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Rapid Multiscale Computational Screening for OLED Host Materials Daniel Sylvinson M. R., Hsiao Fan Chen, Lauren Martin, Patrick J. G. Saris, and Mark E Thompson ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16225 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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ACS Applied Materials & Interfaces

Rapid Multiscale Computational Screening for OLED Host Materials

Daniel Sylvinson M. R., Hsiao-Fan Chen, Lauren M. Martin, Patrick J. G. Saris, Mark E. Thompson* Department of Chemistry, University of Southern California, Los Angeles, California 90089, USA

Keywords: high-throughput screening, materials discovery, electroluminescence, OLED, phosphorescence, phenanthro[9,10-d]imidazole, phenanthro[9,10-d]triazole

Abstract The design of new host materials for phosphorescent OLEDs is challenging because several physical property requirements must be met simultaneously. A triplet energy (ET) higher than that of the chosen emitting dopant, appropriate HOMO/LUMO energy levels, good charge carrier-transport, and high stability are all required. Here, computational methods were used to screen structures to find the most promising candidates for OLED hosts. The screening was carried out in three Tiers. The Tier 1 selection, based on Density Functional Theory calculations identified a set of eight molecular structures with ET > 2.9 eV, suitable for hosting blue phosphorescent dopants such as iridium(III)bis((4,6-di-fluorophenyl)-pyridinato-N,C2’)picolinate (FIrpic). Phenanthro[9,10-d]imidazole was chosen as the starting point for the Tier 2 selection. Thirty-seven unique molecular structures were enumerated by isoelectronic nitrogen transmutation of

up

to

two

CH

fragments

* Corresponding author: [email protected]

of

the

phenanthrene.

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Three

molecules,

i.e.

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imidazo[4,5-f]-phenanthrolines with nitrogens at the 1,10-, 3,8- and 4,7-positions, were selected for Tier 3, which involved the use of molecular dynamics simulations and electron coupling calculations to predict differences in charge transport between the three materials. The three were explored experimentally through synthesis and device fabrication. The singlet, triplet, and frontier orbital energies computed using single molecule DFT calculations (tiers 1 and 2) were consistent with the experimental values in fluid solution, and the multi-scale modeling scheme (tier 3) correctly predicted the poor device performance of one material. We conclude that screening host materials using only single molecule quantum mechanical data was not sufficient to predict whether a given material would make a good OLED host with certainty, however they can be used to screen out materials that are destined to fail due to low singlet/triplet energies or a poor match of the frontier orbital energies to the dopant or transport materials.

Introduction Improvements in organic light emitting diode (OLED) technology for both display and white lighting applications have been realized through the development of both new organic materials and device architectures. Since the first report of heterojunction and doped OLEDs in the late 1980’s,1 significant strides have been made to improve the overall performance of red, green, blue, and white OLEDs. The development of phosphorescent materials capable of harvesting both singlet and triplet electro-generated excitons has yielded maximum internal quantum efficiencies of 100 %.2-6 Devices with effective host-guest emissive layers and supporting transport materials have enabled stable and efficient phosphorescent OLEDs.5-10 Among the important design criteria for blue OLED host materials are high triplet energy, appropriate HOMO/LUMO energy level alignment with respect to the emissive dopant, balanced carrier transport, and robust molecular structure. Finding host materials that satisfy all of these 2 ACS Paragon Plus Environment

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criteria for a given phosphorescent dopant is complex, because these properties are not usually independently tunable with molecular structure. The common approach of synthesizing and characterizing a large number of materials in the hope of uncovering satisfactory candidates is both time consuming and challenging. Here we discuss exploring and screening a chemical space using high-throughput computing as an alternative to the restrictive and time-consuming Edisonian approach. High-throughput computing can accelerate the materials discovery process by allowing for selection of promising structural fragments and elimination of structural moieties that do not meet minimum thresholds for key host parameters before time is spent in the laboratory to prepare and test them. Such an approach has been used extensively in biology, medicine, and catalysis but has only recently been reported for molecular optoelectronic materials.11-16 A recent study reported by the Aspuru-Guzik group describes computationally screening a library of thermally assisted delayed fluorescence (TADF) emitting dopants for OLEDs with an initial size of 1.6 million candidates.15 This study led to a set of 20 TADF based emitters that gave high electroluminescent efficiency. Single molecule modeling studies are well suited to probe emissive dopants for OLEDs, since they are dispersed in the host matrix in the OLED, largely eliminating dopant-dopant interactions. The study presented here is focused on using computational methods to screen host materials for OLEDs, where intermolecular interactions may play a significant role in the properties of the material. Our strategy is to explore a diverse set of candidate structures through a Tiered, multi-scale computational approach by understanding structure-property relationships at each Tier. The conceptual starting point of our search for high triplet energy host materials is triphenylene, which has a triplet energy in the deep-blue range (ET = 425nm, 2.9 eV) as well as a large aromatic system, leading to efficient charge transport.17-18 Additionally, the rigid tetracyclic

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structure inhibits one potential OLED degradation pathway involving exocyclic bond cleavage of the host material, as is widely suspected for the standard phenylcarbazole derivatives.19-20 Due to the synthetic challenges and structural limitations inherent to the synthesis of regiospecifically substituted heterotriphenylenes,21 this report explores a related but more synthetically accessible class of molecular structures in which one of the 6 membered rings of triphenylene is replaced with a 5 membered heterocyclic ring, which can readily be formed from orthoquinone precursors (Scheme 1). This phenanthroheterole based structure is referred to as H2P, due to its two peripheral hexagonal “H-rings” and one pentagonal “P-ring”. The initial screening of candidate materials was carried out with Density Functional Theory (DFT) modeling of the electronic properties of a number of structures with different heterocyclic P-rings (Scheme 1) to identify those with high triplet energies and frontier orbital energies suitable for hosting our desired sky blue dopant phosphor, i.e. iridium(III)bis((4,6-di-fluorophenyl)pyridinato-N,C2’)picolinate (FIrpic). The primary requirement for hosting FIrpic is a triplet energy greater than 2.7 eV22 in order to prevent luminescence quenching by triplet energy transfer. The first Tier of our screening process examined a range of P-ring substitutions to find the heteroatom substitutions that gave the highest triplet energy. A second Tier of screening started with imidazole for the P-ring and involved nitrogen incorporation into the H-rings, leading to the choice of three promising organic host materials to be prepared and tested in FIrpic based phosphorescent OLEDs. A third Tier of screening searched for high charge carrier mobilities by modeling the solid-state properties of the three materials through molecular dynamics (MD) simulations. Computationally inexpensive electron coupling dimer splitting calculations were used to predict charge transport properties from MD simulations, revealing poor electron mobility predicted for one of the three materials. Although our gas phase DFT predictions (Tiers 1 and 2)


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and experimental measurements of the physical properties of the three materials showed them to be very similar, OLEDs prepared with each as a host for a blue phosphorescent dopant gave different performances, as predicted by Tier 3, with external efficiencies ranging from 0.6 % to 9 %.

Experimental Section Quantum mechanics All QM calculations reported in this work were performed using the Materials Science Suite23 of programs developed by Schrödinger LLC. on a 64-core workstation. Density functional theory was used for ground state optimization and estimation of HOMO/LUMO energies, T1 (triplet) state energy and hole/electron reorganization energies (O+, O-) for the molecules. Hole/electron reorganization energies were calculated per the Equations 1 and 2. ߣା ൌ ሺ‫ܧ‬ା଴ െ ‫ܧ‬଴଴ ሻ ൅ ሺ‫ܧ‬଴ା െ ‫ܧ‬ାା ሻ

ሺͳሻ

ߣି ൌ ሺ‫ିܧ‬଴ െ ‫ܧ‬଴଴ ሻ ൅ ሺ‫ܧ‬଴ି െ ‫ ିିܧ‬ሻ

ሺʹሻ

In the above expressions, the superscripts represent the state of the molecule (0, +, and – for neutral, anionic and cationic states respectively) while subscripts indicate the state at which the structure is optimized. For instance, according to this terminology ‫ܧ‬ା଴ represents the energy of the neutral ground state at the cation-optimized geometry. Calculations involving cationic, anionic and the triplet state were performed using the unrestricted Kohn-Sham DFT scheme. Excited singlet state calculations were performed using Time-dependent DFT (TD-DFT). DFT calculations in Tier 1 and Tier 2 were performed using B3LYP/MIDIX and B3LYP/LACV3P** levels for screening, respectively. All DFT calculations were implemented using Schrödinger’s Jaguar24-25 program. Canvas26 was used to visualize and sort results between 5 ACS Paragon Plus Environment


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Tiers.

