear iridium(III) complex is also an “uncharted wilderness”. Moreover, the detailed analysis of radiative decay process (Table 1) reveal that, when the vertical excitation energy levels of two spin forbidden states are lie closely, meanwhile there are spin–orbital coupling interactions between two different occupied metal d orbitals, the intersystem crossing would be strengthened, which eventually lead to the increase of radiative decay rate.⁵⁶

Table 1 Calculated radiative decay constants k_r for 1 and 2 and precursors {Ir(ppy)₂Cl}₂ and {Ir(dfppy)₂Cl}₂

	kr^x (S^−1)	kr^y (S^−1)	kr^z (S^−1)	kr (S^−1)	Quantum yield at 360 nm (Q)
{Ir(dfppy)2Cl}2	1.3 × 10^3	2.0 × 10^3	1.1 × 10^4	4.6 × 10^3
1	2.3 × 10^4	2.9 × 10^4	1.2 × 10^5	5.7 × 10^4	1%
2	2.6 × 10^4	2.6 × 10^5	8.9 × 10^5	3.9 × 10^5	2.3%

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Experimental and computational

All experimental and computational details are available in the ESI.†

Conclusions

In conclusion, two chlorine-bridged dinuclear iridium(III) complexes were synthesized and characterized. The resulting complexes are bright at room temperature, compared to [{Ir(ppy)₂Cl}₂] and [{Ir(dfppy)₂Cl}₂]. This phenomenon is peculiar. To better understand the new peculiar complexes, the aborative experimental and theoretical study has been carried out. Further work on chlorine-bridge precursor was on the way of exploration and development. We expect the implications of these findings to be relevant for the design of more efficient phosphorescence materials in OLED application.

Acknowledgements

The authors are grateful to the financial aid from the China Postdoctoral Science Foundation (Grant No. 2013M541286, 51502285), and the Science and Technology Development of Jilin Province of China (Grant No. 201201078, 20101512, 20110320 and 20140520109JH).

