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Chemical Physics 330 (2006) 166–171

Density functional study of triazole and thiadiazole systems aselectron transporting materials

Emil Jansson *, Prakash Chandra Jha, Hans Agren

Department of Theoretical Chemistry, Royal Institute of Technology, Roslagstullsbacken 15, S-106 91 Stockholm, Sweden

Received 23 May 2006; accepted 9 August 2006Available online 16 August 2006

Abstract

Density functional theory has been used for the calculation of electronic structures, vertical electron affinities and intramolecular reor-ganization energies for bis-aryl substituted triazole and thiadiazole. The results obtained on the basis of the theoretical calculations indi-cate that the HOMO and LUMO energies of the substituted molecules can be tuned by changing the substituents as well as by changingthe center atom. These changes lead to energy shifts in the order of 2–2.5 eV. The calculation and comparison of vertical electron affin-ities and intramolecular reorganization energies confirm that thiadiazole systems are interesting for electron transport properties. Takinga lesson from these substitutions, we further model the systems by twisting the molecular units along the central dihedral angle startingfrom the ideal structure and compare their HOMO–LUMO gap, electron affinity and reorganization energy. We find that by havingsimple substituents at proper positions one can control the reorganization energy, which in turn indicates that electron transport prop-erties can be tuned.� 2006 Elsevier B.V. All rights reserved.

Keywords: Electron transport; Reorganization energy; HOMO–LUMO gap; Oxadiazole; Triazole; Thiadiazole

1. Introduction

A wide range of p-conjugated organic materials hasbeen tested for fabricating organic electroluminescentdevices in recent times [1]. In order to have an efficient suchdevice for commercial applications, the most importantrequirement to fulfill is a balanced charge injection and asufficient carrier mobility of both holes and electrons. Thisin turn means that one needs to look for materials whichpossess desirable ionization-potentials and electron affini-ties at the electron and hole injecting interface in order toget efficient charge injection. The charge carrier materialshould on top of this be environmentally stable to meetthe demands of long lifetime. In order to ensure the men-tioned criteria there are many crucial properties, chemicalas well as physical, which need to be optimized. This can

0301-0104/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.chemphys.2006.08.010

* Corresponding author.E-mail address: emil@theochem.kth.se (E. Jansson).

be ion stability, relaxation energies, luminescence yieldsand molecular orbital (MO) energy levels.

So far, the development of hole-transporting (HT) mate-rials has been superior to electron transporting (ET) ones,something that mainly is due to the fact that many emittingmaterials transport holes and not electrons. This somewhatunbalanced development have caused differences in the effi-ciency between the hole- and electron-transporting materi-als, pushing the recombination zone towards the cathode,yielding a lowering of the electroluminescence efficiencycaused by exciton quenching by the metal cathode [1]. Thisproblem is temporarily solved by the use of low work func-tion cathodes with the drawback that these cathodes aremore easily affected by the surrounding environment. Theneed for better ET materials are therefore a key issue forthe further development for organic light emitting diodes(OLEDs). One needs to develop materials which possesselectron accepting properties such as desirable electronaffinity and with anion radicals that are stable towardsquenching caused by the environment such as molecular

NN

NHY Y

Y={H, NO2, N(CH3)2}

NN

SY Y

NN

OY Y

Fig. 1. Structures of the 2,5-aryl-oxadiazole (top), 2,5-aryl-triazole(middle) and 2,5-aryl-thiadiazole (bottom) systems.

2

1

Cation

VEA

AEA Neutral

Scheme 1. Definitions of calculated quantities.

E. Jansson et al. / Chemical Physics 330 (2006) 166–171 167

oxygen. Also, in more delicate designs where several layersare used, the hole blocking properties of the ET material areof concern.

