Computational and Theoretical Chemistry 963 (2011) 298–305

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Computational and Theoretical Chemistry

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Theoretical studies on structures and spectroscopic properties of a seriesof heteroleptic iridium complexes based on tridentate bis(benzimidazolyl)pyridineligand

Yong Yang, Fu-Quan Bai, Hong-Xing Zhang ⇑, Xin Zhou, Chia-Chung SunState Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 August 2010Received in revised form 25 October 2010Accepted 25 October 2010Available online 3 November 2010

Keywords:IridiumElectronic structuresSpectroscopic propertiesDFT

2210-271X/$ - see front matter � 2010 Elsevier B.V.doi:10.1016/j.comptc.2010.10.032

⇑ Corresponding author. Tel.: +86 431 88498966; faE-mail address: zhanghx@mail.jlu.edu.cn (H.-X. Zh

Electronic structures and spectroscopic properties of series of novel mixed-ligand Ir(III) complexes,[Ir(Mebip)(bpy)Cl]2+ (1), [Ir(Mebip)(ppy)Cl]+ (2), [Ir(Mebip)(ppz)Cl]+ (3) and [Ir(Mebip)(ptz)Cl]+ (4)[bpy = 2,20-bipyridine, ppy = phenylpyridine, ppz = 5-(2-pyridyl)-pyrazole and ptz = 5-(2-pyridyl)-triazole, Mebip = bis(N-methylbenzimidazolyl)pyridine] have been investigated in detail by theoreticalcalculation. The geometries in the ground and excited state for the cationic complexes 1–4 wereoptimized by density functional theory (DFT) and single excitation configuration interaction method(CIS), respectively. The absorptions and emissions of 1–4 in CH3CN solution were calculated usingtime-dependent density functional theory (TDDFT) associated with the polarized continuum model(PCM). With the assistance of the analysis of frontier molecular orbitals, HOMOs of 2–4 mainly compriseIr(III) ion and different chelating ligands, but that of 1 is dominated by Ir(III) ion and Mebip. LUMO andLUMO + 1 of 1–4 predominantly reside on the Mebip ligand. The lowest-lying absorption of 1 at 460 nm(from HOMO to LOMO) is assigned to MLCT and ILCT transitions, while that of 2–4 occurs at 457, 448 and418 nm, respectively, originating from a mixture of MLCT, LLCT and ILCT transitions. The 542 nm phos-phorescence of 1 is assigned as 3MLCT and 3ILCT characters, while the emission for 2–4 at 591, 572and 549 nm is described as 3MLCT, 3LLCT and 3ILCT transitions. Phosphorescent emissions of 1–4 mirror-ing their lowest-lying absorption transitions, display a red shift of 2 > 3 > 4 > 1 which is in line with the rdonor ability of bidentate ligands following an order of ppy(2) > ppz(3) > ptz(4) > bpy(1).

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1. Introduction

Phosphorescent Iridium(III) complexes bearing the cyclometa-lating ligands have been under extensive exploration, because oftheir rich photochemical properties and being applied as high effi-ciency phosphor dopants in the fabrication of electroluminescentdevices [1–5]. In principle, the origin of phosphorescence is a spin-forbidden radiative transition, commonly from the triplet excitedstate to the singlet ground state. However, the strong spin–orbit cou-pling effect on Ir(III) ion partially eliminates the spin-forbidden tran-sition limitation and makes efficient singlet–triplet intersystemcrossing (ISC) more feasible. Hence, the mixing of singlet state intotriplet manifold enhances the probability for dipole-allowed transi-tion and gives rise to highly emissive phosphorescence at room tem-perature. As a general rule, phosphorescent emission of Ir(III)complexes benefits substantially from the charge transfer (CT)transitions, such as MLCT (metal-to-ligand charge transfer), LLCT

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(inter-ligand charge transfer) and ILCT (intra-ligand charge trans-fer). Usually, electron transitions originated from the mixture ofthose three excited states have emissions that span the whole visiblespectrum [6–9]. What is more, organic light emitting device (OLED)fabricated by phosphorescent emitters can theoretically realize aninternal quantum efficiency of 100% in striking contrast to the 25%upper limit for fluorescent counterpart [10–12].

In an attempt to obtain high efficiency and brightness phospho-rescent Ir(III) complexes and in-depth understanding towardsluminous mechanism, many efforts have been under way inboth experimental and theoretical studies [13–21]. For instance,the homoleptic complexes mer-Ir(N^N)3 [N^N = CF3- or tert-butyl-substituted 5-(2-pyridyl)-pyrazole (ppz) or 5-(2-pyridyl)-triazole(ptz)] have been successfully prepared by Chou, Chi and co-work-ers [22]. It has been reported that those species displayed temper-ature-dependent dual phosphorescence and existed ILCT ? LLCTtriplet state conversion accounting for such a particular dual emis-sion. It is worth to note that the coordinated N atom in the azolemoiety can form a stronger chelate interaction owing to its greatacidity in comparison with the analogue of pyridyl. In addition,

Y. Yang et al. / Computational and Theoretical Chemistry 963 (2011) 298–305 299

the p-accepting strength of pyridyl coupled with the high r-donorability of azolate interact directly on the centered Ir(III) ion with aconvenient means for electronic delocalization [23,24].

