Do the Intramolecular π Interactions Improve the Stability of Ionic

Article pubs.acs.org/JPCC

Do the Intramolecular π Interactions Improve the Stability of Ionic, Pyridine-Carbene-Based Iridium(III) Complexes? Rubén D. Costa,*,† Rubén Casillas,† and Joan Cano‡,§ †

Friedrich-Alexander-Universität Erlangen-Nürnberg, Department of Chemistry and Pharmacy& Interdisciplinary Center for Molecular Materials (ICMM), Egerlandstrasse 3, D-91058, Erlangen, Germany ‡ Instituto de Ciencia Molecular (ICMOL) and §Fundació General de la Universitat de València (FGUV), Universitat de València, Paterna, València, Spain ABSTRACT: Throughout the last years one of the most intensive research topics in light-emitting electrochemical cells (LECs) focused on the design of blue-emitting, ionic iridium(III) complexes. To this end, the most recent strategy is the use of carbene-based ancillary ligands. Although blue LECs have been successfully fabricated, the stability has been noted as the main drawback. To overcome this problem, Zhang et al. have recently explored the use of π interactions to enhance the strength of pyridine-carbene-based complexes. The authors suggested that the use of intramolecular π−π stacking interactions by means of pendant phenyl rings to improve the stability of LECs is not as effective as in devices with diimine-based complexes. To interpret this phenomenon clearly, the features of a family of pyridinecarbene-based iridium(III) complexes, in which phenyl groups are sequentially attached to the carbene and the pyridine rings of the ancillary ligand, have been thoroughly studied by using a theoretical approach. Our most valuable findings shed light onto the lack of significant improvement regarding device stability when the pyridine-carbene-based iridium(III) complexes are used. Quite likely, the easy population of the metal-centered 3MC excited states, in which the pyridine ring of the carbene-based ligand is totally decoordinated to the iridium(III) center, is the most plausible explanation for the device behavior. The theoretical study performed in complexes, in which the phenyl substituent is attached to the pyridine ring as well as those with two π−π interactions, clearly confirms that the use of π interactions is not successful in providing a cage conformation in both ground and excited states. This limitation is tentatively attributed to an intrinsic feature when pyridine-carbene-based ligands are utilized.

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self-quenching,20−26 and a reduction of the turn-on time caused by the increased number of mobile anions,27−29 respectively. The latest strategy is the use of π−π interactions to design supramolecular cage Ir-iTMCs for devices that feature stabilities of thousand of hours.9,23,30−38 This approach has been deeply explored in diimine-based Ir-iTMCs in terms of synthesis, photophysical, and theoretical studies and device behavior under different operation conditions during the last 5 years. Although the last breakthroughs in Ir-iTMCs-based LECs ensure their bright future, all of these improvements have been not realized in an LEC with a unique Ir-iTMC. In particular, the final long-term objective is to achieve efficient, stable, whitelight LECs combining either blue or orange emitters in a twocomponent system or blue, green, and red emitters in a threecomponent system. Stable and efficient orange LECs have been reported, but the design of stable, blue-emitting Ir-iTMCs that yield efficient and long-living LECs is the current challenge.3,5−11,19 To this end, the use of carbene derivatives as ancillary ligand emerged as a plausible concept toward novel blue-emitting Ir-

INTRODUCTION The prime strength of ionic iridium(III) complexes, hereafter abbreviated as Ir-iTMCs, is most likely found as phosphorescence materials in LECs.1,2 The latter is described as a singlelayer electroluminescent device consisting of an Ir-iTMC in combination with an ionic liquid or ionic polymers. The presence of small, mobile anions in the active layer provokes that LECs are intrinsically insensitive to the work function of the electrodes employed. Therefore, in contrast with OLEDs, air-stable electrodes, such as gold, silver, or aluminum, can be used, and as a consequence, their encapsulation does not have to be as rigorous as with OLEDs. Besides these unique characteristics that make LECs the simplest kind of electroluminescent devices, the main reason for the new momentum of Ir-iTMCs-based LECs is the possibility to establish clear relationships between intrinsic features of the Ir-iTMCs and the device performance in terms of color, efficiency, turn-on time, and stability.1,2 More specifically, the emission of the Ir-ITMCs can be easily tuned by either using different ancillary ligands3−11 or implementing electron-withdrawing and electron-donating substituents in the main and ancillary ligands.12−19 In the same fashion, the introduction of neutral or charged bulky groups in the periphery of the Ir-iTMCs leads to an enhancement of the efficiency, owing to reduction of the © 2013 American Chemical Society

