Chemical Physics 364 (2009) 39–45

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Chemical Physics

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Push–pull effect on the charge transfer, and tuning of emitting colorfor disubstituted derivatives of mer-Alq3

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

a Faculty of Chemistry, Northeast Normal University, Changchun 130024, Chinab Department of Chemistry, Harbin University, Harbin 150080, Chinac Key Laboratory for Applied Statistics of MOE, Changchun 130024, China

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

Article history:Received 1 July 2009Accepted 15 August 2009Available online 20 August 2009

Keywords:Organic light-emitting diodesMer-Alq3Energy decomposition analysisEmitting materialsReorganization energyMobility

0301-0104/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.chemphys.2009.08.009

* Corresponding author. Tel.: +86 431 85099372; faE-mail address: zhangjingping66@yahoo.com.cn (J

To design innovative and novel optical materials with high mobility, two kinds of disubstituted deriva-tives for meridianal isomer of tris(8-hydroxyquinolinato)aluminum (mer-Alq3) with push–pull (X–Y)substituents have been designed. The structures of tris(4-X-6-Y-8-hydroxyquinolinato)aluminum (type1) and tris(4-Y-6-X-8-hydroxyquinolinato)aluminum (type 2) (where X = –CH3/–NH2 and Y = –CN/–Cl)in the ground (S0) and first excited (S1) states have been optimized at the B3LYP/6-31G* and CIS/6-31G* level of theory, respectively. All the designed derivatives of type 1 show blue shift while most ofthe type 2 derivatives show red shift as compared to the mer-Alq3. The emitting color could be tunedfrom blue to red. We have explained the distribution of HOMOs and LUMOs on different individualligands. The reorganization energies of tris(4-methyl-6-chloro-8-hydroxyquinolinato)aluminum (1),tris(4-methyl-6-cyano-8-hydroxyquinolinato) aluminum (2), tris(4-chloro-6-methyl-8-hydroxyquinoli-nato)aluminum (5) and tris(4-cyano-6-methyl-8-hydroxyquinolinato)aluminum (6) are comparable withmer-Alq3. Thus these derivatives might be good candidates for emitting materials possessing comparablecharge carrier mobility as mer-Alq3.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Organic light-emitting diodes (OLEDs) are of growing interest invarious display applications [1–5]. The meridianal isomer of tris(8-hydroxyquinolinato)aluminum (mer-Alq3) has been used most of-ten because of its good electronic conductivity for organic electro-luminescence (EL) devices [6–12] since the first demonstration of amultilayered efficient EL device by Tang and Van Slyke [13]. Onepopular approach to tuning the emissive color is to introduce elec-tron withdrawing (EWD) and electron donating groups (EDG) intothe hydroxyquinoline ligands [14]. In general, EDG attached to pyr-idine ring causes a blue shift in complex emission while introduc-tion to phenoxide ring causes a red shift [15,16]. High performanceluminescence materials for blue OLEDs can be designed by intro-ducing EWD group at C-6 of the phenoxide ring together with anEDG at C-4 of the pyridyl ring in mer-Alq3 [17]. Modification ofchelating in mer-Alq3 with electron donating or electron with-drawing groups has been also studied by Pohl et al. [18].

The efficiency of charge transport within the organic layer(s)plays a key role. The high-charge mobilities favor recombinationprocesses in the bulk where charges can be confined further by

ll rights reserved.

x: +86 431 85684937.. Zhang).

means of organic–organic interfaces [19]. The charge transfer ratecan be described by Marcus theory via the following equation [20]

W ¼ V2=hðp=kkBTÞ1=2 expð�k=4kBTÞ: ð1Þ

There are two major parameters that determine self-exchange elec-tron-transfer rates and ultimately charge mobility: (i) the electroniccoupling V (transfer integral) between adjacent molecules, whichneeds to be maximized, and (ii) the reorganization energy k, whichneeds to be small for significant transport. The reorganization en-ergy term describes the strength of the electron–phonon (vibration)and can be reliably estimated as twice the relaxation energy of a po-laron localized over a single unit.

