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Synthetic Metals 195 (2014) 54–60

Contents lists available at ScienceDirect

Synthetic Metals

jo ur nal homep age: www.elsev ier .com/ locate /synmet

valuation of various density functionals for predicting thelectrophosphorescent host HOMO, LUMO and triplet energies

ee Kian Onga,c, Kai Lin Woonb,∗, Azhar Ariffina

Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, MalaysiaLow Dimensional Materials Research Center, Department of Physics, University of Malaya, 50603 Kuala Lumpur, MalaysiaItraMAS Corporation. Sdn. Bhd., 542A-B Mukim 1, Lorong Perusahaan Baru 2, Kawasan Perindustrian Perai 13600, Penang, Malaysia

r t i c l e i n f o

rticle history:eceived 11 March 2014eceived in revised form 12 May 2014ccepted 13 May 2014vailable online 2 June 2014

eywords:ensity functional theoryripletOMO

a b s t r a c t

Design of phosphorescent hosts for deep blue emitter with suitable electronic states for high efficiencyelectrophosphorescent light emitting diodes is often carried out by using wet-lab techniques and trialand error. Although, quantum computation has been carried out to study the influence of certain sub-stituents on the materials, an accurate prediction of the highest occupied molecular orbital (HOMO), thelowest unoccupied molecular orbital (LUMO) and the triplet energies (ET) of electrophosphorescent hostsremains challenging. Here, we evaluate different density functionals on a set of electrophosphorescenthosts of different sizes using time-dependent density functional theory. M062X is found to be the bestpredictor for ET. Other methods such as B3LYP and O3LYP are potentially good estimators for HOMOand LUMO levels. By using empirical relationships between the predicted HOMO, LUMO and ET energies

UMOhosphorescencerganic light emitting diode

with respect to the experimental values, more accurate estimations can be obtained. The empirical rela-tionships mean absolute error for ET, LUMO and HOMO energy levels are 0.11 eV, 0.21 eV and 0.17 eV,respectively. These techniques are used to predict a new electron transporting electrophosphorescenthost. These empirical relationships would greatly assist in the accurate prediction of electronic states ofsolution processable electrophophorescent hosts.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Organic light-emitting diodes (OLEDs), especially the phos-horescent ones (PhOLEDs), have been widely studied due to itsotential to reach 100% internal quantum efficiency [1]. This makeshOLEDs a promising alternative as energy efficient displays andolid state lighting. Among the three different colors, red andreen PhOLEDs fabricated by solution processing have been devel-ped with success, while the high efficiency deep-blue PhOLEDsemain scarce [2]. High triplet OLED materials that can be solu-ion processed are highly desirable due to the lower manufacturingost. Other than color display, deep-blue PhOLEDs can be used toroduce white light in solid-state lighting. However, the need ofide triplet bandgap hosts and dopants makes synthesizing high

uantum efficiency deep blue hosts challenging [3]. Furthermore,

creening all potential novel deep blue hosts solely with wet-labechniques and by trial and error is time-consuming and veryxpensive.

∗ Corresponding author. Tel.: +60 3 79674287.E-mail address: ph7klw76@um.edu.my (K.L. Woon).

ttp://dx.doi.org/10.1016/j.synthmet.2014.05.015379-6779/© 2014 Elsevier B.V. All rights reserved.

Reliable and accurate methods for predicting the electronicstates of such molecules would offer a solution for screening awide range of potential deep blue hosts. However, solution pro-cessable hosts are usually large. Accurate predictions of tripletenergy levels (ET) and frontier orbitals of such molecules constitutea significant computational challenge. Time-dependent densityfunctional theory (TD-DFT) is an extension of density functionaltheory and has become one of the most prominent methods forcalculating excited states. It is used in the presence of time-dependent potentials, such as electric or magnetic fields. Externaltime-dependent potentials such as electromagnetic waves can beconsidered a weak perturbation. Hence, the dynamic process suchas transition between two eigenstates can be described. TD-DFTcan be used to extract information such as excitation energies,frequency-dependent response properties and photoabsorptionspectra of a given molecule. TD-DFT is chosen for its compromisebetween accuracy and computational performance for typical chro-mophores of about 100 atoms [4,5]. TD-DFT is an exact theory but

the implementation of it requires selection of an exchange corre-lation function which approximates the model. Hence the resultsare profoundly functional-dependent. Computational studies onOLED materials generally focus on the effect of certain substituents,

B.K. Ong et al. / Synthetic Metals 195 (2014) 54–60 55

Table 1HOMO, LUMO, band gap and triplet energies of selected electrophosphorescent hosts.

