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    c Educons University, Faculty of Sport and Tourism - TIMd logy, Kar

    basedether wof DE(S1rove w

    Received in revised formoctacene, nonacene, decacene and their boron-nitride (BN) analogues, within the framework of densityfunctional theory (DFT). We have also investigated the optoelectronic properties of acenes modied by

    . All rights reserved.

    the research of organic p-conjugated materials due to theirfantastic potential to be the basis of modern organic light-emittingdiodes (OLEDs), organic eld-effect transistors (OFETs) and organicphotovoltaic devices (OPVs) [1e7]. These materials are oftendenoted as organic semiconductors [8].

    e smallest acenesst studied organicsignicant atten-

    miconducting ma-f this molecule are

    used 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 V1 s1. 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* Corresponding author.

    Contents lists availab

    Materials Chemis

    evi

    Materials Chemistry and Physics 170 (2016) 210e217E-mail address: stevan.armakovic@df.uns.ac.rs (S. Armakovic).Optical properties 2015 Elsevier B.V

    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 in

    Benzene, naphtalene and anthracene are thand in the same time belong to a group of the momolecules [9]. Beside them, pentacene receivedtion of the scientic community as an active seterial for application in OFETs [10,11]. Thin lms oOrganic compoundsSemiconductorsAb initio calculationsElectronic structure

    hole 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(S1T1) is much better for BN analogues, having values thatindicate that BN analogues are possible applicable for thermally activated delayed uorescence.4 November 2015Accepted 19 December 2015Available online 28 December 2015

    Keywords:

    BN substitution. Calculated optoelectronic properties encompasses: oxidation and reduction potentials,electron and hole reorganization energies and energy difference between excited rst singlet and tripletstates DE(S1T1). Oxidation and reduction potentials indicate signicantly better stability of BN ana-logues, comparing with their all-carbon relatives. Although higher acenes possess lower electron andUniversity of East Sarajevo, Faculty of Techno

    h i g h l i g h t s

    Optoelectronic properties of structures Oxidation and reduction potentials tog TADF is analyzed through calculation Reorganization energies of acenes imp

    a r t i c l e i n f o

    Article history:Received 18 March 2015http://dx.doi.org/10.1016/j.matchemphys.2015.12.0410254-0584/ 2015 Elsevier B.V. All rights reserved.S, Radnicka 30a, 21000, Novi Sad, Serbiaakaj bb, 75400, Zvornik, Republic of Srpska, Bosnia and Herzegovina

    on higher acenes have been investigated.ith reorganization energies are calculated.T1), which is much better for BN analogues.ith the increase of number of benzene rings.

    a b s t r a c t

    We have investigated optoelectronic properties of higher acenes: pentacene, hexacene, heptacene,b University of Novi Sad, Faculty of Sciences, DeSad, Serbiaa University of Novi Sad, Faculty of Sciences, Department of Physics, Trg Dositeja Obradovica 4, 21000, Novi Sad, Serbiapartment of Chemistry, Biochemistry and Environmental Protection, Trg Dositeja Obradovica 3, 21000, NoviOptoelectronic properties of higher acensubstituted derivatives

    Stevan Armakovic a, *, Sanja J. Armakovic b, Vladim

    journal homepage: www.elss, their BN analogue and

    Holodkov c, Svetlana Pelemis d

    le at ScienceDirect

    try and Physics

    er.com/locate/matchemphys

  • istrhigher 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, Tonshoff 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 uorescence 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 efciently 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 dened, 6-31Gd is used. If however6-31Gd is not dened for certain element, LACV3P is used [51].

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

    OPor RP S OE I (1)

    S. Armakovic et al. / Materials Chemwhere 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 th

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