Kido and Iisumi showed that tris(4-methyl-8-quinolinolato)alu-minum exhibited a larger EL efficiency than that of mer-Alq3 [21].This material was later shown to exhibit higher photolumines-cence (PL) quantum efficiency than that of mer-Alq3 in solutionand in the solid-state [22]. The tris(4-methyl-8-quinolinolato)alu-minum has received attention as an alternative, because it has sig-nificantly higher PL and EL quantum yields [23]. Shi et al. explainedthat fluorination also has a great effect on the photoluminescencequantum yield of the fluorinated mer-Alq3 derivatives [17].

In our previous studies, we have designed some blue and redshifted nitrogen, methyl and fluorinated derivatives and explainedthat position for substitution plays an important role [24–26]. The

Table 1Selected optimized bond lengths in Angstrom (Å) and dipole moment (Debye) former-Alq3 and its disubstituted derivatives at the B3LYP/6-31G* level.

Complexes Al–NA Al–NB Al–NC Al–OA Al–OB Al–OC l

Alq3 2.084 2.126 2.064 1.855 1.881 1.884 4.46931 2.077 2.115 2.058 1.858 1.884 1.886 6.46442 2.079 2.119 2.056 1.856 1.881 1.883 8.73063 2.067 2.096 2.049 1.866 1.893 1.893 7.06644 2.068 2.098 2.048 1.864 1.891 1.891 9.45655 2.084 2.125 2.063 1.853 1.878 1.881 2.60486 2.089 2.136 2.069 1.850 1.874 1.878 0.66317 2.081 2.118 2.062 1.857 1.881 1.883 1.55458 2.085 2.134 2.068 1.854 1.877 1.880 2.5735Expa 2.050 2.087 2.017 1.850 1.860 1.857

a Exp = experimental data of mer-Alq3 from Ref. [46].

40 A. Irfan et al. / Chemical Physics 364 (2009) 39–45

maximum red shift (14 nm) has been observed for tris(7-methyl-8-quinolinolato)aluminum and blue shift (4 nm) for tris(4-methyl-8-quinolinolato)-aluminum [25]. We have also designed blue andgreen light-emitting materials by substituting the mono-fluorineon different positions and explained that the mono-substitutionof fluorine atom has no effect to enhance the intrinsic chargemobility [26].

As mer-Alq3 is very good OLED material so in this study our aimis to design some new novel emitting materials with tunable coloras well as high-charge mobility by introducing EDG and EWDgroups at positions 4 and 6 on 8-hydroxyquinoline ligands, respec-tively. The present work is a detailed theoretical study, i.e., energydecomposition analysis, charge transfer, and optical properties ontwo kinds of disubstituted derivatives of mer-Alq3 with push–pull(X–Y) substituents (where X = –CH3/–NH2 and Y = –CN/–Cl). Thetris(4-X-6-Y-8-hydroxyquinolinato)aluminum depict for EDG (X)on position 4 and EWD group (Y) on position 6 on 8-hydroxyquin-oline ligands (type 1), and tris(4-Y-6-X-8-hydroxyquinolinato)alu-minum represent for EDG on position 6 and EWD on position 4 on8-hydroxyquinoline ligands (type 2).

By using advanced methods of bonding analysis, one can under-stand the bonding situation of molecules [27,28]. The bindinginteractions [29] have been analyzed by using the ADF energy-par-titioning scheme. In the framework of Kohn–Sham Molecular Orbi-tal theory and in conjunction with the fragment approach, one candecompose the bond energy between the fragments of a molecularsystem into contributions associated with the various orbital and

N

O

1

2

7

8

9

11

(b

Al

n* Different

4 CH3, 6 Cl tris(4-methyl-6-chlorolinato)aluminu

4 CH3, 6 CN tris(4-methyl-6-cyanolinato)aluminu

4 NH2, 6 Cl tris(4-amino-6-chlorolinato)aluminu

4 NH2, 6 CN tris(4-amino-6-cyano-linato)aluminu

4 Cl, 6 CH3 tris(6-methyl-4-chlorolinato)aluminu

4 CN, 6 CH3 tris(6-methyl-4-cyano

linato)aluminum

4 Cl, 6 NH2 tris(6-amino-4-chlorolinato)aluminu

4 CN, 6 NH2 tris(6-amino-4-cyano-linato)aluminu

*n= 4 or 6 denotes as positions where “H

and (Cl or CN) as la

A

B

C

(a)

Fig. 1. (a) The geometry of mer-Alq3 with labels A–C for three quinolate ligands (b) thestructure of tris(4-methyl-6-chloro-8-hydroxyquinolinato)aluminum (as an example).

electrostatic interactions, method known as energy decompositionanalysis (EDA). EDA was originally developed by Morokuma [30]and later modified by Ziegler [31]. The energy decomposition anal-ysis has been proved to be an understanding of chemical bonds[32–35]. Detail can be found in Supplementary material and Ref.[36].