Among the wide range of electron transporting materialsreported in the literature, the materials based on oxadiazole[1,3,4-oxadiazole] and silole [1,1 0-dimethylsilacyclopentadi-ene] are supposed to be among the most effective ones.Results of several studies have shown that oxadiazole basedsystems have both efficient electron and hole blocking prop-erties in a variety of molecular architectures [1]. It hasthough also been found that these materials suffer fromnot having a thermotropic liquid crystalline (LC) melts,e.g. high molecular weight derivatives of 1,3,4-oxadiazole[2,3]. Thanks to improvement in the synthetic chemistry[4,5] one has been able to observe changes in the corre-sponding mesophases of the oxadiazole systems when theoxygen atom has been substituted with a sulfur or a nitro-gen atom, creating new interesting compounds. To ourknowledge there are no photoluminescence properties orintramolecular reorganization energies (ki) reported neitherfor thiadiazole nor triazole even though some derivativeshave been synthesized and have displayed semiconductingproperties [6,7].

In this work, we assess bis-aryl substituted 2,5-diphenyl-1,3,4-thiadiazole and 2,5-diphenyl-1,3,4-triazole as potentialprospect molecules for ET materials and their comparisonto oxadiazole. We do this in terms of ki according to thesemi-classical Marcus equation [8], MO energy positionsand photoluminescence wavelengths compared to thealready well studied 2,5-diphenyl-1,3,4-oxadiazole system[9]. The structures are compared in order to detect struc-ture deviation that may cause shifts in the mesophases.As studies where the effect of different side chains whichare added to a backbone already have been reported [10],we focus in this paper on grasping the general trends onhow the highest occupied molecular orbital (HOMO) andthe lowest unoccupied molecular orbital (LUMO) energylevels, ki and fluorescence spectra change when the internaltorsion angle is altered. This general examination incorpo-rates a twist of the dihedral angel between one of the phe-nyl groups and the 1,3,4-oxadiazole thus simulating thetwist caused by a side group. On the basis of the results,we foresee new structure property relations that can guidefurther development of ET materials.

2. Computational details

The systems being examined in this work are bis arylsubstituted triazole, thiadiazole and oxadiazole, seeFig. 1. The phenyl rings at the 2,5 positions are substitutedwith nitrophenyl and N,N-dimethylaminophenyl. Themolecular structures were optimized in gas phase at thedensity functional theory (DFT) level using the hybridB3LYP exchange-correlation functional [11] and the all-electron 6-31G* double-f plus polarization basis set. Forsingle point calculations we used the 6-31+G* double-fplus polarization and diffuse basis set. All calculations have

been performed using the quantum chemistry programGaussian-03 [12]. As mentioned above we use Marcus the-ory [8] to estimate the barrier for electron transport. It hasbeen shown earlier [13] that the activation barrier is pro-portional to the reorganization energy. In this paper wecalculate ki as given by the sum of k1 and k2 shown inScheme 1.

3. Results and discussion

Before we take any decision on the basis of calculatedHOMO–LUMO energies and ki, which relates to the com-bination of the ionization process of the neutral speciesand the electron attaching process of the corresponding cat-ion radical, the validity of the computational method mustbe checked. i.e. it must give reliable results on geometries,ionization potentials as well as electron affinities of thesesystems. When validating the computational methodemployed the question arises to what extent the Kohn–Sham LUMO orbital (KSO) energies could be interpretedas electron affinities and hence a measure of the electroninjection barrier. In case of the Hartree–Fock approxima-tion, Koopmans’ theorem [14] (KT) justifies ionizationpotentials and electron affinities to be estimated on the basisof the frozen MO approximation. The ionization energiesbased on KT are usually a good approximation since the

Fig. 2. B3LYP/6-31+G*-calculated HOMO (left) and LUMO (right)levels of 2,5-diphenyl-1,3,4-oxadiazole, 2,5-diphenyl-1,3,4-triazole and2,5-diphenyl-1,3,4-thiadiazole.