Molecular Dynamics/Electron coupling calculations All MD simulations reported in this work were carried out using the Desmond27 module in the Materials Science Suite on a GPU workstation. The following multi-stage MD simulation protocol was implemented for each of the molecular systems reported in this work: A disordered system of 125 molecules in the simulation box (with periodic boundary conditions) was prepared using the disordered system builder facility available within Schrodinger’s Materials Science Suite23 following which a series of six short MD simulations of 1.2 ns each were performed in the canonical (NVT) ensemble with the temperature between two consecutive stages stepped up by 100 K increments starting from 300 K to 800 K for the final stage. Following the NVT stages, a 30 ns long MD simulation was carried out in the isothermal-isobaric (NPT) ensemble at a fixed temperature and pressure of 300 K and 1.01 atm. In order to expedite the simulation process. Post simulation trajectory analysis on the last 6 ns of the NPT stimulation was performed to log density. The standard deviation of the calculated density was found to be less than 0.5 % of the averaged densities calculated for all the systems ensuring convergence. The OPLS-2005 force-field28 available in the Materials Science Suite was used for all the simulations. In order to calculate the hole/electron hopping rates, charge coupling calculations were performed on the MD equilibrated structures. The hopping rates (ߢ) for charge transfer between two molecules can be calculated using Marcus theory, Equation 3. ߢൌ

ଶ ሺߣ ൅ ߂‫ܩ‬ሻଶ Ͷߨ ଶ ‫ܪ‬௔௕ ቆെ ቇ ݄ ඥͶߨߣ݇஻ ܶ Ͷߣ݇஻ ܶ

ሺ͵ሻ

Here, λ, Hab, ݇஻ and T denote the hole/electron reorganization energy, intermolecular 6 ACS Paragon Plus Environment

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coupling parameter, Boltzmann constant and temperature respectively. ߂‫ ܩ‬is the free energy difference for the charge transfer process which is equal to the enthalpy difference for the process in this case. It should be noted that the reorganization energy, λ is made up of two terms: innersphere reorganization energy (λin) accounting for the change in energy caused by geometric relaxation of a molecule upon addition/removal of an electron and outer-sphere reorganization energy (λout) accounting for relaxation/polarization of the surrounding medium which is expected to be much smaller in rigid solids and is hence neglected in the computation of mobility in most cases. Additionally, λout is more tedious to compute than λin which can be easily derived from gas phase DFT calculations (as described in the methods section). Therefore, in this work λout is neglected and λ in equation (3) becomes λin. The hopping rates are computed by two different approaches in this study. In the first approach, Hab is approximated as half the difference between ଵ

ି the LUMO and LUMO+1 energies of the neutral dimer for electron transfer, ‫ܪ‬௔௕ ൎ ሺ‫ܧ‬௅௎ெைାଵ െ ଶ

‫ܧ‬௅௎ெை ) and similarly half the difference between HOMO and HOMO-1 energies of the dimer for ଵ

ା hole transferǡ ‫ܪ‬௔௕ ൎ ሺ‫ܧ‬ுைெை െ ‫ܧ‬ுைெைିଵ ). These dimer frontier orbital splittings are a measure ଶ

of the intermolecular coupling for the electron and hole transfer processes and are obtained from single point DFT calculations on all dimer pairs in the MD equilibrated structure within a closest approach distance of 4Å between them. This approach is commonly referred to as the EnergySplitting-in-Dimer method in literature29-40. Using the dimer frontier orbital splitting as a surrogate for the coupling parameter vastly reduces computational cost compared to more rigorous treatments, such as Constrained-DFT (CDFT) based approaches. It should be noted that in this approach, the free energy change for the process is not computed and set to zero for all the dimer pairs. It has been shown that this approach can lead to an overestimation of the coupling parameter for non-symmetrical cases and does not account for variation in on-site energies, so while being 7 ACS Paragon Plus Environment


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computationally very inexpensive, it is clearly not the best method to estimate coupling41. In order to confirm the validity of this rather crude approach in describing the charge transport properties of the systems under study, a more sophisticated treatment based on CDFT was implemented42-44. In this approach, Hab is taken as the coupling matrix element between the wavefunction of the initial state where the electron/hole is localized on one of the monomers and the final state where the electron/hole is localized on the other monomer. ߂‫ ܩ‬is approximated as just the computed energy difference between the two states. CDFT is used to localize the electron/hole onto just one monomeric unit. To include electrostatic effects on the charge transfer process from the neighboring molecules, the CDFT calculations were performed with each dimer embedded in a field of atomic point charges obtained from the partial charges (charges from the OPLS2005 Force Field) of the atoms in the neighboring molecules within 4Å from the dimer. The CDFT coupling calculations were performed on 80 randomly selected dimer pairs that are within a distance of 4Å apart. Both the CDFT and dimer frontier orbital splitting coupling calculations were carried out at the B3LYP/LACV3P** level.

Synthesis All starting materials were purchased from Sigma Aldrich and used as received, with the exception of 3,8-phenanthroline-5,6-dione45and 4,7-phenanthroline-5,6-dione46 which were prepared by known procedures. Yields are reported as recovered yield after sublimation. General procedure for synthesis of 1H-imidazo[4,5-f]phenanthrolines: A 50 mL round bottom flask was charged with dione (1 eq.), pivaldehyde (1 eq.), aniline (1.2 eq.), ammonium acetate (10 eq.), and acetic acid. The flask was fitted with a reflux condenser and placed in an oil bath at 130 °C. The reaction mixture was allowed to reflux for 3 h with


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magnetic stirring under ambient atmosphere.

The mixture was allowed to cool to room

temperature and poured into 100 mL 1 M aqueous NaOH. The resulting precipitate was collected by filtration. The precipitate was purified by silica column chromatography (hexane : ethyl acetate 1:3) followed by sublimation at 10-6 torr. 2-(tert-butyl)-1-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline

(10d):

1,10-phenanthroline-5,6-dione (630 mg, 3 mmol), pivaldehyde (258 mg, 3 mmol), aniline (335 mg, 3.6 mmol), ammonium acetate (2.31 g, 30 mmol), and acetic acid (15 mL) were used. Yield 755 mg, 71%. 1H NMR (400 MHz, CDCl3) δ 9.12 (dd, J = 4.1 Hz, J = 1.8 Hz, 1H), 9.05 (dd, J = 7.9 Hz, J = 2.0 Hz, 1H), 8.95 (dd, J = 4.5 Hz, J = 1.8 Hz, 1H), 7.72-7.61 (m, 4H), 7.56-7.61 (m, 2H), 7.61 (dd, J = 8.5 Hz, J = 4.4 Hz, 1H), 6.90 (dd, J = 8.5 Hz, J = 1.8 Hz, 1H), 1.39 (s, 9H);

13

C

NMR (100 MHz, CDCl3) δ 160.50, 148.55, 147.35, 144.63, 144.08, 139.19, 134.17, 130.39, 130.37, 130.06, 129.74, 127.52, 127.32, 123.95, 123.23, 121.92, 119.95, 35.52, 30.53. CHNS: calculated [C, 78.38; H, 5.72; N, 15.90] found: [C, 78.94; H, 5.83; N, 15.25] 2-(tert-butyl)-1-phenyl-1H-imidazo[4,5-f][3,8]phenanthroline

(10c):

3,8-phenanthroline-5,6-dione (1.5 g, 7.14 mmol), pivaldehyde (615 mg, 7.14 mmol), aniline (798 mg, 8.56 mmol), ammonium acetate (5.5 g, 71.4 mmol), and acetic acid (36 mL) were used. 1

H NMR (400 MHz, CDCl3) δ 10.13 (d, J = 0.8 Hz, 1H), 8.81 (d, J = 5.8 Hz, 1H), 8.62 (d, J = 5.8

Hz, 1H), 8.42 (d, J = 5.8 Hz, 1H), 8.35 (d, J = 5.8 Hz, 1H), 8.08 (d, J = 0.8 Hz, 1H), 7.74-7.64 (m, 3H), 7.59-7.54 (m, 2H), 1.43 (s, 9H); 13C NMR (100 MHz, CDCl3) 160.87, 147.08, 144.57, 143.96, 143.59, 139.27, 135.00, 131.49, 130.64, 130.54, 130.32, 129.51, 128.33, 123.13, 119.98, 117.35, 116.28, 35.58, 30.53. CHNS: calculated [C, 78.38; H, 5.72; N, 15.90] found: [C, 77.49; H, 5.86; N, 15.01] 2-(tert-butyl)-1-phenyl-1H-imidazo[4,5-f][4,7]phenanthroline

(10b):



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4,7-phenanthroline-5,6-dione (1.05 g, 5 mmol), pivaldehyde (438 mg, 5 mmol), aniline (558 mg, 6 mmol), ammonium acetate (3.85 g, 50 mmol), and acetic acid (25 mL) were used. Yield 640 mg, 36%. 1H NMR (400 MHz, CDCl3) δ 9.14 (dd, J = 4.5 Hz, J = 1.6 Hz, 1H), 8.87 (dd, J = 8.2 Hz, J = 1.8 Hz, 1H), 8.83 (dd, J = 8.2 Hz, J = 1.8 Hz, 1H), 8.43 (dd, J = 4.4 Hz, J = 1.6 Hz, 1H), 7.60-7.51 (m, 4H), 7.49-7.43 (m, 2H), 7.35 (dd, J = 8.3 Hz, J = 4.3 Hz, 1H), 1.45 (s, 9H); 13C NMR (100 MHz, CDCl3) 161.36, 150.59, 148.70, 143.15, 140.22, 140.03, 138.11, 131.84, 130.91, 130.81, 129.76, 128.89, 128.48, 122.74, 122.16, 120.18, 119.54, 35.66, 30.67. CHNS: calculated [C, 78.38; H, 5.72; N, 15.90] found: [C, 78.39; H, 5.74; N, 15.36] Equipment UV-visible spectra were recorded on a Hewlett-Packard 4853 diode array spectrometer. Photoluminescence spectra were measured using a QuantaMaster Photon Technology International phosphorescence/fluorescence spectrofluorometer. Quantum yield measurements were carried out using a Hamamatsu C9920 system equipped with a xenon lamp, calibrated integrating sphere and model C10027 photonic multi-channel analyzer (PMA). Photoluminescence lifetimes were measured by time-correlated single-photon counting using an IBH Fluorocube instrument equipped with an LED excitation source. UV-visible spectra were recorded in dichloromethane and all the other photophysical measurements were carried out in 2-methyltetrahydrofuran (2-MeTHF). NMR spectra were recorded on Varian 500 and Varian 400 NMR spectrometers and referenced to the residual solvent resonance. Electrochemistry Cyclic voltammetry and differential pulse voltammetry were performed using a VersaSTAT 3 potentiostat. Anhydrous DMF (Aldrich) as used as the solvent under inert atmosphere and 0.1 M tetra(n-butyl)-ammonium hexafluorophosphate (TBAF) was used as the supporting electrolyte. A