2. Computational details

The structures of different disubstituted derivatives of mer-Alq3(see labeling scheme in Fig. 1b) have been optimized in the groundstates (S0) at the B3LYP/6-31G* level [24–26]. To optimize the

3

4

5

6

10

3

)

ligands Complexes

-8-hydroxyquino-m

1

-8-hydroxyquino-m

2

-8-hydroxyquino-m

3

8-hydroxyquino-m

4

-8-hydroxyquino-m

5

-8-hydroxyquino- 6

-8-hydroxyquino-m

7

8-hydroxyquino-m

8

” was substituted by (CH3 or NH2)

beled in Fig. 1b

(c)

ligand labeling for substituted mer-Alq3 complexes considered in this work. (c) The

Fig. 2. Frontier molecular orbitals (FMOs) (0.05 e au�3) for the ground states (S0) of disubstituted type1 derivatives of mer-Alq3.

A. Irfan et al. / Chemical Physics 364 (2009) 39–45 41

geometry of first excited state (S1), configuration interaction withall singly excited determinants (CIS) approach has been applied[26,37–42]. In this study to optimize the geometry of the S1, CIS

was used with 6-31G* basis set. In our previous studies on mer-Alq3 and its derivatives, absorption and emission spectra havebeen calculated by time dependent density functional theory

42 A. Irfan et al. / Chemical Physics 364 (2009) 39–45

(TD-DFT) using PBE0, B3LYP, BLYP, SVWN, and B3PW91 functionalsby adopting 6-31G*, 6-31+G*, and 3-21+G* basis sets. We have alsoexplained that PBE0 gave better and more reliable results and basisset has no significant effect on mer-Alq3 and its derivatives [26].Furthermore, to observe the basis set effect on disubstituted deriv-atives, we have also calculated the absorption and emission spectraof some of the derivatives at the PBE0/6-31+G* level on the struc-tures which were optimized at the B3LYP/6-31+G* and CIS/6-31+G*

level of theories, respectively. The absorption and emission of mer-Alq3 have also been calculated at the PBE0/6-311+G* level of the-ory. We have found that basis set has no significant effect (for de-tail see Supplementary material). Finally, the absorption andemission energies have been calculated at PBE0/6-31G* level oftheory [26]. The reorganization energy which is an importantparameter of the mobility has been computed at the B3LYP/6-31G* level of theory. All the calculations have been carried out withGaussian 03 [43].

Energy decomposition analysis has been performed at theB3LYP/DZP level. Scalar relativistic effects were considered byusing the zero-order regular approximation (ZORA) [44]. The en-ergy decomposition analysis was carried out by using the programpackage ADF [45].

Table 2The HOMO energy (EHOMO), LUMO energy (ELUMO), LUMO + 1 energy (ELUMO+1) andenergy gap (Eg) in eV for S0 states computed at the PBE0//B3LYP/6-31G* level.

Complexes EHOMO ELUMO ELUMO+1 Eg

Alq3 �5.26 �1.65 �1.40 3.861 �5.63 �1.91 �1.67 3.962 �6.05 �2.44 �2.21 3.843 �5.32 �1.33 �1.11 4.214 �5.71 �1.86 �1.66 4.055 �5.51 �1.93 �1.70 3.816 �5.92 �2.84 �2.62 3.307 �5.27 �1.66 �1.44 3.838 �5.67 �2.53 �2.33 3.34

