Pure Hydrocarbon Materials as Highly Efficient Host for White Phosphorescent Organic Light‐Emitting Diodes: A New Molecular Design Approach

Abstract To date, all efficient host materials reported for phosphorescent OLEDs (PhOLEDs) are constructed with heteroatoms, which have a crucial role in the device performance. However, it has been shown in recent years that the heteroatoms not only increase the design complexity but can also be involved in the instability of the PhOLED, which is nowadays the most important obstacle to overcome. Herein, we design pure aromatic hydrocarbon materials (PHC) as very efficient hosts in high‐performance white and blue PhOLEDs. With EQE of 27.7 %, the PHC‐based white PhOLEDs display similar efficiency as the best reported with heteroatom‐based hosts. Incorporated as a host in a blue PhOLED, which are still the weakest links of the technology, a very high EQE of 25.6 % is reached, surpassing, for the first time, the barrier of 25 % for a PHC and FIrpic blue emitter. This performance shows that the PHC strategy represents an effective alternative for the future development of the OLED industry.


General Information
All reactions were performed under a nitrogen atmosphere unless otherwise stated. Dry tetrahydrofuran (THF) was purified by PURE SOLV (Innovative Technology) purification system. 1 H NMR was recorded in chloroform-d (CDCl3) on a Bruker 400 MHz at room temperature. Matrix-Assisted Laser Desorption/Ionization Time of Flight Mass Spectrometry (MALDI-TOF-MS) was measured with a Bruker ultraflextreme MALDI-TOF spectrometer. UV-visible spectra were recorded using an UV-vis spectrophotometer (Lambda 750) in toluene. Molar absorption coefficients (ε) were calculated from the gradients extracted from the plots of absorbance vs concentration with five solutions of different concentrations for each sample. The fluorescent and phosphorescent (liquid nitrogen) spectra were tested by Hitachi F-4600 spectrophotometer, both in toluene solution. The excitation wavelength was 290 nm for both, modulated by a monochromator. Triplet energy levels were calculated from the maximum of the first phosphorescence emission peak, and conversion in electron-volt was obtained with the following formula:

X-Ray analysis
Crystals were picked up with a cryoloop and then frozen at 150 K or 170 K under a stream of dry N2. Data were collected on a D8 VENTURE Bruker AXS diffractometer with Mo-Kα radiation (λ = 0.71073 Å). Structures were solved by direct methods (SIR92) 1 and refined (SHELXL-2014/7) 2 by full-matrix least-squares methods as implemented in the WinGX software package. 3 An empirical absorption (multi-scan) correction was applied. Hydrogen atoms were introduced at calculated positions (riding model) included in structure factor calculation but not refined. Refinement parameters are summarized in Tables S1 and S2. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication data: 1-mtp-SBF (CCDC 2142857) 1-mbp-SBF (CCDC 2168574) and 1-p-SBF (CCDC 1495896). 4 Copies of the data can be obtained free of charge on application to CCDC, 12

Electrochemical analysis
Electrochemical experiments were performed under an argon atmosphere using a Pt disk electrode (diameter 1 mm). The counter electrode was a vitreous carbon rod. The reference electrode was either a silver wire in a 0.1 M AgNO3 solution in CH3CN for the studies in oxidation or a silver wire coated by a thin film of AgI (silver(I)iodide) in a 0.1 M Bu4NI solution in DMF for the studies in reduction. Ferrocene was added to the electrolyte solution at the end of a series of experiments. The ferrocene/ferrocenium (Fc/Fc + ) couple served as an internal standard. The three electrodes cell was connected to a potentiostat/galvanostat (Autolab/PGSTAT101) monitored with the Nova 2.1 Software. Activated Al2O3 was added to the electrolytic solution to remove excess moisture. For further comparison, all potentials are referred to the SCE electrode that was calibrated at -0.405 V vs. Fc/Fc + system. We estimated the electron affinity (EA) or the lowest unoccupied molecular orbital (LUMO) and the ionization potential (IP) or the highest occupied molecular orbital (HOMO) from the redox data. The LUMO level was calculated from the cyclic voltammetry in reduction recorded in DMF (+

Molecular modeling
Full geometry optimization of the ground state (S0) and frequency calculation were performed with Density Functional Theory (DFT) [5][6] using the hybrid Becke-3 0 parameter exchange functional [7][8] and the Lee-Yang-Parr non-local correlation functional 9 (B3LYP) implemented in the Gaussian 16 (Revision B.01) program suite using the 6-31G(d) basis set and the default convergence criterion implemented in the program. Geometry optimization of the first excited triplet state (ET1) was performed using Time-Dependent-Density Functional Theory (TD-DFT) calculations using the B3LYP functional and the 6-31+G(d) basis set. Infrared spectra were calculated on the final geometry to verify that an energy minimum was reached (i.e. no negative frequency). Transition diagrams were obtained through TD-DFT calculations performed using the S3 M06-2X functional and the extended 6-311+G(d, p) basis set on the geometry of S0. T1 to S0 energy transition (ET) was calculated from the difference between the total energy of the molecule in its respective excited triplet state (found through TD-DFT, B3LYP 6-311+G(d, p)) and its ground singlet state (found through DFT, B3LYP 6-311+G(d, p)) in their optimized geometries. Spin density (SD) representation was obtained through TD-DFT calculations performed using the extended 6-311+G(d, p) basis set and a triplet spin on the previously optimized geometry of T1. Calculations were carried out on the OCCIGEN calculator of the Centre Informatique National de l'Enseignement Supérieur (CINES (Montpellier) under project No. 2021-A0100805032). Figures were generated with GaussView 6.0 and GaussSum 3.0.

