Intramolecular Dimerization Quenching of Delayed Emission in Asymmetric D–D′–A TADF Emitters

Understanding the excited-state dynamics and conformational relaxation in thermally activated delayed fluorescence (TADF) molecules, including conformations that potentially support intramolecular through-space charge transfer, can open new avenues for TADF molecular design as well as elucidate complex photophysical pathways in structurally complex molecules. Emissive molecules comprising a donor (triphenylamine, TPA) and an acceptor (triphenyltriazine, TRZ) bridged by a second donor (9,9-dimethyl-9-10-dihydroacridin, DMAC, or phenoxazine, PXZ) are synthesized and characterized. In solution, the flexibility of the sp3-hybridized carbon atom in DMAC of DMAC–TPA–TRZ, compared to the rigid PXZ, allows significant conformational reorganization, giving rise to multiple charge-transfer excited states. As a result of such a reorganization, the TRZ and TPA moieties become cofacially aligned, driven by a strong dipole–dipole attraction between the TPA and TRZ units, forming a weakly charge-transfer dimer state, in stark contrast to the case of PXZ–TPA–TRZ where the rigid PXZ bridge only supports a single PXZ–TRZ charge transfer (CT) state. The low-energy TPA-TRZ dimer is found to have a high-energy dimer local triplet state, which quenches delayed emission because the resultant singlet CT local triplet energy gap is too large to mediate efficient reverse intersystem crossing. However, organic light-emitting diodes using PXZ–TPA–TRZ as an emitting dopant resulted in external quantum efficiency as high as 22%, more than two times higher than that of DMAC–TPA–TRZ-based device, showing the impact that such intramolecular reorganization and donor–acceptor dimerization have on TADF performance.


References S48
Experimental Procedures

General Information for Synthesis
All chemicals and reagents were used as received from commercial sources without purification. Solvents for chemical synthesis were purified by distillation. All chemical reactions were carried out under an argon or nitrogen atmosphere. NMR spectra were measured in CD 2 Cl 2 through Varian (Utility 400) spectrometer at 1 H NMR (400 MHz) and 13 C NMR (100 MHz) respectively.
The synthesis of target molecules PXZ-TPA-TRZ, DMAC-TPA-TRZ, PXZ-Ph-TRZ, and DMAC-Ph-TRZ is shown in Scheme S1. To introduce a triphenylamine or phenyl group onto the phenoxazine or acridine, we followed our reported procedure [1] to synthesize the intermediates PXZ-Br and DMAC-Br. PXZ-Br was converted to PXZ-TPA by using Pd-catalyzed Suzuki-Miyaura coupling reaction with triphenylamino boronic acid (2) and then the removal of the protection group with NaOMe. DMAC-Br was carried out the same Pd-catalyzed coupling reaction condition to give DMAC-TPA. The target molecules, PXZ-TPA-TRZ and DMAC-TPA-TRZ, was then obtained by using the Pd-catalyzed Buchwald-Hartwig amination reaction of donors (PXZ-TPA, DMAC-TPA) and TRZ-Br. TRZ-Br was synthesized following by literature procedure [2] . To probe the through-space charge transfer behavior, we replaced triphenylamine with phenyl group to make the two compounds, PXZ-Ph-TRZ and DMAC-Ph-TRZ, which were synthesized from intermediates PXZ-Ph and DMAC-Ph, where PXZ-Ph and DMAC-Ph underwent the Pd-catalyzed amination reaction with TRZ-Br to afford the desired compounds, PXZ-Ph-TRZ and DMAC-Ph-TRZ. All model compounds, PXZ-Ph-TPA, DMAC-Ph-TPA, PXZ-Ph-Ph, and DMAC-Ph-Ph, were synthesized by Pd-catalyzed Buchwald-Hartwig amination reaction of corresponding intermediates with iodobenzene. All the newly synthesized molecules were purified by column chromatography followed by vacuum sublimation before characterization and device fabrication.

X-ray Crystallography
Crystallographic data were carried out at 200(2) K on a Oxford Gemini A CCD diffractometer using with Mo-Kα radiation (λ = 0.71073Å). Cell parameters and data reduction were retrieved and refined manipulating CrysAlis Pro software on all reflections. The structures were solved and refined with SHELXL programs. The hydrogen atoms were included in calculated positions and refined using a riding mode.

Thermal properties
Thermogravimetric analysis (TGA) was undertaken with a TGA Q500 instrument. The thermal stability of the samples under a nitrogen atmosphere was determined by measuring their 5% weight loss while heating at a rate of 10 o C min -1 . Differential scanning calorimetry (DSC) analyses were performed on a TA Instrument DSC-2920 Low Temperature Difference Scanning Calorimeter at a heating rate of 10 o C min -1 under nitrogen.

