Predicting the Operational Stability of Phosphorescent OLED Host

Article pubs.acs.org/JPCC

Predicting the Operational Stability of Phosphorescent OLED Host Molecules from First Principles: A Case Study Alexandra Ya. Freidzon,*,†,‡ Andrey A. Safonov,† Alexander A. Bagaturyants,†,‡ Dmitry N. Krasikov,§,∥ Boris V. Potapkin,§ Alexey A. Osipov,*,⊥ Alexander V. Yakubovich,⊥ and Ohyun Kwon*,◊ †

Photochemistry Center, Russian Academy of Sciences, ul. Novatorov 7a, Moscow 119421, Russia National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoye shosse 31, Moscow 115409, Russia § Kintech Lab Limited, Third Khoroshevskaya ul. 12, Moscow 123298, Russia ⊥ SAIT-Russia Lab, SRR, Samsung R&D Institute Russia, DMC, SEC, ul. Dvintsev 12, Moscow 127018, Russia ◊ Samsung Advanced Institute of Technology, Samsung Electronics, Company, Limited, 130 Samsung-ro, Yeontong-gu, Suwon-si, Gyeonggi-do 443-803, Korea ‡

S Supporting Information *

ABSTRACT: Low operational stability is the main limiting factor for commercialization of the blue phosphorescent organic light emitting diodes (PhOLEDs). The high energy and long lifetime of triplet excitons in blue PhOLEDs makes them more prone to degradation. Degradation of the host molecules in the emitting layer of PhOLEDs is one of the possible mechanisms leading to the luminosity loss in the course of device operation. Although possible degradation mechanisms are proposed in the literature, predicting the degradation kinetics is not straightforward because the evolution of excited states should be accurately described. We propose a computational scheme to assess the operational stability of PhOLED host materials. Our protocol relies on the usage of the multireference CASSCF/XMCQDPT2 method. In the present work we consider the degradation of four prototypical blue PhOLED host molecules in the charged and excited states as well as the degradation induced by exciton−polaron and exciton− exciton annihilation processes with the focus on breaking of exocyclic C−C or C−N bonds and triazine ring fission. By analyzing the calculated activation energies for different mechanisms we found the least stable states and the most probable dissociation pathways. On the basis of our computations, we derived a stability series for the studied molecules and determine the structural features that provide higher stability with respect to the unimolecular dissociation. operational degradation5). There are two main reasons of the operational degradation of PhOLED proposed in the literature:6−11 destruction of functional molecules and accumulation of defects in the device layers. Possible defects include nonradiative recombination centers,12 luminescence quenchers, and deep charge traps causing formation of a barrier layer and raising the operational voltage.13 Destruction of molecules includes formation of highly reactive species, such as free radicals, and their reaction with neighboring molecules to form defects.14−16 Recently, formation of exciplexes at the interfaces of OLED layers was found.17−20 Although exciplexes can lead to fluorescence quenching,21 such quenching was not observed in OLEDs. Instead, interface exciplexes are used to enhance the luminescence22,23 and tune the emission color.24,25

1. INTRODUCTION Nowadays OLEDs have found their application as active elements in displays of smartphones, tablets, and wearable devices. Special attention of researchers and industry is devoted to phosphorescent organic light emitting diodes (PhOLEDs), since they approach the theoretical limit of the internal quantum efficiency.1,2 The long lifetime of triplet excitons offers the possibility for harvesting both triplet and singlet excitons and converting their energy to light. Besides the efficiency, successful commercial application requires phosphorescent OLEDs to exhibit deep colors and long lifetimes (∼106 h at 1000 cd/m2).3,4 Despite the progress in improvement of the lifetime for green and red PhOLED (with lifetimes ≈ 106 h at 1000 cd m−2), the lifetime of the blue emitting devices is still the limiting factor for their commercial application.5,6 The operational lifetime of OLEDs is limited by reduction of the luminance efficiency in the course of device operation, associated with degradation of the device materials (intrinsic © XXXX American Chemical Society

Received: June 12, 2017 Revised: September 15, 2017 Published: September 20, 2017 A

DOI: 10.1021/acs.jpcc.7b05761 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

2.1), consider degradation mechanisms and derive the stability criteria (section 2.2), and draw a computational scheme (section 2.3); in section 3 we describe the computational methods used; in section 4 we present the results of calculations and analyze different degradation mechanisms for all molecules; finally, in section 5, we summarize the results and provide an outlook.

Modern fabrication methods of OLEDs guarantee efficient encapsulation of the devices.26,27 In an encapsulated device formation and accumulation of defects and nonemissive species occurs by means of intrinsic processes rather than by reactions with water or oxygen.28 Such processes always consist of conversion of the functional molecules into highly reactive species (ions, radicals, or ion−radicals) that may further participate in the complex reactions with molecules from different functional layers of the device. Transformation of a molecule into highly reactive species determines the rate of operational degradation. The possible mechanisms of degradation of molecules in OLED are the following:9,11,29 degradation of charged molecules, degradation of excited molecules, degradation of molecules in charged excited states, and degradation in highly excited states. In the case of PhOLED the degradation of molecules in the charged excited state is considered as the most significant degradation mechanism due to high density of long-lived triplet excitons. In comparison with fluorescent OLED,30−32 the relatively long lifetime of triplet excitons makes PhOLED more susceptible to operational degradation. Synthesis of the new materials and fabrication of the devices for experimental analysis of degradation mechanisms and analysis of stability of potential PhOLED materials is a timeconsuming and costly process. To reduce the corresponding time and expenses, such an analysis has to be preceded by theoretical screening of potential candidates based on some molecular properties. However, it is hardly possible to propose any universal set of molecular properties (or descriptors) that would have an unambiguous relation to the stability of a molecule in an operating OLED. Nevertheless, it may be promising to seek such descriptors within each particular class of molecules, because similar mechanisms can be expected for chemically similar compounds. A reliable theoretical analysis of molecule degradation via different channels demands sophisticated state-of-the-art computational methods that are able to accurately describe ground, charged, and excited states of a molecule and the dissociation energies of such states. One of such methods, timedependent density functional theory (TD-DFT), was used in several works for prediction of the stability of molecules in host and electron-transporting OLED layers.33−35 Taking into account the known drawbacks of DFT-based methods in description of localized excitations and charges, we suggest the use of multireference methods for accurate description of the charge or exciton localization and location of the global minimum on the potential energy surface. In the present work, we analyze the stability of two carbazolebased and two triphenylene-based molecules that are widely used as building cores for host materials in blue PhOLEDs.36 We calculate bond dissociation energies of neutral and charged molecules into different possible fragments and the energies of singlet and triplet excitons in order to derive the activation energies for four degradation mechanisms using the multireference CASSCF/XMCQDPT2 method. Analysis of activation energies allows us to assess the relative stability of four molecules to determine the most probable degradation mechanisms. On the basis of the stability criteria for different degradation mechanisms we conclude on the operational stability of the four molecules and draw recommendations on their possible application. The paper is structured as follows: in section 2, we describe the investigated molecules and degradation channels (section

