Molecular Design Strategies for Color Tuning of Blue TADF Emitters

New thermally activated delayed fluorescence (TADF) blue emitter molecules based on the known donor–acceptor–donor (D–A–D)-type TADF molecule, 2,7-bis(9,9-dimethylacridin-10-yl)-9,9-dimethylthioxanthene-S,S-dioxide (DDMA-TXO2), are reported. The motivation for the present investigation is via the use of rational molecular design, based on DDMA-TXO2, to elevate the organic light emitting diode (OLED) performance and obtain deeper blue color coordinates. To achieve this goal, the strength of the donor (D) unit and acceptor (A) units have been tuned with methyl substituents. The methyl functionality on the acceptor was also expected to modulate the D–A torsion angle in order to obtain a blue shift in the electroluminescence. The effect of regioisomeric structures has also been investigated. Herein, we report the photophysical, electrochemical, and single-crystal X-ray crystallography data to assist with the successful OLED design. The methyl substituents on the DDMA-TXO2 framework have profound effects on the photophysics and color coordinates of the emitters. The weak electron-donating methyl groups alter the redox properties of the D and A units and consequently affect the singlet and triplet levels but not the energy gap (ΔEST). By systematically manipulating all of the aforementioned factors, devices have been obtained with acceptor-substituted III with a maximum external quantum efficiency of 22.6% and Commission Internationale de l’Éclairage coordinates of (0.15, 0.18) at 1000 cd m–2.


S-4
Xantphos (208 mg, 0.036 mmol, 0.09 eq.) and the resulting mixture was bubbled with argon for 20 min, and then transferred via cannula to the previously formed potassium thiolate. The combined mixture was purged with argon for a further 30 min and heated to reflux (160 °C drysyn temperature) for 24 h. TLC of the reaction mixture after cooling to room temperature revealed consumption of the ester starting material (essential for purification). The mixture was diluted with EtOAc (80 mL), washed with H2O (3 × 25 mL), dried over MgSO4, filtered and concentrated under reduced pressure to yield a deep orange oil. The crude product was purified by silica gel column chromatography eluting with CH2Cl2/hexane 1:4 to 1:1 v/v to yield the desired sulfide as a yellow oil (596 mg, 52%). 1

3-methyl((2-(2-hydroxypropan-2-yl)-5-methylphenyl)thio)benzene (D).
Under an atmosphere of argon, a 1.6 M solution of MeLi in diethyl ether (4.51 mL, 7.221 mmol, 3 eq.) was added slowly to a stirred solution of methyl 4-methyl-2-(m-tolylthio)benzoate (C) (655 mg, 2.407 mmol, 1 eq.) in dry THF (20 mL) at −78 °C. The resulting yellow mixture was stirred at −78 °C for 30 min and then returned to room temperature and allowed to stir for a further 1 hour. The reaction mixture was poured over ice, extracted with EtOAc (3 × 10 mL) and washed with brine (3 × 10 mL). A crude 1 H NMR spectrum showed the reaction was not complete, so the crude product was dried under vacuum, backfilled with argon and the experimental procedure was repeated adding further MeLi solution (3.00 mL, 4.81 mmol, 2 eq.). Following quenching and extraction as described above, the organic layer was dried over MgSO4, filtered and concentrated under reduced pressure to yield product as a yellow oil (436 mg, 66%). 1

S-6
To a stirring solution of 2,7-dibromo-3,6,9,9-tetramethylthioxanthene (F) (371 mg, approx. 0.905 mmol, approx. 1 eq.) in AcOH (8 mL) at 80 °C was slowly added H2O2 (35% wt in H2O, 3.90 mL, 45.25 mmol) via the reflux condenser. The reaction mixture was refluxed at 120 °C for 2 h and left to cool to room temperature overnight, resulting in the precipitation of product which was collected by vacuum filtration and washed with H2O (4 × 500 mL) with vigorous mixing in the sinter during each wash. The product was dried at 70 °C followed by drying at ambient temperature under high vacuum to obtain the pure product as a cream-white solid (247 mg, 62%). 1