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silver wire was used as the pseudo reference electrode, and a platinum wire was used as the counter electrode. A glassy carbon rod was used as the working electrode. The redox potentials are based on the values measured using differential pulsed voltammetry and are reported relative to ferrocenium/ferrocene (Cp2Fe+/Cp2Fe) redox couple used as an internal reference while electrochemical reversibility was confirmed by cyclic voltammetry. OLED fabrication and testing OLEDs were fabricated on pre-patterned ITO coated glass substrates (20 ± 5 Ω cm−2, Thin Film Devices, Inc.). Prior to organic deposition the substrates were cleaned by subsequent washes in tergitol solution, water, and acetone followed by a 10 minute UV-ozone treatment. All organic materials were sublimed by thermal gradient sublimation in a 3-zone furnace. Organic layers were deposited by vacuum thermal evaporation at deposition rates of 1-2Å/s at chamber pressures of 10-7 to 10-6 Torr in an EVO Vac 800 deposition system from Angstrom Engineering. Aluminum cathodes were deposited through a shadow mask in a cross bar structure defining device areas of 4mm2. OLED current-power and current-voltage curves, under applied forward bias of 0-12 V, were measured using a Keithley power source meter model 2400, a Newport multi-function optical meter model 1835-C, a low power Newport silicon photodiode sensor model 818-UV and a fiber bundle (used to direct the light into the photodiode). Electroluminescence spectra of the OLEDs were collected with a Photon Technology International QuantaMaster model C-60 fluorimeter at several voltages, between 3-11 V, to ensure emission characteristics remained constant.

Results and Discussion Molecular search strategy For our study, we chose to search for host materials for the well-studied sky blue dopant 11 ACS Paragon Plus Environment


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FIrpic. The choice of this phosphorescent dopant sets limits on the electronic properties for the host material. To achieve a high luminance efficiency for a FIrpic dopant, the triplet energy of the host must be > 2.65 eV. In order to use the FIrpic doped host material in an efficient OLED stack the LUMO of the host needs to be near or above that of FIrpic, so the desired LUMO energies of the host are > -2.1 eV.

Thus, we will use these two criteria to identify the most promising

compounds in our computational searching of the H2P structure space to carry into experimental study.

Isoelectronic,

heteroatomic

transmutations

of

CH

fragments

of

the

parent

cyclopentaphenanthrene structure (Scheme 1) have qualitatively different effects whether they be in the P- or H- rings. For neutral aromatic structures, the H-rings may include pnictogens (only N was considered), while the P-ring may also include chalcogens (O and S were considered). Because the chalcogens, which provide a large chemical space with potentially desirable properties, are only available to the P- ring, our structure search was broken into two Tiers: Tier 1 selection (Figure 1) focused on the heteroatoms of the P-ring heterocycle, while Tier 2 focused on aza substitution in the H-ring, using the best candidates identified in Tier 1. Tier 1 identified the candidates with triplet energies over 2.7 eV, based on a FIrpic dopant. Furthermore, the selection strategy in Tier 1 involved maximizing the LUMO energy, rather than optimizing it based on the LUMO of FIrpic. This strategy anticipates the aza substitution in Tier 2 which will categorically stabilize LUMO energies, so a destabilized LUMO energy from Tier 1 allows room for tunability through the desired range of LUMO energies in Tier 211. The Tier 1 selection involved a survey of 15 P-ring heterocycles and the Tier 2 selection involved 37 unique H-ring aza substitution patterns. By carrying out the selection processes serially, choosing the best candidate in Tier 1 to carry into Tier 2, the number of structures that needed to modeled dropped from 555 (15 * 37), corresponding to all H-ring substitutions for each


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P-ring structure, to 52 (15 + 37) calculations necessary to survey the optimal chemical space of H2P structures. Tier 1 Selection The first Tier selection involved incorporation of oxygen, sulfur and nitrogen for X into the P-ring of the H2P framework to produce the library of chemically relevant structures shown in Figure 1. DFT calculations were performed at the B3LYP/MIDIX level for screening, which is a low level of theory suited for rapid screening. The triplet energies of S and O substituted P-rings were lower than that of N-substituted compounds, with non-chalcogen containing compounds 10-15 having the highest triplet energies. The highest triplet energies were predicted for triazoles 12 and 13, which, while are promising candidates, fail to meet the Tier 1 requirement of maximized LUMO energy. Tier 2 N-substitutions of 12 and 13 would likely result in LUMO energies below the desired range. The target structures were therefore 5, 6, and 10, since they present both high triplet and less negative LUMO energies. Compound 10 was selected because it and substituted versions of it can be readily prepared from phenanthrene-9,10-dione or its aza-substituted analogs, as illustrated in Scheme 2.

It is noteworthy that while 6,6,6,5-membered tetracyclic

imidazo[4,5-f]-1,10-phenanthroline

derivatives

have

been

investigated

as

ligands

in

phosphorescent emitters for OLEDs,47-49 they have never been used as host materials for either fluorescent or phosphorescent OLEDs. Anticipating a crystalline rather than glassy morphology of vapor deposited films of planar 10, as well as potentially recalcitrant synthetic preparations, substituent functional groups were chosen for the imidazole before Tier 2 selection. When R1 = phenyl, the singlet and triplet energies are markedly red shifted, due to conjugation of the phenyl group with the imidazole ring. For example, when R1 = R2 = H, the S1/T1 energies are predicted by DFT to be 3.54/2.96 eV, while when R1 = Ph, R2 = H the S1/T1 energies are predicted to be



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3.33/2.56 eV. Thus, R1 = H or alkyl is preferred for high triplet energy, however, in synthetic trials we found that when R1 is methyl, ethyl, or isopropyl, the reaction does not give 10, but the corresponding alkylidine 2H-imidazole compound instead (Scheme 2 far right). The derivative with R2 = tert-butyl derivative is well behaved due to the lack of α-protons, giving exclusively the desired imidazo-phenanthrene and is therefore the best choice in terms of S1/T1 energies as well as stability and solubility of the compound. In contrast to R1 = phenyl, when R2 is phenyl steric interactions force the aromatic ring out of conjugation, so the orbital and excited state energies are largely unaffected, e.g. R1 = H, R2 = Ph give S1/T1 energies of 3.30/2.89 eV (based on DFT calculations). Tier 2 Selection The Second Tier selection explored nitrogen substitution in the H-rings of 10. Incorporating 0, 1 or 2 nitrogens into the H-rings of the parent phenanthro[4,5-f]imidazole gives a total of 37 unique aza-substituted compounds (see the Electronic Supporting Information, ESI). For the Tier 2 screening, we chose to carryout DFT calculations on the 37 aza-substitutions of 10 with R1 = tert-butyl and R2 = phenyl using the B3LYP hybrid functional and LACV3P** basis set. In Tier 2 we chose a markedly larger basis set than was used in Tier 1, so direct comparisons of theoretically predicted properties to experimentally determined values will be meaningful.

In these

calculations, we predicted a number of parameters, including singlet and triplet energies, as well as molecular orbital energies and surfaces for each of the 37 molecules. The number of nitrogens and the site of nitrogen substitution in the H-rings markedly affects the electronic properties of the H2P molecules. For a complete listing of the parameters predicted in the Tier 2 calculations see the ESI. Figure 2 plots two of the calculated Tier 2 properties: LUMO energy vs. triplet energy. As expected,11,

50

the LUMO is stabilized with nitrogen substitution in the H-ring, shifting


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from -1.0 eV for 10a to a range of -1.2 to -1.4 for a single nitrogen to -1.4 to -1.9 for two nitrogens, with the site of nitrogen substitution markedly affecting the degree of stabilization of the LUMO. The greatest stabilization is seen for substitution in the 3- and 8-postions and the least for substitution at the 2- and 9-positions. The larger range of LUMO energies seen for the molecules with two nitrogens is due to an additive effect when both nitrogens are in a single H-ring. The ortho, meta, and para derivatives together give LUMO energies within a range of roughly 0.2 eV and an average of -1.7 eV, while the materials with a single nitrogen in each H-ring give the same range of LUMO energies, but with an average LUMO energy of -1.55 eV. The site of nitrogen substitution also effects the triplet energy, but not in the same manner as it does the LUMO energies. Substitution of a single nitrogen into the two H-rings, two nitrogens meta in a single H-ring or a single nitrogen in both H-rings gives a minimal change in the triplet energy of the molecule, relative to the unsubstituted compound 10a. In contrast, substitution of two nitrogens into a single H-ring in either an ortho or para disposition lowers the triplet energy substantially below 2.8 eV in nearly every case (the exception has N in the 2,3-positions). The reason for the marked red shift in the ortho- and para-substituted derivatives is a filled-filled interaction51 of the two nitrogen lone pairs in these compounds, leading to a marked destabilization of the out of phase combination of the lone pair orbitals and a resultant narrowing of the HOMO-LUMO gap. This interaction of filled non-bonding orbitals is not seen for the meta-substituted derivative or those with a single nitrogen in each H-ring. The Tier 2 screen also included estimation of the electron and hole reorganization energies for each of the H2P molecules. Reorganization energies are useful parameters to evaluate the kinetics of intermolecular hole and electron hopping and assess charge carrier conduction. A lower reorganization energy reduces the barrier to carrier hopping between molecules in the thin film



