Materials Chemistry and Physics 170 (2016) 210e217 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys Optoelectronic properties of higher acenes, their BN analogue and substituted derivatives * Stevan Armakovic a, , Sanja J. Armakovic b, Vladimir Holodkov c, Svetlana Pelemis d a University of Novi Sad, Faculty of Sciences, Department of Physics, Trg Dositeja Obradovica 4, 21000, Novi Sad, Serbia b University of Novi Sad, Faculty of Sciences, Department of Chemistry, Biochemistry and Environmental Protection, Trg Dositeja Obradovica 3, 21000, Novi Sad, Serbia c Educons University, Faculty of Sport and Tourism - TIMS, Radnicka 30a, 21000, Novi Sad, Serbia d University of East Sarajevo, Faculty of Technology, Karakaj bb, 75400, Zvornik, Republic of Srpska, Bosnia and Herzegovina highlights 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(S1ÀT1), which is much better for BN analogues. Reorganization energies of acenes improve with the increase of number of benzene rings. article info abstract Article history: We have investigated optoelectronic properties of higher acenes: pentacene, hexacene, heptacene, Received 18 March 2015 octacene, nonacene, decacene and their boron-nitride (BN) analogues, within the framework of density Received in revised form functional theory (DFT). We have also investigated the optoelectronic properties of acenes modified by 4 November 2015 BN substitution. Calculated optoelectronic properties encompasses: oxidation and reduction potentials, Accepted 19 December 2015 electron and hole reorganization energies and energy difference between excited first singlet and triplet Available online 28 December 2015 states DE(S1ÀT1). Oxidation and reduction potentials indicate significantly better stability of BN ana- logues, comparing with their all-carbon relatives. Although higher acenes possess lower electron and Keywords: Organic compounds hole reorganization energies, with both best values much lower than 0.1 eV, their BN analogues also have Semiconductors competitive values of reorganization energies, especially for holes for which reorganization energy is also Ab initio calculations lower than 0.1 eV. On the other hand DE(S1ÀT1) is much better for BN analogues, having values that Electronic structure indicate that BN analogues are possible applicable for thermally activated delayed fluorescence. Optical properties © 2015 Elsevier B.V. All rights reserved. 1. Introduction Benzene, naphtalene and anthracene are the smallest acenes and in the same time belong to a group of the most studied organic Higher acenes are structures which consist of linearly fused molecules [9]. Beside them, pentacene received significant atten- benzene rings, possessing interesting electronic properties due to tion of the scientific community as an active semiconducting ma- the conjugated p-electron system. There is a constant interest in terial for application in OFETs [10,11]. Thin films of this molecule are the research of organic p-conjugated materials due to their used as the p-channel in organic transistors [12,13]. Known deriv- fantastic potential to be the basis of modern organic light-emitting ative of pentacene, hexa-perihexabenzocoronene (HBC), have been diodes (OLEDs), organic field-effect transistors (OFETs) and organic also used for the manufacture of FET [14]. Furthermore it is re- photovoltaic devices (OPVs) [1e7]. These materials are often ported that pentacene and rubrene, another typical representative denoted as organic semiconductors [8]. of organic semiconductors, have achieved mobility beyond À À 1.0 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 higher * Corresponding author. than 5 (higher acenes) could be even more useful [15]. Research of E-mail address: [email protected] (S. Armakovic). http://dx.doi.org/10.1016/j.matchemphys.2015.12.041 0254-0584/© 2015 Elsevier B.V. All rights reserved. S. Armakovic et al. / Materials Chemistry and Physics 170 (2016) 210e217 211 higher acenes is also popular due to the fact that these are suc- respectively. The values of S and I were obtained by linear regres- cessfully synthesized. Hexacene and its higher homologues are not sion against experimental OP and RP over wide range of OLED stable, but their existence can be demonstrated in suitable matrixes materials, including hole and electron transporting materials, [11,15], while this acene and heptacene can be synthesized through emitting materials, organics and organometallic complexes [51]. photochemical bisdecarbonylation of bridged R-diketones (Strat- The value of orbital energy is highest occupied molecular orbital ing-Zwanenburg