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Materials Chemistry and Physics 170 (2016) 210e217

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Materials Chemistry and Physics

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

Optoelectronic properties of higher acenes, their BN analogue andsubstituted derivatives

Stevan Armakovi�c a, *, Sanja J. Armakovi�c b, Vladimir Holodkov c, Svetlana Pelemi�s d

a University of Novi Sad, Faculty of Sciences, Department of Physics, Trg Dositeja Obradovi�ca 4, 21000, Novi Sad, Serbiab University of Novi Sad, Faculty of Sciences, Department of Chemistry, Biochemistry and Environmental Protection, Trg Dositeja Obradovi�ca 3, 21000, NoviSad, Serbiac Educons University, Faculty of Sport and Tourism - TIMS, Radni�cka 30a, 21000, Novi Sad, Serbiad University of East Sarajevo, Faculty of Technology, Karakaj bb, 75400, Zvornik, Republic of Srpska, Bosnia and Herzegovina

h i g h l i g h t s

� Optoelectronic properties of structures based on higher acenes have been investigated.� Oxidation and reduction potentials together with reorganization energies are calculated.� TADF is analyzed through calculation of DE(S1�T1), which is much better for BN analogues.� Reorganization energies of acenes improve with the increase of number of benzene rings.

a r t i c l e i n f o

Article history:Received 18 March 2015Received in revised form4 November 2015Accepted 19 December 2015Available online 28 December 2015

Keywords:Organic compoundsSemiconductorsAb initio calculationsElectronic structureOptical properties

* Corresponding author.E-mail address: stevan.armakovic@df.uns.ac.rs (S.

http://dx.doi.org/10.1016/j.matchemphys.2015.12.0410254-0584/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

We have investigated optoelectronic properties of higher acenes: pentacene, hexacene, heptacene,octacene, nonacene, decacene and their boron-nitride (BN) analogues, within the framework of densityfunctional theory (DFT). We have also investigated the optoelectronic properties of acenes modified byBN substitution. Calculated optoelectronic properties encompasses: oxidation and reduction potentials,electron and hole reorganization energies and energy difference between excited first singlet and tripletstates DE(S1�T1). Oxidation and reduction potentials indicate significantly better stability of BN ana-logues, comparing with their all-carbon relatives. Although higher acenes possess lower electron andhole reorganization energies, with both best values much lower than 0.1 eV, their BN analogues also havecompetitive values of reorganization energies, especially for holes for which reorganization energy is alsolower than 0.1 eV. On the other hand DE(S1�T1) is much better for BN analogues, having values thatindicate that BN analogues are possible applicable for thermally activated delayed fluorescence.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Higher acenes are structures which consist of linearly fusedbenzene rings, possessing interesting electronic properties due tothe conjugated p-electron system. There is a constant interest inthe research of organic p-conjugated materials due to theirfantastic potential to be the basis of modern organic light-emittingdiodes (OLEDs), organic field-effect transistors (OFETs) and organicphotovoltaic devices (OPVs) [1e7]. These materials are oftendenoted as organic semiconductors [8].

Armakovi�c).

Benzene, naphtalene and anthracene are the smallest acenesand in the same time belong to a group of the most studied organicmolecules [9]. Beside them, pentacene received significant atten-tion of the scientific community as an active semiconducting ma-terial for application in OFETs [10,11]. Thin films of this molecule areused as the p-channel in organic transistors [12,13]. Known deriv-ative of pentacene, hexa-perihexabenzocoronene (HBC), have beenalso used for the manufacture of FET [14]. Furthermore it is re-ported that pentacene and rubrene, another typical representativeof organic semiconductors, have achieved mobility beyond1.0 cm2 V�1 s�1. This value can be compared even with the amor-phous silicon devices [1].

Besides pentacene, acenes with number of benzene rings higherthan 5 (higher acenes) could be even more useful [15]. Research of

S. Armakovi�c et al. / Materials Chemistry and Physics 170 (2016) 210e217 211

higher acenes is also popular due to the fact that these are suc-cessfully synthesized. Hexacene and its higher homologues are notstable, but their existence can be demonstrated in suitable matrixes[11,15], while this acene and heptacene can be synthesized throughphotochemical bisdecarbonylation of bridged R-diketones (Strat-ing-Zwanenburg reaction). On the other side, synthesis of octaceneand nonacene during experiments under conventional conditionsat room temperature are not possible. However, using cryogenicmatrix-isolation techniques, T€onshoff and Bettinger have demon-strated that successful synthesis octacene and nonacene is possible[15].