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and efficient carrier conduction is more favorable for a given material. The compounds in the Tier 2 screen ranged from low to moderate reorganization energies for holes (0.24-0.30 eV) and electrons (0.18-0.40 eV), suggestive of efficient carrier transport in the H2P family (see the ESI for a full listing). We chose to focus on three imidazo[4,5-f]phenanthroline derivatives 10b, 10c and 10d for characterization and study in blue OLEDs due to their high triplet energies and favorable reduction potentials. Additionally, these three materials have symmetric nitrogen substitution patterns in the H-rings, so only a single regioisomer will be formed for each derivative by our synthetic approach. Figure 3 shows the DFT optimized ground state geometries along with the HOMO and LUMO orbitals for 10a-10d. The orbital density distribution of the HOMOs and LUMOs for 10b-10d are very similar. The LUMOs are localized on the phenanthroline while the HOMOs involve both the phenanthroline and imidazole fragments. The addition of tert-butyl and phenyl groups have a negligible effect on the composition of the HOMO or LUMO of these compounds. For comparison, the HOMO and LUMO are shown for 10a as well. The HOMO matches that seen for 10b-10d, but the LUMO of 10a is localized on the phenyl group, rather than the phenanthroline, due to the relative difficulty of reducing phenanthrene compared to phenanthroline. The LUMO+1 orbital of 10a is a good match to the LUMO orbital of 10b-10d. Nitrogen substitution in the H-rings stabilizes the phenanthrene system, such that the phenanthroline orbitals fall below the phenyl based S-orbitals for 10b-10d.

Synthesis and Physical Characterization of H2P Host Materials Compounds 10b-10d were synthesized in 60-80 % yields as illustrated in Scheme 2, where the phenanthrene-dione precursor is replaced with the appropriate phenanthroline-dione. The H2P compounds exhibit intense π-π* absorptions between 250 and 290 nm. A weak tail from 320 to 16 ACS Paragon Plus Environment

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400 nm was observed for 10b and 10d, whereas a more intense and distinct absorption is seen for 10c in the same spectral region. The absorption features at longer wavelengths suggest a degree of charge transfer character, which is strongest for 10c. Solutions of 10b, 10c, and 10d have emission maxima at 392, 402, and 396 nm, respectively, at room temperature. Singlet excited state energies corresponding to the energy of the short wavelength edge of the emission band (intensity = 0.1ൈOmax) for the three compounds are 3.47, 3.20 and 3.42 eV, respectively. The triplet energies (ET) for 10b, 10c, and 10d at 2.91, 2.76, and 2.83 eV, respectively, were determined from the high energy edge of the phosphorescence spectra measured in 2-MeTHF at 77 K. There is a good correspondence between the calculated and experimental triplet energies. The calculated triplet energies are within 0.05 eV for 10a, 10b and 10d and within 0.13 eV for 10c. Triplet energies obtained from neat solids are lower than those in solution. Triplets in the solid state were found to be ET = 2.65 eV for 10b, 2.47 eV for 10c and 2.62 eV for 10d (Figure 4). The

triplet

energy

depression in the solid state, of approximately 250 meV, for all three materials could not have been predicted from the modeling in Tiers 1 and 2, prompting us to continue with a third Tier (vide infra). The photoluminescent quantum yields ()PL) of 10% FIrpic-doped films are 0.61, 0.40 and 0.20 for 10d, 10b, and 10c, respectively. The photoluminescence efficiency of a FIrpic doped film of 10b-10d will limit the internal quantum efficiency for OLEDs made with each host material. The low )PL for doped films of 10c is due to the low triplet energy of that host, such that significant quenching of FIrpic emission is expected. However, 10b with the highest triplet energy did not provide the highest )PL, likely due to a lower average triplet energy in the solid state than that of 10d, leading to partial quenching of FIrpic emission in 10b. Cyclic voltamograms of 10a-10d showed quasi-reversible reduction waves, however, only 17 ACS Paragon Plus Environment


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10d gives a quasi-reversible oxidation, whereas oxidation of 10b and 10c are irreversible (see Table 1 for potentials). DFT calculations suggest that reduction occurs predominantly on the phenanthroline fragment and oxidation involves the imidazole ring (Figure 3).

The

electrochemical potentials are consistent with these predictions. The imidazole groups are not significantly affected by the nitrogen position in the H-rings, leading to a closely spaced set of oxidation potentials for 10b-10d (1.11 to 1.17 V).

In contrast, the phenanthroline fragment is

significantly affected by the nitrogen position, leading a more disparate set of reduction potentials (-2.25 V to -2.54 V). Placement of nitrogens in the 3-, 8- positions of the phenanthroline gives 10c the lowest reduction potential. HOMO and LUMO levels in Table 1 were calculated from redox potentials using previously published correlations.52-53 The HOMO and LUMO values for 10a-10d were computed using the adiabatic scheme (implemented in the Materials Science Suite) at the B3LYP/LACV3P** level where single-point energies were computed for the cation, anion and neutral species using the finite-element PoissonBoltzmann solver (PBF) continuum solvent model54-55 with DMF as the solvent on the corresponding gas-phase optimized structures. The HOMO/LUMO energies calculated using this procedure are in reasonable agreement with experimental values as seen in Table 1, differing by 0.2-0.3 eV, however, the trends in the values between the four compounds experimentally were reproduced in the theoretical values.

Tier 3 selection The gas phase calculations in Tiers 1 and 2 agree well with the properties of the three host compounds in fluid solution. However, these single-molecule calculations cannot predict the solid-state properties of the bulk materials. The decrease in triplet energy from solution to neat solid (vide supra) prompted us to include a third Tier of modeling to address solid–state properties.


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Because modeling excited state energies in amorphous solids is challenging, and because triplet energy depression were similar for the three materials, attention was turned to charge transport, the critical role of a host material. In order to screen for the performance of 10b-10d as host materials, we carried out theoretical studies of the three materials as amorphous solids using MD and electron coupling DFT calculations.

The search criterion for this Tier 3 screen is a

maximization of charge mobility. MD simulations were performed on a cell of 125 molecules of each host material with periodic boundary conditions in order to model solid state morphologies. The final density for each simulation was between 1.11-1.14 g/cm3. As a first approximation of charge mobility, the number of π-π interactions for 10b, 10c and 10d were counted, with more interactions presumably being favorable for charge transport. Here, the S-S interactions are classified into two types: 1) face-face interactions where two aromatic rings are within a distance of 4.4 Å and a maximum angle of 30q with respect to each other, and 2) edge-face interactions where two aromatic rings are within a distance of 5.5 Å with a minimum subtended angle of 60q between them. The number of S-S contacts includes the total number of face-face and edge-face interactions, giving a total of 324, 409, and 364 for 10b, 10c, and 10d, respectively. Compound 10c has substantially more face-to-face S-contacts than either 10b or 10d, while 10d has more face-to-edge S-contacts than the other two compounds. This preliminary analysis suggests a trend in charge transport as: 10c > 10d > 10b. A similar trend is seen for the center of mass (COM) radial distribution functions (RDF), as shown in Figure 5. The COM RDFs reflect differences in solid state center-to-center distances, which could have an effect on charge transport in the host material. The 10b RDF clearly shows differences in distances in the range of 4-6 Ǻ. There is approximately a 35% and 50% difference in height between the nearest-neighbor peak maximum of the 10b RDF and the nearest-neighbor peak maximum of 10d and 10c RDFs, respectively. This 19 ACS Paragon Plus Environment


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indicates a lower proportion of short center-to-center distances in bulk 10b with respect to the other H2Ps. A plausible explanation for the dissimilar morphologies of 10b from 10c and 10d involves the dipole moments of each molecule. In all three cases the permanent dipole moment for each molecule lies in the H2P plane, however, in 10c and 10d the dipole extends from the imidazole ring into the phenanthroline, while in 10b the dipole is largely within the imidazole ring (Figure 3). In the amorphous solid, the molecules tend to form dimer pairs with adjacent dipoles in a roughly antiparallel orientation. For 10c and 10d, closely spaced dimers can be formed with antiparallel dipoles, however, for 10b overlapping the dipoles of adjacent molecules are sterically hindered by the tert-butyl and phenyl groups on the imidazoles. Complimentary to structural analysis of MD simulations, a quantum mechanical analysis was performed. Dimer frontier orbital splitting electron coupling calculations (see methods section) were performed on the MD equilibrated structure for all dimer pairs within a contact distance of 4Å from each other (note that this is the closest atom-atom distance for any pair, not the center to center distance or radius) amounting to a total of 838, 857, and 880 dimer pairs for 10b, 10c, and 10d respectively). These calculations were used to estimate the charge carrier hopping rates according to equation (3) to assess variations in charge transport between neat host materials. Figure 6 shows histograms of the calculated hole and electron hopping rates. Notably, the distribution of electron hopping rates for 10b is significantly narrower and the rates are slower than those of 10c and 10d. There were no significant differences in hole hopping rates between the three host materials. This suggests that on average the dimer pairs formed for 10b give comparable HOMO-HOMO overlap to those of 10c and 10d, but poorer LUMO-LUMO overlap compared to 10c and 10d.