2. MODELS 2.1. Investigated Molecules and Fragmentation Channels. The mechanism of OLED degradation may depend on the temperature, vibrational relaxation, molecular packing and molecular mobility in amorphous layers, and interface interactions. The temperature governs the dissociation rate through the Arrhenius equation, and vibrational relaxation influences the pre-exponential factor of the rate equation (see section 2.2). In addition, vibrational relaxation of excited or charged molecules can lead to intramolecular rearrangements. The latter reactions are not considered in this work due to the lack of experimental data on such rearrangements. Such factors as molecular packing and mobility and interface interactions affect the possibility of intermolecular processes, such as reactions of highly reactive species (excited or charged molecules and molecular fragments) with neighboring molecules. Although the reactions of molecular radical cations or anions with neighboring molecules M± + M → (M−M)± are possible, their products were not observed in the experiments.6 Instead, the products of interaction of molecular fragments with neighboring molecules were found.6 Therefore, fragmentation of the molecules constituting the layer should be the first step of chemical degradation. We study the stability of four potential host molecules: two carbazole-based phosphorescent OLED host molecules, 9(diphenyl-1,3,5-triazin-2-yl)-9H-carbazole (DPTZCarb) and 9(3,5-diphenylphenyl)-9H-carbazole (DPPCarb), and two triphenylene-based ones, 2,4-diphenyl-6-(triphenylen-2-yl)-1,3,5triazine (DPTZTriphen) and 2-(3,5-diphenylphenyl)triphenylene (DPPTriphen). The structural formulas of the molecules are shown in Figure 1. These molecules are built of typical fragments used in either fluorescent or phosphorescent OLED host materials36 and, therefore, can suffer from all types of operational degradation. In general, there are several ways for degradation of these molecules: dissociation of endocyclic bonds, detachment of one of the fragments (dissociation of exocyclic bonds), or chemical attack by surrounding molecules. Below we describe the choice of fragmentation channels for our analysis. We exclude from consideration the dissociation of endocyclic bonds in benzene rings, because aromatic C−C bonds in the πconjugated structures in most cases have bond dissociation energies (BDE) higher than formally single bonds between fragments (especially exocyclic C−X bonds).29 However, we consider dissociation of endocyclic C−N bonds in the triazine rings, because s-triazine derivatives are known to undergo photodissociation.37−41 The fragmentation channels involving exocyclic bonds are shown with red lines in Figure 1; triazine ring fission channel is shown in Figure 2. For the degradation channels involving exocyclic bonds we assume that the initial step of degradation is breaking of a chemical bond between the fragments and analyze the ability for such bonds to dissociate. This assumption is quite strong; however, we speculate that the reactivity of certain molecular bonds should be related to their BDEs, since the reactions that B


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than in 106 h of operation at T = 70 °C (the minimum temperature OLED has to withstand during operation6). Such luminance loss corresponds to accumulation of the defects in an amount of nearly 0.1% of the molecular density.6 Therefore, the degradation rate of host molecules has to be low enough to sustain at least 106 h of operation before accumulation of 0.1% defects. The rate constant kD of dissociation for neutral molecules in the ground state can be assessed from solution of the first-order kinetic equation d[M]/dt = −kD[M], where [M] is the density of molecules. The rate constant that causes degradation of 0.1% of molecules after 106 h of operation is kD ≈ 3 × 10−13 s−1. The corresponding activation energy evaluated from the Arrhenius equation kD = k0·exp(−EA/kT) with k0·≈ 1012 s−1 (typical bond vibration frequency42,43) is EA ≈ 1.7 eV. The BDEs of typical OLED molecules in the ground state are much higher than 1.7 eV. However, the molecules in the recombination and chargetransport layers of an operating device are mainly in the charged or excited state due to the charge transport44 and emission processes. Thus, the molecules constituting the host of the recombination layer, such as those discussed in the present paper (Figure 1), can exist in states of all three types. Below, we consider several degradation mechanisms that could contribute to the overall OLED degradation:9,15,29 (i) degradation in a charged state, (ii) degradation in an excited state, (iii) degradation in a charged excited state (exciton− polaron annihilation, EPA), and (iv) degradation in a highly excited state (exciton−exciton annihilation, EEA). In this work, we analyze the relative stability of the four molecules against the degradation via all these mechanisms. 2.2.1. Degradation of Molecules in Charged States. In general, charged states of molecules are less stable than the corresponding neutral state. A molecule in the charged state can undergo a bond rupture or enter into radical reactions with surrounding molecules due to thermal activation. One of the best known examples is the degradation of charged Alq3 molecules in both anionic45 and cationic46 states. The stability of a molecule with respect to dissociation in a charged state can be estimated by solution of the kinetic equation d[M]/dt = −kD[MP], where [MP] is the density of polarons. Taking t = 106 h, [M] = 1021 cm−3, and [MP] = 1019 cm−3 similar to the Alq3 layer,47 we obtain kD ≈ 3 × 10−11 s−1 and EA ≈ 1.5 eV. Therefore, let us assume that the molecule is stable against hole or electron current through the layer if the lowest BDE of the molecule in the charged state is higher than 1.5 eV. 2.2.2. Degradation of Molecules in Excited States. Let us consider the possible mechanism of bond cleavage for a molecule in the lowest excited triplet or singlet (T1 or S1) states. The depth of the potential energy well associated with bond dissociation can be estimated as a difference between the ground state BDE and the energy of the excited state

Figure 1. Structural formulas of (a) 9-(diphenyl-1,3,5-triazin-2-yl)-9Hcarbazole (DPTZCarb), (b) 9-(3,5-diphenylphenyl)-9H-carbazole (DPPCarb), (c) 2,4-diphenyl-6-(triphenylen-2-yl)-1,3,5-triazine (DPTZTriphen), and (d) 2-(3,5-diphenylphenyl)triphenylene (DPPTriphen). Studied fragmentation channels are marked with a red line.

Figure 2. Fission of triazine rings into three nitriles. Broken bonds are shown with red lines.

involve bonds with high BDEs would require the formation of transition states with higher energies. In other words, molecular fragments resulting from channels with lower BDEs should be more likely to form than fragments resulting from channels with higher BDEs, and therefore, such intermediates should be abundant in the primary dissociation products. We leave the accurate verification of this assumption for further work but mention situations where the assumption could fail, e.g., for excited highly reactive radicaloid states. For each molecule we study two fragmentation channels. One channel is common for all molecules, namely, detachment of the phenyl ring. Additionally, for carbazole-based molecules we consider detachment of the carbazolyl moiety, while for triphenylene-based molecules we consider detachment of the triphenylenyl moiety. 2.2. Degradation Mechanisms. Let us consider OLED to be stable if it loses one-half of the initial luminance not earlier

ΔEexc = BDE − Eexc

(1)

where ΔEexc is the depth of the potential energy well for the excited state, BDE is the bond dissociation energy of a molecule in the ground state, and Eexc is the adiabatic energy of the excited state (see Figure 3 for the scheme of potential energy surfaces (PESs)). More accurate thresholds for ΔEexc values can be obtained using the particular shape of PES for bond dissociation, device operating conditions, surrounding molecules, etc. In addition, reverse processes of bond formation from predissociated states, C


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hot excited state (so-called hot-molecule dissociation mechanism48). The depth of the potential energy well for EPA dissociation can be estimated as the difference between the BDE of the charged molecule and the exciton energy ΔE EPA = BDE+ / − − Eexc

(2)

where ΔEEPA is the depth of the potential energy well for the EPA mechanism, BDE± is the bond dissociation energy of the charged molecule, and Eexc is the exciton energy. We consider the “the worst case scenario”, since the excited terms Dn with n > 1 are not always degenerate at the infinite separation of fragments. Although the mismatch between the exciton energy and the excitation energies of the charged molecules can suppress EPA, the excitation spectra of charged molecules are usually quite dense and such mismatch is unlikely. It is rather difficult to estimate the necessary depth of the potential energy well in excited charged states. There are a variety of mechanisms of bond dissociation in excited charged states; on the other hand, these states are extremely short lived and rare, because the probability of the formation of excited charged states is proportional to the product of the charge and exciton densities. However, it is reasonable to propose that BDE in a charged state should be higher than the T1 state energy to avoid at least spontaneous bond cleavage. 2.2.4. Degradation in Highly Excited States. In an operating OLED, the energy transfer between excitons may lead to the formation of an excited state with an energy twice the exciton energy (Figure 5).49 The probability to form such