Besides benzene as a building block of acenes, borazine can beviewed as a building block of acene boron-nitride (BN) analogues.Benzene and borazine are typical representatives of planar aro-matic organic and inorganic molecules, containing six p electronswhich are delocalized over the six-membered ring [16,17]. BNsubstitution of carbon basedmaterials is frequently used procedurefor obtaining materials with improved physico-chemical proper-ties. This procedure is also known as BN/CC isosterism and meansreplacement of a C]C unit with the isosteric BeNunit [18,19]. Morethan 60 years ago Dewar started with the synthesis of BN isosteresof simple polycyclic aromatic hydrocarbons (PAH). These and laterworks resulted in BN isosteres of naphtalene, phenantrene,anthracene, pyrene, benz [a]anthracene, chrysene, etc [20e35].Cyclo BN-acenes have been in the focus of many research groupsdealing with both theoretical and experimental studies [36e41].On the other side, although BN analogues of acenes have not beenexperimentally synthetized yet, there are studies dealing withpolymerization of borazine which might be very useful for thesepurposes [42e44].

After dealing with interesting curved organic molecule, suma-nene [45e49], in this work we decided to investigate the opto-electronic properties of planar organic structures - higher aceneswhich consist of linearly fused benzene rings (ranging 5 to 10, frompentacene to decacene), their BN analogues and hybrid structuresobtained by BN substitution of higher acenes. Namely, we calcu-lated oxidation and reduction potentials (OP and RP respectively)and electron and hole reorganization energies (ERE and HRErespectively). We also investigated the potential of all investigatedstructures in this work for application in OLED devices from theaspect of thermally activated delayed fluorescence mechanism(TADF). TADF mechanism is important as it provides a possibility todesign precious-metal-free organic molecules which would serveas the basis for OLED devices.

2. Computational details

All density functional theory (DFT) calculations were performedwith Jaguar 8.7. program as implemented in and correspondingoptoelectronics module [50]. Optoelectronic properties are calcu-lated within screening method [51] which include correctionsbased on experimental data thanks to which relatively small basisset can be efficiently used. Namely, this method utilizes MIDI! basisset (which is in Jaguar program denoted as MIDIX), which producesresults similar in quality as 6-31Gd, but on the other side muchimproved over 3-21Gd. However, since MIDI! doesn't havecoverage for many elements in the periodic table, by default for anyelement for which MIDI! is not defined, 6-31Gd is used. If however6-31Gd is not defined for certain element, LACV3P is used [51].

Concerning the OP and RP they were calculated within Koop-mans approximation using the following equation:

OPðor RPÞ ¼ S� OEþ I (1)

where S, OE and I denotes slope, orbital energy and intercept,

respectively. The values of S and I were obtained by linear regres-sion against experimental OP and RP over wide range of OLEDmaterials, including hole and electron transporting materials,emitting materials, organics and organometallic complexes [51].The value of orbital energy is highest occupied molecular orbital(HOMO) energy from the neutral molecule for the OP, and thelowest unoccupied molecular orbital (LUMO) energy for the RP.These values were developed using B3LYP with the default basisset, which is MIDI! in this case. This means that these values are notsuitable for other functionals and basis sets, other than B3LYP andMIDI!. Precisely, the value of slopes for OP and RP were �17.50and �22.50 V, respectively, while the values of intercept for OP andRP were �2.17 and �0.35 V, respectively [51]. This methodologywas already used in our previous work [52].

3. Results and discussion

3.1. Oxidation and reduction potentials

It is well known that pentacene's drawback is related to its poorstability and low solubility in organic solvents, even beside its largecarrier mobilities [53e56]. This drawback is due the fact thatpentacene is easily oxidized in air, while poor solubility is a result ofstrong intermolecular forces due to the p-stacking [53]. Thus, it wasinteresting for us to investigate the RP/OP, which can be interpretedas the tendency of structure to gain/loose electrons and thus tobecome oxidized/reduced, of higher acenes and their BN analogues.For RP, themore positive/less negative potential, themore likely thereduction is to occur, while for OP the more negative/less positivethe potential is, the more likely the oxidation is to occur. Obtainedresults are presented in Fig. 1, while Fig. 2 contains OP and RP of BNsubstituted acenes. Structures of BN substituted acenes arenumerous (total of 57 structures) and for the sake of clarity wepresented these hybrid structures in Figs. S1eS6 of supplementarymaterials.