168 E. Jansson et al. / Chemical Physics 330 (2006) 166–171

lack of correlation tend to cancel the missing relaxation. Onthe other hand, electron affinities experience in general con-siderably larger deviations from experiments (even givingthe wrong sign) since then correlation tend to add to themissing relaxation. Formally, KT does not apply to DFTsince the KS orbital energies do not carry any physicalmeaning. However, using the Janak’s theorem [15] which‘‘provides a meaning for the eigenvalues of the Kohn–Shamequation’’, Perdew [16] has shown a connection betweenionization potentials and electron affinities to the HOMOand LUMO energies, respectively, in DFT. On the basiswe have used B3LYP which is expected to provide bandgap energies which lie between pure density functionalsand Hartree–Fock results [17]. Regardless of the limitationsand technical problems, KSO energies calculated by meansof density functional theory have been successfully used incomparing and predicting ionization potentials as well aselectron affinities of a wide range of experimentally reportedmaterials [13,18]. On top of this, the ability of DFT methodsto include electron correlation (at least partially) into theformalism, make the KSO energies attractive. To get anidea about the correlation between the self-consistent fieldenergies and KT-derived electron affinities, we performeda regression analysis, including all compounds, whichresults in a linear fit (y = 1.005 + 1.390) with an offset of1.39 eV which is consistent with previous results obtainedby Risko et al. [9]. At least for these systems it seems thattaking the interpretation of the LUMO level, as the electronaffinity in a ‘‘Koopmans theorem like’’ association is rea-sonable if yet not completely rigorous. Keeping these pointsin mind, we have evaluated the electron affinities and ioni-zation potentials and compared them to molecular orbitalbased estimates reported in the literature wherever possible.

3.1. Molecular orbital, ionization potential and electron

affinities

The calculated HOMO and LUMO energy levels for thesystems studied in this work are presented in Table 1. Wecompare our calculated HOMO–LUMO energies withthose of the corresponding oxadiazole systems reportedin the literature [9]. We have selected N,N-dimethylamin-ophenyl and nitrophenyl as substituents which are elec-

Table 1B3LYP/6-31+G*-calculated HOMO and LUMO levels of the bis aryl substitu

Compound Di-substituent

Oxadiazolea N,N-DimethylaminophenylPhenylNitrophenyl

Triazole N,N-DimethylaminophenylPhenylNitrophenyl

Thiadiazole N,N-DimethylaminophenylPhenylNitrophenyl

a Reference [9].

tro-donating and electron-accepting, respectively, to getan idea about the tuning parameters for the HOMO andLUMO levels which are of importance if one wants tomatch the Fermi energy of an electrode. As seen in Table1 the calculated HOMO level for all the three systems alongwith their substituents fall within a small range of 2 eV.The oxadiazole molecular orbital is the most stable one.

The HOMO and LUMO levels are delocalized over theentire molecules for all different substitutions, see Fig. 2 forthe phenyl systems. For the central ring the HOMO molec-ular level is characterized by bonding orbitals between the3,4-nitrogen and the 2,5-carbon, whereas the LUMO ischaracterized with a bonding orbital between the 3,4-nitro-gens. The oxadiazole systems have the most stabilizedHOMO and substituting the center atom with nitrogen orsulfur destabilizes the HOMO. The LUMO level of triazoledestabilizes while it stabilizes in the case of thiadiazole. TheHOMO and LUMO levels are affected in different mannerwith respect to the aryl substituents in all the three differentcases irrespective of the attachment of the end group atboth ends. The HOMO level becomes shifted by �1.2–

ted compounds

HOMO (eV) LUMO (eV)

�5.09 �1.18�6.44 �1.96�7.49 �3.67

�4.90 �0.90�6.14 �1.67�7.17 �3.52

�5.01 �1.48�6.39 �2.24�7.42 �3.71

E. Jansson et al. / Chemical Physics 330 (2006) 166–171 169

1.4 eV upwards when substituting with N,N-dimethylamin-ophenyl and �1 eV downwards in case of nitrophenyl sub-stitution. The effect on the LUMO levels are stronger whenthe electron accepting group is present, i.e. stabilizing it by�1.5–1.9 eV. The results for the HOMO and LUMO ener-gies of all the three systems with different substituents indi-cate that even with a simple substitution we can tune thegap by 2–2.5 eV. They also suggest that oxadiazole basedmaterials is the best materials to use for hole-blockingproperties.