More recently, Haga and co-workers have synthesized a series ofnovel mixed-ligand Ir(III) complexes with formula [Ir(Mebip)(L)X]n+

and [Ir(Mebib)(L)X]n+, where Mebip = bis(N-methylbenzimidazol-yl)pyridine, Mebib = bis(N-methylbenzimidazolyl)benzene, L = 2,20-bipyridine (bpy) or phenylpyridine (ppy) derivatives, X = Cl, Br, I,CN, CH3CN or –CCPh and n = 0, 1 or 2, and their photophysical andphotochemical properties have been carefully examined [25,26].Upon Mebib introduced to replace Mebip ligand, the higher phospho-rescent quantum efficiency was observed as a result of stronger ligandfield induced by the Ir–C bond which increases the d–d energy gap andhence the radiationless transitions can be suppressed [1,27]. How-ever, incontrast to [Ir(Mebip)2]3+ and [Ir(Mebib)(Mebip)]2+, the mixedbidentate and tridentate ligands of complexes [Ir(Mebip)(L)Cl]n+ exhi-bit a relative higher emission yield. Moreover, [Ir(Mebip)(L)Cl]n+

served as cationic complexes could be synthesized under mild condi-tion, and have a good solubility in polar solvents and even in aqueousmedia [25,28–30].

From the above background, a detailed theoretical study on ser-ies of the mixed-ligand coordination environments of the Ir(III) ionsurrounded by both tridentate and bidentate ligands, [Ir(Mebip)(bpy)Cl]2+ (1), [Ir(Mebip)(ppy)Cl]+ (2), [Ir(Mebip)(ppz)Cl]+ (3) and[Ir(Mebip)(ptz)Cl]+ (4), was carried out by ab initio and densityfunctional theory (DFT). It is worth mentioning that the negativelycharged bidentate ligands ppz and ptz were introduced withoutany terminal substituents, such a design strategy cannot only allowan accessible synthesis in experiment [24] but also get a plainunderstanding how they exert an influence on spectroscopic prop-erties and assignments of charge transfer in theory. Accordingly,with this Ir(N)5Cl- and Ir(N)4CCl-type structural modification, wecarried out the present work here, aimed at establishing electronicstructure of 1–4 and understanding their various spectroscopicbehaviors, as well as investigating the relationship between thespectra and the bidentate ligands. We hope that these investiga-tions will be found useful in long-term design of highly emissiveIridium phosphorescent materials.

2. Computational methods

Based on the single-crystal X-ray crystallography [25,26], thegeometries in the ground and excited states for the cationic Irid-ium(III) complexes 1–4 were fully optimized using density func-tional theory (DFT) at the B3LYP level [31–33] and singleexcitation configuration interaction approach (CIS) [34–36],respectively. In several recent theoretical study cases, calculationsemploying these methods have been well in agree with the corre-sponding experimental observations [20,37–42], and equallyshown the feasibility here for such an Ir(III) complexes system.On the basis of the optimized geometry structures, time-dependentdensity functional theory (TDDFT) [43–45] with the same functionaland basis set along with the polarized continuum model (PCM)[46,47] were then performed to determine the absorption andemission properties in CH3CN media at room temperature. Inaddition, the analysis of the associated frontier molecular orbitalswas carried out to obtain the nature of the electronic transitions.

Further, as depicted in Fig. 1, complexes 1–4 were adopted C1

symmetry in the calculation which means that no symmetryrestriction was imposed on these species, and to a certain extentgetting closer to the experimental determinations. In order toenhance the computational efficiency, substituent groups such astrifluoromethyl or tert-butyl attached to the bidentate ligandswere not taken into account in the article, due to they commonlyhave a relatively weak effect on phosphorescent properties in com-parison to the cyclometalated ligands [24,26,48].

With regard to the choice of basis sets involving in the calcula-tion, the quasi-relativistic pseudo-potentials of Ir(III) ion proposedby Hay and Wadt [49,50] with 17 valence electrons were em-ployed, and a ‘‘double-n’’ quality basis set LANL2DZ associated withthe pseudo-potential was adopted. As an approximation for thesimplified description of Ir(III) ion, the relativistic effective poten-tial (ECP) was used to replace the inner core electrons, thus leavingthe outer core (5s25p6) electrons and the (5d6) valence electronsfor Ir(III) ion. Moreover, one additional f-type polarization functionwas implemented on Ir(III) ion (a = 0.938) [51]. The 6-31G(d) basissets were taken for Cl, N, C and H atoms. Consequently, the basissets for Ir(III) ion (8s6p3d/3s3p2d), Cl (3s3p/2s2p), C and N(10s5p/3s2p), and H (4s/2s) were used for the description of thevalence shell, respectively. Take 2 for instance, there have been629 basis functions and 292 electrons involving in the calculation.All calculations were accomplished using Gaussian03 softwarepackage [52].

