Thermally activated delayed fluorescence emitters showing wide-range near-infrared piezochromism and their use in deep-red OLEDs

Organic small molecules exhibiting both thermally activated delayed fluorescence (TADF) and wide-ranging piezochromism (Δλ > 150 nm) in the near-infrared region have rarely been reported in the literature. We present three emitters MeTPA-BQ, tBuTPA-BQ and TPPA-BQ based on a hybrid acceptor, benzo[g]quinoxaline-5,10-dione, that emit via TADF, having photoluminescence quantum yields, ΦPL, of 39–42% at photoluminescence (PL) maxima, λPL, of 625–670 nm in 2 wt% doped films in 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP). Despite their similar chemical structures, the PL properties in the crystalline states of MeTPA-BQ (λem = 735 nm, ΦPL = 2%) and tBuTPA-BQ (λem = 657 nm, ΦPL = 11%) are significantly different. Further, compounds tBuTPA-BQ and TPPA-BQ showed a significant PL shift of ∼98 and ∼165 nm upon grinding of the crystalline samples, respectively. Deep-red organic light-emitting diodes with MeTPA-BQ and tBuTPA-BQ were also fabricated, which showed maximum external quantum efficiencies, EQEmax, of 10.1% (λEL = 650 nm) and 8.5% (λEL = 670 nm), respectively.

Air-sensitive reactions were performed under a nitrogen atmosphere using Schlenk techniques, no special precautions were taken to exclude air or moisture during work-up and crystallization.
Anhydrous tetrahydrofuran, acetonitrile, and toluene were obtained from an MBraun SPS5 solvent purification system.Flash column chromatography was carried out using silica gel (Silica-P from Silicycle, 60 Å, 40-63 µm).Analytical thin-layer-chromatography (TLC) was performed with silica plates with aluminum backings (250 µm with F-254 indicator).TLC visualization was accomplished by 254/365 nm UV lamp.HPLC analysis was conducted on a Shimadzu LC-40 HPLC system.HPLC traces were performed using an ACE Excel 2 C18 analytical column.HPLC and gel permeation chromatography (GPC) was conducted on a Shimadzu LC-40 HPLC system.
GPC trace was performed using a Shim-pack GPC-803 column with THF as mobile phase. 1 H, and 13 C spectra were recorded on a Bruker Advance spectrometer (400 MHz for 1 H, 101 or 125 MHz for 13 C).The following abbreviations have been used for multiplicity assignments: "s" for singlet, "d" for doublet, "t" for triplet, "dd" for doublet of doublets and "m" for multiplet. 1H and 13 C NMR spectra were referenced residual solvent peaks with respect to TMS (δ = 0 ppm).Melting points were measured using open-ended capillaries on an Electrothermal 1101D Mel-Temp apparatus and are uncorrected.High-resolution mass spectrometry (HRMS) was performed at the University of Edinburgh.Elemental analyses were performed by the School of Geosciences at the University of Edinburgh.PXRD was performed at room temperature using a PANalytical Empyrean diffractometer in Bragg-Brentano geometry using Cu Kα1 radiation with a primary beam monochromator (λ = 1.54060Å).
Theoretical Calculations.All ground-state optimizations have been carried out at the Density Functional Theory (DFT) level with Gaussian16 4 using the PBE0 functional 5 and the 6-31G(d,p) basis set 6 .Excited-state calculations have been performed at Time-Dependent DFT (TD-DFT) within the Tamm-Dancoff approximation (TDA) 7 using the same functional and basis set as for ground state geometry optimization.Spin-orbit coupling matrix elements (ξ) were calculated based on the optimized singlet excited state geometry.Molecular orbitals were visualized using GaussView 6.0 8 .Calculations were automated using an in-house designed software package, Silico, which uses a number of 3 rd party libraries and programs, including extraction and processing of results: cclib ).An Ag/Ag + electrode was used as the reference electrode while a platinum electrode and a platinum wire were used as the working electrode and counter electrode, respectively.The redox potentials are reported relative to a saturated calomel electrode (SCE) with a ferrocenium/ferrocene (Fc/Fc + ) redox couple as the internal standard (0.46 V vs SCE) 12 .
Photophysical measurements.Optically dilute solutions of concentrations on the order of 10 -5 or 10 -6 M were prepared in spectroscopic or HPLC grade solvents for absorption and emission analysis.Absorption spectra were recorded at room temperature on a Shimadzu UV-2600 double beam spectrophotometer with a 1 cm quartz cuvette.Molar absorptivity determination was verified by linear regression analysis of values obtained from at least four independent solutions at varying concentrations with absorbance ranging from 0.48 to 0.17 for MeTPA-BQ, 0.15 to 0.37 for tBuTPA-BQ and 0.54 to 0.19 for TPPA-BQ.
Toluene solutions were degassed via three freeze-pump-thaw cycles.Steady-state emission and time-resolved emission spectra were recorded at 298 K using an Edinburgh Instruments FS5 fluorimeter.Thin films were then spin-coated on a quartz substrate using a spin speed of 1500 rpm for 60 s.Absolute photoluminescence quantum yields (ΦPLs) were determined using an integrating sphere that is equipped with an FS5 spectrometer.The ΦPLs were measured in air and N2 environment by purging the integrating sphere with N2 gas flow for 2 min.Steady-state PL spectra were measured using a xenon lamp as the source.Time-gated PL spectra (delayed emission/phosphorescence) were measured using a pulsed microsecond flash lamp by the multichannel scaling (MCS) mode in FS5.The time-gated PL spectra for the samples were collected between 1-9 ms (λexc = 450 nm).Temperature-dependent (100 to 298 K) measurements were performed using an Oxford Instruments OPTISTAT DN-V cryostat controlled by an Oxford Instruments Mercury iTC temperature controller connected to the FS5 spectrometer.Samples were allowed to equilibrate at each temperature before measurements were conducted.
The singlet-triplet energy splitting (∆EST) in 2-MeTHF was estimated from the onset of steadystate and phosphorescence emission at 77 K (λexc = 450 nm).Phosphorescence spectra collected by using pulsed microsecond flashlamp (1-9 ms).Prompt fluorescence lifetimes were measured using a picosecond pulsed diode laser (375 nm).Phosphorescence lifetimes were measured using a pulsed xenon microsecond flash lamp.Samples were ground mechanically using a mortar and pestle for 5 min.Steady-state PL (λexc = 450 nm) and time-resolved measurements, prompt lifetime using TCSPC (λexc = 375 nm) and delayed lifetime using microsecond flash lamp (λexc = 340 nm), were conducted for the as-prepared and ground samples.
Fitting of time-resolved luminescence measurements: Time-resolved PL measurements were fitted to a sum of exponentials decay model, with chi-squared (χ 2 ) values between 1 and 2, using the EI FLS980 software.Each component of the decay is assigned a weight, (wi), which is the contribution of the emission from each component to the total emission.
The average lifetime was then calculated using the following: • Two exponential decay model: where A1 and A2 are the preexponential-factors of each component.
• Three exponential decay model: with weights defined as where A1, A2 and A3 are the preexponential-factors of each component.
Toluene (30 mL) was added to the mixture under an N2 atmosphere.The mixture was stirred at S12 100 °C for 12 h, then cooled to room temperature and quenched with brine (80 mL).The product was extracted with CH2Cl2, washed several times with water (2 × 100 mL), dried over anhydrous Na2SO4 and concentrated in vacuo.The crude mixture was purified by silica gel flash column chromatography using DCM : hexane = 2:1 as the eluent to afford the desired compound as a white solid.Yield: 74%.Rf: 0.33 (DCM : hexane = 2:1 on silica gel).Mp: 252-254 °C.