It can be seen in Fig. 1 that OP and RP change subsequently withthe increased number of benzene rings. Changes of OP and RPindicate that both oxidation and reduction are more likely to occur,when going from pentacene to decacene. This significantly in-fluences the stability of higher acenes and it is in agreement withexperimentally known facts that higher acenes are instable andharder to synthetize when compared to pentacene [57]. Mentionedissues related to oxidation of pentacene results in problematicprocession of pentacene, which finally consequences in the factthat thermal vapor deposition method should be used [53].

OP and RP of acenes substituted with BN are presented in Fig. 2.It can be seen that with introduction of BN fine adjustments of OPand RP can be made in both directions. In other words, for allinvestigated higher acene in this work there is always at least oneBN hybrid structurewith increased and decreased value of OP or RP.

Concerning the BN analogues of higher acenes, OP and RP arepractically constant, with the values of 1.96 and �3.15 eV respec-tively, no matter how many six member BN rings they contain.What more, both OP and RP are having values which indicate thatBN analogues are significantly less prone to oxidation and reduc-tion than higher acenes, leading to significantly higher stability ofBN analogues of higher acenes comparing to their all-carbonrelatives.

Oxygen quenching is closely related with oxidation and reduc-tion properties of some molecule. This phenomenon is a commonproblem when electrical material is exposed to air and severelyaffect the luminescence, electroluminescence and electronicproperties of material. Improved oxidation and reduction proper-ties according to results presented in this chapter indicate that BNanalogues of higher acenes should be significantly less prone to the

Fig. 1. OP and RP of higher acenes.

Fig. 2. OP and RP of higher acenes modified by BN substitution.

S. Armakovi�c et al. / Materials Chemistry and Physics 170 (2016) 210e217212

negative effects of oxygen quenching.It is known fact that replacement of two carbon atoms by one

boron and one nitrogen atom leads to the formation of systems thatare isoelectronic with their all-carbon analogues. This inducescharge polarization because of which dipole moment is inducedand further changes the electronic, optical and stability properties[58]. Implementation of BN units is also important because meth-odologies for experimental synthesis of BN higher non-linearacenes have been advised [59].

3.2. Reorganization energies

High charge mobilities and efficient charge injection are themain prerequisites for production of efficient electronic material[60]. Related to charge transport, at room temperatures the mostimportant mechanism is hopping mechanism. In this case chargehopping rate, KET, is the main quantity that regulates the chargecarrier mobility and represents the rate constant or the hoppingrate for charge transport between adjacent molecules. Using the

S. Armakovi�c et al. / Materials Chemistry and Physics 170 (2016) 210e217 213

Marcus theory approach, this quantity can be expressed as [61,62]:

KET ¼ 4 p2

h1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

4 p l kB Tp t2 exp

� �l

4 kB T

�(2)

where l is reorganization energy, while t represents the chargetransfer integral (or charge coupling). It should be noted that weshall regard to the results of Marcus theory as a first approximationand the obtained values of chargemobilities shall be treated only asqualitative trends, due to the simplicity of used models [63,64].Observing equation (2) it is clear that for better electric materialperformance reorganization energy should be minimized, whilecharge coupling should be maximized. Reorganization energiesconsist of two contributions; the inner and the outer one. The innercontribution, li is determined by fast changes in molecular geom-etry and the outer contribution, l0, is determined by slow variationsin polarization of surrounding medium. The outer contribution iscommonly neglected [60e62]. The importance of the reorganiza-tion energies lies in the fact that this parameter can be used forassessment of the impact of charge (hole and electron) injection[65]. Reorganization energies can be calculated according tofollowing formulas:

l1 ¼ E0�G*�� E0

�G0

�; (3)

l2 ¼ E*�G0

�� E*

�G*�; (4)

li ¼ l1 þ l2 (5)

where E0(G0) and E*(G*) are the ground state energies of the neutraland ionic states, respectively. E0(G*) is the energy of the neutralmolecule at the optimal ionic geometry, while E*(G0) is the energyof the charged state at the optimal geometry of the neutralmolecule.

Methodology used in optoelectronic module of SchrodingersMaterial Suite is significantly improved from the aspect of effi-ciency, since the calculations are much faster. Firstly, we wanted tocheck the obtained results for the sole pentacene molecule.Calculated HRE and ERE for pentacene are 0.092 eV and 0.127 eVrespectively, which is in excellent agreement with the resultsprovided in reference [66] where the values of 0.092 and 0.131 eVwere reported respectively for HRE and ERE. Very similar resultswere obtained in references [67,68], as well.