3. Results and discussion

3.1. Optimized geometries in the ground state and triplet excited state

The optimized structures of complexes 1–4 in the ground stateare illustrated in Fig. 1 as well as the key atom labeling, and thecorresponding selected geometrical parameters both in the groundand excited state together with X-ray crystal data are provided inTable 1 [25,26]. With the aid of analyzing the metal–ligand inter-action, the indirect explanations for the electron transition charac-ters could be offered in a qualitative way. The calculated resultsreveal that the coordination of central Ir(III) ion displays a dis-torted octahedral arrangement for all complexes, as shown inFig. 1. From the investigations reported by Haga and Liu, etc[26,53,54], there may exist the steric isomerism to both 3 and 4,namely the Ir–N4 bond in the Ir–pyridylazole moiety lies oppositethe Ir–Cl bond. However, calculations confirm that the structuresof 3 and 4 are readily adopted relative to their isomers, due tothe energies of the isomers are 0.40 and 0.38 eV higher than thoseof 3 and 4, respectively.

As given in Table 1, the optimized geometrical data in theground state are correlated with the corresponding experimentalvalues on the whole [25,26]. Generally, the difference in bondlengths of the metal connected to surrounding atoms as in thepresent case is related to their trans bonding and the cooperativeeffect [26,55]. The Ir–Cl bond of 2 is located trans to the Ir–C5 bondin the Ir–ppy moiety, while those of 1, 3 and 4 lie in the trans posi-tion of the coordinating N5 atom of the bidentate ligands. TheseIr–Cl bond lengths for 1, 3 and 4 (range from 2.387 to 2.428 Å)are much shorter than that of 2 (2.505 Å), indicating that the Ir–N5 bond exerts a stronger bonding interaction at the trans positionin comparison with the trans effect induced by the Ir–C5 bond.Likewise, the greater donor strength of azoles in 3 and 4 relativeto that of pyridyl fragment of bpy in 1 should be responsible forthe elongation of Ir–Cl bond being trans to the azoles. On the otherhand, being a neutral ligand, bpy binds to the Ir(III) ion with the da-tive interaction; whereas chelation between the anionic 2-pyridylazole ligand and the Ir(III) ion possesses higher bonding strengthin comparison with former, giving rise to a minor contraction ofIr–N5 of 3 and 4. Regarding the Ir–N bond in the Ir–Mebip moiety,the two symmetric Ir–N (N1, N3) bonds on the imidazolyl fragmentare the same distance for each complex and range from 2.053 to2.064 Å for all four complexes, which are longer than the Ir–N2bond lengths (1.982–2.000 Å). The shortening of Ir–N2 bond canreasonably be explained by structural restriction of the tridentateligand [25]. Deviated from the measured value by 4.0�, the N4–C3–C4–N5 (C5) dihedral angles of 0.0� clearly demonstrate that the

Fig. 1. Optimized geometrical structures of complexes 1–4 in the ground state at the B3LYP/LANL2DZ level.

Table 1Selected optimized geometrical parameters for [Ir(Mebip)(bpy)Cl]2+ 1, [Ir(Mebip)(ppy)Cl]+ 2, [Ir(Mebip)(ppz)Cl]+ 3 and [Ir(Mebip)(ptz)Cl]+ 4 in the ground and the first tripletexcited state, together with the experimental values of 1 and 2.

Parameter 1 2 3 4 1 2

1A 3A 1A 3A 1A 3A 1A 3A Exptla Exptlb

Bond length (Å)Ir–Cl 2.387 2.390 2.505 2.510 2.428 2.437 2.420 2.428 2.338(3) 2.475(3)Ir–N1 2.064 2.079 2.053 2.076 2.054 2.076 2.054 2.076 2.039(8) 2.037(8)Ir–N2 2.000 2.006 1.982 2.010 1.984 2.004 1.984 2.002 1.991(8) 1.972(7)Ir–N3 2.064 2.079 2.053 2.076 2.054 2.076 2.054 2.076 2.032(9) 2.02(1)Ir–N4 2.077 2.105 2.085 2.105 2.099 2.122 2.109 2.132 2.046(9) 2.059(7)Ir–N5 2.070 2.090 2.007 2.004 2.011 2.012 2.049(9)Ir–C5 2.029 2.036 2.00(1)