Received: February 7, 2013 Revised: March 18, 2013 Published: April 11, 2013 8545

dx.doi.org/10.1021/jp401380b | J. Phys. Chem. C 2013, 117, 8545−8555

The Journal of Physical Chemistry C

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iTMCs.6,8−10 This strategy was first proposed by Yang et al. 2 years ago, and it is founded on the very high lowest unoccupied molecular orbital (LUMO) of the carbene ligand that allows us to obtain emission originated from the main cyclometalated ligand.6 Indeed, this design leads a straightforward tuning of the emission maximum from near-UV to red regions by using classical tuning strategies on the main ligand.8 The main drawback is the low device stability when this type of compound is used as the primary active component in LECs. To tackle this issue, Zhang et al. have recently explored the possibility of extrapolating the use of π−π interactions in pyridine-carbene-based Ir-iTMCs that feature bluish or greenish emission in solution and solid state.9 Contrary to what was expected, the device stability is enhanced but not as significant as that observed in devices with diimine-based Ir-iTCMs, in which the π-staking interactions are present.23,30,32,34−36 To elucidate the underlying reasons of the aforementioned finding, we provide a comprehensive theoretical study of a family of four pyridine-carbene-based Ir-iTMCs with different number of π−π interactions in this work (Figure 1). More specifically, the simplest complex used as a reference compound is [Ir(ppy)2(pymi)]+ (1) (ppy = 2-phenylpyridine and pymi = 1-pyridyl-3-methylimidazolin-2-ylidene-C,C2′). Following, two complexes with a phenyl substituent attached to either the carbene ring or to the pyridine ring that performs an intramolecular π-stacking interaction with the main ligands are studied. These compounds are noted as [Ir(ppy)2(pyphmi)]+ (2) and [Ir(ppy)2(phpymi)]+ (3), where pyphmi = 1-pyridyl-3-phenylimidazolin-2-ylidene-C,C2′ and phpymi = 1-(4-phenyl-pyridyl)-3-methylimidazolin-2-ylideneC,C2′. Finally, the compound [Ir(ppy)2(phpyphmi)]+ (4) (phpyphmi = 1-(4-phenyl-pyridyl)-3-phenylimidazolin-2-ylidene-C,C2′) with two π−interactions, that is, two phenyl substituents attached to the pyridine and carbene moieties, closes the family of pyridine-carbene-based Ir-iTMCs investigated in the current work. Importantly, compounds 1 and 2 were those recently reported by Zhang et al.,9,39 while 3 and 4 have not yet been sensitized.

1. Quite likely, the easy population of the 3MC states, in which the pyridine ring of the ancillary ligand is totally decoordinated to the iridium(III) center, is the most plausible explanation for the device behavior. Indeed, the theoretical study performed in 3 and 4, in which the phenyl substituent is attached to the pyridine ring, clearly confirms that the use of π interactions is not successful in providing a cage conformation in both ground and excited states. This limitation is tentatively attributed as an intrinsic feature of pyridine-carbene-based ligands.

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RESULTS AND DISCUSSION Computational Details. Density functional theory (DFT) calculations were carried out with the A.01 revision of the Gaussian 09 program package. 40 All calculations were performed without symmetry considerations and considering solvent effects within the SCRF (self-consistent reaction field) theory using the polarized continuum model (PCM) approach to model the interaction with the solvent.41,42 For this purpose, we selected acetonitrile because it is the most commonly used solvent in the photophysical characterization of Ir-iTMCs. Given the vast number of functionals available for DFT calculations in the field of coordination complexes, the groundstate geometry of complex 2 was optimized by using different functionals: the popular Becke’s three-parameter B3LYP hybrid functional that includes the Lee−Yang−Parr correlation functional,43−45 the combination of Becke’s functional with Perdew−Wang’s 1991 gradient-corrected correlation functional (B3PW91),46−48 the Perdew−Wang exchange functional as modified by Adamo and Barone combined with PW91 correlation (mPW1PW91),49 and finally the hybrid functional using the 1998 revised form of the PBE exchange and correlation functional (PBEh1PBE).50,51 As the basis set, the 6-311G** for C, H, F, and N atoms52 and the “double-ζ” quality LANL2DZ basis set for the Ir element were utilized.53 The best functional was selected in terms of: (i) comparing the geometrical parameters calculated for the coordination sphere of the iridium(III) center in the singlet ground state of the complex with the X-ray crystallographic data, (ii) the computational time needed to perform the geometry optimization, and (iii) the quality of the simulated absorption spectrum calculated within the time-dependent (TD-DFT) approach. The latter was performed taking into account 50 excited states. Figure 2 shows the differences between the theoretical values calculated for the bond lengths and bond angles of the iridium(III) coordination sphere using the different functionals and the experimental X-ray data. Deviations from experimental values are found to be larger for calculations performed B3LYP functional. In contrast, B3PW91, mPW1PW91, and PBEh1PBE appear to behave similarly and provide values that are closer to the experimental values. This is also valid when analyzing the π−π interactions by means of centroid−centroid distance between the pendant phenyl ring and the phenyl ring of the main ligand. The derivation of the predicted value with the experimental X-ray data was noted as 0.25, 0.27, 0.30, and 0.32 Å for PBEh1PBE, mPW1PW91, B3PW91, and B3LYP, respectively. A second criteria to select the best functional was the computational time needed to complete the geometrical optimization. On one hand, calculations using B3LYP were three times less efficient than the others. PBEh1PBE and mPW1PW91 were faster than using B3PW91, saving a 31 and 12% in computation time, respectively. The PBEh1PBE was therefore revealed as the most efficient in computation time.