reaction). On the other side, synthesis of octacene (HOMO) energy from the neutral molecule for the OP, and the and nonacene during experiments under conventional conditions lowest unoccupied molecular orbital (LUMO) energy for the RP. at room temperature are not possible. However, using cryogenic These values were developed using B3LYP with the default basis matrix-isolation techniques, Tonshoff€ and Bettinger have demon- set, which is MIDI! in this case. This means that these values are not strated that successful synthesis octacene and nonacene is possible suitable for other functionals and basis sets, other than B3LYP and [15]. MIDI!. Precisely, the value of slopes for OP and RP were À17.50 Besides benzene as a building block of acenes, borazine can be and À22.50 V, respectively, while the values of intercept for OP and viewed as a building block of acene boron-nitride (BN) analogues. RP were À2.17 and À0.35 V, respectively [51]. This methodology Benzene and borazine are typical representatives of planar aro- was already used in our previous work [52]. matic organic and inorganic molecules, containing six p electrons which are delocalized over the six-membered ring [16,17].BN 3. Results and discussion substitution of carbon based materials is frequently used procedure for obtaining materials with improved physico-chemical proper- 3.1. Oxidation and reduction potentials ties. This procedure is also known as BN/CC isosterism and means replacement of a C]C unit with the isosteric BeN unit [18,19]. More It is well known that pentacene's drawback is related to its poor than 60 years ago Dewar started with the synthesis of BN isosteres stability and low solubility in organic solvents, even beside its large of simple polycyclic aromatic hydrocarbons (PAH). These and later carrier mobilities [53e56]. This drawback is due the fact that works resulted in BN isosteres of naphtalene, phenantrene, pentacene is easily oxidized in air, while poor solubility is a result of anthracene, pyrene, benz [a]anthracene, chrysene, etc [20e35]. strong intermolecular forces due to the p-stacking [53]. Thus, it was Cyclo BN-acenes have been in the focus of many research groups interesting for us to investigate the RP/OP, which can be interpreted dealing with both theoretical and experimental studies [36e41]. as the tendency of structure to gain/loose electrons and thus to On the other side, although BN analogues of acenes have not been become oxidized/reduced, of higher acenes and their BN analogues. experimentally synthetized yet, there are studies dealing with For RP, the more positive/less negative potential, the more likely the polymerization of borazine which might be very useful for these reduction is to occur, while for OP the more negative/less positive purposes [42e44]. the potential is, the more likely the oxidation is to occur. Obtained After dealing with interesting curved organic molecule, suma- results are presented in Fig. 1, while Fig. 2 contains OP and RP of BN nene [45e49], in this work we decided to investigate the opto- substituted acenes. Structures of BN substituted acenes are electronic properties of planar organic structures - higher acenes numerous (total of 57 structures) and for the sake of clarity we which consist of linearly fused benzene rings (ranging 5 to 10, from presented these hybrid structures in Figs. S1eS6 of supplementary pentacene to decacene), their BN analogues and hybrid structures materials. obtained by BN substitution of higher acenes. Namely, we calcu- It can be seen in Fig. 1 that OP and RP change subsequently with lated oxidation and reduction potentials (OP and RP respectively) the increased number of benzene rings. Changes of OP and RP and electron and hole reorganization energies (ERE and HRE indicate that both oxidation and reduction are more likely to occur, respectively). We also investigated the potential of all investigated when going from pentacene to decacene. This significantly in- structures in this work for application in OLED devices from the fluences the stability of higher acenes and it is in agreement with aspect of thermally activated delayed fluorescence mechanism experimentally known facts that higher acenes are instable and (TADF). TADF mechanism is important as it provides a possibility to harder to synthetize when compared to pentacene [57]. Mentioned design precious-metal-free organic molecules which would serve issues related to oxidation of pentacene results in problematic as the basis for OLED devices. procession of pentacene, which finally consequences in the fact that thermal vapor deposition