Bond angle (�)N2–Ir–N3 79.5 79.4 79.6 79.2 79.7 79.4 79.7 79.4 78.9(3)N2–Ir–N4 179.3 177.3 178.7 177.6 175.1 173.8 175.4 173.9 178.5(4)N4–Ir–N5 78.7 78.3 78.4 78.1 78.2 77.9 81.0(4)N4–Ir–C5 79.8 79.8N1–Ir–N3 159.0 158.7 159.1 158.4 159.4 158.7 159.4 158.8 156.3(2)

Dihedral angle (�)N2–C1–C2–N3 �0.9 �1.4 �0.8 �2.0 0.6 1.1 0.3 0.8 �1(1)N4–C3–C4–N5 0.0 0.0 0.0 0.0 0.0 0.0 4(1)N4–C3–C4–C5 0.0 0.0

a,bFrom Refs. [25,26], respectively.

300 Y. Yang et al. / Computational and Theoretical Chemistry 963 (2011) 298–305

bidentate ligands have a planar conformation. The slight discrepancyof geometries between the calculated and the measured values isreasonable, because the former results are obtained adopting thefree molecular in the gas phase, whereas the latter ones are exam-ined in the crystalline state.

On the basis of the optimized S0 structures, the triplet excitedstate (T1) geometric structures for 1–4 are optimized using the

CIS method and the selected geometrical parameters are also listedin Table 1. As shown in Table 1, all bond lengths follow the almostidentical variation trend for 1–4, being elongated more or less inthe T1 state in contrast to those in the S0 state, except for thenegligible contraction for the Ir–N5 bond of 3. Additionally, theN2–Ir–N4 bond angle distorts somewhat from its equilibrium S0

state about 1.1–2.0�, and the N2–C1–C2–N3 dihedral angle still

Fig. 2. Simulated absorption spectra of 1 (black/dashed line), 2 (red/dotted line), 3(green/solid line) and 4 (blue/dash-dotted line) in CH3CN at room temperature fromthe data calculated with the TDDFT method (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.).

Y. Yang et al. / Computational and Theoretical Chemistry 963 (2011) 298–305 301

approximates to 0� in the T1 states which can be attributed to thelesser twisting motion of Mebip ligand. These above alterations ofgeometries between ground and excited states induced by theelectronic promotion will have a fundamental influence on thetransition assignments as well as molecular orbital compositions.

3.2. Absorption spectra and the S0 molecular orbital properties

The selected absorptions in CH3CN along with the correspond-ing oscillator strengths, the main configuration (with larger CIcoefficients), the assignments of transition as well as the experi-mental values are summarized in Table 2 [25,26]. The simulatedGaussian type absorption curves (wavelength vs. oscillator strength)are shown in Fig. 2. Further, an analysis of frontier molecularorbital (FMO) in S0 is performed to gain detailed insights into theelectronic transition characters and is offered in Tables 3–6.

As depicted in Fig. 2, the spectral profiles for 1 and 2 are generalaccordance with the experimental illustrations [25,26], and there isa strong spectral resemblance between 3 and 4. The lowest-energyabsorptions for 1–4 occur in the region of 420–460 nm with smal-ler oscillator strengths characterized as the mixed ligand-centeredand MLCT transitions. In the case of 1, the 146A ? 147A (fromHOMO to LUMO) transition appears at 460 nm correlated well withthe experimental observation of 452 nm [25], which is originatedfrom [p(Mebip) + dxz(Ir)] ? [p�(Mebip)] excitations, thus the low-est-energy absorption of 1 can be assigned into ILCT/MLCT transi-tions. Besides having a nearly identical charge transfer transitionnatures to 1 at 460 nm, the lowest-energy absorptions at 457,448 and 418 nm for 2–4, respectively, possess still an additionalLLCT character derived from [p(ppy)]/[p(ppz)]/[p(ptz)] ? [p�(Me-bip)] transitions, even with a larger degree than their respectiveILCT contribution. The above results manifest that the lowest-energy absorptions have a notable increase of MLCT participationcoupling with the enhanced LLCT contribution. Upon the enhance-ment of the extent of electron density population distributing overthe entire molecular framework, these lowest-lying absorptionsdisplay a distinct blue-shift in the order 4 > 3 > 2 > 1. Thus, it wouldbe expected that the closer energy levels of the chromophores of 4

Table 2Absorptions of 1–4 in CH3CN media at room temperature determined by the TDDFT meth

Transition Confign (CI coeff) k/(nm) (E

Singlet ? singlet1 X1A ? A1A 146A ? 147A (0.68064) 460 (2.69

X1A ? B1A 145A ? 147A (0.66864) 378 (3.26X1A ? C1A 145A ? 149A (0.47784) 319 (3.87X1A ? D1A 146A ? 151A (0.45769) 292 (4.27

139A ? 148A (0.34300)

