Communication Cite This: Organometallics XXXX, XXX, XXX−XXX
Emission Tuning of Ir(N∧C)2(pic)-Based Complexes via Torsional Twisting of Picolinate Substituents Ross J. Davidson,* Yu-Ting Hsu, Dmitry Yufit, and Andrew Beeby* Department of Chemistry, Durham University, South Road, Durham DH1 3LE, U.K.
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S Supporting Information *
ABSTRACT: Pyridine-2-carboxylate (pic) has been employed extensively as a blue-shifting ancillary ligand in the production of cyclometalated iridium complexes used in OLEDs, but surprisingly, further elaboration of this ligand has largely been unexplored. In this work we demonstrate a simple and versatile route for modifying picolinate ligands coordinated to iridium. Reacting a μ-chloro iridium(C∧N) dimer (where C∧N is a phenylpyridinebased ligand) with 4-bromopicolinic acid (HpicBr) yields the corresponding iridium(C∧N)2(picBr) complexes, which were readily modified by a Suzuki− Miyaura reaction to give the corresponding aryl-substituted picolinate complexes. The luminescent behavior of these complexes shows that by restricting the torsional angle between the substituent and pic the emission can be shifted by up to 77 nm.
T
a modest blue shift in the emission.15−17 Recently, we demonstrated the versatility of Ir(PPy)2(pic-Br) complexes by performing Sonogashira couplings on them. These complexes had low ΦPL values, but we were able to demonstrate that the 4-position of pic was the most chemically reactive position on the pic and that substitution at this point had a significant effect on the complexes’ emission properties.18 In this communication, we demonstrate the substitution of the picolinate in Ir(X2ppy)2(pic) with aromatic groups in which the torsional angle between the substituent and pic can be restricted. This was achieved by way of a Suzuki−Miyaura coupling, with an assortment of arylboronic acids selected to provide varying degrees of rotational hindrance and conjugation. Two compounds, Ir(PPy)2(pic-4-Br) and Ir(F2PPy)2(pic-4-Br), were reacted with phenylboronic acid, [1,1′-biphenyl]-4-ylboronic acid, o-tolylboronic acid, and (2,3,5,6-tetramethylphenyl)boronic acid, using Pd(PPh3)4 and K2CO3 in dry dimethylformamide (DMF) solution, to yield the corresponding Ir(X-PPy)2(pic-4-Ar) complexes (see Scheme 1). The DMF was readily removed by precipitation of the complexes by the addition of water, affording an easy route to purification. For complexes 1−3 and 5−7, the yields were high (60− 78%), typical of unhindered Suzuki−Miyaura couplings, with little difference observed between the 2-phenylpyridine (ppy)and 2-(2,4-difluorophenyl)pyridine (F2ppy)-based complexes. However, the yields for complexes 4 and 8 were significantly reduced to 23% and 19%, respectively, attributed to the steric hindrance of (2,3,5,6-tetramethylphenyl)boronic acid. This
he high phosphorescent quantum yield (ΦPL), microsecond lifetimes, and the capacity to harvest both singlet and triplet excitons make iridium(III) bis(2-phenylpyridine) complexes particularly attractive materials to use for organic light-emitting diodes (OLEDs),1 biological probes,2 oxygen sensors,3,4 and other applications. They also have a significant advantage over other emissive systems, in that their emission color can be readily tuned through simple modifications. For iridium bis(phenylpyridine) (ancillary ligand), the addition of either an electron-withdrawing group to the phenylate or an electron-donating group to the pyridine blue-shifts the emission. This is due to a stabilizing of the HOMO largely localized to the phenyl/metal and destabilizing the LUMO localized to the pyridine, respectively.5−9 Additionally, the choice of ancillary ligand can also alter the emission properties.10 The most prominent example of this is observed when replacing an acetylacetonate with a picolinate, resulting in a blue shift of the emission. FIrpic is one of the most famous examples of this and has become the benchmark to which all blue emissive iridium complexes are compared (λemis 468 nm, ΦPL = 0.67). However, FIrpic is particularly insoluble and therefore not suitable for solution-processed OLEDs. To address this problem, derivatives of FIrpic have been created, typically by substituting the phenylpyridine ligand (N∧C) with solubilizing groups: e.g., tert-butyl and mesitylene.11,12 However, despite extensive success with this approach, only brief investigations have been made on modifications of the picolinate ligand (pic). The few examples of these changes have largely been focused on the addition of electron acceptor and donor groups. The addition of an electron acceptor results in the LUMOs being localized to the pic, often favoring 3CT emission.13,14 The addition of electron-donating groups raises the energy of pic, resulting in reduced pic character for the LUMO and gives © XXXX American Chemical Society