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While feasible for this study, transport calculations carried out for a large number of dimer pairs of molecules is too time consuming and impractical to be repeated for a large number of different materials. In order to develop a more rapid, high-throughput Tier 3 materials screen, we sought to test the validity of a smaller scale estimate of the hopping rates for each of the materials using a truncated random subset of dimer pairs. Calculations were repeated for twenty random dimers from the exhaustive set of dimer pairs with a spacing of 4 Ǻ or less. The distribution of hopping rates for the smaller sets mirrors what we observed for the exhaustive calculations (Figure 6), suggesting that a less rigorous calculation could be used in the future to compare the range of hoping rates expected for different materials. The calculated hopping rates of 500 randomly selected dimer pairs from the exhaustive dimer splitting coupling calculations of each system were then used to compute charge carrier mobilities (Table 2). The mobilities were also computed for the hopping rates (80 dimers) calculated from the CDFT approach in order to validate the simple dimer-splitting method. A simple transport model proposed by Goddard, et. al56 was used to compute the mobilities. In this model, the carrier mobility is described by the Einstein relation (Equation 4) ߤ௛Ȁ௘ ൌ

݁‫ܦ‬ ݇஻ ܶ

ሺͶሻ

where D, the charge diffusivity is calculated from the hopping rates (ߢ௜ ) obtained from the coupling calculations, Equations 5 and 6. ଵ

‫ ܦ‬ൌ σ௜ ‫ݎ‬௜ଶ ߢ௜ ܲ௜ ଺

఑೔

ܲ௜ ൌ σ

೔ ఑೔

(5) ሺ͸ሻ

The index i runs over all dimer pairs for which coupling calculations were done.



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Both coupling methods predict that the electron mobility of 10b would be significantly lower than that of 10c and 10d. The dimer-splitting method was found to predict a greater disparity in the electron mobility of 10c/10d versus 10b. Also, the hole mobility of 10c is predicted by the CDFT approach is much larger than that of 10b and 10d compared to the dimersplitting method. The reason for this disparity between the two methods may be attributed to the fact that the dimer-splitting approach does not account for the Gibbs free energy change (߂‫ ܩ‬in eqn. 3) associated with the charge-transfer process in different dimer pairs which can be substantial in disordered systems.57-58 It is important to stress that the dimer-splitting method is only useful in qualitative estimates of the coupling, used calculating carrier mobilities to rankorder the members of the structurally similar molecules of a Tier 3 set for this property. The average coupling parameters computed using the two methods are in reasonably good agreement with each other (see SI). The low electron mobility for 10b is consistent with its greater centerof-mass intermolecular spacing leading to a lower average hoping rate for the 500 dimer pairs examined. Tier 3, performed at various levels of theory, predicts that 10b may display poor electron mobility in the solid state and therefore fails to meet the selection criterion. It should be noted that the crude mobility calculations presented above are used here to qualitatively gauge the trends in the transport properties of the candidates and are not expected to be quantitatively accurate. More sophisticated multi-scale methods like Kinetic Monte Carlo (KMC) based approaches among others have been developed to adequately compute mobilities of amorphous organic materials but tend to be more computationally expensive.57-63 After 3 Tiers of screening on a 555 membered structure space, 2 materials were selected: 10c and 10d. For the benefit of validating the above theoretical methods, 10b was also investigated experimentally as an example of a poorly performing material.


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OLED Fabrication and Testing We employed a relatively simple configuration for OLED devices with H2P host materials: ITO/NPD (20 nm)/mCP (5 nm)/H2P: FIrpic (10%, 30 nm)/BCP (50 nm)/LiF (1 nm)/Al (NPD = N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine, mCP = 1,3-di(9H-carbazol-9yl)benzene , BCP = 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline). NPD/mCP functioned as the hole-transporting layers, and BCP was introduced as both hole-blocking and electron-transporting layer (energy level diagram is given in Figure 7a). Figure 7b shows the current density-voltage-brightness (J-V-L) characteristics of these devices. The highest current is observed for the devices based on 10c, with comparable light output at a given voltage observed for 10c and 10d, leading to higher external quantum efficiency (EQE) for 10d than 10c. The maximum EQE values are 0.7 %, 5.0 % and 9.0 %, for devices with 10b, 10c and 10d, respectively. It is useful to compare these OLEDs to FIrpic based OLEDs with a similar architecture, but a conventional host material. N,N’-dicarbazolyl-4,4’-biphenyl (CBP) is a common OLED host with a triplet energy comparable to 10c (2.6 eV) is used as the host in a device architecture very similar to the one used here the peak EQE was reported as 6.1%.64 Shifting to a host material with a triplet energy of 2.9 eV, i.e. mCP, puts the host triplet close to those of 10b and 10d. The FIrpic doped mCP OLED gave a peak efficiency of 7.5%.64 The efficiencies of OLEDs based on 10c and 10d are comparable to those made with similar triplet energy carbazole-based hosts, while the efficiency of 10 based devices fall well short of comparable carbazole-based OLED. In order to understand the source of the differences in device efficiencies, we roughly deconvoluted the EQE into four limiting factors as shown in Equation 7. The photoluminescent quantum yield (ΦPL) was measured for phosphor doped host films and is assumed to be unchanged



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in the device. The usable exciton fraction (χ) takes into account the statistical branching ratio of electro-generated singlets to triplets with respect to the emissive species, which is unity for a phosphorescent dopant such as Firpic. The outcoupling factor (ηe) accounts for losses due to wave-guiding and plasmon absorption, and is usually 0.2-0.3.65 Lastly, the charge recombination factor (ηr) is the ratio of excitons formed to injected charge carrier pairs, sometimes referred to as charge balance. Charge recombination factors that are less than unity are due to differential charge injection or transport between electrons and holes. Table 3 lists the parameters from Equation 7 for each host. The EQE and ΦPL were measured from devices and thin films, respectively. The χ and ηe are assumed to be 1 and 0.2, respectively. The ηr is calculated from the other four parameters.

Ȱ୉୐ ൌ Ȱ୔୐ ߯ߟ୰ ߟୣ

(7)

The highest efficiency is seen for the device with a 10d based emissive layer. The device performance with 10c is noteworthy since the device exhibited an EQE at its theoretical limit. Considering a 20 % )PL and out-coupling of 0.2, the maximum achievable efficiency for the OLEDs is no greater than 4%, assuming that the dopant is isotropically dispersed in the 10c host.66 This suggests near unit efficiency for carrier recombination in these devices. The 10b-based device exhibited the lowest device efficiency among the three hosts despite a 40% QY in a 10% FIrpic-doped film, due to a very low charge recombination factor. The trend in J-V characteristics shows current densities of 10c > 10d > 10b at a given voltage, agreeing well with the Tier 3 screening. Furthermore, the turn-on voltage (voltage at a brightness of 0.1 cd/m2) for devices utilizing 10b is ca. 2 V higher than those for devices with 10c and 10d. The turn-on voltage is tied to both injection barriers and the carrier mobilities. The dependence on carrier mobility is due to the need for both carriers to diffuse into the EML prior to recombination to avoid exciplex 24 ACS Paragon Plus Environment

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formation at the interfaces between the EML and the transport layers. We expect the barriers to charge injection of the three host materials to be the same, so the principal factor controlling the turn-on voltage is likely the carrier mobilities of the devices. The trend in turn-on voltages is consistent with charge carrier mobilities within the devices of 10c, 10d > 10b. By inspection of the energy level diagram (Figure 7a) and the nature of charge transport in other blue OLEDs,67 the dopant (FIrpic) mediates hole transport, while the host is expected to mediate electron transport. To test the predictions of the MD study with regards to electron transport characteristics of the three hosts, we fabricated electron-only devices with a structure of ITO/BCP (10 nm)/H2P (40 nm)/LiF:Al. In this device architecture, holes are blocked so the current passing through the organic layers is purely an electron current. Figure 8 shows the current versus voltage characteristics of the electron-only devices for the three materials. As anticipated, the trend in the J-V characteristics shows current densities of 10c > 10d > 10b at a given voltage, and the device with 10b gave a current that is roughly 3 orders of magnitude lower than those of devices with either 10c or 10d.

Summary: In this paper, we have described a computational approach to screen a family of 6,6,6,5-membered tetracyclic materials for use as host materials in blue emitting phosphorescent OLEDs (PHOLEDs). Using computational methods to accelerate the materials discovery process, we identified the best candidates from a large library of different host structures before preparing them. Synthesizing and physically testing only those candidate materials with a high likelihood of success minimizes the time taken to discover useful materials. The screening of the materials was based on DFT and TDDFT calculations for the candidate molecules in the gas phase and was used to identify the materials that gave the most promising triplet and LUMO energies for blue 25 ACS Paragon Plus Environment


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PHOLEDs. The criteria used to choose the host material also depends on the other materials used to fabricate the blue OLEDs. Here we chose to search for host materials that are optimally designed for a FIrpic dopant, an NPD/mCP hole transporting stack and a BCP electron transporting layer. If it were desirable to change the other materials in the PHOLED, i.e. the emissive dopant or transport materials, the screening process could be reassessed to find the best host material for the new set of PHOLED materials.

It is important to stress that the DFT/TDDFT calculations

would not need to be run again, as the new set of PHOLED materials would simply change the parameters used to screen the database of calculated molecular properties. One could take this process further and consider the materials in this library for another application all together. If one were interested in finding materials for photovoltaic applications for example, a different set of search criteria could be used to screen the same database of calculated properties to find the best candidate materials for that application. In the context of the Tier 1 and Tier 2 screening approach reported here, the Tier 2 screen might need to be repeated with a different candidate or candidates advanced from Tier 1. The screening methods based on gas phase computational modeling allowed us to identify the materials with suitable triplet and LUMO energies to serve as host materials for blue PHOLEDs.

The calculations gave excellent agreement with the experimental parameters

measured in fluid solution.