3. Results and discussion

3.1. Ground states

3.1.1. Molecular geometriesFig. 1a is labeled with A–C designating the three different quin-

olate ligands of mer-Alq3. The structure is such that the central Alatom (+3 formal oxidation state) is surrounded by the three quin-olate ligands in a pseudo-octahedral configuration with the A- andC-quinolate nitrogens and the B- and C-quinolate oxygens trans toeach other. The molecular models used in our calculations havebeen obtained by systematic substitution of –CH3/–NH2 and –CN/–Cl at positions 4 and 6 (see labeling scheme) on each ligandas shown in Fig. 1. In Table 1 and Table S1 (Supplementary mate-rial), we gave selected geometrical parameters of the mer-Alq3and its disubstituted derivatives AlL3, where both optimized andexperimental results [46] are listed for comparison. Maximumlengthened or shortened in the bond lengths has been describedhere. The calculated bond length of Al–N1 is shortened in the type1 disubstituted derivatives, the decrease has been found in bondlength to be 0.03 Å in 3, 0.028 Å in 4, 0.011 Å in 1, and 0.008 Å in2 as compared to parent molecule (mer-Alq3). The calculated Al–O bond is slightly lengthened in disubstituted derivatives of type1, i.e., 0.012 Å lengthened in 3, 0.009 Å in 4, while negligible devi-ation has been found for 1 and 2. We have found that the shorten-ing of Al–N1 bond is in following order: 3 > 4 > 1 > 2. On the otherhand, Al–N1 bond is slightly lengthened in 6 and 8, i.e., 0.01 Å and0.008 Å, respectively. The Al–N1 bond length slightly shortened inthe 7, while 5 has almost the same Al–N1 bond length as that ofmer-Alq3. It has been observed that Al–O bond is slightly short-ened in 6 and 8. By introducing the EDG on position 4 on the li-gands, shortening of Al–N1 bond has been found in type 1derivatives. It is due to the more electronic density on N1 as EDGis also on pyridine side. Thus N1 attracts Al toward itself resultingdecrease the Al–N1 bond length. Maximum Al–N1 bond lengthshortening has been found in 3 and 4, because in both of the com-plexes strongly activating –NH2 group is at position 4 which do-nates more electrons toward N1. By introducing strong EWDgroups at position 4, Al–N1 bond length increases because EWDgroups attract the electrons of N1 toward itself and decreases theelectronic density on N1 resulting in increase of the Al–N1 bondlength as in 6 and 8. As far as 5 is concerned, weakly deactivating

group –Cl is at position 4 and weakly activating –CH3 is at position6 of the ligands, balancing the effect of each other so no change inbond length has been observed. In 7 weakly deactivating group –Clis at position 4 while strongly activating –NH2 group is at position6, withdrawing effect is less than the donating so in this complexAl–N1 bond is slightly shortened.

3.1.2. Dipole momentThe ground state dipole moment (l) values for all the com-

pounds studied in this work, computed at the B3LYP/6-31G* levelof theory are given in Table 1 as well. In our previous study [24],we explained that ‘‘CH”/N substitution affect dipole momentdependently on the substituted group. The same effect has beenobserved here that by substituting EDG at position 4 and EWGon position 6 (type 1) leads to l > 6 and by reverting substitution(type 2) l < 3. The tendency of dipole moment in disubstitutedderivatives is 4 > 2 > 3 > 1 > mer-Alq3 > 5 > 8 > 7 > 6.

3.1.3. Electronic structureIt has already been reported that the HOMOs are localized

mostly on A-ligand while LUMOs are localized on B-ligand inmer-Alq3 and its derivatives [24–26]. The distribution pattern ofHOMOs and LUMOs of disubstituted derivatives of mer-Alq3 inthe S0 states shown in the Supplementary material (Fig. S1) whichsuggests the localization of molecular orbitals. The HOMOs and LU-MOs in the disubstituted derivatives of mer-Alq3 show the similartrend of localization on A- and B-ligands, respectively. It can beseen that HOMOs are located on the phenoxide oxygen and theC-5, C-7, and C-8 of A-ligand, while LUMOs are located on the pyr-idyl ring of B-ligand. As shown in Fig. 2 (distribution pattern ofHOMOs and LUMOs + 1 for type 1) and Fig. S2, Supplementarymaterial (distribution pattern of HOMOs and LUMOs + 1 for type2) that LUMOs + 1 is mostly localized on C-ligand and on the N,C1, and C3 (pyridyl ring) of A-ligand. Among different disubstitutedderivatives of type 1, as shown in Table 2, the trend of HOMOsenergies (EHOMO) is as follows: 3 > 1 > 4 > 2, while in LUMOs ener-gies (ELUMO) is as: 3 > 4 > 1 > 2. The similar tendency has beenfound in the energies of LUMOs + 1 (ELUMO+1). The gap energieshave been calculated between the difference of EHOMO and ELUMO+1

because in the derivatives studied here, assignments for theabsorptions are from HOMO to LUMO + 1 (for details see Supple-mentary material). The trend of gap energies (Eg) in disubstitutedderivatives of type 1 is 3 > 4 > 1 > 2. The general trend for type 2is that the EHOMO, ELUMO, ELUMO+1, and Eg all decrease in the samesequence 7 > 5 > 8 > 6.

