ARTICLE pubs.acs.org/JPCA
Theoretical Study of Absorption and Emission Properties of Green and Yellow Emitting Iridium(III) Complexes Amel Kadari,†,‡ Aurelien Moncomble,§ Ilaria Ciofini,§ Meziane Brahimi,† and Carlo Adamo*,§ †
Laboratoire Physico-Chimie Theorique et Chimie Informatique, Faculte de Chimie, USTHB, BP 32 Al-alia, Babezzouar, Alger, Algeria Departement de Chimie, Universite Mouloud Maamri, Tizi Ouzou, Algeria § lectrochimie, Chimie des Interfaces et Modelisation pour l'E nergie, E cole Nationale Superieure de Chimie de Paris, Laboratoire d'E Paris, France ‡
bS Supporting Information ABSTRACT: Iridium(III) complexes are among the most used phosphorescent materials for the development of organic light emitting diodes (OLEDs). In this work, the photophysical properties of a family of complexes based on phenyldiazine ligands were studied. Their ground state geometric and electronic structures as well as their absorption and emission spectra were investigated by the means of density functional theory (DFT) and time-dependent DFT (TD-DFT). An extremely good agreement between the computed and experimental values is obtained, thus suggesting that the computational protocol here applied could be used for the in silico screening and design of new Ir-based emitting complexes.
1. INTRODUCTION Cyclometalated complexes containing iridium(III) metal center(s) have attracted much attention in the past decade as new materials for the development of organic light emitting diodes (OLEDs) for displays and low energy lighting devices.17 Furthermore, the same complexes found interesting bioapplications as labeling agents and phosphorescent sensors.814 Their most appealing properties are related to their peculiar luminescent properties,15 such as high emission quantum yields and long excited state lifetimes, photostability, and thermal stability. Not surprisingly, an increasingly large number of cyclometalated Ir(III) complexes has been synthesized in the past decade with the goal of optimizing the phosphorescence efficiency and tuning the emission color by chemically functionalizing the ligands.1,7,1518 Furthermore, since the absorption and emission properties of these systems are strongly related to a subtle interplay between their electronic and structural features at both the ground and excited states, it is not surprising that several theoretical studies have been devoted to the analysis of the photophysical properties of Ir(III) at ab initio (especially density functional theory (DFT)) level.1933 Indeed, while efficient complexes emitting blue or red light have already been subject to intensive experimental and theoretical study, Ir(III) compounds emitting in the green or the yellow are still quite rare, and only recently the teams of H. Guo and D. Zhou34 were able to extend the color tunability of iridium(III) complexes to these wavelengths, using phenyldiazine ligands rarer than the well-known phenylpyridine derivatives. These complexes are constituted by a pseudooctahedral Ir(III) center coordinated by acetylacetonate ligand (acac) and two phenyldiazines (either pyrimidine (PPM) or pyrazine (MPPZ)) which can be fluorinated or hydrogenated, giving rise r 2011 American Chemical Society
to the four complexes depicted in Figure 1 that will be the subject of the present theoretical study. Their properties, at both the ground and excited states, were investigated by means of DFT and time-dependent DFT (TD-DFT) methods to gain insights into their photophysical behavior with the final aim of setting up a computational protocol enabling the in silico prediction and design of new purposely tailored Ir(III) complexes with selected luminescent properties. Several previous works have already pointed out the very good performances of DFT and TD-DFT in the description and prediction of ground and excited state properties of both organic and inorganic systems, especially when using hybrid exchange correlation functionals and properly including solvent effects (see, for instance, ref 35 and references therein). Nonetheless, some failures of DFT and TD-DFT are also nowadays quite well established, and in particular, it is recognized that, even when using hybrid functionals, particular care should be taken when dealing with charge transfer excitations in the case of a real throughspace electron transfer.36 Specific diagnostic indexes of eventual pathological cases have also been developed in this framework.37,38 The paper is structured as follows: after a description of the computational protocol applied (section 2), the geometric and electronic features of the four complexes at the ground state will be discussed (section 3.1). Next, the absorption and emission spectra will be investigated (section 3.2), and some general remarks and conclusions on the computational procedure will be drawn (section 4). Received: July 22, 2011 Revised: August 26, 2011 Published: September 02, 2011 11861
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Figure 1. Schematic drawings of the four studied complexes. MPPZ, 2-phenyl-3-methylpyrazine; DFMPPZ, 2-(2,4-difluorophenyl)-3-methylpyrazine; PPM, 2-phenylpyrimidine; DFPPM, 2-(2,4-difluorophenyl)pyrimidine.