Figure 3. Schematic representation of the PES for the ground, S0, and the first triplet and singlet excited states T1 and S1. At large separation of fragments the energies of T1 and S1 converge to the same value because the overlap of electronic wave functions between dissociating fragments vanishes in the case of homolytic dissociation without drastic structural rearrangement (typical for organic molecules9,15).

which we do not consider in the present model, can also contribute to this estimate. To estimate the minimum depth of the potential energy well of the excited state of a stable molecule, ΔEexc, we use the kinetic equation d[M]/dt = −kD[M*], where [M*] is the density of excitons. The triplet exciton density can be as high as [M*] = 1019 cm−3 (see, e.g., ref 47), while the density of singlet excitons is usually at least 2 orders of magnitude lower. Therefore, in assumption of constant exciton density, we arrive to the minimal potential well depth ΔEexc ≈ 1.5 eV for triplet and ΔEexc ≈ 1.4 eV for singlet excitons. The discussed mechanism clarifies to some extent the difficulty of the design of long-lived deep blue emitting materials. Indeed, triplet energies of the blue phosphorescent OLED molecules are at least 2.8 eV. Taking into account the minimal necessary threshold of ∼1.5 eV to stabilize the bond, the lower limit of the ground state BDE of host molecules can be estimated as ∼4.3 eV. 2.2.3. Degradation of Molecules in Charged Excited States. Degradation through exciton−polaron annihilation mechanism corresponds to the process of energy transfer from an exciton to the charged molecule (Figure 4). Due to energy transfer, the charged molecule in the excited state can dissociate directly (if excited to the purely repulsive term) or via internal conversion and subsequent dissociation from the

Figure 5. Schematic diagram of PESs for the ground and excited states upon triplet−triplet annihilation.

states is proportional to the squared exciton density. Dissociation of a molecule via a highly excited state is called an exciton−exciton annihilation (EEA) degradation mechanism. Since the lifetime of triplet excitons is much longer than that of singlet excisions and their concentration is higher, the processes of triplet−triplet exciton annihilation (TTA) prevail. The excited state resulting from TTA can decay via internal conversion or intersystem crossing with intermediate states, and some of the intermediate states can be repulsive or loosely bounded. It is hardly possible to analyze all of the intermediate states being populated in the course of de-excitation to the T1 or S1 states and draw conclusions on the probability of bond dissociation in each of these states. However, it is reasonable to propose that if the doubled triplet exciton energy is lower than the BDE in the ground state, the energy released in TTA is insufficient to cause bond cleavage. Otherwise, if doubled triplet

Figure 4. Schematic diagram of EPA mechanism: triplet or singlet exciton (wavy arrow line) transfers its energy Eexc to the charged molecule in the ground state D0 and forms an excited doublet state Dn. D


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below zero. In this case, high charge density will facilitate degradation, but the molecule can be used at low charge densities. For exciton−exciton annihilation, instability is established if one of the values of ΔETTA = BDE − 2·ET1 is below zero. In this case, high exciton density is harmful for the molecule, but it can be used at low exciton density. If all values of ΔEEPA and ΔETTA are positive, the molecule is regarded as stable and can be recommended for general use as OLED host. We consider dissociation of all exocyclic bonds in the target molecules in the ground (BDE), excited singlet and triplet (ΔEexc), and charged (BDE±) states. For charged states, we consider two channels corresponding to different radical−ion pairs (Scheme 1). These energies show the stability of the corresponding states. Next, ΔEEPA and ΔETTA are calculated using eqs 2 and 3. These energies show the stability of the molecules with respect to EPA and TTA dissociation, that is, hot-molecule mechanisms. Finally, for the specific case of triazine derivatives, we calculate the energy of triazine ring fission in the ground and excited states.

energy is higher than BDE, the energy could be transmitted to the channel leading to bond dissociation. Therefore, the depth of the potential energy well for TTA dissociation can be estimated as the difference between the BDE and twice the exciton energy ΔE TTA = BDE − 2E T1

(3)

where ΔETTA is the depth of the potential energy well for the TTA mechanism, BDE is the bond dissociation energy in the ground state, and ET1 is the triplet energy. 2.3. Computational Scheme. In summary, the entire computational scheme is shown in Chart 1. First, the bonds Chart 1. Flow Chart of the Computational Scheme for the Analysis of Host Stabilitya

3. COMPUTATIONAL METHODS In order to analyze the probability of degradation via different degradation mechanisms described in section 2.2, we estimate the energy of dissociation (the depth of the potential well) from the calculated BDEs and energies of excited states (eqs 1−3). For this purpose, first we calculate the BDEs for various dissociation channels in the neutral, negatively, and positively charged states of the molecules as a total energy difference between the separated fragments and the ground state of a molecule in the neutral or charged state. If there are several local energy minima on a PES and the corresponding state is rather long lived to allow for thermal quasi-equilibrium, the degradation rate is proportional to the bond dissociation probability from the global minimum on the PES. Therefore, it is important to locate the state corresponding to the global minimum and to use its energy for estimation of the dissociation barrier. We do not consider nonequilibrium conditions when the populations of the local minima do not obey the Boltzmann distribution and depend on the history of the system. In organic molecules excitation and excess charge is most frequently localized on individual fragments, as it was demonstrated, for example, for organometallic complexes.50−54 To locate the energy minimum on PES of a charged or an excited molecule, one needs a computational method that is able to correctly reproduce localization of a charge and excitations within a molecule. It is known that density functional theory suffers from the so-called “delocalization error”.55,56 This error results in unphysically low energies for delocalized electronic states and commonly leads to overestimated delocalization of electron density.57 Though this deficiency is overcome to some extent by using long-rangecorrected or range-separated functionals, even these improved functionals do not provide a good description of electronic structure for systems, especially with unpaired electrons.58 Artificial charge localization forced in the “constrained DFT” method59 still provides charge localization on one of the molecules participating in a charge-transfer process rather than on one of its fragments. On the other hand, multireference methods, though more computationally demanding, are able to

a

The activation energy threshold of 1.5 eV accepted for the dissociation from the low-energy states ensures degradation of 0.1% of molecules after 106 h of operation (see section 2.2). Zero activation energy (barrierless dissociation) is assumed for the hot-molecule mechanisms.

most likely to dissociate are chosen based on the literature data or chemical intuition. Next, bond dissociation energies are calculated for each chosen bond in the ground, excited, and charged states. To do this the geometries of the corresponding states and molecular fragments should be optimized. If any one of the calculated BDEs is below the threshold value of 1.5 eV, the molecule is regarded as unstable with respect to this dissociation channel. Such molecule cannot be recommended as OLED host. If all of the dissociation energies are above zero, degradation according to hot-molecule mechanisms should be considered. For exciton−polaron annihilation, instability is established if one of the values of ΔEEPA = BDE± − Eexc is E


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The Journal of Physical Chemistry C Scheme 1. Reactions Considered in the Paper with the Example of DPTZCarba

a

(a) Possible channels of exocyclic bonds dissociation; (b) triazine ring fission.