Received: April 2, 2018
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DOI: 10.1021/acs.organomet.8b00194 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
complexes varied significantly as the steric hindrance increased, starting with the phenyl and going to the o-tolyl and duryl groups. The emission of the complexes was blue-shifted, 56 nm for F2ppy complexes (5 and 8, taking the highest emission) and 77 nm for ppy complexes (1 and 4), and changed from broad, featureless emissions (3CT in character) to sharp, LCbased emissions (see Figure 2). The difference is further exaggerated at low temperature (77 K, see the Supporting Information) with complex 2 showing a single broad 3CT emission spanning ca. 200 nm while 8 shows five resolved emission bands that are LC in nature. This change was attributed to the degree of conjugation between the pic pyridine and aromatic group. By extension of the conjugation of the pic ligand the orbital energies of the corresponding LUMOs are lowered, resulting in pic making a greater contribution to the emission: i.e., greater MLCT for complexes 1, 2, 5, and 6. However, when the torsional angle between the aryl substituent and pic is increased, this conjugation is reduced, resulting in the complexes behaving more like the parent Ir(X2ppy)(pic) complex. Such behavior has been previously demonstrated when similar substitutions were made on the ppy ligand of an Ir(N∧C)2(pic) system.19 However, changes of this extent have not been reported for such modifications to other parts of the complex. Although kr values for the complexes were similar to those of previously reported alkyne-substituted complexes,18 the nonradiative rates (knr) values were only 8−16% of the value of those observed in the alkyne complexes. This difference is attributed to the reduced freedom of rotation about the substituents, which resulted in ΦPL being 1 order of magnitude higher. This difference was further exaggerated for the F2ppy complexes, with an additional reduction in knr values. This reduction was due to the greater LC character of the F2ppy complexes, where the lowest T1 state will be more localized to the one chromophoric ligand.20 This result was in contrast to results for ppy complexes, where the excited state was more 3 CT in character, and potentially gave rise to more vibration modes that contributed to radiationless deactivation. The photophysical behavior of complex 6 was distinctly different, with an emission lifetime of 16.58 μs (τ0 = 31.76 μs). On the basis of the work of Rodriguez and Guo this phenomenon is attributed to reversible electronic energy transfer (REET),21−23 where the triplet energy of 4-([1,1′-biphenyl]4-yl)picolate was within the thermally accessible range of the iridium−ppy excited state.24 Density functional theory (DFT) calculations showed negligible contribution to the highest occupied molecular
Scheme 1. Complexes 1 (X = H, Ar = Ph), 2 (X = H, Ar = Biphen), 3 (X = H, Ar = o-Tol), 4 (X = H, Ar = Dur), 5 (X = F, Ar = Ph), 6 (X = F, Ar = Biphen), 7 (X = F, Ar = o-Tol), and 8 (X = F, Ar = Dur)