Note that Tiers 1 and 2 were not sufficient to predict device

performance. Extending our modeling studies of the three synthesized materials into the solid state in Tier 3, using MD simulations and modeling of the resultant equilibrated amorphous solids, predicted the differences in the carrier transport we observed in the materials. While the two host materials identified after Tier 3 screening (10c and 10d) appeared to be well suited as PHOLED host materials, the gas phase modeling studies did not include condensed phase polarization or


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packing effects. We observed a marked red shift in the triplet energy between the gas/solution phase triplet energies to those in the solid state, to the extent that emission from Firpic was either partially (10d) or mostly (10c) quenched. We are currently exploring further multi-scale modeling methods to develop an expanded Tier 3 screen that will predict triplet energies of organic materials in the solid state. With such a multi-scale “solid-state” method in hand we can use a computational approach to accelerate materials discovery of compounds with triplet energies high enough to act as host materials in deep blue PHOLEDs. The total chemical space of materials considered in our Tier 1/Tier 2 screening process was 555 structures, however, it could be expanded to much larger chemical space. Recent reports from other groups have shown a similar computational screening approach used with libraries that are several orders of magnitude larger than the one considered here.12, 14-15 In a study from the AspuruGuzik group, the researchers identified 20 candidate TADF emitters for PHOLEDs from a 1.6 million member library, prepared them and demonstrated external efficiencies as high as 20% for one of them, comparable to some of the best Ir-phosphor based PHOLEDs. It is important to stress that this study involved dopants, which will be present in the OLED at low concentration and thus have no interactions between dopants. This is an important distinction from the thin film host materials considered here, where intermolecular interactions mediate energy and electron transport. Gas phase modeling studies are sufficient to predict PHOLED performance for an emissive dopant, such as a TADF material, but condensed phase studies are needed for materials that will transport charge or excitons in the device. In the present study, we used a multi-scale approach involving MD in conjunction with DFT calculations to model the solid state properties of the three materials that we prepared and studied, however, this would not be practical for libraries with thousands of members or more. We are currently working to develop methods to



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streamline the condensed phase modeling studies, with the goal to rapidly identify new host and transport materials for optoelectronic applications.

Supplemental Materials: Electronic supplementary information (ESI) available: Cyclic voltammetry, list of molecules in Tier 2 library, FMO plots for unsubstituted H2Ps, list of calculated properties for molecules in Tier 2 library and average Hab values computed using the dimer-splitting and CDFT approaches.

Acknowledgements:

This work was supported by grant DE-EE0007077 of the US

Department of Energy, and Universal Display Corporation. The authors would like to thank Schrödinger Inc. for access to their Materials Suite family of software tools, used extensively here, and Dr. Mathew Halls for helpful discussions.

Associated Content: One of the authors (Thompson) has a financial interest in the Universal Display Corporation.


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References: 1. Tang, C. W.; VanSlyke, S. A., Organic Electroluminescent Diodes. Applied physics letters 1987, 51 (12), 913-915. 2. Baldo, M. A.; O'brien, D.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M.; Forrest, S., Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices. Nature 1998, 395 (6698), 151-154. 3. Baldo, M.; Lamansky, S.; Burrows, P.; Thompson, M.; Forrest, S., Very High-Efficiency Green Organic Light-Emitting Devices Based on Electrophosphorescence. Appl. Phys. Lett. 1999, 75, 4. 4. O’brien, D.; Baldo, M.; Thompson, M.; Forrest, S., Improved Energy Transfer in Electrophosphorescent Devices. Appl. Phys. Lett. 1999, 74 (3), 442-444. 5. Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R., Nearly 100% Internal Phosphorescence Efficiency in an Organic Light-Emitting Device. Journal of Applied Physics 2001, 90, 5048-5051. 6. Kwong, R. C.; Nugent, M. R.; Michalski, L.; Ngo, T.; Rajan, K.; Tung, Y.-J.; Weaver, M. S.; Zhou, T. X.; Hack, M.; Thompson, M. E., High Operational Stability of Electrophosphorescent Devices. Appl. Phys. Lett. 2002, 81 (1), 162-164. 7. Tsutsui, T.; Yang, M.-J.; Yahiro, M.; Nakamura, K.; Watanabe, T.; Tsuji, T.; Fukuda, Y.; Wakimoto, T.; Miyaguchi, S., High Quantum Efficiency in Organic Light-Emitting Devices with Iridium-Complex as a Triplet Emissive Center. Jpn. J. Appl. Phys. 1999, 38 (12B), L1502. 8. Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E., High-Efficiency Organic Electrophosphorescent Devices with Tris (2-Phenylpyridine) Iridium Doped into ElectronTransporting Materials. Applied Physics Letters 2000, 77 (6), 904. 9. Adachi, C.; Baldo, M. A.; Forrest, S. R.; Lamansky, S.; Thompson, M. E.; Kwong, R. C., High-Efficiency Red Electrophosphorescence Devices. Applied Physics Letters 2001, 78 (11), 1622-1624. 10. Adachi, C.; Kwong, R.; Forrest, S. R., Efficient Electrophosphorescence Using a Doped Ambipolar Conductive Molecular Organic Thin Film. Org. Electron. 2001, 2 (1), 37-43. 11. Halls, M. D.; Djurovich, P. J.; Giesen, D. J.; Goldberg, A.; Sommer, J.; McAnally, E.; Thompson, M. E., Virtual Screening of Electron Acceptor Materials for Organic Photovoltaic Applications. New Journal of Physics 2013, 15 (10), 105029. 12. Aspuru̺Guzik, A.; Adams, R.; Baldo, M.; Aguilera̺Iparraguirre, J.; Gµmez̺ Bombarelli, R. In 34.4: Invited Paper: Combinatorial Design of Oled̺Emitting Materials, SID Symposium Digest of Technical Papers, Wiley Online Library: 2015; pp 505-506. 29 ACS Paragon Plus Environment


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13. Pyzer-Knapp, E. O.; Suh, C.; Gómez-Bombarelli, R.; Aguilera-Iparraguirre, J.; AspuruGuzik, A., What Is High-Throughput Virtual Screening? A Perspective from Organic Materials Discovery. Annual Review of Materials Research 2015, 45, 195-216. 14. Hachmann, J.; Olivares-Amaya, R.; Atahan-Evrenk, S.; Amador-Bedolla, C.; SánchezCarrera, R. S.; Gold-Parker, A.; Vogt, L.; Brockway, A. M.; Aspuru-Guzik, A., The Harvard Clean Energy Project: Large-Scale Computational Screening and Design of Organic Photovoltaics on the World Community Grid. The Journal of Physical Chemistry Letters 2011, 2 (17), 2241-2251. 15. Gomez-Bombarelli, R.; Aguilera-Iparraguirre, J.; Hirzel, T. D.; Duvenaud, D.; Maclaurin, D.; Blood-Forsythe, M. A.; Chae, H. S.; Einzinger, M.; Ha, D.-G.; Wu, T.; Markopoulos, G.; Jeon, S.; Kang, H.; Miyazaki, H.; Numata, M.; Kim, S.; Huang, W.; Hong, S. I.; Baldo, M.; Adams, R. P.; Aspuru-Guzik, A., Design of Efficient Molecular Organic Light-Emitting Diodes by a HighThroughput Virtual Screening and Experimental Approach. Nat Mater 2016, 15 (10), 1120-1127. 16. Shin, Y.; Liu, J.; Quigley, J. J.; Luo, H.; Lin, X., Combinatorial Design of Copolymer Donor Materials for Bulk Heterojunction Solar Cells. ACS Nano 2014, 8 (6), 6089-6096. 17. Clark, W.; Litt, A. D.; Steel, C., Triplet Lifetimes of Benzophenone, Acetophenone, and Triphenylene in Hydrocarbons. J. Am. Chem. Soc. 1969, 91 (19), 5413-5415. 18. Togashi, K.; Nomura, S.; Yokoyama, N.; Yasuda, T.; Adachi, C., Low Driving Voltage Characteristics of Triphenylene Derivatives as Electron Transport Materials in Organic LightEmitting Diodes. J. Mater. Chem. 2012, 22 (38), 20689-20695. 19. Kondakov, D.; Lenhart, W.; Nichols, W., Operational Degradation of Organic LightEmitting Diodes: Mechanism and Identification of Chemical Products. J. Appl. Phys. 2007, 101 (2), 024512. 20. Schmidbauer, S.; Hohenleutner, A.; König, B., Chemical Degradation in Organic Light̺ Emitting Devices: Mechanisms and Implications for the Design of New Materials. Adv. Mater. 2013, 25 (15), 2114-2129. 21. Saris, P. J. G.; Thompson, M. E., Gram Scale Synthesis of Benzophenanthroline and Its Blue Phosphorescent Platinum Complex. Organic Letters 2016, 18 (16), 3960-3963. 22. Adachi, C.; Kwong, R. C.; Djurovich, P.; Adamovich, V.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R., Endothermic Energy Transfer: A Mechanism for Generating Very Efficient HighEnergy Phosphorescent Emission in Organic Materials. Appl. Phys. Lett. 2001, 79 (13), 20822084. 23.

Materials Science Suite 2016-4, Schrödinger, Llc, New York, Ny, 2016.

24. Bochevarov, A. D.; Harder, E.; Hughes, T. F.; Greenwood, J. R.; Braden, D. A.; Philipp, D. M.; Rinaldo, D.; Halls, M. D.; Zhang, J.; Friesner, R. A., Jaguar: A High-Performance Quantum Chemistry Software Program with Strengths in Life and Materials Sciences. Int. J. Quantum Chem 2013, 113 (18), 2110-2142. 30 ACS Paragon Plus Environment

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25.

Jaguar, Schrödinger, Llc, New York, Ny, 2016.

26.

Canvas, Schrödinger, Llc, New York, Ny, 2016.

27.