Information. In order to avoid convergence to artifact structures during geometry optimization, we choose active spaces and a state-averaging scheme so that the state-averaged electron densities of symmetric or chemically equivalent fragments in the ground state are not distorted. After geometry optimization with the SA-CASSCF method, we calculate the energies of the resulting states using the XMCQDPT2 method with the same active spaces and state-averaging scheme as for CASSCF calculations and the effective Hamiltonian of 30 states. For geometry optimization of molecular fragments we use the state-specific CASSCF (SS-CASSCF) method (active spaces for different molecular fragments and states are presented in Table S2 of the Supporting Information), while for calculation of total energies of fragments we use the statespecific XMCQDPT2 method with the effective Hamiltonian of 10 states. To calculate the thermodynamic stability of triazine derivatives (DPTZCarb and DPTZTriphen) with respect to fragmentation into three neutral ground-state nitrile molecules, we considered the same calculated S0, T1, and S1 states of DPTZCarb and DPTZTriphen and ground states of benzonitrile BN (SS-CASSCF(10,10)), N-cyanocarbazole CCN (SS-

distinguish between localized and delocalized states and reproduce correct charge localization. Therefore, in our work we use a multireference methodology, namely, complete active space self-consistent field (CASSCF) methodology together with extended multiconfiguration quasidegenerate second-order perturbation theory (CASSCF/ XMCQDPT2),60−63 in order to obtain as accurate values of BDEs as possible. We perform calculations with the 6-31G(d,p) basis set using the Firefly QC package,64 which is partially based on the GAMESS (US) source code.65 Before calculating the total energy of the molecule and separated fragments, we performed the geometry optimization of a corresponding molecule or fragment. To avoid excessive computational cost of CASSCF calculations and to find a tradeoff between the computational cost and the accuracy, we carefully choose the active space and the state-averaging scheme. Below we describe the choice of active spaces for geometry optimization and total energy calculations for molecules and fragments. For molecules we use the state-averaged complete active space self-consistent field (SA-CASSCF) method for statespecific gradients66 with active spaces and state-averaging schemes summarized in Table S1 of the Supporting F


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The Journal of Physical Chemistry C CASSCF(12,12)), and 2-triphenylenecarbonitrile TCN (SSCASSCF(12,12)) (see Supporting Information for notation).

Table 1. BDEs of Molecules in the Neutral State and in the Global Minima of Charged States carbazolyl detachmenta

4. RESULTS AND DISCUSSION For each molecule we calculate BDEs for dissociation channels described in section 2.1 (detachment of phenyl, carbazolyl, or triphenylenyl moieties). For every molecule we analyze different charged states (anions and cations with different charge localization) and several low lying excited states (singlets and triplets with different localization of excitations). Details of the obtained charge and excitation localization in different states are presented on Figures S1−S4 of the Supporting Information. In the case of dissociation of neutral molecules, the products are neutral molecular fragments, while in the case of dissociation of charged molecules, one of the products is charged. 4.1. Degradation of Charged States. Careful examination of the electron density distribution in the charged (both cationic and anionic) states of the studied molecules showed that the unpaired electron in these radical ionic species is not localized on a certain atom. Instead, it is delocalized over large fragments of the molecule (see Figures S1−S4 of the Supporting Information). This delocalization makes these species unreactive in bimolecular reactions with their neighbors and, therefore, safe under normal operation. Next, we analyze the degradation of these molecules in the charged states by comparing calculated BDEs of charged molecules (Table 1). On the basis of this comparison we find that BDEs of molecular anions are smaller than BDEs of neutral molecules, while BDEs of cations are generally higher than BDEs of neutral molecules. Typical BDEs of the cationic states are 4.3−7.5 eV, while those of the anionic states can be as low as 1.5 eV. Therefore, the least stable states are anions for all considered molecules. Nevertheless, all BDEs are positive, which indicates only slow degradation under normal operation. The relative stability of our set of four molecules can be analyzed by comparing BDEs of the most probable dissociation channels for each molecule (see Table 1). By comparing these lowest BDEs we can arrange the molecules in a series DPPCarb < DPTZCarb < DPPTriphen ≤ DPTZTriphen by increasing their stability, which shows that both carbazole-based molecules are less stable than triphenylene-based ones. The degradation of carbazole-based molecules happens via detachment of a carbazolyl group. 4.2. Degradation of Excited States. To analyze the degradation of molecules in the excited states, we compare ΔEexc values calculated for different singlet and triplet excited states (Table 2). There are no obvious trends in ΔEexc for different excited states of each particular molecule, though singlet states are expectedly less stable than triplets. Comparison of the lowest ΔEexc of different molecules reveals the same stability trend as in the case of degradation from the charged state, DPTZCarb ≤ DPPCarb < DPPTriphen ≤ DPTZTriphen, and detachment of carbazolyl group is also the most probable among all considered dissociation channels. The important difference from the dissociation of charged states is very low ΔEexc values for the most probable dissociation channels of carbazole-based molecules. The ΔEexc values of carbazole-based molecules are below our rough estimations of ΔEexc critical values obtained in section 2.2. Therefore, carbazole-based molecules fail to meet the stability criterion formulated in section 2.2 and cannot be used as phosphorescent OLED host molecules, at least for single-host devices.

molecule

fragments

DPTZ• + Carb• DPTZ• + Carb− b DPTZ− + Carb• DPTZCarb+ DPTZ• + Carb+ DPTZ+ + Carb• DPPCarb DPP• + Carb• − DPPCarb DPP• + Carb− b DPP− + Carb• DPPCarb+ DPP• + Carb+ DPP+ + Carb• triphenylenyl detachmenta DPTZTriphen DPTZ• + Triphen• DPTZTriphen− DPTZ• + Triphen− DPTZ− + Triphen• + DPTZTriphen DPTZ• + Triphen+ DPTZ+ + Triphen· DPPTriphen DPP• + Triphen• − DPPTriphen DPP• + Triphen− DPP− + Triphen• DPPTriphen+ DPP• + Triphen+ DPP+ + Triphen•

DPTZCarb DPTZCarb−

phenyl detachmenta

BDE (eV)

fragments

BDE (eV)

4.1 2.5

Ph· + PTZCarb• Ph• + PTZCarb−

5.5 4.3

3.6 5.1 4.6 4.2 1.5 2.8 5.0 4.3

Ph− + PTZCarb• Ph• + PTZCarb+ Ph+ + PTZCarb• Ph• + DPCarb• Ph• + DPCarb− Ph− + DPCarb• Ph• + DPCarb+ Ph+ + DPCarb•

5.8 5.8 7.5 5.4 3.9 4.6 5.3 7.1

5.4

Ph• + PTZTriphen• Ph• + PTZTriphen− b Ph− + PTZTriphen• Ph• + PTZTriphen+ Ph+ + PTZTriphen• Ph• + DPTriphen• Ph• + DPTriphen− b Ph− + DPTriphen• Ph• + DPTriphen+ Ph+ + DPTriphen•

4.8

5.8 5.8 6.2 5.3 6.2 5.3 5.5 7.4 6.4

4.5 5.9 5.3 6.2 5.3 4.3 5.1 6.2 7.1

a

For each charged molecule two reactions with differently charged products are analyzed. bMost probable dissociation channels for each molecule.

DPTZTriphen molecule is on the edge of stability according to our criterion, while DPPTriphen meets our criterion and has to be tested against other degradation channels. 4.3. Degradation of Charged Excited States. In order to assess the possibility of exciton−polaron dissociation, we calculate the activation energy of exciton−polaron dissociation using BDEs of the ionic species (Table 1) and the energies of excitons of the same molecules (Table 3). Our exciton energies calculated for DPTZCarb agree well with the experimental data on the absorption, fluorescence, and phosphorescence of this compound.67 The equilibrium (relaxed) excitons have the lowest energies (adiabatic exciton energy), while the unrelaxed just generated excitons can be considered hot, and their energies depend on the state from which they were generated. For example, in Table 3 we provide the energy of excitons generated through light absorption (vertical exciton energy). Relaxation of hot excitons occurs simultaneously or even faster than the exciton diffusion in the layer. Therefore, the collision of a polaron with the unrelaxed hot excitons is less probable and dropped from consideration in the rest of the paper. Calculated activation energies of exciton−polaron dissociation (Table 4) suggest that negatively charged carbazolylcontaining molecules are highly susceptible to EPA degradation G