Molecular structure Experimental data References

HOMO (eV) LUMO (eV) Triplet energy (eV) Bandgap (eV)

BCP

−6.4 −3.0 2.6 3.4 [17]

TAPC

−5.1 −2.0 2.87 3.1 [22]

PO14

−7.45 −2.50 3.14 4.95 [23]

HM-A1

−5.59 −2.56 2.84 3.03 [18,21]

CBP

−6.2 −2.17 2.56 4.03 [15]

PO1

−7.2 −2.9 2.72 4.3 [14]

mCP

−5.8 −2.4 2.9 3.4 [19]

DHM-A2

−5.37 −2.46 2.7 2.91 [21]

NPD

−5.4 −2.4 2.26 3.0 [20]

56 B.K. Ong et al. / Synthetic Metals 195 (2014) 54–60

Table 1 (Continued)

Molecular structure Experimental data References

HOMO (eV) LUMO (eV) Triplet energy (eV) Bandgap (eV)

TCTA

−5.2 −2.2 2.85 3.0 [16]

Table 2HOMO, LUMO, band gap and triplet energies of TCTA calculated from various functionals using TD-DFT.

Method HOMO (eV) LUMO (eV) Band gap (eV) Triplet energy (eV)

B3LYP −5.40 −1.43 3.97 2.51CAM-B3LYP −6.64 −0.32 6.32 2.31BHandHLYP −6.32 −0.42 5.90 1.85O3LYP −5.13 −1.50 3.63 2.62X3LYP −5.40 −1.33 4.07 2.49�B97a −7.58 0.53 8.11 1.97�B97XD −7.15 0.26 7.41 2.45M062X −6.47 −0.66 5.81 2.83M062X//O3LYP −6.72 −0.61 6.11 2.88

srtmtssdtr[ceeitha

gcoiTBreEuerps

2

i

(T1), the HOMO and LUMO energies. Generally, the computationalcost for chromophores that are less than 50 atoms is around 1–2days (16 processors, 1 GB memory) and a week for larger chro-mophores. Due to time restriction and the need to identify suitable

Experimental −5.20 −2.20

a Job not terminated normally.

uch as the carbazoles, fluorene, phosphine oxides, and anthracenesather than searching for the most suitable computational methodo predict electronic properties of OLED materials [6–10]. Further-

ore, the assessment of TD-DFT is restricted to small compounds orhe subunits but not the whole OLED material [11]. A more exten-ive research, where TD-DFT was compared with Hartree–Fock (HF)imulations to predict optical excitations of organic oligomers, wasone by A. Pogantsch et al. [12]. Again, the study is confined tohiophene subunits. Recently, a semi-empirical method, PM6, waseported but only analysis of ground-state properties is discussed13]. Appropriate selection of the exchange correlation is oftenrucial to grasp the chemical sound conclusions. Furthermore, thestimation of ET of the electrophosphorescent hosts against differ-nt density functionals remains scarce. High ET energy levels aremportant for the deep blue emitter as the host must be higherhan that of the guest to favor exothermic energy transfer from theost to the guest. Estimation of the ET energy level with chemicalccuracy (0.1 eV) may be highly desirable.

TD-DFT was selected in order to calculate frontier orbitals ener-ies and ET in vacuum. The goal was to determine exchangeorrelation functions that give the closest theoretical frontierrbital energies and ET to the experimental results. However, exper-mental results are often done in solid state or in a dilute solvent.he calculated results might differ from the experimental results.ulk solvent effects can be incorporated in the calculation but it willesult in unnecessary computational cost. Hence, we also developmpirical relationships between the predicted HOMO, LUMO andT energies with respect to the experimental values. This will allows to predict more accurately the calculated electronic states oflectrophosphorescent hosts. Determination of such a relationshipequires TD-DFT calculations on a set of representative electrophos-horescent hosts. In this investigation, we use a set of 10 moleculesince their respective electronic states are well characterized.