2 X1A ? A1A 146A ? 148A (0.65980) 457 (2.71X1A ? B1A 142A ? 147A (0.58505) 362 (3.49X1A ? C1A 145A ? 149A (0.62715) 302 (4.21X1A ? D1A 146A ? 155A (0.62487) 239 (5.28

3 X1A ? A1A 143A ? 145A (0.68351) 448 (2.76X1A ? B1A 140A ? 145A (0.62477) 353 (3.54X1A ? C1A 137A ? 145A (0.67776) 312 (4.01X1A ? D1A 133A ? 144A (0.57285) 261 (4.63

4 X1A ? A1A 143A ? 145A (0.66586) 418 (2.96X1A ? B1A 141A ? 144A (0.66955) 359 (3.36X1A ? C1A 137A ? 145A (0.67193) 313 (4.01X1A ? D1A 133A ? 144A (0.54973) 262 (4.65

136A ? 146A (0.30624)

Singlet ? triplet1 1A ? 3A 146A ? 147A (0.69820) 525 (2.362 1A ? 3A 146A ? 148A (0.59690) 528 (2.353 1A ? 3A 143A ? 145A (0.57437) 515 (2.404 1A ? 3A 143A ? 144A (0.68293) 553 (2.24

a From Refs. [25,26].

will decrease the transition dipole moment and lead to possibleradiationless pathways [56,57].

As seen from the analysis on FMOs, virtual orbitals at lower en-ergy level of all four complexes are predominantly localized on thecyclometalated ligands, especially on the p�-based Mebip. LUMOand LUMO + 1 are mainly Mebip-based, indicating that the p�orbitalof Mebip is located at lower energy level than those of N^N/C^N,which can be ascribed to extendedp conjugation of Mebip and henceresults in an evidently decreased energy level of corresponding vir-tual orbitals [58]. Furthermore, the lack of metal composition in theunoccupied MOs should reduce the transition probability for theoccurrence of MC- and/or LMCT-based absorption, the result ofwhich makes the possible radiationless deactivation avertible [24].In Table 2, the absorptions with the largest oscillator strength of1–4 appear at 378, 362, 353 and 359 nm, respectively, where 1 and2 are closely comparable to their corresponding experimental values

od, together with the experimental values.

/eV) Oscillator Assignment kexptl (nm)a

) 0.0173 ILCT/MLCT 452) 0.3780 ILCT 354) 0.1839 ILCT 315) 0.1289 LLCT/MLCT

MLCT/ILCT/LLCT

) 0.0196 MLCT/LLCT/ILCT) 0.2239 ILCT/LLCT 348) 0.1509 ILCT/LLCT 313) 0.1220 MLCT/LLCT/ILCT 242

) 0.0217 MLCT/LLCT/ILCT) 0.2696 MLCT/ILCT/LLCT) 0.1914 ILCT/LLCT/MLCT) 0.1796 LLCT/ILCT/MLCT

) 0.0282 MLCT/LLCT/ILCT) 0.2833 ILCT) 0.1760 ILCT/LLCT/MLCT) 0.1925 LLCT/ILCT/MLCT

MLCT/ILCT/LLCT

) ILCT/MLCT) MLCT/LLCT/ILCT 510) MLCT/LLCT/ILCT) MLCT/LLCT/ILCT

Table 3Molecular orbital compositions (%) of 1 in the ground state.

Orbital Energy (eV) MO composition Main bond type Ir component

Ir Cl Mebip bpy

151A �1.7682 1.0 0.1 1.0 97.9 p�(bpy)149A �2.8605 1.7 0.1 97.1 1.1 p�(Mebip)148A �2.9345 4.6 0.1 3.7 91.5 p�(bpy)147A �3.2390 5.2 1.2 91.5 2.1 p�(Mebip)

HOMO–LUMO energy gap146A �6.6734 30.3 4.1 63.3 2.3 p(Mebip) + d(Ir) 29.8dxz

145A �7.0255 3.4 1.4 94.9 0.3 p(Mebip)139A �7.8277 60.2 0.0 15.4 24.3 d(Ir) + p(bpy) + p(Mebip) 59.5dxy

Table 4Molecular orbital compositions (%) of 2 in the ground state.

Orbital Energy (eV) MO composition Main bond type Ir component

Ir Cl Mebip ppy

155A 0.0011 0.4 0.0 82.5 17.1 p�(Mebip) + p�(ppy)149A �1.7527 4.3 0.1 3.7 91.9 p�(ppy)148A �2.5293 2.0 0.0 97.7 0.2 p�(Mebip)147A �2.8276 7.2 2.6 88.7 1.5 p�(Mebip)

HOMO–LUMO energy gap146A �5.8546 44.3 7.2 19.2 29.3 d(Ir) + p(ppy) + p(Mebip) 43.6dxz

145A �6.4960 4.8 0.0 25.1 70.1 p(ppy) + p(Mebip)142A �6.8073 8.2 0.0 79.6 12.2 p(Mebip) + p(ppy)

Table 5Molecular orbital compositions (%) of 3 in the ground state.