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The Journal of Physical Chemistry C Table 2. ΔEexc for Dissociation of Excited States Calculated Using eq 1 carbazolyl detachment molecule/state DPTZCarb/T1a DPTZCarb/T2 (n−π*)a DPTZCarb/T3a DPTZCarb/T4a DPTZCarb/S1a DPTZCarb/S2a DPTZCarb/S3a DPTZCarb/S4 (n−π*)a DPPCarb/T1a DPPCarb/T2a DPPCarb/T3a DPPCarb/S1a DPPCarb/S2a DPTZTriphen/T1a DPTZTriphen/T2a DPTZTriphen/T3a DPTZTriphen/S1a DPTZTriphen/S2a DPPTriphen/T1a DPPTriphen/T2a DPPTriphen/T3a DPPTriphen/S1a DPPTriphen/S2a DPPTriphen/S3a

fragments

energies for these molecules are slightly lower than for dissociation from the excited state. However, the necessity of a polaron to meet the exciton somewhat reduces the probability of EPA dissociation and can make it comparable to or lower than the probability of excited state dissociation. Experimental studies of intrinsic degradation of phosphorescent OLEDs have shown the dominant role of EPA in the formation of defects.30,31 In archetypical stacked OLEDs, the EPA process is more intense at the interfaces between the holetransporting and emissive layers, where the concentrations of positive polarons and singlet excitons are high.68 To reduce the EPA probability, one should design the device layers in a way to minimize the overlap between the exciton and the electron densities in the material. Another option could be to dope the material with the molecules stable in the anionic state that have a slightly deeper lowest unoccupied molecular orbital energy (by ∼0.2 eV) to facilitate electron transfer primarily via admixed molecules. Suppression of EPA was recently achieved by special design of OLEDs with a gradient host−dopant concentration profile in the broadened emissive layer proposed in refs 69, 70, and 71, which significantly increased the lifetime of phosphorescent OLEDs.4 4.4. Degradation of Highly Excited States. Finally, we analyze the TTA degradation mechanism using ΔETTA values calculated with eq 3 (see Table 5). DPTZTriphen molecule is the most stable to TTA degradation because of its low triplet energy. Other molecules have high triplet energy, which causes negative ΔETTA values and, therefore, promotes fast dissociation of a highly excited state. Even the DPPTriphen molecule, which is stable for other degradation mechanisms, turned out to be unstable for TTA degradation. Our results suggest that TTA-mediated degradation can be the most harmful process for phosphorescent OLED host molecules and can affect even relatively stable molecules that withstand other degradation mechanisms. It is therefore reasonable to make efforts to suppress TTA-mediated degradation. This may be achieved by (i) creating an evenly distributed recombination zone by tuning the carrier mobilities (layer morphology by design), (ii) reducing the exciton density in the recombination zone, (iii) reducing the exciton formation rate on the host molecules by using dopants with higher electron affinity or lower ionization potential, and (iv) using molecules with the BDE higher than double the triplet energy. 4.5. Degradation of Triazine Rings. We checked the thermodynamical stability of both triazine derivatives, DPTZCarb and DPTZTriphen, with respect to the process of ring fission into three nitrile molecules (two benzonitrile and N-cyanocarbazole or 2-triphenylenecarbonitrile, respectively) (Figure 2, Table 6). Indeed, both molecules are stable in the ground state but unstable in the excited (triplet and singlet) states, with DPTZTriphen being less stable. These are the thermodynamic stability estimates, which cannot be used immediately for assessing the kinetic stability, as we did for bond breaking reactions, as well as for comparing the rate of ring fission with the rate of bond breaking. When a bond is broken into two radicals or (in a charged state) into a radical and an ion, we assume fast relaxation of (probably excited) products to their ground states, which is very likely for radicals and ions. In the case of excited-state ring fission, one (or even more) of the products can be formed in the excited state. For example, a molecule in the excited triplet state will break into one triplet and two singlet ground-state fragments. If one of the products is formed in the triplet state,

phenyl detachment

ΔEexc (eV)

ΔEexc (eV)

fragments

DPTZ• + 1.0b Ph• + Carb• PTZCarb• • DPTZ + 0.7 Ph• + Carb• PTZCarb• • DPTZ + 0.6 Ph• + Carb• PTZCarb• DPTZ• + 0.6 Ph• + Carb• PTZCarb• DPTZ• + 0.8b Ph• + Carb• PTZCarb• • DPTZ + 0.7 Ph• + Carb• PTZCarb• b • DPTZ + 0.8 Ph• + Carb• PTZCarb• DPTZ• + 0.7 Ph• + Carb• PTZCarb• DPP• + Carb• 0.7 Ph• + DPCarb• DPP• + Carb• 0.8 Ph• + DPCarb• b • • DPP + Carb 1.1 Ph• + DPCarb• DPP• + Carb• 0.4 Ph• + DPCarb• DPP• + Carb• 0.7b Ph• + DPCarb• triphenylenyl detachment DPTZ• + 3.1 Ph• + Triphen• PTZTriphen• DPTZ• + 2.4 Ph• + Triphen• PTZTriphen• • DPTZ + 2.5 Ph• + Triphen• PTZTriphen• • DPTZ + 2.0 Ph• + Triphen• PTZTriphen• DPTZ• + 2.1 Ph• + Triphen• PTZTriphen• DPP• + 2.8 Ph• + Triphen• DPTriphen• • DPP + 2.9 Ph• + Triphen• DPTriphen• • DPP + 3.1 Ph• + Triphen• DPTriphen• DPP• + 2.5 Ph• + Triphen• DPTriphen• DPP• + 2.5 Ph• + Triphen• DPTriphen• • DPP + 1.9 Ph• + Triphen• DPTriphen•

2.4 2.1 2.0 1.9 2.1 2.1 2.2 2.0 1.9 2.0 2.4 1.6 1.9 2.4b 1.7 1.8 1.3 1.4b 1.9 2.0 2.1b 1.6b 1.6b 0.9

a

For each singlet and triplet state different exciton localizations are analyzed. bMost probable dissociation channels from the global minimum of each singlet and triplet state.

Table 3. Lowest Energies of Singlet and Triplet Excitons ET1 (eV)

ES1 (eV)

host

vertical

adiabatic

vertical

DPTZCarb DPPCarb DPTZTriphen DPPTriphen

3.51 3.91 2.89 3.52

3.09 3.08 2.34 3.17

3.78 4.35 3.68 4.69

adiabatic (τrad, μs) 3.31 3.50 3.37 3.70

(1.565) (0.024) (0.204) (0.002)

upon collisions with low-energy triplet excitons (see Table 4). At the same time, positive polarons of these hosts can be promoted to dissociation only with hot (mostly singlet) excitons. Triphenylene-containing molecules are stable to EPA in anionic and cationic states. The EPA activation H


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Table 4. ΔEEPA Values Calculated Using eq 2 for Singlet and Triplet Adiabatic Excitons Compared with ΔEexc Values for the Most Probable Channels ΔEEPA (eV)

a

host

BDE− − ES1

BDE− − ET1

BDE+ − ES1

BDE+ − ET1

most probable ΔEexc (eV)

DPTZCarb DPPCarb DPTZTriphen DPPTriphen

−0.8 −2.1a 1.0 0.6

−0.6 −1.7a 2.1 1.1

1.0 0.8 1.9 2.1

1.2 0.9 2.0 2.6

1.0 1.1 1.4 1.6

a

a

Negative activation energies suggest barrierless dissociation.

