Journal of Molecular Structure: THEOCHEM 956 (2010) 61–65

Contents lists available at ScienceDirect

Journal of Molecular Structure: THEOCHEM

journal homepage: www.elsevier .com/locate / theochem

Toward rational designing of n-type materials: Theoretical investigationsof mer-Alq3 derivatives

Ahmad Irfan a, Ruihai Cui b,c, Jingping Zhang a,*

a Faculty of Chemistry, Northeast Normal University, Changchun 130024, Chinab Department of Chemistry, Harbin University, Harbin 150080, Chinac Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, China

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

Article history:Received 5 February 2010Received in revised form 15 June 2010Accepted 24 June 2010Available online 3 July 2010

Keywords:mer-Alq3Frontier molecular orbitalsIonization potentialElectron affinityReorganization energiesEnergy decomposition analysis

0166-1280/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.theochem.2010.06.028

* Corresponding author. Tel.: +86 431 85098652; faE-mail addresses: zhangjingping66@yahoo.com.cn

Zhang).

The ground state geometries of the CN and OCH3 derivatives of the meridianal isomer of tris(8-hydrox-yquinolinato)aluminum (mer-Alq3) were calculated by density functional theory. The absorption spectrawere computed at the TD-PBE0/6-31G� level. We have observed that position for substitution playsimportant role for absorption properties. The cyano derivatives make the LUMO energy levels lowerand the electron affinity increase, thus these derivatives would enhance the electron injection ability.The ionization potentials and electron affinities showed that cyano derivatives would be better holeblockers than methoxy derivatives. The reorganization energies indicate that CN derivatives wouldenhance the electron mobility while introduction of OCH3 has no effect in the enhancement of electronmobility. We explained the distribution of highest occupied molecular orbitals (HOMOs) and lowestunoccupied molecular orbitals (LUMOs) on different individual ligands.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

In 1987, Tang and Van Slyke [1] reported the first efficient, lowmolecular weight organic light-emitting device (OLED) with twolayers and using the metaloquinolate tris(8-hydroxyquinolina-to)aluminum (Alq3) as the electron-transport material and emittinglayer. In the following decades, OLEDs have become the highly com-petitive alternative of full color flat panel display [2–4]. There aretwo geometric isomers of Alq3, the facial tris(8-hydroxyquinolina-to)aluminum (fac-Alq3) and meridianal (mer-Alq3) forms with C3and C1 symmetries, respectively. The mer-Alq3 has become the pro-totype of a whole class of electroluminescent units currently in usein OLEDs [5]. Both experimental [6,7] and theoretical [7] studiesindicate that the meridianal isomer is more stable energeticallyand generally the major constituent of thin films. Therefore, thecalculations performed here will exclusively consider mer-Alq3.The electronic structures of mer-Alq3-like complexes can be tunedby adding substituents to quinolate ligand. By attaching electrondonating group (EDG) to pyridine ring causes a blue shift in complexemission [8–10] while introduction to benzene ring causes a redshift [9,10].

ll rights reserved.

x: +86 431 85099521., zhangjp162@nenu.edu.cn (J.

Highest occupied molecular orbital (HOMO) and lowest unoc-cupied molecular orbital (LUMO) play important role in the chargetransfer properties. Energy decomposition analysis that is the bondenergy between the fragments of a molecular system into contri-butions associated with the various orbital and electrostatic inter-actions [11] of the bonding between the fragments AlL2

+ (Al alongwith any two ligands among ligand-A, ligand-B, and ligand-C) anda single ligand Li

� (Li = A, B or C ligand) has been performed toexplain the distribution of HOMO and LUMO on individual ligands.The preparation energy DEprep and the interaction energy DEint isknown as bond energy DE (Eq. (1)).

DE ¼ DEprep þ DEint ¼ DEprep; geo þ DEprep; el þ DEint ð1Þ

The interaction energy DEint is further decomposed into threephysically meaningful terms.