. Computational methodology

The chemical structures of the electrophosphorescent hostsncluding their HOMO, LUMO and ET are investigated theoretically

3.00 2.85

in this work and are depicted in Table 1 [14–23]. The HOMO andLUMO levels in Table 1 are obtained by cyclic voltammetry. TheET in Table 1 are usually obtained as the first vibronic mode ofthe corresponding phosphorescence spectra at a temperature lessthan 77 K. The aromatic cores consist mainly of carbazole, phos-phine oxide, biphenyl and triphenylamine. Calculations are donewith the Gaussian09 [24] program installed in the Symmetric MultiProcessing (SGiAltix4700, 64x Dual Core Intel Itanium 2 64 bitsProcessors) at University of Malaya High Performance Computingand Academic GRID (2× Quad Core Intel(R) Xeon(R) CPU E5450 @3.00 GHz). All structures were optimized at the ground state (So)using spin restricted DFT 6-311G(d,p). Then spin restricted TD-DFT 6-311G(d,p) is used to obtain the first excited triplet state

Fig. 1. MSE and MAE of ET, LUMO and HOMO for M062X//B3LYP, M062X//O3LYP,O3LYP and B3LYP.

B.K. Ong et al. / Synthetic Metals 195 (2014) 54–60 57

F O3LYPc

fh[[tpiew

3

3

a

ig. 2. Plots of computed LUMO, HOMO and ET for M062X//B3LYP, M062X//O3LYP,

orresponding R2 are shown on the top left corner.

unctionals, only 8 DFT functionals are selected. These includeybrid functionals (B3LYP [25], BHandHLY [26], O3LYP [27], X3LYP28], M062X [29]) and range-separated functionals (CAM-B3LYP30], �B97 [31], �B97XD [32]). These functionals were appliedo tris(4-carbazoyl-9-ylphenyl)amine (TCTA) as training sets. Fourromising functionals are selected and applied to the OLED hosts

n Table 1. The band gap was determined here to be the differ-nce between the HOMO and LUMO energies. The calculated resultsere compared with literature.

. Results and discussion

.1. Training sets

The training set data are obtained and 4 potential function-ls are identified for further investigation. These functionals are

and B3LYP against experimental values. The linear regression equations and their

B3LYP, O3LYP, M062X//B3LYP, and M062X//O3LYP. Results showedthat B3LYP and O3LYP are both good functionals to estimate fron-tier orbital energies. M062X is among the best in estimating ETas shown in Table 2. The O3LYP functionals are similar to theB3LYP functionals, but the O3LYP uses better optimized exchangefunctionals developed by Handy and Cohen [33]. In contrast withB3LYP, O3LYP has a reduced HF-exchange contribution, a largercoefficient multiplying OPTX functional in place of the standardXB88 exchange and a new Vosko, Wilks, and Nusair’s local cor-relation [34]. The M062X functional is a non locality functionalwith twice the amount of nonlocal exchange, parametrized onlyfor nonmetals. It treats opposite-spin and parallel-spin correla-

tion differently [35]. This functional has terms that depend onspin-up and spin-down electron densities, spin density gradientsand spin kinetic energy densities. In phosphorescence, electronschange their spin direction creating triplet states. Hence, M062X is

5 tic Metals 195 (2014) 54–60

aBwsiMtturctEt

3

XfutibitprwtlnlaTnuasau(besif

MsToM0oeMllwacraMc

8 B.K. Ong et al. / Synthe

good estimator of electronic process that involves spin-flipping.3LYP and O3LYP use electron density and its first derivativeshile M062X includes the second derivatives of the electron den-

ity in their exchange-correlation functionals. Although M062Xs potentially more accurate, further terms in the expansion of

062X functional increase the computational time. To shortenhe amount of time used to calculate the ET, the TCTA structurehat is optimized with O3LYP was submitted for direct calculationsing M062X functionals (M062X//O3LYP). The same method wasepeated with B3LYP to generate M062X//B3LYP. It was noted thatalculations that were done with B3LYP and O3LYP on other elec-rophosphorescent hosts have the tendency to underestimate theT. M062X//B3LYP and M062X//O3LYP have a better estimation ofhe electrophosphorescent hosts triplet energies.