In Table 2 we report calculated adiabatic electron affin-ities (AEA) together with the (VEA) vertical electron affin-ities and ki. The AEA gives an idea about the stability ofthe radical ion molecule towards quenching caused bymolecular oxygen. The calculated AEA for molecular oxy-gen is �0.59 eV [19] at the DFT/B3LYP level of theory. Asseen in Table 2, the calculated AEA for N,N-dimethylam-inophenyl substituted triazole is positive making this thesystem most vulnerable for oxygen quenching. The remain-ing systems all have negative AEA, nitrophenyl substitutedthiadiazole having the most negative AEA is thus the moststable system towards oxygen quenching. The comparisonof all three systems with different substituents tells us veryclearly that thiadiazole based systems could be the bestmaterial against oxygen quenching since it have the lowestAEA in all cases. The triazole system, on the other hand,needs to be complemented with an electron accepting sub-stituent in order to effectively avoid quenching.

3.2. Reorganization energies

So far, we have discussed charge injection and lightemission properties as a function of the molecular struc-ture, while we have not addressed the electron transportingproperties. No matter if one uses the semi-classical Marcustheory [8] or the Bixon–Jortner model [20] to explain theelectron transport property in OLEDs, it is clear that thereorganization energy plays an important role when itcomes to controlling the electron transfer rates. The reor-ganization energy is defined as the sum of the intramolecu-lar and intermolecular energy change during a chargetransfer in a donor–acceptor complex. The intramolecular

Table 2B3LYP/6-31+G*-calculated vertical (VEA) and adiabatic (AEA) electron affinthe total intramolecular reorganization energy (ki)

Compound Di-substituent VEA (eV)

Oxadiazolea N,N-Dimethylaminophenyl 0.11Phenyl �0.53Nitrophenyl �2.36

Triazole N,N-Dimethylaminophenyl 0.36Phenyl �0.28Nitrophenyl �2.24

Thiadiazole N,N-Dimethylaminophenyl �0.20Phenyl �0.82Nitrophenyl �2.45

a Reference [9].

reorganization (ki) energy is composed by the relaxationof the electron-donating molecule (k1) and the relaxationof the electron-accepting molecule (k2). The intermolecularreorganization energy refers to the relaxation of the med-ium in which the donor and acceptor are placed. Both con-tributions are expected to be of the same order ofmagnitude [21] and we stress that a low ki is needed inorder to get a high electron transfer rate. In this paperwe have calculated ki as given in Scheme 1 and we have col-lected the results in Table 2. It is obvious from Table 2 thatboth the triazole and thiadiazole systems are having prom-isingly low ki and better than oxadiazole, irrespective of thesubstituents attached at the two ends. In fact, the reorgani-zation energy ki of phenyl substituted triazole and thiadiaz-ole is 0.28 eV which is slightly less than the most widelystudied [22,23] hole-transport material N,N 0-diphenyl-N,N 0-bis(3-methylphenyl)-([1,1 0])-4,4 0-diamine (TPD)which has a ki of 0.29 eV. Another important characteris-tics of the triazole and thiadiazole systems is that the ki isstable no matter if an electron accepting or an electrondonation group is added having an energy span of0.05 eV compared to energy span of 0.43 eV of the oxadiaz-ole systems.