DEint ¼ DEelst þ DEpauli þ DEorb ð2Þ

DEelst gives the electrostatic interaction energy between thefragments which are calculated with the frozen electron densitydistribution of A and B in the geometry of the complex AB. It canbe considered as an estimate of the electrostatic contribution tothe bonding interactions. DEpauli, refers to the repulsive interac-tions between the fragments which are caused by the fact, thattwo electrons with the same spin cannot occupy the same regionin space. The term comprises the four-electron destabilizing inter-actions between occupied orbitals. The last term in Eq. (2) gives the

62 A. Irfan et al. / Journal of Molecular Structure: THEOCHEM 956 (2010) 61–65

stabilizing orbital interactions, DEorb, is the energy change due tothe relaxation of the wave function to its final form through elec-tron pair bonding, charge transfer and polarization. DEorb, can beconsidered as an estimate of the covalent contributions to thebonding. Thus, the ratio DEelst/DEorb indicates the electrostatic/covalent character of the bond. Detail can be found in Ref. [11].With the aim to design blue and red shifted materials having highmobility, we have designed mer-Alq3 derivatives by substitutingEDG (OCH3) and electron withdrawing (EWD) group (CN) at differ-ent positions on all the ligands.

2. Computational details

All the calculations have been carried out with Gaussian 98[12]. The structures of all the derivatives of mer-Alq3 (see labelingscheme in Fig. 1b) have been optimized in the ground state (S0) atthe B3LYP/6-31G� level, which has been proved to be an efficientapproach for mer-Alq3 and its derivatives [13–15]. The absorptionspectra were calculated by time dependent density functional the-ory (TD-DFT) at PBE0/6-31G� level which give good and accurateresults as compared to B3LYP, BLYP, B3PW91 and SVWN [15].The vertical and adiabatic ionization potentials, vertical and adia-batic electronic affinities and reorganization energies [16] havebeen calculated at B3LYP/6-31G� level. Energy decompositionanalysis has been carried out by using the B3LYP/DZP level [15].Scalar relativistic effects were considered by using the zero-orderregular approximation (ZORA) [17] on optimized structures atthe B3LYP/6-31G� level. The calculations for energy decompositionanalysis have been carried out with the program package ADF [18].

3. Results and discussion

3.1. Molecular geometries

Fig. 1(a) is labeled with A–C designating the three differentquinolate ligands of mer-Alq3. The structure is such that the

N

O

1

2 3

4

5

67

8

9 10

11

3

(b)

Al

n Different ligands Complexes 3 4 5 6 3 4 5 6

tris(3-cyano-8-hydroxyquinolinato)aluminum tris(4-cyano-8-hydroxyquinolinato)aluminum tris(5-cyano-8-hydroxyquinolinato)aluminum tris(6-cyano-8-hydroxyquinolinato)aluminum tris(3-methoxy-8-hydroxyquinolinato)aluminum tris(4-methoxy-8-hydroxyquinolinato)aluminum tris(5-methoxy-8-hydroxyquinolinato)aluminum tris(6-methoxy-8-hydroxyquinolinato)aluminum

1 2 3 4 5 6 7 8

n= 3, 4,.., 6 substituted “H” position by “CN” and “OCH3” as labeled in Fig. 1(b)

A

B

C

(a)

Fig. 1. (a) The geometry of mer-Alq3 with labels A–C for three quinolate ligands (b)the ligand labeling for mer-Alq3 substituted complexes considered in this work.

central Al atom (+3 formal oxidation state) is surrounded by thethree quinolate ligands in a pseudooctahedral configuration withthe A- and C-quinolate nitrogens and the B- and C-quinolate oxy-gens trans to each other. The molecular models used in our calcu-lations, obtained by systematic substitution in positions 3, 4, 5, and6 (see labeling scheme) in each ligand are shown in Fig. 1. Table 1present the selected geometrical parameters of the mer-Alq3 alongwith 1 and 5 as representatives, together with their correspondingpristine molecules, where both optimized and experimental results[19] are listed for comparison. Geometrical parameters from 1 to 8can be found in Table S1.

3.2. Frontier molecular orbitals (FMOs) analysis of S0 states andabsorption properties

The HOMOs are localized mostly on A-ligand while LUMOs arelocalized on B-ligand in mer-Alq3 and its derivatives [13–15]. TheFMOs distributions of cyano and methoxy derivatives of mer-Alq3at the S0 states shown in Figs. 2 and S1, suggest a localization ofmolecular orbitals. The HOMOs and LUMOs in all the derivatives ofmer-Alq3 showed the similar trend of localization at A and B ligands,respectively. The HOMO, LUMO, and gap energies of CN and OCH3