Orbital Energy (eV) MO composition Main bond type Ir component

Ir Cl Mebip ppz

145A �2.5957 1.7 0.0 98.2 0.1 p�(Mebip)144A �2.9043 5.9 1.7 91.0 1.4 p�(Mebip)

HOMO–LUMO energy gap143A �5.9640 38.8 6.7 16.4 38.1 d(Ir) + p(ppz) + p(Mebip) 38.4dxz

140A �6.8083 37.5 22.6 36.3 3.6 d(Ir) + p(Mebip) + p(Cl) 35.2dyz

137A �7.1776 14.3 23.0 60.5 2.2 p(Mebip) + p(Cl) + d(Ir) 13.3dyz

133A �8.1790 20.3 48.2 24.8 6.6 p(Cl) + p(Mebip) + d(Ir) 19.8dyz

Table 6Molecular orbital compositions (%) of 4 in the ground state.

Orbital Energy (eV) MO composition Main bond type Ir component

Ir Cl Mebip ptz

146A �2.0126 5.4 0.1 3.8 90.7 p�(ptz)145A �2.6539 1.6 0.0 98.3 0.1 p�(Mebip)144A �2.9791 5.7 1.6 91.2 1.5 p�(Mebip)

HOMO–LUMO energy gap143A �6.2064 39.9 7.5 26.3 26.4 d(Ir) + p(ptz) + p(Mebip) 39.7dxz

141A �6.8742 2.6 0.8 89.5 7.1 p(Mebip)137A �7.2396 17.9 27.0 51.5 3.6 p(Mebip) + p(Cl) + d(Ir) 16.6dyz

136A �7.4138 61.7 2.4 16.7 19.2 d(Ir) + p(ptz) + p(Mebip) 61.2dxy

133A �8.2922 21.0 46.0 26.6 6.4 p(Cl) + p(Mebip) + d(Ir) 20.4dyz

302 Y. Yang et al. / Computational and Theoretical Chemistry 963 (2011) 298–305

of 354 and 348 nm [25,26]. As for 1, the absorption at 378 nm has aMebip-based charge transfer and strongly resembles that of 4 at359 nm in nature and, both absorptions are attributed to [p(Me-bip)] ? [p�(Mebip)] transition as ILCT character. HOMO-4 (142A)of 2 mainly consists of 79.6% p(Mebip) and 12.2% p(ppy), so the exci-tation from HOMO-4 to LUMO (147A) is substantially contributed by[p(Mebip) + p(ppy)] ? [p�(Mebip)] transitions and is responsiblefor the absorption at 362 nm, whereas the 353 nm absorption for3 is assigned to a mixture of MLCT/ILCT/LLCT generated from[dyz(Ir) + p(Mebip) + p(Cl)] ? [p�(Mebip)] transitions. These results

here clearly demonstrate that the alteration of bidentate ligands inthis series has a significant effect on absorption/charge transferproperties as well as tuning the emission (vide infra) despite thecoordination environments around Ir(III) ion are quite similar.

The calculated higher absorptions of 1 and 2 are at 319 nm(3.87 eV) and 302 nm (4.21 eV), respectively. Both absorptionsare equally contributed from the 145A ? 149A excitation similarto their lower-lying absorptions (378 and 362 nm, respectively)in character, for which such absorption of 1 has a Mebip-basedILCT nature, while that of 2 can be described as a [p(ppy) +

Table 7Phosphorescent emissions of 1–4 in CH3CN at room temperature calculated with the TDDFT method, together with the experimental values.

Transition Confign (CI coeff) E/nm (eV) Assignment kexptl (nm)a

1 3A ? 1A 147A ? 146A (0.69601) 542 (2.29) 3ILCT/3MLCT 5472 3A ? 1A 147A ? 146A (0.68706) 591 (2.10) 3MLCT/3LLCT/3ILCT 6103 3A ? 1A 144A ? 143A (0.68048) 572 (2.17) 3MLCT/3LLCT/3ILCT4 3A ? 1A 144A ? 143A (0.68216) 549 (2.26) 3MLCT/3ILCT/3LLCT

a From Refs. [25,26].

Table 8Molecular orbital compositions (%) in the excited state for complexes 1–4 at the B3LYP/LANL2DZ level.

