Optoelectronic Properties of Higher Acenes, Their BN Analogue and Substituted Derivatives

Materials Chemistry and Physics 170 (2016) 210e217

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Materials Chemistry and Physics

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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(S1T1), 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(S1T1). Oxidation and reduction potentials indicate significantly better stability of BN analogues, 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(S1T1) 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 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 17.50 Besides benzene as a building block of acenes, borazine can be and 22.50 V, respectively, while the values of intercept for OP and viewed as a building block of acene boron-nitride (BN) analogues. RP were 2.17 and 0.35 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 method should be used [53]. 2. Computational details 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 OP All density functional theory (DFT) calculations were performed and RP can be made in both directions. In other words, for all with Jaguar 8.7. program as implemented in and corresponding investigated higher acene in this work there is always at least one optoelectronics module [50]. Optoelectronic properties are calcu- BN hybrid structure with increased and decreased value of OP or RP. lated within screening method [51] which include corrections Concerning the BN analogues of higher acenes, OP and RP are based on experimental data thanks to which relatively small basis practically constant, with the values of 1.96 and 3.15 eV respec- set can be efficiently used. Namely, this method utilizes MIDI! basis tively, no matter how many six member BN rings they contain. set (which is in Jaguar program denoted as MIDIX), which produces What more, both OP and RP are having values which indicate that fi results similar in quality as 6-31Gd, but on the other side much BN analogues are signi cantly less prone to oxidation and reduc- fi improved over 3-21Gd. However, since MIDI! doesn't have tion than higher acenes, leading to signi cantly higher stability of coverage for many elements in the periodic table, by default for any BN analogues of higher acenes comparing to their all-carbon element for which MIDI! is not defined, 6-31Gd is used. If however relatives. 6-31Gd is not defined for certain element, LACV3P is used [51]. Oxygen quenching is closely related with oxidation and reduc- Concerning the OP and RP they were calculated within Koop- tion properties of some molecule. This phenomenon is a common mans approximation using the following equation: problem when electrical material is exposed to air and severely affect the luminescence, electroluminescence and electronic OPðor RPÞ¼S OE þ I (1) properties of material. Improved oxidation and reduction properties according to results presented in this chapter indicate that BN where S, OE and I denotes slope, orbital energy and intercept, analogues of higher acenes should be significantly less prone to the 212 S. Armakovic et al. / Materials Chemistry and Physics 170 (2016) 210e217

Fig. 1. OP and RP of higher acenes.

Fig. 2. OP and RP of higher acenes modified by BN substitution. negative effects of oxygen quenching. 3.2. Reorganization energies It is known fact that replacement of two carbon atoms by one boron and one nitrogen atom leads to the formation of systems that High charge mobilities and efficient charge injection are the are isoelectronic with their all-carbon analogues. This induces main prerequisites for production of efficient electronic material charge polarization because of which dipole moment is induced [60]. Related to charge transport, at room temperatures the most and further changes the electronic, optical and stability properties important mechanism is hopping mechanism. In this case charge [58]. Implementation of BN units is also important because meth- hopping rate, KET, is the main quantity that regulates the charge odologies for experimental synthesis of BN higher non-linear carrier mobility and represents the rate constant or the hopping acenes have been advised [59]. rate for charge transport between adjacent molecules. Using the S. Armakovic et al. / Materials Chemistry and Physics 170 (2016) 210e217 213

Marcus theory approach, this quantity can be expressed as [61,62]: even 0.04 eV. Pentacene is known as hole conductor, however, here is demonstrated that higher acenes could be considered as good p2 l electron conductors as well, since both ERE and HRE have signifi- ¼ 4 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 2 KET t exp (2) cantly low values. h 4 plkB T 4 kB T Regarding the reorganization energies, for BN analogues of where l is reorganization energy, while t represents the charge higher acenes results are also interesting, Fig. 4. Namely, BN ana- fi transfer integral (or charge coupling). It should be noted that we logues can be also considered as potentially ef cient electronic fi shall regard to the results of Marcus theory as a first approximation materials since reorganization energies are signi cantly low. Same and the obtained values of charge mobilities shall be treated only as as for higher acenes, reorganization energies of their BN analogues qualitative trends, due to the simplicity of used models [63,64]. decrease with the increased number of rings. Namely, going from Observing equation (2) it is clear that for better electric material pentacene to decacene BN analogue ERE decreases from 0.16 eV to performance reorganization energy should be minimized, while 0.12 eV, while HRE decreases from 0.11 to 0.07 eV. fi charge coupling should be maximized. Reorganization energies Signi cantly reduced values of HRE and ERE of both acenes and consist of two contributions; the inner and the outer one. The inner their BN analogues enables better charge transfer rate between these molecules, according to Equation (2). Besides reorganization contribution, li is determined by fast changes in molecular geom- energy, charge transfer rate is principally regulated by the charge etry and the outer contribution, l0, is determined by slow variations in polarization of surrounding medium. The outer contribution is coupling, t, which should be maximized in order for charge carriers commonly neglected [60e62]. The importance of the reorganiza- to have better mobility. This parameter is spatially dependent; it tion energies lies in the fact that this parameter can be used for describes the strength of the interactions between adjacent mole- assessment of the impact of charge (hole and electron) injection cules and generally the closer the molecules are the higher is the t. [65]. Reorganization energies can be calculated according to In this work it is demonstrated that reorganization energies of following formulas: investigated structures are suitable for application in optoelectronic devices, while their spatial distribution can be modified À Á further in order to improve the values of charge coupling. In this l ¼ E0 G* E0 G0 ; (3) 1 work we have also investigated reorganization energies of acenes modified by BN substitution and these results are presented in À Á Fig. 5. l ¼ E* G0 E* G* ; (4) 2 Results presented in Fig. 5 indicate that BN substitution of higher acenes could also lead to the improvement of ERE and HRE, l ¼ l þ l however this improvement cannot be regarded as significant. BN i 1 2 (5) substitution improved the HRE for hexacene, namely in the case of * * where E0(G0) and E (G ) are the ground state energies of the neutral Hex@BN3 hybrid the HRE was lowered for 0.01 eV. On the other * and ionic states, respectively. E0(G ) is the energy of the neutral side, BN substitution improved the ERE for pentacene and hepta- * molecule at the optimal ionic geometry, while E (G0) is the energy cene (hybrids Pen@BN2 and Hep@BN7) when this quantity was of the charged state at the optimal geometry of the neutral lowered also for 0.01 eV. molecule. Methodology used in optoelectronic module of Schrodingers 3.3. Thermally activated delayed fluorescence Material Suite is significantly improved from the aspect of efficiency, since the calculations are much faster. Firstly, we wanted to Modern flat panel displays and solid-state lighting technology check the obtained results for the sole pentacene molecule. are based on OLEDs because they possess high electroluminescence Calculated HRE and ERE for pentacene are 0.092 eV and 0.127 eV efficiency. Their potential for practical applications is significant respectively, which is in excellent agreement with the results also because they can be manufactured on different substrates at provided in reference [66] where the values of 0.092 and 0.131 eV moderate temperatures [69]. According to spin statistics, in OLEDs were reported respectively for HRE and ERE. Very similar results singlet and triplet excitons are formed in 1:3 ratio. On the other side were obtained in references [67,68], as well. it is known that only singlet excitons are responsible for light Calculated results of higher acenes are presented in Fig. 3 and emitting, while triplet excitons relax by releasing heat. Phospho- clearly indicate that ERE and HRE significantly improve from pen- rescent emitters such as iridium or platinum based complexes are tacene to decacene. Namely, ERE subsequently decreases from commonly utilized in production of high efficiency OLEDs [70e72]. 0.13 eV to 0.6 eV, while HRE decreases subsequently from 0.09 eV to Employment of precious metal complexes enables the usage of