are tabulated in Table 2, the trend of HOMOs energies (EHOMO) is asfollows: 7 > 6 > 5 > 8 > mer-Alq3 > 2 > 1 > 4 > 3, while in LUMOsenergies (ELUMO) is as: 6 > 5 > 7 > 8 > Alq3 > 1 > 4 > 3 > 2. The trendof gap energies (Eg) in mer-Alq3 derivatives is 6 > 5 > 8 > Alq3 � 3 >4 > 1 > 7 > 2. The work function of Al is �4.3 eV [20] and the LUMOenergy level of mer-Alq3 is �1.65 eV (see Table 2), the injectionenergy is around 2.65 eV (=�1.65 � (�4.3)) from the electrode Alto the mer-Alq3 surface. The barrier for electron transport acrossthe interface is high, which indicates that mer-Alq3 has large energybarrier for charge injection, leading to somewhat high turn-on volt-age for mer-Alq3. Therefore, it is necessary to lower the LUMO levelto enhance the electron injection ability. From Table 2 it can be foundthat the CN derivatives effects make the LUMO energy levels lower;i.e., in 1 (0.94 eV), 2 (1.32 eV), 3 (0.98 eV) and 4 (0.96 eV).

The computed and experimental [21] absorption wavelengthshave been reported in Table 2. The 1, 2, and 7 are red shifted, i.e.,28 nm, 96 nm and 52 nm while 3 and 6 are blue shifted, i.e., 8 nmand 32 nm, respectively. The 4, 5, and 8 are negligible blue shifted to-ward absorption wavelengths. The position for substitution playsimportant role as we have observed that EWD group CN at position3 and 4 show red shift while substitution at position 5 lead to blueshift which is in good agreement with Burrows et al. [22]. It has beenobserved that EDG on pyridine ring causes blue shift while on ben-zene ring lead to red shift; our theoretical investigations are in goodagreement with already reported work [8–10]. Generally, the com-plexes having higher energy gap than parent molecule show blueshift while small energy gap derivatives show red shifts.

3.3. Charge transfer properties

The charge transport can be depicted by a particle diffusion pro-cess [23,24] coupled with the Marcus theory of the electron trans-fer rate for a self-exchange reaction process [25–27].

K ¼ V2=hðp=kkBTÞ1=2 expð�k=4kBTÞ ð3Þ

The intermolecular transfer integral (V) and the reorganizationenergy (k) are the main parameters that determine self-exchangeelectron-transfer (ET) rate. The reorganization energy is further di-vided into two parts: k1 and k2, where k1 corresponds to the geom-etry relaxation energy of one molecule from neutral state tocharged state, and k2 corresponds to the geometry relaxation en-ergy from charged state to neutral one [28,29].

k ¼ k1 þ k2 ð4Þ

Table 1Selected optimized bond lengths (Å) and bond angles (�) for mer-Alq3, its cyano (1) and methoxy (5) derivatives at the B3LYP/6-31G� level.

Cplxa Al–NA Al–NB Al–NC Al–OA Al–OB Al–OC <1c <2d <3e

Alq3 2.084 2.126 2.064 1.855 1.881 1.884 171.5 172.6 166.61 2.092 2.137 2.070 1.851 1.872 1.878 170.9 172.8 166.15 2.087 2.131 2.067 1.854 1.877 1.882 170.1 172.8 165.9Exp.b 2.050 2.087 2.017 1.850 1.860 1.857 173.8 171.5 168.2

a Cplx, complexes.b Experimental data from Ref. [19].c <1 = NA–Al–NC.d <2 = NB–Al–OA.e <3 = OC–Al–OB.

Fig. 2. Frontier molecular orbitals (FMOs) (0.05 e au�3) for the ground states (S0) of 1 and 5.

Table 2The calculated HOMO energy (EHOMO), LUMO energy (ELUMO), energy gap (Eg) andabsorption wavelengths (ka) in eV for S0 states computed at the TD-PBE0/6-31G�level.

Complexes EHOMO ELUMO Eg f ka Exp. (ka)a

Alq3 �5.26 �1.65 3.61 0.0759 410 3871 �6.04 �2.59 3.45 0.0485 438 –2 �6.02 �2.97 3.05 0.0778 506 –3 �6.24 �2.63 3.61 0.1364 402 –4 �6.18 �2.61 3.55 0.0850 407 –5 �5.09 �1.44 3.65 0.0658 408 –6 �4.97 �1.13 3.84 0.1149 378 –7 �4.75 �1.44 3.31 0.0689 462 –8 �5.15 �1.52 3.63 0.0605 408 –

a Exp., The experimental absorption wavelengths (ka) from Ref. [21]; f, oscillatorstrength.