.2. Benchmark data

In terms of HOMO and LUMO levels, the B3LYP, O3LYP and3LYP functionals produce closer experimental values than other

unctionals tested here. The relatively good performance of the pop-lar B3LYP/6-31G* model chemistry has been used for calculatinghe HOMO and LUMO levels for various organic semiconduct-ng materials. However, these three functionals over-estimate theandgaps with O3LYP showing the most promising estimate. This

s consistent with the recent results obtained by L. Ling et al. onriphenylamine-fluorene copolymers [36]. While for the ET, M062Xroduces the closest value while the HOMO and LUMO levels wereather far from experimental values. This is consistent with theork of D. Jacquemin et al., where a set of small molecules with

he largest being naphthalene, is benchmarked with mean abso-ute error (MAE) of 0.07 eV [37]. TCTA is about 5 times larger thanaphthalene and the accuracy of the result is remarkable. Indeed

arge HF exchange contributions such as M062X (54% HF exchange)re required for an accurate prediction of the triplet energy [38].he next closest is O3LYP followed by B3LYP. It is important toote that the ET of electrophosphorescent hosts are often obtainedsing B3LYP functionals [39]. Although M062X offers excellentccuracy, it demands high computational cost for large moleculesuch as TCTA without pre-optimized molecular structure at highccuracy. Hence, we pre-optimized the ground state of the TCTAsing O3LYP before submitting a direct calculation using M062Xreferred as M062X//O3LYP). As expected, a high accuracy ET cane obtained using M062X//O3LYP. In order to obtain a meaningfulvaluation of the accuracy of different functionals, a comparisonet of electrophosphorescent materials are required. It would benteresting to compare values calculated with experimental dataor the molecules listed in Table 1.

Four different methods (B3LYP, O3LYP, M062X//O3LYP and062X//B3LYP) are used to evaluate the accuracy of electronic

tates of different electrophosphorescent hosts as listed in Table 1.he MAE and mean signed error (MSE) is given in Fig. 1. These meth-ds tend to underestimate the triplet energy. M062X/O3LYP and062X/B3LYP are the best triplet energy predictors with MAEs of

.18 eV and 0.21 eV respectively. MSEs of −0.12 eV and −0.18 eV arebtained for M062X/O3LYP and M062X/B3LYP respectively. How-ver, both methods are the worst predictors for LUMO levels withAEs of more than 1 eV. B3LYP is the best predictor for HOMO

evels with an MAE of 0.28 eV. However, the error is considerablyarge for the LUMO with an MAE of 0.64 eV. Linear regressions

ere performed on the data sets from each methodology and thesere plotted in Fig. 2. These regressions are used to correct empiri-ally the electronic states. By adding the regression slope and the

egression intercept, all methods have significantly smaller errorss shown in Fig. 3 as the “corrected MAE”. Although the “correctedAE” has lower error, it is important to note that the correlation

oefficient for an estimation of the LUMO level is poor (R2 ∼ 0.4).

Fig. 3. “Corrected MAE” of ET, LUMO and HOMO for M062X//B3LYP, M062X//O3LYP,O3LYP and B3LYP.

However, the correlation coefficient for the HOMO is rather good(R2 ∼ 0.7). ET shows the highest correlation with O3LYP having anR2 of 0.76. The “corrected MAE” for ET falls between 0.11 and 0.16 eVmaking the regression method a promising method for estimatingthe triplet energy.