methyl 5-methyl-2-(p-tolylamino)benzoate (H)
This molecule is known in the literature and was made by an alternative route. 1 H NMR data for the product is consistent with literature data. 5 Methyl 2-amino-5-methylbenzoate (4.25 g, 25.72 mmol, 1.1 eq.) and Cs2CO3 (11.43 g, 35.10 mmol, 1.5 eq.) were added to a dry 250 mL 2-necked round-bottomed flask fitted with reflux condenser, which was evacuated for 15 min and backfilled with argon. 4-bromotoluene (2.87 mL/4.00 g, 23.39 mmol, 1 eq.), was then added via syringe. Dry toluene (100 mL) was added via syringe, and the reaction mixture was bubbled vigorously with argon for 15 min. Pd3(OAc)5NO2 (105 mg, 158 µmol, 0.7 mol%) and Xantphos (541 mg, 94 µmol, 4 mol%) were finally added and the reaction bubbled with argon for a further 15 min. The reaction mixture was heated to 100 °C with stirring for 18 h. The reaction mixture was allowed to cool to ambient temperature and water (100 mL) followed by CH2Cl2 (150 mL) was added. The organic layer was separated and the reaction mixture was extracted with additional CH2Cl2 (3 × 100 mL). All organic extracts were combined, washed with brine (2 × 100 mL), dried with MgSO4 and filtered. Solvent was removed under reduced pressure and the crude residue was purified by silica gel column chromatography gradient 20% CH2Cl2/hexane (v/v) followed by 40% and finally 60% CH2Cl2. Removal of solvent under reduced pressure gave product with a single minor impurity that was removed by HPLC (Waters XBridge C18 -50 g column, flow rate: 17 mL min −1 , solvent system: isocratic 80% MeCN:H2O). Removal of MeCN under reduced pressure was followed by extraction of the aqueous mixture with CH2Cl2 (2 × 50 mL). Drying with MgSO4, filtering and removing solvent under reduced pressure gave pure product (2.98 g, 50%). This molecule is known and was made by a modified literature route where MeLi was used in the first step instead of MeMgBr. 1 H NMR data for the product is consistent with literature data. 6 To a stirring solution of methyl 5-methyl-2-(p-tolylamino)benzoate (2.31 g, 9.05 mmol, 1 eq.) in THF (50 mL) at −78 °C was added MeLi (10.2 mL, 3.1 M in diethoxymethane, 3.5 eq.) dropwise over 15 min. The reaction mixture was maintained at −78 °C for 30 min and was then allowed to warm to ambient temperature, at which point the mixture was stirred for an additional 1 h. TLC analysis of a quenched portion of reaction mixture (in H2O layered with Et2O) showed conversion to the intermediate which was not isolated. The reaction mixture was slowly quenched with H2O (50 mL) and CH2Cl2 (100 mL) was added. The organic layer was separated from the mixture and the aqueous layer was extracted with CH2Cl2 (2 × 100 mL). All organic extracts were combined, dried with MgSO4 and filtered. The solvent was removed under reduced pressure give a crude mixture. To the crude dried mixture (≈ 2.0 g) was then added toluene (7 mL) and the mixture was stirred for 5 min to allow dissolution of the thick crude oil. H3PO4 (40 mL) was then added and the mixture was stirred vigorously at room temperature for 4 h. The reaction mixture was diluted with CH2Cl2 (100 mL) and H2O (200 mL). The organic layer was separated and the mixture was extracted with CH2Cl2 (2 × 100 mL). All organic extracts were combined, washed with brine (2 × 100 mL), dried with MgSO4 and filtered. Solvent was removed under reduced pressure and the crude residue was purified by silica gel column chromatography eluting with 10% CH2Cl2/hexane (v/v) followed by 20% CH2Cl2. Removal of solvent under reduced pressure yielded pure product as a cream-white solid (1.52 g, 71%).

2,7-Bis(2,7,9,9-tetramethyl-acridin-10-yl)-9,9-dimethylthioxanthene-S,S-dioxide (IV)
2,7-dibromo-9,9-dimethylthioxanthene-S,S-dioxide (0.098 g, 0.236 mmol, 1 eq.) and 2,7,9,9tetramethyl-10H-acridine (J) (0.112 g, 0.472 mmol, 2 eq.) were dried under vacuum for 30 min in a two-neck 100 mL round-bottomed flask fitted with a reflux condenser. 4 The flask was back-filled with argon and dry toluene (10 mL) was added. The reaction mixture was bubbled with argon for 30 min, then Pd2(dba)3·CHCl3 (12 mg, 11 μmol, 0.05 eq.) and HP t Bu3BF4 (7 mg, 24 μmol, 0.1 eq.) was added, and the reaction mixture was bubbled with argon for a further 30 min. NaO t Bu (68 mg, 0.708 mmol, 3 eq.) was added under a high flow of argon and the reaction was then heated to 115 C (drysyn kit temperature) with stirring for 16 h. At the end of the reaction the mixture was filtered through a celite pad, and the pad was washed with CH2Cl2 (100 mL). Water (50 mL) was added to the filtered solution and the organic layer was then separated. The aqueous layer was extracted further with CH2Cl2 (2 × 50 mL) and all organic extracts were combined, dried with MgSO4 and filtered. The solvent was evaporated under reduced pressure, and the crude mixture was purified by silica gel column chromatography eluting with 50:50 v/v CH2Cl2: hexane increasing to 100% CH2Cl2 in 10% increments. Removal of solvent under reduced pressure gave a white solid, which was then sonicated in pentane (5 mL) and filtered and dried to give pure product (90 mg, 52% yield). Crystals suitable for X-ray diffraction were obtained by slow evaporation from toluene in a sealed NMR tube over a period of several months.

b. Experimental Methods
Three types of samples were studied in this work: solutions in toluene and in DCM solvents

d. Electrochemistry
Cyclic voltammetry (CV) measurements were performed in order to obtain the HOMO level of each material ( Figure S1). The measurements were carried out using a BAS CV50W electrochemical analyzer fitted with a three-electrode system consisting of a glassy carbon (ϕ = 3 mm) working electrode, and Pt wire counter and quasi reference electrodes. Experiments were conducted at a scan rate of 100 mV s -1 . Experiments were conducted in dry deoxygenated THF with 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte. All voltammograms presented were referenced using a separate internal ferrocene voltammograms. These experiments have allowed direct determination of the HOMO level, while the LUMO was estimated from the measured HOMO and the bandgap obtained using absorption spectra. HOMO was estimated using ferrocene as an internal reference. The oxidative wave of IV was shown to be reversible. The HOMO levels for all derivatives are very similar, whereas the LUMO energy varies slightly.

S-24
a HOMO estimated from onset of the oxidation wave in CV. b As the reduction potential is out of the accessible range of the THF solvent window, the LUMO energy was calculated from HOMO -Eg. The optical band gap (Eg) was determined from the onset of the UV-Vis absorption band in DCM solvent.