X-ray crystallography
X-ray diffraction data for MeTPA-BQ and tBuTPA-BQ were collected at 125 K using a Rigaku MM-007HF High Brilliance RA generator/confocal optics [Cu Kα radiation (λ = 1.54187Å)] with XtaLAB P200 diffractometer.Intensity data were collected using ω steps accumulating area detector images spanning at least a hemisphere of reciprocal space.Data for both compounds were collected using CrystalClear 13 and processed (including correction for Lorentz, polarization and absorption) using CrysAlisPro. 14Structures were solved by direct methods (SIR2011 15 ) and refined by full-matrix least-squares against F 2 (SHELXL-2019/3 16 ).Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were refined using a riding model.Two of the t-butyl groups in tBuTPA-BQ showed rotational disorder, and the methyl sites were split over two locations, the minor component of one disordered t-butyl group requiring isotropic refinement.The disordered atoms were refined with restraints to bond distances and thermal motion.All calculations were performed using the Olex2 Table S2.Particle (blue) and hole (red) for S1, T1 and T2 states estimated from S0 equilibrium geometries.
-7  mbar.A pre-patterned glass substrate coated with indium-doped tin oxide (ITO) was cleaned sequentially by ultrasonication in acetone and isopropanol for 15 minutes.The temperature of the ultrasonication bath was set at 60-70 °C.The cleaned substrate was exposed to oxygen plasma for 3 min to remove all dust and organics on the ITO surface and to increase the work function of the ITO anode for better hole injection from the anode to the organic layer.The substrate was loaded in the thermal evaporator.The organic layers were deposited at a rate of 0.3-1.0Å/s, monitored using a quartz crystal.The electron injection layer, LiF, was deposited at a rate of 0.05 Å/s, while the Al cathode was deposited initially with a rate of 0.5 Å/s to obtain 10 nm thickness and after that, the rate of the Al cathode was increased to 3 Å/s.Two custom-made shadow masks were used to define the area of the evaporations.The organic layers and LiF were evaporated with the same shadow mask, but Al was evaporated with the other mask.The active area of the OLED was 2 mm 2 , determined by the spatial overlap of the anode and cathode electrodes.All the devices were encapsulated with glass lids and UV epoxy resin inside a N2-filled globe box.The luminance-current-voltage characteristics were measured in an ambient environment using a Keithley 2400 source meter and a homemade photodiode circuit connected to a Keithley 2000 multimeter for the voltage reading.The external quantum efficiency was calculated assuming the Lambertian emission pattern for the OLEDs.The electroluminescence spectra were recorded by an Andor DV420-BV CCD spectrometer.

Figure S26 .
Figure S26.TRPL spectra of (a) sublimed (b) as-prepared and (c) ground samples of tBuTPA-BQ (indicating no PL decay observed for these samples).
Figure S28.Absorption spectra of the as prepared and ground samples of the tBuTPA-BQ and TPPA-BQ.
V and pulse amplitude, width, and period of 50 mV, 0.05, and 0.5 s, respectively.Samples were prepared as DCM solutions, which were degassed by sparging with MeCN-saturated argon gas for 5 minutes prior to measurements.All measurements were performed using 0.1 M DCM solution of tetra-n-butylammonium hexafluorophosphate ([ n Bu4N]PF6]

Table S1 .
17interface.Selected crystallographic data are presented in TableS1.CCDC 2296442-2296443 contains the supplementary crystallographic data for this paper.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.Selected crystallographic data.

Table S5 .
Decay components of prompt emission and delayed emission and average lifetimes of of MeTPA-BQ, tBuTPA-BQ and TPPA-BQ in doped CBP films.

Table S6 .
EL performance of representative deep-red TADF OLEDs with emission peak between 650 and 670 nm.