explaining the homo and lumo distribution on individual ligands in mer-alq3 and its “ch”/n...

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Journal of Molecular Structure: THEOCHEM 850 (2008) 79–83

Explaining the HOMO and LUMO distribution on individual ligandsin mer-Alq3 and its ‘‘CH’’/N substituted 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, China

c Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, China

Received 6 September 2007; received in revised form 16 October 2007; accepted 18 October 2007Available online 4 November 2007

Abstract

HOMO and LUMO (FMOs) play important role in the optical properties of meridianal isomer of tris(8-hydroxyquinolino)aluminum(mer-Alq3) and its derivatives. The frontier molecular orbitals (FMOs) also play a vital role in the process of charge transport. It isurgent to find the reason of FMO distribution pattern among the ligands. The structures of mer-Alq3 and its ‘‘CH’’/N substituted deriv-atives have been optimized at the B3LYP/6-31G* level. Energy decomposition analysis has been performed at the B3LYP/DZP level. Theresults of energy-partitioning analysis of ground states are discussed. It has been explained that HOMOs are on A-ligands due to weakerelectrostatic interaction energy between La-AlLbLc fragments while LUMOs are on B-ligands due to weaker orbital interaction energybetween Lb-AlLaLc fragments.� 2007 Elsevier B.V. All rights reserved.

Keywords: mer-Alq3; ‘‘CH’’/N substituted derivatives; FMOs distribution pattern; Energy partitioning analysis; Instantaneous interaction energy

1. Introduction

Among light emitting materials, meridianal isomer oftris(8-hydroxyquinolino)aluminum (mer-Alq3) has beenused most often because of its good electronic conductivityand strong electroluminescent (EL) emission [1]. The elec-tronic structures of Alq3-like complexes can be tuned byadding substituents to quinolate ligand, changing the ener-gies of the filled or vacant orbitals [2]. Recent advances,such as enhancement of EL efficiency and color tuningthrough molecular doping [3,4] and microcavity-assistedcolor tuning and collimation [5] have increased the rangeof applications.

In general, electron donating groups such as methylattached to pyridine ring causes a blue shift in complexemission [6] while introduction of alkyls to benzene ring

0166-1280/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.theochem.2007.10.029

* Corresponding author.E-mail address: [email protected] (J. Zhang).

causes a red shift [7]. The presence of electron-withdrawinggroups (EWGs) such as chloro- [8] and cyano [9] groups inthe 5- or 7-position of the benzene ring results in almostnegligible emission shifts, while strong EWGs such as sul-fonamide (–SO2NR2) result in significantly blue-shiftedemission [10]. If the 8-hydroxyquinoline ligands of Alq3are prepared with an electron donating group in the 4 posi-tion of the quinolate ligand (para to N), the vacant orbitalsare raised in energy corresponding a blue shifted emission[2].

In our previous study, B3LYP/6-31G* approach hasbeen used to optimize and compare the bond lengths, bondangles, and HOMO–LUMO energy gap for mer-Alq3 andits ‘‘CH’’/N substituted derivatives [11]. Energy partition-ing analysis has been performed for mer-Alq3 [12,13]. Byusing modern methods of bonding analysis, it is possibleto obtain insight into the bonding situation of moleculeswhich agrees with the physical mechanism of chemicalbond formation. One such method is the energy partition-ing scheme that is available in the program package ADF

mailto:[email protected]

Fig. 1. (a) The geometry of mer-Alq3 with labels A-C for three quinolateligands (b) the ligand labeling for mer-Alq3 and substituted complexesconsidered in this work.

80 A. Irfan et al. / Journal of Molecular Structure: THEOCHEM 850 (2008) 79–83

[14]. It is based on the work of Morokuma [15], this workwas later pursued by Ziegler and Rauk [16]. The energypartitioning analysis has been proven to give an under-standing of chemical bonds [17].

In this contribution, the energy partioning analyses ofthe bonding between the metal fragments AlL2

+ and a sin-gle ligand L� in Alq3 and its ‘‘CH’’/N substituted deriva-tives in their ground states have been performed, with theaim to rationalize the distribution pattern of HOMOsand LUMOs for this kind of complexes.

2. Theoretical background of bond dissociation energy

The bond dissociation energy DEe between two frag-ments A and B is given by the sum of DEint and the frag-ment preparation energy DEprep.

DEe ¼ DEint þ DEprep ð1Þ

DEprep is the energy that is necessary to promote the frag-ments from their equilibrium geometry and electronicground state to the geometry and electronic state that theyhave in the optimized structure. The instantaneous interac-tion energy can be divided into three parts:

DEint ¼ DEelstat þ DEpauli þ DEorb ð2Þ

DEelstat gives the electrostatic interaction energy betweenthe fragments. It can be considered as an estimate of theelectrostatic contribution to the bonding interactions.DEpauli gives the repulsive interactions between the frag-ments. The last term in Eq. (2) gives the stabilizing orbitalinteractions, DEorb, which can be considered as an estimateof the covalent contributions to the bonding. Thus, the ra-tio DEelstat/DEorb indicates the electrostatic/covalent char-acter of the bond. Detail can be found in Ref. [18].