Calculated results of higher acenes are presented in Fig. 3 andclearly indicate that ERE and HRE significantly improve from pen-tacene to decacene. Namely, ERE subsequently decreases from0.13 eV to 0.6 eV, while HRE decreases subsequently from 0.09 eV to

Fig. 3. Electron and hole reorganiza

even 0.04 eV. Pentacene is known as hole conductor, however, hereis demonstrated that higher acenes could be considered as goodelectron conductors as well, since both ERE and HRE have signifi-cantly low values.

Regarding the reorganization energies, for BN analogues ofhigher acenes results are also interesting, Fig. 4. Namely, BN ana-logues can be also considered as potentially efficient electronicmaterials since reorganization energies are significantly low. Sameas for higher acenes, reorganization energies of their BN analoguesdecrease with the increased number of rings. Namely, going frompentacene to decacene BN analogue ERE decreases from 0.16 eV to0.12 eV, while HRE decreases from 0.11 to 0.07 eV.

Significantly reduced values of HRE and ERE of both acenes andtheir BN analogues enables better charge transfer rate betweenthese molecules, according to Equation (2). Besides reorganizationenergy, charge transfer rate is principally regulated by the chargecoupling, t, which should be maximized in order for charge carriersto have better mobility. This parameter is spatially dependent; itdescribes the strength of the interactions between adjacent mole-cules and generally the closer the molecules are the higher is the t.In this work it is demonstrated that reorganization energies ofinvestigated structures are suitable for application in optoelec-tronic devices, while their spatial distribution can be modifiedfurther in order to improve the values of charge coupling. In thiswork we have also investigated reorganization energies of acenesmodified by BN substitution and these results are presented inFig. 5.

Results presented in Fig. 5 indicate that BN substitution ofhigher acenes could also lead to the improvement of ERE and HRE,however this improvement cannot be regarded as significant. BNsubstitution improved the HRE for hexacene, namely in the case ofHex@BN3 hybrid the HRE was lowered for 0.01 eV. On the otherside, BN substitution improved the ERE for pentacene and hepta-cene (hybrids Pen@BN2 and Hep@BN7) when this quantity waslowered also for 0.01 eV.

3.3. Thermally activated delayed fluorescence

Modern flat panel displays and solid-state lighting technologyare based on OLEDs because they possess high electroluminescenceefficiency. Their potential for practical applications is significantalso because they can be manufactured on different substrates atmoderate temperatures [69]. According to spin statistics, in OLEDssinglet and triplet excitons are formed in 1:3 ratio. On the other sideit is known that only singlet excitons are responsible for lightemitting, while triplet excitons relax by releasing heat. Phospho-rescent emitters such as iridium or platinum based complexes arecommonly utilized in production of high efficiency OLEDs [70e72].Employment of precious metal complexes enables the usage of

tion energies of higher acenes.

Fig. 4. ERE and HRE of BN analogues of higher acenes.

Fig. 5. ERE and HRE of higher acenes modified by BN substitution.

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normally non-radiative triplet excitons and thus improve theoverall electroluminescence efficiency [72,73], which results ininternal quantum efficiency close to 100% in OLEDs based onphosphorescency PHOLEDs [69]. Unfortunately, utilization ofprecious metals impose some challenges such as high pricing,which reduces the chances for OLEDs to have more significant rolein the market [69]. Thus, it is imperative to seek for ways how toproduce emitters which are not based on expensive precious metalcomplexes. In this regard, TADF mechanism could be important for

the improvement of efficiency of OLEDs because it might enableachievement of 100% internal quantum efficiency withoutemployment of phosphorescent metal-organic complexes.

In order to assess the potential of the certain molecular struc-ture as an efficient OLED material it is very useful to quantify thecrucial parameter that determines the effectiveness of the TADF e

the energy separation between the lowest excited singlet (S1) andtriplet (T1) state, DE(S1�T1). If DE(S1�T1) has suitably low value,then singlet state S1 can be populated thermally at ambient

Table 1DE(S1�T1) of higher acenes and their BN analogues.