Notes and references

1. M. S. Lowry and S. Bernhard, Chem.–Eur. J., 2006, 12, 7970.
2. P. T. Chou and Y. Chi, Chem.–Eur. J., 2007, 13, 380.
3. R. C. Evans, P. Douglas and C. J. Winscom, Coord. Chem. Rev., 2006, 250, 2093.
4. E. Holder, B. M. W. Langeveld and U. S. Schubert, Adv. Mater., 2005, 17, 1109.
5. Y. R. Sun, N. C. Giebink, H. Kanno, B. W. Ma, M. E. Thompson and S. R. Forrest, Nature, 2006, 440, 908.
6. M. A. Baldo, M. E. Thompson and S. R. Forrest, Nature, 2000, 403, 750.
7. Y. You and S. Y. Park, Dalton Trans., 2009, 1267.
8. Q. Wang and D. G. Ma, Chem. Soc. Rev., 2010, 39, 2387.
9. L. X. Xiao, Z. J. Chen, B. Qu, J. X. Luo, S. Kong, Q. H. Gong and J. Kido, Adv. Mater., 2011, 23, 926.
10. Y. C. Zhu, L. Zhou, H. Y. Li, Q. L. Xu, M. Y. Teng, Y. X. Zheng, J. L. Zuo, H. J. Zhang and X. Z. You, Adv. Mater., 2011, 23, 4041.
11. K. K. W. Lo, M. W. Louie and K. Y. Zhang, Coord. Chem. Rev., 2010, 254, 2603.
12. L. S. Natrajan, A. Toulmin, A. Chew and S. W. Magennis, Dalton Trans., 2010, 39, 10837.
13. Q. Zhao, M. X. Yu, L. X. Shi, S. J. Liu, C. Y. Li, M. Shi, Z. G. Zhou, C. H. Huang and F. Y. Li, Organometallics, 2010, 29, 1085.
14. W. L. Jiang, Y. Gao, Y. Sun, F. Ding, Y. Xu, Z. Q. Bian, F. Y. Li, J. Bian and C. H. Huang, Inorg. Chem., 2010, 49, 3252.
15. L. Murphy, A. Congreve, L. O. Palsson and J. A. G. Williams, Chem. Commun., 2010, 46, 8743.
16. M. X. Yu, Q. Zhao, L. X. Shi, F. Y. Li, Z. G. Zhou, H. Yang, T. Yia and C. H. Huang, Chem. Commun., 2008, 2115.
17. K. K. W. Lo, K. Y. Zhang, S. K. Leung and M. C. Tang, Angew. Chem., Int. Ed., 2008, 47, 2213.
18. E. Baranoff, H. J. Bolink, F. De Angelis, S. Fantacci, D. Di Censo, K. Djellab, M. Gratzel and M. K. Nazeeruddin, Dalton Trans., 2010, 39, 8914.
19. L. Donato, C. E. McCusker, F. N. Castellano and E. Zysman-Colman, Inorg. Chem., 2013, 52, 8495.
20. S. Bettington, M. Tavasli, M. R. Bryce, A. S. Batsanov, A. L. Thompson, H. A. A. Attar, F. B. Dias and A. P. Monkman, J. Mater. Chem., 2006, 16, 1046.
21. M. Y. Wong, G. Xie, C. Tourbillon, M. Sandroni, D. B. Cordes, A. M. Z. Slawin, I. D. W. Samuel and E. Zysman-Colman, Dalton Trans., 2015, 44, 8419.
22. J. Fernández-Cestau, N. Giménez, E. Lalinde, P. Montaño, M. T. Moreno and S. Sánchez, Organometallics, 2015, 34, 1766.
23. V. Chandrasekhar, B. Mahanti, P. Bandipalli and K. Bhanuprakash, Inorg. Chem., 2012, 51, 10536.
24. A. Auffrant, A. Barbieri, F. Barigelletti, J. Lacour, P. Mobian, J. P. Collin, J. P. Sauvage and B. Ventura, Inorg. Chem., 2007, 46, 6911.
25. A. Auffrant, A. Barbieri, F. Barigelletti, J. P. Collin, L. Flamigni, C. Sabatini and J. P. Sauvage, Inorg. Chem., 2006, 45, 10990.
26. V. L. Whittle and J. A. G. Williams, Dalton Trans., 2009, 3929.
27. V. L. Whittle and J. A. G. Williams, Inorg. Chem., 2008, 47, 6596.
28. K. J. Arm and J. A. G. Williams, Dalton Trans., 2006, 2172.
29. E. S. Andreiadis, D. Imbert, J. Pécaut, A. Calborean, I. Ciofini, C. Adamo, R. Demadrille and M. Mazzanti, Inorg. Chem., 2011, 50, 8197.
30. C. Adachi, R. C. Kwong, P. Djurovich, V. Adamovich, M. A. Baldo, M. E. Thompson and S. R. Forrest, Appl. Phys. Lett., 2001, 79, 2082.
31. R. J. Holmes, B. W. D'Andrade, S. R. Forrest, X. Ren, J. Li and M. E. Thompson, Appl. Phys. Lett., 2003, 83, 3818.
32. G. A. Djurovich, P. I. Djurovich and R. J. Watts, Inorg. Chem., 1993, 21, 32.
33. A. D. Ryabov, Chem. Rev., 1990, 90, 403.
34. A. E. Shilov and G. B. Shul’pin, Chem. Rev., 1997, 97, 2879.
35. H. Werner, Dalton Trans., 2003, 3829.
36. I. Omae, Coord. Chem. Rev., 2004, 248, 995.
37. Y. Chi and P. T. Chou, Chem. Soc. Rev., 2010, 39, 638.
38. Q. J. He, J. L. Shi, X. Z. Cui, J. J. Zhao, Y. Chen and J. Zhou, J. Mater. Chem., 2009, 19, 3395.
39. Y. H. Yang, J. H. Cui, M. T. Zheng, C. F. Hu, S. Z. Tan, Y. Xiao, Q. Yang and Y. L. Liu, Chem. Commun., 2012, 48, 380.
40. L. Flamigni, A. Barbieri, C. Sabatini, B. Ventura and F. Barigelletti, Top. Curr. Chem., 2007, 281, 143.
41. B. Giese, Acc. Chem. Res., 2000, 33, 631.
42. D. N. Beratan, J. N. Betts and J. N. Onuchic, Science, 1991, 252, 1285.
43. D. Gust, T. A. Moore and A. L. Moore, Acc. Chem. Res., 1993, 26, 198.
44. F. D. Lewis, R. L. Letsinger and M. R. Wasielewski, Acc. Chem. Res., 2001, 34, 159.
45. N. S. Hush, Coord. Chem. Rev., 1985, 64, 135.
46. G. L. Closs and J. R. Miller, Science, 1988, 240, 440.
47. J. P. Sauvage, J.-P. Collin, J.-C. Chambron, S. Guillerez, C. Coudret, V. Balzani, F. Barigelletti, L. De Cola and L. Flamigni, Chem. Rev., 1994, 94, 993.
48. D. M. Guldi, Chem. Soc. Rev., 2002, 31, 22.
49. G. McLendon, T. Guarr, M. McGuire, K. Simolo, S. Strauch and K. Taylor, Coord. Chem. Rev., 1985, 64, 113.
50. K. S. Schanze, D. B. MacQueen, T. A. Perkins and L. A. Cabana, Coord. Chem. Rev., 1993, 122, 63.
51. G. McLendon and R. Hake, Chem. Rev., 1992, 92, 481.
52. M. R. Wasielewski, Chem. Rev., 1992, 92, 435.
53. D. Gust, T. A. Moore and A. L. Moore, Acc. Chem. Res., 2001, 34, 40.
54. D. Hanss, J. C. Freys, G. Bernardinelli and O. S. Wenger, Eur. J. Inorg. Chem., 2009, 4850.
55. J. C. Freys, G. Bernardinelli and O. S. Wenger, Chem. Commun., 2008, 4267.
56. L.-M. Xie, F.-Q. Bai, W. Li, Z.-X. Zhang and H.-X. Zhang, Phys. Chem. Chem. Phys., 2015, 17, 10014.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10577g

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