3.3. Excitation energies

Besides the electron transporting properties we have alsoperformed time-dependent DFT (TDDFT) calculations inorder to evaluate the excitation energies which can bedirectly compared to experimental absorption peaks underequivalent conditions. We have performed these calcula-tions on the phenyl systems. The excitation wavelength ofthe S0! S1 (pp*) transition of oxadiazole, triazole and thi-adiazole are found to be 304.1, 304.0 and 329.7 nm, respec-tively. The experimentally reported absorption wavelengthis 278.00 nm for vaporized 2,5-diphenyl-1,3,4-oxadiazole,we refer to the experimental observation mentioned inthe paper by Borisevich et al. [24] and the referencestherein. Sato et al. [25] studied absorption on polymer sys-tems based on 2,5-diphenyl-1,3,4-thiadiazole which all havean absorption maximum at 320.5–331.0 nm. We attributethe deviations between calculations and experiment to

ities, relaxation energies for the neutral (k1) and radical-ion (k2) as well as

AEA (eV) k1 (eV) k2 (eV) ki (eV)

�0.26 0.36 0.37 0.73�0.68 0.15 0.15 0.30�2.54 0.17 0.18 0.35

0.22 0.15 0.14 0.29�0.42 0.14 0.14 0.28�2.42 0.15 0.18 0.32

�0.34 0.15 0.14 0.29�0.96 0.14 0.14 0.28�2.62 0.16 0.17 0.33

NN

O

Fig. 3. The dihedral angle of 2,5-diphenyl-1,3,4-oxadiazole subjected tothe twist.

Fig. 4. B3LYP/6-31+G*-calculated HOMO (left) and LUMO (right)levels for 2,5-diphenyl-1,3,4-2,5-oxadiazole at untwisted (top) and 90�twisted phenyl group (bottom).

170 E. Jansson et al. / Chemical Physics 330 (2006) 166–171

solvent effects and our monomer approximation. Unfortu-nately, we have not been able to find experimental absorp-tion spectra for triazole and thiadiazole systems and in thiscase our calculations may only be treated as predictions forfuture experiments.

3.4. Bending and twisting of the molecular structure

It is well known that five-membered rings such as theones studied in this paper in general are not as well suitedfor device materials since they do not form the requiredmesogenic phases. This may be caused by a number of rea-sons and one of them is that five-membered rings, in con-trast to six-membered ones, deviate from linearity [26]. Ithas been shown that thiadiazoles analogues of oxadiazolesystems can form an LC phase while the oxadiazoles donot. The calculated angle h(2-5-R), where R correspondsto the phenyl group, shows a structure shift towards linear-ity when going from the oxadiazole (h = 158.3�) to the thi-adiazole (h = 170.5�) systems, which is one of the reasonswhy the thiadiazole systems show different mesogenicphases. This has also been confirmed by experiment [27].For the triazole system we observe the same trend(h = 162.7�) if not as pronounced as for thiadiazole.

To elucidate how the properties change during a confor-mational twist of the dihedral angle, as shown in Fig. 3,which may be caused by high pressure or by an attachedgroup [28], we have performed calculations at differentdihedral angles ranging from 0� to 90� in steps of 15�. Thisstudy is only qualitatively valid if the attached groups donot change the fundamental chemical identity of the back-bone and our study will then work more as a guideline ofwhat can be expected under this condition. We havedevoted the study to 2,5-diphenyl-1,3,4-oxadiazole, theresults are gathered in Table 3. The effect of the twist angleon the HOMO–LUMO gap, AEA as well as the reorgani-zation energy is summarized in Table 3. As the twist angleincreases from 0� to 90� the HOMO energy level decreases

Table 3B3LYP/6-31+G*-calculations on 2,5-diphenyl-1,3,4-oxadiazole. Relaxation ereorganization energy (ki) vertical (VEA) and adiabatic (AEA) electron affinit

Dihedral angle (�) HOMO (eV) LUMO (eV) VEA (eV)