A. Irfan et al. / Journal of Molecular Structure: THEOCHEM 956 (2010) 61–65 63

In the evaluation of k, the two terms were computed directlyfrom the adiabatic potential energy surfaces [30].

k ¼ k1 þ k2 ¼ Eð1ÞðXþÞ � Eð0ÞðXþÞh i

þ Eð1ÞðXÞ � Eð0ÞðXÞh i

ð5Þ

Here, E(0)(X), E(0)(X+) are the ground-state energies of the neutraland charged states, E(1)(X) is the energy of the neutral molecule atthe optimized charged geometry and E(1)(X+) is the energy of thecharged state at the geometry of the optimized neutral molecule.

It is well known that injection barrier for hole and electron ofthe semiconducting material and work function of the electrodecorresponds to the energy difference between the ionization po-tential or electron affinity, respectively. The vertical and adiabaticionization potentials, vertical and adiabatic electronic affinitiesand reorganization energies are tabulated in Table 3. To achievehigh electron affinity for organic materials to improve electroninjection is a challenge for the applications of organic materialsin OLED. The high electron affinity and high ionization potentialare the important characteristics of the hole blocking materials[31,32]. The adiabatic electron affinities of CN derivatives of mer-Alq3 are 1.55–1.92 eV and vertical electron affinities are 1.45–

Table 3Calculated vertical and adiabatic ionization potentials, vertical and adiabaticelectronic affinities and reorganization energies in eV at B3LYP/6-31G� level.

Complexes IPa EAa IPv EAv k (h) k (e)

mer-Alq3 6.40 0.59 6.52 0.46 0.242 0.2761 6.95 1.54 7.04 1.46 0.227 0.1872 6.92 1.92 7.02 1.82 0.248 0.2383 7.13 1.58 7.20 1.47 0.175 0.2644 7.08 1.55 7.17 1.45 0.233 0.2375 5.99 0.42 6.09 0.28 0.239 0.3546 5.87 0.12 5.97 �0.01 0.259 0.3337 5.61 0.42 5.76 0.28 0.343 0.3288 6.04 0.49 6.13 0.36 0.205 0.306

64 A. Irfan et al. / Journal of Molecular Structure: THEOCHEM 956 (2010) 61–65

1.86 eV which are higher than the parent molecule. The ionizationpotentials of CN derivatives are 6.92–7.13 eV which are higherthan that of mer-Alq3 (6.40 eV) mean that these derivatives wouldbe better hole blocker than the parent molecule. We have foundthat for CN derivatives of mer-Alq3 cause energy lowering in theenergy levels of the lowest unoccupied molecular orbitals (LUMO)(see Table 2) and the electron affinity (EA) increase, which wouldfacilitate injection of the electron carrier from the metal electrode.More precisely, electron injection barrier would decrease. The mer-Alq3 is electron transfer material so we have compared the k (e) ofdesigned derivatives with parent molecule. The electron reorgani-zation energies of CN derivatives (1–4) are smaller than the parentmolecule which indicates that these would be better electrontransfer materials. The ionization potentials and electron affinitiesof methoxy derivatives are smaller than the parent molecule. Theionization potentials and electron affinities showed that mer-Alq3 would be better hole blockers than methoxy derivatives. Gen-erally, the electron affinities of methoxy derivatives are smallerwhile electron reorganization energies are larger than the parentmolecule, see Table 3. These results showed that methoxy deriva-tives would have no effect to enhance the intrinsic charge transfermobility. Moreover, we have also investigated the effect of substit-uents on the charge transfer by NBO analysis. The EWD group CNshows decreased charge transfer in the aromatic rings while EDGOCH3 shows increased charge transfer see Table S2 which is ingood agreement with the Lee et al. [33]. They pointed out that sub-stitution of strong EWD group at facial position decreased chargetransfer whereas strong EDG shows increased charge transferwhile substitution of strong EWD group at axial position increasedcharge transfer toward the axial aromatic ring whereas strong EDGshows decreased charge transfer.