Cyclic Voltammetry
The electrochemical properties of all the products were measured by cyclic voltammetry by using a CHI619B potentiostat. The oxidation potential was determined by cyclic voltammetry using 0.1M nBu 4 NPF 6 (TBAPF 6 ) in CH 2 Cl 2 as a supporting electrolyte and a scan rate of 100 mV s -1 . The reduction potential was recorded using 0.1M nBu 4 NClO 4 (TBAP) in DMF as a supporting electrolyte and a scan rate of 100 mV s -1 . A standard 3electrode cell comprising silver/silver chloride (Ag/AgCl), a platinum wire and a glassy carbon electrode as the reference, counter, and working electrodes, respectively, were used. All potentials were recorded versus Ag/AgCl (saturated) as a reference electrode. Oxidation of the ferrocene/ferrocenium (Fc/Fc + ) redox couple in CH 2 Cl 2 /TBAPF 6 occurs at E′o = + 0.47 V and reduction of the ferrocene/ferrocenium (Fc/Fc+ ) redox couple in DMF/TBAP occurs at E′o = + 0.51 V vs. Ag/AgCl (saturated). HOMO and LUMO levels were calculated from the oxidation / reduction half-wave potential with the formula:

Steady state characterisations
Steady-state absorption and photoluminescence spectra in solution are obtained at concentration of 10 µmg/ml using Shimadzy UV-3600 UV-VIS-NIR spectrometer and Jobin Yvon Horiba Fluromax-3 respectively. For aerated and degassed emission, the emission is first measured before undergoing at least four repeat cycles of freeze/thaw/pump on a custom made 1 cm path-length quartz degassing cuvette stoppered with a Young's tap.
1.7 Time-resolved spectroscopy 0.1% concentration by weight is prepared using zeonex as a host dissolved in toluene. The mixture is then dropcast onto the sapphire substrate. Time-resolved spectra are acquired by exciting the sample with N 2 laser (LTBMNL 100, Lasertechnik Berlin) at 10 Hz at 337 nm due to its superior power stability. Sample emission is directed onto a spectrograph and gated iCCD camera (Stanford Computer Optics). For the low temperature measurement, Janis Research VNF-100 cryostat is used in conjunction with a Lakeshore 332 temperature controller. The solutions underwent at least four freeze/thaw/pump cycles on a custom made 1 cm path-length quartz cuvette .Phosphorescence is obained by cooling the sample to 80 K with with a gate time delay of 90 ms and 5 ms gate window. For the TRZ:TPA exciplex system, equal molar amounts of TRZ:TPA were added at 1% weight to a zeonex chloroform solution before drop-casting onto a sapphire substrate. The sample is excited at 10 Hz Nd:YAG Laser (EKSPA)

Computation
To search for conformer, Chemdraw 3D is used to run the molecular dynamics at 400 K with 2 kcal/atom/ps heating rate, sampling every 2000 steps in order to generate 50 samples. The conformers are then minimized using Gaussian 09W at PM6 level and the structure of the molecules are ranked based on thier Gibbs Free Energy The conformations having the lowest energy and yet distinct structure are then extracted to be further optimized using increasing basis set (ie, sto-3g, 3-21g, 6-31g and finally cc-pVDZ) using density functional theory (DFT).These structures remains distinct with respect to each other after DFT claculations confirming the existance of the conformations. Range-separated hybrid functional using long-ranged corrected LC-ɯPBE is tuned so that J 2 is minimum where J 2 is given as where ε H is the HOMO energy and IP is the ionization potential as applied in neutral (N) and anion (N+1) systems . IP-tuning is performed using Terachem 1.9 software [3] and GPU server that had 64 GB RAM installed to support eight Tesla K10 graphic cards. Vertical excited and vertical fluorescence states are obtained using TD-DFT/ LC-ɯPBE// cc-pVDZ at optimal ɯ. All calculations are done in vacuum. This tuned range-separated LC-ωPBE* functional is used () in order to minimize localization/delocalization error for CT states and was found to be a better predictor for vertical excitation (VE) energy of the singlet state (E VE (S 1 )) for CT molecules despite the fact that it tends to over-estimate E VE (S 1 ) [4] The results are visualized using VMD software. Reduced density gradient is calculated at the ground state and visualized using Multiwfn software [5] .