Fig. 3. Electron and hole reorganization energies of higher acenes. 214 S. Armakovic et al. / Materials Chemistry and Physics 170 (2016) 210e217

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

Fig. 5. ERE and HRE of higher acenes modified by BN substitution. normally non-radiative triplet excitons and thus improve the the improvement of efficiency of OLEDs because it might enable overall electroluminescence efficiency [72,73], which results in achievement of 100% internal quantum efficiency without internal quantum efficiency close to 100% in OLEDs based on employment of phosphorescent metal-organic complexes. phosphorescency PHOLEDs [69]. Unfortunately, utilization of In order to assess the potential of the certain molecular struc- precious metals impose some challenges such as high pricing, ture as an efficient OLED material it is very useful to quantify the which reduces the chances for OLEDs to have more significant role crucial parameter that determines the effectiveness of the TADF e in the market [69]. Thus, it is imperative to seek for ways how to the energy separation between the lowest excited singlet (S1) and produce emitters which are not based on expensive precious metal triplet (T1) state, DE(S1T1). If DE(S1T1) has suitably low value, complexes. In this regard, TADF mechanism could be important for then singlet state S1 can be populated thermally at ambient S. Armakovic et al. / Materials Chemistry and Physics 170 (2016) 210e217 215

Table 1 significantly in some cases. For each higher acene investigated in D E(S1 T1) of higher acenes and their BN analogues. this case there was always at least one hybrid structure with D Higher acenes DE(S1T1) BN analogues of DE(S1T1) E(S1 T1) parameter lowered for at least 0.2 eV. The highest [eV] Higher acenes [eV] reduction of the DE(S1T1) parameter by BN introduction Pentacene 1.41 Pentacene 0.25 happened for pentacene, when this parameter changed from Hexacene 1.33 Hexacene 0.30 1.41 eV to 0.78 eV. Heptacene 1.27 Heptacene 0.15 On the other side, all BN analogues of higher acenes investigated Octacene 1.21 Octacene 0.15 in this work are having DE(S T ) values below the threshold of Nonacene 1.15 Nonacene 0.15 1 1 Decacene 1.11 Decacene 0.15 0.37 eV indicating that all of them could be potentially applicable in the area of OLED materials, Table 1. In general, low DE(S1T1) is possible for structures that contain spatially separated donor and acceptor moieties. However, ac- temperature from the energetically lower-lying triplet state. This cording to Fermi's golden rule this leads to the low radiative decay process consequences in the, so called, thermally activated delayed 3 1 rate (kr) which value should be at least reasonably high enough. fluorescence (TADF). If DE(S1T1) is larger than ca. 3 10 cm (or Thus, it can be concluded that highly luminescent TADF material is 0.37 eV) a thermal population of the singlet state S1 is not effective. determined by the combination of low DE(S1T1), <~0.1 eV and Obtained results are presented in Table 1. reasonably high radiative decay rate, >106 s 1. These two conflicted Results obtained in this study indicate that TADF can be hardly conditions indicate the necessity of careful balancing of frontier achieved with higher acenes, Table 1. Namely, DE(S1T1) subse- molecular orbitals [73], which is separate and tedious task. quently decreases from 1.41 eV to 1.11 eV when going from pentacene to decacene. Although DE(S1T1) significantly decreases from pentacene to decacene, values of this parameter are still high 4. Conclusion above the threshold of 0.37 eV. Same conclusion can be drawn for acenes modified by BN substitution, Fig. 6. Introduction of BN to We have demonstrated that higher acenes possess outstanding structure of higher acenes has decreased the DE(S1T1) optoelectronic properties, while their BN analogues possess suitable stability properties beside very competitive optoelectronic

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