3.4. Energy decomposition analysis

To investigate the nature of metal–ligand interaction in cyanoand methoxy derivatives of mer-Alq3, we have carried out an en-ergy decomposition analysis (EDA). The EDA results of the studiedderivatives of mer-Alq3 are given in Table S3. Where the DEelst%and DEorb% denotes percentage contribution to the total attractiveinteraction, respectively [34,35]. The electrostatic energies (DEelst)of 1, 2, 3, 4, 5, 6, 7, and 8 are 61.77%, 61.36%, 61.33%, 61.63%,61.55%, 61.59%, 61.34%, and 61.68%, while orbital interaction ener-gies (DEorb) are 38.23%, 38.64%, 38.67%, 38.37%, 38.45%, 38.41%,38.66%, and 38.32% between the fragments AlL2

+ and a single li-gand Li

�, respectively. These results show that the electrostaticcharacter is larger than covalent character which shows the similartrend with our previously reported work [15].

In 1, the DEelst of LA–AlLBLC, LB–AlLALC, and LC–AlLALB fragmentsare �207.04, �213.77, and �215.07 kcal/mol, respectively. TheDEorb of LA–AlLBLC, LB–AlLALC, and LC–AlLALB fragments are�131.18, �129.01, and �133.96 kcal/mol, respectively. Note thatthe DEelst between LA–AlLBLC fragments is weaker than those of

LB–AlLALC and LC–AlLALB fragments, while DEorb between LB–AlLALC

fragments is weaker than LA–AlLBLC and LC–AlLALB fragments. Over-all arrangement for three ligands is responsible to the weakerDEelstat

between LA–AlLBLC fragments whereas weaker DEorb is betweenLB–AlLALC fragments. The weaker DEelstat between LA–AlLBLC frag-ments results in the HOMO localization on the A-ligand, whereasweaker DEorb between LB–AlLALC fragments results in the LUMOlocalization on B-ligand. The same trend has been found in othersubstituted derivatives of mer-Alq3 (2, 3, 4, 5, 6, 7, and 8).

4. Conclusions

By attaching electron donating group (EDG) to pyridine ringcauses a blue shift and introduction to benzene ring causes a redshift while electron withdrawing group (EWG) on pyridine ringcauses red shift and on benzene ring causes blue one. The CN deriv-atives (1, 2, 3, and 4) cause energy lowering in the energy levels ofthe lowest unoccupied molecular orbitals and the electron affinityincrease, thus these derivatives would enhance the electron injec-tion ability which would facilitate injection of the electron carrierfrom the metal electrode. The electron reorganization energies ofCN derivatives showed that these derivatives would enhance theelectron mobility while introduction of methoxy has no effect toenhance the electron mobility. Position for substitution playsimportant role; this study would be helpful to design and synthe-size the new materials of different wavelengths. The energydecomposition analysis shows that the metal–ligand interactionsof all the derivatives of mer-Alq3 have a higher electrostatic char-acter than covalent character. The HOMOs are localized on A-li-gand due to the weaker DEelst between LA–AlLBLC fragmentswhile the LUMOs are localized on B-ligand because LB–AlLALC frag-ments have weaker DEorb.

Acknowledgements

Financial supports from the NSFC (Nos. 50873020; 20773022),NCET-06-0321 are gratefully acknowledged. We appreciativelyacknowledge the assistance of Rizwan Ali during our manuscriptwriting. A. Irfan acknowledges the financial support from Ministryof Education, Pakistan and China Scholarship Council.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.theochem.2010.06.028.

References

[1] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913.[2] J.H. Choi, K.H. Kim, S.J. Choi, H.H. Lee, Nanotechnology 17 (2006) 2246.[3] Y. Sun, N.C. Giebink, H. Kanno, B. Ma, M.E. Thompson, S.R. Forrest, Nature 440

(2006) 908.[4] E.L. Williams, K. Haavisto, J. Li, G.E. Jabbour, Adv. Mater. 19 (2007) 197.[5] P. Addy, D.F. Evans, R.N. Sheppard, Inorg. Chim. Acta 127 (1987) L19.[6] G.P. Kushto, Y. Iizumi, J. Kido, Z.H. Kafafi, J. Phys. Chem. 104 (2000) 3670.[7] A. Curioni, M. Boero, W. Andreoni, Chem. Phys. Lett. 294 (1998) 263.[8] J. Yu, Z. Chen, Y. Sakuratani, H. Suzuki, M. Tokita, S. Miyata, Jpn. J. Appl. Phys. 38