Thermal analysis
Thermal Gravimetric Analysis (TGA) was carried out by using a METTLER TOLEDO TGA1 instrument. TGA curves were measured at 10 °C min −1 from 30 °C to 700 °C under nitrogen flux. Differential Scanning Calorimetry (DSC) was carried out by using a METTLER TOLEDO DSC1 unit instrument equipped with an intracooler. DSC traces were measured at 10 °C min −1 , 2 heating/cooling cycles were successively carried out under nitrogen flux.

Device fabrication and characteristics
All organic compounds except the product of this paper were purchased from Lumtec Co., Ltd. ITO substrates (185 nm, R = 10 Ω/square for blue PhOLEDs, 135 nm, R = 15 Ω/square for white PhOLEDs; activating area 0.1 cm 2 ; glass substrate, 32 mm × 32 mm × 0.7 mm) with patterned electrodes were ultrasonically cleaned in deionized water, acetone and ethanol for 15 min. Before loading into the vacuum chamber, the substrates were treated by UV ozone for 15 min at 40 °C. OLED devices were fabricated by depositing materials in a thermal evaporator chamber under a vacuum of 4.0 × 10 −6 Torr. The organic layers and cathode were defined by shallow masks placed under the substrates (0.15 cm 2 ), which define an active area of 0.1 cm 2 . The evaporation rates and thicknesses of organic layers (2-4 Å s −1 ) and aluminum cathode (4-8 Å s −1 ) were monitored with oscillating quartz crystals. The devices were well encapsulated with glued glass lids before testing. A programmable spectra scan photometer (PHOTO RESEARCH, PR 655) connected to a KEITHLEY 2400 source meter was used to test the EL and luminescence characteristics, and the current-voltage characteristics. 4 The synthesis and characterization of 1-p-SBF can be found in previous works. 4

1-Quaterphenyl-9,9'-spirobifluorene [1-mqp-SBF]
1-Quaterphenyl-9-fluorenone 4,4,5,5-tetramethyl-2-(5'-phenyl-[1,1':3',1''-terphenyl]-3yl)-1,3,2-dioxaborolane was synthesized using a previously reported synthetic procedure. 10 1-Bromo-9H-fluoren-9-one (0.800 g, 3.09 mmol, 1.0 eq), 4,4,5,5tetramethyl-2-(5'-phenyl- [    3 ) for 1-mtp-SBF. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor.   108.88(11) 4 Molecular Modeling Figure S1. Representation of the energy levels and the main molecular orbitals involved in the electronic transitions of 1-p-SBF obtained by TD-DFT m06-2x and the 6-311+G(d, p) basis set on the geometry of S0, shown with an isovalues of 0.04 [e bohr-3 ] 1/2 (for clarity purpose, only the main contribution for each transition is shown, see details in Table below). Figure S2. Representation of the energy levels and the main molecular orbitals involved in the electronic transitions of 1-mbp-SBF obtained by TD-DFT m06-2x and the 6-311+G(d, p) basis set on the geometry of S0, shown with an isovalues of 0.04 [e bohr-3 ] 1/2 (for clarity purpose, only the main contribution for each transition is shown, see details in Table below).  Table below). Figure S4. Representation of the energy levels and the main molecular orbitals involved in the electronic transitions of 1-mqp-SBF obtained by TD-DFT m06-2x and the 6-311+G(d, p) basis set on the geometry of S0, shown with an isovalues of 0.04 [e bohr -3 ] 1/2 (for clarity purpose, only the main contribution for each transition is shown, see details in Table below).  Table S13. Atomic coordinates of 1-p-SBF at the fundamental state after geometry optimization (DFT b3lyp/6-31g(d)). Atom X (Å)           Figure S20. Determination of the quantum yield (cyclohexane, RT) of 1-mbp-SBF (2 measurements).     Figure S22. Determination of the quantum yield (cyclohexane, RT) of 1-mqp-SBF (2 measurements).    Platinum disk (Ø: 1mm) working electrode. a) One cycle reaching only the first oxidation; b) ten cycles reaching the second oxidation wave up to 2.3 V (electrodeposition process); c) five cycles reaching only 1.6 V (the reversible p-doping process of the deposit); d) ten cycles reaching the second oxidation wave up to 2.09 V (electrodeposition process); e) five cycles reaching only 1.6 V (the reversible p-doping process of the deposit).   in which V is the driving voltage, L is the thickness of the thin layer, ε0 is the permittivity of the free space, ε is the relative dielectric constant (estimated to be 3.0 here), μ0 is the zero-field mobility and β is Poole-Frenkel factor. The thickness L equals to 100 nm, and the zero-field mobility of the compounds was calculated and summarized in Table 1 in the manuscript.