2. COMPUTATIONAL DETAILS All calculations were carried out at the DFT level using the Gaussian 09 package.39 The PBE0 exchange-correlation functional40,41 was used throughout. Iridium was described using the HayWadt effective core potential and the associated double-ζ basis set,42 while the 6-31G(d,p) Pople basis set43 was used for all other atoms. This level of theory has been shown to give reliable results for both structural and electronic features of transition metal complexes including excitation energies.25,44 The structure of the singlet ground state (S0) of all the complexes was fully optimized both in a vacuum and in solution (dichloromethane) in the absence of any symmetry constraints. The nature of the stationary points (minima) was confirmed by subsequent vibrational frequency analysis. Solvent effects (dichloromethane) were taken into account using the polarizable continum model (PCM45) as implemented in the Gaussian program (IEF-PCM formalism46). The 40 lowest lying vertical excitations were computed at the TD-DFT level using the same functional and basis set both in a vacuum and in dichloromethane. Absorption spectra were then simulated by a Gaussian convolution using a fixed full width at half-minimum (fwhm) of 0.3 eV. To investigate the phospholuminescence properties, all the complexes were also fully optimized in their lowest triplet spin state (T1), without symmetry constrains, both in a vacuum and in dichloromethane. On such structures, phosphorescence emission wavelengths were thus computed using a ΔSCF approach as previously reported in the literature.47,48 3. RESULTS AND DISCUSSION 3.1. Geometry and Electronic Study of Ground State. The main structural parameters obtained for the four investigated iridium(III) complexes, both in a vacuum and in dichloromethane, are collected in Table 1 in the case of the lowest lying singlet (S0) and triplet (T1, discussed in section 3.2) states. From the analysis of the ground state structural data it is rather clear that the Ir atoms are, as expected, octahedrally coordinated.
Table 1. Computed Bond Lengths (Å) in a Vacuum and in Dichloromethane (in Parentheses) for the S0 and T1 States I
IrN
II
III
IV
S0
T1
S0
T1
S0
T1
S0
T1
2.003
1.986
2.014
1.974
2.019
1.999
2.017
1.949
(2.006) (1.988) (2.017) (1.974) (2.022) (1.999) (2.020) (1.955) IrN0 IrC
2.143 2.170 2.141 2.142 2.161 2.194 2.144 2.153 (2.145) (2.176) (2.144) (2.145) (2.163) (2.198) (2.145) (2.163) 1.988
1.951
1.988
1.981
1.994
1.955
1.992
1.989
(1.989) (1.951) (1.988) (1.981) (1.995) (1.958) (1.992) (1.991) IrC0
1.993
1.994
1.996
1.990
1.998
2.001
1.995
2.000
(1.994) (1.997) (1.997) (1.991) (1.999) (2.004) (1.996) (1.995) IrO1 2.164
2.163
2.152
2.160
2.161
2.159
2.153
2.157
(2.164) (2.153) (2.150) (2.157) (2.159) (2.147) (2.149) (2.123) IrO2 2.061 2.078 2.055 2.067 2.055 2.070 2.056 2.065 (2.063) (2.076) (2.057) (2.067) (2.059) (2.068) (2.059) (2.058)
The first coordination sphere of the metal atom is not strongly affected either by the change in the aromatic skeleton of the ligand (that is changing from pyrazine to pyrimidine based ligand) or by its fluorination, with the four complexes showing very similar IrN, IrO, and IrC distances (Table 1). Furthermore, solvent effects are predicted to be negligible on the first coordination sphere of the metal atom, with the maximal change between the gas phase and dichloromethane computed structures being below 0.01 Å. A more careful analysis of the geometric parameters actually shows a small asymmetry between the two aromatic ligands concerning the IrN distances (of about 0.14 Å). A similar deviation from an ideal octahedral coordination is observed for the two IrO bonds involving the acac ligand, although less pronounced of about only 0.10 Å. On the other hand, the IrC bonds are computed to be practically equivalent. The computed structural parameters and, in particular, the computed IrN and IrC distances are indeed very close 11862
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Table 2. Computed Highest Occupied Molecular Orbital Energies (EHOMO), Lowest Unoccupied Molecular Orbital Energies (ELUMO), and Gap Energies Eg (eV) in a Vacuum and in Dichloromethane (in Parentheses) Together with the Corresponding Experimental Values I exptla
calcd
EHOMO 5.31 5.43
II exptla
5.64 5.89
(5.66) ELUMO 3.13 1.64 2.18
3.79 (3.86)
a
Figure 2. Computed highest occupied molecular orbitals (HOMOs) and lowest occupied molecular orbitals (LUMOs) of complexes IIV (isovalue 0.04 au).