3.2. Electronic structure of the first excited states

Fig. 3 (HOMOs and LUMOs distribution pattern of type 1 deriv-atives) and Fig. S3, Supplementary material (HOMOs and LUMOsdistribution pattern of type 2 derivatives) show that HOMOs are lo-cated on the phenoxide ring while LUMOs are located on the pyr-

Fig. 3. Frontier molecular orbitals (FMOs) (0.05 e au�3) for the excited states (S1) of disubstituted type1 derivatives of mer-Alq3.

A. Irfan et al. / Chemical Physics 364 (2009) 39–45 43

idyl ring of A-ligand for S1 states. In Table 3, we only listed the cal-culated energies for the EHOMO and ELUMO eigenvalues, and Eg forthe investigated compounds at S1 states. From the results pre-

sented in Table 3, a general trend for type 1 is that EHOMO and ELUMO

decrease in the sequence of 3 > 1 > 4 > 2, and 2 > 3 ffi 4 > 1, respec-tively. The tendency in the Eg is as: 4 > 3 > 1 ffi 2. In type 2 deriva-

Table 3The HOMO energy (EHOMO), LUMO energy (ELUMO) and HOMO–LUMO energy gap (Eg)in eV for S1 states at the TD-PBE0//CIS/6-31G* level.

Complexes EHOMO ELUMO Eg

Alq3 �4.87 �1.65 3.221 �5.25 �1.99 3.392 �5.68 �1.67 3.323 �4.99 �1.89 3.474 �5.40 �1.89 3.565 �5.09 �1.91 3.186 �5.50 �2.81 2.707 �4.94 �1.66 3.298 �5.36 �2.51 2.85

Table 4Calculated absorption (ka) and emission (ke) wavelengths (nm) of mer-Alq3 and itsdisubstituted derivatives at the TD-PBE0/6-31G* level.

Complexes f ka Exp (ka)a f ke Exp (ke)b

Alq3 0.0759 410 387 0.0441 523 5151 0.1071 392 – 0.0692 477 –2 0.1146 401 – 0.0771 473 –3 0.1629 354 – 0.0756 461 –4 0.1661 365 – 0.1051 432 –5 0.0129 422 – 0.0550 528 –6 0.0413 503 – 0.0420 678 –7 0.0239 412 – 0.0535 506 –8 0.0838 487 – 0.0468 622 –

f = Oscillator strength.a Exp = the experimental absorption wavelengths (ka) from Ref. [17].b Exp = the experimental emission wavelengths (ke) from Ref. [17].

44 A. Irfan et al. / Chemical Physics 364 (2009) 39–45

tives, the tendency of the EHOMO, ELUMO, and Eg for S1 states is7 > 5 > 8 > 6.

Table 5Calculated intramolecular reorganization energies of mer-Alq3 and its disubstitutedderivatives (in eV) for hole k(h) and electron k(e) at the B3LYP/6-31G* level.

Complexes k(h) k(e) Complexes k(h) k(e)

Alq3a 0.242 0.276 5 0.274 0.2611 0.245 0.270 6 0.242 0.2372 0.238 0.231 7 0.261 0.3403 0.304 0.428 8 0.218 0.3104 0.277 0.347

a Data from Ref. [48].