Desmond Molecular Dynamics System, D. E. Shaw Research, New York, Ny, 2016.

28. Banks, J. L.; Beard, H. S.; Cao, Y. X.; Cho, A. E.; Damm, W.; Farid, R.; Felts, A. K.; Halgren, T. A.; Mainz, D. T.; Maple, J. R.; Murphy, R.; Philipp, D. M.; Repasky, M. P.; Zhang, L. Y.; Berne, B. J.; Friesner, R. A.; Gallicchio, E.; Levy, R. M., Integrated Modeling Program, Applied Chemical Theory (Impact). J. Comput. Chem. 2005, 26 (16), 1752-1780. 29. Huang, J. S.; Kertesz, M., Intermolecular Transfer Integrals for Organic Molecular Materials: Can Basis Set Convergence Be Achieved? Chem. Phys. Lett. 2004, 390 (1-3), 110-115. 30. Lemaur, V.; Da Silva Filho, D. A.; Coropceanu, V.; Lehmann, M.; Geerts, Y.; Piris, J.; Debije, M. G.; Van de Craats, A. M.; Senthilkumar, K.; Siebbeles, L. D. A.; Warman, J. M.; Bredas, J. L.; Cornil, J., Charge Transport Properties in Discotic Liquid Crystals: A Quantum-Chemical Insight into Structure-Property Relationships. J. Am. Chem. Soc. 2004, 126 (10), 3271-3279. 31. Hutchison, G. R.; Ratner, M. A.; Marks, T. J., Intermolecular Charge Transfer between Heterocyclic Oligomers. Effects of Heteroatom and Molecular Packing on Hopping Transport in Organic Semiconductors. J. Am. Chem. Soc. 2005, 127 (48), 16866-16881. 32. Kwon, O.; Coropceanu, V.; Gruhn, N. E.; Durivage, J. C.; Laquindanum, J. G.; Katz, H. E.; Cornil, J.; Bredas, J. L., Characterization of the Molecular Parameters Determining Charge Transport in Anthradithiophene. J. Chem. Phys. 2004, 120 (17), 8186-8194. 33. Newton, M. D., Quantum Chemical Probes of Electron-Transfer Kinetics - the Nature of Donor-Acceptor Interactions. Chem. Rev. 1991, 91 (5), 767-792. 34. Cornil, J.; Calbert, J. P.; Bredas, J. L., Electronic Structure of the Pentacene Single Crystal: Relation to Transport Properties. J. Am. Chem. Soc. 2001, 123 (6), 1250-1251. 35. Cheng, Y. C.; Silbey, R. J.; da Silva, D. A.; Calbert, J. P.; Cornil, J.; Bredas, J. L., ThreeDimensional Band Structure and Bandlike Mobility in Oligoacene Single Crystals: A Theoretical Investigation. J. Chem. Phys. 2003, 118 (8), 3764-3774. 36. da Silva, D. A.; Kim, E. G.; Bredas, J. L., Transport Properties in the Rubrene Crystal: Electronic Coupling and Vibrational Reorganization Energy. Adv. Mater. 2005, 17 (8), 1072-+. 37. Jordan, K. D.; Paddonrow, M. N., Long-Range Interactions in a Series of Rigid Nonconjugated Dienes .1. Distance Dependence of the Pi+,Pi- and Pi+Star,Pi-Star Splittings Determined by Abinitio Calculations. J. Phys. Chem. 1992, 96 (3), 1188-1196. 38. Paddon-Row, M. N.; Jordan, K. D., Analysis of the Distance Dependence and Magnitude of the Pi+, Pi- and Pi+Asterisk, Pi-Asterisk Splittings in a Series of Diethynyl[N]Staffanes - an Abinitio Molecular-Orbital Study. J. Am. Chem. Soc. 1993, 115 (7), 2952.



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39. Liang, C. X.; Newton, M. D., Abinitio Studies of Electron-Transfer - Pathway Analysis of Effective Transfer Integrals. J. Phys. Chem. 1992, 96 (7), 2855-2866. 40. Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.; Silbey, R.; Bredas, J.-L., Charge Transport in Organic Semiconductors. Chem. Rev. 2007, 107 (4), 926-952. 41. Valeev, E. F.; Coropceanu, V.; da Silva, D. A.; Salman, S.; Bredas, J. L., Effect of Electronic Polarization on Charge-Transport Parameters in Molecular Organic Semiconductors. J. Am. Chem. Soc. 2006, 128 (30), 9882-9886. 42. Wu, Q.; Voorhis, T. V., Extracting Electron Transfer Coupling Elements from Constrained Density Functional Theory. The Journal of Chemical Physics 2006, 125 (16), 164105. 43. Wu, Q.; Van Voorhis, T., Constrained Density Functional Theory and Its Application in Long-Range Electron Transfer. Journal of Chemical Theory and Computation 2006, 2 (3), 765774. 44. Ratcliff, L. E.; Grisanti, L.; Genovese, L.; Deutsch, T.; Neumann, T.; Danilov, D.; Wenzel, W.; Beljonne, D.; Cornil, J., Toward Fast and Accurate Evaluation of Charge on-Site Energies and Transfer Integrals in Supramolecular Architectures Using Linear Constrained Density Functional Theory (Cdft)-Based Methods. J. Chem. Theory Comput. 2015, 11 (5), 2077-2086. 45. Botana, E.; Da Silva, E.; Benet-Buchholz, J.; Ballester, P.; de Mendoza, J., Inclusion of Cavitands and Calix[4]Arenes into a Metallobridged Para-(1h-Imidazo[4,5-F][3,8]Phenanthrolin2-Yl)-Expanded Calix[4]Arene. Angew. Chem. Int. Ed. 2007, 46 (1-2), 198-201. 46. Imor, S.; Morgan, R. J.; Wang, S.; Morgan, O.; Baker, A. D., An Improved Preparation of 4,7-Phenanthrolino-5,6:5͛,6͛-Pyrazine. Synth. Commun. 1996, 26 (11), 2197-2203. 47. Tordera, D.; Pertegas, A.; Shavaleev, N. M.; Scopelliti, R.; Orti, E.; Bolink, H. J.; Baranoff, E.; Graetzel, M.; Nazeeruddin, M. K., Efficient Orange Light-Emitting Electrochemical Cells. J. Mater. Chem. 2012, 22 (36), 19264-19268. 48. Zhang, F.; Si, C.; Wei, D.; Wang, S.; Zhang, D.; Li, S.; Li, Z.; Zhang, F.; Wei, B.; Cao, G.; Zhai, B., Solution-Processed Organic Light-Emitting Diodes Based on Yellow-Emitting Cationic Iridium(Iii) Complexes Bearing Cyclometalated Carbene Ligands. Dyes Pigm. 2016, 134, 465471. 49. Zhao, G.-W.; Hu, Y.-X.; Chi, H.-J.; Dong, Y.; Xiao, G.-Y.; Li, X.; Zhang, D.-Y., High Efficient Oleds Based on Novel Re(I) Complexes with Phenanthroimidazole Derivatives. Opt. Mater. 2015, 47, 173-179. 50. Winkler M, H. K. N., Nitrogen-Rich Oligoacenes: Candidates for N-Channel Organic Semiconductors. J. Am. Chem. Soc. 2007 129, 1805. 51. Knight, E. T.; Myers, L. K.; Thompson, M. E., Structure and Bonding in Group 4 Metallocene Acetylide and Metallacyclopentadiene Complexes. Organometallics 1992, 11 (11), 3691-3696. 32 ACS Paragon Plus Environment

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52. D'Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.; Polikarpov, E.; Thompson, M. E., Relationship between the Ionization and Oxidation Potentials of Molecular Organic Semiconductors. Org. Electron. 2005, 6 (1), 11-20. 53. Djurovich, P. I.; Mayo, E. I.; Forrest, S. R.; Thompson, M. E., Measurement of the Lowest Unoccupied Molecular Orbital Energies of Molecular Organic Semiconductors. Org. Electron. 2009, 10 (3), 515-520. 54. Tannor, D. J.; Marten, B.; Murphy, R.; Friesner, R. A.; Sitkoff, D.; Nicholls, A.; Ringnalda, M.; Goddard, W. A.; Honig, B., Accurate First Principles Calculation of Molecular ChargeDistributions and Solvation Energies from Ab-Initio Quantum-Mechanics and Continuum Dielectric Theory. J. Am. Chem. Soc. 1994, 116 (26), 11875-11882. 55. Marten, B.; Kim, K.; Cortis, C.; Friesner, R. A.; Murphy, R. B.; Ringnalda, M. N.; Sitkoff, D.; Honig, B., New Model for Calculation of Solvation Free Energies: Correction of SelfConsistent Reaction Field Continuum Dielectric Theory for Short-Range Hydrogen-Bonding Effects. J. Phys. Chem. 1996, 100 (28), 11775-11788. 56. Wen, S.-H.; Li, A.; Song, J.; Deng, W.-Q.; Han, K.-L.; Goddard, W. A., First-Principles Investigation of Anistropic Hole Mobilities in Organic Semiconductors. The Journal of Physical Chemistry B 2009, 113 (26), 8813-8819. 57. Bassler, H., Charge Transport in Disordered Organic Photoconductors - a Monte-Carlo Simulation Study. Phys. Status Solidi B-Basic Res. 1993, 175 (1), 15-56. 58. Friederich, P.; Symalla, F.; Meded, V.; Neumann, T.; Wenzel, W., Ab Initio Treatment of Disorder Effects in Amorphous Organic Materials: Toward Parameter Free Materials Simulation. Journal of Chemical Theory and Computation 2014, 10 (9), 3720-3725. 59. Masse, A.; Coehoorn, R.; Bobbert, P. A., Universal Size-Dependent Conductance Fluctuations in Disordered Organic Semiconductors. Physical Review Letters 2014, 113 (11), 5. 60. Friederich, P.; Gomez, V.; Sprau, C.; Meded, V.; Strunk, T.; Jenne, M.; Magri, A.; Symalla, F.; Colsmann, A.; Ruben, M.; Wenzel, W., Rational in Silico Design of an Organic Semiconductor with Improved Electron Mobility. Advanced Materials 2017, 29 (43), 7. 61. Friederich, P.; Meded, V.; Poschlad, A.; Neumann, T.; Rodin, V.; Stehr, V.; Symalla, F.; Danilov, D.; Ludemann, G.; Fink, R. F.; Kondov, I.; von Wrochem, F.; Wenzel, W., Molecular Origin of the Charge Carrier Mobility in Small Molecule Organic Semiconductors. Advanced Functional Materials 2016, 26 (31), 5757-5763. 62. Kordt, P.; van der Holst, J. J. M.; Al Helwi, M.; Kowalsky, W.; May, F.; Badinski, A.; Lennartz, C.; Andrienko, D., Modeling of Organic Light Emitting Diodes: From Molecular to Device Properties. Advanced Functional Materials 2015, 25 (13), 1955-1971. 63. Kwiatkowski, J. J.; Nelson, J.; Li, H.; Bredas, J. L.; Wenzel, W.; Lennartz, C., Simulating Charge Transport in Tris(8-Hydroxyquinoline) Aluminium (Alq3). Physical Chemistry Chemical Physics 2008, 10 (14), 1852-1858. 33 ACS Paragon Plus Environment