approach should be viable for any arylboronic acid, making it possible to attach aromatic groups to the complex that may not be stable during iridium coordination. X-ray crystallography of complexes 1, 3, and 5−8 showed the typical coordination behavior of the Ir(N∧C)2(pic) complex with the iridium, with a coordination sphere of “N3C2O” and ppy pyridines arranged in a trans geometry (see Figure 1). The variations of substituents in the pic-4-Ar ligand do not noticeably affect the coordination geometry of the iridium atom. The dihedral angles between the pic pyridine ring and the aryl ring varied significantly, with those of the phenyl rings of complexes 1 and 5 being 30.12−37.55°, while the tolyl rings of complexes 3 and 7 have dihedral angles of 42.28−48.94° and the duryl rings of complex 8 have a dihedral angle of 81.42°. Using electrochemical analysis, each of the complexes was shown to have a single oxidation attributed to the Ir(III)/ Ir(IV) redox couple, with no observable reduction within the solvent window of dichloromethane. The oxidation was governed by modifications of the phenylpyridine, with complexes 1−4 having potentials of 0.49−0.51 VFc/Fc+, while complexes 5−8 had potentials of 0.83−0.84 VFc/Fc+. This result shows that modifications to the pic have a negligible effect on the energies of the HOMO. Each of the complexes displayed almost identical electronic absorption behavior, with metal−ligand charge transfer (MLCT) bands ranging from 500 to 370 nm, and π → π* bands ranging from 370 to 250 nm (see Table 1). Complexes 2 and 6 showed enhancement of π → π* originating from the biphenyl groups. The emission behavior of each of the
Figure 1. X-ray crystal structure of compounds 1 (left) and 8 (right). B
DOI: 10.1021/acs.organomet.8b00194 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics Table 1. Emission Wavelength, PLQY, and Lifetimes for the Iridium Complexes 1−8a λemis complex
λabs (ε, 104 L mol−1 cm−1)b
DCM, RTb
Me-THF, RT (77 K)c
PLQY (Φ)b
lifetime (μs)b
kr (105 s−1) knr (105 s−1)
pure radiative lifetime (τ0, μs)
T1 (eV)b
595
582 (495, 532, 565 sh)
0.154
0.338
4.56
25.0
2.194
2.48
520 sh, 593
620 (620)
0.182
0.429
4.24
19.1
2.357
2.56
506 sh, 535
0.441
5.53
17.1
1.807
2.60
0.403
1.095
3.68
5.45
2.717
2.62
5
261 (5.03), 380 (0.58)
0.711
1.394
5.10
2.07
1.960
2.76
6
275 (6.22), 302 (4.33)
471 sh, 525 br, 550 540
0.315
0.288
31.762
2.71
7
258 (5.24), 380 (0.62)
469, 494
8
254 (4.74), 381 (0.55)
469, 496
495, 531, 568 sh (492, 527, 567 sh) 515, 539 sh (494, 529, 569 sh) 474, 500 (459, 481 sh, 492, 518, 531) 473 sh, 517, 548 (516, 554, 597) 469, 499 (461, 491, 525 br, 567 sh) 472, 497 (459, 482 sh, 491, 520, 531)
0.244
4
268 (4.84), 330 (1.58), 401 (0.57) 264 (5.89), 296 (4.81), 425 (0.58) 265 (5.17), 328 (1.56), 400 (0.53) 265 (3.78), 398 (0.42)
1 2 3
518
0.522
16.58
0.565
1.792
3.15
2.43
3.171
2.79
0.469
1.543
3.04
3.44
3.29
2.76
The radiative (kr) and nonradiative (knr) values were calculated according to the equations kr = Φ/τ and knr = (1 − Φ)/τ, from the quantum yields Φ and the lifetime values τ. bRecorded in DCM, crecorded in Me-THF.
a
Figure 2. Emission spectra of complexes 1−4 (left) and 5−8 (right), recorded in dichloromethane (DCM).
orbital (HOMO) from either the pic or aryl substituent. However, the LUMO was localized to both pic and substituent, with an aryl substituent (Ph, Biphen, o-Tol, or Dur) contribution ranging from 29% (2) and 27% (6) to 2% for 4 and 8, again reinforcing the idea that by increasing conjugation of the substituent the pic becomes more significantly involved in the photophysical behavior of the system. In conclusion, through the substitution of Ir(N∧C)2(pic) complexes, PLQYs can be greatly enhanced in comparison to alkyne analogues. Through the incorporation of steric hindrance, it is also possible to electronically decouple substituents from the Ir(N∧C)2(pic) complex, resulting in a new means of tuning emission color.
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Accession Codes
CCDC 1832981−1832986 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for R.J.D.:
[email protected]. *E-mail for A.B.:
[email protected]. ORCID
Ross J. Davidson: 0000-0003-3671-4788
ASSOCIATED CONTENT
Notes
S Supporting Information *
The authors declare no competing financial interest.
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00194. Cartesian coordinates of calculated structures (XYZ) Experimental and synthetic procedures, analytical, NMR spectra, crystallographic data, orbital contributions, low temperature emission spectra and electrochemical data (PDF)
ACKNOWLEDGMENTS R.J.D. gratefully acknowledge the EPSRC (EP/K007548/1) for funding of this work.
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REFERENCES
(1) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nature 2000, 403 (6771), 750−753.