Table 5. ΔETTA Values Calculated Using eq 3

5. CONCLUSION We performed analysis of the relative stability for four small organic host molecules (DPPCarb, DPTZCarb, DPPTriphen, DPTZTriphen) during operation of phosphorescent OLEDs with respect to uninomolecular bond dissociation with activation energies obtained from ab initio multireference calculations. After analysis of four types of degradation mechanisms we found that the dissociation of molecules in the charged states is not harmful for all considered molecules. Molecules in the excited states dissociate much faster, and carbazole-based molecules were found to be unstable against dissociation from the neutral excited and negatively charged excited states. Triphenylene-based molecules are stable against dissociation from the lowest excited states, while one of them, DPPTriphen, is unstable against dissociation from the highly excited state formed by the triplet−triplet annihilation process. Therefore, only one molecule from our test set, DPTZTriphen, meets the stability criteria for all considered degradation mechanisms. The results obtained allow us to arrange molecules into a qualitative stability series (DPPCarb < DPTZCarb < DPPTriphen < DPTZTriphen) and to determine structural features that lead to improved stability of a molecule. Thus, a molecule is strengthened with respect to considered dissociation reactions if the carbazolyl moiety is replaced by the triphenylenyl one or if the central phenyl ring is replaced by a 1,3,5-triazine moiety. The most stable molecule, DPTZTriphen, contains a central 1,3,5-triazine ring and a triphenylenyl moiety. At the same time, both triazine-based hosts are unstable with respect to triazine ring fission in the excited state, which makes them undesirable as hosts in recombination layers, where exciton density is high. It is difficult to accurately estimate the relative probability of different degradation channels, because it depends not only on the properties of the material itself but also on the OLED operating conditions, e.g., the spatial distribution of charge carriers or excitons. For accurate simulation of degradation processes via different mechanisms under operational conditions and in order to pinpoint the most probable degradation mechanisms, a detailed kinetic model is required. This model can be partially fed with the data calculated in our work. Experimental verification of the theoretical predictions, extension of the theoretical formalism toward description of intermolecular reactions, and verification on a larger set of OLED molecules might be the subject for future work.

ΔETTA (eV) DPTZCarb DPPCarb DPTZTriphen DPPTriphen

carbazolyl detachment −2.08 −1.96 triphenylenyl detachment 0.72 −0.14

phenyl detachment −0.58 −0.76 phenyl detachment 0.12 −1.04

Table 6. BDE of the Studied Triazines with Respect to Ring Fission state

BDE (eV) DPTZCarb → 2BN + CCN

S0 T1 S1

2.2 −0.9 −1.1 DPTZTriphen → 2BN + TCN

S0 T1 S1

1.1 −1.2 −2.3

its energy is higher, and as a total, the reaction will be endothermic, because the relaxation of the triplet product to its ground state is relatively slow. Similarly, a molecule in the excited singlet state can break into one singlet excited- and two singlet ground-state fragments. In this case relaxation of the singlet excited fragment will be relatively fast. Therefore, triplet states can be thermodynamically unstable but kinetically stable with respect to ring fission, but singlet excited states will be unstable both thermodynamically and kinetically. That is why for more accurate estimates one should calculate not only the ground states of the ring fission products but also their triplet states as well. In addition, we did not estimate the stability of the charged states of DPTZCarb and DPTZTriphen with respect to the ring fission process. In this case, one has to calculate the charged states of the fission products. A detailed mechanism of photodissociation of 1,3,5-triazine is presented in ref 40. Even for such a small molecule, the actual mechanism is rather complicated and involves numerous intermediates and a great variety of products, including the channel similar to that considered above (decomposition into three HCN molecules). In the present work we only demonstrate that even the lowest excited states of DPTZCarb and DPTZTriphen are unstable with respect to such processes, and it makes these molecules undesirable as hosts in the recombination layers of OLED.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b05761. I


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(3) Segal, M.; Mulder, C.; Celebi, K.; Singh, M.; Rivoire, K.; Difley, S.; Van Voorhis, T.; Baldo, M. A. Spin and Device Engineering for Blue Organic Light Emitting Devices. Proc. SPIE 2008, 699912. (4) Zhang, Y.; Lee, J.; Forrest, S. R. Tenfold Increase in the Lifetime of Blue Phosphorescent Organic Light-Emitting Diodes. Nat. Commun. 2014, 5, 5008. (5) Kondakov, D. Y.; Lenhart, W. C.; Nichols, W. F. Operational Degradation of Organic Light-Emitting Diodes: Mechanism and Identification of Chemical Products. J. Appl. Phys. 2007, 101, 024512. (6) Scholz, S.; Kondakov, D.; Lüssem, B.; Leo, K. Degradation Mechanisms and Reactions in Organic Light-Emitting Devices. Chem. Rev. 2015, 115, 8449−8503. (7) Zamani Siboni, H.; Aziz, H. Triplet-Polaron Quenching by Charges on Guest Molecules in Phosphorescent Organic Light Emitting Devices. Appl. Phys. Lett. 2012, 101, 063502. (8) Klubek, K. P.; Tang, C. W.; Rothberg, L. J. Investigation of Blue Phosphorescent Organic Light-Emitting Diode Host and Dopant Stability. Org. Electron. 2014, 15, 1312−1316. (9) So, F.; Kondakov, D. Degradation Mechanisms in SmallMolecule and Polymer Organic Light-Emitting Diodes. Adv. Mater. 2010, 22, 3762−3777. (10) Reineke, S.; Baldo, M. A. Recent Progress in the Understanding of Exciton Dynamics within Phosphorescent OLEDs. Phys. Status Solidi A 2012, 209 (12), 2341−2353. (11) Sandanayaka, A. S. D.; Matsushima, T.; Adachi, Ch. Degradation Mechanisms of Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence Molecules. J. Phys. Chem. C 2015, 119 (42), 23845−23851. (12) Kondakov, D. Y.; Sandifer, J.; Tang, C.; Young, R. Nonradiative Recombination Centers and Electrical Aging of Organic LightEmitting Diodes: Direct Connection Between Accumulation of Trapped Charge and Luminance Loss. J. Appl. Phys. 2003, 93, 1108−1119. (13) Kondakov, D. Y. Direct Observation of Deep Electron Traps in Aged Organic Light Emitting Diodes. J. Appl. Phys. 2005, 97, 024503. (14) Scholz, S.; Walzer, K.; Leo, K. Analysis of Complete Organic Semiconductor Devices by Laser Desorption/Ionization Time-OfFlight Mass Spectrometry. Adv. Funct. Mater. 2008, 18, 2541−2547. (15) Schmidbauer, S.; Hohenleutner, A.; Konig, B. Chemical Degradation in Organic Light-Emitting Devices: Mechanisms and Implications for the Design of New Materials. Adv. Mater. 2013, 25, 2114−2129. (16) Kondakov, D. Y.; Pawlik, T. D.; Nichols, W. F.; Lenhart, W. C. Free-Radical Pathways in Operational Degradation of OLEDs. J. Soc. Inf. Disp. 2008, 16, 37−46. (17) Wang, J. F.; Kawabe, Y.; Shaheen, S. E.; Morrell, M. M. E.; Jabbour, G.; Lee, P. A.; Anderson, J.; Armstrong, N. R.; Kippelen, B.; Mash, E. A.; et al. Exciplex Electroluminescence from Organic Bilayer Devices Composed of Triphenyldiamine and Quinoxaline Derivatives. Adv. Mater. 1998, 10, 230−233. (18) Carvelli, M.; van Reenen, A.; Janssen, R. A. J.; Loebl, H. P.; Coehoorn, R. Org. Electron. 2012, 13, 2605−2614. (19) Hung, W.-Y.; Fang, G.-C.; Lin, S.-W.; Cheng, S.-H.; Wong, K.T.; Kuo, T.-Y.; Chou, P.-T. The First Tandem, All-Exciplex-based WOLED. Sci. Rep. 2015, 4, 5161. (20) Kaplunov, M. G.; Nikitenko, S. N.; Krasnikova, S. S. In Organic Light Emitting Devices; Singh, J., Ed.; InTech, 2012. (21) Liang, J. X.; Nguyen, Q. L.; Matsika, S. Exciplexes and Conical Intersections Lead to Fluorescence Quenching in π-Stacked Dimers of 2-Aminopurine with Natural Purine Nucleobases. Photochem. Photobiol. Sci. 2013, 12, 1387−1400. (22) Cherpak, V.; Stakhira, P.; Minaev, B.; Baryshnikov, G.; Stromylo, E.; Helzhynskyy, I.; Chapran, M.; Volyniuk, D.; Hotra, Z.; Dabuliene, A.; et al. Mixing of Phosphorescent and Exciplex Emission in Efficient Organic Electroluminescent Devices. ACS Appl. Mater. Interfaces 2015, 7, 1219−1225. (23) Ivaniuk, Kh.; Cherpak, V.; Stakhira, P.; Hotra, Z.; Minaev, B.; Baryshnikov, G.; Stromylo, E.; Volyniuk, D.; Grazulevicius, J. V.; Lazauskas, A.; et al. Highly Luminous Sky-Blue Organic Light-Emitting