Page 34 of 46

64. Holmes, R. J.; Forrest, S. R.; Tung, Y.-J.; Kwong, R. C.; Brown, J. J.; Garon, S.; Thompson, M. E., Blue Organic Electrophosphorescence Using Exothermic Host–Guest Energy Transfer. Appl. Phys. Lett. 2003, 82 (15), 2422-2424. 65. Gather, M. C.; Reineke, S., Recent Advances in Light Outcoupling from White Organic Light-Emitting Diodes. PHOTOE 2015, 5 (1), 057607-057607. 66. Flämmich, M.; Gather, M. C.; Danz, N.; Michaelis, D.; Bräuer, A. H.; Meerholz, K.; Tünnermann, A., Orientation of Emissive Dipoles in Oleds: Quantitative in Situ Analysis. Org. Electron. 2010, 11 (6), 1039-1046. 67. Zhang, Y.; Lee, J.; Forrest, S. R., Tenfold Increase in the Lifetime of Blue Phosphorescent Organic Light-Emitting Diodes. Nat Commun 2014, 5.


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Schemes

Scheme 1.

Scheme 2.



Figures

-0.6 -0.8

LUMO (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 46

P-ring substitution O and N,O S and N,S N only

6 5 10 1

-1.0

2 9

-1.2

7 11

3 4

14

-1.4 2.4

13

8 2.6

12

15 2.8

3.0

Triplet Energy (eV)

Figure 1: (top) Elemental enumeration of O, S, NH and CH in the “P-ring” of the H2P structure. (bottom) Lowest unoccupied molecular orbital energy vs. triplet energy for iterative cyclopentaphenanthrene structures.


Page 37 of 46

-0.8 -1.0

LUMO (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1.2 -1.4

parent 10a one N Two N: One N in each ring ortho meta para

10a

10d 10b

-1.6 -1.8 -2.0 2.0

10c

2.2

2.4

2.6

2.8

3.0

Triplet Energy (eV)

Figure 2: LUMO vs. triplet energy for second tier iteration of aza-substitution in phenanthrene section the H-rings of the parent phenantho[4,5-f]imidazole. Compounds 10a-10d are illustrated by colored circles. The identities of the other compounds in the screen are given in the SI.



Page 38 of 46

Figure 3: Highest occupied molecular orbital and lowest unoccupied molecular orbital diagrams calculated at the B3LYP/LACV3P** level of theory. The permanent dipole moment for each molecule is illustrated in the images at the top.


Page 39 of 46

Energy in eV 5 4.5

(a)

4

3.5

3

2.5

2

Absorbance (a.u.)

10b

Absorbance (a.u.) Absorbance (a.u.)

(c)

0.4

0.8

0.4

0.0

0.0

10c 0.8

Absorption Emission (RT) 0.8 Emission (77K)

0.4

0.4

0.0

0.0

10d 0.8

Absorption Emission (RT) 0.8 Emission (77K)

0.4

0.4

0.0

250

300

350

400

450

500

550

Emission Intensity (a.u.) Emission Intensity (a.u.)

(b)

Absorption Emission (RT) Emission (77K)

0.8

Emission Intensity (a.u.)

0.0

600

650

Wavelength (nm)

Energy (eV)

(d)

3

2.8

2.6

2.4

2.2

2

1.0

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10b 10c 10d

0.8 0.6

1.8

2.62 eV

0.4 2.65 eV 2.47 eV

0.2 0.0 400

450

500

550

600

650

700

Wavelength (nm)

Figure 4: Absorption (black line), fluorescence (blue line), and cryogenic (77 K) spectra (red line) of (a) 10b, (b) 10d, and (c) 10c. In each subfigure, the bottom axis refers to wavelength and the top axis denotes energy in eV. (d) Solid state phosphorescence spectra: collected during the 0.1-1.0 ms period after the excitation pulse to remove contribution from fluorescence . 39 ACS Paragon Plus Environment


10b 10c 10d

1.5

1.0

g(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 46

0.5

0.0 3

4

5

6

7

8

r (Å)

Figure 5: Center of mass radial distributions [g(r)] from MD simulations of three host materials, obtained by averaging over 30ns.


Page 41 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6: Histogram plots showing the distribution of hole and electron hopping rates extracted from the frontier dimer orbital splitting coupling calculations of both the exhaustive dimer set and the smaller 20 dimer subset for the three host materials (top: 10b, middle: 10c, bottom: 10d)



(a) -2

-2 LiF:Al

Energy (eV)

-3

-3 NPD

BCP

-4

10b

ITO

-4

10c 10d

-5

-5

-6

-6 mCP

-7

-7

(b)

300

10b 10c 10d

250

2

)

1000

Brightness (cd/m

100

200

10

150

1

100

0.1

0

2

4

6

8

Voltage (V)

2

0.01

50

Current Density (mA/cm )

0 12

10

(c)

External Quantum Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 46

10

1

10b 10c 10d 0.1 0.1

1

10

100 2

Current Density (mA/cm

)

Figure 7: (a) Energy level diagram of the OLEDs (blue line is FIrpic). The energies of the three different hosts are illustrated next to each other, but each device contained only a single host material. (b) shows J-V-L characteristics and (c) is a plot of EQE versus current density. ACS Paragon Plus Environment

42

Page 43 of 46

0.01 1E-3

Current (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1E-4 1E-5 1E-6

10b 10c 10d

1E-7 1E-8 0

2

4

6

8

10

12

Voltage (V)

Figure 8: Current-voltage plots for electron only devices, i.e. ITO/BCP (10 nm)/H2P (40 nm)/LiF:Al.



Page 44 of 46

Tables Table 1. Summary of calculated and experimental physical properties for 10b, 10c and 10c.

10a

10 b 10 c 10d a

Eox/Ered

HOMO (eV)

(V/V)a

Calc.

Exp.d

Calc.

Exp.d

0.89b/-2.97b

-5.47

-5.85

-1.04

1.15c/-2.54b

-5.91

-6.21

1.11c/-2.25b

-5.88

1.17b/-2.51b

-5.82

O(+)e (eV)

O(-) f (eV)

-1.27

0.24

-1.54

-1.76

-6.15

-1.77

-6.24

-1.41

LUMO (eV)

ET (eV)

)PLg

Calc.

Solution

Solid

0.21

2.90

2.86

2.70

' (sol-sol) 0.16

0.27

0.40

2.91

2.92

2.65

0.27

0.40

-2.10

0.25

0.30

2.84

2.71

2.47

0.24

0.20

-1.79

0.28

0.22

2.90

2.85

0.23

0.61

Oxidation and reduction potentials vs. ferrocene/ferrocenium redox couple.

b

Quasi-reversible. 51, 52

2.62 c

irreversible.

d

--

HOMO and LUMO

e

energy levels were calculated from the redox potential with published correlation. Calculated hole reorganization energy. f g Calculated electron reorganization energy. Photoluminescent quantum yield of vacuum deposited films containing 10% FIrpic-doped H2P.

Table 2. Number of S-S contacts and calculated mobilities for 10b, 10c and 10c. Isotropic mobility (x10-4 m2/Vs) Dimer-splitting CDFT

S-contacts Total 10b 10c 10d

324 409 364

S-ff (faceface) 77 142 72

S-ef (edgeface) 247 267 292

S-ff/S-ef

μh

μe

μh

0.31 0.53 0.25

4.45 6.47 6.16

0.67 3.46 5.34

20.3 27.5 14.9

μe 23.4 53.8 41.7


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Table 3: Efficiency parameters of the OLEDs for 10b, 10c and 10d. (ηr was computed assuming F =1 and Ke = 0.2)

Host

EQE (%)

)PL

Charge recombination factor (K Kr)

10b

0.7

0.4

0.09

10c

4

0.2

1

10d

9

0.6

0.75



Computational screening of organic materials is used to find optimal candidates for OLED applications. 89x34mm (150 x 150 DPI)

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Rapid Multiscale Computational Screening for ... - ACS Publications

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