3.3. Application on novel OLED materials

Some of the more interesting OLED materials are TCTAderivatives with diphenylphosphine oxide. Tris[4-(3,6-diphenylphosphinoyl-9H-carbazoyl-9-yl)phenyl]amine (T6POCA)is a new TCTA derivative with six diphenylphosphine oxides.TD-B3LYP 6-311G(d,p) calculation is carried out on T6POCA.The HOMO, LUMO and ET energy levels are −5.61 eV, −1.84 eVand 2.44 eV respectively. Using the empirical relationship,the calculated results for HOMO, LUMO, and ET energy lev-els are corrected to be (−5.90 ± 0.19) eV, (−2.61 ± 0.21) eV, and(2.92 ± 0.14) eV. The ET and bandgap are expected to be very similarto TCTA and (9-(4-(bis(4-(9H-carbazol-9-yl)phenyl)amino)-phenyl)-9H-carbazol-3-yl) diphenylphosphine oxide (TCTAPO)(EHOMO = −5.30 eV, ELUMO = −1.83 eV, ET = 2.83 eV) synthesized byX. Yang et al. [40]. This observation supports that the electron-withdrawing diphenylphosphine oxide group only lowers theHOMO and LUMO energy levels, but give little changes to the ETand the bandgap. Compared to TCTAPO, the presence of five extradiphenylphosphine oxide units in T6POCA leads to deeper HOMOand LUMO levels. This would give better hole-blocking propertiesand enhanced electron injection. This material will be suitablefor an electron transporting layer for solution processable bluePHOLEDs.

Unlike TCTA where the electron delocalization at the HOMO ismore evenly distributed, the electron delocalization for T6POCA isconcentrated at the core as shown in Fig. 4. The LUMO levels for bothmolecules, however, are concentrated at the triphenylamine core.From the electrostatic potential map, the carbazole chromophoresof TCTA are very electronegative while the triphenylamine core ishighly electropositive reducing the net dipole moment (0.03 D). InT6POCA, the six electron withdrawing diphenylphosphine oxidespull all the electrons toward the oxygen atoms (red) as depictedin Fig. 5. The triphenylamine core becomes less electropositivelycharged (cyan) while the carbazole chromophores are neutral(green). This accounts for the high net dipole moment (3.76 D) and

indicates that T6POCA is very polar.

A novel dispirofluorene (2,1-b)-indenofluorene has been syn-thesized and computed at B3LYP//6-311G(d,p) [9]. Using ourderived empirical equation, the HOMO and LUMO levels are

B.K. Ong et al. / Synthetic Metals 195 (2014) 54–60 59

Fig. 4. Chemical structures, HOMO and LUMO of TCTA and T6POCA. The fronti

pw−

4

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[[[[[

[

[

[

[18] C.G. Lee, S. Park, R.S. Ruoff, A. Dodabalapur, Appl. Phys. Lett. 95 (2009)023304/1–023304/23304.

[19] M.S. Lin, S.J. Yang, H.W. Chang, Y.H. Huang, Y.T. Tsai, C.-C. Wu, S.H. Chou, E.

Fig. 5. Electrostatic potential map of TCTA and T6POCA.

redicted to be (−5.9 ± 0.19) eV and (−2.1 ± 0.21) eV respectivelyhile the values obtained by cyclic voltammetry are −5.73 eV and2.1 eV. These values are within the predicted range.

. Conclusion

M062X is found to be the best predictor for ET while other meth-ds such as B3LYP and O3LYP are potentially good estimators forOMO and LUMO levels. Using linear regression, a more accuratestimation is obtained. The “corrected” results of B3LYP, O3LYP,062X//B3LYP, and M062X//O3LYP show great promise in the esti-ation of the ET energy level with an MAE of less than 0.16 eV.e applied these techniques to predict a new electron trans-

orting electrophosphorescent host. Using these simple empiricalelationships, the electronic properties of a wide range of phos-horescent hosts, especially the large solution processable hostsan be estimated with sufficient accuracy. This is very useful forcreening a large set of potential hosts and reducing the amount of

ime wasted in synthesizing non-optimized materials in the wet-ab. [

er orbitals plots are calculated with B3LYP, 6-311G(d,p) at ground state.

Acknowledgements

This research is funded by ItraMAS Sdn. Bhd. (PV002-2013),Chancellery High Impact Research Grant (UM.C/625/1/HIR/195)and (UM.C/625/1/HIR/208) Malaysia Long Term Research GrantScheme (LR003-2011A), and Postgraduate Research Fund (PPP)(PG021-2013A) by University of Malaya. We would like to thankPusat Teknologi Maklumat Universiti Malaya and Academic GRIDfor high performance computer access. Special thanks to Mr.Safwan (UM PTM) and Mr. Farhan (Academic GRID) for providingtechnical support.

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