Figure 1. Chemical structures of the pyridine-carbene-based complexes. The numbering used to describe the structural parameters of the Ir(III) coordination sphere is also shown.

The major thrust of this work is the establishment of the scenario of excited states in this novel family of pyridinecarbene-based Ir-iTMCs. Special emphasis was put on elucidating the structural changes in both ground and emitting excited states upon introducing phenyl groups in the ancillary ligand as well as the role of the triplet metal centered (3MC) excited states that are responsible of the degradation process by means of ligand exchange reaction with nucleophilic components. More specifically, our findings shed light onto the lack of significant improvement regarding device stability when 2 is used as a main component compared with LECs with 8546

dx.doi.org/10.1021/jp401380b | J. Phys. Chem. C 2013, 117, 8545−8555

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computational time and reliable data in terms of optimized molecular structure and description of the excited states. Theoretical Description of the Ground State. The geometries of the complexes 1−4 in the electronic ground state (S0) were fully optimized by using the PBEh1PBE functional, and the values obtained for selected bond lengths are listed in Table 1. As seen in Figure 2, the calculated theoretical distances Table 1. Selected Bond Distances (in angstroms) Calculated for Complexes 1, 2, 3, and 4 in S0, T1, and the MetalCentered (3MC) States compound

bonds

S0

T1

1

Ir−N1 Ir−N2 Ir−N3 Ir−C1 Ir−C2 Ir−C3 Ir−N1 Ir−N2 Ir−N3 Ir−C1 Ir−C2 Ir−C3 Ir−N1 Ir−N2 Ir−N3 Ir−C1 Ir−C2 Ir−C3 Ir−N1 Ir−N2 Ir−N3 Ir−C1 Ir−C2 Ir−C3

2.069 2.062 2.192 2.052 2.009 2.083 2.072 2.059 2.192 2.052 2.009 2.084 2.072 2.058 2.301 2.061 2.001 2.073 2.079 2.058 2.298 2.058 2.000 2.083

2.046 2.073 2.224 2.013 2.002 2.103 2.052 2.078 2.217 1.969 2.050 2.094 2.048 2.069 2.340 2.022 1.999 2.090 2.047 2.068 2.356 2.017 1.998 2.093

2

3

4

Figure 2. Calculated differences between theoretical bond lengths (upper part) and bond angles (lower part) computed for the Ir(III) coordination sphere of complex 2 using different functionals and X-ray crystallographic data.

As a final criteria to scrutinize the best functional, singlet TDDFT calculations were performed using the above-proposed functionals and the structure optimized with PBEh1PBE. Figure 3 displays the simulated absorption spectra obtained for complex 2. As a matter of fact, all functionals provide very similar absorption spectra. In summary, the PBEh1PBE functional is concluded to be the most appropriate one, offering the best efficiency in

3

MCcarbenea 2.054 2.059 2.029 2.030 2.069 2.056 2.057 2.029 2.031 2.064 2.059 2.054 2.031 2.029 2.069 2.056 2.056 2.032 2.028 2.063

3

MCppy 2.897 2.199 2.175 2.070 2.012 2.026 2.910 2.190 2.179 2.070 2.011 2.019 2.890 2.243 2.234 2.063 2.011 2.050 2.778 2.272 2.248 2.059 2.011 2.054

a

Ir−N1 distance is not given for the 3MCcarbene state because the pyridine ring of the ancillary ligand is decoordinated in this state.