C
DOI: 10.1021/acs.organomet.8b00194 Organometallics XXXX, XXX, XXX−XXX
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Organometallics (2) Lo, K. K.-W.; Li, S. P.-Y.; Zhang, K. Y. New J. Chem. 2011, 35 (2), 265−287. (3) Jana, A.; Crowston, B. J.; Shewring, J. R.; McKenzie, L. K.; Bryant, H. E.; Botchway, S. W.; Ward, A. D.; Amoroso, A. J.; Baggaley, E.; Ward, M. D. Inorg. Chem. 2016, 55 (11), 5623−5633. (4) Jiang, X.; Peng, J.; Wang, J.; Guo, X.; Zhao, D.; Ma, Y. ACS Appl. Mater. Interfaces 2016, 8 (6), 3591−3600. (5) De Angelis, F.; Fantacci, S.; Evans, N.; Klein, C.; Zakeeruddin, S. M.; Moser, J.-E.; Kalyanasundaram, K.; Bolink, H. J.; Grätzel, M.; Nazeeruddin, M. K. Inorg. Chem. 2007, 46 (15), 5989−6001. (6) Di Censo, D.; Fantacci, S.; De Angelis, F.; Klein, C.; Evans, N.; Kalyanasundaram, K.; Bolink, H. J.; Grätzel, M.; Nazeeruddin, M. K. Inorg. Chem. 2008, 47 (3), 980−989. (7) Takizawa, S.-y.; Nishida, J.-i.; Tsuzuki, T.; Tokito, S.; Yamashita, Y. Inorg. Chem. 2007, 46 (10), 4308−4319. (8) Avilov, I.; Minoofar, P.; Cornil, J.; De Cola, L. J. Am. Chem. Soc. 2007, 129 (26), 8247−8258. (9) Henwood, A. F.; Zysman-Colman, E. Chem. Commun. 2017, 53 (5), 807−826. (10) Li, J.; Djurovich, P. I.; Alleyne, B. D.; Yousufuddin, M.; Ho, N. N.; Thomas, J. C.; Peters, J. C.; Bau, R.; Thompson, M. E. Inorg. Chem. 2005, 44 (6), 1713−1727. (11) Kozhevnikov, V. N.; Zheng, Y.; Clough, M.; Al-Attar, H. A.; Griffiths, G. C.; Abdullah, K.; Raisys, S.; Jankus, V.; Bryce, M. R.; Monkman, A. P. Chem. Mater. 2013, 25 (11), 2352−2358. (12) Laskar, I. R.; Hsu, S. F.; Chen, T. M. Polyhedron 2006, 25 (5), 1167−1176. (13) Zhou, Y.; Li, W.; Liu, Y.; Zeng, L.; Su, W.; Zhou, M. Dalton Trans. 2012, 41 (31), 9373−9381. (14) Li, H.; Winget, P.; Risko, C.; Sears, J. S.; Bredas, J. L. Phys. Chem. Chem. Phys. 2013, 15 (17), 6293−6302. (15) Baranoff, E.; Jung, I.; Scopelliti, R.; Solari, E.; Gratzel, M.; Nazeeruddin, M. K. Dalton Trans. 2011, 40 (26), 6860−6867. (16) Bolink, H. J.; Coronado, E.; Garcia Santamaria, S.; Sessolo, M.; Evans, N.; Klein, C.; Baranoff, E.; Kalyanasundaram, K.; Graetzel, M.; Nazeeruddin, M. K. Chem. Commun. 2007, 31, 3276−3278. (17) Park, H. J.; Choi, H. J.; Seo, H. W.; Hyun, M. H.; Yoon, U. C. J. Photopolym. Sci. Technol. 2012, 25 (2), 171−174. (18) Davidson, R.; Hsu, Y.-T.; Bhagani, C.; Yufit, D.; Beeby, A. Organometallics 2017, 36 (15), 2727−2735. (19) Davidson, R.; Hsu, Y.; Batchelor, T.; Yufit, D.; Beeby, A. Dalton Trans. 2016, 45 (28), 11496−11507. (20) Rausch, A. F.; Thompson, M. E.; Yersin, H. J. Phys. Chem. A 2009, 113 (20), 5927−5932. (21) Medina-Rodriguez, S.; Denisov, S. A.; Cudre, Y.; Male, L.; Marin-Suarez, M.; Fernandez-Gutierrez, A.; Fernandez-Sanchez, J. F.; Tron, A.; Jonusauskas, G.; McClenaghan, N. D.; Baranoff, E. Analyst 2016, 141 (10), 3090−3097. (22) Ma, L.; Guo, S.; Sun, J.; Zhang, C.; Zhao, J.; Guo, H. Dalton Trans. 2013, 42 (18), 6478−6488. (23) Sun, J.; Zhong, F.; Zhao, J. Dalton Trans. 2013, 42 (26), 9595− 9605. (24) Lavie-Cambot, A.; Lincheneau, C.; Cantuel, M.; Leydet, Y.; McClenaghan, N. D. Chem. Soc. Rev. 2010, 39 (2), 506−515.
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DOI: 10.1021/acs.organomet.8b00194 Organometallics XXXX, XXX, XXX−XXX