0 �6.44 �1.96 �0.5315 �6.46 �1.94 �0.5230 �6.50 �1.89 �0.4645 �6.57 �1.82 �0.3860 �6.66 �1.75 �0.2975 �6.74 �1.69 �0.1890 �6.79 �1.66 �0.08

while the LUMO energy level increases. This creates a lar-ger band gap going from 4.48 to 5.13 eV and thus indirectlyalters the emission wavelengths. The effect on the HOMOand LUMO MO levels are shown in Fig. 4. The twist local-izes the HOMO and LUMO levels as the dihedral angleincreases and at maximum distortion they no longer haveany density at the twisted phenyl group. By looking atthe absorption wavelength, which is blue shifted by51 nm when going from the untwisted structure to the90� twisted structure, we conclude that the emission shouldbe shifted in the same manner. As the twist angle increases,the VEA as well as AEA increases, which must be consid-ered with respect to the AEA of oxygen already discussedabove. The shift of these quantities are 0.45 and 0.42 eVfor VEA and AEA, respectively, which in the case of 2,5-diphenyl-1,3,4-oxadiazole is crucial since the AEA movesabove the AEA of oxygen. ki increases as we change thedihedral angle, saturating around a 45� twist at 0.38 eV.

4. Conclusions

The research on ET materials is crucial for the furtherdevelopment of efficient organic electroluminescentdevices. Motivated by this and recent progresses in syn-thetic chemistry [4,5] of thiadiazole and triazole systemsas candidates for ET materials, we have performed a den-sity functional study of the intramolecular reorganizationenergy and HOMO–LUMO gap in these systems. We haveshown that tuning of HOMO and LUMO levels of bis-arylsubstituted triazole and thiadiazole effectively can be

nergies for the neutral (k1) and radical-ion (k2). Total intramolecularies. k refers to the S0–S1 optical excitation gap

AEA (eV) k1 (eV) k2 (eV) ki (eV) k (nm)

�0.68 0.15 0.15 0.30 304�0.67 0.15 0.15 0.30 295�0.62 0.16 0.16 0.32 290�0.55 0.18 0.17 0.38 282�0.45 0.19 0.17 0.37 272�0.36 0.21 0.18 0.39 260�0.26 0.20 0.18 0.38 253

E. Jansson et al. / Chemical Physics 330 (2006) 166–171 171

accomplished by changing the substituents but also bychanging the center atom. We have shown that 2,5-diphe-nyl-triazole has almost identical photophysical characteris-tics, in terms of absorption wavelength, as its oxygenanalogue. Coming to the problem with oxygen quenchingthe triazole systems radical ions are only stable in the pres-ence of an electron accepting substituent. These character-istics make the triazole systems more vulnerable for oxygenquenching compared to the oxadiazole and thiadiazole sys-tem. Besides being the best resistant against oxygenquenching the thiadiazole shows many other interestingcharacteristics. One is the stabilized LUMO levels whichis of importance for design of electron accepting materialsfor usage in organic electronics [29]. The 2,5-diphenyl-1,3,4-thiadiazole structure is the one closest to linearitycompared to the triazole and oxadiazole analogues, whichprobably leads to improved liquid crystal characteristics.Both thiadiazole and triazole analogues of the oxadiazolesystems are showing smaller and more stable intramolecu-lar reorganization energies with respect to different bis-arylsubstitutions and are hence expected to have larger electrontransfer rates. We have indicated the implications for theelectron transfer as well as the photoluminescence proper-ties when twisting the 2,5-phenyl-1,3,4-oxadiazole systemalong the central dihedral angle starting from the idealstructure. Increased intramolecular reorganization energyand a blue shifted emission wavelength are then expected.We hope that these results have shed some light on struc-ture property relations between substitution and electroncarrier rate and/or hole blocking ability, and that they willguide further development of ET materials based on thi-adiazole and triazole systems.

Acknowledgments

One of the author, P.C.J. acknowledge financial supportfrom Wenner-Gren Foundations for supporting through apostdoctoral Grant. We acknowledge the use of computa-tional resources at the National Supercomputer Centre(NSC) in Linkoping, Sweden.

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