3.3. Photophysical properties

In Table 4, we have reported the calculated and experimental[17] absorption and emission results. The 1, 3, and 4 showed blueshifts of absorption spectra ca. 18 nm, 56 nm, and 45 nm while 5, 6,and, 8 showed red shifts of the absorption spectra ca. 12 nm,93 nm, and 77 nm compared with calculated mer-Alq3 value,respectively. The ka of 2 and 7 are almost equal to mer-Alq3. Theiremission spectra being blue shifted, i.e., 1, ca. 46 nm, 2, 50 nm, 3,62 nm, and 4, 91 nm. Thus these derivatives are good candidatefor blue (1, 2, and 3) and violet (4) light-emitting materials. Theemission spectra of 6 and 8 have been found to be red shifted,i.e., ca. 155 nm and ca. 99 nm, respectively, suggesting good con-testant for red and orange emitting materials. The 5 has the samevalue of emission spectra as mer-Alq3. The 7 is blue shifted ca.17 nm compared to parent molecule. Here, we found that strongEWD group (CN) on position 4 on the ligands enhancing the redshift. The absorption spectra of 1, 3, and 4 are blue shifted becausethe gap energies increase whereas 5, 6, and 8 are red shifted due tothe decrease in the gap energies, the ka of 2, and 7 are same as com-pared with the parent molecule because of the same gap energiesin S0 (see Section 3.1.3). The emission spectra of 1, 2, 3, 4, and 7 areblue shifted due to the increase in gap energies while 6, and 8 arered shifted due to the decrease in gap energies, the ke of 5 and theparent molecule are almost same as the gap energies are almostsame in S1 (see Section 3.2). In general these results reflect thetrends of the gap energies difference discussed in the previous Sec-tions 3.1.3 and 3.2. It can be seen that in mer-Alq3 and its disubsti-tuted derivatives, the assignments for absorption are HOMO toLUMO + 1 while for emission are LUMO to HOMO (see Supplemen-tary material). An interesting feature was observed that the impor-tant calculated red and blue shifts correspond to the cases where

the Al–N bonds are affected in the same direction, i.e., lengtheningor shortening at the same time. The lengthening of Al–N bondscause red shift while shortening result blue shift (see Section3.1.1).

3.4. Energy decomposition analysis

To investigate the nature of metal–ligand interaction in disub-stituted derivatives of mer-Alq3, we have carried out an energydecomposition analysis (EDA). In Tables S2 and S3, Supplementarymaterial key results of bonding analysis for interaction betweenone quinolate ligand ðL�i Þ and AlLþ2 ðLi ¼ A; B or CÞ fragments havebeen displayed. It is shown in Tables S2 and S3 (Supplementarymaterial), that the trend of DEelstat is La–AlLbLc > Lb–AlLaLc > Lc–Al-LaLb, while DEorb is Lb–AlLaLc > La–AlLbLc > Lc–AlLaLb in disubsti-tuted derivatives of mer-Alq3, i.e., weaker DEelstat is between La–AlLbLc fragments while weaker DEorb is between Lb–AlLaLc frag-ments. Thus in all the disubstituted derivatives of mer-Alq3, theHOMOs are localized on A-ligand due to the weaker DEelstat be-tween La–AlLbLc fragments, while LUMOs are localized on B-liganddue to the weaker DEorb between Lb–AlLaLc fragments. For detailsee Refs. [25,26].

3.5. Charge transfer properties

The reorganization energy (k) includes the molecular geometrymodifications when an electron is added or removed from a mole-cule (inner reorganization) as well as the modifications in the sur-rounding medium due to polarization effects (outerreorganization). Here, we focus inner reorganization energy, whichreflects the geometric changes in the molecules when going fromthe neutral to the ionized state and vice versa. It is the sum oftwo relaxation energy terms [47]: (1) the difference between theenergies of the neutral molecule in its equilibrium geometry andin the relaxed geometry characteristic of the ion and (2) the differ-ence between the energies of the radical ion in its equilibriumgeometry and in the neutral geometry.

Lin et al. calculated the reorganization energy of mer-Alq3(reorganization energy for hole k(h) = 0.242 eV and reorganizationenergy for electron k(e) = 0.276 eV) at the B3LYP/6-31G* level. Onthe basis of reorganization energy it seems that mer-Alq3 is holetransfer material which is contradiction with the experiment thenLin et al. also calculated transfer integral and explained as electron-transfer material [48]. To compare the results of reorganization en-ergy of disubstituted derivatives of mer-Alq3, we have also calcu-lated the reorganization energies of disubstituted derivatives ofmer-Alq3 at the same level of theory. The mer-Alq3 is electron-transfer material so we have compared the k(e) of disubstitutedderivatives with parent molecule. As shown in Table 5 that k(e)are 0.270 eV, 0.231 eV, 0.261 eV and 0.237 eV for 1, 2, 5 and 6,respectively. The k(e) of 1 and 5 are the same as parent moleculewhile 2 and 6 are smaller. The reorganization energies of 3, 4, 7and 8 are higher than that of mer-Alq3. To calculate the mobility,reorganization energy and transfer integral are important parame-