Active spaces and state-averaging schemes used for multireference calculations of the molecules and molecular fragments; energies of the ground, excited, and ionic states of the studied molecules in their respective geometries with corresponding singly occupied orbitals showing charge or exciton localization; electron affinities, ionization potentials, and triplet and singlet excitation energies for these molecules (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Alexandra Ya. Freidzon: 0000-0002-7473-7692 Present Address ∥

D.N.K.: First Solar, Incorporated, 28101 Cedar Park Boulevard, Perrysburg, Ohio 43551, United States. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding from the Russian Science Foundation (project no. 1443-00052) and the Competitiveness Program of National Research Nuclear University “MEPhI” is acknowledged. Analysis of literature data and design of computer experiments was financially supported by the Russian Science Foundation. Calculations were financially supported by the Competitiveness Program of National Research Nuclear University “MEPhI”. Calculations were performed using the facilities of the Joint Supercomputer Center of Russian Academy of Sciences, the Supercomputing Center of Lomonosov, Moscow State University, and high-performance computing resources of the federal center for collective usage at NRC “Kurchatov Institute”.

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ABBREVIATIONS OLED, organic light-emitting device; PES, potential energy surface; BDE, bond dissociation energy; EPA, exciton−polaron annihilation; EEA, exciton−exciton annihilation; TTA, triplet− triplet exciton annihilation; CASSCF, complete active space self-consistent field; XMCQDPT, extended multiconfiguration quasi-degenerate perturbation theory; DPTZCarb, 9-(diphenyl1,3,5-triazin-2-yl)-9H-carbazole; DPPCarb, 9-(3,5-diphenylphenyl)-9H-carbazole; DPTZTriphen, 2,4-diphenyl-6-(triphenylen-2-yl)-1,3,5-triazine; DPPTriphen, 2-(3,5diphenylphenyl)triphenylene

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REFERENCES

(1) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nearly 100% Internal Phosphorescence Efficiency in an Organic LightEmitting Device. J. Appl. Phys. 2001, 90, 5048−5051. (2) Minaev, B.; Baryshnikov, G.; Agren, H. Principles of Phosphorescent Organic Light Emitting Devices. Phys. Chem. Chem. Phys. 2014, 16, 1719−1758. J


Article


(44) Yamaguchi, Y.; Fujiyama, T.; Tanaka, H.; Yokoyama, M. Material Design of Organic Thin Films for Bipolar Charge Transport. Chem. Mater. 1990, 2, 341−342. (45) Luo, Y.; Aziz, H.; Xu, G.; Popovic, Z. D. Similar Roles of Electrons and Holes in Luminescence Degradation of Organic LightEmitting Devices. Chem. Mater. 2007, 19, 2079−2083. (46) Aziz, H.; Popovic, Z. D.; Hu, N.-X.; Hor, A.-M.; Xu, G. Degradation Mechanism of Small Molecule-Based Organic LightEmitting Devices. Science 1999, 283, 1900−1902. (47) Kasemann, D.; Bruckner, R.; Frob, H.; Leo, K. Organic LightEmitting Diodes under High Currents Explored by Transient Electroluminescence on the Nanosecond Scale. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 115208. (48) Nakashima, N.; Yoshihara, K. Role of Hot Molecules Formed by Internal Conversion in UV Single-Photon and Multiphoton Chemistry. J. Phys. Chem. 1989, 93, 7763−7771. (49) Kondakov, D. Y. Triplet−Triplet Annihilation in Highly Efficient Fluorescent Organic Light-Emitting Diodes: Current State and Future Outlook. Philos. Trans. R. Soc., A 2015, 373, 20140321. (50) Safonov, A. A.; Bagaturyants, A. A. Charge Localization and Charge Transfer in the Bebq2Monomer and Dimer. J. Mol. Model. 2014, 20, 2397−2404. (51) Freidzon, A. Ya.; Safonov, A. A.; Bagaturyants, A. A. Theoretical Study of the Spectral and Charge-Transport Parameters of an Electron-Transporting Material Bis(10-Hydroxybenzo[h]Qinolinato) Beryllium (Bebq2). J. Phys. Chem. C 2015, 119, 26817−26827. (52) Emelina, T. B.; Freidzon, A. Ya.; Bagaturyants, A. A.; Karasev, V. E. Electronic Structure and Energy Transfer in Europium(III)− Ciprofloxacin Complexes: A Theoretical Study. J. Phys. Chem. A 2016, 120, 7529−7537. (53) Freidzon, A. Ya.; Scherbinin, A. V.; Bagaturyants, A. A.; Alfimov, M. V. Ab Initio Study of Phosphorescent Emitters Based on RareEarth Complexes with Organic Ligands for Organic Electroluminescent Devices. J. Phys. Chem. A 2011, 115, 4565−4573. (54) Romanova, K. A.; Freidzon, A. Ya.; Bagaturyants, A. A.; Galyametdinov, Y. G. Ab Initio Study of Energy Transfer Pathways in Dinuclear Lanthanide Complex of Europium(III) and Terbium(III) Ions. J. Phys. Chem. A 2014, 118, 11244−11252. (55) Mori-Sanchez, P.; Cohen, A. J.; Yang, W. T. Localization and Delocalization Errors in Density Functional Theory and Implications for Band-Gap Prediction. Phys. Rev. Lett. 2008, 100 (14), 146401. (56) Cohen, A. J.; Mori-Sanchez, P.; Yang, W. T. Challenges for Density Functional Theory. Chem. Rev. 2012, 112 (1), 289−320. (57) Johnson, E. R.; Mori-Sanchez, P.; Cohen, A. J.; Yang, W. T. Delocalization Errors in Density Functionals and Implications for Main-Group Thermochemistry. J. Chem. Phys. 2008, 129 (20), 204112. (58) Tentscher, P. R.; Arey, J. S. Binding in Radical-Solvent Binary Complexes: Benchmark Energies and Performance of Approximate Methods. J. Chem. Theory Comput. 2013, 9 (3), 1568−1579. (59) Wu, Q.; Van Voorhis, T. Direct Optimization Method to Study Constrained Systems within Density Functional Theory. Phys. Rev. A: At., Mol., Opt. Phys. 2005, 72 (2), 024502. (60) Roos, B. O. In Radiation Induced Molecular Phenomena in Nucleic Acids, Challenges and Advances in Computational Chemistry and Physics; Shukla, M. K., Leszczynski, J., Eds.; Springer Science+Business Media B.V.: Berlin, Heidelberg, 2008; Vol. 5, pp 125−156. (61) Schmidt, M. W.; Gordon, M. S. The Construction and Interpretation of MCSCF Wavefunctions. Annu. Rev. Phys. Chem. 1998, 49, 233−266. (62) Nakano, H. Quasidegenerate Perturbation Theory with Multiconfigurational Self-Consistent-Field Reference Functions. J. Chem. Phys. 1993, 99, 7983−7992. (63) Granovsky, A. A. Extended Multi-Configuration QuasiDegenerate Perturbation Theory: The New Approach to Multi-State Multi-Reference Perturbation Theory. J. Chem. Phys. 2011, 134, 214113. (64) Granovsky, A. A. Firefly, version 8.1.0; http://classic.chem.msu. su/gran/firefly/index.html (accessed Aug 24, 2017).