of the coordination sphere of the iridium(III) central compare well with the X-ray data reported for 2 and suggest a nearoctahedral coordination for all complexes. In general, this notion is valid for all complexes independently of the phenyl substitution. In particular, theoretical calculations predict three remarkable features in the molecular structure of 1−4. First, in line with previously reported works,3,8,9 the length of the iridium−carbon (main ligand) bond (Ir−C1 in Table 1 and Figure 1) is in all cases longer than the Ir−C2 bond, owing to the strong trans effect of the carbene ring in the ancillary ligand. Second, upon attaching phenyl groups at both carbene and pyridine rings, the Ir−C3 (ancillary ligand) bond is barely affected and is of ∼2.08 Å for all complexes. Third, with and without phenyl substitution in the carbene ring (1 and 2) the Ir−N3 bond is 2.19 Å, but it increases to 2.30 Å in 3 and 4 when adding the phenyl ring to the 6-position of the pyridine ring. A likely rationale could be the different type of π−π interactions between the phenyl ring of the main ligand and the pendant phenyl ring attached either to the carbene or to the pyridine of the ancillary ligand. However, independently of where the pendant phenyl is attached, the face-to-face πstacking interactions are very similar to calculated centroid− centroid distances of ∼3.90 Å. Normally this type of π

Figure 3. Simulated absorption spectra of complexes 1−4 by using different functionals in acetonitrile. 8547

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distortion of the ancillary ligand seen in the dihedral C−N−C− N of pyridine-carbene-based Ir-iTMCs effectively weakens the metal−ligand bond (Ir−N3 bond in Table 1), the energy stabilization of the LUMO must be ascribed to the localization on only the ancillary ligand. Simulated Absorption Spectra. On the basis of the optimized ground-state geometry, the TD-DFT method was used to calculate the absorption properties of 1−4. The singlet excited states with oscillator strength ( f) superior to 0.05 are listed in Table 2 together with the vertical excitation energies (λ), the dominant monoexcitations, and the nature of the electronic transition. In addition, fitted Lorentzian-type absorption curves with the calculated absorption data are shown in Figure 5. Seen from experimental data, the absorption spectra of 2 are divided in two regions, namely, from the ultraviolet up to ∼350 nm with maxima at 265 and 310 nm and from 350 nm extending to the visible region with maxima at 375 nm.9 In line with the experimental data, theoretical calculations reproduce the same features. First, the lowest energy band peaks at ∼376 nm and is mainly described as a HOMO → LUMO (S1, f = 0.02) and HOMO → LUMO+1 (S2, f = 0.03) transitions and, as such, the nature of the band is a mixture between metal-to-ligand, ligand-to-ligand, and intraligand charge transfer ( 1 MLCT, 1 LLCT, and 1 ILCT, respectively). Second, the next two bands are also wellreproduced by the theoretical calculations. The singlet excited state S7 is responsible of the first band centered at ∼316 nm that is described as a combination of 1MLCT, 1LLCT, and 1 ILCT monoelectronic transitions. The second band is mainly governed by S29 (f = 0.14), S30 ( f = 0.28), and S35 (f = 0.11) excited states that peak at 262, 260, and 251 nm, respectively. These states evolve electronic transitions from the HOMO-1, HOMO-2, and HOMO-5 to LUMO and LUMO+5 and might also be considered as a mixture of 1MLCT, 1LLCT, and 1ILCT bands rather than the typical assumption of spin-allowed π−π transitions centered only on the ligands. A very similar scenario of singlet excited states is predicted for complex 1, as shown in Table 2 and Figure 5. This fact is expected because the substitution of the methyl group by a pendant phenyl ring does not have a profound impact on both the electronic and the molecular structures vide supra. In contrast, interesting differences are noted in the absorption spectra of 3 and 4 compared with those of 1 and 2, quite likely due to the aforementioned changes in the electronic structure caused by the phenyl substitution in the pyridine ring. In detail, the lowest energy region (>350 nm) is ruled by S1 (HOMO → LUMO) and S2 (HOMO → LUMO +1) excited states that are well-defined as 1MLCT/1LLCT and 1 MLCT/1ILCT transitions, respectively. In line with the strong reduction of the HOMO−LUMO energy gap described for 4, theoretical calculations predict a red-shifted lowest absorption band (∼20 nm) compared with the other complexes (Table 2). Interestingly, in the high-energy region (