3. Computational details

The structures of mer-Alq3 (Fig. 1a) and its ‘‘CH’’/Nsubstituted derivatives (see labeling scheme in Fig. 1b)are optimized in the ground state (S0) at the B3LYP/6-31G* level as in our previous work [11]. Fig. 1(a) is labeledwith A–C designating the three different quinolate ligands.The structure is such that the central Al atom (+3 formaloxidation state) is surrounded by the three quinolateligands in a pseudooctahedral configuration with the A-and C-quinolate nitrogens and the B- and C-quinolate oxy-gens trans to each other. In our previous study, we per-formed energy decomposition analysis of the chemicalbonds for mer-Alq3 at the BP86/TZ2P level [12,13]. As itis deemed that hybrid functional such as B3LYP [19] givesbetter accuracy than that of local functional as BP86, so inthis study the calculation has been carried out by using theB3LYP functional, with a valence basis set of DZP withuncontracted Slater-type Orbital [20]. Scalar relativisticeffects were considered by using the zero-order regularapproximation (ZORA) [21] on optimized structures atthe B3LYP/6-31G* level. The calculations for energy

decomposition analysis were carried out with the programpackage ADF [18,22].

The bonding interactions between the metal fragmentAlL2

+ and a single ligand Li� have been analyzed with

the energy decomposition scheme of the program packageADF which is based on the energy decomposition analysis(EDA) method of Morokuma [15] and the extended transi-tion state (ETS) partitioning scheme of Ziegler [16].

4. Results and discussion

4.1. Frontier molecular orbitals (FMOs) analysis of S0 state

A number of theoretical studies of the electronic struc-ture of mer-Alq3 have been reported [23–25]. The FMOsdistribution of the S0 states shown in Fig. 2 suggests alocalization of molecular orbitals. In present work, theHOMO in mer-Alq3 is localized mainly at A-ligand,whereas LUMO is localized mainly at B-ligand. This pat-tern is in agreement with a previous DFT study publishedby Curioni et al. [26]. The HOMOs and LUMOs in the‘‘CH’’/N substituted derivatives of mer-Alq3 show thesame trend of localization at A and B ligands, respectively.The HOMO of each ‘‘CH’’/N substituted molecules ismainly localized on residual phenoxide side of the ligandA with a relatively significant contribution from the nitro-gen atom. The LUMO of each ‘‘CH’’/N substituted mole-cules is localized on pyridyl side of ligand B with a large

Fig. 2. Frontier molecular orbitals (FMOs) (0.05 e au�3) for the ground states (S0) of mer-Alq3 and its ‘‘CH’’/N substituted derivatives.

A. Irfan et al. / Journal of Molecular Structure: THEOCHEM 850 (2008) 79–83 81

contribution from the nitrogen. The same trend has beenfound in our previous work [11].

In the comparison of HOMOs and LUMOs energy gap,it was found that the greatest increase on HOMO eigen-value (0.03 au at the B3LYP/DZP level) was obtained forAl-5N3. Within LUMO eigenvalues, the greatest decrease

was observed for Al-4N3 (0.027 au with B3LYP/DZP).These values compare well with the calculated results of0.03 au for both HOMO and LUMO [11]. The energygap in Al-5N3 between HOMO and LUMO is the largestone (0.1357 au) and the smallest is in Al-4N3 (0.1083 au)as presented in Table 1.

Table 1The HOMO, LUMO Eigenvalues and gap energies (Eg) computed atB3LYP/DZP level

Complexes HOMO (au) LUMO (au) Eg (eV)

Alq3 �0.2039 �0.0851 3.233Al-3N3 �0.2207 �0.1000 3.284Al-4N3 �0.2210 �0.1127 2.947Al-5N3 �0.2356 �0.0999 3.693Al-6N3 �0.2197 �0.1046 3.132Al-7N3 �0.2244 �0.0999 3.388

82 A. Irfan et al. / Journal of Molecular Structure: THEOCHEM 850 (2008) 79–83

4.2. Energy partitioning analysis

To investigate the nature of metal–ligand interaction inmer-Alq3 and its ‘‘CH’’/N substituted derivatives, we carriedout an energy partitioning analysis. In Table 2 importantresults of bonding analysis for interaction between one quin-olate ligand (Li

�) and AlL2+ (Li = A, B or C) fragments have

been displayed. Electrostatic energies (DEelstat) of mer-Alq3,Al-3N3, Al-4N3, Al-5N3, Al-6N3, and Al-7N3 are 62.03%,62.18%, 62.05%, 62.4%, 62.13%, and 62.44% while orbitalinteraction energies (DEorb) are 37.97%, 37.82%, 37.94%,37.6%, 37.87%, and 37.56%, respectively. The results showthat the electrostatic energy plays a more important rolefor the metal–ligand binding than for the orbital interaction,i.e., the metal–ligand interactions have a larger electrostaticcharacter than covalent character.