Diodes Based on the Bis[(1,2)(5,6)]indoloanthracene Emissive Layer. J. Phys. Chem. C 2016, 120, 6206−6217. (24) Cherpak, V.; Stakhira, P.; Minaev, B.; Baryshnikov, G.; Stromylo, E.; Helzhynskyy, I.; Chapran, M.; Volyniuk, D.; TomkutéLuksiene, D.; Malinauskas, T.; et al. Efficient “Warm-White” OLEDs Based on the Phosphorescent bis-Cyclometalated Iridium(III) Complex. J. Phys. Chem. C 2014, 118, 11271−11278. (25) Chapran, M.; Angioni, E.; Findlay, N. J.; Breig, B.; Cherpak, V.; Stakhira, P.; Tuttle, T.; Volyniuk, D.; Grazulevicius, J. V.; Nastishin, Y. A.; et al. An Ambipolar BODIPY Derivative for a White Exciplex OLED and Cholesteric Liquid Crystal Laser toward Multifunctional Devices. ACS Appl. Mater. Interfaces 2017, 9, 4750−4757. (26) Park, J.-S.; Chae, H.; Chung, H. K.; Lee, S. I. Thin Film Encapsulation for Flexible AMOLED: A Review. Semicond. Sci. Technol. 2011, 26, 034001. (27) Kim, S.; Kwon, H.-J.; Lee, S.; Shim, H.; Chun, Y.; Choi, W.; Kwack, J.; Han, D.; Song, M.; Kim, S.; et al. Low-Power Flexible Organic Light-Emitting Diode Display Device. Adv. Mater. 2011, 23, 3511−3516. (28) Schaer, M.; Nüesch, F.; Berner, D.; Leo, W.; Zuppiroli, L. Water Vapor and Oxygen Degradation Mechanisms in Organic Light Emitting Diodes. Adv. Funct. Mater. 2001, 11, 116−121. (29) Kondakov, D. Y. In OLED Fundamentals: Materials, Devices, and Processing of Organic Light-Emitting Diodes; Gaspar, D. J., Polikarpov, E., Eds.; CRC Press, Taylor & Francis Group: Boca Raton, 2015; pp 339−364. (30) Giebink, N.; D’Andrade, B.; Weaver, M.; Mackenzie, P. B.; Brown, J. J.; Thompson, M. E.; Forrest, S. R. Intrinsic Luminance Loss in Phosphorescent Small-Molecule Organic Light Emitting Devices due to Bimolecular Annihilation Reactions. J. Appl. Phys. 2008, 103, 044509. (31) Giebink, N.; D’Andrade, B.; Weaver, M.; Brown, J.; Forrest, S. Direct Evidence for Degradation of Polaron Excited States in Organic Light Emitting Diodes. J. Appl. Phys. 2009, 105, 124514. (32) Forrest, S. R. Excitons and the Lifetime of Organic Semiconductor Devices. Philos. Trans. R. Soc., A 2015, 373, 20140320. (33) Lin, N.; Qiao, J.; Duan, L.; Li, H.; Wang, L.; Qiu, Y. Achilles Heels of Phosphine Oxide Materials for OLEDs: Chemical Stability and Degradation Mechanism of a Bipolar Phosphine Oxide/Carbazole Hybrid Host Material. J. Phys. Chem. C 2012, 116, 19451−19457. (34) Lin, N.; Qiao, J.; Duan, L.; Wang, L.; Qiu, Y. Molecular Understanding of the Chemical Stability of Organic Materials for OLEDs: A Comparative Study on Sulfonyl, Phosphine-Oxide, and Carbonyl-Containing Host Material. J. Phys. Chem. C 2014, 118, 7569−7578. (35) Hong, M.; Ravva, M. K.; Winget, P.; Brédas, J.-L. Effect of Substituents on the Electronic Structure and Degradation Process in Carbazole Derivatives for Blue OLED Host Materials. Chem. Mater. 2016, 28, 5791−5798. (36) Yook, K. S.; Lee, J. Y. Organic Materials for Deep Blue Phosphorescent Organic Light-Emitting Diodes. Adv. Mater. 2012, 24, 3169−3190. (37) Ondrey, G. S.; Bersohn, R. Photodissociation Dynamics of 1,3,5Triazine. J. Chem. Phys. 1984, 81, 4517. (38) Gejo, T.; Harrison, J. A.; Huber, J. R. Three-Body Photodissociation of 1,3,5-Triazine. J. Phys. Chem. 1996, 100, 13941−13949. (39) Osamura, Y.; Unno, M.; Hashimoto, K. Theoretical Study of the Photodecomposition of s-Triazine. J. Am. Chem. Soc. 1987, 109, 1370− 1372. (40) Dyakov, Y. A.; Mebel, A. M.; Lin, S. H.; Lee, Y. T.; Ni, C.-K. Photodissociation of 1,3,5-Triazine: An Ab Initio and RRKM Study. J. Phys. Chem. A 2007, 111, 9591−9599. (41) Savee, J. D.; Mozhayskiy, V. A.; Mann, J. E.; Krylov, A. I.; Continetti, R. E. The Role of Excited-State Topology in Three-Body Dissociation of sym-Triazine. Science 2008, 321 (5890), 826−830. (42) Eyring, H. The Activated Complex in Chemical Reactions. J. Chem. Phys. 1935, 3, 107−115. (43) Laidler, K.; King, C. Development of Transition-State Theory. J. Phys. Chem. 1983, 87 (15), 2657−2664. K


Article

The Journal of Physical Chemistry C (65) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. J.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S.; et al. General Atomic and Molecular Electronic Structure System. J. Comput. Chem. 1993, 14, 1347−1363. (66) Granovsky, A. A. An Efficient Approach to Compute StateSpecific Nuclear Gradients for a Generic State-Averaged MultiConfiguration Self Consistent Field Wavefunction. J. Chem. Phys. 2015, 143, 231101. (67) An, Z.; Zheng, C.; Tao, Y.; Chen, R.; Shi, H.; Chen, T.; Wang, Z.; Li, H.; Deng, R.; Liu, X.; et al. Stabilizing Triplet Excited States for Ultralong Organic Phosphorescence. Nat. Mater. 2015, 14, 685−690. (68) Song, D.; Zhao, S.; Luo, Y.; Aziz, H. Causes of Efficiency RollOff in Phosphorescent Organic Light Emitting Devices: Triplet-Triplet Annihilation Versus Triplet-Polaron Quenching. Appl. Phys. Lett. 2010, 97, 243304. (69) Lei, G.; Wang, L.; Qiu, Y. Improved Performance of Electrophosphorescent Organic Light-Emitting Diode by Graded Doped Emissive Layer. Jpn. J. Appl. Phys. 2004, 43, L1226. (70) Chin, B. D.; Lee, S.-H.; Kim, J. K.; Lee, C. H. Role of Carrier Mobility, Exciton Diffusion, and Their Interplay for Charge Balance and Improved Properties of Organic Electrophosphorescent Device. Proc. SPIE 2006, 633315. (71) Liu, Z.; Helander, M. G.; Wang, Z.; Lu, Z. Highly Efficient Two Component Phosphorescent Organic Light-Emitting Diodes Based on Direct Hole Injection into Dopant and Gradient Doping. Org. Electron. 2013, 14, 852−857.

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