Orbital Energy (eV) MO composition Main bond type Ir component

Ir Cl Mebip N^N/C^N

1 147A �3.2327 5.0 0.9 91.8 2.3 p�(Mebip)146A �6.6045 24.2 2.9 71.4 1.5 p(Mebip) + d(Ir) 23.7dxz

2 147A �2.7813 6.1 2.2 90.4 1.3 p�(Mebip)146A �5.8535 44.9 7.5 21.3 26.3 d(Ir) + p(ppy) + p(Mebip) 44.1dxz

3 144A �2.8597 5.2 1.4 92.1 1.3 p�(Mebip)143A �5.9686 40.7 7.2 19.2 32.8 d(Ir) + p(ppz) + p(Mebip) 40.0dxz

4 144A �2.9367 5.1 1.4 92.2 1.3 p�(Mebip)143A �6.2143 39.6 7.4 34.4 18.6 d(Ir) + p(Mebip) + p(ptz) 39.2dxz

Y. Yang et al. / Computational and Theoretical Chemistry 963 (2011) 298–305 303

p(Mebip)] ? [p�(ppy)] transition with ILCT/LLCT natures. In thecase of the pyridylazole-based complexes 3 and 4, this type ofabsorption around 310 nm has a combined ILCT/LLCT/MLCTcharacters which is assigned to [p(Mebip) + p(Cl) + dyz(Ir)] ?[p�(Mebip)] transition. On the other hand, the highest energyabsorptions are remarkably blue-shifted following an order of2 > 3–4 > 1. The major blue-shift of [Ir(Mebip)(ppy)Cl]+2 comparedwith [Ir(Mebip)(bpy)Cl]2+ 1, [Ir(Mebip)(ppz)Cl]+ 3 and [Ir(Me-bip)(ptz)Cl]+ 4 can be attributed to the replacement of N^N withC^N ligand: the stronger r donor property of Ir–C5 bond and thereduction of total charge, which facilitate the electron densityincorporation between metal ion and ligands and thus give riseto the enhancement of metal composition involved in the MLCTstate. Consequently, MLCT state combined with the ligand-centeredp–p� manifold make the singlet-to-triplet intersystem crossingaccessible due to the stronger spin–orbit coupling of Ir(III) ion,

Fig. 3. Emissions of 1–4 originated from the transitions ca

and hence the mixing between singlet and triplet excited statesenhances the dipole-allowed transition probability [24,59,60].

Moreover, we also investigate the vertical triplet absorptions for1–4, which occur at 525, 528, 515 and 553 nm, respectively, pos-sessing the mixed ligand-centered and 3MLCT transition properties(Table 2). As for 2, a weak absorption band around 510–530 nm inthe experiment is supposed as a 3MLCT band [26], however, thecalculation manifests that the mixture of singlet and triplet MLCTstates should be accounted for it.

3.3. Phosphorescent properties and bidentate ligand effects in CH3CNmedia

On the basis of the optimized triplet excited state geometries,phosphorescent emissions of all four complexes in CH3CN solutionare obtained by TDDFT method, and the emissions coupled with

lculated using the TDDFT method in CH3CN solution.

304 Y. Yang et al. / Computational and Theoretical Chemistry 963 (2011) 298–305

the transition assignments as well as the experimental values[25,26] are compiled in Table 7. To gain further insights into thetransition mechanism, the compositions of primal frontier molecu-lar orbitals involving in the emission are presented in the Table 8.

From Table 7, we can see that the emissions for 1–4 are at 542,591, 572 and 549 nm, respectively, being distinguishable and con-sistent well with the corresponding experimental values of 547and 610 nm for 1 and 2 [25,26]. As listed in Table 8, the emissionsof 1–4 are all originated from LUMO ? HOMO transition. TheLUMO of the four complexes dwells basically on the Mebip ligand,while the HOMO is contributed from different chromophores withlarger Ir(III) ion composition. With regard to 1, the HOMO (146A) ismainly composed of 71.4% p(Mebip) and 23.7% dxz(Ir), thus147A ? 146A transition responsible for the 542 nm emission is as-signed to a mixed 3ILCT and 3MLCT origin. In sharp contrast to 1, itis worthy to note that bidentate ligand of 2–4 possesses a relativelyhigher composition in HOMO. This result can be ascribed to theintroduction of a single Ir–Nbpy dative bond in 1, which forms arather weak interaction between bpy chelate and Ir(III) ion andleads to a notable reduction of electron density on the bpy. Addi-tionally, calculations indicate that the metal composition in HOMOhas a strong tendency to increase accompanying the greatly en-hanced donor strengths of bidentate ligands (2 > 3 > 4 > 1). Withthe largest 3MLCT participation, complex 2 at 591 nm emissiongenerates from [p�(Mebip)] ? [dxz(Ir) + p(ppy) + p(Mebip)] transi-tion with 3MLCT/3LLCT/3ILCT character. Similar charge transfercharacters are found in 3 and 4, their LUMOs (144A) are primarilyoriented on the Mebip moiety, while the respective HOMO (143A)is mainly constitutive of [40.0% dxz(Ir), 32.8% p(ppz) and 19.2%p(Mebip)] and [39.2% dxz(Ir), 34.4% p(Mebip) and 18.6% p(ptz)].Accordingly, both LUMO ? HOMO transitions as the leading con-figuration possess a combination of 3MLCT/3LLCT/3ILCT nature.