In mer-Alq3, the DEelstat of La-AlLbLc, Lb-AlLaLc andLc-AlLaLb fragments are �210.00, �216.59 and,�217.46 kcal/mol, respectively. The DEorb of La-AlLbLc,Lb-AlLaLc and Lc-AlLaLb fragments are �131.43,�130.07, and �132.71 kcal/mol, respectively. Note that

Table 2Energy partitioning analysis of mer-Alq3 and its ‘‘CH’’/N substituted derivatLi� and AlLjLk

+ (all values in kcal/mol)a

Complexes Fragments Energy components

DEint DEpauli

Alq3 La-AlLbLc �199.30 142.13Lb-AlLaLc �200.17 146.49Lc-AlLaLb �202.47 147.7

Al-3N3 La-AlLbLc �199.92 138.72Lb-AlLaLc �200.42 143.01Lc-AlLaLb �202.32 144.35





a If Li� is one of the three ligands then AlLjLk

+ will be Al along with the tw

the DEelstat between La-AlLbLc fragments is weaker thanthose of Lb-AlLaLc and Lc-AlLaLb fragments while DEorb

between Lb-AlLaLc fragments is weaker than La-AlLbLc

and Lc-AlLaLb fragments. Overall arrangement for threeligands is responsible to the weaker DEelstat betweenLa-AlLbLc fragments whereas weaker DEorb is betweenLb-AlLaLc fragments. The weaker DEelstat betweenLa-AlLbLc fragments results in the HOMO localizationon the A-ligand, whereas weaker DEorb between Lb-AlLaLc

fragments results in the LUMO localization on B-ligand.These results are in good agreement with our previouswork at the BP86/TZ2P level [12,13].

In Al-3N3, DEelstat of La-AlLbLc, Lb-AlLaLc andLc-AlLaLb fragments are �209.35, �214.44, and�215.90 kcal/mol, while DEorb of La-AlLbLc, Lb-AlLaLc

and Lc-AlLaLb fragments are �129.29, �128.98, and�130.77 kcal/mol, respectively. The DEelstat betweenLa-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. Thesame trend has been found in other substituted derivativesof mer-Alq3 (Al-4N3, Al-5N3, Al-6N3, and Al-7N3).It is shown in Table 2 that the trend of DEelstat isLa-AlLbLc > Lb-AlLaLc > Lc-AlLaLb while DEorb isLb-AlLaLc > La-AlLbLc > Lc-AlLaLb in mer-Alq3 and its‘‘CH’’/N substituted derivatives, i.e., weaker DEelstat

is between La-AlLbLc fragments while weaker DEorb isbetween Lb-AlLaLc fragments. Thus in mer-Alq3 and its‘‘CH’’/N substituted derivatives, the HOMOs are localizedon A-ligand due to the weaker DEelstat between La-AlLbLc

fragments, while LUMOs are localized on B-ligand due tothe weaker DEorb between Lb-AlLaLc fragments.

ives in electronic ground state at the B3LYP/DZP level, using fragments

DEelstat DEorb DEelstat,% DEorb,%

�210.00 �131.43 61.51 38.49�216.59 �130.07 62.48 37.52�217.46 �132.71 62.11 37.89�209.35 �129.29 61.82 38.18�214.44 �128.98 62.45 37.55�215.90 �130.77 62.28 37.72�210.87 �131.62 61.57 38.43�216.32 �130.00 62.46 37.54�218.26 �133.01 62.14 37.86�208.04 �128.24 61.87 38.13�214.84 �127.06 62.84 37.16�215.02 �129.23 62.46 37.54�210.05 �131.45 61.51 38.49�216.43 �128.94 62.67 37.33�218.35 �132.64 62.21 37.79�208.03 �128.82 61.76 38.24�215.82 �126.59 63.03 36.97�217.87 �130.57 62.53 37.47

o others.

A. Irfan et al. / Journal of Molecular Structure: THEOCHEM 850 (2008) 79–83 83

5. Conclusions

The ground state structures of mer-Alq3 and its ‘‘CH’’/N substituted derivatives have been optimized at theB3LYP/6-31G* level of theory. Energy partitioning analy-sis has been performed by using the B3LYP/DZP. Thetrend of DEelstat and DEorb in La-AlLbLc, Lb-AlLaLc andLc-AlLaLb fragments and distribution of HOMO andLUMO on the basis of these energies agree very well withour already calculated results. The energy partitioninganalyses show that the metal–ligand interactions of mer-Alq3 and its ‘‘CH’’/N substituted derivatives have a higherelectrostatic character than covalent character. TheHOMOs are localized on A-ligand due to the weakerDEelstat between La-AlLbLc fragments while the LUMOsare localized on B-ligand because Lb-AlLaLc fragmentshave weaker DEorb. This study will be helpful to investigatethe reason of HOMO-LUMO distribution on individualdifferent ligands in other derivatives of mer-Alq3 and, per-haps in other molecules possessing FMO individuallocalization.

Acknowledgements

Financial supports from the NSFC (No.50473032:20773022) and NCET-06-0321 are gratefullyacknowledged. A. Irfan acknowledges the financial supportfrom Ministry of Education, Pakistan.

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explaining the homo and lumo distribution on individual ligands in mer-alq3 and its “ch”/n...

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