(1999) 6762.[9] L.S. Sapochak, A. Padmaperuma, N. Washton, F. Endrino, G.T. Schmett,

J. Marshall, D. Fogarty, P.E. Burrows, S.R. Forrest, J. Am. Chem. Soc. 123(2001) 6300.

[10] S. Anderson, M.S. Weaver, A.J. Hudson, Synth. Met. 111 (2000) 459.[11] G. te Velde, F.M. Bickelhaupt, E.J. Baerends, C. Fonseca Guerra, S.J.A. van

Gisbergen, J.G. Snijders, T. Ziegler, J. Comput. Chem. 22 (2001) 931.[12] M.J. Frisch et al., Gaussian 98, Gaussian, Inc., Pittsburgh, PA, 1998.[13] G. Gahungu, J. Zhang, J. Phys. Chem. B 109 (2005) 17762.[14] J. Zhang, G. Frenking, J. Phys. Chem. A 108 (2004) 10296.[15] A. Irfan, R. Cui, J. Zhang, Theor. Chem. Acc. 122 (2009) 275.[16] Y. Zhang, X. Cai, Y. Bian, X. Li, J. Jiang, J. Phys. Chem. C 112 (2008) 5148.[17] E. van Lenthe, A. Ehlers, E.J. Baerends, J. Chem. Phys. 110 (1999) 8943.

A. Irfan et al. / Journal of Molecular Structure: THEOCHEM 956 (2010) 61–65 65

[18] ADF 2006.01, SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam,The Netherlands.

[19] M. Brinkmann, G. Gadret, M. Muccini, C. Taliani, N. Masciocchi, A. Sironi, J. Am.Chem. Soc. 122 (2000) 5147.

[20] H.B. Michaelon, J. Appl. Phys. 48 (1977) 4729.[21] Y.W. Shi, M.M. Shi, J.C. Huang, H.Z. Chen, M. Wang, X.D. Liu, Y.G. Ma, H. Xu,

B. Yang, Chem. Commun. (2006) 1941.[22] P.E. Burrows, Z. Shen, V. Bulovic, D.M. McCarty, S.R. Forrest, J.A. Cronin, M.E.

Thompson, J. Appl. Phys. 79 (1996) 7991.[23] Y.B. Song, C.A. Di, X.D. Yang, S.P. Li, W. Xu, Y.Q. Liu, L.M. Yang, Z.G. Shuai, D.Q.

Zhang, D.B. Zhu, J. Am. Chem. Soc. 128 (2006) 15940.[24] L.B. Schein, A.R. McGhie, Phys. Rev. B 20 (1979) 1631.[25] R.A. Marcus, Annu. Rev. Phys. Chem. 15 (1964) 155.[26] R.A. Marcus, Rev. Mod. Phys. 65 (1993) 599.

[27] J.L. Bredas, J.P. Calbert, D.A. da Silva Filho, J. Cornil, Proc. Natl. Acad. Sci. USA 99(2002) 5804.

[28] N.E. Gruhn, D.A. da Silva Filho, T.G. Bill, M. Malagoli, V. Coropceanu, A. Kahn,J.L. Brédas, J. Am. Chem. Soc. 124 (2002) 7918.

[29] J.R. Reimers, J. Chem. Phys. 115 (2001) 9103.[30] V. Coropceanu, T. Nakano, N.E. Gruhn, O. Kwon, T. Yade, K. Katsukawa, J.L.

Brédas, J. Phys. Chem. B 110 (2006) 9482.[31] T.H. Huang, W.T. Whang, J.Y. Shen, Y.S. Wen, J.T. Lin, T.H. Ke, L.Y. Chen, C.C. Wu,

Adv. Funct. Mater. 16 (2006) 1449.[32] S.O. Jung, Y.H. Kim, S.K. Kwon, H.Y. Oh, J.H. Yang, Org. Electron. 8 (2007) 349.[33] E.C. Lee, B.H. Hong, J.Y. Lee, J.C. Kim, D. Kim, Y. Kim, P. Tarakeshwar, K.S. Kim, J.

Am. Chem. Soc. 127 (2005) 4530.[34] J. Frunzke, M. Lein, G. Frenking, Organometallics 21 (2002) 3351.[35] C. Esterhuysen, G. Frenking, Theor. Chem. Acc. 111 (2004) 381.