to those experimentally obtained by X-ray measurements (ca. 2.043/2.036 Å and 1.997 Å, respectively) for analogous binuclear Ir complexes containing phenylpyridine ligands.47 Concerning the ligands, both phenylpyrazine and phenylpyrimidine ligands are computed to be planar in all the complexes, with the only significant exception being represented by complex II. For this system a distortion of bicyclic phenylpyrazine ligands (DFMPPZ) is computed with a dihedral angle between the two aromatic cycles of about 17° (see the Supporting Information, Figure SI.1). This distortion can be reasonably attributed in the case of the DFMPPZ ligand to the steric repulsion between the closely lying methyl group, carried by the pyrazine ring and absent in complexes III and IV, and the fluorine atom, functionalizing the phenyl ring and absent in complex I. From the point of view of the electronic structure, all the systems display the typical features of complexes containing a d6 metal in a pseudooctahedral environment. In particular, the three highest occupied molecular orbitals (HOMOs) are mainly localized on the iridium atom (although sizable contributions from the ligands can be pointed out) while the lowest unoccupied molecular orbitals (LUMOs) are mainly localized on the π-system of phenyldiazine ligands (see Figure 2). Due to the asymmetry of the first coordination sphere, indeed the three highest occupied MOs, corresponding to the t2g orbitals in a perfect octahedral environment, are not exactly degenerate: the HOMO and HOMO 1 are practically isoenergetic while the HOMO 2 is lying ca. 0.3 eV below them. From the point of view of the orbital composition, the contribution of the d metal atomic orbital to both the HOMO and HOMO 1 is about 40% for all complexes, ranging from 37% for II to 41% for complex III, with the rest being represented by the phenyldiazine ligand. For comparison, the contribution of iridium centered atomic orbitals to the LUMO is only about 5%, ranging from 4.6% for III to 6.6% for complex I. This contribution is mainly arising from the 5dz2 of the Ir. The HOMO energy can be experimentally derived from electrochemistry experiments (Table 234). From a qualitative point of view, both the computed and the experimental data consistently underly the sizable electron-withdrawing character of the fluorine substituents, as clearly pointed out by the lower energies (of about 0.3 eV) computed for the HOMOs of the fluorinated compounds II and IV.
exptla
3.39 1.88 4.01 (4.01)
IV exptla
2.93 1.41
(5.95) 3.12 1.62
(1.56) 2.34
3.98 (4.10)
calcd
5.65 5.80
(5.66)
(1.95) 2.25
calcd
5.27 5.39
(5.96)
(1.80) Eg
calcd
III
(1.67) 2.53
4.18 (4.28)
Experimental data from ref 34.