To intuitively describe phosphorescent emission, electron den-sity diagrams related to the key transition orbitals for 1–4 are illus-trated in Fig. 3. It can be seen from the delineation in Fig. 3 that theemission wavelengths of 1–4 are red-shifted according to the order2 > 3 > 4 > 1, which is in proportion to the donor property of biden-tate ligands following the order ppy(2) > ppz(3) > ptz(4) > bpy(1).For complexes 2–4, with the direct involvement of 3LLCT state inthe transition, HOMO level is raised significantly greater in extentthan LUMO one. As a consequence, the reduction of HOMO–LUMOenergy gap results in a longer emission wavelength relative to 1,nevertheless, by reason of the widely distributed electron densityof triplet ligand-to-ligand charge transfer state, and mixing MLCTwith this state may increase the radiative lifetime and thus thepartially forbidden transitions would likely lower the phosphores-cence quantum efficiency [5]. For the experimental measurements,complex 2 has a much longer radiative lifetime (s = 2 ms) withcomparatively smaller quantum efficiency (U = 0.16) than thoseof 1 (s = 5.7 ls, U = 0.19) [25,26], which can be tentatively ratio-nalized by our discussions here. Since there exist the identicalsymmetry and transition characters between the lowest-energyabsorption and emission, phosphorescence should be the reverseprocess of the lowest-energy absorption. The calculated Stokesshifts between the vertical triplet absorption and correspondingemission display complexes 2 and 3 bring a larger shift than 1and 4, as a result of increased participation of bidentate ligand,supporting the electronic configurations of emissions for 2 and 3which possess a greater mixture of 3MLCT and ligand-to-ligandcharge transfer states. Emission of 2 with the largest contributionof dxz(Ir) orbital involved in HOMO corresponds well with theexperimental emissive spectrum which is generally broad and fea-tureless, because emission from a large extent of 3MLCT state isgenerally broad and featureless, whereas a predominantly ligand-centered excited state typically gives highly structured emission[6,26].

4. Conclusions

With an aim of obtaining a better comprehension of the under-lying mechanism of phosphorescence, detailed calculations for 1–4were performed using DFT method. Theoretical studies of electronicstructures of the ground and excited state, the charge transfertransition characters of absorptions and emissions for complexes1–4 are carefully reported here. The calculated results manifestthat the optimized geometries in the ground state agree well withthe corresponding experimental values. The minor geometricaldifference in this series is induced by the differing donor strength ofbidentate chelating ligand and the trans effect. To a certain extent,the metal–ligand bonds are generally weakened upon electronicexcitation, implying electron density is redistributed among thechromophores relative to the ground state.

The calculated spectroscopic data indicate that LUMO andLUMO + 1 are predominantly localized on the Mebip ligand; hencethe transitions involving both absorptions and emissions are mainlyligand-based mixing with partial d(Ir) to p�(Mebip) charge transfercharacters. The lowest-lying absorption of 460 nm for 1 is character-ized as ILCT/MLCT transitions, whereas the lowest-lying absorptionsfor 2–4 at 457, 448 and 418 nm possess a mixture of MLCT/LLCT/ILCTcharacters. Those lowest-lying absorptions are red-shifted moder-ately in the order 4 > 3 > 2 > 1. The emission of 1 at 545 nm comesfrom a combination of 3ILCT and 3MLCT excited states, while suchemissions for 2–4 at 591, 572 and 549 nm are attributed to the mixed3MLCT/3LLCT/3ILCT transitions. A modest red shift of phosphorescentemissions is observed for 1–4 following the order 2 > 3 > 4 > 1, whichcorresponds to the donor strength of bidentate ligand increasing inthe order ppy(2) > ppz(3) > ptz(4) > bpy(1). The calculated Stokesshifts for 1–4 are 0.07, 0.25, 0.23 and �0.02 eV, respectively. Basedon the previous studies [6,20], emissions of 2 and 3 should stem froma increased participation of bidentate ligand due to both complexeshave a larger Stokes shift. In the experiment, phosphorescence of 2has a lower quantum efficiency and much long-lived radiative relax-ation than 1, which should be the consequence of greater mixture of3MLCT and ligand-to-ligand charge transfer states of 2 largely involv-ing the emission process. Therefore, the alternation of differingelectronic effect bidentate ligands has a major effect on the absorptionproperties and achieves a fine tuning of emission for this series ofMebip-based Iridium complexes. We hope these theoretical studieshere can provide some practical help in the design of highly efficientphosphorescent materials.

Acknowledgments

This work is supported by the National Science Foundation ofChina (Grant Nos. 20973076 and 20573042) and the Research Fundfor the Doctoral Program of Higher Education (Project No.200801831004).

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