A. Irfan et al. / Chemical Physics 364 (2009) 39–45 45

ters. In order to investigate the transfer integral, crystal data is re-quired which is not available, so we just calculated one of theimportant parameter of mobility, i.e., reorganization energy. Onthe basis of reorganization energy, mobility of 1, 2, 5 and 6 deriv-atives will be comparable with mer-Alq3. In general, substitutionsof EDGs favor the p-channel materials while introduction of EWDgroups promote the n-channel materials [49]. From Table 5, wefound that the hole reorganization energies of all the derivativesexcept 3 are almost same as mer-Alq3. The introduction of stronglyactivating –NH2 group at position 4 (3 and 4) or position 6 (7 and 8)boost the electron reorganization energies in the derivatives com-pared with mer-Alq3. We have found that by introducing weaklyactivating –CH3 group on position 4 (1 and 2) or position 6 (5and 6) along with EWDs –Cl/–CN, hole reorganization energies aswell as electron reorganization energies are almost same as paren-tal molecule. Introduction of weakly deactivating group –Cl doesnot affect the strength of reorganization energies for 1 and 5; itmay be due to that weakly activating group –CH3 and weakly deac-tivating group –Cl counter balance the effect of each other (see Ta-ble 5). The electron reorganization energies of 2 and 6 are smallerthan mer-Alq3 may be due to the strongly deactivating group –CNbut the effect is not significant.

4. Conclusions

The ground state structures of disubstituted derivatives of mer-Alq3 have been optimized at the B3LYP/6-31G* level of theory. Incomparison to the pristine mer-Alq3, the energy gap increases inthe disubstituted type 1 derivatives, while decreases in tris(6-methyl-4-cyano-8-hydroxyquinolinato)aluminum (6) and tris(6-amino-4-cyano-8-hydroxyquinolinato)aluminum (8). The signifi-cant blue shifts have been observed for tris(4-amino-6-chloro-8-hydroxyquinolinato)aluminum (3) and tris(4-amino-6-cyano-8-hydroxyquinolinato)aluminum (4), and red shifts for 6 and 8. Wehave designed some luminescence materials of mer-Alq3 deriva-tives in order to cover the whole visible region (from blue to red)for OLEDs by introducing strong electron withdrawing groups atC-6, electron donating groups at C-4 and by reverting their posi-tions on the ligands. The energy decomposition analysis shows thatthe metal–ligand interactions of disubstituted derivatives of mer-Alq3 have a higher electrostatic character than covalent character.The HOMOs are localized on A-ligand due to the weaker electro-static energy (DEelstat) between La–AlLbLc fragments while the LU-MOs are localized on B-ligand because Lb–AlLaLc fragments haveweaker orbital interaction energy (DEorb). The tris(4-methyl-6-chloro-8-hydroxyquinolinato)aluminum (1), tris(4-methyl-6-cya-no-8-hydroxyquinolinato)aluminum (2), tris(6-methyl-4-chloro-8-hydroxyquinolinato)aluminum (5), and tris(6-methyl-4-cyano-8-hydroxyquinolinato)aluminum (6) are considered to be goodcandidates for blue (1, 2), green (5), and red (6) emitting materialswith higher mobility similar to mer-Alq3.

Acknowledgements

Financial supports from the NSFC (Nos. 50873032 and20773022), NCET (NCET-06-0321) and NENU (NENU-STB07007)are gratefully acknowledged. A. Irfan acknowledges the financialsupport from China Scholarship Council (CSC) and Ministry of Edu-cation (MoE), Pakistan.

Appendix A. Supplementary material

The Frontier molecular orbitals (FMOs) distribution pattern ofHOMOs and LUMOs of disubstituted derivatives, basis set effect

on optical properties and energy decomposition analysis can befound in the Supporting Information. Supplementary data associ-ated with this article can be found, in the online version, atdoi:10.1016/j.chemphys.2009.08.009.

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