Device Fabrication and measurement
All organic materials used in experiments (except for the TADF emitters) were purchased from Lumtec, Inc. All compounds were subjected to temperature-gradient sublimation under high vacuum before use. OLEDs were fabricated on the ITO-coated glass substrates with multiple organic layers sandwiched between the transparent bottom indium-tin-oxide (ITO) anode and the top metal cathode. All material layers were deposited by vacuum evaporation in a vacuum chamber with a base pressure of ≤ 10 -6 torr. The deposition system permits the fabrication of the complete device structure in a single vacuum pump-down without breaking vacuum. Indium tin oxide (ITO), MoO 3 , LiF, and Al were the anode, hole-injection layer, electron-injection layer and cathode, respectively. Di-[4-(N,N-ditolyl-amino)-phenyl]-cyclohexane (TAPC) and N,N-dicarbazolyl-3,5-benzene (mCP) were hole-transport layers (HTL). The bipolar and large-triplet-energy mCPCN host constituted the emitting layer (EML). Tris-[3-(3-pyridyl)mesityl]borane (3TPYMB) was the electron-transport layer (ETL) [6] The deposition rate of organic layers was kept at 0.1-0.2 nm/s. The doping was conducted by co-evaporation from separate evaporation sources with different evaporation rates. The active area of the device is 1 x 1 mm 2 , as defined by the shadow mask for cathode deposition. The current-voltage-brightness (I-V-L) characterization of the light-emitting devices was performed with a source-measurement unit (SMU) and a spectroradiometer (DMS 201, AUTRONIC-MELCHERS GmbH). EL spectra of devices were collected by a calibrated CCD spectrograph. The external quantum efficiencies of devices were determined by collecting the total emission fluxes with a calibrated integrating-sphere measurement system and by measuring the angular distribution of the emission spectra and intensities. S10     : IP tuning of LC-ɯPBE for the materials studied. It seems that minimum J 2 , ɯ seems to concentrate around 0.15-0.20. ε H is the HOMO energy and IP is the ionization potential as applied in neutral (N) and anion (N+1) systems. J min for the compounds studied are ~ 0.01 eV or less.      Thermogravimetric and Differential Scanning Calorimetry Analysis.

Supplementary Figures and Tables
Thermal stability of these new molecules PXZ-TPA-TRZ, DMAC-TPA-TRZ, PXZ-Ph-TRZ, and DMAC-Ph-TRZ were analyzed by thermogravimetric analysis (TGA). The decomposition temperature (T d ) corresponding to 5% weight loss of PXZ-TPA-TRZ, DMAC-TPA-TRZ, PXZ-Ph-TRZ, and DMAC-Ph-TRZ was determined to be 361, 372, 303, and 315 o C, respectively ( Figure S20). The data were summarized in Table S4. All of these molecules display good thermal stability suitable for the preparation of thin films by thermal evaporation. In addition, the analysis by differential scanning calorimetry (DSC) indicated that PXZ-TPA-TRZ and DMAC-TPA-TRZ exhibit a glass transition temperature (T g ) of 122 and 141 o C, respectively ( Figure S21).

Cyclic voltammetry
The electrochemical properties of PXZ-TPA-TRZ, DMAC-TPA-TRZ, PXZ-Ph-TRZ, and DMAC-Ph-TRZ were measured by cyclic voltammetry (CV) as shown in Figure S22. CV of all the molecules display a quasireversible oxidation and reduction, indicating their bipolar electrochemical properties. The corresponding HOMO/LUMO energy levels of PXZ-TPA-TRZ, DMAC-TPA-TRZ, PXZ-Ph-TRZ, and DMAC-Ph-TRZ were deduced from on-set of their oxidation/reduction potentials referred to the redox potential of ferrocene to be -5.16/-2.79, -5.20/-2.71, -5.27/-2.70, and -5.53/-2.64, respectively. Compared to DMAC-TPA-TRZ, it is obvious that PXZ as a donor bridge leads to a higher HOMO level and a lower LUMO level as observed in PXZ-TPA-TRZ. This result indicates that stronger electron-donating character of PXZ than that of DMAC. The observation of small energy band gap in PXZ-TPA-TRZ suggests that a better conjugation in this D-D'-A molecule. The similar result is also observed in ortho-phenyl substituted molecules, where PXZ-Ph-TRZ exhibits a higher HOMO level and a small energy band gap as compared to those of DMAC-Ph-TRZ. The ortho-triphenylamine-substituted molecule PXZ-TPA-TRZ exhibits a higher HOMO level than that of phenyl-substituted counterpart PXZ-Ph-TRZ. This result reveals that the introduction of triphenylamino group onto PXZ can increase the electron-donating ability. The same electronic effect is also observed in DMAC cases.    Table S1: Crystallographic data of PXZ-TPA-TRZ, PXZ-Ph-TRZ and DMAC-Ph-TRZ Table S2: Integrated intensity along with the ratio between degassed and aerated obtained from Figure S3 Molecule solvent degassed aerated degassed/aerated The fluorescence decay is usually well fitted by the sum of two exponentials, one describing the PF and DF decays, see equation below. The Φ DF /Φ PF ratio is then easily determined to give reverse intersysten crossing rate (K RISC )

!(ns)
PXZ-TPA -TRZ    a UV-Vis on-set spectra measured in MCH solution. b Fluorescence maximum wavelength in MCH solution. c Lifetime of the prompt and the delayed component measured in a zeonex matrix. d HOMO level calculated from onset of the oxidation potential; LUMO level calculated from onset of the reduction potential e The glass transition temperature; Decomposition temperature corresponding to 5% weight loss in the thermogravimetric analyses.