As a consequence, the HOMOLUMO gap increases substantially in the case of the fluorine substituents as previously shown in the literature for other Ir(III) complexes such as the ones containing phenylpyridine, bipyridine,47,49 or phenylquinazoline50 ligands. From a quantitative point of view, a good linear correlation between the computed HOMO energy values and the experimental ones can indeed be found (R = 0.98; see Figure SI.2 in the Supporting Information), thus allowing proposing the level of theory here applied as potentially useful for the prediction of the oxidation potential of new related compounds. A substantial linear correlation (R = 0.91; see Figure SI.3 in the Supporting Information) is also found between the computed gap and experimental gap although the computed HOMOLUMO energy difference is substantially larger than the observed optical gap (of about 1.6 eV on average). This finding is per se not surprising, and it simply reflects the lack of relaxation effects that should lower the LUMO energy. As a matter of fact, when considering the results at the TD-DFT level, the computed gap values are in the range 2.753.25 eV, only 0.7 eV larger than the experimental ones. Of note it is also interesting to point out that while the predicted properties (HOMO energy and gap) for complexes I, III, and IV strongly correlate with the experimental values, complex II shows somehow a systematic slightly erratic behavior (see Figures SI.2 and SI.3 in the Supporting Information). 3.2. Absorption and Emission Spectra. The experimental UVvisible spectra of the four Ir complexes analyzed (Figure 334) show intense absorptions below 300 nm and less intense features in the 300375 nm energy range, giving rise to two main structured absorption bands with maxima around 260 and 330 nm, respectively (Table 3). Both bands actually correspond to metal to ligand charge transfer (MLCT) transitions from the Ir t2g type orbitals to the pyrazine/pyrimidine ring of the phenyldiazine ligand, consistent with the previously detailed electronic structures of the complexes. The simulated absorption spectra (computed in dichloromethane) are also in depicted Figure 3. The corresponding computed vertical transitions are reported in the Supporting Information. In general, the overall features of the spectra are well reproduced by the calculations in terms of both position and shape of the band. The computed absorption maxima (reported in Table 3) are in excellent agreement with experimental data with errors about 0.2 eV for the band occurring at higher energies and of 0.4 eV for the one occurring at the lowest energy. The error in 11863
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Table 4. Computed Atomic Polar Tensor (APT) Charges on the Ir Atoms for Studied Complexes Ir APT charge I
0.14
II III
0.19 0.22
IV
0.23
Table 5. Computed and Experimentala Phosphorescence Energies (in nm) λtheo
a
Figure 3. Experimental (solid lines, from ref 34) and simulated (dashed lines) absorption spectra of complexes IIV.
Table 3. Main Experimentala and Computed Absorption Maxima (in nm)b I a
II calcd
exptl
258
252
322
(250) 291
exptl
(292)
a
III a
calcd
exptl
258
256
327
(254) 310 (306)
IV a
calcd
exptl
261
268
256
269
341
(264) 354
330
(264) 334
(339)
calcd
(324)
a
Experimental data from ref 34. b Values computed in dichloromethane are reported in parentheses.
the computed maxima of the pyrimidine-containing complexes (PPM family) is actually only ca. 0.2 eV for both maxima. In the case of pyrazine-based complexes (I and II), the simulated spectra show the presence of bands with maxima at 379 and 382 nm, respectively, corresponding to the band experimentally observed for both systems at 410 nm. Also, in this case, these transitions have mainly a MLCT character.
λexp
vacuum
CH2Cl2
CH2Cl2
I
588
594
575
II
600
600
546
III IV
539 527
539 523
527 496
Experimental data from ref 34.
A sizable red shift of pyrimidine complexes (III and IV) when compared to pyrazine analogues (I and II) is also computed in agreement with the experimental data and with the known acceptor properties of the formers. This finding is also reflected by the higher atomic polar tensor (APT) charge51 on the iridium atom computed for complexes III and IV compared to the two others (Table 4). To investigate the phosphorescence process, the structure of the first excited triplet state (T1) was also optimized and the results are collected in Table 1. The T1 state is of MLCT character with spin density localized on the metal center and on the pyrazine/pyrimidine ring of the phenyldiazine ligand. Although the triplet relaxation leads to some changes in bond lengths, these latter are very minor and imply structural rearrangement on the order of, for instance, only 0.07 Å on the IrN bond in the case of complex IV. Indeed, all conclusions drawn for the ground state structural features still hold. Comparison of the bond lengths computed in a vacuum and in dichloromethane (Table 1) also did not show any significant change, with the largest variation in bond length being below 0.03 Å. Phosphorescence emission energies are reported in Table 5. In general, very good agreement between experimental and computed data is obtained, with the absolute error being in the range 0.050.12 eV, if one excepts the pathological case of complex II, for which a larger error is computed (0.2 eV). Indeed, even when including the results obtained for complex II, a good correlation between computed and experimental emission energies is obtained (R = 0.9; see Figure SI.4 in the Supporting Information) and, in agreement with the experimental trend, a blue shift observed when going from pyrazine to pyrimidine derivatives is computed.