Higher acenes DE(S1�T1)[eV]

BN analogues ofHigher acenes

DE(S1�T1)[eV]

Pentacene 1.41 Pentacene 0.25Hexacene 1.33 Hexacene 0.30Heptacene 1.27 Heptacene 0.15Octacene 1.21 Octacene 0.15Nonacene 1.15 Nonacene 0.15Decacene 1.11 Decacene 0.15

S. Armakovi�c et al. / Materials Chemistry and Physics 170 (2016) 210e217 215

temperature from the energetically lower-lying triplet state. Thisprocess consequences in the, so called, thermally activated delayedfluorescence (TADF). If DE(S1�T1) is larger than ca. 3� 103 cm�1 (or0.37 eV) a thermal population of the singlet state S1 is not effective.Obtained results are presented in Table 1.

Results obtained in this study indicate that TADF can be hardlyachieved with higher acenes, Table 1. Namely, DE(S1�T1) subse-quently decreases from 1.41 eV to 1.11 eV when going from pen-tacene to decacene. Although DE(S1�T1) significantly decreasesfrom pentacene to decacene, values of this parameter are still highabove the threshold of 0.37 eV. Same conclusion can be drawn foracenes modified by BN substitution, Fig. 6. Introduction of BN tostructure of higher acenes has decreased the DE(S1�T1)

Fig. 6. DE(S1�T1) parameter of higher a

significantly in some cases. For each higher acene investigated inthis case there was always at least one hybrid structure withDE(S1�T1) parameter lowered for at least 0.2 eV. The highestreduction of the DE(S1�T1) parameter by BN introductionhappened for pentacene, when this parameter changed from1.41 eV to 0.78 eV.

On the other side, all BN analogues of higher acenes investigatedin this work are having DE(S1�T1) values below the threshold of0.37 eV indicating that all of them could be potentially applicable inthe area of OLED materials, Table 1.

In general, low DE(S1�T1) is possible for structures that containspatially separated donor and acceptor moieties. However, ac-cording to Fermi's golden rule this leads to the low radiative decayrate (kr) which value should be at least reasonably high enough.Thus, it can be concluded that highly luminescent TADF material isdetermined by the combination of low DE(S1�T1), <~0.1 eV andreasonably high radiative decay rate, >106 s�1. These two conflictedconditions indicate the necessity of careful balancing of frontiermolecular orbitals [73], which is separate and tedious task.

4. Conclusion

We have demonstrated that higher acenes possess outstandingoptoelectronic properties, while their BN analogues possess suit-able stability properties beside very competitive optoelectronic

cenes modified by BN substitution.

S. Armakovi�c et al. / Materials Chemistry and Physics 170 (2016) 210e217216

properties. OP and RP potentials of higher acenes indicate thatreactivity increases subsequently when going from pentacene todecacene. On the other hand, OP and RP of BN analogues of higheracenes are practically the same for all investigated structures, withvalues that clearly indicate much improved stability over their all-carbon relatives. OP and RP of BN substituted higher acenes indi-cate that this type of substitution can lead to the fine adjustment ofOP and RP.

Values of ERE and HRE with both values below 0.1 eV indicateoutstanding optoelectronic properties of higher acenes, while theirBN analogues are having somewhat larger values, but still very lowand competitive. Namely, the HRE of some BN analogues is alsolower than 0.1 eV. BN substitution of higher acenes also lead tosmall improvement of reorganization energies, ~0.01 eV, for somehigher acenes (HRE lowered for hexacene, while ERE lowered forpentacene and heptacene).

Significant attention was paid to TADF and the main parameterthat dictates this property, the DE(S1�T1). For higher acenes valuesof this parameter turned out to be too high, actually they are highabove the threshold value of 0.37 eV. BN substitution of higheracenes lead to significant improvement of DE(S1�T1) parameter,however it is far above the desired threshold of 0.37 eV. TheDE(S1�T1) parameter was the lowest for pentacene BN hybrid, withthe value of 0.78 eV. However, for all BN analogues of higher acenesDE(S1�T1) has a significantly lower value than the mentionedthreshold of 0.37 eV.

Consequently, a combination of good stability and optoelec-tronic properties together with excellent DE(S1�T1) values farbeyond the needed threshold is what makes BN analogues ofhigher acenes a very interesting electronic material, while BNsubstitution can be useful for fine adjustments, from the aspect oftheoretical analysis.

Acknowledgment

This work has been done thanks to the support received fromSchr€odinger, Inc. Presented research was done within the frame-work of projects supported by the Ministry of Education, Scienceand Technological Development of Serbia, grant numbersON171039 and TR34019.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.matchemphys.2015.12.041.

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