Axially Chiral Biphenyl Compound‐Based Thermally Activated Delayed Fluorescent Materials for High‐Performance Circularly Polarized Organic Light‐Emitting Diodes

Abstract To boost intrinsic circularly polarized luminescence (CPL) properties of chiral emitters, an axially chiral biphenyl unit is inlaid in thermally activated delayed fluorescent (TADF) skeleton, urging the participation of chiral source in frontier molecular orbital distributions. A pair of enantiomers, (R)‐BPPOACZ and (S)‐BPPOACZ, containing the cyano as electron‐withdrawing moieties and carbazole and phenoxazine as electron‐donating units are synthesized and separated. The circularly polarized TADF enantiomers exhibit both high photoluminescence quantum yield of 86.10% and excellent CPL activities with maximum dissymmetry factor |g PL| values of almost 10−2 in solution and 1.8 × 10−2 in doped film, which are among the best values of previously reported small chiral organic materials. Moreover, the circularly polarized organic light‐emitting diodes based on the TADF enantiomers achieve the maximum external quantum efficiency of 16.6% with extremely low efficiency roll‐off. Obvious circularly polarized electroluminescence signals with |g EL| values of 4 × 10−3 are also recorded.


Fabrication and measurement of CP-OLEDs
Indium-tin-oxide (ITO) coated glass with a sheet resistance of 10 Ω sq -1 was used as the anode substrate. Prior to film deposition. All the organic layers were deposited with the rate of 0.1 nm s -1 under high vacuum (≤ 2×10 -5 Pa). The doped layers were prepared by coevaporating dopant and host material from two individual sources, and the doping concentrations were modulated by controlling the evaporation rate of dopant. LiF and Al were deposited in another vacuum chamber (≤ 8.0×10 -5 Pa) with the rates of 0.01 and 1 nm s -1 , respectively, without being exposed to the atmosphere. Device performances were measured by using a programmable Keithley source measurement unit with a silicon photodiode. The EL spectra were measured with a calibrated Hitachi F-7000 fluorescence spectrophotometer.
Based on the uncorrected EL fluorescence spectra, the Commission Internationale de l'Eclairage (CIE) coordinates were calculated using the test program of Spectrascan PR650 spectrophotometer. The EQEs of EL devices were calculated based on the photo energy measured by the photodiode, the EL spectrum and the current pass through the device. The circularly polarized electroluminescence (CPEL) spectra were measured on a Jasco CPL-300 spectrophotometer.

Measurement of the chiroptical properties in thin film and the g EL of the device
Firstly, thin films and devices were fabricated through vacuum evaporation in same conditions, for example under the pressure of 2 × 10 -5 Pa and at the temperature of 195 o C for BPPOACZ. Secondly, all CPL and CPEL characteristics were measured by using JASCO CPL-300. Thin films are small enough to be sandwiched in CHUCKING APPLIANCE and no adjustment was necessary during the measurements to keep the light path parallel to avoid refraction. A distance of 5.5 cm between sample and EXCITATION can achieve high signal to noise. Devices were too high from the light path when sandwiched in CHUCKING APPLIANCE, so appropriate adjustments were needed. A 5.5 cm wide hard card was abutted against the side of EXCITATION. Then an UNFIXED SLIDE was lowered the height and abutting against the other side of the hard card. And the devices could be sandwiched in CHUCKING APPLIANCE as high as the light path. After a baffle covering the xenon lamp from EXCITATION, devices could be turned on with an external power. All contacts during the adjustment were very tight to keep light path parallel. Thirdly, the CPPL spectra were measured on a Jasco CPL-300 spectrophotometer with "Standard" sensitivity at 200 nm/min scan speed and respond time of 2.0 s employing "slit" mode. Following are photos and sketches of the measurements.

Preparation procedure for BPCNF
The solution of 2-bromo-3-fluorobenzonitrile (4.00 g, 20.0mmol), copper powder (1.27g, 20.0 mmol) in dry DMF (20 mL) was stirred for 12 h at 120℃. After cooling down to room temperature, the suspension mixture was poured into water (100 mL), and the aqueous layer was extracted with dichloromethane (3×20 mL), which was washed with water and concentrated under vacuum. The product was the purified by flash chromatography (EA/PE, 1:6) to get the white solid 1.87 g (78%). 1

Preparation procedure for BPPOAF
The suspension mixture of phenoxazine (1.0 g, 5.5 mmol) and sodium hydride (242 mg, 6.05 mmol) in dry DMF (30 mL) was stirred for 0.5 h at room temperature. After adding BPCNF (1.20 g, 5 mmol), the mixture was heated at 80 ℃ for 12 h, and then was poured into water (100 mL). The aqueous layer was extracted with ethyl acetate (3×20 mL), and the combined organic phase was washed with water (50 mL) and concentrated under vacuum.
The crude product was the purified by flash chromatography (EA/PE, 1:6) to get the yellow solid 1.25 g (62%). 1

Preparation procedure for (rac)-BPPOACZ
The suspension mixture of carbazole (551.8 mg, 3.3 mmol), and sodium hydride (145 mg, 3.63 mmol) in dry DMF (15 mL) was stirred for 0.5 h at room temperature. After adding BPPOAF (1.25 g, 3 mmol), the mixture was stirred for 12 h at 80 ℃ and poured into water (60 mL), which was extracted with ethyl acetate (3×10 mL). The organic phase was washed with water (30 mL) and concentrated under vacuum. The crude product was purified by flash chromatography (EA/PE, 1:6) to get the pale yellow solid 1.14 g (69%). The pale yellow solid was purified by sublimation to get pale yellow crystal (50%) and then the crystal was improved purity by sublimation with 90% yield. 1

Copies of 1 H NMR, 19 F NMR, 13 C NMR and MALDI-TOF spectra of new
compounds.                   [eV]

DFT calculations and electrochemical measurement.
[eV] [eV] [eV] [eV] E ST (adiabatic) [b] [eV] (rac)-    Figure S11. a) PL decay curves of (rac)-BPPOACZ in toluene under nitrogen atmosphere at 288 K; b) PL decay curves of powder of (rac)-BPPOACZ at 300 K (blue line), 200 K (green line), 100 K (red line).   We investigated the efficiency roll-off behaviours by taking triplet-triplet annihilation (TTA) into account. The TTA model represents an exciton-exciton quenching process and the current density dependent EQE in the TTA model is expressed by equation (1): where η TT is the EQE in the presence of TTA, η 0 is the maximum EQE, J 0 is the current density where EQE reaches half of the maximum value.