4. CONCLUSION A performing computational procedure based on DFT and TD-DFT for the evaluation of absorption and emission spectra of Ir(III) complexes is here presented and validated for a family of systems containing phenyldiazine ligands. Both the absolute values and the trends in photophysical properties are actually well described by the computational procedure here applied and 11864
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’ ASSOCIATED CONTENT
bS
Supporting Information. Sketches of the optimized geometries of the four studied complexes; plots of the experimental versus computed HOMO energies, gaps, and emission energies; Cartesian coordinates of all complexes; computed excitations for all complexes; complete ref 39. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work has been supported by the Ministere de l'Enseignement Superieur et de la Recherche Scientifique (MESRS) d'Algerie. ’ REFERENCES (1) Lowry, M. S.; Bernhard, S. Chem.;Eur. J. 2006, 12, 7970–7977. (2) Chou, P. T.; Chi, Y. Chem.;Eur. J. 2007, 13, 380–395. (3) Evans, R. C.; Douglas, P.; Winscom, C. J. Coord. Chem. Rev. 2006, 250, 2093–2126. (4) Holder, E.; Langeveld, B. M. W.; Schubert, U. S. Adv. Mater. 2005, 17, 1109–1121. (5) Sun, Y. R.; Giebink, N. C.; Kanno, H.; Ma, B. W.; Thompson, M. E.; Forrest, S. R. Nature 2006, 440, 908–912. (6) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nature 2000, 403, 750–753. (7) You, Y.; Park, S. Y. Dalton Trans. 2009, 1267–1282. (8) Lo, K. K. W.; Louie, M. W.; Zhang, K. Y. Coord. Chem. Rev. 2010, 254, 2603–2622. (9) Natrajan, L. S.; Toulmin, A.; Chew, A.; Magennis, S. W. Dalton Trans. 2010, 39, 10837–10846. (10) Zhao, Q.; Yu, M. X.; Shi, L. X.; Liu, S. J.; Li, C. Y.; Shi, M.; Zhou, Z. G.; Huang, C. H.; Li, F. Y. Organometallics 2010, 29, 1085–1091. (11) Jiang, W. L.; Gao, Y.; Sun, Y.; Ding, F.; Xu, Y.; Bian, Z. Q.; Li, F. Y.; Bian, J.; Huang, C. H. Inorg. Chem. 2010, 49, 3252–3260. (12) Murphy, L.; Congreve, A.; Palsson, L. O.; Williams, J. A. G. Chem. Commun 2010, 46, 8743–8745. (13) Yu, M. X.; Zhao, Q.; Shi, L. X.; Li, F. Y.; Zhou, Z. G.; Yang, H.; Yia, T.; Huang, C. H. Chem. Commun. 2008, 2115–2117. (14) Lo, K. K. W.; Zhang, K. Y.; Leung, S. K.; Tang, M. C. Angew. Chem., Int. Ed. 2008, 47, 2213–2216. (15) Flamigni, L.; Barbieri, A.; Sabatini, C.; Ventura, B.; Barigelletti, F. Top. Curr. Chem. 2007, 281, 143–203. (16) Endo, A.; Suzuki, K.; Yoshihara, T.; Tobita, S.; Yahiro, M.; Adachi, C. Chem. Phys. Lett. 2008, 460, 155–157. (17) Coppo, P.; Plummer, E. A.; De Cola, L. Chem. Commun. 2004, 1774–1775. (18) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2003, 125, 7377–7387. (19) Li, X.; Minaev, B.; Agren, H.; Tian, H. Eur. J. Inorg. Chem. 2011, 16, 2517–2524.
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